Application of an intramolecular dipolar cycloaddition to an asymmetric synthesis of the fully oxygenated tricyclic core of the stemofoline alkaloids

doi:10.1016/j.tet.2008.02.008

Tetrahedron. Author manuscript; available in PMC 2009 April 21.

Published in final edited form as:

Tetrahedron. 2008 April 21; 64(17): 3629–3641.

doi: 10.1016/j.tet.2008.02.008

PMCID: PMC2352155

NIHMSID: NIHMS44224

Application of an intramolecular dipolar cycloaddition to an asymmetric synthesis of the fully oxygenated tricyclic core of the stemofoline alkaloids

Ryan J. Carra,^a Matthew T. Epperson,^b and David Y. Gin^a^*

^aMemorial Sloan-Kettering Cancer Center, 1275 York Avenue, New York, NY, 10065, USA

^bDepartment of Chemistry, University of Illinois, Urbana, IL, 61801, USA

* Corresponding author. Tel.: +1-646-888-2555; fax: +1-646-422-0416; e-mail: gind/at/mskcc.org

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Abstract

An intramolecular non-stabilized azomethine ylide dipolar cycloaddition was applied toward the first non-racemic synthesis of the fully-oxygenated bridged pyrrolizidine core (45) of (+)-stemofoline (1) in eleven steps from a commercially available starting material.

1. Introduction

Stemofoline (1, Figure 1) was first isolated from the stems and leaves of Stemona japonica by Irie and coworkers in 1970.¹ X-Ray crystallographic analysis of the hydrobromide salt of 1 revealed a pyrrolizidine ring system containing a two-carbon bridge connecting C7 and C9a and an angular butyl side chain at C3. Stemofoline also contains a spiroketal functionality at C8 and (Z)-olefin bridge from a tetrahydrofuran ring to a conjugated butenolide moiety. The rigid pentacyclic core of 1 possesses three heteroatoms and seven contiguous stereogenic centers.²^,³

Figure 1

Selected stemofoline alkaloids.

Of the eleven members of the stemofoline alkaloids which have been reported to date,⁴^–⁸ most differ only in the oxidation state of the C3 chain (cf. 1 versus 2 or 3) or the (E)- vs. (Z)-configuration of the C11–C12 alkene (1 versus 4). Notably, Sekine and coworkers reported isolation of 2, which they named asparagamine A, from the roots of Asparagus racemosus.⁴ Later studies suggested that Sekine had actually isolated 2 from Stemona collinsae, which is commonly confused with A. racemosus.⁵ Parallel studies found 2 to be a major component of the ethanolic extracts of S. collinsae,⁶ along with 19-(S)-hydroxystemofoline (3).⁵ Since it appears to have no connection to the Asparagus plant genus, 2 is now commonly referred to as didehydrostemofoline.

The Thai plant from which Sekine and coworkers isolated didehydrostemofoline (2) had traditionally been administered to pregnant women to arrest premature uterine contractions, and it was hypothesized that 2 was the active agent. Indeed, 2 was found not only to inhibit oxytocin-induced labor in pregnant rats but also to exhibit dose-dependent in vivo activity against Kato-III human gastric carcinoma cells.⁹ More recent explorations have focused on the potential of Stemona alkaloids as natural insecticides, as 1–3 were found to exhibit insecticidal and growth-inhibitory activity against the neonate larvae of Spodoptera littoralis.⁵^,¹⁰ Additionally, 1 and 2 have shown insecticidal and antifeedant activity against the larvae of the diamondback moth, a vegetable crop pest.⁶

Owing to their complex molecular architecture and intriguing biological activity, the stemofoline alkaloids have attracted considerable synthetic attention,¹¹^–¹⁴ culminating in two racemic total syntheses.¹⁵^,¹⁶ The studies reported herein comprise the first non-racemic synthesis of the fully oxygenated tricyclic core of the stemofoline alkaloids.

2. Results and Discussion

The utility of azomethine ylide [3+2] cycloadditions for the construction of highly-substituted pyrrolidine rings has been demonstrated by its implementation in the total synthesis of numerous alkaloid natural products.¹⁷^–²¹ Non-stabilized azomethine ylides are often generated by desilylation of iminium salts derived from the O-activation of amides¹⁷^,²² or the N-alkylation of vinylogous imidates.²³ In a related strategy, we recently reported the use of N-(trimethylsilyl)methyl vinylogous amides as precursors to azomethine ylides suitable for the preparation of substituted pyrrolizidines and indolizidines.¹³ This method proceeded by O-activation of (trimethylsilyl)methyl vinylogous amides such as 5 with trifluoromethanesulfonic (triflic) anhydride, followed by desilylation with the anhydrous fluoride source tetrabutylammonium triphenyldifluorosilicate (TBAT) to generate azomethine ylides such as 6, which underwent [3+2] cycloadditions with a variety of electron-poor olefins (Scheme 1). Extension of the method to an intramolecular mode led to a synthesis of the C2-deoxy bridged pyrrolizidine core of the stemofoline alkaloids. Herein we disclose the synthesis of the fully-oxygenated core of this family of alkaloids, overcoming notable stereochemical impedances in installing the C2 oxygen functionality.

Scheme 1

Dipolar Cycloadditions of azomethine ylides derived from vinylogous amides.

Two synthetic routes were explored (Scheme 2), one involving an intermolecular mode of the azomethine ylide cycloaddition, and the other focusing on an intramolecular variant. In the former (Scheme 2A), vinylogous amide 8, incorporating the requisite C2-OR group, was envisioned to be a suitable precursor to azomethine ylide formation, followed by endo-selective [3+2] cycloaddition with methyl acrylate to provide pyrrolizidine 9. Subsequent trans-annular Dieckmann condensation could then effect formation of the C8–C9 bond to provide bridged pyrrolizidine 10, which directly maps onto the aza-polycyclic substructure within stemofoline (1). Alternatively in an intramolecular cycloaddition approach to the target (Scheme 2B), the more elaborate vinylogous amide 11, incorporating both the C2-OR group and the C6-C7 dipolarophile, could be used to generate the azomethine ylide to trap the pendant electron-poor olefin, providing the suitably functionalized bridged pyrrolizidine 12 directly.

Scheme 2

Proposed synthesis of C2 oxygenated bridged pyrrolizidine core of the stemofoline alkaloids via inter- or intramolecular dipolar azomethine ylide cycloaddition.

The intermolecular cycloaddition-Dieckmann approach began with Arndt–Eistert homologation of the readily available N-benzyloxycarbonyl-L-glutamic acid-5-methyl ester (13),²⁴ which provided bis(methyl ester) 14 (79%, Scheme 3). Selective α-hydroxylation of the ester distal to the N-functionality with Davis oxaziridine 15 allowed for introduction of the C2-oxygen group, providing the secondary alcohol 16 as a single diastereomer (52%). Hydrogenolysis of the benzyl carbamate to the corresponding primary amine was followed by lactamization in hot pyridine. Verification of stereochemistry was not conducted until silylation of the secondary alcohol in 16 to provide the lactam 17 (95%, 3 steps), whose cis-stereochemical relationship was established by a nOe signal between the C9a and C2 methine proton resonances. However, alkylation of 17 with (chloromethyl)trimethylsilane proceeded with epimerization of the C2 stereocenter to yield its C2-R configuration (60%).²⁵ Nevertheless, this was anticipated to be a welcome occurrence given that this C2 substituent can be envisioned to serve as a favorable stereochemical determinant in the key cycloaddition. Lactam 18 was converted to the corresponding (E)-vinylogous amide 19 via a two-pot sequence involving thionation with Lawesson’s reagent (95%) followed by condensation of the resulting thiolactam with 1-bromo-2-butanone and phosphine-mediated Eschenmoser sulfide contraction (71%).

Scheme 3

a) EtOCOCl, Et₃N, THF, 0 °C; CH₂N₂, Et₂O, −10 to 23 °C, 87%; b) AgOAc, MeOH, 23 °C, 91%; c) LHMDS, THF, −78°C, 52%; d) H₂ (1 atm), 10% Pd/C, MeOH, 23 °C; e) pyridine, 100 °C; f) TBSCl, imidazole, (more ...)

In accordance with the cycloaddition method previously developed in our laboratories, vinylogous amide 19 was treated sequentially with Tf₂O and TBAT in the presence of the dipolarophile methyl acrylate (Scheme 4). It was anticipated that the α-oriented C2-OTBS ether would bias approach of the dipolarophile onto the β-face of the transient azomethine ylide in an endo transition state to provide the cycloadduct 9, originally depicted in Scheme 2A. Although a regio- and endo-selective cycloaddition ensued, it proceeded with an unexpected stereochemical outcome with the isolation of the pyrrolizidine 22 (55%, Scheme 4). The structure of 22, a result consistent with in situ C2-epimerization leading to α-approach of the dipolarophile, was verified by a battery of NMR techniques, most notably nOe signals between the C9a and C2 methine protons, as well as between the vinyl and C7 methine protons. It is likely that the initially formed iminium triflate 20 underwent epimerization to the more thermodynamically stable cis-stereoisomer 21, in which both the C9a and C2 substituents occupied pseudo-equatorial orientations in an envelope conformation. Subsequent approach of the dipolarophile to the cis-substituted azomethine ylide corresponding to 21, from the face opposite the C9a and C2 substituents in a sterically governed cycloaddition, led to the formation of pyrrolizidine 22.

Scheme 4

Intermolecular dipolar azomethine ylide cycloaddition proceeds with C2 epimerization.

Support for the rationale of C2-epimerization at the iminium stage of the reaction was obtained by in situ NMR monitoring of the O-triflation process of the trans-substituted vinylogous amide 19 (Scheme 5) in CDCl₃. Clean formation of the corresponding vinylogous iminium triflate was observed within 1 hr, providing a 4:1 mixture of the trans- and cis-iminium triflates 20 and 21, respectively. After extending the NMR reaction time to 4.5 hr, further C2-epimerization occurred to provide a 1:2 ratio of 20:21, signaling the thermodynamic preference for the cis-iminium triflate 21. Unfortunately, all attempts to modify reaction conditions to avoid C2-epimerization, or to effect C9a- or C3-epimerization via reversible β-elimination, were unsuccessful.²⁶ Notably, the non-epimerizeable C9a and C3 substituents in the observed cycloadduct 22 occupied the same face of the pyrrolizidine ring system; thus, it was not possible to execute the transannular Dieckmann condensation for the advancement of the synthesis to stemofoline (1).

Scheme 5

C2 Epimerization of iminium triflate.

Given the stereochemical impasse in the intermolecular cycloaddition approach, efforts then focused on the intramolecular cycloaddition strategy utilizing the novel vinylogous amide 11 (Scheme 2B). In this route, face-selective approach of the pendant dipolarophile in the substrate 11 would be dictated solely by the C9a-configuration irrespective of the stereoconfiguration at C2. Indeed, the success of a related intramolecular cycloaddition in our earlier model study with a simplified substrate devoid of C2-oxygenation boded well for this strategy.¹³ This approach began with N-methoxymethyl amide 22, available in three steps from N-benzyloxycarbonyl-L-glutamic acid-5-methyl ester.¹³ Hydroxylation of the lithium enolate of 23 (Scheme 6) with Davis oxaziridine 15 provided the secondary alcohol 24 as a single diastereomer (57%). Hydrogenolysis of the benzyl carbamate (Pd/C, H₂) preceded spontaneous lactamization to provide 25 (94%). Selective protection of the secondary alcohol as a TBS ether (91%) was followed by N-alkylation of the lactam with (chloromethyl)trimethylsilane, which proceeded with C2 epimerization, providing a 7:1 mixture (70% overall) favoring the trans-lactam isomer 27 over its cis-counterpart 26, whose structure was verified by the presence of a C9a-C2 methine nOe. Lactam 27 was elaborated to the (E)-vinylogous amide 29 by thionation with Lawesson’s reagent (86%) and subsequent Eschenmoser sulfide contraction (52%, Scheme 7). The dipolarophile was installed by treatment of 29 with ethynylmagnesium chloride and trapping the resulting reactive ynone with ethane thiol, providing the trans vinylogous thioester 30 (57%, 2 steps), thereby setting the stage for an intramolecular azomethine ylide cycloaddition.

Scheme 6

a) LHMDS, THF, −78°C, 57%; b) 1 atm H₂, 10% Pd/C, MeOH, 23 °C, 94%; c) TBSCl, imidazole, DMF, 23 °C, 91%; d) NaH, TMSCH₂Cl, DMF, 23 °C, 70%.

Scheme 7

a) Lawesson's reagent, PhMe, 23 °C, 86%; b) 1-bromo-2-butanone, 60 °C; Ph₃P, Et₃N, CH₂Cl₂, 23 °C, 52%; c) HCCMgCl, THF, 23 °C; d) EtSH, Et₃N, CH₂Cl₂, 23 °C, 57% (2 steps).

In this key reaction, clean formation of iminium triflate 31 was observed by ¹H-NMR upon treatment of vinylogous amide 30 with Tf₂O; however, only an intractable mixture of products was obtained after addition of TBAT. Based on analogy to the C2 epimerization of iminium triflate 20 (cf. Scheme 4), it is likely that fluoride anion, prior to desilylating iminium triflate 31, acted as a general base to effect C2 epimerization, resulting in a rapid equilibrium favoring iminium 34, in which the C9a and C2 substituents would both occupy pseudo-equatorial positions in the envelope conformation. The observed lack of productive cycloaddition might arise from the absence of any appreciable quantity of ylide 32 that could react to form the desired cycloadduct 33 (Scheme 8, Path A). Rather, the reaction was dominated by the cis-iminium 34, kinetically favoring decomposition pathways over cycloaddition. This outcome could be rationalized by an energetically disfavored cycloaddition transition state 35 (Scheme 8, Path B), incorporating severe C9a-C2 1,3-diaxial interactions, giving way to other, as yet unidentified, unproductive reaction manifolds.

Scheme 8

C2 Epimerization of Iminium Triflate.

Given the likelihood of C2-epimerization in 31 imparting a detrimental effect on the intramolecular cycloaddition strategy, a modification of this approach was explored, employing a cycloaddition substrate incapable of C2-epimerization. In this context, iminium triflate 37 (Scheme 9), in which the C1 and C2 oxygen substituents are tethered as part of a cis-fused isopropylidene ketal, was pursued. In this intermediate, C2 epimerization would be prohibited as a result of formation a high-energy trans-fused [3.3.0]-bicyclo system.

Scheme 9

Proposed Dipolar Azomethine Ylide Cycloaddition.

Synthesis of the newly designed cycloaddition substrate began with treatment of the commercially available (−)-2,3-O-isopropylidine-D-erythronolactone (39) with (trimethylsilyl)methyl amine (97%, Scheme 10). Parikh-Doering oxidation of the resulting primary alcohol furnished a hemiaminal, which was acetylated to afford aminal 40 (69%, 2 steps). Lewis-acid promoted Mannich reaction between aminal 40 and the trimethylsilylketene acetal of ethyl acetate (41, 72%) was followed by hydrolysis of the resulting ester to provide carboxylic acid 42 (96%). Thionation of the lactam functionality within 42 was accomplished by heating in the presence of Lawesson’s reagent (95%), and the C8-carboxyl group in the resulting product was then converted to the corresponding N-methoxy-N-methyl amide 43 (62%). Eschenmoser sulfide contraction (81%), and then treatment with ethynylmagnesium bromide and ethane thiolate (58%, 2 steps) installed the required functionality for the proposed intramolecular [3+2] azomethine ylide cycloaddition. In this event, sequential treatment of 44 with triflic anhydride and TBAT provided the desired polycyclic alkaloid 45 in 71% yield. This result comprises the first non-racemic synthesis of the fully-oxygenated bridged pyrrolizidine core of the stemofoline alkaloids.

Scheme 10

a) TMSCH₂NH₃Cl, Et₃N, THF, 70°C, 97%; b) SO₃·pyr, Et₃N, DMSO, 23 °C; c) Ac₂O, pyridine, 23 °C, 69% (2 steps); d) TMSOTf, CH₂Cl₂, 23 °C, 72%; e) LiOH, THF, H₂O, 23 °C, 96%; f) Lawesson's reagent, PhMe, 65°C, (more ...)

Further advancement of the cycloadduct 45 (Scheme 11) involved reduction of its enol triflate functionality under a hydrogen atmosphere in the presence of Pd/C to provide pyrrolizidine 46 (89%), which contains the saturated butyl side chain present in (+)-stemofoline (1). Installation of a two-carbon functionality at the equatorial position of C9 was accomplished by enolate alkylation of ketone 46 with ethyl iodoacetate, followed by C9 equilibration (DBU, heat), to afford the ethyl ester 47 (58%, 2 steps). Treatment of 47 with aqueous hydrochloric acid effected deprotection of the isopropylidene group and hydrolysis of the ester (96%), providing a versatile intermediate 48 to explore strategies for butenolide construction and C8 ketal closure en route to the stemofoline alkaloids.

Scheme 11

a) 1 atm H₂, 10% Pd/C, MeOH, 23 °C, 89%; b) LDA, ICH₂CO₂Et, THF, HMPA, 0 °C; c) DBU, PhMe, 80°C, 58% (2 steps); d) 2.5M HCl, THF, 60 °C, 96%.

3. Conclusion

A key observation regarding the dependence of α-stereochemistry in the azomethine ylide generation and intramolecular dipolar cycloaddition of vinylogous amide 44 led to the development of a successful non-racemic synthesis of the bridged pyrrolizidine core of the stemofoline alkaloids.

4. Experimental Section

4.1. General

All reactions were performed in flame-dried modified Schlenk (Kjeldahl shape) flasks fitted with a glass stopper under a positive pressure of argon, unless otherwise noted. Air- and moisture-sensitive liquids and solutions were transferred via syringe. Organic solutions were concentrated by rotary evaporation below 30 °C. Flash column chromatography was performed employing 230–400 mesh silica gel. Thin-layer chromatography (analytical and preparative) was performed using glass plates pre-coated to a depth of 0.25 mm with 230–400 mesh silica gel impregnated with a fluorescent indicator (254 nm). Infrared (IR) spectra were obtained using a Perkin Elmer Spectrum BX spectrophotometer or a Bruker Tensor 27 referenced to a polystyrene standard. Data are presented as the frequency of absorption (cm⁻¹). Proton and carbon-13 nuclear magnetic resonance (¹H NMR or ¹³C NMR) spectra were recorded on a Varian 400, a Varian 500, Varian Inova 500 NMR, or a Bruker Avance III spectrometer; chemical shifts are expressed in parts per million (δ scale) downfield from tetramethylsilane and are referenced to the residual protium in the NMR solvent (CHCl₃: δ 7.26 for ¹H NMR, δ 77.16 for ¹³C NMR; CD₃OD: δ 3.30 for ¹H NMR, δ 49.00 for ¹³C NMR). Data are presented as follows: chemical shift, multiplicity (s = singlet, bs = broad singlet, d = doublet, bd = broad doublet, t = triplet, m = multiplet and/or multiple resonances), coupling constant in Hertz (Hz), integration, assignment.

4.2. Dimethyl 3-(benzyloxycarbonylamino)hexanedioate (14)

To a stirred solution of 2-benzyloxycarbonylamino-pentanedioic acid 5-methyl ester (13) (4.45 g, 15.1 mmol, 1.0 equiv) and triethylamine (2.31 mL, 16.5 mmol, 1.1 equiv) in tetrahydrofuran (80 mL) at −10 °C was added via syringe ethyl chloroformate (1.60 mL, 16.5 mmol, 1.1 equiv). The white suspension was stirred at −10 °C for one hour before the addition of diazomethane (0.58 M solution in diethyl ether, 80 mL, 47 mmol, 3.1 equiv). The bright yellow suspension was stirred at −10 °C for one hour and then 23 °C for three hours. The reaction mixture was then diluted with diethyl ether (600 mL) and washed with water (1 x 150 mL). The organic layer was separated, dried over magnesium sulfate, filtered, and concentrated. Purification by silica gel flash chromatography (50% hexanes in ethyl acetate) provided 4-benzyloxycarbonylamino-6-diazo-5-oxo-hexanoic acid methyl ester (4.21 g, 87%) as a yellow solid. R_f = 0.43 (50% hexanes in ethyl acetate); ¹H NMR (CDCl₃) δ 7.37 – 7.30 (m, 5H, Ph), 5.60 (d, 1H, J = 9.2 Hz, NH), 5.50 (br s, 1H, CHN₂), 5.09 (s, 2H, CH₂Ph), 4.31 (m, 1H, CH₂CH₂CHC(O)CHN₂), 3.67 (s, 3H, CO₂CH₃), 2.46 (m, 1H, CH₂CO₂CH₃), 2.39 (m, 1H, CH₂CO₂CH₃), 2.16 (m, 1H, CH₂CH₂CHC(O)CHN₂), 1.85 (m, 1H, CH₂CH₂CHC(O)CHN₂); FTIR (neat film, NaCl) 3327, 2953, 2110, 1731, 1640, 1525, 1440, 1336 cm⁻¹; HRMS (FAB) m/z: Calcd for C₁₅H₁₈N₃O₅ (MH⁺) 320.1246, found 320.1247. Silver acetate (320 mg, 1.92 mmol, 0.20 equiv) was added to a stirred solution of 4-benzyloxycarbonyl-amino-6-diazo-5-oxo-hexanoic acid methyl ester (3.07 g, 9.61 mmol, 1.0 equiv) in dry methanol (50 mL). The resulting brown suspension was stirred at room temperature for 12 h. The reaction mixture was then filtered through Celite and the filtrate concentrated. Purification by silica gel flash chromatography (40% ethyl acetate in hexanes) afforded 14 (2.83 g, 91%) as a white powder. R_f = 0.48 (50% ethyl acetate in hexanes); ¹H NMR (500 MHz, CDCl₃) δ 7.39 – 7.29 (m, 5 H, Ph), 5.32 (d, 1 H, J = 9.2 Hz, NH), 5.09 (m, 2 H, CH₂Ph), 4.01 (m, 1 H, CH₂CHNHCbz), 3.67 (s, 3 H, CO₂CH₃), 3.66 (s, 3 H, CO₂CH₃), 2.58 (t, 2 H, J = 4.6 Hz, CH₂CO₂CH₃), 2.41 (t, 2 H, J = 7.2 Hz, CH₂CO₂CH₃), 1.89 (m, 2 H, CH₂CH₂CO₂CH₃); ¹³C NMR (126 MHz, CDCl₃) δ 173.8, 172.0, 156.1, 136.6, 128.7, 128.4, 128.3, 67.4, 66.9, 52.0, 47.9, 39.1, 31.0, 29.5; FTIR (neat film, NaCl) 3350, 2953, 1737, 1691, 1530, 1438, 1245, 1056, 740, 699 cm⁻¹; HRMS (FAB) m/z: Calcd for C₁₆H₂₂N₁O₆ (MH⁺) 324.1447, found 324.1446.

4.3. 4-Benzyloxycarbonylamino-2-hydroxy-hexanedioic acid dimethyl ester (16)

A stirred solution of 14 (776 mg, 2.40 mmol, 1.0 equiv) and 3-phenyl-2-(toluene-4-sulfonyl)-oxaziridine (15) (990 mg, 3.60 mmol, 1.5 equiv) in dry tetrahydrofuran (30 mL) at −78 °C was added dropwise via cannula to a −78 °C solution of lithium bis(trimethylsilyl)amide (1.20 g, 7.20 mmol, 3.0 equiv) in tetrahydrofuran (20 mL). The bright yellow solution was then stirred at −78°C for 30 minutes and then quenched by the addition of dry methanol (10 mL). The resulting solution was then warmed to room temperature, diluted with diethyl ether (500 mL) and washed with 1.0 N aqueous hydrochloric acid (1 x 100 mL) and saturated aqueous sodium bicarbonate (1 x 100 mL). The organic layer was then separated, dried over magnesium sulfate, filtered, and concentrated. Purification by silica gel flash chromatography (33% hexanes in ethyl acetate) provided 16 (419 mg, 52%) as a colorless oil. R_f = 0.49 (33% hexanes in ethyl acetate); ¹H NMR (500 MHz, CDCl₃) δ 7.39 – 7.29 (m, 5 H, Ph), 5.69 (br d, 1 H, J = 9.3 Hz, NH), 5.10 (br s, 2 H, CH₂Ph), 4.28 (br m, 2 H, CHOH and CHNHCbz), 3.77 (s, 3 H, CO₂CH₃), 3.68 (s, 3 H, CO₂CH₃), 3.54 (br d, 1 H, J = 4.7 Hz, OH), 2.66 (t, 2 H, J = 5.7 Hz, CH₂CO₂CH₃), 2.12 (ddd, 1 H, J = 14.0, 10.2, 2.6 Hz, CH₂CHNHCbz), 1.77 (ddd, 1 H, J = 14.2, 10.9, 3.7, CH₂CHNHCbz); ¹³C NMR (126 MHz, CDCl₃) δ 174.7, 172.1, 156.8, 136.6, 128.8, 128.5, 128.4, 68.1, 67.3, 52.8, 52.1, 45.3, 38.8, 38.7; FTIR (neat film, NaCl) 3368, 2954, 1734, 1732, 1692, 1531, 1439, 1251, 1217, 1111, 1056, 741 cm⁻¹; HRMS (FAB) m/z: Calcd for C₁₆H₂₂N₁O₇ (MH⁺) 340.1396, found 340.1395.

4.4. [4-(tert-Butyl-dimethyl-silanyloxy)-5-oxo-pyrrolidin-2-yl]-acetic acid methyl ester (17)

A 100-mL round bottom flask containing 16 (419 mg, 1.23 mmol) and 10% palladium on carbon (ca. 20 mg) in methanol (20 mL) was charged with hydrogen via balloon, and the mixture was stirred at room temperature under the hydrogen atmosphere for 2 h. The reaction mixture was then filtered through Celite and concentrated. The crude amino alcohol was then dissolved in pyridine (20 mL) and heated to 100 ºC to effect lactamization. After 24 h the solution was cooled to room temperature and tert-butyldimethylsilyl chloride (445 mg, 2.96 mmol, 2.4 equiv) and imidazole (419 mg, 6.15 mmol, 5.0 equiv) were added. The solution was stirred at room temperature for 4 h and concentrated in vacuo. Purification by silica gel flash chromatography (33% hexanes in ethyl acetate) provided 17 (336 mg, 95%) as a white solid. R_f = 0.40 (40% hexanes in ethyl acetate); ¹H NMR (500 MHz, CDCl₃) δ 6.44 (br s, 1 H, NH), 4.25 (t, 1 H, J = 7.5 Hz, CH₂CHOTBS), 3.84 (m, 1 H, CH₂CHN), 3.71 (s, 3 H, CO₂CH₃), 2.58 (m, 3 H, CH₂CO₂CH₃ and CH₂CHOTBS), 1.66 (dt, 1 H, J = 12.9, 7.3 Hz, CH₂CHOTBS), 0.91 (s, 9 H, TBS t-butyl), 0.16 (s, 3 H, TBS methyl), 0.14 (s, 3 H, TBS methyl); ¹³C NMR (126 MHz, CDCl₃) δ 175.9, 172.1, 70.3, 52.2, 46.9, 41.2, 37.6, 26.0, 18.3, −4.34, −5.06; FTIR (neat film, NaCl) 3234, 2949, 1738, 1694, 1473, 1391, 1296, 1252, 1162, 1056, 840, 777 cm⁻¹; HRMS (FAB) m/z: Calcd for C₁₃H₂₆N₁O₄Si₁ (MH⁺) 288.1631, found 288.1632.

4.5. [4-(tert-Butyl-dimethyl-silanyloxy)-5-oxo-1-trimethylsilanylmethyl-pyrrolidin-2-yl]-acetic acid methyl ester (18)

To a stirred solution of 17 (853 mg, 2.97 mmol, 1.0 equiv) in dry dimethylformamide (8 mL) was added sodium hydride (60% suspension in mineral oil, 131 mg, 3.26 mmol, 1.1 equiv) and the resulting suspension was stirred at room temperature for 1 h. Chloromethyltrimethylsilane (1.20 mL, 8.91 mmol, 3.0 equiv) was added and the resulting suspension was stirred for an additional 3 h at room temperature. The reaction mixture was diluted with de-ionized water (40 mL) and extracted with diethyl ether (1 x 200 mL). The organic extracts were dried over magnesium sulfate, filtered, and concentrated. Purification by silica gel flash chromatography (20% ethyl acetate in hexanes) provided 18 (666 mg, trans-isomer only, 60%) as a colorless oil. R_f = 0.60 (33% ethyl acetate in hexanes); ¹H NMR (400 MHz, CDCl₃) δ 4.21 (dd, 1 H, J = 7.4, 4.7 Hz, CH₂CHOTBS), 3.80 (ddt, 1 H, J = 9.9, 7.3, 4.5, Hz, CH₂CHN), 3.72 (s, 3 H, CO₂CH₃), 3.20 (d, 1 H, J = 15.4 Hz, TMSCH₂N), 2.86 (dd, 1 H, J = 15.8, 4.4 Hz, CH₂CO₂CH₃), 2.49, (dd, 1 H, J = 15.7, 9.9 Hz, CH₂CO₂CH₃), 2.46 (dt, 1 H, J = 13.4, 7.3 Hz, CH₂CHOTBS), 2.33 (d, 1 H, J = 15.3 Hz, TMSCH₂N), 1.68 (dt, 1 H, J = 13.4, 4.6 Hz, CH₂CHOTBS), 0.90 (s, 9 H, TBS t-butyl), 0.15 (s, 3 H, TBS methyl), 0.14 (s, 3 H, TBS methyl), 0.086 (s, 9 H, TMS); ¹³C NMR (126 MHz, CDCl₃) δ 172.6, 171.6, 70.5, 54.4, 52.1, 38.9, 35.8, 32.3, 25.9, 18.4, −1.28, −4.31, −5.05; FTIR (neat film, NaCl) 2954, 1740, 1699, 1437, 1363, 1324, 1251, 1199, 1152, 1127, 983, 840, 780 cm⁻¹; HRMS (FAB) m/z: Calcd for C₁₇H₃₆N₁O₄Si₂ (MH⁺) 374.2183, found 374.2184.

4.6. [4-(tert-Butyl-dimethyl-silanyloxy)-5-(2-oxo-butylidene)-1-trimethylsilanyl-methyl-pyrrolidin-2-yl]-acetic acid methyl ester (19)

To a stirred solution of 18 (666 mg, 1.78 mmol, 1.0 equiv) in dry toluene (10 mL) at room temperature was added Lawesson’s reagent (397 mg, 0.981 mmol, 0.55 equiv). The resulting yellow suspension was stirred at 65 ºC for 1 h. The reaction mixture was then concentrated in vacuo. Purification by silica gel flash chromatography (16% ethyl acetate in hexanes) provided [4-(tert-Butyl-dimethyl-silanyloxy)-5-thioxo-1-trimethylsilanylmethyl-pyrrolidin-2-yl]-acetic acid methyl ester (657 mg, 95%) as a colorless oil. R_f = 0.59 (20% ethyl acetate in hexanes); ¹H NMR (500 MHz, CDCl₃) δ 4.53 (dd, 1 H, J = 6.6, 2.9 Hz, CH₂CHOTBS), 4.10 (dddd, 1 H, J = 9.8, 6.9, 4.1, 2.9 Hz, CH₂CHN), 3.94 (d, 1 H, J = 14.7 Hz, TMSCH₂N), 3.73 (s, 3 H, CO₂CH₃), 2.90 (dd, 1 H, J = 15.9, 4.1 Hz, CH₂CO₂CH₃), 2.83 (d, 1 H, J = 14.7 Hz, TMSCH₂N), 2.71, (dd, 1 H, J = 15.9, 9.8 Hz, CH₂CO₂CH₃), 2.43 (dt, 1 H, J = 13.5, 6.9 Hz, CH₂CHOTBS), 1.79 (dt, 1 H, J = 13.5, 2.9 Hz, CH₂CHOTBS), 0.90 (s, 9 H, TBS t-butyl), 0.20 (s, 3 H, TBS methyl), 0.18 (s, 3 H, TBS methyl), 0.14 (s, 9 H, TMS); ¹³C NMR (126 MHz, CDCl₃) δ 198.8, 171.3, 80.0, 62.8, 52.3, 38.7, 38.4, 36.7, 25.9, 18.3, −0.63, −3.95, −5.03; FTIR (neat film, NaCl) 2953, 1740, 1492, 1437, 1363, 1319, 1251, 1199, 1153, 1129, 1091, 842, 780 cm⁻¹; HRMS (FAB) m/z: Calcd for C₁₇H₃₆N₁O₃S₁Si₂ (MH⁺) 390.1954, found 390.1955. To a stirred solution of [4-(tert-butyl-dimethyl-silanyloxy)-5-thioxo-1-trimethylsilanylmethyl-pyrrolidin-2-yl]-acetic acid methyl ester (96.2 mg, 0.247 mmol, 1.0 equiv) in dichloromethane (2 mL) was added 1-bromo-2-butanone (30 μL, 0.27 mmol, 1.1 equiv). The colorless solution was then concentrated in vacuo and the resulting oil was heated at 60 ºC for 20 minutes. The resulting white solid was dissolved in dichloromethane (2 mL) and triphenylphosphine (71.1 mg, 0.271 mmol, 1.1 equiv) and triethylamine (40 μL, 0.27 mmol, 1.1 equiv) were added at room temperature. The resulting yellow solution was then stirred for 1 h at room temperature and concentrated. Purification by silica gel flash chromatography (13% ethyl acetate in benzene) provided 19 (75.3 mg, 71%, (E)-isomer only) as a white solid. R_f = 0.32 (15% ethyl acetate in hexanes); ¹H NMR (500 MHz, C₆D₆) δ 6.01 (d, 1 H, J = 5.2 Hz, CH₂CHOTBS), 4.83 (s, 1 H, vinyl H), 3.67 (ddt, 1 H, J = 9.4, 7.4, 4.4 Hz, CH₂CHN), 3.34 (s, 3 H, CO₂CH₃), 2.88 (dd, 1 H, J = 15.7, 9.4 Hz, CH₂CO₂CH₃), 2.68 (dd, 1 H, J = 15.8, 4.4 Hz, CH₂CO₂CH₃), 2.45 (d, 1 H, J = 15.7 Hz, TMSCH₂N), 2.41 (q, 2 H, J = 7.4 Hz, CH₂CH₃), 2.28 (d, 1 H, J = 15.7 Hz, TMSCH₂N), 1.75 (m, 2 H, CH₂CHOTBS), 1.27 (t, 3 H, J = 7.4 Hz, CH₂CH₃), 1.01 (s, 9 H, TBS t-butyl), 0.51 (s, 3 H, TBS methyl), 0.40 (s, 3 H, TBS methyl), −0.078 (s, 9 H, TMS); ¹³C NMR (126 MHz, C₆D₆) δ 195.2, 172.1, 163.3, 89.9, 72.4, 61.6, 51.5, 39.2, 37.8, 37.2, 36.1, 26.6, 18.8, 10.4, −0.79, −3.69, −5.00; FTIR (neat film, NaCl) 2929, 2856, 1739, 1651, 1564, 1437, 1371, 1314, 1251, 1209, 1126, 1098, 853, 780 cm⁻¹; HRMS (FAB) m/z: Calcd for C₂₁H₄₂N₁O₄Si₂ (MH⁺) 428.2652, found 428.2651.

4.7. 7-(tert-Butyl-dimethyl-silanyloxy)-5-methoxycarbonylmethyl-7a-(2-trifluoro-methanesulfonyloxy-but-1-enyl)-hexahydro-pyrrolizine-1-carboxylic acid methyl ester (22)

To a stirred solution of 19 (9.2 mg, 0.022 mmol, 1.0 equiv) in dichloromethane (500 μL) at room temperature was added via syringe trifluoromethanesulfonic anhydride (4.0 μL, 0.024 mmol, 1.1 equiv). The resulting yellow solution was stirred for 15 minutes at room temperature before the addition of methyl acrylate (20 μL, 0.22 mmol, 10.0 equiv) and tetrabutylammonium triphenyldifluorosilicate (12.8 mg, 0.0237 mmol, 1.1 equiv). The resulting dark red solution was stirred at room temperature for 1 h. The reaction mixture was then concentrated in vacuo. Purification by silica gel flash chromatography (17% ethyl acetate in hexanes) provided 22 (6.8 mg, 55%, (Z)-isomer only) as a colorless oil. R_f = 0.50 (17% ethyl acetate in hexanes); ¹H NMR (500 MHz, C₆D₆) δ 6.08 (br s, 1 H, vinyl H), 4.17 (dd, 1 H, J = 11.2, 6.0 Hz, CH₂CHOTBS), 3.39 (s, 3 H, CO₂CH₃), 3.36 (s, 3 H, CO₂CH₃), 3.19 (td, 1 H, J = 12.1, 5.1 Hz, CH₂N), 3.10 (m, 1 H, CH₂CHN), 2.90 (dd, 1 H, J = 16.5, 5.3 Hz, CH₂CO₂CH₃), 2.80 (dd, 1 H, J = 12.2, 6.1 Hz, CHCO₂CH₃), 2.66 (ddd, 1 H, J = 12.1, 7.2, 1.0 Hz, CH₂N), 2.62 (dd, 1 H, J = 16.5, 8.5 Hz, CH₂CO₂CH₃), 2.24 (m, 1 H, CH₂CHOTBS), 2.13 (m, 1 H, CH₂CH₃), 2.00 (m, 1 H, CH₂CH₃), 1.92 (m, 1 H, CH₂CHN), 1.50 (m, 1 H, CH₂CHOTBS), 1.48 (m, 1 H, CH₂CHN), 0.91 (s, 9 H, TBS t-butyl), 0.86 (t, 3 H, J = 7.3 Hz, CH₂CH₃), 0.066 (s, 3 H, TBS methyl), −0.028 (s, 3 H, TBS methyl); FTIR (neat film, NaCl) 2957, 1736, 1438, 1413, 1260, 1202, 1144, 1033, 978, 884, 779 cm⁻¹; HRMS (FAB) m/z: Calcd for C₂₃H₃₉N₁O₈F₃S₁Si₁ (MH⁺) 574.2118, found 574.2119.

4.8. Iminium triflates 20 and 21

To a stirred solution of 19 (19.3 mg, 0.0450 mmol, 1.0 equiv) in CDCl₃ (1 mL, freshly distilled from CaH₂ under N₂ at atmospheric pressure) at room temperature was added via syringe trifluoromethanesulfonic anhydride (9.0 μL, 0.050 mmol, 1.1 equiv). The resulting pale orange solution was stirred for 1 h at room temperature. Examination of the reaction mixture by ¹H NMR revealed both 20 and 21 (>95% conversion, as a 4:1 mixture of 20 to 21), each formed as the (Z)-isomer with respect to the enol triflate double bond. The solution was then stirred at room temperature for an additional 4.5 h. Examination of the reaction mixture by ¹H NMR revealed both 20 and 21 (>95% conversion, as a 1:2 mixture of 20 to 21). Iminium triflate 20: ¹H NMR (500 MHz, CDCl₃) δ 7.12 (br s, 1 H, vinyl H), 5.55 (dd, 1 H, J = 6.9, 3.2 Hz, CH₂CHOTBS), 4.61 (ddt, 1 H, J = 9.1, 7.5, 3.8 Hz, CH₂CHN), 4.29 (d, 1 H, J = 14.5 Hz, TMSCH₂N) 3.78 (s, 3 H, CO₂CH₃), 3.40 (d, 1 H, J = 14.5 Hz, TMSCH₂N), 3.23 (dd, 1 H, J = 16.9, 4.2 Hz, CH₂CO₂CH₃), 2.91 (dd, 1 H, J = 17.0, 9.1 Hz, CH₂CO₂CH₃), 2.86 (dt, 1 H, J = 13.9, 7.5 Hz, CH₂CHOTBS), 2.73 (m, 1 H, CH₂CH₃), 2.61 (m, 1 H, CH₂CH₃), 2.05 (dt, 1 H, J = 13.8, 3.3 Hz, CH₂CHOTBS), 1.32 (t, 3 H, J = 7.3 Hz, CH₂CH₃), 0.89 (s, 3 H, TBS methyl), 0.88 (s, 9 H, TBS t-butyl), 0.86 (s, 3 H, TBS methyl), 0.26 (s, 9 H, TMS). Iminium triflate 21: ¹H NMR (500 MHz, CDCl₃) δ 6.50 (br s, 1 H, vinyl H), 5.41 (t, 1 H, J = 7.6 Hz, CH₂CHOTBS), 4.89 (dtd, 1 H, J = 8.9, 7.5, 4.8 Hz, CH₂CHN), 4.18 (d, 1 H, J = 14.7 Hz, TMSCH₂N), 3.77 (s, 3 H, CO₂CH₃), 3.39 (d, 1 H, J = 14.7 Hz, TMSCH₂N), 3.11 (dd, 1 H, J = 16.5, 4.8 Hz, CH₂CO₂CH₃), 2.86 (dt, 1 H, J = 12.9, 7.5 Hz, CH₂CHOTBS), 2.78 (dd, 1 H, J = 16.5, 8.9 Hz, CH₂CO₂CH₃), 2.77 (m, 1 H, CH₂CH₃), 2.62 (m, 1 H, CH₂CH₃), 1.89 (dt, 1 H, J = 12.9, 7.8 Hz, CH₂CHOTBS), 1.30 (t, 3 H, J = 7.4 Hz, CH₂CH₃), 1.01 (s, 3 H, TBS methyl), 0.89 (s, 9 H, TBS t-butyl), 0.85 (s, 3 H, TBS methyl), 0.31 (s, 9 H, TMS).

4.9. 4-Benzyloxycarbonylamino-2-hydroxy-5-(methoxy-methyl-carbamoyl) pentanoic acid methyl ester (24)

To a stirred solution of 23 (2.51 g, 7.36 mmol, 1.0 equiv) and 3-phenyl-2-(toluene-4-sulfonyl)-oxaziridine (15) (3.04 g, 11.0 mmol, 1.5 equiv) in dry tetrahydrofuran (80 mL) at −78 °C was added dropwise via cannula a cold (−78 °C) solution of lithium bis(trimethylsilyl)amide (2.71 g, 16.2 mmol, 2.2 equiv) in tetrahydrofuran (80 mL). The bright yellow solution was then stirred at −78°C for 20 minutes and then quenched by the cannula addition of a solution of camphor sulfonic acid (6.67 g, 29.4 mmol, 4.0 equiv) in tetrahydrofuran (40 mL). The resulting solution was then warmed to room temperature, diluted with diethyl ether (500 mL) and washed with 1.0 N aqueous hydrochloric acid (1 x 100 mL) and saturated aqueous sodium bicarbonate (1 x 100 mL). The organic layer was then separated, dried over magnesium sulfate, filtered, and concentrated. Purification by silica gel flash chromatography (ethyl acetate) provided 24 (1.50 g, 57%) as a white solid. R_f = 0.45 (ethyl acetate); [α]²⁴D = −16.5° (c = 1.08, CHCl₃); ¹H NMR (CDCl₃) δ 7.41 – 7.30 (m, 5 H, Ph), 6.13 (br d, 1 H, J = 8.6 Hz, NH), 5.10 (br s, 2 H, CH₂Ph), 4.28 (br m, 2 H, CH₂CHOH and CH₂CHNHCbz), 3.76 (s, 3 H, NOCH₃), 3.66 (s, 3 H, CO₂CH₃), 3.16 (s, 3 H, NCH₃), 2.87 (br d, 1 H, J = 15.3 Hz, CH₂CHNHCbz), 2.69 (br dd, 1 H, J = 16.3, 4.4 Hz, CH₂CHNHCbz), 2.21 (t, 1 H, J = 11.8 Hz, CH₂CHOH), 1.75 (br m, 1 H, CH₂CHOH); ¹³C NMR (CDCl₃) δ 174.7, 172.3, 157.0, 136.6, 128.7, 128.3, 128.2, 68.2, 67.1, 61.5, 52.6, 45.3, 39.1, 35.8, 32.0; FTIR (neat film, NaCl) 3340, 2953, 1736, 1719, 1642, 1528, 1438, 1390, 1254, 1111, 1053, 996, 741 cm⁻¹; HRMS (FAB) m/z: Calcd for C₁₇H₂₅N₂O₇ (MH⁺) 369.1662, found 369.1663.

4.10. 2-(4-Hydroxy-5-oxo-pyrrolidin-2-yl)-N-methoxy-N-methyl-acetamide (25)

A suspension of 24 (1.25 g, 3.39 mmol) and 10% palladium on carbon (ca. 200 mg) in methanol (50 mL) was charged with hydrogen via balloon, and the mixture was stirred at room temperature under the hydrogen atmosphere for 1 h. The reaction mixture was then filtered through Celite and concentrated at 35 °C using a rotary evaporator. Purification by silica gel flash chromatography (10% methanol in ethyl acetate) provided 25 (642 mg, 94%) as a white solid. R_f = 0.21 (10% methanol in ethyl acetate); [α]²⁴D = +4.3° (c = 1.24, CHCl₃); ¹H NMR (500 MHz, CDCl₃) δ 6.80 (br s, 1 H, NH), 4.33 (t, 1 H, J = 8.6 Hz, CH₂CHOH), 4.15 (br s, 1 H, OH), 3.89 (br m, 1 H, CH₂CHN), 3.69 (s, 3 H, NOCH₃), 3.18 (s, 3 H, NCH₃), 2.86 (br d, 1 H, J = 16.1 Hz, CH₂C(O)N(CH₃)OCH₃), 2.66 (br m, 1 H, CH₂CHOH), 2.54 (dd, 1 H, J = 16.8, 9.7 Hz, CH₂C(O)N(CH₃)OCH₃), 1.70 (dt, 1 H, J = 12.9, 8.3 Hz, CH₂CHOH); ¹³C NMR (126 MHz, CDCl₃) δ 177.9, 172.2, 89.3, 61.2, 52.7, 38.5, 36.1, 31.4; FTIR (neat film, NaCl) 3391, 2930, 1698, 1643, 1434, 1393, 1304, 1181, 1118, 993 cm⁻¹; HRMS (FAB) m/z: Calcd for C₈H₁₅N₂O₄ (MH⁺) 203.1032, found 203.1033.

4.11. Lactams 26 and 27

To a stirred solution of 25 (642 mg, 3.17 mmol, 1.0 equiv) and imidazole (1.08 g, 15.9 mmol, 5.0 equiv) in anhydrous dimethylformamide (30 mL) at room temperature was added tert-butyldimethylsilyl chloride (957 mg, 6.35 mmol, 2.0 equiv). The resulting solution was stirred at room temperature for 24 h. The reaction mixture was then diluted with de-ionized water (250 mL) and extracted with diethyl ether (2 x 300 mL). The organic extracts were then combined, dried over magnesium sulfate, filtered, and concentrated. Purification by silica gel flash chromatography (ethyl acetate) provided 2-[4-(tert-Butyl-dimethylsilanyloxy)-5-oxo-pyrrolidin-2-yl]-N-methoxy-N-methyl acetamide (913 mg, 91%) as a colorless oil. R_f = 0.43 (ethyl acetate); [α]²⁴D = +15.3° (c = 0.50, CHCl₃); ¹H NMR (500 MHz, CDCl₃) δ 6.45 (br d, 1 H, J = 8.6 Hz, NH), 4.23 (t, 1 H, J = 7.6 Hz, CH₂CHOTBS), 3.83 (br m, 1 H, CH₂CHN), 3.65 (s, 3 H, NOCH₃), 3.15 (s, 3 H, NCH₃), 2.76 (dd, 1 H, J = 16.9, 2.6 Hz, CH₂C(O)N(CH₃)OCH₃), 2.56 (m, 2 H, CH₂C(O)N(CH₃)OCH₃ and CH₂CHOTBS), 1.66 (dt, 1 H, J = 13.0, 7.2 Hz, CH₂CHOTBS), 0.88 (s, 9 H, TBS t-butyl), 0.13 (s, 3 H, TBS methyl), 0.11 (s, 3 H, TBS methyl); ¹³C NMR (126 MHz, CDCl₃) δ 175.7, 172.1, 70.3, 61.4, 46.9, 39.4, 37.6, 32.1, 25.9, 18.4, −4.4, −5.1; FTIR (neat film, NaCl) 3293, 2954, 1715, 1660, 1472, 1390, 1300, 1252, 1151, 1054, 997, 861, 839 cm⁻¹; HRMS (FAB) m/z: Calcd for C₁₄H₂₉N₂O₄Si₁ (MH⁺) 317.1897, found 317.1898. To a stirred solution of 2-[4-(tert-butyl-dimethylsilanyloxy)-5-oxo-pyrrolidin-2-yl]-N-methoxy-N-methyl acetamide (215 mg, 0.679 mmol, 1.0 equiv) in dry dimethylformamide (6 mL) was added sodium hydride (60% suspension in mineral oil, 30.0 mg, 0.747 mmol, 1.1 equiv) and the resulting suspension was stirred at room temperature for 1.5 h. Chloromethyltrimethylsilane (290 μL, 2.04 mmol, 3.0 equiv) was added and the resulting suspension was stirred for an additional 18 h at room temperature. The reaction mixture was diluted with de-ionized water (50 mL) and extracted with diethyl ether (1 x 500 mL). The organic extracts were dried over magnesium sulfate, filtered, and concentrated. Purification by silica gel flash chromatography (40% ethyl acetate in hexanes) provided 26 and 27 (191 mg as a separable 7:1 mixture favoring 27, 70% total) as colorless oils. Lactam 26: R_f = 0.52 (50% hexanes in ethyl acetate); [α]²⁴D = +28.2° (c = 1.18, CHCl₃); ¹H NMR (500 MHz, CDCl₃) δ 4.32 (t, 1 H, J = 7.2 Hz, CH₂CHOTBS), 4.00 (ddt, 1 H, J = 9.9, 6.9, 3.6 Hz, CH₂CHN), 3.69 (s, 3 H, NOCH₃), 3.21 (d, 1 H, J = 15.2 Hz, TMSCH₂N), 3.19 (s, 3 H, NCH₃), 2.82 (dd, 1 H, J = 15.4, 3.5 Hz, CH₂C(O)N(CH₃)OCH₃), 2.36, (dd, 1 H, J = 15.4, 9.9 Hz, CH₂C(O)N(CH₃)OCH₃), 2.29 (d, 1 H, J = 15.2 Hz, TMSCH₂N), 2.14 (dt, 1 H, J = 13.1, 6.9 Hz, CH₂CHOTBS), 2.07 (ddd, 1 H, J = 13.2, 7.5, 3.5 Hz, CH₂CHOTBS), 0.89 (s, 9 H, TBS t-butyl), 0.14 (s, 3 H, TBS methyl), 0.13 (s, 3 H, TBS methyl), 0.10 (s, 9 H, TMS); ¹³C NMR (126 MHz, CDCl₃) δ 172.8, 171.4, 69.9, 61.5, 53.8, 41.8, 36.2, 35.6, 32.3, 26.0, 18.5, −1.32, −4.23, −4.91; FTIR (neat film, NaCl) 2954, 1697 (C=O), 1664 (C=O), 1462, 1418, 1388, 1362, 1250, 1172, 1114, 1002, 856, 840, 779 cm⁻¹; HRMS (FAB) m/z: Calcd for C₁₈H₃₉N₂O₄Si₂ (MH⁺) 403.2448, found 403.2449. Lactam 27: R_f = 0.67 (50% hexanes in ethyl acetate); [α]²⁴D = −8.1° (c = 3.03, CHCl₃); ¹H NMR (400 MHz, CDCl₃) δ 4.21 (dd, 1 H, J = 7.4, 4.4 Hz, CH₂CHOTBS), 3.91 (ddt, 1 H, J = 9.8, 7.0, 4.0 Hz, CH₂CHN), 3.68 (s, 3 H, NOCH₃), 3.20 (d, 1 H, J = 15.3 Hz, TMSCH₂N), 3.19 (s, 3 H, NCH₃), 2.95 (dd, 1 H, J = 16.2, 3.8 Hz, CH₂C(O)N(CH₃)OCH₃), 2.63, (dd, 1 H, J = 16.2, 9.8 Hz, CH₂C(O)N(CH₃)OCH₃), 2.49 (dt, 1 H, J = 14.3, 7.5 Hz, CH₂CHOTBS), 2.35 (d, 1 H, J = 15.3 Hz, TMSCH₂N), 1.65 (dt, 1 H, J = 13.5, 4.1 Hz, CH₂CHOTBS), 0.89 (s, 9 H, TBS t-butyl), 0.14 (s, 3 H, TBS methyl), 0.13 (s, 3 H, TBS methyl), 0.081 (s, 9 H, TMS); ¹³C NMR (126 MHz, CDCl₃) δ 172.6, 171.9, 70.7, 61.5, 54.4, 42.0, 36.7, 36.1, 32.4, 25.9, 18.4, −1.28, −4.35, −4.99; FTIR (neat film, NaCl) 2954, 1698, 1667, 1464, 1417, 1388, 1361, 1250, 1173, 1116, 1004, 841, 780 cm⁻¹; HRMS (FAB) m/z: Calcd for C₁₈H₃₉N₂O₄Si₂ (MH⁺) 403.2448, found 403.2449.

4.12. 2-[4-(tert-Butyl-dimethylsilanyloxy)-5-thioxo-1-trimethylsilanylmethyl-pyrrolidin-2-yl]-N-methoxy-N-methyl acetamide (28)

To a stirred solution of 27 (37.1 mg, 0.0921 mmol, 1.0 equiv) in dry toluene (1 mL) at room temperature was added Lawesson’s reagent (18.6 mg, 0.0460 mmol, 0.51 equiv). The resulting yellow suspension was stirred at room temperature for 21 h. The reaction mixture was then concentrated in vacuo. Purification by silica gel flash chromatography (33% ethyl acetate in hexanes) provided 28 (33 mg, 86%) as a colorless oil. R_f = 0.47 (33% ethyl acetate in hexanes); ¹H NMR (400 MHz, CDCl₃) δ 4.54 (dd, 1 H, J = 6.5, 2.5 Hz, CH₂CHOTBS), 4.21 (ddt, 1 H, J = 8.7, 6.6, 3.4 Hz, CH₂CHN), 3.95 (d, 1 H, J = 14.5 Hz, TMSCH₂N), 3.67 (s, 3 H, NOCH₃), 3.20 (s, 3 H, NCH₃), 3.02 (dd, 1 H, J = 16.4, 3.8 Hz, CH₂C(O)N(CH₃)OCH₃), 2.85, (d, 1 H, J = 14.5 Hz, TMSCH₂N), 2.83 (dd, 1 H, J = 16.4, 8.7 Hz, CH₂C(O)N(CH₃)OCH₃), 2.47 (dt, 1 H, J = 13.5, 7.1 Hz, CH₂CHOTBS), 1.78 (dt, 1 H, J = 13.3, 2.8 Hz, CH₂CHOTBS), 0.89 (s, 9 H, TBS t-butyl), 0.20 (s, 3 H, TBS methyl), 0.19 (s, 3 H, TBS methyl), 0.14 (s, 9 H, TMS); FTIR (neat film, NaCl) 2953, 1666, 1472, 1416, 1388, 1360, 1250, 1153, 1128, 1082, 1005, 908, 884, 842, 779 cm⁻¹; HRMS (FAB) m/z: Calcd for C₁₈H₃₉N₂O₃S₁Si₂ (MH⁺) 419.2220, found 419.2219.

4.13. 2-[4-(tert-Butyl-dimethyl-silanyloxy)-5-(2-oxo-butylidene)-1-trimethylsilanyl-methyl-pyrrolidin-2-yl]-N-methoxy-N-methyl acetamide (29)

To a stirred solution of 28 (24.7 mg, 0.0589 mmol, 1.0 equiv) in dichloromethane (1 mL) was added 1-bromo-2-butanone (7.0 μL, 0.065 mmol, 1.1 equiv). The colorless solution was then concentrated in vacuo. The colorless oil was heated neat at 60 ºC for 10 minutes. The resulting white solid was dissolved in dichloromethane (1 mL) and triphenylphosphine (17.0 mg, 0.0648 mmol, 1.1 equiv) and triethylamine (9.0 μL, 0.065 mmol, 1.1 equiv) were added. The resulting yellow solution was then stirred for 21 h at room temperature and concentrated. Purification by silica gel flash chromatography (33% ethyl acetate in hexanes) provided 29 (14.1 mg, 52%, (E)-isomer only) as a colorless oil. R_f = 0.31 (33% ethyl acetate in hexanes); ¹H NMR (500 MHz, CDCl₃) δ 5.82 (d, 1 H, J = 5.5 Hz, CH₂CHOTBS), 4.78 (s, 1 H, vinyl H), 3.92 (m, 1 H, CH₂CHN), 3.65 (s, 3 H, NOCH₃), 3.19 (s, 3 H, NCH₃), 2.94 (m, 2 H, CH₂C(O)N(CH₃)OCH₃), 2.84 (d, 1 H, J = 15.6 Hz, TMSCH₂N), 2.68 (d, 1 H, J = 15.6 Hz, TMSCH₂N), 2.27 (q, 2 H, J = 7.3 Hz, CH₂CH₃), 2.13 (ddd, 1 H, J = 13.2, 8.8, 5.1 Hz, CH₂CHOTBS), 1.83 (d, 1 H, J = 13.2 Hz, CH₂CHOTBS), 1.10 (t, 3 H, J = 7.4 Hz, CH₂CH₃), 0.83 (s, 9 H, TBS t-butyl), 0.26 (s, 3 H, TBS methyl), 0.12 (s, 9 H, TMS), 0.10 (s, 3 H, TBS methyl); FTIR (neat film, NaCl) 2954, 1664, 1558, 1462, 1412, 1386, 1305, 1250, 1117, 853, 779 cm⁻¹; HRMS (FAB) m/z: Calcd for C₂₂H₄₅N₂O₄Si₂ (MH⁺) 457.2918, found 457.2918.

4.14. 1-[4-(tert-Butyl-dimethyl-silanyloxy)-5-(2-oxo-butylidene)-1-trimethylsilanyl-methyl-pyrrolidin-2-yl]-4-ethylsulfanyl-but-3-en-2-one (30)

To a stirred solution of 29 (15.2 mg, 0.0333 mmol, 1.0 equiv) in dry tetrahydrofuran (1 mL) at room temperature was added ethynylmagnesium chloride (0.5 M solution in tetrahydrofuran, 400 μL, 0.200 mmol, 6.0 equiv) and the resulting yellow solution was stirred for 1 h. The reaction mixture was then filtered through a short pad of silica gel (ethyl acetate) and the filtrate concentrated. The resulting bright yellow oil was then dissolved in dichloromethane (1 mL). Ethanethiol (3.0 μL, 0.036 mmol, 1.1 equiv) and triethylamine (10 μL, 0.073 mmol, 2.2 equiv) were then added at room temperature. The resulting solution was stirred at room temperature for 8 h and then concentrated in vacuo. Purification by silica gel flash chromatography (25% ethyl acetate in hexanes) provided 30 (9.2 mg, 57%, (E)-isomer only) as a pale yellow oil. R_f = 0.52 (33% ethyl acetate in hexanes); ¹H NMR (500 MHz, CDCl₃) δ 7.66 (d, 1 H, J = 16.0 Hz, CH₃CH₂SCH=CH-C=O), 6.11 (d, 1 H, J = 16.0 Hz, CH₃CH₂SCH=CH-C=O), 5.81 (d, 1 H, J = 5.4 Hz, CH₂CHOTBS), 4.79 (s, 1 H, vinyl H), 3.96 (br m, 1 H, CH₂CHN), 3.10 (dd, 1 H, J = 16.9, 7.8 Hz, CH₂C(O)CH=CHSCH₂CH₃), 2.98 (dd, 1 H, J = 16.9, 4.6 Hz, CH₂C(O)CH=CHSCH₂CH₃), 2.84 (d, 1 H, J = 15.6 Hz, TMSCH₂N), 2.80 (q, 2 H, J = 7.3 Hz, SCH₂CH₃), 2.57 (d, 1 H, J = 15.6 Hz, TMSCH₂N), 2.29 (q, 2 H, J = 7.4 Hz, CH₂CH₃), 2.04 (m, 1 H, CH₂CHOTBS), 1.69 (m, 1 H, CH₂CHOTBS), 1.38 (t, 3 H, J = 7.3 Hz, SCH₂CH₃), 1.14 (t, 3 H, J = 7.4 Hz, CH₂CH₃), 0.86 (s, 9 H, TBS t-butyl), 0.26 (s, 3 H, TBS methyl), 0.12 (s, 9 H, TMS), 0.078 (s, 3 H, TBS methyl); FTIR (neat film, NaCl) 2928, 1653, 1557, 1458, 1366, 1250, 1207, 1082, 839 cm⁻¹; HRMS (FAB) m/z: Calcd for C₂₄H₄₆N₁O₃S₁Si₂ (MH⁺) 484.2737, found 484.2735.

4.15. Acetic acid 2,2-dimethyl-6-oxo-5-trimethylsilanylmethyl-tetrahydro-[1,3]dioxolo[4,5-c]pyrrol-4-yl ester (40)

To a solution of lactone 39 (4.9 g, 31 mmol, 1.0 equiv) in tetrahydrofuran (30 mL) was added triethylamine (5.6 mL, 40 mmol, 1.3 equiv) and C-trimethylsilanyl-methylammonium hydrochloride (5.6 g, 40 mmol, 1.3 equiv). The flask was sealed under argon and heated to 70 °C for 18 hours. After cooling to room temperature, the resulting orange suspension was transferred to a separatory funnel containing deionized water (300 mL) and extracted with dichloromethane (3 x 300 mL). The combined organic extracts were dried over sodium sulfate, filtered, and concentrated in vacuo providing 5-hydroxymethyl-2,2-dimethyl-[1,3]dioxolane-4-carboxylic acid trimethylsilanylmethyl-amide (7.8 g, 97%) as a brown solid which was used without further purificaion. R_f = 0.16 (33% ethyl acetate in hexanes); ¹H NMR (500 MHz, CDCl₃) δ 6.66 (br s, 1H, CONH), 4.62 (d, 1H, J = 7.6 Hz, COCHCHCH₂), 4.54 (ddd, 1H, J = 8.6, 7.7, 4.6 Hz, COCHCHCH₂), 3.81 (dd, 1H, J = 11.8, 4.6 Hz, COCHCHCH₂), 3.55 (dd, 1H, J = 11.8, 8.7 Hz COCHCHCH₂), 2.96 (dd, 1H, J = 15.4, 7.0 Hz, NCH₂), 2.66 (dd, 1H, J = 15.0, 5.0 Hz, NCH₂), 1.54 (s, 3H, C(CH₃)₂), 1.40 (s, 3H, C(CH₃)₂), 0.10 (s, 9H, Si(CH₃)₃); ¹³C NMR (126 MHz, CDCl₃) δ 170.6, 110.1, 77.7, 77.4, 61.9, 29.4, 27.2, 24.6, −2.6; FTIR (neat film, NaCl) 3272, 2987, 2956, 1644 cm⁻¹; HRMS (ESI) m/z: Calcd for C₁₁H₂₄NO₄Si (MH⁺) 262.1474, observed 262.1471. To a solution of 5-hydroxymethyl-2,2-dimethyl-[1,3]dioxolane-4-carboxylic acid trimethylsilanylmethylamide (4.0 g, 15.3 mmol, 1.0 equiv) in dimethylsulfoxide (30 mL) was added triethylamine (15 mL, 107 mmol, 7.0 equiv). Sulfer trioxide-pyridine complex (9.7 g, 61 mmol, 4.0 equiv) was added via cannula in a solution in dimethylsulfoxide (30 mL). After stirring at room temperature for 6 hours, the resulting clear orange solution was transferred to a separatory funnel containing ice water (500 mL) and extracted with dichloromethane (3 x 700 mL). The combined organic extracts were concentrated in vacuo to a volume of 500 mL and washed sequentially with a 1:1 solution of 1N aqueous hydrochloric acid and brine (1 x 100 mL) and a 4:1 solution of brine and saturated aqueous sodium bicarbonate (1 x 100 mL). The organic layer was dried over sodium sulfate, filtered and concentrated in vacuo, affording the desired hemiaminal, which was carried forward without further purification. The hemiaminal was dissolved in pyridine (30 mL) and acetic anhydride (7.3 mL, 77 mmol, 5.0 equiv) was added to the solution. After stirring for 42 hours at room temperature, the resulting clear brown solution was transferred to a separatory funnel containing ethyl acetate (500 mL). The organic layer was washed sequentially with 0.5 M aqueous hydrochloric acid (2 x 200 mL) and a 9:1 solution of brine and saturated aqueous sodium bicarbonate (1 x 200 mL). The organic layer was dried over magnesium sulfate, filtered, concentrated in vacuo and purified by silica gel column chromatography (33% ethyl acetate in hexanes), affording aminal 40 (3.2 g, 69%) as a pale yellow oil. R_f = 0.51 (50% ethyl acetate in hexanes); ¹H NMR (500 MHz, CDCl₃) δ 6.01 (s, 1H, NCHCHCH), 4.74 (d, 1H, J = 6.0 Hz, NCHCHCH), 4.49 (d, 1H, J = 5.6 Hz, NCHCHCH), 3.12 (d, 1H, J = 15.3, NCH₂), 2.46 (dd, 1H, J = 15.3, 0.7 Hz, NCH₂), 2.11 (s, 3H, COCH₃), 1.44 (s, 3H, C(CH₃)₂), 1.36 (s, 3H, C(CH₃)₂), 0.11 (s, 9H, Si(CH₃)₃); ¹³C NMR (126 MHz, CDCl₃) δ 185.1, 153.3, 113.8, 86.9, 76.2, 32.5, 27.0, 25.7, 21.1, 8.7, −1.6; FTIR (neat film, NaCl) 2954, 1746, 1719 cm⁻¹; HRMS (ESI) m/z: Calcd for C₁₃H₂₃NNaO₅Si (MNa⁺) 324.1243, observed 324.1247.

4.16. (2,2-Dimethyl-6-oxo-5-trimethylsilanylmethyl-tetrahydro-[1,3]dioxolo[4,5-c]pyrrol-4-yl)-acetic acid (42)

To a solution of aminal 40 (154 mg, 0.511 mmol, 1.0 equiv) in dichloromethane (5 mL) was added silyl ketene acetal 41 (207 mg, 1.02 mmol, 2.0 equiv) followed by trimethylsilyl trifluoromethanesulfonate (18 μL, 0.102 mmol, 0.20 equiv). After stirring at room temperature for 26 hours, the reaction was quenched with saturated aqueous sodium bicarbonate (1 mL) and the mixture transferred to a separatory funnel containing deionized water (50 mL). Following extraction with dichloromethane (3 x 100 mL) the combined organic extracts were dried over sodium sulfate, filtered, concentrated in vacuo, and purified by silican gel column chromatography (20% ethyl acetate in dichloromethane), affording (2,2-Dimethyl-6-oxo-5-trimethylsilanylmethyl-tetrahydro-[1,3]dioxolo[4,5-c]pyrrol-4-yl)-acetic acid ethyl ester (121 mg, 72%) as a colorless oil. R_f = 0.37 (50% ethyl acetate in hexanes); ¹H NMR (500 MHz, CDCl₃) δ 4.72 (d, 1H, J = 6.0 Hz, COCHCHCHCH₂), 4.60 (d, 1H, J = 5.9 Hz, COCHCHCHCH₂), 4.14 (q, 2H, J = 7.1, OCH₂CH₃), 3.86 (dd, 1H, J = 7.5, 3.2 Hz, COCHCHCHCH₂), 3.33 (d, 1H, J = 15.4 Hz, NCH₂), 2.70 (dd, 1H, J = 16.2, 3.3 Hz, COCHCHCHCH₂), 2.55 (dd, 1H, J = 16.2, 7.6 Hz, COCHCHCHCH₂), 2.21 (dd, 1H, J = 15.4, 0.7 Hz), 1.45 (s, 3H, C(CH₃)₂), 1.36 (s, 3H, C(CH₃)₂), 1.25 (t, 3H, OCH₂CH₃), 0.12 (s, 9H, Si(CH₃)₃); ¹³C NMR (126 MHz, CDCl₃) δ 170.4, 170.0, 112.4, 77.4, 77.3, 61.4, 60.7, 35.1, 32.0, 27.1, 25.5, 14.3, −1.4; FTIR (neat film, NaCl) 2986, 1734, 1696 cm⁻¹; HRMS (ESI) m/z: Calcd for C₁₅H₂₈NO₅Si (MH⁺) 330.1737, observed 330.1724. To a solution of (2,2-Dimethyl-6-oxo-5-trimethylsilanylmethyl-tetrahydro-[1,3]dioxolo[4,5-c]pyrrol-4-yl)-acetic acid ethyl ester (2.38 g, 7.22 mmol, 1.0 equiv) in THF (130 mL) was added 1.0 M aqueous lithium hydroxide (21.7 mL, 21.7 mmol, 3.0 equiv). After stirring at room temperature for three hours the resulting pale yellow suspension was transferred into a separatory funnel containing 500 mL 0.5 M aqueous hydrochloric acid. The aqueous layer was extracted with dichloromethane (3 x 600 mL). The combined organic extracts were dried over sodium sulfate, filtered and concentrated, providing 42 (2.09 g, 96%) as a pale yellow foam. R_f = 0.20 (ethyl acetate); ¹H NMR (500 MHz, CDCl₃) δ 4.85 (d, 1H, J = 5.8 Hz, COCHCH), 4.65 (d, 1H, J = 5.9 Hz, COCHCH), 3.84 (dd, 1H, J = 6.6, 3.3 Hz, NCHCH₂), 3.74 (app t, 1H, J = 6.5 Hz, CO₂H), 3.31 (d, 1H, J = 15.3 Hz, NCH₂), 2.71 (dd, 1H, J = 16.5, 3.3 Hz, NCHCH₂), 2.64 (dd, 1H, J = 16.5, 6.6 Hz, NCHCH₂), 2.24 (d, 1H, J = 15.3 Hz, NCH₂), 1.42 (s, 3H, C(CH₃)₂), 1.34 (s, 3H, C(CH₃)₂), 0.10 (s, 9H, Si(CH₃)₃); ¹³C NMR (126 MHz, CDCl₃) δ 172.3, 171.8, 112.3, 77.6, 68.1, 61.2, 34.4, 32.2, 27.0, 25.5, −1.4; FTIR (neat film, NaCl) 3448, 2989, 1729, 1659 cm⁻¹; HRMS (ESI) m/z: Calcd for C₁₃H₂₃NO₅SSi (M⁺) 302.1424, observed 302.1425.

4.17. 2-(2,2-Dimethyl-6-thioxo-5-trimethylsilanylmethyl-tetrahydro-[1,3]dioxolo[4,5-c]pyrrol-4-yl)-N-methoxy-N-methyl-acetamide (43)

A solution of lactam 42 (1.68 g, 5.57 mmol, 1.0 equiv) in toluene (56 mL) was charged with Lawesson’s reagent (1.24 g, 3.07 mmol, 0.55 equiv), sealed and heated to 65 °C for 16 hours. The resulting pale yellow solution was concentrated in vacuo and purified directly by silica gel column chromatography (dichloromethane followed by 3% acetic acid in ethyl acetate) to afford (2,2-Dimethyl-6-thioxo-5-trimethylsilanylmethyl-tetrahydro-[1,3]dioxolo[4,5-c]pyrrol-4-yl)-acetic acid (1.68 g, 95%) as a pale yellow oil. R_f = 0.44 (3% acetic acid in ethyl acetate); ¹H NMR (500 MHz, CDCl₃) δ 4.96 (dd, 1H, J = 5.4, 1.3 Hz, NCHCHCH), 4.58 (d, 1H, J = 5.4 Hz, NCHCHCH), 4.23 (dd, 1H, J = 8.1, 3.8 Hz, NCHCHCH), 4.20 (d, 1H, J = 14.8 Hz, NCH₂TMS), 3.08 (dd, 1H, J = 16.8, 3.8 Hz, HOC=OCH₂), 2.88 (dd, 1H, J = 16.8, 8.1 Hz, HOC=OCH₂), 2.56 (dd, 1H, J = 14.8, 1.4 Hz, NCH₂TMS), 2.11 (s, 1H, HOC=OCH₂), 1.46 (s, 3H, C(CH₃)₂), 1.37 (s, 3H, C(CH₃)₂), 0.18 (s, 9H, Si(CH₃)₃); ¹³C NMR (126 MHz, CDCl₃) δ 197.0, 193.6, 113.0, 86.4, 78.0, 67.4, 44.8, 38.3, 27.2, 25.8, −0.7; FTIR (neat film, NaCl) 2954, 1693, 1491 cm⁻¹; HRMS (ESI) m/z: Calcd for C₁₃H₂₄NO₄SSi (MH⁺) 318.1195, observed 318.1180. To a solution of (2,2-Dimethyl-6-thioxo-5-trimethylsilanylmethyl-tetrahydro-[1,3]dioxolo[4,5-c]pyrrol-4-yl)-acetic acid (1.75 g, 5.51 mmol, 1.0 equiv) in dichloromethane (28 mL) was added sequentially N,O-dimethylhydroxylamine hydrochloride (1.07 g, 11.0 mmol, 2.0 equiv), 1-[3-(dimethylamino)propyl]-3-ethylcarbodiimide hydrochloride (2.11 g, 11.0 mmol, 2.0 equiv), and triethylamine (3.85 mL, 27.6 mmol, 5.0 equiv). After stirring 15 hours at room temperature, the resulting brown suspension was transferred to a separatory funnel containing 300 mL saturated aqueous ammonium chloride and extracted with dichloromethane (3 x 500 mL). The combined organic fractions were dried over sodium sulfate, filtered, concentrated in vacuo and filtered through a plug of silica gel with ethyl acetate. Concentration afforded 43 (1.24 g, 62%) as a pale yellow oil. R_f = 0.66 (ethyl acetate); ¹H NMR (500 MHz, CDCl₃) δ 4.96 (dd, 1H, J = 5.5, 1.1 Hz, NCHCHCH), 4.59 (d, 1H, J = 5.4 Hz, NCHCHCH), 4.28 (dd, 1H, J = 8.9, 3.7 Hz, NCHCHCH), 4.12 (d, 1H, J = 14.7 Hz, NCH₂TMS), 3.68 (s, 3H, NOCH₃), 3.17 (s, 3H, NCH₃), 2.84 (dd, 1H, J = 16.7, 3.5 Hz, (MeO)NMeC=OCH₂), 2.63 (dd, 1H, J = 16.5, 8.9 Hz, (MeO)NMeC=OCH₂), 2.59 (dd, 1H, J = 14.6, 1.2 Hz, NCH₂TMS), 1.44 (s, 3H, C(CH₃)₂), 1.34 (s, 3H, C(CH₃)₂), 0.15 (s, 9H, Si(CH₃)₃); ¹³C NMR (126 MHz, CDCl₃) δ 196.6, 169.8, 112.6, 86.5, 78.5, 67.6, 61.7, 38.2, 32.3, 27.3, 25.7, 14.6, −0.7; FTIR (neat film, NaCl) 2953, 1655, 1491 cm⁻¹; HRMS (ESI) m/z: Calcd for C₁₅H₂₈N₂O₄SSi (MH⁺) 361.1617, observed 361.1620.

4.18. 1-[2,2-Dimethyl-6-(2-oxo-butylidene)-5-trimethylsilanylmethyl-tetrahydro-[1,3]dioxolo[4,5-c]pyrrol-4-yl]-4-ethylsulfanyl-but-3-en-2-one (44)

To a solution of 43 (607 mg, 1.68 mmol, 1.0 equiv) in acetonitrile (34 mL) was added 1-bromo-2-butanone (180 μL, 1.77 mmol, 1.05 equiv) at room temperature. After 22 hours, triphenylphosphine (485 mg, 1.85 mmol, 1.1 equiv) was added followed by triethylamine (260 μL, 1.85 mmol, 1.1 equiv). The resulting pale orange solution was stirred for 90 minutes at room temperature before being concentrated in vacuo to 5 mL. The crude reaction solution was purified directly by silica gel column chromatography (50% hexane in ethyl acetate ramped to 100% ethyl acetate) affording 2-[2,2-Dimethyl-6-(2-oxo-butylidene)-5-trimethylsilanylmethyl-tetrahydro-[1,3]dioxolo[4,5-c]pyrrol-4-yl]-N-methoxy-N-methyl-acetamide (545 mg, 81%) as a pale yellow oil. R_f = 0.09 (ethyl acetate); ¹H NMR (500 MHz, C₆D₆) δ 6.22 (d, 1H, J = 6.0 Hz, NCHCHCH), 4.96 (s, 1H, vinyl H), 4.39 (d, 1H, J = 6.0 Hz, NCHCHCH), 4.17 (dd, 1H, J = 8.4, 4.2 Hz, NCHCHCH), 2.85 (s, 3H, NOCH₃), 2.71 (s, 3H, NCH₃), 2.55 (d, 1H, J = 15.3 Hz, NCH₂TMS), 2.41 (q, 1H, J = 7.5 Hz, CH₃CH₂C=O), 2.39 (q, 1H, J = 7.6 Hz, CH₃CH₂C=O), 2.34 (m, 1H, (MeO)NMeC=OCH₂), 2.23 (d, 1H, J = 15.5 Hz, NCH₂TMS), 2.10 (dd, 1H, J = 16.1, 8.3 Hz, (MeO)NMeC=OCH₂), 1.39 (s, 3H, C(CH₃)₂), 1.23 (s, 3H, C(CH₃)₂), 1.21 (t, 3H, J = 7.3 Hz, CH₃CH₂C=O), 0.04 (s, 9H, Si(CH₃)₃); ¹³C NMR (126 MHz, C₆D₆) δ 194.7, 170.6, 160.2, 110.9, 91.3, 81.0, 80.2, 66.0, 60.6, 36.6, 35.1, 33.1, 31.6, 27.2, 25.3, 9.7, −1.4; FTIR (neat film, NaCl) 2937, 1656, 1556 cm⁻¹; HRMS (ESI) m/z: Calcd for C₁₉H₃₅N₂O₅Si (MH⁺) 399.2315, observed 399.2327. To a solution of 2-[2,2-Dimethyl-6-(2-oxo-butylidene)-5-trimethylsilanylmethyl-tetrahydro-[1,3]dioxolo[4,5-c]pyrrol-4-yl]-N-methoxy-N-methyl-acetamide (545 mg, 1.37 mmol, 1.0 equiv) in tetrahydrofuran (23 mL) at room temperature was added ethynylmagnesium chloride (0.5 M solution in tetrahydrofuran, 6.6 mL, 3.28 mmol, 2.4 equiv). After stirring for two hours, the reaction solution was filtered through a plug of silica gel with ethyl acetate. Concentration of the solution via rotary evaporation to about 10 mL volume was followed by dilution with dichloromethane to about 30 mL total volume. After repeating this procedure four times, ethanethiol (111 μL, 1.50 mmol, 1.1 equiv) was added at room temperature followed by triethylamine (360 μL, 2.60 mmol, 1.9 equiv). The reaction solution was stirred for 19 hours at room temperature, then poured into a separatory funnel containing 150 mL dichloromethane and partitioned with 100 mL deionized water. The aqueous layer was extracted with dichloromethane (2 x 150 mL) and the combined organic fractions were dried over sodium sulfate, filtered and concentrated in vacuo, affording 44 (337 mg, 58%) as a pale yellow oil. R_f = 0.24 (diethyl ether); ¹H NMR (500 MHz, C₆D₆) δ 7.50 (d, 1H, J = 15.3 Hz, CH₃CH₂SCH=CHC=OCH₂), 6.16 (d, 1H, J = 5.8 Hz, NCHCHCH), 5.86 (d, 1H, J = 15.3 Hz, CH₃CH₂SCH=CHC=OCH₂), 4.95 (s, 1H, CH₃CH₂C=OCH=C), 4.22 (d, 1H, J = 6.0 Hz, NCHCHCH), 4.17 (dd, 1H, J = 8.2, 4.2 Hz, NCHCHCH), 2.50 (d, 1H, J = 15.8 Hz, NCH₂TMS), 2.41 (q, 2H, J = 7.9 Hz, CH₃CH₂SCH=CHC=OCH₂), 2.40 (q, 2H, J = 7.8 Hz, CH₃CH₂SCH=CHC=OCH₂), 2.19 (dd, 1H, J = 17.1, 4.1 Hz, CH₃CH₂SCH=CHC=OCH₂), 2.15 (q, 2H, J = 7.2 Hz, CH₃CH₂C=OCH=C), 2.14 (d, 1H, J = 15.4 Hz, NCH₂TMS), 1.92 (dd, 1H, J = 17.1, 8.4 Hz, CH₃CH₂SCH=CHC=OCH₂), 1.39 (s, 3H, C(CH₃)₂), 1.22 (s, 3H, C(CH₃)₂), 1.21 (t, 3H, J = 7.5 Hz, CH₂CH₃), 0.82 (t, 3H, J = 7.5 Hz, CH₂CH₃), 0.04 (s, 9H, Si(CH₃)₃); ¹³C NMR (126 MHz, C₆D₆) δ 194.8, 192.2, 160.3, 147.3, 122.5, 111.0, 91.0, 81.0, 80.2, 65.5, 40.8, 36.6, 35.1, 27.2, 26.0, 25.3, 13.5, 9.7, −1.4; FTIR (neat film, NaCl) 2963, 1648, 1553 cm⁻¹; HRMS (ESI) m/z: Calcd for C₂₁H₃₅NO₄SSi (MH⁺) 426.2134, observed 426.2143.

4.19. Bridged pyrrolizidine 45

To a solution of 44 (1.32 g, 3.10 mmol, 1.0 equiv) in chloroform (220 mL) at −45 °C was added trifluoromethanesulfonic anhydride (570 μL, 3.41 mmol, 1.1 equiv). The resulting colorless solution was stirred at −45 °C for 15 minutes, warmed to 0 °C and stirred for 20 minutes, then recooled to −45 °C. Tetrabutylammonium triphenyldifluorosilicate (1.84 g, 3.41 mmol, 1.1 equiv) was added and the solution was allowed to warm to room temperature slowly over 15 hours. The resulting clear, purple solution was concentrated in vacuo and purified by silica gel column chromatography (1% methanol in dichloromethane, ramped to 2% methanol in dichloromethane), providing 45 (1.07 g, 71%) as a colorless oil. R_f = 0.30 (diethyl ether); ¹H NMR (500 MHz, C₆D₆) δ 5.21 (s, 1H, vinyl H), 4.99 (d, 1H, J = 3.4 Hz, COCH₂CHCHCH), 4.06 (d, 1H, J = 5.8 Hz, COCH₂CHCHCH), 3.49 (s, 1H, COCHCHCH₂), 3.24 (d, 1H, J = 5.5 Hz, COCH₂CHCHCH), 3.02 (d, 2H, J = 6.1 Hz, COCHCHCH₂), 2.73 (t, 1H, J = 6.3 Hz, COCHCHCH₂), 2.32 (quartet, 2H, J = 7.4 Hz, C(OTf)CH₂CH₃), 2.25 (quartet, 2H, J = 7.4 Hz, SCH₂CH₃), 1.79 (dd, 1H, J = 16.0, 6.3 Hz, COCH_2(ax)CHCHCH), 1.60 (d, 1H, J = 16.0 Hz, COCH_2(eq)CHCHCH), 1.54 (s, 3H, C(CH₃)₂), 1.19 (s, 3H, C(CH₃)₂), 1.03 (t, 3H, J = 7.4 Hz, SCH₂CH₃), 0.98 (t, 3H, J = 7.4 Hz, C(OTf)CH₂CH₃); ¹³C NMR (126 MHz, C₆D₆) δ 203.9, 150.4, 120.0, 118.9, 112.0, 83.4, 80.2, 79.3, 66.9, 64.3, 56.1, 48.6, 39.8, 27.6, 26.6, 26.3, 25.3, 14.6, 10.7; FTIR (neat film, NaCl) 2965, 1712, 1560, 1430, 1260 cm⁻¹; [α]²⁴D = −213.0 (c 0.034, CHCl₃); HRMS (ESI) m/z: Calcd for C₁₉H₂₇F₃NO₆S₂ (MH⁺) 486.1232, observed 486.1234.

4.20. Bridged pyrrolizidine 46

To a solution of 45 (350 mg, 0.721 mmol) in methanol (300 mL) was added 10% palladium on carbon (1.00 g) and the resulting black suspension was charged with hydrogen gas via balloon and stirred under an atmosphere of hydrogen for 18 hours. Filtration through Celite, concentration in vacuo, and purification of the residue by silica gel column chromatography (50% ethyl acetate in hexanes) provided 46 (214 mg, 89%) as a colorless oil. R_f = 0.46 (ethyl acetate); ¹H NMR (500 MHz, C₆D₆) δ 4.37 (d, 1H, J = 6.0 Hz, COCH₂CHCHCH), 4.04 (d, 1H, J = 6.0 Hz, COCH₂CHCHCH), 3.27 (d, 1H, J = 6.0 Hz, COCH₂CHCHCH), 3.13 (dd, 1H, J = 14.0, 5.1 Hz, COCHCHCH₂), 3.03 (s, 1H, COCHCHCH₂), 2.97 (dd, 1H, J = 14.0, 8.3 Hz, COCHCHCH₂), 2.63 (dd, 1H, J = 8.3, 5.1 Hz, COCHCHCH₂), 2.12 (q, 2H, J = 7.4 Hz, SCH₂CH₃), 2.05 (m, 2H, CH₂(CH₂)₂CH₃), 1.81 (dd, 1H, J = 15.8, 6.2 Hz, COCH₂CHCHCH), 1.56 (d, 1H, J = 15.8 Hz, COCH₂CHCHCH), 1.50 (s, 3H, C(CH₃)₂), 1.49 – 1.33 (m, 4H, , CH₂(CH₂)₂CH₃), 1.09 (s, 3H, C(CH₃)₂), 0.93 (t, 3H, J = 7.4 Hz, CH₂CH₃), 0.90 (t, 3H, J = 7.2 Hz, CH₂CH₃); ¹³C NMR (126 MHz, CDCl₃) δ 207.7, 111.5, 82.7, 81.0, 79.5, 79.2, 65.0, 64.7, 56.8, 40.5, 29.9, 28.3, 26.3, 26.0, 24.9, 23.1, 15.1, 14.0; FTIR (neat film, NaCl) 2958, 2927, 1712, 1459, 1379 cm⁻¹; HRMS (ESI) m/z: Calcd for C₁₈H₃₀NO₃S (MH⁺) 339.1868, observed 339.1868.

4.21. Bridged pyrrolizidine 47

A solution of 46 (13 mg, 0.038 mmol, 1.0 equiv) in tetrahydrofuran (600 μL) was cooled to 0 °C and charged with a solution of 0.2 M LDA in tetrahydrofuran (260 μL, 1.2 equiv). After stirring for one hour, ethyl iodoacetate (5.4 μL, 0.046 mmol, 1.2 equiv) was added and the solution was stirred for a further 30 minutes at 0 °C, then quenched with saturated aqueous sodium bicarbonate. The product was extracted from deionized water (5 mL) with ethyl acetate (3 x 10 mL), and the combined organic layers were dried over sodium sulfate, filtered and concentrated in vacuo. The residue was purified by silica gel column chromatography (29% ethyl acetate in hexanes, ramped to 50% ethyl acetate in hexanes, providing 47 (9.5 mg) as a 1:1 mixture of C-12 epimers. This mixture was dissolved in toluene (500 μL) and charged with DBU (13.3 μL, 0.089 mmol, 4.0 equiv) and the resulting solution was stirred at 80 °C for 16 hours. Elution of the solution through a plug of silica gel with ethyl acetate provided, upon concentration, 46 (9.5 mg, 58% over two steps) as a colorless oil. R_f = 0.32 (50% ethyl acetate in hexanes); ¹H NMR (500 MHz, C₆D₆) δ 4.32 (d, 1H, J = 6.1 Hz, COCHCHCHCH), 4.09 (d, 1H, J = 6.1 Hz, COCHCHCHCH), 3.95 (q, 1H, J = 7.2 Hz, OCH₂CH₃), 3.93 (q, 1H, J = 7.2 Hz, OCH₂CH₃), 3.39 (d, 1H, J = 5.7 Hz, COCHCHCH₂), 3.20 (d, 2H, J = 6.6 Hz, CH₂CO₂Et), 3.15 (s, 1H, COCHCHCHCH), 3.09 (ddd, 1H, J = 8.8, 5.5, 4.1 Hz, COCHCHCH₂), 3.03 (t, 1H, J = 6.6 Hz, COCHCHCHCH), 2.74 (dd, 1H, J = 16.9, 8.8 Hz, COCHCHCH₂), 2.16 – 2.02 (m, 4H, SCH₂CH₃, CH₂(CH₂)₂CH₃), 1.70 (dd, 1H, J = 16.9, 4.1 Hz, COCHCHCH₂), 1.52 (t, 3H, C(CH₃)₂), 1.51 – 1.34 (m, 4H, CH₂(CH₂)₂CH₃), 1.11 (s, 3H, C(CH₃)₂), 0.96 (t, 3H, J = 7.1 Hz, CH₂CH₃), 0.93 (t, 3H, J = 7.3 Hz, CH₂CH₃), 0.91 (t, 3H, J = 7.4 Hz, CH₂CH₃); ¹³C NMR (126 MHz, CDCl₃) δ 207.1, 171.8, 111.4, 81.3, 79.2, 79.0, 69.6, 63.7, 61.2, 57.0, 49.3, 44.2, 30.0, 29.8, 28.5, 26.5, 26.1, 24.9, 23.1, 15.2, 14.3, 14.1; FTIR (neat film, NaCl) 2960, 2927, 1732, 1708, 1560, 1412 cm⁻¹; HRMS (ESI) m/z: Calcd for C₂₂H₃₆NO₅S (MH⁺) 425.2236, observed 425.2233.

4.22. Bridged pyrrolizidine 48

A solution of 47 (2.6 mg, 0.0061 mmol) was charged with 2.5 M aqueous hydrochloric acid (750 μL), sealed under argon and heated to 60 °C for 3.5 hours. Concentration in vacuo provided 48 (2.1 mg, 96%) as a colorless film. ¹H NMR (500 MHz, CD₃OD) δ 4.54 (dd, 1H, J = 14.1, 8.6 Hz, COCHCHCH₂), 4.39 (d, 1H, J = 5.3 Hz, COCHCHCHCH), 4.23 (d, 1H, J = 6.0 Hz, COCHCHCHCH), 4.05 (d, 1H, J = 5.3 Hz, COCHCHCHCH), 3.78 (dd, 1H, J = 8.6, 5.2 Hz, COCHCHCH₂), 3.68 (m, 2H, COCHCHCH₂, COCHCHCHCH), 2.88 (dd, 1H, J = 17.4, 7.4 Hz, CH₂CO₂H), 2.78 (q, 2H, J = 7.4 Hz, SCH₂CH₃), 2.43 (dd, 1H, J = 17.4, 5.6 Hz, CH₂CO₂H), 2.08 (m, 2H, CH₂(CH₂)₂CH₃), 1.47 (m, 4H, CH₂(CH₂)₂CH₃), 1.33 (t, 3H, J = 7.4 Hz, SCH₂CH₃), 1.01 (t, 3H, J = 7.2 Hz, CH₂(CH₂)₂CH₃); FTIR (neat film, NaCl) 3411, 3137, 2960, 2925, 2868, 1727, 1711, 1158 cm⁻¹; HRMS (ESI) m/z: Calcd for C₁₇H₂₈NO₅S (MH⁺) 358.1688, observed 358.1690.

Supplementary Material

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Acknowledgments

This research was supported by the NIH-NIGMS (GM67659) and Merck. MTE thanks NSF and AstraZeneca for predoctoral fellowships.

Footnotes

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References and Notes

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25. The trans-configuration was inferred from comparison of coupling constant values to that of 27 (vide infra), as well as the lack of nOe signal between the C9a and C2 methine protons. No positive diagnostic nOe signals were observed.

26. Cycloaddition conditions involving chloroform, dichloromethane, acetonitrile and benzene as solvent, TBAT and CsF as fluoride sources and desilylation temperatures ranging from −78 ºC to 23 ºC failed to prevent C2 epimerization.