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Nat Commun. 2017; 8: 679.
Published online 2017 September 25. doi:  10.1038/s41467-017-00787-y
PMCID: PMC5612996

Stereoselective one-pot synthesis of polypropionates

Abstract

Polypropionates—motifs with alternating methyl and hydroxy groups—are important segments of many natural products possessing high bioactivity and therapeutic value. Synthetic access to these structures remains an area of intensive interest, focusing on the establishment of the contiguous stereocentres and a desire for operational simplicity. Here we report an efficient strategy for the stereoselective assembly of polypropionates with three or four stereocentres through a three-step relay process that include Diels–Alder reaction, silylenol ether hydrolysis and Baeyer–Villiger oxidation. The stereochemistry and functionality of the resulting polypropionates depend on the substitution pattern of the diene and dienophile substrates of the Diels–Alder cycloaddition. More importantly, the relay sequence is effectively performed in one pot, and the product could potentially undergo the same sequence for further elaboration. Finally, the C1–C9 segment of the macrolide etnangien is constructed with four of the six stereogenic centres established using the relay sequence.

Introduction

Polyketides are a large class of structurally diverse secondary metabolites isolated from fungi, bacteria and sponges1. They exhibit a wide range of biological activities, including antibiotic, antitumour, antifungal, cholesterol-lowering, antiparasitic and immunosuppressive properties1, 2. In particular, several polyketide-containing natural products, such as the antibiotics erythromycin3 and monensin A4 and the antitumour discodermolide5 (Fig. 1), have displayed clinical significance. A common feature of natural products with polyketide structures are polypropionate segments characterized by alternating methyl and hydroxy groups6. The multiple stereogenic centres displayed in these segments provide for large numbers of possible stereochemical permutations. A stereotetrad motif (structure with four contiguous stereogenic centres), for example, could have 16 possible diastereomers. Consequently, the preparation of polypropionates has been a longstanding interest in the synthesis community610.

Fig. 1
Polypropionate-containing natural products. The major polypropionate segments are marked by dashed polygons

Numerous synthetic approaches have been developed to address the architectural complexity of polypropionates. Forging a single C–C bond into adjacent methyl and hydroxy-bearing stereogenic centres could be accomplished by established aldol reactions11, but crotylation1214, allenylation15, epoxide opening6, [2 + 2]-cycloaddition16 and many other methods6 are available. Although the application of these methods in iterative protocols allowed for more complex polypropionate subunits, additional step-consuming transformations are required for each subsequent C–C bond extensions. In addition, the stereochemical control of the newly created stereocentres over the course of the propionate chain elongation is often complicated. Attractive alternatives, especially considering atom and step economy, are multicomponent domino (or cascade) reactions1719 that could create more than one C–C bond, and hence, multiple stereocentres in a single-pot operation. Nevertheless, only particular sets of stereoisomers could be achieved in these procedures. Thus, further development of convenient entries to several possible stereotriads, stereotetrads or stereopentads, as well as their related structures that exploit operationally simple strategies in library design and drug discovery of polypropionate-containing molecules are still highly desirable.

Motivated by the intensive interest towards the expedient synthesis of polypropionates, we herein devise an alternative stereoselective strategy from simple and readily available precursors. This approach involves sequential Diels–Alder cycloaddition of silylenol ethers and enones acting as the respective dienes and dienophiles, silylenol ether hydrolysis to afford a cyclic ketone and Baeyer–Villiger oxidation to enable double oxygen insertions with stereoretention at key positions. Ultimately, the relay sequence provides a suitable access into polypropionates with three or four contiguous stereocentres both in stepwise and one-pot manner. Using this procedure, the synthesis of the C1–C9 segment of the macrolide natural product etnangien is also successfully demonstrated.

Results

Synthetic strategy

Our synthetic design follows the formation of the seven-membered lactone 1, harbouring three to four stereocentres (Fig. 2). The embedded oxygen functionalities at C4 and C6 positions of lactone 1 can be traced back to cyclohexanone 2 by Baeyer–Villiger oxidation. Analysis of compound 2 shows possible access from a pivotal Diels–Alder cycloaddition of silyloxydiene 3 and dienophile 4. Previous studies have demonstrated the strong effect of the terminal substitution of dienes and dienophiles in the exo- and endo-stereochemical outcome of the Diels–Alder reaction20, 21. Thus, we anticipate a favourable control of stereoselectivity during the formation of the cycloadduct. Finally, the required dienes, and in certain cases, the dienophiles could be afforded from the commercially or readily available enone 5. This synthetic approach benefits from the inherent flexibility of the Diels–Alder reaction and the ready availability of various dienes and dienophiles, with the potential option to grow the polypropionate chains having the desired diastereomeric relationships. In addition, the transformations of the diene 3 and dienophile 4 towards the lactone 1, that is, the Diels–Alder/Baeyer–Villiger relay process, could be performed using compatible reagents to permit not only stepwise handling but also one-pot operation. Furthermore, with synthetically versatile groups at the terminal positions of the corresponding open form of the lactone 1, it is, in principle, possible to reiterate the process through compound 6 carrying the familiar enone structure.

Fig. 2
Our retrosynthetic analysis for the synthesis of polypropionates. The asterisks represent possible stereogenic centres. TBS tert-butyldimethylsilyl

Evaluation of the Diels–Alder/Baeyer–Villiger relay process

To examine the proposed relay concept, we first applied the designed procedure to the prototypical enones 7 and 9 in a stepwise manner (Fig. 3a). Deprotonation of enone 7 with lithium hexamethyldisilazide followed by silyl trapping supplied the silylenol ether 8 in 89% yield ((Z,E)/(E,E) = 86/14). Alternatively, use of triethylamine as base to permit the same reaction led to lower yield (82%) and diastereoselectivity for the desired (Z,E)-isomer (85/15). The cycloaddition of diene 8 and dienophile 9 is expected to produce the exo-adduct based on the methyl substitution at C1 of 8 and Cβ of 9 21 (for the carbon designations of dienes and dienophiles used in this paper, refer to Fig. 2). Thus, the boron trifluoride-catalysed Diels–Alder reaction effectively provided the product 10 in excellent yield and exo-selectivity. Other Lewis acid catalysts, such as copper(II) triflate, scandium(III) triflate, trimethylsilyl triflate, dimethylaluminum chloride and tin tetrachloride, also gave similar exo-preference, but only in moderate-to-good yields (Supplementary Table 1). Notably, the supposed Diels–Alder cycloadduct between the (E,E)-isomer of 8 and the enone 9 was not observed in these cases. The desilylation of 10 could be achieved by tetrabutylammonium fluoride (TBAF), but acid hydrolysis offers better compatibility with Baeyer–Villiger oxidation in light of our desire for a one-pot operation. Consequently, treatment with trifluoroacetic acid (TFA) for 20 min delivered the corresponding dione, which was directly treated with meta-chloroperoxybenzoic acid (mCPBA) for the double oxygen insertion to afford the expected ε-lactone 11 carrying a polypropionate syn,syn,syn-stereotetrad in 75% yield (two steps, one pot). The structure of 11 was unambiguously confirmed by X-ray crystallographic analysis (Fig. 3b, Supplementary Table 2, Supplementary Data 1) and further characterized by nuclear magnetic resonance (NMR) spectroscopy (Supplementary Methods). Curiously, the exposure of compound 10 to TBAF led to a complete epimerization at the α-carbon leading to compound 12 (77%) as a single diastereomer, providing a possible point of access to another stereotetrad structure.

Fig. 3
Stepwise preparation of polypropionate 14. a Synthetic route. b X-ray crystal structure of 11. DBU 1,8-diazabicyclo[5.4.0]undec-7-ene; LiHMDS lithium hexamethyldisilazide; mCPBA, meta-chloroperoxybenzoic acid; TBAF, tetrabutylammonium fluoride; Tf, triflyl; ...

Treatment of the lactone 11 with catalytic sodium methoxide in methanol resulted in deacetylation and transesterification, supplying the γ-lactone 13 (96%). This cyclic system should offer more stability than the initial seven-membered ring structure and could serve as key intermediate with various potential for further manipulation. With the intent to explore the possibility of reiterating the tandem Diels–Alder/Baeyer–Villiger reaction process, we examined the elaboration of compound 13 to further generate an advanced enone structure. The operation started from the silyl group protection of the free hydroxy group (97%), followed by nucleophilic addition to the carbonyl using allylmagnesium chloride. Subsequent base-promoted double bond isomerization ultimately afforded the desired enone 14 (78% in two steps). This and similar compounds offer prospective applications for the assembly of longer and more complex polypropionate chains.

After successfully demonstrating stereotetrad formation via Diels–Alder cycloaddition and Baeyer–Villiger oxidation in stepwise manner, we then explored the feasibility of our envisioned one-pot operation (Table 1). Pleasingly, the Diels–Alder exo-adduct, formed in situ from compounds 8 and 9 by the catalytic assistance of boron trifluoride, was smoothly converted to the desired ε-lactone 11 in 63% yield upon successive treatment with TFA and mCPBA in the same vessel (entry 1). For cycloaddition with enone 15, we found that copper(II) triflate provided the best product yield and selectivity for the expected endo-adduct20 (Supplementary Table 1). Thus, the sequential Diels–Alder cycloaddition of 8 and 15, acid hydrolysis and Baeyer–Villiger oxidation supplied the ε-lactone 16 in 53% yield for the one-pot process (entry 2). Further application of the one-pot procedure to different diene and enone combinations all provided satisfactory results (entry 3–12). Curiously, when the phenyl-substituted 32 was employed as starting enone (entries 11 and 12), minor amounts of the separable corresponding γ-lactone (34 and 36), apparently resulting from acid-induced translactonization22, was acquired along with the expected ε-lactone (33 and 35). The structures of the duly formed lactones were confirmed by NMR spectroscopy as well as X-ray crystallographic analysis for compounds 16, 23, 29 and 34 (Supplementary Fig. 1, Supplementary Tables 36, Supplementary Data 25). Treatment of all the lactone products with sodium methoxide in methanol resulted into the corresponding γ-lactone akin to compound 13 with variously substituted stereogenic centres (Supplementary Table 7).

Table 1
One pot operation for the tandem Diels–Alder and Baeyer–Villiger transformations

Preparation of the C1–C9 segment of etnangien

To demonstrate the utility of the tandem one-pot Diels–Alder/Baeyer–Villiger relay process for the assembly of polypropionates leading to more complex molecular architectures, we targeted the stereohexad structure present at the C1–C9 subunit of the macrolide etnangien (Fig. 1). Etnangien, an antibiotic isolated from the myxobacterium Sorangium cellulosum, is a potent inhibitor of the RNA polymerase of Gram-positive bacteria23. Our synthesis began with the Weinreb amidation (85%) of the commercially available (S)-3-phenylbutanoic acid (37) (Fig. 4a). Nucleophilic allyl addition to the amide, followed by double-bond isomerization established the familiar enone structure 38. Silylation of this enone fashioned the desired 1,4-disubstituted diene 39 in 82% yield ((Z,E)/(E,E) = 70/30).

Fig. 4
Synthesis of the C1–C9 fragment of etnangien. a Synthetic route. b X-ray crystal structure of 41. c X-ray crystal structure of 42. Ac, acetyl, DMAP, N,N-dimethyl-4-aminopyridine; PCC, pyridinium chlorochromate

With required diene 39 in hand, the formation of the target ε-lactone through the crucial relay process was first performed in a stepwise fashion. Thus, the Diels–Alder cycloaddition of the diene 39 and enone 9 employing the boron trifluoride-catalysed condition produced two exo- and two endo-cycloadduct diastereoisomers (exo/endo = 84/16) of which the desired silylenol ether and exo-adduct 40 was obtained as the major diastereomer in 65% yield; the other diastereomers were collectively afforded in 25% yield (see the Supplementary Methods for details). A preliminary X-ray analysis of the racemic version of 40 supported the structure of this major diastereomer (Supplementary Fig. 1, Supplementary Table 8, Supplementary Data 6). The facial selectivity of the Diels–Alder reaction towards 40 is perceived as a result of the asymmetric induction of the stereocentre attached to C1 of the diene (Supplementary Fig. 2). Having the desired stereochemistry and the necessary carbon atoms of the target subunit, the exo-adduct 40 was subjected to sequential hydrolysis and Baeyer–Villiger oxidation utilizing TFA and mCPBA, respectively, generating the required ε-lactone 41 in 75% yield as a single isomer. X-ray crystallography confirmed the structure of 41 with the prescribed relative configuration of the contiguous stereocentres on the seven-membered lactone ring (Fig. 4b, Supplementary Table 9, Supplementary Data 7). After the successful stepwise attempt, we next pursued the transformation in one pot. To our satisfaction, the three-step sequence also proceeded smoothly and provided the expected lactone 41 in 40% overall yield. Treatment of 41 with sodium methoxide resulted to deacetylation and subsequent conversion into the γ-lactone 42 (96%). Again, an X-ray analysis supported the structure and relative configuration of the stereopentad motif of the crystalline 42 (Fig. 4c, Supplementary Table 10, Supplementary Data 8).

With implicitly positioned terminal groups, 42 was further modified to form the more elaborated C1–C9 fragment of etnangien, particularly using a Birch reduction/ozonolytic cleavage sequence24. Thus, exposure of the alcohol 42 to lithium metal in the presence of ammonia25 reduced the lactone into a terminal alcohol and the phenyl group into a 1,4-cyclohexadiene structure. A side product of this reaction, with the phenyl group remaining unscathed, was further converted to 42 using the same Birch reduction in moderate yield (Supplementary Methods). Silylation of the alcohol groups led to compound 43. Ozonolysis transformed the 1,4-cyclohexadiene portion of 43 into the keto/cis-enol pair as verified by NMR spectroscopy of 44 with likely stabilization by hydrogen bonding. Treatment of 44 with hydrogen fluoride in pyridine provided for partial desilylation and subsequent enol ether formation via a lactol intermediate. Acetylation of the so-formed 45 followed by Luche reduction26 established the alcohol 47 as a single diastereosomer in excellent yield. Final silylation and oxidation provided the desired C1–C9 fragment of etnangien containing six stereocentres (48). This fragment possesses the synthetic versatility that would enable further elaboration towards etnangien and other more advanced molecules.

Discussion

We have developed an efficient approach to stepwise and one-pot stereoselective synthesis of polypropionate structures through a three-step Diels–Alder cycloaddition/Baeyer–Villiger oxidation relay process. The highly substrate-controlled Diels–Alder reaction, facilitated by the Lewis acid catalysts boron trifluoride and copper(II) triflate, enabled the establishment of the necessary cyclohexene ring featuring the silylenol ether and three-to-four stereogenic centres. Following an acid hydrolysis to provide the aptly positioned dicarbonyl groups, Baeyer–Villiger oxidation with mCPBA formed the lactone and ester functionalities. Using this procedure, a range of polypropionate stereoisomers with high stereochemical flexibility and substantial skeletal diversity was accessed. The amenability of our strategy for a one-pot operation offers a rapid and convenient method to the many possible polypropionate motifs as demonstrated herein. Importantly, the resulting products not only feature the polyketide subunits, but also render functionalities that could, in principle, be further transformed for use in subsequent C–C bond forming events or other elaborations to more complex structures. Further extension of this methodology to provide more comprehensive access to other biologically significant polypropionate motifs is ongoing and will be reported in due course.

Methods

One-pot preparation of compound 41

Freshly distilled ketone 9 (0.24 mL, 2.38 mmol) in CH2Cl2 (2 mL) was added by syringe to a solution of BF3·OEt2 (12 µL, 99 μmol) in CH2Cl2 (2 mL) at −78 °C under argon atmosphere. After stirring at −78 °C for 10 min, diene 9 ((Z,E)/(E,E) = 70/30, 0.600 g, 1.99 mmol) in CH2Cl2 (2.0 mL) was added dropwise, and the reaction was allowed to stir for 12 h at the same temperature. TFA (0.18 mL, 2.38 mmol) was next added in one portion, and the reaction was allowed to warm to 0 °C. After 20 min at 0 °C, the resulting mixture was poured into a solution of mCPBA (77%, 3.56 g, 15.9 mmol) in CH2Cl2 (20 mL) at 0 °C. The reaction was warmed to ambient temperature and stirred for 24 h. The reaction mixture was slowly quenched with satd. Na2S2O3(aq) at 0 °C, and the crude organic portion was extracted with CH2Cl2. The combined organic layer was washed with satd. NaHCO3(aq) and brine, dried over MgSO4, filtered and concentrated under reduced pressure. Purification via flash column chromatography (n-hexane/ethyl acetate = 4/1) provided the ε-lactone 41 (0.169 g, 40%) as a colourless solid. [α]23 D = −7.28 (c 10, CHCl3); m.p. 130–131 °C (recrystallized from n-hexane/CHCl3); Infrared Spectrum (thin film): ν 2973, 1734, 1637, 1454, 1239, 1183, 1087, 1020, 767, 703 cm–1; 1H NMR (600 MHz, CDCl3): δ 7.28 (t, J = 7.5 Hz, 2H; Ph-H), 7.21 (t, J = 7.2 Hz, 1H; Ph-H), 7.12 (d, J = 7.8 Hz, 2H; Ph-H), 4.70 (d, J = 10.6 Hz, 1H; ε-methine-H), 4.67 (s, 1H; γ-methine-H), 3.28 (dd, J = 14.0, 6.7 Hz, 1H; α-methylene-H), 2.99 (dq, J = 13.4, 6.7 Hz, 1H; benzylic H), 2.44 (dd, J = 14.0, 5.2 Hz, 1H; α-methylene-H), 2.10 (br, 1H; β-methine-H), 1.90 (s, 3H, CO2CH3), 1.56–1.54 (m, 1H; δ-methine-H), 1.38 (d, J = 6.7 Hz, 3H; CH3), 1.14 (d, J = 7.7 Hz, 3H; CH3), 1.01 (d, J = 7.6 Hz, 3H; CH3); 13C NMR (150 MHz, CDCl3): δ 173.6 (C), 168.9 (C), 143.1 (C), 128.9 (CH × 2), 127.2 (CH × 2), 127.2 (CH), 80.9 (CH), 77.7 (CH), 41.9 (CH), 37.4 (CH), 35.1 (CH2), 32.6 (CH), 21.1 (CH3), 20.2 (CH3), 16.9 (CH3), 10.1 (CH3); High Resolution Mass Spectrum (Electrospray Ionisation): m/z calcd for C18H24O4Na ([M + Na]+): 327.1572, found: 327.1580. An X-ray analysis of the crystal supported the structure of 41 and the relative configuration of its contiguous chiral centres ((Fig. ((Fig.4b,4b, CCDC 1556026).

The complete experimental details and compound characterization data can be found in the Supplementary Methods. For the NMR spectra of the compounds in this article, see Supplementary Figs. 3102.

Data availability

The X-ray crystallographic coordinates for compounds 11, 16, 23, 29, 34, 40, 41 and 42 in this study have been deposited at the Cambridge Crystallographic Data Centre (CCDC), under deposition numbers CCDC 1511846, CCDC 1511847, CCDC 1511848, CCDC 1511849, CCDC 1511880, CCDC 1511850, CCDC 1556026 and CCDC 1556034, respectively. These data can be obtained free of charge from the CCDC via www.ccdc.cam.ac.uk/data_request/cif. All data that support the findings of this study are available from the corresponding author upon reasonable request.

Acknowledgements

This work was supported by the Ministry of Science and Technology (MOST 104-0210-01-09-02, MOST 104-2628-M-001-001, MOST 105-0210-01-13-01, MOST 105-2745-M-001-003-ASP) and Academia Sinica.

Author contributions

Author contributions

S.-C.H. conceived the project and supervised staffs. G.-M.H. performed the synthetic experiments. M.M.L.Z. and G.-M.H. wrote the manuscript. All authors discussed the results and commented on the manuscript.

Notes

Competing interests

The authors declare no competing financial interests.

Footnotes

Electronic supplementary material

Supplementary Information accompanies this paper at doi:10.1038/s41467-017-00787-y.

Publisher's note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

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