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Logo of nihpaAbout Author manuscriptsSubmit a manuscriptHHS Public Access; Author Manuscript; Accepted for publication in peer reviewed journal;
 
J Am Chem Soc. Author manuscript; available in PMC 2010 October 21.
Published in final edited form as:
PMCID: PMC2791109
NIHMSID: NIHMS148145

Total Synthesis and Stereochemical Assignment of (–)-Ushikulide A

Abstract

We report the determination of the full stereostructure of (–)-ushikulide A (1), a spiroketal containing macrolide by total synthesis. Ushikulide A (1) was isolated from a culture broth of Streptomyces sp. IUK-102 and exhibits potent immunosuppressant activity (IC50 = 70 nM). To embark upon an ushikulide A synthesis, a tentative assignment was made based on analogy to cytovaricin (2), a related macrolide isolated from a culture of Streptomyces diastatochromogenes whose full structure was previously established via synthesis and X-ray crystallography. This report delineates studies on several key steps, namely a direct aldol reaction catalyzed by the dinuclear zinc ProPhenol complex, a metal catalyzed spiroketalization, as well as application of an unprecedented asymmetric alkynylation of a simple saturated aldehyde with methyl propiolate to prepare the nucleophilic partner for a Marshall-Tamaru propargylation. These studies culminated in the first total synthesis and stereochemical assignment of (–)-ushikulide A and significantly extended the scope of the above-mentioned methodologies.

Keywords: Ushikulide A, cytovaricin, spiroketal, macrolactone, dinuclear zinc aldol, Suzuki cross-coupling, Marshall/Tamaru propargylation

Introduction

In spite of the tremendous advances in modern spectroscopic methods, organic synthesis continues to play a pivotal role in the elucidating the full structure of complex natural products.1 This method has the advantage that even in the absence of a firm structural assignment, a combination of logic and spectroscopic comparison can arrive at the correct structure. Previous work in our group led to the assignment of amphidinolide A after a total of eleven diastereomers were prepared synthetically.2 Thus we were encouraged to consider the synthesis of other natural products of uncertain stereochemistry.

Ushikulide A (1), a newly isolated3 and stereochemically undefined member of the oligomycinrutamycin family was chosen out of many possible synthetic targets based on several considerations. Due to the stereochemical complexity of such natural products randomly preparing diastereomers would lead to an exceedingly tedious task (ushikulide A contains 14 stereogenic centers - leading to 214 = 16384 stereoisomers). Therefore, a reasonable amount of likely stereochemical information was required before synthetic studies could be initiated. Ushikulide A (1) has remarkable resemblance to cytovaricin (2), a natural product isolated from the same species of bacteria, whose full structure was established through X-ray crystallography4 and confirmed via synthesis.5 Furthermore, careful comparison of the NMR spectra of these structures revealed close similarity between the spiroketal regions of 1 and 2 (Figure 2). While this was clearly insufficient evidence to firmly assign the structure of ushikulide, it appeared at the outset that eight of the stereocenters (carbons 16, 19, 20, 21, 23, 26, 27 and 29) were either clearly analogous to cytovaricin or their configuration should be apparent after the first synthetic diastereomer was prepared. Specifically, stereochemistry at C16 could be lost through base catalyzed isomerization of the β,γ-unsaturated ketone yielding ushikulide B (which contains an E olefin between C15 and C16), while the stereochemistry at C29 (which is isolated from the macrolactone) could be assigned so long as the spiroketal portion of the synthetic material had the correct configuration. Only six centers remained, a task roughly comparable to our successful synthesis of amphidinolide A.2 In the absence of significant quantities of a natural product for degradation studies, synthesis is the only method to reliably assign structure in the event that X-ray crystallography proves untenable, as was the case for 1.6 We hope that this full account7 will provide a deeper understanding of the chemistry employed in this study while also outlining the power of synthesis for the determination of structure.

Figure 2
Comparison of Cytovaricin and Ushikulide A

Synthetic Planning

A flexible approach that could be amended to prepare any diastereomer of 1 was envisioned (Scheme 1). As stereochemistry of the spiroketal was pivotal, it seemed well advised to split the spiroketal (3) away from the aliphatic portion (4) via Suzuki coupling and esterification. While sp2-sp2 Suzuki coupling is a well established disconnection for diene containing members of this family,8 our retrosynthesis relied on a less common sp2-sp3 Suzuki reaction9 to access the C14-C15 olefin of ushikulide A. For spiroketal fragment 3, a Marshall/Tamaru propargylation would provide the homopropargylic alcohol that could be elaborated to the C15-C14 olefin. The metal catalyzed spiroketalization, pioneered by Utimoto10 represents a highly atom-economical and redox-neutral11 approach to the desired spiroketal ring system. Utilization of an alkyne as an orthogonally reactive surrogate12 for the desired spiroketal is strategically advantageous as it enabled two carbon-carbon bond forming reactions without requiring the use of additional protecting group manipulations or activating groups, which certainly would have been required if more conventional carbonyl chemistry had been employed. Either 5 or 6 could potentially lead to the desired spiroketal however, only the former case appeared likely to give the desired regioselectivity (See 6 to 7, Scheme 1). Application of our dinuclear zinc aldol methodology13 was expected to simplify the aliphatic fragment into ketone 9 and aldehyde 10. Due to the similar steric bulk of the methyl and allyl groups at C10 conventional aldol methodologies would not be expected to give acceptable diastereoselectivity for this reaction. However, we envisioned that through catalyst control this highly convergent strategy would become tenable.

Scheme 1
Retrosynthetic Analysis of Ushikulide A

Synthesis of the Spiroketal Fragment

Our studies began with the preparation of several aldehydes to fulfill the role of synthon 8 (16ad, Scheme 2). To accomplish this, a Crimmins14 aldol reaction between thiazolidinethione 14 and three known aldehydes (13a-c)15 was employed. Reaction of the (Z)-titanium enolate derived from 14 with aldehydes 13a-c resulted in essentially complete diastereoselectivity and quantitative yield for each case. Reduction of aldol adduct 15a to the β-hydroxy aldehyde was complicated by facile epimerization of the α-stereogenic center on silica gel (not shown). However, after protection of the secondary alcohol, mono-reduction proceeded uneventfully to yield the desired aldehydes (16a-d) in good yield over three steps.

Scheme 2
Preparation of Several Aldehyde Partners through a Crimmins Aldol Reaction

Preparation of an alkyne fragment to fulfill the role of synthon 11 began with a known Noyori reduction of methyl-3-oxopentanoate (Scheme 3).16 Formation of the p-methoxybenzyl ether and mono-reduction with diisobutylaluminum hydride gave aldehyde 17 in good yield. Addition of the lithium anion of TMS acetylene in the presence of a chelating Lewis acid (Et2AlCl) gave a mixture of propargyl alcohols 18a and 18b in low diastereoselectivity and yield (~2:1 at C27, Scheme 3). Separation of these epimers on a preparative scale was prohibitively difficult, particularly at this early stage in the synthesis. As an alternative, a number of crotylations were surveyed, starting with the conditions of Keck,17 which afforded a mixture of predominantly two diastereomers (2:1 - 19a : 19b) that were also inseparable by flash chromatography on silica gel. Varying the Lewis acid (SnCl4, CH2Cl2, Entry 2) led to only a negligible increase in diastereoselectivity. Fortunately, ample precedent suggested that this problem could be overcome by reagent-control.18 In our case, using either antipode of Brown’s19 (Z)-crotyl-B(Ipc)2 reagent led to excellent control of stereochemistry for the preparation of 19a or 19b in >20:1 d.r. by 1H NMR analysis for both cases. Preparation of alkyne 20 was completed in three additional steps (silylation, hydroboration-iodination and nucleophilic substitution). Because no chromatographic separation of diastereomers was required, this route proved to be highly scalable and more than 10 grams of 20 were ultimately prepared.

Scheme 3
Preparation of the Alkyne Fragment

The stereochemistry of 19a was also confirmed via formation of a cyclic para-mehoxybenzyl acetal (Equation 1). Exposure of 19a to DDQ in the presence of molecular sieves 20 furnished acetal 21, which could be cleanly separated from an initial mixture of diastereomers in 60% isolated yield. NOE irradiation of the benzylic acetal proton showed strong enhancements at C26 and C29, consistent with the indicated chair form and demonstrating the 1,3 diol to be of an anti stereochemical relationship.

equation image
(1)

With the two fragments in hand, our attention turned to nucleophilic addition of alkyne 20 to the aldehyde fragment 16c (Equation 2). The addition proceeded with low diastereoselectivity; however, separation of the C21 epimers was readily accomplished by silica gel chromatography. Desilylation with fluoride (HF•pyridine or n-Bu4NF) proceeded only very slowly at one of the two silyl groups, but exposure of either 22a or 22b to hydrochloric acid yielded either epimeric diol in good yield (23a or 23b).

equation image
(2)

The stereochemistry of each triol was assigned by the method of Rychnovsky (Equation 3).21 Protection of 1,3-diols 23a and 23b was accomplished with 2-methoxypropene and PPTS, yielding acetonides 24 and 25. gHSQC analysis of either acetonide confirmed the relative stereochemistry with syn acetonide 24 displaying large differences in the acetonide 13C NMR shifts, consistent with a chair conformation, while anti acetonide 25 displayed nearly equivalent 13C NMR shifts consistent with a half-chair conformation.

equation image
(3)

Attempts were also made to improve the diastereoselectivity of the alkyne addition reaction (Scheme 4). Moderate Felkin-Anh selectivity could be obtained in the presence of lithium bromide and molecular sieves (6:1 d.r., Entry 2). However, obtaining the chelation controlled product (22b) proved not to be feasible under the reaction conditions examined. Switching the C19 tert-butyldimethylsilyl group to a methoxymethyl ether (16d, Entry 4) gave some improvement of the diastereoselectivity; unfortunately the resulting epimers (27a and 27b) were very recalcitrant to chromatographic separation causing this route to be impractical.

Scheme 4
Attempted Diastereoselective Alkyne Addition

The low selectivity of the alkynylation led us to consider two possible alternatives. The first would be addition of the alkyne to a Weinreb amide, followed by diastereoselective Noyori reduction,22 while the second would be inversion of the syn 1,3-diol to converge both epimers to the anti stereochemistry. Because of the ease of chromatographic separation, we ultimately elected to pursue the latter course (Scheme 5). Alternatives to the 1,3-dithiane protecting group were also examined as the sulfur atoms it contained might cause catalyst poisoning during the key spiroketalization reaction. Three aldehydes (16a-c) were subjected to alkyne addition with the lithium acetylene derived from 20. After separation of the resulting epimers the anti product was esterified with retention (BzCl, pyridine), while the syn product was esterified under conditions leading to inversion (PPh3, DEAD, BzOH, toluene). All three pairs of epimers converged, yielding propargyl benzoates 28a-c. The TBDPS group (28a) proved too acid-labile and yielded predominantly triol 29a when subjected to the deprotection conditions developed previously (HCl, H2O, THF). Fortunately, both 28b and 28c could be cleanly desilylated yielding diols 29b and 29c respectively.

Scheme 5
Modification of the Spiroketalization Substrate

With the triol 23b in hand, we examined application of a metal catalyzed spiroketalization for the preparation of spiroketal 26. Coordination of a carbophilic Lewis acid (PdII) to the alkyne present in the starting material should lead to oxypalladation, generating an enol-ether and ultimately producing the desired spiroketal. Unfortunately, Utimoto’s conditions (Pd[CH3CN]2Cl2, THF, CH3CN, reflux)10 failed entirely for the preparation of 26 (Equation 4). When acetone was substituted as solvent with the same catalyst, a mixture of acetonide 25 along with an unidentified product was obtained. All other PdII catalyzed conditions that were examined afforded only decomposition after prolonged heating.

equation image
(4)

Further investigation of the spiroketalization reaction focused on triol 30b and propargyl benzoate 29b as precursors to spiroketal 31a (Scheme 6). Palladium catalysis again provided only limited reactivity (Entry 1), prompting us to examine platinum23 and gold. 24 While platinum also failed (Entry 2), gold(I)-chloride gave complete conversion at room temperature, but almost none of the desired spiroketal (31a) was observed. Instead elimination product 32 was isolated in 61% yield (Entry 3). Examination of other gold catalysts (such as AuClPPh3 with or without added silver salts) failed to resolve this difficulty. Under nearly identical conditions propargyl benzoate 29b gave a 17% yield of spiroketal 31b along with 47% of elimination product 32 (Entry 4). Examination of several Brønsted acid additives showed that the distribution of products could be biased toward the formation of 31b in the presence of PPTS (Entry 5). In the presence of stronger acid (CSA, Entry 6) the elimination problem was significantly exacerbated and the overall yield decreased significantly. Variation of the solvent caused significant perturbation to the ratio of products. Moving from MTBE to THF (Entry 7) significantly improved the yield of 31b, but continuing to an even more polar solvent, such as methanol increased the elimination problem relative to THF. Out of the conditions surveyed, the best were those outlined in Entry 7 and these were used for all further spiroketalization reactions.

Scheme 6
Optimization of the Spiroketalization Reaction

Aponick25 reported the preparation of unsaturated 1,7-dioxaspiro[5.5]undec-4-ene spiroketals via a gold catalyzed dehydrative cyclization of monopropargylic triols. Thus in retrospect, it is not surprising that triol 30b was so prone to elimination. However, the leaving group ability of benzoate appears to be greater than a hydroxyl group at least in a thermodynamic sense (pKa 15.7 vs. 4.3 in water).26 At first, we were somewhat puzzled by this counterintuitive observation. This issue is also convoluted by the fact that it is unclear whether the gold cation is lost via protonaton prior to or following the elimination. The mechanism below (Scheme 7) assumes loss of gold cation prior to formation of the spiroketal. While we cannot rule out that enol ether 33 or 34 could induce elimination directly after protonolysis of the gold cation, the heightened propensity of the hydroxy group to eliminate suggests that gold is involved in the elimination step prior to protonolysis. This premise is consistent with the fact that weak acid disfavors elimination, as acid should favor protonolysis of the gold cation before elimination can occur. The length the reaction time did not alter the ratio of 31b and 32, implying that 31b does not convert to 32 directly via reionization and solvolysis. The first report of a gold catalyzed spiroketalization24 presented several reactions in which 5-exo-dig cyclization dominated over a possible 6-endo-dig pathway. Therefore, it seems more likely that the C27 hydroxyl group attacks first via a 6-exo dig pathway to form intermediate 33 or 34, although we cannot rule out a 6-endo cyclization of the C19 hydroxyl group, as it would lead to the same product. Nucleophilic addition to C23 in preference to C22 is enforced by the benzoyloxy or hydroxy group at C21, which through its inductive electron withdrawing nature disfavors a cationic intermediate at C22.27 Calculations have indicated that this oxy-metalation consists of a syn addition across the alkyne, resulting from prior coordination of the hydroxy group to the gold Lewis acid (see intermediates 33 and 34).28 One possible rationalization for the benzoyl groups lessened propensity toward elimination would be formation of a 6-membered chelate (35), in which the σ* orbital of the C21-O bond is orthogonal to both the π-system and the Au-C σ bond, leading to a kinetically inert conformation. Intermediate 34 instead encourages elimination through coordination of the hydroxyl group to the gold center, resulting in formation allenyl enol ether 37, which could undergo reversible protonation to form oxonium 38, explaining the formation of the cis olefin geometry required for 32.

Scheme 7
Mechanism of the Spiroketalization Reaction

Because we had assumed that triol 30b would be a superior spiroketalization substrate, it had not been anticipated that the benzoyl group would be present in the product. This led to unexpected protecting group difficulties as the benzoyl group was not compatible with conditions known to selectively cleave benzyl groups in the presence of p-methoxybenzyl groups (lithium 4,4′-di-tertbutylbiphenyl, THF, −78 °C).29 Hydrogenolysis was unsatisfactory, and after surveying a fair number of conditions only a 40% yield of the desired alcohol (39) could be isolated (Equation 5).

equation image
(5)

As a result, the 1,3 dithiane group was reinvestigated and to our delight it proved to be an effective substrate for the spiroketalization (Scheme 8). In analogy to the spiroketalization with the benzyl ether, employing triol 30c in the spiroketalization led almost exclusively to elimination product 42. In contrast, benzoate 29c led at slightly elevated temperature and increased catalyst loading (50 °C, 10 mol% AuCl, THF) to spiroketal 41 in 63% isolated yield. Discrimination of the protecting groups was now trivial and methyl iodide in buffered aqueous acetonitrile gave aldehyde 45 directly.

Scheme 8
Completion of the Spiroketal Fragment

Our need to access mesylate 44 prompted us to examine the asymmetric alkynylation of isovaleraldehyde with methyl propiolate, a very challenging transformation30 that was unprecedented before our communication.7 Implementation of our zinc alkynylation method 31 led to the isolation of 43 in high yield and excellent enantioselectivity, the first such transformation reported for a simple aliphatic aldehyde. Under identical conditions, TMS acetylene fails entirely due to its increased basicity, which causes destructive polymerization of the aldehyde by a competing aldol pathway. Methyl propiolate serves as a very effective acetylene equivalent as saponification and decarboxylation were easily accomplished,32 yielding 44 after mesylation. Propargylation of 45 with 44 under the conditions of Marshall33 afforded a 3.7:1 mixture of separable propargyl epimers (C17) in a combined 90% yield. The stereochemistry of 46a is assigned by Marshall’s model in which propargyl mesylate 44 undergoes oxidative addition with palladium(0) with inversion, followed by transmetallation to diethyl zinc with retention. The resulting allenylzinc undergoes nucleophilic addition to aldehyde 45 via coordination of the aldehyde to zinc. Thus, stereochemistry at C16 is completely controlled, whereas C17 is formed as a mixture favoring 46a through a Zimmerman-Traxler transition state in which the aldehyde group occupies a pseudo-equatorial position. Both epimers were independently carried forward to the complete spiroketal fragment, as the C17 stereochemistry is ultimately irrelevant and will be removed by oxidation later in the synthesis. After some experimentation, it proved best to incorporate the sensitive vinyl iodide last, giving the completed spiroketal fragment (47) in 16 linear steps from methyl 3-oxopentanoate.

Spiroketal product 41 was examined extensively by various NMR experiments (gCOSY, gHSQC, gHMBC, ROESY, Figure 3). The expected NOE correlations (H19 to H27 and H19 to H21) and coupling constants (JC26-C27 = 2.5 Hz [equatorial-axial], JC21-C22 = 4.5 Hz [axial-equatorial] 12.0 Hz [axial-axial], JC19-C20 = 2.5 Hz [axial-equatorial]) were observed within the bicyclic framework of spiroketal 41 consistent with the indicated structure. The strong resemblance of these data to the NMR spectrum of authentic ushikulide A (Figure 2) was taken to be a very positive sign and improved the likelihood that our synthetic spiroketal had the same relative stereochemistry as the natural product.

Figure 3
NOE and Coupling Constant Analysis of the Spiroketal

Synthesis of the Aliphatic Fragment

A first generation approach toward an aliphatic fragment to fulfill the role of synthon 4 began from the known dibromide 48 (available in three steps from the (S)-Roche ester, Scheme 9).20,34 Exposing dibromide 48 to n-BuLi led to the expected Fritsch-Buttenberg-Wiechell rearrangement35 and the resulting lithium acetylide was quenched with N-methoxy-N-methylacetamide to yield ynone 49 in 76% yield. This ketone gave good reactivity in the direct zinc aldol reaction13 with aldehyde 5013d to yield aldol adduct 51 as a single diastereomer by 1H NMR analysis. Unfortunately, hydrosilylation36 of this product under the standard conditions (Cp*Ru(CH3CN)3PF6, BDMSH, acetone) caused deprotection of the diethoxyketal. Changing the solvent to dichloromethane failed to remedy the problem. Therefore, aldehyde 53 was prepared in enantiopure form via an allylation reported by Mukaiyama.37 This aldehyde also gave good yield and diastereoselectivity in the zinc aldol reaction to afford aldol adduct 54, again as a single diastereomer. The alkyne moiety of 54 was reduced in a syn fashion via a palladium-catalyzed reduction,38 yielding enone 55. Lindlar reduction was also attempted on ynone 54 or the corresponding syn 1,3-diol after reduction of the ketone function. Both substrates gave preferential reduction of the terminal double bond, demonstrating the superior selectivity of homogeneous palladium catalysis in this case. Enone 55 was then reduced to the syn diol and protected as a cyclic acetonide (syn stereochemistry of 56 and 64 was consistent with Rychnovsky’s 13C shift analysis).21 Epoxidation of 56 gave no diastereoselectivity and modest chemoselectivity for the more substituted olefin to yield epoxides 57 and 58 as a separable but unassigned mixture of diastereomers. Before any attempt was made to remedy this diastereoselectivity issue, the epoxide opening had to be demonstrated. Unfortunately, several attempts to accomplish this transformation failed to yield 59, instead cleaving the p-methoxybenzyl group. While this general strategy still might have provided the completed aliphatic fragment,39 a more direct strategy was already showing promise (vide infra).

Scheme 9
First Approach Toward the Aliphatic Fragment

One potential method to prepare the aliphatic fragment would be to couple a fully saturated aldehyde and ketone through the direct aldol methodology. Utilization of such an unstabilized ketone enolate with nearly 1:1 stoichiometry had not previously been demonstrated in this reaction (acetone and acetophenone are commonly employed as solvent or in excess [5-10 equivalents]). The requisite methyl ketone 61 was prepared easily via Wacker oxidation40 of the known41 homoallylic alcohol 60 (Scheme 10). By using a preformed lithium enolate of ketone 61 with aldehyde 53 the expected aldol product could be generated, but low stereoselectivity was observed as expected (~1:1 d.r., Entry 1). The standard zinc aldol conditions failed entirely to give the desired product and only the reduced primary alcohol (63) and recovered ketone (61) were observed (Entry 2). Presumably this reduction must proceed via a Meerwein/Ponndorf/Verley pathway. Employing t-BuOH instead of i-PrOH resulted in some yield of the desired product (62, Entry 3), while switching the solvent to 1,4-dioxane and increasing the catalyst loading gave an improved yield of aldol 62 in excellent stereoselectivity (Entry 4). Stoichiometric ProPhenol and diethylzinc failed to increase this yield and instead led to contamination of the product with various side products (Entry 5). Fortunately, the aldehyde and ketone could be recovered and resubjected to the reaction conditions, leading to an acceptable yield (65%) after three total runs. With ketone 62 in hand syn-reduction and acetonide formation gave 64, which was converted to aliphatic fragment 65 in three additional steps including a Horner/Wadsworth/Emmons olefination.

Scheme 10
Second Generation and Completion of the Aliphatic Fragment

Further attempts to optimize the aldol reaction were fruitless and short of modifying the ligand there seemed little hope for solving the problem of catalyst turnover that required several recycling iterations. To this end, we considered another aldol disconnection, wherein the C7-C8 bond instead of the C8-C9 would be formed in the aldol reaction. To attempt this, preparation of methyl ketone 67 was required rather than aldehyde 53. The ideal method to accomplish this would be a stereoselective allylation of 2,3-butanedione (Equation 6). This reaction was unknown in the literature and a number of attempts were made to accomplish it, which are outlined below. Unfortunately, the best enantioselectivity obtained was 45% ee, (Entry 5) too low to carry forward with the desired route.

equation image
(6)

Alternatively aldehyde 53 was transformed to silyl ether 68 in three steps. Addition of methyl magnesium bromide gave a mixture of secondary alcohols, which were oxidized without further purification. Finally, formation of the lithium enolate and silylation afforded the desired silyl enol ether.

equation image
(7)

Aldehyde partner 69 was prepared via oxidative cleavage of 60. Moderate yield and diastereoselectivity were obtained in the Mukaiyama, aldol reaction42 using a slight excess of 68 in toluene with boron trifluoride diethyl etherate to yield the Felkin-Anh aldol product (70).43 Reduction of this hydroxy ketone afforded the syn-diol, which was converted to an acetonide (64) identical to that prepared in our previous strategy. Alternatively, the PMB group was cyclized to the p-methoxyphenyl acetal (71). Upon irradiation of the benzylic proton strong NOE correlations were observed to H5 and H7. Coupling constant analysis (JH5-H6 = 2.0Hz [axial-equatorial] and JH6-H7 = 2.0Hz [axial- equatorial]) was also consistent with the chair form indicated, further supporting that the expected stereochemistry had been observed in both aldol reactions.

Completion of the Synthesis

Completion of the synthesis rested on two key bond-forming reactions – an sp3-sp2 Suzuki coupling44 and an esterification. While a great many syntheses have relied on macrolactonization, 45 only a few have utilized Suzuki coupling (particularly an sp3-sp2 coupling) for the formation of macrocycles.46 We therefore elected to attempt the Suzuki macrocyclization first in order to explore its potential in this arena (Scheme 12). Yamaguchi esterification47 allowed for the direct coupling of 47a and 65 to yield ester 72. Hydroboration of 72 (9-BBN, THF) gave the expected preference for reaction of the terminal olefin as observed by 1H NMR analysis. However, under a variety of conditions, which were later shown to be viable in an intermolecular reaction, macrocycle 73 could not be detected or isolated. The borate described above was injected via syringe pump to a solution containing palladium catalyst, base and solvent (palladium sources: Pd[PPh3]4, PdCl2•(dppf) and Pd2(dba)3•CHCl3; bases: Cs2CO3, TlOEt; solvents: dioxane, THF, DMF each with co-solvent of water). In many of these attempts the vinyl iodide was clearly consumed, unfortunately a number of products were formed, none of which corresponded to the macrocycle 73, which was successfully isolated later on.

Scheme 12
Completion of the Synthesis

The alternative pathway (Suzuki coupling then macrolactonization) was attempted. From the unsaturated acid (65) hydroboration was feasible with 2.5 equivalents of 9-BBN. Immediate gas evolution gave evidence that the carboxylic acid was reacting with one equivalent of 9-BBN, however, this fast reaction was followed by consumption of the terminal olefin. The crude borate (74) was added along with vinyl iodide 47a to a flask containing PdCl2•(dppf), triphenylarsine and cesium carbonate.48 At room temperature this gave seco-acid 75 directly.

Several conditions were applied to the seco-acid (75) to effect macrolactonization. Yonemitsu’s modification of the Yamaguchi esterification protocol8b failed to give any product (2,4,6-trichlorobenzoyl chloride, Et3N, 4-DMAP, benzene). Applying the Wasserman-Kita esterification to macrolaconization49 gave the first successful isolation of macrocycle 73 in ~30% yield, but attempts to improve the yield were unsuccessful (ethoxy acetylene, 2 mol% [RuCl2(p-cymene)]2, toluene, then 10 mol% CSA). The conditions of Shiina,50 gave the best result (65%) for preparation of macrolactone 73. Following cyclization, the secondary tert-butyldimethylsilyl ether was cleaved employing buffered HF•pyridine and the resulting alcohol was oxidized with Dess-Martin periodinane. The final deprotection required DDQ to oxidatively cleave the p-methoxybenzyl ethers and then acid to hydrolyze the cyclic acetonide. It proved to be essential to perform the oxidative deprotection first, as prior deprotection of the acetonide led to undesired cyclization during oxidative removal of the PMB group (PMP acetal or p-methoxybenzoate formation). When the PMB groups were removed first, the resulting triol could be cleanly isolated without interference from neighboring groups. In the final acid catalyzed deprotection the starting acetonide triol quickly interconverted to two other compounds of similar polarity as evidenced by TLC analysis (presumably these were migration of the acetonide to the C5 or C10 oxygens respectively), however, on prolonged stirring at room temperature (~4 hrs) these three compounds converged to a single spot of indistinguishable Rf when compared to an authentic sample of ushikulide A. Upon isolation, this compound exhibited identical 1H, 13C, IR, HRMS, and HPLC properties when compared to the same authentic sample. Measurement of the optical rotation {[α]D (24 °C): −12° (c 0.28, MeOH)} confirmed that the absolute stereochemistry was the same as that of natural ushikulide A {[α]D (24 °C): −13° (c 0.50, MeOH)}. In total, these data confirmed our tentative assignment and led us to assign the complete structure of (–)-ushikulide A as depicted in Scheme 12.

Summary

We have developed a flexible synthetic route, which establishes the full structure of (–)-ushikulide A for the first time. This route, which requires 21 linear and 40 total steps, provides the natural product in 2.2% yield over the longest linear sequence a significant accomplishment given the complexity of 1. The key gold-catalyzed spiroketalization to yield 41 is the first example of a metal-catalyzed spiroketalization in which regioselectivity is controlled by the electronic bias of the alkyne. This implementation extends the methodology to 1,7-dioxaspiro[5.5]undecane ring systems, a motif found in many natural products. Application of our zinc aldol methodology to aldehyde 53 and ketone 61 gave excellent control of diastereoselectivity, although catalyst turnover still needs to be improved. Control of diastereoselectivity in this manner allowed for an otherwise unavailable synthetic disconnection resulting in the preparation of aliphatic fragment 65 in only 11 linear steps. Finally, effective use of Suzuki coupling, Shiina macrolactonization and the Marshall/Tamaru propargylation all contributed to the overall success of this synthetic endeavor.

This synthesis confirmed the ‘educated guess’ that was made when we began our synthesis, however, it is likely that almost any diastereomer of ushikulide might have been prepared via this general strategy. Furthermore, a large number of unassigned natural products could be targeted by this method. In total, there are at least 12 natural products as close in similarity to ushikulide as cytovaricin,51 and many other unknown natural products could be approached in such a manner. Each specific case would lead to a slightly different implementation of this method. For example, access to greater amounts of material would allow for degradation studies, whereas a less conformational flexible target would allow for more computational analysis than was possible here. The success of this project is a testament to the power of organic synthesis in the structural elucidation of complex natural products. Further studies to apply and improve upon this strategy in the synthesis of other members of the oligomycin-rutamycin family are underway and will be reported in due course.

Experimental Section

(2S,3S,4S,6R,8S)-2-((1,3-dithian-2-yl)methyl)-8-((R)-2-(4-methoxybenzyloxy)butyl)-3,9-dimethyl-1,7-dioxaspiro[5.5]undecan-4-yl benzoate (41)

A solution of diol 29c (1.1492g, 1.82mmoles) in dry THF (0.05M, 36mL) is degassed (2x freeze/pump/thaw) and allowed to warm under argon. Gold (I) chloride (42.3mg, 0.18mmoles) and PPTS (45.7mg, 0.18mmoles) are added and degassing is repeated for 2 additional cycles. The reaction is then warmed to 50 °C for a period of 22 hours. After this time the solvent is removed under reduced pressure, a drop of triethyl amine is added, and the residue is purified by flash chromatography (9:1 hexane/THF) yielding unsaturated spiroketal 41 (325.6mg, 28%) followed by the desired spiroketal product 42 (729.6mg, 63%).

Data for 41

Rf = 0.23 (9:1 Hexane/THF); [α]D (23 °C): −5.2° (c=0.71, CHCl3); IR (neat): 2936, 1719 (sharp) cm−1;1H NMR (500 MHz, C6D6): δ 8.17-8.16 (m, 2H), 7.39 (d, J=8.5Hz, 2H), 7.16-7.07 (m, 3H), 6.85 (d, J=8.5Hz, 2H), 5.83 (dt, 1H, J=4.7, 12.1Hz), 4.56 (d, J=11.4Hz, 1H), 4.50 (d, J=11.4Hz, 1H), 4.34 (ddd, 1H, J=3.4, 3.6, 11.4Hz), 4.26 (dd, 1H, J=5.9, 8.5Hz), 4.19 (dt, J=2.4, 6.8Hz, 1H), 3.71-3.68 (m, 1H), 3.31 (s, 3H), 2.43-2.24 (m, 4H), 2.09 (t, J=7.0Hz, 1H), 1.92-1.84 (m, 3H), 1.73-1.58 (m, 4H), 1.50-1.20 (m, 8H), 1.14 (t, J=7.4Hz, 3H), 0.98 (d, J=6.9Hz, 3H), 0.87 (d, J=6.9Hz, 3H); 13C NMR (125 MHz, C6D6): δ 165.3, 159.4, 132.8, 132.1, 129.8, 129.1, 113.9, 97.7, 78.3, 71.0, 69.9, 67.4, 54.7, 44.4, 39.3, 37.5, 36.0, 35.8, 30.5, 30.3, 30.2, 29.9, 29.9, 27.4, 26.8, 25.9, 11.4, 9.6, 5.4 ppm; HRMS (ESI+) calc’d for C35H48NaO6S2 (M+Na)+ 651.2790, found 651.3694.

(R)-methyl 4-hydroxy-6-methylhept-2-ynoate (43)

A flask containing (S,S) ProPhenol (479.1mg, 0.75mmoles) dissolved in dry toluene (50mL) is sequentially treated with methyl propiolate (1.87mL, 21.0mmoles) followed by dimethyl zinc (1.2M in hexane, 17.5mL, 21.0mmoles). After 30 minutes isovaleraldehyde is injected (804μL, 7.5mmoles). The reaction is transferred to the cold room (ca 4 °C) and stirred for a period of 60 hours. Concentrated aqueous ammonium chloride (50mL) and diethyl ether (50mL) are added and the reaction is transferred to a separatory funnel. The organic phase is dried over magnesium sulfate, filtered and concentrated under reduced pressure (ca 100 torr). The resulting solution is purified by flash chromatography (4:1 PE/Et2O) and concentrated under reduced pressure (~100 torr) to yield propargyl alcohol 44a as a clear oil (1.1193g, 88%).

Data for 4352

Enantiomeric excess is determined to be 95% on Chiracel OD column; 98:2 heptane/isopropanol; Flowrate=0.8mL/min; Detector set to 220nm; Rt(major S): 21.6min, Rt(minor R): 23.67min; Rf=0.30 (2:1 Hexanes/Et2O); [α]D (23 °C): 13.8° (c=0.93, CHCl3); IR (neat): 3419, 2959, 2237, 1719, 1436, 1254, 1070 cm−1; 1H NMR (500 MHz, CDCl3): δ 4.52 (t, 1H, J=7.0Hz), 3.77 (s, 3H), 2.70 (s, broad, 1H), 1.89-1.81 (m, 1H), 1.70 (ddd, J=7.0, 7.5, 13.5Hz, 1H), 1.60 (ddd, J=7.0, 7.0, 14.0Hz, 1H), 0.94 (d, J=7.0Hz, 3H), 0.94 (d, J=7.0Hz, 3H) ppm; 13C NMR (125 MHz, CDCl3) δ 153.9, 88.8, 75.9, 60.5, 52.8, 45.6, 24.4, 22.4, 22.1 ppm.

(2S,3S,4R,6R,8S)-2-((2R,3S)-3-ethynyl-2-hydroxy-5-methylhexyl)-8-((R)-2-(4-methoxybenzyloxy)butyl)-3,9-dimethyl-1,7-dioxaspiro[5.5]undecan-4-yl benzoate (46)

Aldehyde 45 (132.9mg, 0.25mmoles) and propargyl mesylate 44 (71.5mg, 0.38mmoles) are dissolved in dry THF (1.7mL) and cooled to −78 °C. A solution of palladium acetate (5.6mg, 25.1μmoles) and triphenylphosphine (6.6mg, 25.1μmoles) in THF (1mL) is transferred into the reaction flask via syringe, followed by a solution of diethyl zinc in hexanes (0.75mL, 1M, 0.75mmoles). The reaction is allowed to warm to −20 °C for a period of 14hrs, after which time it is poured into a saturated solution of ammonium chloride. The organic phase is diluted with ethyl acetate, separated and dried over sodium sulfate. Following filtration and concentration the crude residue is purified by flash chromatography (19:1 toluene/EtOAc) yielding the major diastereomer 46a (113.2 mg, 71%) followed by its C-17 epimer 46b (30.0mg, 19%).

Data for 46a

Rf=0.20 (19:1 Toluene/EtOAc); [α]D (23 °C): −69° (c=0.93, CHCl3); IR (neat): 3505 (broad), 3305, 2955, 1718 (sharp), 1514 cm−1; 1H NMR (500 MHz, CDCl3): δ 8.04 (d, J=7.2Hz, 2H), 7.57 (t, J=7.4Hz, 1H), 7.45 (t, J=7.6Hz, 2H), 7.29 (d, J=8.5Hz, 2H), 6.85 (d, J=8.6Hz, 2H), 5.54 (td, J=12.0, 4.8Hz, 1H), 4.50 (d, J=11.4Hz, 1H), 4.45 (d, J=11.2Hz, 1H), 4.17 (m, 1H), 4.02-3.95 (m, 1H), 3.87 (s, 1H), 3.78 (s, 3H), 3.73 (dd, J=9.7, 3.0Hz, 1H), 3.64 (dt, J=10.0, 5.4Hz, 1H), 2.34-2.22 (m, 1H), 2.17-2.10 (m, 1H), 2.04 (d, J=2.3Hz, 1H), 1.99-1.90 (m, 3H), 1.84-1.73 (m, 2H), 1.73-1.41 (m, 9H), 1.38 (dd, J = 14.36, 1.78 Hz, 1H), 1.17-1.05 (m, 1H), 0.97 (d, J=6.9Hz, 3H), 0.96 (d, J=7.3Hz, 3H), 0.95-0.90 (m, 6H), 0.84 (d, J=6.6Hz, 3H) ppm; 13C NMR (125 MHz, CDCl3): δ 165.7, 158.8, 132.9, 131.3, 130.5, 129.5, 128.7, 128.4, 113.7, 98.2, 84.6, 76.9, 73.0, 71.6, 70.8, 70.6, 69.3, 68.2, 55.2, 39.0, 37.0, 36.6, 36.2, 35.7, 30.1, 29.1, 26.6, 26.2, 25.8, 23.6, 21.1, 11.2, 9.2, 5.6 ppm; HRMS (ESI+) calculated for C39H54O7Na (M+Na)+ 657.3766 found 657.3767.

(2S,3R,4S,7R,8R)-1-(tert-butyldimethylsilyloxy)-7-hydroxy-3,8-bis(4-methoxybenzyloxy)-2,4,8-trimethylundec-10-en-5-one (62)

A solution of (S,S) ProPhenol (590.2mg, 0.92mmoles) in dioxane (8mL) is treated with diethyl zinc (1M in hexanes, 1.84mL) at ambient temperature. A separate flask is charged with ketone 61 (913.6mg, 2.31mmoles), aldehyde 53 (759.5mg, 3.24mmoles) and activated 4Å powdered molecular sieves (500mg). After 30 minutes the catalyst solution is transferred into the reaction flask at ambient temperature. The reaction is stirred for 45 hours and quenched with 0.1N HCl (50mL) and diluted with ethyl acetate (100mL). The solution is filtered and the organic phase is washed with an addition portion of 0.1N HCl followed by sodium bicarbonate (25mL portions). The organic phase is dried over sodium sulfate, filtered and concentrated. Flash chromatography (85:15 Hexane/EtOAc) gives aldol adduct 62 (518.5mg, 35%) as a single diastereomer along with a 1.4:1 mixture of the aldehyde and ketone (1.026g, 60%). Repetition of the procedure above 2 additional times (identical relative stoichiometry) with recovered starting material gives a total of 65% yield based on recovered starting material.

Data for 62

Rf = 0.33 (4:1 Hexane/EtOAc); [α]D (23 °C): +28° (c=0.28, CH2Cl2); IR (neat): 3525 (broad), 2931, 1703 (sharp), 1514 cm−1; 1H NMR (500 MHz, CDCl3): δ 7.21 (d, J=9.3Hz, 4H), 6.84 (d, J=8.7Hz, 4H), 5.94-5.82 (m, 1H), 5.20-5.05 (m, 2H), 4.43-4.39 (m, 4H), 4.19-4.06 (m, 1H), 3.88-3.71 (m, 7H), 3.54-3.40 (m, 2H), 3.04-2.82 (m, 2H), 2.63-2.48 (m, 2H), 2.45-2.32 (m, 1H), 1.73-1.69 (m, 1H), 1.21 (s, 3H), 1.14 (d, J=6.7Hz, 3H), 0.85 (d, J=6.8Hz, 3H), 0.90 (s, 9H), 0.03 (s, 3H), 0.03 (s, 3H) ppm; 13C NMR (125 MHz, CDCl3): δ 213.9, 159.1, 133.7, 131.2, 130.8, 129.3, 128.8, 117.9, 113.8, 113.7, 79.7, 78.5, 73.9, 71.7, 65.5, 63.8, 55.3, 50.0, 43.5, 39.0, 38.8, 29.7, 25.9, 19.6, 18.2, 12.6, 11.5, −5.4, −5.5; HRMS (ESI+) calculated for C36H56O7NaSi (M+Na)+ 651.3693 found 651.3694.

Seco Acid (75)

Carboxylic acid 65 (64.8mg, 0.109mmoles) and 9-BBN (33.1mg, 0.271mmoles) are dissolved in THF (1mL) and stirred under a nitrogen atmosphere for a period of 14 hours. The solvent is removed under reduced pressure and water is injected (60μL). In a separate flask vinyl iodide 47 (109.5mg, 0.141mmoles) is dissolved in DMF (2mL) and the solvent is degassed (2 × freeze/pump/thaw). PdCl2(dppf) (7.9mg, 10.9μmoles), cesium carbonate (106.5mg, 0.327mmoles) and triphenyl arsine (3.3mg, 11.0μmoles) are added to the flask containing 74. The DMF solution of 47 is transferred into the reaction flask and the reaction is stirred at ambient temperature under argon for 21 hours. The reaction is then cooled in an ice bath, 30% aqueous peroxide (2mL) is injected dropwise. After warming to ambient temperature for 30 minutes, the reaction is poured into sodium thiosulfate (5mL) acidified with 1M sodium bisulfate (pH=2, ~15mL) and further diluted with water (30mL). The aqueous solution is washed with ethyl acetate (3×25mL) and the combined organic extracts are dried over sodium sulfate, filtered and concentrated. Flash chromatography (gradient 10% to 20% to 35% chloroform/EtOAc with 1% methanol) gives seco-acid 75 as a white foam (89.1mg, 67%).

Data for 75

Rf=0.35 (2:1 chloroform/EtOAc with 1% methanol); [α]D (23 °C): −16° (c=0.26, benzene); IR (neat): 3419 (broad), 2956, 1697 cm−1; 1H NMR (500 MHz, CDCl3): δ 7.37-7.18 (m, 6H), 7.10 (dd, J=15.7, 7.9Hz, 1H), 6.91-6.85 (m, 6H), 5.83 (d, J=15.8Hz, 1H), 5.40 (dd, J=14.3, 7.5Hz, 1H), 5.19 (dd, J=15.2, 8.9Hz, 1H), 4.63-4.34 (m, 7H), 4.25-4.13 (m, 1H), 3.88-3.76 (m, 12H), 3.71-3.63 (m, 1H), 3.51-3.38 (m, 2H), 2.82-2.68 (m, 1H), 2.27-2.15 (m, 1H), 2.12-2.00 (m, 2H), 1.95-1.81 (m, 2H), 1.82-1.44 (m, 17H), 1.39 (s, 1H), 1.36 (s, 3H), 1.20-1.30 (m, 2H), 1.14 (s, 3H), 1.12 (d, J=6.8Hz, 3H), 0.98 (d, J=6.9Hz, 3H), 0.98-0.93 (m, 9H), 0.88 (s, 9H), 0.82 (d, J=6.6Hz, 3H), 0.81-0.77 (m, 6H), 0.07 (s, 6H) ppm; 13C NMR (125 MHz, CDCl3): δ. 170.2, 159.1, 159.0, 158.7, 154.4, 132.4, 132.0, 131.1, 130.8, 129.2, 128.8, 128.3, 119.9, 113.7, 113.7, 113.7, 98.4, 97.5, 82.5, 77.9, 74.1, 73.7, 73.7, 73.5, 70.7, 69.6, 68.2, 67.3, 64.3, 55.2 (3C), 47.1, 41.1, 40.0, 39.4, 37.4, 36.4, 34.6, 30.3, 30.1, 29.8, 28.3, 27.8, 27.4, 27.0, 26.8, 26.5, 26.0, 25.6, 24.3, 22.6, 20.6, 19.3, 18.1, 17.5, 14.9, 13.6, 10.9, 9.8, 9.3, −4.1, −4.3 ppm; HRMS (ESI+) calculated for C73H114O14NaSi (M+Na)+ 1265.7876 found 1265.7885.

Macrocycle (73)

A solution of seco-acid 75 (84.3mg, 68.8μmoles) in dry DCE (5mL) is transferred to a gastight syringe and injected into a flask containing 2-nitrobenzoic anhydride (freshly recrystallized, 38.6mg, 0.105mmoles) and 4-DMAP (18.0mg, 0.126mmoles) in additional DCE (55mL) over a period of 18 hours. After 2 additional hours the reaction is poured into a separatory funnel containing saturated sodium bicarbonate (30mL) and ethyl acetate (90mL). The aqueous phase is separated and the organic phase is washed with ammonium chloride and sodium bicarbonate (30mL portions). The organic phase is then dried over sodium sulfate, filtered and concentrated. Flash chromatography (95:5 toluene/ethyl acetate) gives the macrocycle as a white foam (55.4mg, 65%).

Data for 73

Rf=0.54 (9:1 toluene/EtOAc); [α]D (23 °C): −20° (c=0.10, chloroform); IR (neat): 2926, 1718, 1514 cm−1; 1H NMR (500 MHz, CDCl3): δ 7.29-7.21 (m, 6H), 6.88-6.82 (m, 6H), 6.83 (dd, J=15.6, 7.9Hz, 1H), 5.83 (d, J=15.8Hz, 1H), 5.54-5.28 (m, 2H), 5.19 (dd, J=15.2, 9.3Hz, 1H), 4.58-4.35 (m, 6H), 4.00-3.95 (m, 1H), 3.85-3.83 (m, 1H), 3.82-3.75 (m, 10H), 3.67-3.61 (m, 1H), 3.48-3.34 (m, 2H), 3.18 (d, J=7.0Hz, 1H), 2.78-2.61 (m, 1H), 2.12-1.91 (m, 4H), 1.90-1.77 (m, 2H), 1.79-1.42 (m, 19H), 1.39 (s, 3H), 1.36 (s, 3H), 1.28-1.22 (m, 2H), 1.14 (s, 3H), 1.12 (d, J=6.8Hz, 3H), 0.98 (d, J=6.9Hz, 3H), 0.96-0.90 (m, 6H), 0.88 (s, 9H), 0.85-0.79 (m, 3H), 0.78-0.74 (m, 6H), 0.07 (s, 3H), 0.03 (s, 3H) ppm; 13C NMR (125 MHz, CDCl3): δ. 164.5, 159.1, 158.9, 158.7, 150.6, 132.5, 131.2, 130.9, 129.4, 129.2, 128.6, 128.2, 122.2, 113.7, 113.7, 113.6, 98.4, 97.4, 84.4, 78.2, 77.9, 74.2, 73.9, 73.4, 70.7, 69.7 (3C), 67.4, 63.7, 55.2 (3C), 47.1, 42.3, 42.1, 37.5, 36.0, 34.3, 33.7, 33.0, 30.2, 30.1, 29.9, 29.7, 26.9, 26.4, 26.0, 25.5, 24.3, 22.6, 21.3, 20.3, 19.5, 18.1, 17.7, 10.9, 9.1, 7.8, 4.7, −4.2, −4.4 ppm; HRMS (ESI+) calculated for C73H112O13NaSi (M+Na)+ 1247.7770 found 1247.7766.

(–)-Ushikulide A (1)

A plastic vial is charged with THF (10mL), pyridine (5.7mL) and HF·pyridine (Aldrich, 3.0mL). To a second plastic vial containing macrocycle 73 (10.6mg, 8.7μmoles) is added the stock solution prepared above (5mL). The reaction is stirred under nitrogen for 26 hours and diluted with ethyl acetate (100mL) and diluted with water (25mL). Finally, the biphasic mixture is neutralized with saturated sodium bicarbonate (~20mL). The organic phase is then washed with copper sulfate, water, and brine (30mL portions). The organic phase is then dried over sodium sulfate, filtered and concentrated. Flash chromagraphy (9:1 toluene/EtOAc) gives the alcohol as a white foam.

The alcohol prepared above is immediately dissolved in dichloromethane (1mL) and Dess-Martin reagent is added (15.0mg). The milky white solution is stirred for 4 hours and quenched with sodium thiosulfate (5mL) and diluted with diethyl ether (40mL). The organic phase is washed with sodium bicarbonate (2×15mL) dried over sodium sulfate, concentrated and used directly in the next step without further purification.

The crude ketone is dissolved in dichloromethane (3mL) and water (0.5mL). DDQ (10.2mg) is added at ambient temperature and the red solution is stirred vigorously for 45 minutes. Silica gel (300mg) is added along with additional dichloromethane (10mL). The solution is concentrated to dryness and the residual solid is loaded onto a short column of silica gel. The product elutes (single fraction collected, 9:1 DCM/EtOAc) separating easily from the red catechol byproduct. After concentration the crude triol is suspended in a 3:2 mixture of glacial acetic acid and water (3mL) and the reaction is stirred vigorously under nitrogen for 5 hours. The solvent is removed in vacuo, and the residue is dissolved three times in toluene and concentrated to dryness (for the first cycle a minimum amount of methanol is added to give a homogeneous solution). The resulting oil is purified by flash chromatography (10:1 chloroform/methanol) yielding 1 as a white powder (3.3mg, 52% over 3 steps).

Data for Synthetic (–)-Ushikulide A53

Rf=0.30 (9:1 chloroform/methanol); [α]D (23 °C): −12° (c=0.28, CH3OH); IR (neat): 3440 (broad), 1711 (sharp), 1463, 1276, 985 cm−1; 1H NMR (600 MHz, CD3OD): δ 6.76 (dd, J=15.7, 10.1Hz, 1H), 5.81 (d, J=15.7Hz, 1H), 5.60 (ddd, J=15.0, 9.4, 4.2Hz, 1H), 5.44 (dd, J=15.4, 8.8Hz, 1H), 5.30 (ddd, J=11.6, 5.0, 5.0Hz, 1H), 4.33 (t, broad, J=8.2Hz, 1H), 4.09 (m, 1H), 4.02 (m, 1H), 3.78 (m, 1H), 3.62 (J=dd, 9.5, 1.5Hz, 1H), 3.18 (J=dd, 11.0, 2.0Hz, 1H), 3.07 (ddd, J=15.1, 9.0, 8.0Hz, 1H), 2.78 (dd, J=17.6, 6.6Hz, 1H), 2.73 (J=dd, 17.4, 8.1Hz, 1H), 2.46 (m, 1H), 2.23 (m, 1H), 2.19 (m, 1H), 2.15-2.00 (m, 3H), 1.80-1.40 (m, 18H), 1.28 (m, 1H), 1.14 (d, J=6.6Hz, 3H), 1.06 (s, 3H), 0.98 (t, J=7.3Hz, 3H), 0.94 (d, J=7.1Hz, 3H), 0.91 (d, J=6.8Hz, 3H), 0.87 (d, J=7.1Hz, 3H), 0.85 (d, J=7.0Hz, 3H), 0.79 (d, J=7.0Hz, 3H) ppm; 13C NMR (125 MHz): δ. 212.5, 166.5, 152.1, 136.6, 129.5, 122.7, 98.8, 81.3, 75.9, 75.4, 74.9, 71.2, 70.5, 69.0, 67.1, 57.8, 42.9, 41.6, 41.9, 41.3, 39.7, 38.4, 36.3, 36.3, 34.6, 34.5, 33.0, 32.0, 30.6, 27.5, 26.6, 24.1, 23.2, 22.7, 22.3, 17.9, 11.4, 10.2, 6.1, 4.2 ppm; HRMS (ESI+) calculated for C40H68NaO10 (M+Na)+ 731.4710, found 731.4700.

Figure 1
Tentative Stereochemical Assignment of (–)-Ushikulide A
Scheme 11
An Alternative Aldol Disconnection in the Synthesis of the Aliphatic Fragment

Supplementary Material

1_si_001

2_si_002

Acknowledgement

We gratefully acknowledge K. Takahashi for providing an authentic sample of ushikulide A and D. A. Evans for providing spectra of cytovaricin. We also acknowledge S. Lynch for his invaluable help in the acquisition of 2-D NMR data. We thank the Stanford Graduate Fellowship Program and NIH (GM 13598) for their financial support of our program, Johnson Matthey for their supply of precious metal salts and Aldrich for ProPhenol.

Footnotes

Supporting Information Available: Detailed experimental procedures, full characterization of all products, and comparison NMR spectra. This material is available free of charge on the Internet at http://pubs.acs.org.

References and Footnotes

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(53) The. 1H NMR of synthetic ushikulide A was compared to an authentic sample generously provided by K. Takahashi. The authentic spectrum was obtained on the same instrument (600 Mhz) and at the same concentration (~2 mg/mL) as our synthetic sample. All protons are within 0.01 ppm, and the coupling constants for all peaks also match. The 13C spectra from the literature is in excellent agreement (within 0.1 ppm) with our synthetic sample. See Supporting Information for further details and spectral data. For published data see Reference 5