Epothilone intermediates

The synthesis of an epothilone model system via an alternative C9-C10 disconnection was first examined by Danishefsky in 1997. However, extension of this C9-C10 strategy to a fully functionalized epothilone intermediate was not successful, demonstrating the limitations of RCM with the early catalysts A and B [116]. In 2002, Sinha and Sun disclosed the stereoselective total syntheses of epoA (238a) and epoB (238b) by the RCM of epoxy compounds 242 in the presence of catalyst C (Scheme 50) [117]. The reaction furnished an inconsequential mixture of isomers 243 (E/Z 1 1) in high yield. Subsequent selective hydrogenation of the newly formed double bond followed by deprotection led to epothilones A and B. 

Despite comprehensive studies, extension of this C9,C10 strategy to the preparation of a fully functionalized epothilone intermediate proved unattainable, demonstrating limitations of the RCM process [14]. 

Table 1. Selected epothilone intermediates and analogs synthesized via ringclosing metathesis... 

Figure 5. Proposed scheme for the biosynthesis of epothilone intermediates from modules 3-5 in E. coli. The segment containing modules 3. 4, 5 and the TE domain were comtructed in an expression vector and expressed as described in the text.

Scheme 10.8 Biosynthesis of epothilone. Individual PKS domains are represented as circles and individual NRPS domains as hexagons. Acyl carrier proteins (ACPs) and thiola-tion domains (T) are posttranslationally modified by a phos-phopantetheinyl group to which the biosynthetic intermediates are covalently bound throughout the chain assembly. The thioesterase domain (TE) cyclizes the fully assembled carbon chain to give the 16-membered lactone. Following dehydration of Cl 2—Cl 3 to give epothilones C and D, the final step in epothilone biosynthesis is the epoxidation of the C12=C13 double bond by the cytochrome P450 enzyme P450epol<. KS ketosyn-thase KS(Y) active-site tyrosine mutant of KS AT acyltransfer-ase C condensation domain A adenylation domain ...

The reduction of the isoxazoline ring after the cycloaddition was not successful with the usual reagents (see p. 532), but Sml2 accomplished the reaction. In contrast to the epoxidation used as the final step in most of the other epothilone A syntheses, the epoxide was introduced through a sulfite intermediate. Deprotection of C(15) leads to intramolecular displacement at the sulfite with formation of the epoxide (Steps E-3 and E-4). 

Epothilones are a class of molecules that show anticancer activity. Production of a synthetic intermediate was investigated through the action of an esterase on various sterically hindered 3-hydroxy esters [76]. No initial activity was observed, so a Pseudomonasfluorescens esterase was transformed into a mutator strain Epicurian coli and screened using an indicator in the growth plates that would produce a red color if hydrolysis occurred. An ee of 25% was achieved from a variant containing two mutations. 

After extensive developmental studies, [35] the final crucial element in our most recent synthesis of epothilone B involves an asymmetric catalytic reduction of the C3 ketone of 67 proceeding via a modified Noyori procedure (Scheme 2.8, 67—>68). In the event, Noyori reduction of ketone 67 afforded the desired diol 68 with excellent diasteresdectivity (>95 5). The ability to successftdly control the desired C3 stereochemistry of the late stage intermediate 68 permitted us to introduce the Cl-C7 fragment into the synthesis as an achiral building block. 

He also performed the KR of aldol intermediate 19 in the Sinha-Lemer synthesis of epothilone A on a 1.2-g scale, thus affording the natural dextrorotatory enantiomer in 47% yield and 98% ee s = 107) (Scheme 8) [82]. 

Scheme 8 Preparation of an epothilone A intermediate using Fu s planar chiral 4-DMAP ent-16 [82]...

In NRPS, the cyclization domain catalyzes cyclization of the side-chain nucleophile from a dipeptide moiety such as AA-Ser or AA-Cys (AA = amino acids) to form a tetrahedral intermediate, followed by dehydration to form oxazolines and thiazolines (Scheme 7.1) [20]. The synthesis of a 2-methyl oxazoline from threonine follows a similar mechanism. Once a heterocycle is formed, it can be further modified by reductase to form tetrahydro thiazolidine in the case of pyochelin biosynthesis. Conversely, oxidation of the dehydroheterocycles lead to heteroaro-mahc thiazoles or oxazoles as in the case of epothilone D (Figure 7.2) [21]. 

In comparison to other aldolases, DERA has a rather broad substrate range. DERA-catalyzed aldol reactions were used to get an access to key intermediates for epothilones (Fig. 36) [194]. According to retrosynthetic analysis, both fragments of the molecule could be obtained from aldol building blocks, and two out of seven stereocenters were established enzymatically. For the southern part of epothilone A,... 

The natural products epothilone A and B are structurally different from taxol but have similar anticancer activity. Significantly, they have been reported to be much more active against cell lines exhibiting multiple-drug resistance [26], Taylor and co-workers at the University of Notre Dame have recently published an elegant, formal total synthesis of epothilone A [27], In this work, the authors used the CLC form of Burkholderia cepacia (formerly Pseudomonas cepacia) lipase (ChiroCLEC -PC) to resolve a key alcohol intermediate by selective acylation with vinyl acetate in /-butyl methyl ether (Fig. 6). The enantioselectivity was >20 1 at 47% conversion and efficiently provided gram quantities of the desired (R) alcohol. Since the unreacted (S) alcohol can easily be epimerized by a simple oxidation-reduction sequence and the catalyst reused without significant loss in activity, the method is ideally suited for scale-up. 

An example that illustrates the potential of this catalytic C-C bond-forming process to build up key structural subunits of natural products is shown in Scheme 2. The reaction of acetophenone with aldehyde 18 in the presence of 8 mol% catalyst 1 affords the aldol adduct 19 in 70% yield and 93% ee, which is subsequently transformed into 20, a key intermediate of the anticancer agent epothilone A [8b]. Similarly, Scheme 3, the aldol reaction of hydroxyacetylfuran 21 with valeralde-hyde in the presence of 5 mol% catalyst 3 produces syn diol 23 with high efficiency [10d]. Further chemical elaboration of 23 leads to 24, a key intermediate in the synthesis of (+)-boronolide, a folk medicine with emetic and anti-malaria activity. 

Scheme 2. Synthesis ofthe key intermediate en route to epothilone A based on a catalytic, asymmetric direct aldol addition of acetophenone (5.7 mmol scale) and an a-branched acyclic aldehyde, (a) 1 (8mol%), KHMDS...

Ruthenium-catalyzed olefin cross-metathesis (ring-closing metathesis, RGM) between terminal alkenes and vinyl-boronic acid or esters has recently been developed for the synthesis of ( )-l-alkenylboron compounds from alkenes.459,460 The efficiency of protocol was proved in the synthesis of a key intermediate of epothilone 490 292 461 (Equation (84)). The vinyl boronate was given almost exclusively the trans-adduct. 

In the stereoselective synthesis of epothilone A (11), Carreira used a syn -reduction methodology in the synthesis of the key intermediate (14)9 (Scheme 4.If). Reduction of the isoxazoline 12 with samarium iodide at 0 C in THF gave the ketone 13. Narasaka reduction of the P-hydroxy ketone 13 using triethylb-orane/sodium borohydride afforded the. syn-diol 14 in high yield and with high diastereoselectivity. 

A particularly successful synthesis of Epothilone A is based on two DERA-cata-lyzed steps. In these two of the seven stereocentres of Epothilone A were established. When a racemic aldehyde was released in situ from its acetal, DERA converted only the R-enantiomer into the stable cyclic hemiacetal. This is a combined kinetic resolution and carbon-carbon bond formation yielding a building block with two chiral centers. Since the alcohol function was oxidized, the optical information obtained from the kinetic resolution was lost. Thus, for the overall yield it would have been better if DERA had displayed no stereoselectivity towards the acceptor (Scheme 5.32). In the DERA-catalyzed synthesis of another part of Epothilone A DERA is again highly stereoselective. Fortunately its preference is for the S-enan-tiomer of the acceptor aldehyde, the enantiomer that has to be submitted to the carbon-carbon bond formation in order to obtain the desired building block, again a stable hemiacetal (Scheme 5.32). Indeed, both DERA-catalyzed reactions yield open chain products that form stable cyclic hemiacetals. This ensures that the equilibria of these aldol reactions are shifted towards the desired products. Further synthetic manipulations converted these intermediates into Epothilone A [55]. 

Using precursor-directed biosynthesis, Boddy et a/. were able to effectively reconstitute epothilone biosynthesis in E. coli. A pentaketide NAC thioester intermediate... 

Several of the aldol products obtained were readily converted to their corresponding esters by Baeyer-Villiger oxidation. These results also are summarized in Table 16. Ester 66 was further transformed into key epothilone A intermediate 69 and also a key synthetic intermediate 70 for bryostatin 7. What is the mechanism of these direct catalytic asymmetric aldol reactions using LLB-II It is apparent that self-assembly of LLB and KOH occms, because of the formation of a variety of aldol products in high ee and yields. In addition, the NMR and LDI-TOF(-i-)MS spectra of LLB KOH show the occurrence of rapid exchange between Li and K. We have already found that LPB[LaK3tris(binaphthoxide)] itself is not a useful catalyst for aldol reactions, and that the complexes LPB KOH or LPB LiOH give rise to much less satisfactory results. 

While there are many natural products that are either NRPS or PKS derived, there are multidomain megasynthases that contain both NRPS and PKS biosynthetic features. Examples of such biosynthetic motifs are epothilone, jamaicamide, and the endiyne C-1027.33-35 At NRPS-PKS interfaces in these systems, the condensation of malonyl takes place with an amino acid that was activated by an A domain. Alternatively, an amino acid loaded on a T domain condenses with an elongating polyketide. Since the substrates and intermediates in NRPS, PKS, or hybrid NRPS/PKS biosynthesis introduce mass changes on T domains, they are ideal candidates for investigation by MS. 

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