Enantioselective fermentative reduction

The most important contributions to the synthesis of the optical isomers of zeaxanthin have been achieved by the research group of F. Hoffmann-La Roche. From 6-oxoisophorone (56) as starting material, the chiral centre was introduced by an enantioselective fermentative reduction of the double bond with baker s yeast. Subsequent diastereoselective and regioselective reduction of the resulting diketone 57 gave a mixture of epimeric hydroxyketones, (3R)-58 and (3S)-58, which could readily be separated by Craig distribution (Scheme 14). 

Reduction of 1-nitro-1-alkenes with fermenting Baker s yeast proceeds enantioselectively to give optically active nitroalkanes (Eq. 3.53).81... 

Optically pure a, . (R)-3-hydroxybutanoates can be obtained by alcoholysis of poly-(R)-3-hydroxybutanoate, a fermentation product of fructose by Alcaligones eutrophus.4 (S)-Ethyl 3-hydroxybutanoate in 84-87% ee can be synthesized in 57-67% yield on a decagram-scale by an Organic Syntheses procedure6 using bakers yeast reduction of ethyl 3-oxobutanoate with the aid of sucrose.7 In order to obtain enantioselectivity as high as 95-97% ee, the substrate concentration should be kept below 1 g/L.6... 

Chiral alcohols are valuable products mainly as building blocks for pharmaceuticals or agro chemicals or as part of chiral catalysts. Cheap biotransformation methods for the selective reduction of particular ketone compounds are known for many years rather catalyzed by fermentation than with isolated enzymes. Products prepared with whole cells such as baker s yeast often lack high enantioselectivity and there were several attemps to use isolated enzymes. Resolution of racemates with hydrolases are known in some cases but very often the reduction of the prochiral ketone using alcohol dehydrogenases are much more attractive. 

In 1985, we reported that reduction of a prochiral 1,3-diketone A (Figure 3.6) with fermenting baker s yeast (Saccharomyces cerevisiae) was enantioselective to give (5)-hydroxy ketone B of 98-99% ee.26 I noticed that the Baeyer-Villiger oxidation of B would furnish (S)-hydroxylactone, a building block synthetically equivalent to the terminal epoxide moiety of (+)-JH III. This idea was used for the synthesis of (+)- and (-)-JH III in 1987.27... 

Figure 3.9 summarizes our synthesis of the enantiomers of JH I in 1988.28 If we want to employ the same strategy as used for the synthesis of JH III, the synthesis of JH I demands the execution of the diastereo-and enantioselective reduction of a prochiral 1,3-diketone A. Unfortunately, reduction of A with fermenting baker s yeast was nondiastereoselective, giving both B and C. After extensive screening of yeasts, Pichia terricola KI 0117 donated by Kirin Brewery Co. was found to achieve highly stereoselective reduction to give >99% of B with 99% ee. Since both of the 3,5-dinitrobenzoates D and E were crystalline, these could be purified by recrystallization, and 100% pure E was secured. Conversion of E to F was achieved by methanolysis followed by acetonide formation. The enantiomeric purity of F was proved to be ca. 100% ee by HPLC analysis of bis-MTPA ester G derived from F. 

In an alternate process for the preparation of the C-13 paclitaxel side chain, the enantioselective enzymatic hydrolysis of racemic acetate ci5 -3-(acetyloxy)-4-phenyl-2-azetidinone 38 (Eignre 16.10B), to the corresponding (S)-alcohol 39 and the nnreacted desired (l )-acetate 38 was demonstrated [63] nsing lipase PS-30 from Pseudomonas cepacia (Amano International Enzyme Company) and BMS lipase (extracellnlar lipase derived from the fermentation of Pseudomonas sp. SC 13856). Reaction yields of more than 48% (theoretical maximnm yield 50%) with EEs greater than 99.5% were obtained for the (R)-38. BMS lipase and lipase PS-30 were immobilized on Accnrel polypropylene (PP), and the immobilized lipases were reused (10 cycles) without loss of enzyme activity, productivity, or the EE of the product (R)-38. The enzymatic process was scaled up to 250 L (2.5 kg substrate input) using immobilized BMS lipase and lipase PS-30. Prom each reaction batch, R-acetate 38 was isolated in 45 mol% yield (theoretical maximum yield 50%) and 99.5% EE. The (R)-acetate was chemically converted to (R)-alcohol 39. The C-13 paclitaxel side-chain synthon (2R,3S-37 or R-39) produced by either the reductive or resolution process could be coupled to bacattin III 34 after protection and deprotection to prepare paclitaxel by a semisynthetic process [64]. 

The reduction of carbon-carbon double bonds to single bonds has been studied with various substrates. For example, Ohta et al. demonstrated that the reduction of a number of 1-nitro-l-alkenes by fermenting baker s yeast was enantioselective, resulting in the formation of optically active 1-nitroalkanes as shown in Fig. 10.13(a).On the other hand, Fuganti et al. reduced a,/3-unsaturated y-lactones to produce enantiomerically pure (-1-)-(J )-goniothalamin (Fig. 10.13(b)), which shows central nervous system activity. They also performed the kinetic resolution of the corresponding embryotoxic epoxide with yeast. 8... 

The ability of yeasts such as baker s yeast Saccharomyces cerevisiae) was utilized extensively by chemists to reduce carbonyl compounds to alcohols [70,71]. 2,2-Dimethylcy-clohexane-l,3-dione (56) can be reduced with fermenting baker s yeast to give the (5)-hydroxy ketone 5 (Fig. 27), which was employed extensively in terpene synthesis as shown in Fig. 5 [23]. Diastereo- and enantioselective reduction of 57 Wiih Pichia terricola KI 0117 yielded 6 (Fig. 27) [35], which was converted to both (+)-JH I [35] and (—)-JH I [72,73] (Fig. 6). Reduction of 57 with baker s yeast was nondiastereoselective [35]. 

Scheme 32 outlines a high yielding approach to the enantiopure appetite suppressant drug 77. The reduction of the unsaturated aldehyde 74 was reported to occur with modest enantioselectivity in normal fermenting conditions with baker s yeast [155]. When the biotransformation is performed at very low substrate concentration, the e.e. can be raised to more than 90%, suggesting that incomplete enantioselectivity is due to the action of enzymes operating on the same substrate with opposite stereochemical preference [115]. However, an efficient transformation can be performed if the substrate concentration is controlled with the addition of absorbing hydrophobic resins. At 5 g/L 97% recovery and 98% e.e. was obtained. The halohydrin 75 obtained was transformed into the epoxide 76 and finally into the enantiopure 2-/ -benzylmorpholine 77, the more active enantiomer with appetite suppressant activity [26]. 

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