Enantioselective enzymatic deprotection

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]. 

Alternatively, enzymatic resolution of 61 by hydrolysis or of 62 by enzymatic esterification could be achieved with >99% ee and enantioselectivities of E>200, e.g. hydrolysis with common lipases like CAL-B or BCL (Amano PS) [86-88]. Wittig reaction and deprotection led to 64. Enzymatic resolution is also possible at the stage of C15-racemic 65 [86-88]. 

Electroenzymatic reactions are not only important in the development of ampero-metric biosensors. They can also be very valuable for organic synthesis. The enantio- and diasteroselectivity of the redox enzymes can be used effectively for the synthesis of enantiomerically pure compounds, as, for example, in the enantioselective reduction of prochiral carbonyl compounds, or in the enantio-selective, distereoselective, or enantiomer differentiating oxidation of chiral, achiral, or mes< -polyols. The introduction of hydroxy groups into aliphatic and aromatic compounds can be just as interesting. In addition, the regioselectivity of the oxidation of a certain hydroxy function in a polyol by an enzymatic oxidation can be extremely valuable, thus avoiding a sometimes complicated protection-deprotection strategy. 

Several other practical syntheses of enantiopure amino acid derivatives have been accomplished recently from substrate 35 (Chart 10.6). The Imperiali group has used two techniques following PTC alkylations that occurred with modest enantioselectivity (50-53% ee). The first involved fractional recrystallization followed by subsequent deprotection/reprotection to give 39 (>99% ee). In the second method, enzymatic hydrolysis of the amino acid methyl ester with alkaline protease and then nitrogen acylation gave 40 (99% ee) [16]. Several other publications that deal with related purification techniques have appeared [17-19]. 

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