Prochiral stereoselective acylation

Enzymatic desymmetrization of prochiral or meso-alcohols to yield enantiopure building blocks is a powerful tool in the synthesis of natural products. For example, a synthesis ofconagenin, an immunomodulator isolated from a Streptomyces, involved two enzymatic desymmetrizations [149]. The syn-syn triad of the add moiety was prepared via a stereoselective acylation of a meso-diol, whereas the amine fragment was obtained by the PLE-catalyzed hydrolysis of a prochiral malonate (Figure 6.56). 

The stereoselective acylation of a prochiral chromanedimethanol derivative by VA in the presence of CAL-B gave the corresponding (S)-monoester in high enantiomeric... 

Esterases, proteases, and some lipases are used in stereoselective hydrolysis of esters bearing a chiral or a prochiral acyl moiety. The substrates are racemic esters and prochiral or meso-diesters. Pig liver esterase (PLE) is the most useful enzyme for this type of reaction, especially for the desymmetrization of prochiral or meso substrates. 

Immobilized PLE was applied to promote stereoselective acetylation of prochiral bis(hydroxymethyl)methyl-phenylgermane 106 (R = Me) with vinyl acetate as a solvent and acyl donor. Later on, the same group reported that each enantiomer of hydridogermane monoacetates 107 (R = H) was obtained either via acetylation of the bis-hydroxy derivative 106 (R = H) or hydrolysis of the corresponding diacetate 108 (R = H). In both methods, porcine pancreatic lipase was used and, obviously, each reaction led to a different enantiomer of 107 (Equation 51). ... 

This technology has been apphed as part of the total synthesis of myx-alamide A (Scheme 56) [139]. The stereoselective aldol reaction between aldehyde 218 and the propionate 219 dehvered, after reduction, protection, and acylation, ester 220 as a single isomer. After -silyl ketene acetal formation a [3,3]-sigmatropic rearrangement accompanied by 1,3-chirality transfer took place. This, together with the uniform prochirality at the double bonds of the... 

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