Vinyl esters irreversible lipase-catalyzed transesterifications

Kinetic Resolution by Transesterification. Asymmetric transformation involving acylation of chiral alcohols is by far the most common example of kinetic resolution by lipase-catalyzed transesterification, most commonly with irreversible vinyl esters. This field is now becoming the most widely applied technique involving lipases. Recent reports of the numerous secondary alcohol substrates include various monocyclic (eq 6) andacyclic compounds, cyanohydrins, sulfones, and glycals, to name a few. 

Lozano et al. [64—67], and Reetz, Leitner and co-workers [68, 69], simultaneously reported SCCO2-IL biphasic, biocatalytic systems for lipase-catalyzed transesterifications using vinyl esters as the transesterification agent [Eq. (13)]. Vinyl butyrate was used because the product vinyl alcohol tautomerizes to acetaldehyde and hence the reaction is irreversible. 

Miyazawa, T. Kurita, S. Shimaoka, M. Ueji, S. Yamada, T. Resolution of racemic carboxylic acids via the lipase-catalyzed irreversible transesterification of vinyl esters. Chirality 1999, 11 554-560. 

Most of lipase-catalyzed acylations of sugars in organic solvents have been reported as transesterification rather than esterification reactions. The displacement of the equilibrium towards products has been accomplished by using activated acyl donors [58] such as 2,2,2-trichloroethyl esters and, more often, enol esters. The use of enol esters, such as a vinyl or an isopropenyl ester, was, in fact, first reported in lipase-catalyzed reactions with sugars [59]. In the reaction, an unstable enol is liberated which instantaneously tautomerizes to the corresponding aldehyde or ketone, making the reaction irreversible. 

B. Lipase-Catalyzed Irreversible Transesterifications by Means of Vinyl Esters and Similar Reagents... 

Regioselective acylations of polyhydroxylated compounds such as carbohydrates, glycerols, steroids, or alkaloids have been carried out with lipases, esterases, and proteases [13, 20]. One example is the Candida antartica lipase (immobilized on acrylic resin) catalyzed monoacylation of the signalling steroid ectysone (1) giving selectively the 2-C)-acetate 2 (eq. (1)). Using vinyl acetate for this transesterification the reaction was irreversibly pushed to the product side, since the liberated enol instantaneously isomerizes to acetaldehyde [21]. The sometimes unfavorable aldehyde is avoided when 1-ethoxyvinyl acetates [22], trichloro- or -fluoroethyl esters [23 a, b], oxime esters [23 c] or thioesters [23 d] are employed for the quasi-irreversible reaction courses. 

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