Biotransformations

This report reviews significant developments in applications of biological catalysis in synthetic organic chemistry for the year 2008. Amongst the highlights, a carboxylesterase with specificity for tertiary alcohols has been engineered for inverted enantioselectivity, and the halohydrin dehalogenase HheC has been applied to the formation of chiral oxazolidinones using cyanate as a nucleophile. There have been notable advances in the screening and application of ketoreductases (KREDs) for both chiral reductions and the quantitative deracemisation of secondary alcohols. A KRED has also been used in combination with palladium catalysis to perform a two step Suzuki-coupling/carbonyl reduction in one-pot in aqueous medium. Studies on an unusual Baeyer-Villiger monooxygenase from Xanthobacter have revealed its ability to epoxidise a non-activated alkene. There has been renewed interest in transaminases for the production of chiral amines from prochiral ketones, including the imaginative use of both pyruvate decarboxylase and α-amino acid dehydrogenase to solve equilibrium issues that militate against quantitative conversion. In the area of carbohydrate-transforming enzymes, a new glycosynthase has been developed for the irreversible formation of glycoside links to glucuronic acid, and a glycosyltransferase from Helicobacter pylori has been described that catalyses the formation of S-glycosides. Finally, in silico enzyme design has been coupled with directed evolution techniques to deliver an enzyme which catalyses a reaction not known in Nature, the Kemp elimination, with far-reaching implications for the possible design of enzymatic catalysts in the future.