α-fluoroamides in biotransformations

Classically hydrolytic enzymes, such as lipases, are considered to exhibit selectivity at a stereogenic centre, based on the steric nature of the adjacent substituent. In this study we report on the selectivity of hydrolytic enzymes (acylase's and protease's) on compounds containing both hydrogen and fluorine atoms at the stereogenic centre. A selectivity of 96% de is reported for the hydrolysis of an α-fluoroamide using Aspergillus melleus acylase. The K,„ and Ymax values are reported for each diastereomer, which demonstrate a ten-fold preference for the {S, S)-isomer over the (S-S)-isomer. A similar phenomenon is established for the analogous α-chloroamide and a comparison of reaction rates for non-halogenated amides is made. This effect has been rationalised by; a) the fluorine substituent controlling of the conformation of the amide bond and b) the stabilisation of the enzyme / amide intermediate. Furthermore a selection of racemic a-fluoroamides were prepared and their solid state conformations determined by X-ray crystallography. The C-F and the amide bonds are found to orientate in a very specific manner with respect to each other, consistent with a very specific stereoelectronic alignment. Attempts at preparing the elusive dipeptide with a fluorine atom alpha to the amide bond proved unsuccessful, however, one attempt led to the formation of a dimeric compound for which X-ray crystal analysis secured the structure. (^19)F variable temperature NMR was used to examine the barrier to rotation that existed for this dimeric compound. [brace not closed]