Switching from S- to R-selectivity in the Candida antarctica lipase B-catalyzed ring-opening of omega-methylated lactones : tuning polymerizations by ring size

Novozym 435-catalyzed ring-opening of a range of w-methylated lactones demonstrates fascinating differences in rate of reaction and enantioselectivity. A switch from S- to R-selectivity was obsd. upon going from small (ring sizes ?7) to large lactones (ring sizes >=8). This was attributed to the transition from a cisoid to a transoid conformational preference of the ester bond on going from small to large lactones. The S-selectivity of the ring-opening of the small, cisoid lactones was low to moderate, while the R-selectivity of the ring-opening of the large transoid lactones was surprisingly high. The S-selectivity of the ring-opening of the small, cisoid lactones combined with the established R-selectivity of the transesterification of (aliph.) secondary alcs. prevented polymn. from taking place. Ring-opening of the large, transoid lactones was R-selective with high enantioselectivity. As a result, these lactones could be polymd., without exception, by straightforward kinetic resoln. polymn., yielding the enantiopure R-polyester with excellent enantiomeric excess (>99%)