Switching from S- to R-selectivity in the Candida antarctica lipase B-catalyzed ring-opening of omega-methylated lactones: tuning polymerizations by ring size |
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Authors: | van Buijtenen Jeroen van As Bart A C Verbruggen Marloes Roumen Luc Vekemans Jef A J M Pieterse Koen Hilbers Peter A J Hulshof Lumbertus A Palmans Anja R A Meijer E W |
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Institution: | Laboratory of Macromolecular and Organic Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands. |
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Abstract: | Novozym 435-catalyzed ring-opening of a range of omega-methylated lactones demonstrates fascinating differences in rate of reaction and enantioselectivity. A switch from S- to R-selectivity was observed upon going from small (ring sizes or=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 (aliphatic) secondary alcohols prevented polymerization from taking place. Ring-opening of the large, transoid lactones was R-selective with high enantioselectivity. As a result, these lactones could be polymerized, without exception, by straightforward kinetic resolution polymerization, yielding the enantiopure R-polyester with excellent enantiomeric excess (>99%). |
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