Asymmetric Silylation: Extension to Kinetic Resolution

Significance

A catalytic protocol for the kinetic resolution of secondary syn-1,2-diols (rac-1a) as well as 1,2-diols bearing a primary alcohol (rac-1b) through highly site- and enantioselective silylation has been developed. Chiral imidazole derivative 2 can distinguish between two hydroxy sites of a substrate, either two secondary hydroxy groups or a primary and a secondary/tertiary alcohol. The conditions have been optimized towards maximal unreacted substrate (ent-1a, ent-1b) enantiomeric purity. In most cases, isomeric silyl ethers (ent-4a, ent-4b) are generated to low extent (in general <2%). Only with substrates rac-1a containing an ester moiety the site selectivity is decreased due to intramolecular hydrogen bonding as proposed by the authors (see transition state B).