Catalytic enantioselective silylation of acyclic and cyclic triols: application to total syntheses of cleroindicins D, F, and C.

Abstract

Catalysts that promote site-selective modification of poly-functional molecules can be of significant utility in selective chemical synthesis.[1] Of particular importance are chiral catalysts that initiate enantioselective functionalization of polyoxygenated molecules[2]—entities commonly found among biologically active agents. Herein, we present methods for the enantioselective silylations[3] of acyclic and cyclic 1,2,3-triols [Eq. (1)]; the transformations are promoted by a readily available small-molecule catalyst and afford silyl ethers that bear a neighboring diol moiety in up to > 99: 98% ee). Enantiomerically enriched silyl-protected triols obtained by the protocols described in this report cannot be selectively prepared by catalytic or stoichiometric dihydroxylations[4] (including the directed variants).[5] The utility of the new catalytic processes is demonstrated in the context of enantioselective total syntheses of cleroindicins D, F, and C, natural products isolated from Clerodendum indicum, a plant used in China to battle malaria and rheumatism.[6, 7]