Stereocontrolled [4+2]-annulation accessing dihydropyrans: synthesis of the C1a-C10 fragment of kendomycin.

[reaction: see text] Development of new organosilane reagents bearing C-centered chirality where the stereocenter is fully substituted, and their use in the stereocontrolled synthesis of cis- and trans-dihydropyrans containing a trisubstituted olefin is described. The reagents participate in Lewis acid promoted [4+2]-annulations providing useful levels of selectivity with both aliphatic and aromatic aldehydes. A stereoselective synthesis of the C1a-C10 fragment of kendomycin (1) is also described.