An Evans-Tishchenko-ring-closing metathesis approach to medium-ring lactones.

A new approach to the synthesis of medium-ring lactones is reported based on sequential Evans-Tishchenko and ring-closing metathesis (RCM) reactions. High diastereoselectivity (>95:5) is demonstrated in the Evans-Tishchenko reaction of unsaturated aldehydes with unsaturated beta-hydroxy ketones, and conditions for the RCM cyclization of the resultant dienes have been optimized to give high yields of medium ring lactones. The synthetic utility of this sequence is demonstrated through generation of the fully functionalized core of octalactin A. [reaction: see text].