Stereocontrolled synthesis of heteroannular acetals from functionalized 1,5-enynes via radical cascades - Construction of quaternary and 1,2-diquaternary centres in polycyclic systems

Sequential, radical-mediated cyclizations of 1,5-enynes have been evaluated with respect to structure, stereochemistry and reactivity. In series A 5-exo-trig,6-endo-dig cascades are favoured. The unusual 6-endo-dig cyclization is dictated by ring strain. Diastereomeric series B tolerates leakage into the 5-exo-trig, 5-exo-dig sequence, especially if a quaternary carbon centre is established during the first five-ring cyclization. The cascade allows construction of four and more chiral centres in a single convergent step. The influence of E/Z geometry of the olefinic component is also evaluated. (C) 2000 Published by Elsevier Science Ltd.