While reaction of the bromoalkene with a 5-membered ketal 6a with tributyltin hydride gave only the acyclic product 18a, reaction of the corresponding bromoalkene with a 6-membered ketal 6b gave good yields of the cyclobutane 17b, in a novel ketal ring size effect. Also the gem-dicarboalkoxy effect was operative in these systems, eg, cyclization of the bromo alkene triester 11a afforded reasonable yields of the cyclobutane 19.
