Tandem radical cyclization-based strategy for the synthesis of oxa- and aza-cages: a case of fragmentation versus cyclization

Abstract

Tandem radical cyclizations involving two successive radical cyclizations result in the formation of the oxa- and aza-cage compounds. Cyclization with the ketones or olefins without stabilizing groups led to oxa-cages via a fragmentation reaction. A new strategy for the synthesis of oxa- and aza-cage compounds based on tandem radical cyclizations is described. The iodides 1 lead to oxa-cages 3 after two tandem radical cyclizations. The ester 10aa on reaction with n -Bu 3 SnH and AIBN gives rise to the oxa-cage 12aa after two tandem 5- exo - trig cyclizations. On the other hand, reaction of the ketones 17aa and 21 under similar conditions furnished the oxa-cages 20aa and 23 , respectively, via a double 5- exo - trig tandem radical cyclization followed by fragmentation.