Carbonylative radical cyclization approaches to tri- and tetra-quinanes: sequential formation of three, four and five carbon-carbon bonds

Tandem radical cyclizations and carbonylative tandem cyclizations of 5,5-disubstituted cyclopentadienes provide a variety of interesting products. Standard tandem cyclization of 9 provides an epimeric mixture of angular triquinanes, but carbonylative cyclization of the related precursors 7, 18 and 22 provides new products resulting from a ‘round trip’ radical rearrangement that occurs by a carbonylation, three successive cyclizations and a fragmentation. Double carbonylation of 25 occurs with triple cyclization and no fragmentation to produce tetracycle 26 as the principal product.