The scope and limitations of 1,3-stannyl shift-promoted intramolecular cyclizations of alpha-stannyl radicals with a formyl group

Alpha-tributylstannyl radicals can be generated from the corresponding bromides or xanthates. These radicals undergo efficient intramolecular 1,5-cyclizations with a formyl group. The resulting beta-stannyl alkoxy radicals proceed through a 1,3-stannyl shift from carbon to oxygen to afford beta-stannyloxy radicals. This novel rearrangement is most likely irreversible and serves as a driving force to promote the cyclizations. Although the cyclization rates can be accelerated when the formyl group carries alpha-dimethyl substituents, unfortunately beta-scission of the alkoxy radicals becomes competitive with the 1,3-stannyl shift. The beta-stannyloxy radicals can be employed in further cyclizations to obtain tandem cyclization products.