A Novel 1,3-Stannyl Shift Promoted Intramolecular Cyclizations of α-Stannyl Radicals with a Formyl Group
摘要:
[GRAPHICS]Reactions of alpha-stannyl bromides and xanthates with tributyltin hydride generate a stannyl radicals, Intramolecular cyclizations of these radicals with a formyl group afford gamma-stannyl alkoxy radicals that undergo a 1,3-stannyl shift from carbon to oxygen. The carbon radicals obtained can be trapped inter- or intramolecularly. Approximately, the rates of 5-exo cyclizations of alpha-stannyl radicals with a formyl group and terminal olefin are similar.
and serves as a driving force to promote the cyclizations. Although the cyclization rates can be accelerated when the formyl group carries α-dimethyl substituents, unfortunately β-scission of the alkoxy radicals becomes competitive with the 1,3-stannyl shift. The β-stannyloxy radicals can be employed in further cyclizations to obtain tandem cyclization products.