CYCLIZATION REACTIONS OF ALLYLIC O-STANNYL KETYLS

This is a summary of an investigation of the tributyltin hydride-induced cyclization reactions of unsaturated ketones with electronically deficient olefins. This reaction was initiated by an O-stannyl ketyl formed by the addition of a tributyltin radical to a carbonyl, which has both anionic and radical character. The intramolecular coupling produced functionalized cyclopentanes, bearing two synthetically useful carbon appendages. An activating or electron-withdrawing function on the alkene was essential to the cyclization. A dilution study revealed that excellent anti stereoselectivities (>50:1) could be achieved, and this was attributed to a reversible cyclization. Another goal of this study was to separate the radical reactivity from the anionic reactivity of the O-stannyl ketyl by the participation of labile functional groups and external electrophiles. The presence of minor products and enolate-trapping studies demonstrated that the anionic character of the ketyl could be utilized in the form of a tin enolate. This work represents the first free radical- and reagent-based approach to the study of the intramolecular hydrodimerization of activated alkenes.