Stannykarbonylation of Hydroxy-substituted Alkynes. Intramoleuclar Trapping of α-Ketenyl Radicals by a Hydroxy Group Leading to Lactols

摘要

Radical carbonylation reactions are becoming a powerful tool for the introduction of carbon monoxide into organic molecules. We have previously reported that free-radical mediated stannylcarbonylation of alkynes provides a useful entry to α,β-unsaturated acyl radicals, which are likely to exist in an equilibrium with α-ketenyl radicals. Intra- and intermolecular trapping attempts of α,β-unsaturated acyl radicals/α-ketenyl radicals led us to postulate that ketenyl carbonyl has an electrophilic nature sufficient enough to allow for the attack of nitrogen nucleophiles, such as imines, oxazolines, and amines. While stannylcarbonylation of amino-substituted alkynes gave lactams, we were curious at knowing if the stannylcarbonylation of hydroxy-substituted alkynes would give the corresponding lactones or not, since nucleophilicity of alcohols is not so strong as that of amines. We report herein the results of the free-radical mediated stannylcarbonylation reaction of 5-hydroxyalkynes, which gave cyclized carbonylated products, lactols, as principal carbonylated products. Interestingly recent work of Nishii, Tanabe, and coworkers reported that lactols can be formed by radical carbonylation of cyclopropyl radicals, through the trapping of intermediary acyl radicals by an internal hydroxyl group.