REDUCTIVE CYCLIZATION OF CARBON-CENTERED GLYCINE RADICALS - A NOVEL SYNTHETIC ROUTE TO CYCLIC ALPHA-AMINO-ACIDS

Reductive cyclizations (tributyltin hydride, AIBN) of several alpha-(phenylthio)glycine derivatives with a 3-alkenyl substituent at nitrogen are reported. These reactions proceed via 2-aza-5-alken-1-yl radicals as intermediates which bear electron-withdrawing carbonyl substituents at the radical center and at nitrogen. Such radicals can be considered as relatively stable captodative radicals, but are reactive enough for olefin cyclization. The main products usually arise from 5-exo cyclization and are structurally interesting analogues of proline. Varying amounts of pipecolic acid analogues via 6-endo cyclization are also obtained in some cases. A similar cyclization of an acetylene is successful, whereas a nitrile fails to cyclize.