INTRAMOLECULAR CYCLIZATION OF ALLYLIC PROPIOLATES MEDIATED BY THE ADDITION OF STANNYL RADICALS - A NEW SYNTHETIC ROUTE TO ALPHA-METHYLENE-GAMMA-BUTYROLACTONES - LEE, E; KO, SB; JUNG, KW; CHANG, MH (PERGAMON-ELSEVIER SCIENCE LTD, 1989)