Abstract
Reaction of bisacetoxy pyranone 9 with Et3N gave 3-oxidopyrylium ylide 10, which underwent a stereo- and regiospecific [5 + 2] cycloaddition with alpha-methylenebutyrolactone to afford 16 (34%). Treatment of 16 with Cs2CO3 resulted in hydrolysis of the lactone and acetate and conjugate addition of the hydroxyethyl group to the enone. Lactonization on acidification afforded 4 (57%), completing a two-step, formal synthesis of polygalolides A and B.