The key THF derivative (9a) for an enantioselective synthesis of amphidinolideX/Y was obtained from 1a via a selenoetherification reaction. In fact, among the cyclization methods investigated, the highest yield and stereocontrol were achieved at -78 degrees C with PhSeCl/EtiPr2N from diols 1a (anti-Z) and 1b (anti-E) and with PhSeCl/ZnBr2 from diols 1c (syn-Z) and 1d (syn-E). Also, surprisingly, use