PALLADIUM-CATALYZED CYCLIZATION OF O-ALKYNYLPHENOLS WITH ALLYL CARBONATES - A REGIOSELECTIVE SYNTHESIS OF 2-SUBSTITUTED-3-ALLYLBENZO[B]FURANS

2-Substituted-3-allylbenzo[b]furans 3 can be prepared from o-alkynylph enols 1 and allyl carbonates 2 through a palladium-catalyzed O-allylat ion/cyclization sequence. Two basic procedures have been developed: a stepwise method based on the isolation of O-allyl derivatives 4 and th eir subsequent cyclization to 3 (procedure A) and a one-pot reaction o mitting the isolation of 4 (procedure B). The cyclization of 4 in the presence of the electron-rich sterically-encumbered ligand tris(2,4,6- trimethoxyphenyl)phosphine (ttmpp) exhibits remarkable regioselectivit y in that 3-allyibenzofurans in which the benzofuryl unit is bound to the less substituted allyl terminus are formed almost exclusively. Som e loss of the stereochemistry of the carbon-carbon double bond is obse rved.