Palladium-catalyzed synthesis of 2E-[(methoxycarbonyl)methylene] tetrahydrofurans:: oxidative cyclization-methoxycarbonylation of 4-yn-1-ols versus cycloisomerization-hydromethoxylation

4-Yn-1-ols bearing a terminal triple bond undergo oxidative cyclization-alkoxycarbonylation in methanol at 70 degrees C and 100 atm of a 9:1 mixture of carbon monoxide and air in the presence of catalytic amounts of [PdI4](2-) in conjunction with an excess of KI to give 2E-[(methoxycarbonyl)methylene]tetrahydrofurans in good yields. A competing reaction, cycloisomerization-hydromethoxylation leading to 2-methoxy-2-methyltetrahydrofurans, can be easily curtailed by increasing the KI excess. The latter products can be prepared from 4-yn-1-ols and methanol in high yields using the same catalytic system and without KI excess in the absence of carbon monoxide. (C) 2000 Elsevier Science S.A. All rights reserved.