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°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.