Structures of New [4+2]-Cycloadducts from Diels–Alder Reactions of Methyl 2-Oxo-2 H -pyran-5-carboxylate with 2-Methyl- and 2-Methoxy-1,3-butadienes

Abstract

A Diels–Alder reaction of methyl 2-oxo-2H-pyran-5-carboxylate (1) with isoprene gave two stereoisomers, methyl 8-methyl-8-vinyl- and 8-isopropenyl-3-oxo-2-oxabicyclo[2.2.2]oct-5-ene-6-carboxylates. On the other hand, only a stereoisomer from a reaction of 1 with 2-methoxy-1,3-butadiene, methyl 8-methoxy-8-vinyl-3-oxo-2-oxabicyclo[2.2.2]oct-5-ene-6-carboxylate was obtained. The stereostructures of these adducts were determined by NMR spectral data and single-crystal X-ray analyses.