Selective ring-expansion of oxirane into tetrahydropyran tetrahydrofuran by reagent-controlled conditions

Ring-expansion of methyl 2-bromomethyl-5-oxiranylpent-2-enoate (7) with var ious acids was studied: treatment with Lewis acids (TiBr4, MgBr2 and ZnBr2) afforded the tetrahydrofuran derivative 4-Br as a major product, while rea ction with silver salt (AgNO3/KPF6), which is known to have a high halide a ffinity, gave preferentially the tetrahydropyran 3-ONO2 derivatives as the major products.