Single and double ring closing metathesis in the formation of dihydropyrans and bisoxacyclic systems with a quaternary centre

Abstract

Starting from α-hydroxy carboxylic acid esters, allyl homoallyl ethers with a quaternary centre become available in a one-pot reaction via a sequence of O-allylation–Wittig-rearrangement–O-allylation. Elaboration of the side chain provides various precursors for dihydropyrans and bisoxacyclic systems, which become available by single or double ring closing metathesis. Double ring closing metathesis of the substrates investigated in this study is highly regioselective to give dihydropyrans linked via a C–C-bond to a dihydrofuran or an oxepine. The regioisomeric fused products are not formed in the reaction.