Stereoconvergent [1,2]- and [1,4]-Wittig Rearrangements of 2‑Silyl-6- aryl-5,6-dihydropyrans: A Tale of Steric vs Electronic Regiocontrol of Divergent Pathways

ABSTRACT: The regiodivergent ring contraction of diaste-reomeric 2-silyl-5,6-dihydro-6-aryl-(2H)-pyrans via [1,2]- and [1,4]-Wittig rearrangements to the corresponding α-silylcyclo-pentenols or (α-cyclopropyl)acylsilanes favor the [1,4]-path-way by ortho and para directing groups in the aromatic appendage and/or by sterically demanding silyl groups. The [1,2]-pathway is dominant with meta directing or electron-poor aromatic moieties. Exclusive [1,2]-Wittig rearrangements are observed when olefin substituents proximal to the silyl are present. cis and trans diastereomers exhibit different reactivities, but converge to a single [1,2]- or [1,4]-Wittig product with high diastereoselectivity and yield. Since their discovery more than 70 years ago,1 Wittig rearrangements have evolved into powerful tools for the isomerization of α-metalated ethers into alkoxides. Wittig rearrangements can proceed through a concerted, orbital