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 diastereomeric 2-silyl-5,6-dihydro-6-aryl-(2<i>H</i>)-pyrans via [1,2]- and [1,4]-Wittig rearrangements to the corresponding α-silylcyclopentenols or (α-cyclopropyl)acylsilanes favor the [1,4]-pathway by <i>ortho</i> and <i>para</i> directing groups in the aromatic appendage and/or by sterically demanding silyl groups. The [1,2]-pathway is dominant with <i>meta</i> directing or electron-poor aromatic moieties. Exclusive [1,2]-Wittig rearrangements are observed when olefin substituents <i>proximal</i> to the silyl are present. <i>cis</i> and <i>trans</i> diastereomers exhibit different reactivities, but converge to a single [1,2]- or [1,4]-Wittig product with high diastereoselectivity and yield