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

The regiodivergent ring contraction
of diastereomeric 2-silyl-5,6-dihydro-6-aryl-(2H)-pyrans
via [1,2]- and [1,4]-Wittig rearrangements to
the corresponding α-silylcyclopentenols or (α-cyclopropyl)acylsilanes
favor the [1,4]-pathway 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.

非对映异构的2-硅基-5,6-二氢-6-芳基-(2H)-吡喃经[1,2]-与[1,4]-维蒂希重排发生区域发散性环收缩，生成对应的α-硅基环戊烯醇或(α-环丙基)酰硅烷。当芳基取代基带有邻位、对位定位基，或使用空间位阻较大的硅基时，反应更倾向于沿[1,4]-路径进行；当芳环为间位定位基或缺电子芳环基团时，反应则以[1,2]-路径为主。若硅基邻近存在烯烃取代基，则仅发生[1,2]-维蒂希重排。顺式与反式非对映异构体虽表现出不同的反应活性，但最终均能以高非对映选择性和优异收率，得到单一的[1,2]-或[1,4]-维蒂希重排产物。