Ring Expansion of Cyclic α-Ethynyl Sulfonium Ylides by [2, 3]-Sigmatropic Rearrangement : Formation of Thiocin, Thionin, and Thiecin Derivatives

Abstract

The cyclic 1-ethoxycarbonylmethyl-2-ethynyl sulfonium salts (15Aa-d, 15Ba-d, and 15Ca-d), prepared from thiolane (12A), thiane (12B), and thiepane (12C), were treated with 1, 5-diazabicyclo[5.4.0]undec-5-ene (DBU) to result in ring expansion, giving thiocins (18Aa-d), thionins (18Ba-d), and thiecins (18Ca-d), respectively, presumably via the allenic intermediates (17) derived from the initially formed sulfonium ylides (16) by [2, 3]-sigmatropic rearrangement. Similarly, the 1- (27a-c) and 3-benzothionins (30a-c) were obtained from 2-ethynylthiochromans (21) and 1-ethynylisothiochromans (22), respectively, via the sulfonium ylides (25, 28) and the allenic compounds (26, 29) successively.