Stereochemical Course of Wittig Rearrangements of Dihydropyran Allyl Propargyl Ethers

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• 作者：Minoru Isobe ; Wei-Chung Chang ; Pei-Kang Tsou ; Chatchawan Ploysuk ; Chin-Hui Yu
• 刊名：Journal of Organic Chemistry
• 出版年：2015
• 出版时间：June 19, 2015
• 年：2015
• 卷：80
• 期：12
• 页码：6222-6237
• 全文大小：779K
• ISSN：1520-6904

文摘

[2,3]-Wittig rearrangements of sugar-derived dihydropyran allyl propargyl ethers located at the 2- or 4-position have been studied as useful means for extending the carbon chains of the 4- or 2-position with chirality transfer. The stereochemical course of these reactions depends on the following factors: (1) deprotonation of pro-R or pro-S-H, (2) equilibration of the lithiated stereogenic carbanion, (3) conformational inversion during the rearrangement, and (4) concerted [2,3]- or [1,2]-Wittig rearrangement. In some cases, a stepwise mechanism that involves the allyl-C鈥揙 bond cleavage is shared as the first step by both the [2,3]- and [1,2]-Wittig rearrangements. The stereochemical courses of the rearrangements are compared among the lithiated reactants to determine the reaction pathways. These mechanisms in the polyoxygenated dihydropyran ring system were further supported by DFT calculations.