α,β-Unsaturated Gold(I) Carbenes by Tandem Cyclization and 1,5-Alkoxy Migration of 1,6-Enynes: Mechanisms and Applications.

Pilar Calleja,María Moreno,Antonio M Echavarren,Óscar Pablo,Morgane Gaydou,Anthony Pitaval,Beatrice Ranieri,Mihai Raducan

doi:10.1002/chem.201602347

Abstract

1,6‐Enynes bearing OR groups at the propargyl position generate α,β‐unsaturated gold(I)‐carbenes/ gold(I) stabilized allyl cations that can be trapped by alkenes to form cyclopropanes or 1,3‐diketones to give products of α‐alkylation. The best migrating group is p‐nitrophenyl ether, which leads to the corresponding products without racemization. Thus, an improved formal synthesis of (+)‐schisanwilsonene A has been accomplished. The different competitive reaction pathways have been delineated computationally.

Highlights

The study of gold(I)-catalyzed reactions of 1,n-enynes has led to the discovery of a wealth of cyclization modes, including mechanistically intriguing skeletal rearrangements and nucleophilic addition reactions to cycloaddition processes.[1]
We recently found that 1,6-enynes such as 1 bearing propargyl alcohols, ethers, or silyl ethers react with gold(I) catalysts through the usual type of highly delocalized cyclopropyl gold(I) intermediates 2,[2] which undergo a new type of 1,5-migration of the OR groups to generate species 3[3] that we postulated as intermediate between a,b-unsaturated gold(I) carbenes and gold(I)-stabilized allyl cations.[2,4]
Intermediates 3 can react with electron-rich alkenes to form the corresponding cyclopropanes,[3] a reaction which is characteristic of gold(I) carbenes 2.[6,7,8] We demonstrated the potential of this tandem cyclization/1,5OR migration/cyclopropanation to form products 6 that were

Summary

Full Paper

& Au-Catalyzed Reactions a,b-Unsaturated Gold(I) Carbenes by Tandem Cyclization and 1,5Alkoxy Migration of 1,6-Enynes: Mechanisms and Applications. Pilar Calleja,[a] Óscar Pablo,[a] Beatrice Ranieri,[a] Morgane Gaydou,[a] Anthony Pitaval,[a] María Moreno,[a] Mihai Raducan,[a] and Antonio M. Abstract: 1,6-Enynes bearing OR groups at the propargyl position generate a,b-unsaturated gold(I)-carbenes/ gold(I) stabilized allyl cations that can be trapped by alkenes to form cyclopropanes or 1,3-diketones to give products of aalkylation. The best migrating group is p-nitrophenyl ether, which leads to the corresponding products without racemization. An improved formal synthesis of (+)-schisanwilsonene A has been accomplished. The different competitive reaction pathways have been delineated computationally

Introduction

Selection of the best OR migration group

Mechanistic discussion

Conclusions