Ammonium Ylide Mediated Cyclization Reactions.

Lukas Roiser,Mario Waser,Katharina Zielke

doi:10.1002/ajoc.201800091

Lukas Roiser, Mario Waser + Show 1 more

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https://doi.org/10.1002/ajoc.201800091

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Abstract

The use of readily accessible ammonium ylides for (asymmetric) transformations, especially cyclization reactions, has received considerable attention over the past two decades. A variety of highly enantioselective protocols to facilitate annulation reactions have recently been introduced as an alternative to other common methods including S-ylide-mediated strategies. It is the intention of this short review to provide an introduction to this field by highlighting the potential of ammonium ylides for (asymmetric) cyclization reactions as well as to present the limitations and challenges of these methods.

Highlights

Ylides are well known and have been systematically described for more than a century.[1,2,3] These unique molecules, which are characterized by a carbanionic site that is directly attached to a positively charged heteroatom,[4] have emerged as versatile and frequently used reagents for organic syntheses, especially since the seminal reports of Wittig and co-workers that describe the use of phosphonium ylides in olefination reactions.[2]
KGaA, Weinheim ports that describe the use of ammonium ylides suggest that cyclization reactions do not readily occur with these systems compared with those of S-ylides
Over the last two decades, a variety of different approaches clearly show these initial assumptions to be wrong, and the use of in situ generated ammonium ylides has emerged as a powerful complementary strategy for cyclization reactions

Summary

Introduction

Ylides are well known and have been systematically described for more than a century.[1,2,3] These unique molecules, which are characterized by a carbanionic site that is directly attached to a positively charged heteroatom (an onium group),[4] have emerged as versatile and frequently used reagents for organic syntheses, especially since the seminal reports of Wittig and co-workers that describe the use of phosphonium ylides in olefination reactions.[2]. The most commonly employed ylides are phosphonium ylides for olefination reactions[5] along with sulfonium ylides for cyclopropanations, epoxidations, and aziridinations.[6] Besides these, As-, Se-, and Te-ylides have been used to some, but a lesser, extent.[7] One family of compounds that has attracted considerable attention for (asymmetric) transformations over the course of the last two decades are N-ylides.[8,9,10,11,12] As a result, one can distinguish between N(sp2)-based ylides such as azomethine,[9] pyridinium,[10] and triazolium ylides[11] and N(sp3)based quaternary ammonium ylides.[8] The latter family are especially interesting for asymmetric transformations, as the use of simple achiral tertiary amines and readily available chiral tertiary amines (e.g., naturally occurring cinchona alkaloids) have recently resulted in several highly efficient protocols for diastereoselective and enantioselective transformations. Over the course of the last 20 years, N-ylides have become increasingly important, especially when it comes to enantioselective variations, where the chiral tertiary amines are more readily available than the chiral sulfides.[17]

Cyclopropanations

Epoxidations

Aziridinations

Conclusions

Findings

Conflict of interest