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Synthesis 2012; 44(15): 2295-2309
DOI: 10.1055/s-0031-1289788
review
© Georg Thieme Verlag Stuttgart · New York

NHCs in Asymmetric Organocatalysis: Recent Advances in Azolium Enolate Generation and Reactivity

James Douglas
a   EaStCHEM, School of Chemistry, University of St Andrews, North Haugh, St Andrews, KY16 9ST, UK
,
Gwydion Churchill
b   AstraZeneca, Process R&D, Macclesfield, Cheshire, SK10 2NA, UK, Fax: +44(1334)463806   Email: ads10@st-andrews.ac.uk
,
Andrew D. Smith*
a   EaStCHEM, School of Chemistry, University of St Andrews, North Haugh, St Andrews, KY16 9ST, UK
› Author Affiliations
Further Information

Publication History

Received: 17 April 2012

Accepted after revision: 03 May 2012

Publication Date:
13 July 2012 (online)

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Abstract

Organocatalysis represents a synthetic paradigm that has grown exponentially in popularity over the last decade, arguably becoming one of the most desired synthetic methods for the creation of enantiomerically enriched products. Within this field the use of N-heterocyclic carbenes (NHCs) has seen appreciable research activity with the realisation of many novel reaction types. This review provides a comprehensive account of recent advances in the generation and reactivity of azolium enolates with exclusive focus on those processes rendered asymmetric through the use of chiral NHCs.

1 Introduction: NHCs in Organocatalysis

1.1 Azolium Enolates

2 Azolium Enolate Generation via Ketenes

2.1 Asymmetric Formal [2+2] Cycloadditions

2.2 Asymmetric Formal [3+2] Cycloadditions

2.3 Asymmetric Formal [4+2] Cycloadditions

2.4 Asymmetric Protonation and Halogenation

3 Azolium Enolate Generation via α-Functionalised Aldehydes

4 Azolium Enolate Generation via Enals

5 Activated Esters as Azolium Enolate Precursors

6 Conclusions and Outlook

Key words

N-heterocyclic carbene - azolium enolate - organocatalysis, ketene - α-functionalised aldehydes - Enals - activated esters
 

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