Development of Highly Enantioselective Oxazaborolidinone Catalysts for the Reactions of Acyclic α,β-Unsaturated Ketones

Abstract

Chiral Lewis acid catalysis for ketones is one of the ­active areas in enantioselective reactions to expand the scope of substrates previously limited to aldehydes and bidentate carbonyl compounds. In this account, we describe the development of new oxazaborolidinone (OXB) catalysts derived from allo-threonine for asymmetric Mukaiyama-Michael and Diels-Alder reactions of acyclic α,β-unsaturated ketones. The scope of these reactions is outlined with particular emphasis on the origin of the enantioselective activation by the oxazaborolidinone catalysts.

1 Introduction

2 Asymmetric Mukaiyama-Michael Reactions

2.1 Design of the Oxazaborolidinone Catalysts

2.2 Evaluation of Intrinsic Enantioselectivity

2.3 Suppression of the Silyl Cation Catalyzed Racemic ­Pathway

2.4 Use of a Phenolic Additive

2.5 Use of an Ethereal Additive

2.6 O-Dimethylsilyl Ketene S,O-Acetal

2.7 Scope and Limitations

2.8 Activated Complex Model

2.9 Michael Reaction of Propionate-Derived Nucleophiles

3 Asymmetric Diels-Alder Reactions

3.1 Oxazaborolidinone Catalysis in Asymmetric Diels-Alder Reactions

3.2 Origin of s-cis-anti Conformation

3.3 Origin of High Activity

4 ¹H NMR Spectroscopic Study of Oxazaborolidinone-Amine Complexes

4.1 Top-Face Selectivity of Pyridines

4.2 Electrostatic Interaction for Top-Face Coordination

5 Summary

asymmetric catalysis - Lewis acids - Michael additions - Diels-Alder reactions

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