An Enantioselective Claisen Rearrangement Catalyzed by N-Heterocyclic Carbenes
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TLDR
Investigations demonstrate that the counterion of the azolium salt plays a key role in the formation of the catalytically active species in the synthesis of enantioenriched kojic acid derivatives.Abstract:
In the presence of a chiral azolium salt (10 mol %), enols and ynals undergo a highly enantioselective annulation reaction to form enantiomerically enriched dihydropyranones via an N-heterocyclic carbene catalyzed variant of the Claisen rearrangement. Unlike other azolium-catalyzed reactions, this process requires no added base to generate the putative NHC-catalyst, and our investigations demonstrate that the counterion of the azolium salt plays a key role in the formation of the catalytically active species. Detailed kinetic studies eliminate a potential 1,4-addition as the mechanistic pathway; the observed rate law and activation parameters are consistent with a Claisen rearrangement as the rate-limiting step. This catalytic system was applied to the synthesis of enantioenriched kojic acid derivatives, a reaction of demonstrated synthetic utility for which other methods for catalytic enantioselective Claisen rearrangements have not provided a satisfactory solution.read more
Citations
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Enantioselective Claisen Rearrangements with a Hydrogen-Bond Donor Catalyst
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Asymmetric Claisen rearrangement
Hisanaka Ito,Takeo Taguchi +1 more
TL;DR: In this review article, recent advances in the asymmetric Claisen rearrangement are described.