[4+2]Cycloaddition of Ketenes with N-Benzoyldiazenes Catalyzed by N-Heterocyclic Carbenes

摘要

The cycloaddition reactions of ketenes are useful approaches to heterocyclic compounds.Recently,N-heterocyclic carbenes(NHCs)were found to be efficient catalysts for the[2+2]ketene/imine,ketene/aldehyde,and[4+2]ketene/enone cycloaddition reactions.Herein we report an unprecedented enantioselective[4+2]cycloaddition reaction of ketenes with N-benzoyldiazenes catalyzed by chiral NHCs to give 1,3,4-oxadiazin-6-ones.In addition to its potential biological activity,this heterocycle is highly functionalized and is a useful intermediate in organic synthesis.For an example,it can be regarded as the masked a,a-disubstituted amino acid derivative.Interestingly,Berlin and Fu reported a[2+2]cycloaddition of ketenes with N,N'-dimethoxycarbo-nyldiazenes catalyzed by their planar-chiral 4-pyrrolidinopyr-idine derivatives[Eq.(1)].The different mode of reaction([4+2]versus[2+2]cycloaddition)prompted us to report our results herein.