Stereoselectivity of Cydoaddition of<i>N</i>-(Cyanomethyl)- and<i>N</i>-(α-Cyanobenzyl)imines with Olefinic Dipolarophiles. Synthetic Equivalents of Nitrile Ylide 1,3-Dipoles
N-(Cyanomethyl)- and N-(α-cyanobenzyl)imines derived from a variety of aldehydes and ketones can tautomerize into N-protonated azomethine ylides which undergo cycloadditions with olefinic dipolarophiles. These cycloadditions are often accompanied by the elimination of HCN, mostly in a stereospecific manner, showing these imines to be synthetic equivalents of nonstabilized nitrile ylides. Stereoselectivity
The Synthesis of Pyrroles and Dihydropyrroles by 1,3-Dipolar Cyclisations of<i>N</i>-Arylmethylene[(benzotriazol-1-yl)arylmethyl]amines
作者:Alan R. Katritzky、Gregory J. Hitchings、Xiaohong Zhao
DOI:10.1055/s-1991-26595
日期:——
N-Arylmethylene[(benzotriazol-1-yl)arylmethyl]amines 1 are prepared by the reaction of arylaldehydes, benzotriazole, and ammonia in dry ethanol. Deprotonation of these imines with butyllithium yields N-lithiated azomethine ylides 2 which readily undergo 1,3-dipolar cycloaddition with a large range of dipolarophiles. The cyclisations are regiospecific and generally exhibit high stereoselectivity. Benzotriazolate is eliminated during the course of the reaction, and the method thereby provides a route to 2,5-diaryl substituted pyrroles, 3,4-dihydro-2H-pyrroles and c-ring fused 3,4-dihydro-2H-pyrroles.
Metallic Base-Induced Cycloadditions of<i>N</i>-(1-Cyanoalkyl)imines via<i>N</i>-Metalated Azomethine Ylides: Enhanced Reactivity and High Regio- and Stereoselectivity
Lithiation of N-(1-cyanoalkyl)imines with LDA generates new N-lithiated azomethine ylide 1,3-dipoles which show enhanced reactivity toward dipolarophiles. They undergo exclusively regio- and stereoselective 3+2 cycloaddition reaction with α,β-unsaturated esters to give 1-pyrrolines after the elimination of LiCN. Metallic bases other than LDA can be also effective. Such high regio- and stereoselectivity is explained by the involvement of N-metalated azomethine ylides.