Stereoselective synthesis with and without organometallics

Abstract

The first part of this paper deals with stereochemical studies related to the Diels-Alder reactions of 2-azadienes. 2-Azadienes could be readily prepared from carboxylic acid chlorides and imine derivatives. Their reactions with electron-poor dienophiles showed an interesting chemical dichotomy : cyclic dienophiles mainly yielded endo-adducts while acyclic dienophiles reacted with a high exo-selectivity. This selectivity could be further increased by using Lewis acids which are compatible with the reactive azadienes. Highly selective asymmetric cycloadditions were observed with unsaturated amides derived from C2 symmetric pyrrolidines. Less satisfactory results were obtained with Evans' and Oppolzer's dienophiles. The second part deals with stereoselective cyclopentannulations of enones. It is shown that a three-carbon reagent bearing an orthoester at C1 and a sulphone at C2 reacts as a 1,3-dipole equivalent with enones to give a cyclopentannulation product. The reaction is general and of great synthetic value. Our efforts to develop an asymmetric version of this reaction have been successful : racemic sulphonamide-orthoesters have been shown to effect the cyclopentannulation of cyclic enones with high facial selectivity.