On the stereochemical outcome of the reaction between (–)-chorismic acid and diazomethane: absolute proof of stereochemistry of the major pyrazoline by X-ray crystallography of a cyclopropane based derivative

Abstract

Reinvestigation of the reaction between (–)-chorismic acid and diazomethane in diethyl ether on a larger scale has shown that the previously reported single pyrazoline based product is accompanied by a stereoisomer that result from the addition of diazomethane to the more hindered α-face of the chorismate 1,2-double bond (α:β addition ca. 1:6). Thermolysis of the two cycloadducts at 80 °C afforded a pair of cyclopropane derivatives with stereochemistry that could not be confidently assigned using data from coupling constants alone. NOE data allowed a more confident assignment of the stereochemistry of the two cyclopropanes; the β-cyclopropyl derivative was saponified and hydrolysed to yield a bicyclo[4.1.0]hept-2-ene-1-carboxylic acid derivative that was unequivocally shown to possess (1S,4R,5R,6R)-stereochemistry by an X-ray crystallographic study.