Stereochemistry of thermolytic base-catalysed decarboxylation to form conjugated diene–acids: synthesis using ethylidenemalonic ester condensation

Abstract

The condensation of aromatic aldehydes with ethylidenemalonic ester in the presence of benzyltrimethylammonium hydroxide leads to 2(E),4(E)-half-esters, which are decarboxylated in refluxing pyridine to 2(E),4(E)-esters. When decarboxylated by thermolysis in quinoline at 130 °C cinnamylidenemalonic acid gives almost pure 5-phenylpenta-2(Z),4(E)-dienoic acid which slowly stereomutates, but on continued heating at 170 °C it passes over to give almost pure 2(E),4(E)-acid. In pyridine near its boiling point, however, the malonic acid is converted into a 64 : 36 mixture of 2(Z),4(E)-:2(E),4(E)-acids, the composition of which does not change on continued refluxing.

The use of carboxy-labelled dideuteriomalonic acid in the pyridine reaction leads to 5-phenylpenta-2(Z),4(E)-dienoic acid and its 2(E),4(E)-stereoisomer, each having similar ∼ 2 : 1 α/γ deuterium labelling. The latter stereoisomer does not arise by stereomutation, and a dual pathway originating from a common deuteriated lactone is proposed. Decarboxylation of the deuteriomalonic acid in quinoline at 130 °C, giving almost pure 2(Z),4(E)-dienoic acid with ∼ 2 : 1 α/γ labelling, involves only one of the pathways.

The ethylidenemalonic acid method is suitable for the preparation of 2(E),4(E)-half-esters and 5-phenylpenta-2(Z),4(E)-dienoic acids having both electron-withdrawing as well as electron-releasing aryl substituents. 2(Z),4(E)-Sorbic acid can also be made from the corresponding malonic acid by quinoline-catalysed decarboxylation, whereas the classical pyridine-catalysed Doebner reaction forms almost entirely 2(E),4(E)-sorbic acid.