Thermal electrocyclic ring-opening of cyclobutenes: stereoselective routes to functionalised conjugated ( Z,E)- and ( E,E)-2,4-dienals

Abstract

The cyclobutenecarbaldehyde 12 undergoes thermal electrocyclic ring-opening at low temperature, producing the (2 Z,4 E)-hexadienal 13 exclusively. By contrast, unsymmetrical derivatives of cis-3-cyclobutene-1,2-dicarboxylic acid undergo ring-opening at 80–110°C with low levels of stereoselectivity, which vary according to the balance of the electronic and, to a lesser extent, the steric nature of the substituents located on the rehybridising carbon atoms.