Formation of β-lactams by the reaction of an acid chloride, a Schiff base and a tertiary amine seems to involve multiple pathways some of which are very fast at higher temperatures. When β-lactam formation is conducted in open vessels in unmodified domestic microwave ovens, high level irradiation leads to preferential formation of trans β-lactams in several cases when the Schiff base is derived from an aryl aldehyde rather than glyceraldehyde acetonide.