Synthesis of δ-damascone [trans-1-(2,6,6-trimethylcyclohex-3-enyl)but-2-en-1-one] and β-damascenone [trans-1-(2,6,6-trimethylcyclohexa-1,3-dienyl)but-2-en-1-one]

Abstract

Diels–Alder addition of penta-1,3-diene to 3-bromo-4-methylpent-3-en-2-one catalysed by aluminium chloride gave stereoisomeric 4-acetyl-4-bromo-3,5,5-trimethylcyclohexenes (11) and (12), dehydrobromination of which led to 1-acetyl-2,6,6-trimethylcyclohexa-1,3-diene (8). Condensation of this dienone (8) with acetaldehyde and dehydration of the resulting aldol (19) produced β-damascenone (1). The same sequence of reactions on the adduct (5) of penta-1,3-diene and 4-methylpent-3-en-2-one led to δ-damascone (20).

Use of copper(I) bromide in dimethylformamide is suggested for cis-dehydrobromination of α-bromo-ketones.