Straightforward Synthesis of Nonconjugated Cyclohex-3-enone and Conjugated 4-Methylenecyclohex-2-enone Derivatives

The synthesis of nonconjugated cyclohex-3-enones via the regiodivergent cobalt-catalysed Diels-Alder reactions of 2-(trimethylsilyloxy) buta-1,3-diene with alkynes and hydrolysis of the dihydroaromatic intermediates is described. The application of bidentate phosphine ligands versus pyridine-imine ligands led to the regioselective formation of one out of the two possible regioisomeric products when terminal or unsymmetrically substituted alkynes were applied. The cyclohex-3-enone products were isolated mostly in good yields without isomerisation of the carbon-carbon double bond into conjugation based on the mild reaction conditions. The use of acetoxymethyl-substituted alkynes led to the conjugated 4-methylenecyclohex-2-enones after deprotection of the silyl enol ether.