Synthesis of 2,3,4,6,7,8‐Hexahydro‐5 H ‐1‐benzopyran‐5‐ones and 3‐(6‐Oxo‐1‐cyclohexenyl)alkanoic Acids by Reduction of 4,6,7,8‐Tetrahydro‐2 H ‐1‐benzopyran‐2,5(3 H )‐diones

Comparative results on the reduction of 4,6,7,8‐tetrahydro‐7,7‐dimethyl‐2 H ‐1‐benzopyran‐2,5(3 H )‐diones 1 are reported. Hydride reduction (LiAlH 4 in Et 2 O or NaBH 4 in i‐PrOH) affords 2,3,4,6,7,8‐hexahydro‐5 H ‐1‐benzopyran‐5‐ones 5 in 30‐60% isolated yield. Photochemical reduction of 1b and 1d (direct irradiation at λ = 300 or 254 nm in i‐PrOH, or sensitized irradiation in acetone/i‐PrOH or benzene/i‐PrOH) gives 3‐(6‐oxo‐1‐cyclohexenyl)alkanoic acids 6 in 50–80%, while 1c affords the isomeric 3‐(4,4‐dimethyl‐6‐oxo‐1‐cyclohexenyl)‐4‐methyl‐4‐pentenoic acid ( 9 ) in 73% isolated yield. Electrochemical reduction (Hg, CH 3 CN, Bu 4 N + ClO   4 − , −2.6 V vs. Ag/Ag + ) requires more than 4 Farad/mol for the consumption of 1 without any major product being detected.