A General Method for the Preparation of 4-Acyl-Cyclohex-3-Enones and Their Derivatives

Abstract

The metal-ammonia reduction of methoxylated aromatic compounds (the Birch reduction), followed by the acid catalysed hydrolysis of the resultant methoxycyclohexadienes, provides a convenient route to cyclohexenones but suffers from the limitation that oxygen atoms are hydrogenolysed completely from the benzylic position. This has been attributed¹ to the dianion produced by the addition of two electrons undergoing rapid cleavage to the benzylic anion and an oxide anion. It has been shown that the introduction of electron-releasing substituents such as methoxyl² or methyl³ into the 4-position of the ring gives some retention of oxygen in benzyl alcohol derivatives. However, an attempt⁴ to maintain the oxidation level of a carbonyl group by protecting 4-methoxyacetophenone as its dioxolane derivative (1) was unsuccessful since reduction gave (2) as the major product. This result has been confirmed but it would appear that the use of this particular protecting group was an unfortunate choice.