Formation of Phenols from an Acyclic Precursor

A review has appeared on this subject (ref.56). Remarkably little work has been reported on the conversion of Cg acyclic intermediates and homologous substances to phenolic compounds althou in the extant investigatbns in the last decade considerable ingenuity has been shown as in the following two thermal methods. Flash vacuum pyrolysis of 1-[(E,E)-hexa-2,4-diene-1-oxoj- 

The eneynone, 3-ethyl-5-oxo-oct-3-ene-6-yne in toluene solution upon heating with collidine 4-toluenesulphonate at 250°C, formed 3,4-dimethyl- 5-ethylphenol in 82% yield (ref.58). Although strictly this synthesis is relevant to the preparation of alkylphenols, it is conceivable that the parent Cg enynone, or a derivative,could be used for phenol itself. 

Cycloheptanophenols have been formed from 1-formyl-2-(2-oxopropyl)-cyclohept-1-enes by the route shown. The 2-enol of 2-formylcycloheptanone as the trimethylsilyl ether reacted with an allylic Grignard reagent to afford, upon acidification, an intermediate which underwent Wacker oxidation to a ketoaldehyde. Cyclisation of this with methanolic potassium hydroxide gave the product (ref. 59). 

Cyclisation of a stirred mixture of 3-methyi-6-(6-methyi-5-hepten-2-yi)-2,4-hexadienoic acid in refluxing acetic anhydride containing fused sodium acetate during 30 hours led to 5-methyl-2-(6-methyl-5-hepten-2-yl)phenyl acetate in 70% yield (ref.60). 

A structurally related acid, 3-methoxycarbonyl-5-methylhexa-3,5-dienoic acid in dichloromethane solution with N-methylmorpholine and diphenylphosphinic chloride in the same solvent at -23°C was stirred for 0.5 hour and triethylamine added to the mixture which was then allowed to reach ambient temperature over approximately 1.5 hours. Methyl 3-hydroxy-5-methylbenzoate was obtained in 50% yield (ref.61) by a process having some similarity to Horner-Emmons methodology. 

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