Dealkylation and Dealkoxylation

A number of examples of dealkylations by chemical and catalytic methods are given in Table 4.7 (refs.89-93). A very general method for the cleavage of phenolic ethers consists in their reaction under reflux during 36 hours with an excess of 37% hydrobromic acid and a small amount of hexadecylammonium bromide. Phenyl n-pentyl ether gave phenol in 71% yield (ref.94). 

Selective cleavage, so often a prerequisite in synthesis, of an aryl benzyl ether was effected in 80% yield with the dibenzyl ether of estrane-3,17-diol in chloroform containing N,N-dimethylaniline by the addition of powdered aluminium chloride, stirring during 30 mins, at ambient temperature, and work-up following quenching with M hydrochloric acid (ref.95). 

The heterocycle 3-methyl-1,2-benzisoxazole, whilst structurally distinct from the phenolic aryl ethers underwent cleavage at the 0-N bond by slow addition in tetrahydrofuran to 1M lithium diethylamide at -75 C followed by isolation of crude 1,6-dihydropyridazine and its oxidation with manganese dioxide over 1 hour to give 3,5-bis(2-hydroxyphenyl)pyridazine in 61% yield (ref.96). 

Anisole added to 1,2-dichloroethane complex containing boron tribromide dimethyl sulphide complex (Nj) and the mixture refluxed (12 h)to give phenol. 

To N-methylaniline added first dropwise at 65° to stirred sodium hydride in xylene containing hexamethylphosphoramide (1 equiv.) after 1/4 h, 1,2,4-trimethoxybenzene in xylene and mixture at 85 ° (6 h) to afford Me 

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