REDUCTIVE FREE-RADICAL ALKYLATIONS AND CYCLIZATIONS MEDIATED BY 1-ALKYLCYCLOHEXA-2,5-DIENE-1-CARBOXYLIC ACIDS

A range of 1-alkylcyclohexa-2,5-diene-1-carboxylic acids were prepared by Birch reduction-alkylation of benzoic acid and their efficiency as mediators of alkyl radical chain addition and cyclisation processes w as investigated. Reductive alkylations were respectably successful, ev en with only one or two equivalents of alkene, for secondary, tertiary and benzylic radicals. Reaction of [2-(cyclohex-2-enyloxy)ethyl]cyclo -hexa-2,9-diene- 1-carboxylic acid yielded the product of exo-trig-cyc lisation, i.e. 7-oxabicyclo[4.3.0]nonane, in a yield comparable to tha t obtained from the tributyltin hydride induced cyclisation of 3-(2'-i odoethoxy)-cyclohexene. This, together with the isolation of both exo- and endo-cyclisation products from 2-ylmethoxy)ethyl]cyclohexa-2,5-di ene-1-carboxylic acid established that ring closures could also be sat isfactorily mediated with these reagents. Preparations were completely free of metal contaminants and direct reduction of the alkyl radicals , prior to addition or cyclisation, was completely absent, However, th e desired products were accompanied by alkylbenzenes, together with by -products from the initiator decompositions, and this complicated work -up. Failure to obtain -yloxy)cyclohexyl]cyclohexa-2,5-diene-1-carboxy lic acid in Birch reductive alkylations with trans-1-iodo-2-(prop-2-yn -1-yloxy)cyclohexane (and the corresponding bromide) indicated a limit ation on precursor synthesis. The Birch reduction-alkylation was not o f universal applicability and was suppressed for alkyl halides having beta-substituents.