Position-selective oxochromium (VI) oxidation of activated arylmethylenes

ORGN 132

Frederick A. Luzzio, faluzz01@athena.louisville.edu, Marek T. Wlodarczyk, mtwlod01@gwise.louisville.edu, and Joel P. Ott, jpott001@athena.louisville.edu. Department of Chemistry, University of Louisville, 2320 South Brook Street, Louisville, KY 40292
The 2,2'bipyridinium chlorochromate (BPCC)/peroxide reagent system is effective for both oxidative cleavage of benzylidene acetals to hydroxybenzoyl esters and selective oxidation of phthalimidines to phthalimides. More recently, Barrett's employment of the BPCC system in the total synthesis of the preussomerins and Fuchs' reports of oxochromium (VI)-catalyzed C-H oxidations has prompted our evaluation of various BPCC-based systems as chemoselective IBX types of oxidants for benzylic positions of activated aryls. The selectivity of such oxidants is in high demand when considering key oxidative steps in the recent syntheses of the cancer cell growth inhibitory epicatechins by Kozikowski and Tuckmantel in which a number of benzylic methylene oxidants were cited. We have constructed a number of substrates containing biaryl, biaryl/cycloalkyl and arylalkyl groups where at least one aryl contains activating substituents and have noted a distinct position selectivity when exposing these substrates to the BPCC/MCPBA reagent system. Using the BPCC/peroxide system, the substrates containing protecting groups were also tolerant in delivering the corresponding carbonyl compounds as a result of methylene oxidation. An array of substrates, their product yields, various reaction conditions and a mechanistic discussion will be presented.