Alcohols, secondary, oxidation with manganese dioxide

Enhancing the utility of the allq lation-rearrangenient sequence in synthesis, the Evans group addressed the problems of a-versus-y alkylation as well as low anion reactivity by employing heterocyclic sulfides as the alleviation substrates tScheme IR.IfiE For instance, allylic imidazolyl sulfide 56 could be allqvlated efficiently, reaction at the a-position being favored by a chelated but reactive allyl lithium intermediate. Oxidation of 57 to the allylic sulfoxide and treatment with a secondary amine thiophile provided allylic alcohol 58 in high yield and with excellent stereoselectivity at the trisubstituted alkene. Allylic oxidation with manganese dioxide completed a synthesis of the sesquiterpene nuciferal (59). ... 

The selective oxidation of a secondary alcohol in the presence of a tertiary amine function has been carried out with manganese dioxide (equations 43 and 44) in the context of an alkaloid synthesis. ... 

Several references have appeared on the use of solid-phase oxidants. Solid potassium permanganate-copper sulphate mixtures oxidize secondary alcohols to ketones in high yield, and pyridinium chromate or chromic acid on silica gel are described as convenient off-the-shelf reagents for oxidation of both primary and secondary alcohols. Anhydrous chromium trioxide-celite effects similar transformations only when ether is present as co-solvent. An excellent review, with over 400 references, on supported oxidants covers the use of silver carbonate-celite, chromium trioxide-pyridine-celite, ozone-silica, chromyl chloride-silica, chromium trioxide-graphite, manganese dioxide-carbon, and potassium permanganate-molecular sieve. 

The readily available and stable compound barium manganate has been shown to be an efficient oxidizing agent for the oxidation of primary and secondary alcohols to carbonyl compounds. It has similar activity to manganese dioxide but is claimed to be better for the preparation of certain aldehydes e.g. the furan aldehyde (11) is produced in 80% yield whereas with Mn02 the yield is less than 20%. 

---