Benzyl alcohols 4- pyridinium chlorochromate

The uses of the versatile oxidant pyridinium chlorochromate have been reviewed. Pyridinium chlorochromate has been used for the oxidation of methyl 5-hydroxypentanoate to the aldehyde, an intermediate useful in the synthesis of leukotrienes, and the same reagent oxidatively cleaves secondary vicinal diols to the corresponding aldehydes. The reactivity of the chlorochromate ion as an oxidant can be influenced by the counter-ion. For example, the 4-dimethylaminopyridinium salt is a mild, selective reagent for the oxidation of allylic and benzylic alcohols. Pyridinium fluorochromate oxidizes primary and secondary alcohols to aldehydes and ketones respectively in dichloromethane solution, and shows a less pronounced acidity compared with the chloro-... 

The C2-symmetric epoxide 23 (Scheme 7) reacts smoothly with carbon nucleophiles. For example, treatment of 23 with lithium dimethylcuprate proceeds with inversion of configuration, resulting in the formation of alcohol 28. An important consequence of the C2 symmetry of 23 is that the attack of the organometallic reagent upon either one of the two epoxide carbons produces the same product. After simultaneous hydrogenolysis of the two benzyl ethers in 28, protection of the 1,2-diol as an acetonide ring can be easily achieved by the use of 2,2-dimethoxypropane and camphor-sulfonic acid (CSA). It is necessary to briefly expose the crude product from the latter reaction to methanol and CSA so that the mixed acyclic ketal can be cleaved (see 29—>30). Oxidation of alcohol 30 with pyridinium chlorochromate (PCC) provides alde-... 

Imidazolium dichromate is a selective oxidant for allylic and benzylic hydroxy groups. (Allylic alcohols are oxidized faster than benzylic alcohols.) The selectivity over saturate alcohols is similar to that of 4-(dimethylamino)pyridinium chlorochromate. DMF is recommended as the solvent for oxidations, since it appears that the choice of solvent is critical to obtaining high yields. This reagent has also been observed to cause some ( )/(Z)-isomerization during the oxidation of allylic alcohols. 

Oxidations by pyridinium chlorochromate resemble those by dipyridine chromium(VI) oxide, both in scope and the mild conditions required. At room temperature, primary alcohols give aldehydes [604, 605], secondary alcohols afford ketones [605], allylic and benzylic methylene groups are oxidized to carbonyl groups [606, 6d7], enol ethers are converted into esters [608] or lactones [609], trimethylsilyl ethers of diphenols are transformed into quinones [610], and alkylboranes are converted into aldehydes (yll]. 

Reaction of estrone methyl ether with 2,2-dimethylpropane-l, 3-diol in the presence of a catalytic amount of acid leads to derivative 26-1, in which the ketone at 17 is protected as an acetal (Scheme 3.26). Treatment of this intermediate with pyridinium chlorochromate leads to oxidation of the Cg benzylic carbon atom to a carbonyl group (26-2). Potassium tert-butoxide abstracts a proton from the adjacent methylene at C7 alkylation of the resulting anion with 4-(A, A -dimethyl)butyl iodide gives 26-3 as a mixture of diastereomers. The carbonyl group is next reduced to an alcohol by means of sodium borohydride (26-4). Dehydration of the newly introduced hydroxyl group is arguably facilitated by the adjacent aromatic ring (26-5). Aqueous acid removes the 17-acetal to afford 26-6, which is in essence an equilinin derivative. 

The oxidations of benzyl alcohols and substituted mandelic acids by pyridinium chlorochromate have been described. This oxidant is a complex of Cr03,pyridine and hydrochloric acid, and is known to convert alcohols into carbonyl compounds in high yield. In the reaction with benzyl alcohol the reaction is... 

Amongst the new deactivated chromium(VI) reagents which have been described are 3- and 4-carboxypyridinium dichromate, derived from nicotinic and isonicotinlc acid respectively, which oxidize benzylic and allylic alcohols, and a pyridinium chlorochromate-... 

Allylic and benzylic primary and secondary alcohols are more easily oxidized, and a number of reagents selective for these are in use, including freshly precipitated manganese dioxide, silver carbonate, dichlorodicyanoquinone, and potassium ferrate. 4-(Dimethylamino) pyridinium chlorochromate is mild and selective as demonstrated in Equation 6.26 [44]. 

Suitable mechanisms have been proposed following determination of the kinetic and activation parameters for oxidation of 2-naphthol and cyclic ketones by nicotinium dichromate some a-amino acids by tripropylammonium fluorochromate " distyryl ketone by quinaldinium fluorochromate methanol by benzyltriethylammonium chlorochromate catalysed by 1,10-phenanthroline substituted benzyl alcohols by tetraethylammonium bromochromate L-cysteine by pyridinium bromochromate lactic acid and 3,5-dimethyl-2,6-diaryl piperidin-4-one oximes by pyridinium chlorochromate allyl alcohol by IDC benzophenoxime by bispyridine silver(I) dichromate and alkyl phenyl sulfides by cetyltrimethylammonium dichromate. A non-linear Hammett plot obtained for the oxidation of substituted benzyl alcohols by IDC has been attributed to the operation of substituent effect on two steps of the proposed mechanism. " Kinetic and activation parameters for oxidation of o-toluidine and of A-methyl-2,6-diphenyl piperidin-4-one oxime and its 3-alkyl derivatives by sodium dichromate have been determined and suitable mechanisms have been suggested. Micellar catalysis in the 1,10-phenanthroline-promoted chromic acid oxidation of propanol... 

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