Pyridinium chlorochromate catalytic

The complex chromium salts [9-14] have a reactivity and selectivity comparable with that of pyridinium chlorochromate. Used at a catalytic level, the salts are more efficient than when stoichiometric amounts are used. The best results are obtained under soliddiquid conditions (10.3.5.F, when X = HS04 or F ) good conversions are also obtained with triflates and tosylates. Surprisingly, although relatively efficient under soliddiquid conditions, poorer conversions usually result from the use of the chloride and bromide salts under liquiddiquid two-phase conditions. 

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. 

S)-Citramalic acid (1029) is readily reduced to triol 1030 with either diborane [222] or borane methylsulfide-trimethylborate [223]. Conversion of 1030 to acetonide 1031 can be accomplished either with acetone in the presence of a catalytic quantity of perchloric acid (73% yield from 1029) [222] or with acetone [224] or 2,2-dimethoxypropane [223] in the presence of copper sulfate (48% yield from 1029). Oxidation of the alcohol with pyridinium chlorochromate furnishes aldehyde 1032 in 72% yield [223]. 

Reductive Silylation ofp-Qninones. Me6Si2 in combination with a catalytic amount of iodine converts /7-quinones (30) into l,4-bis(trimethylsiloxy)arenes (31) in almost quantitative yields (eq 15). The products are a protected form of quinones, which are regenerated by oxidation with pyridinium chlorochromate (PCC)P... 

Piancatelli G, Scettri A, D Auria M (1977) The oxidation of furan derivatives with pyridinium chlorochromate a novel synthesis of 6-hydroxy-27f-pyran-3(67f)-ones. Tetrahedron Lett 18 (25) 2199-2200. doi 10.1016/S0040-4039(01)83720-5 Pleshakov VG, Abedin MZ, Sergeeeva ND, Prostakov NS (1983) Formation of 9-methylisoindolo [l,2-c]quinoxaline by catalytic dehydrocyclization of 2-(2,4-dimethylphenyl)quinoxaline. J Org Chem, USSR (Engl Transl) 19(3) 586-587... 

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