Pyridinium dichromate catalytic

The avermectins also possess a number of aUyflc positions that are susceptible to oxidative modification. In particular the 8a-methylene group, which is both aUyflc and alpha to an ether oxygen, is susceptible to radical oxidation. The primary product is the 8a-hydroperoxide, which has been isolated occasionally as an impurity of an avermectin B reaction (such as the catalytic hydrogenation of avermectin B with Wilkinson s rhodium chloride-triphenylphosphine catalyst to obtain ivermectin). An 8a-hydroxy derivative can also be detected occasionally as a metaboUte (42) or as an impurity arising presumably by air oxidation. An 8a-oxo-derivative can be obtained by oxidizing 5-0-protected avermectins with pyridinium dichromate (43). This also can arise by treating the 8a-hydroperoxide with base. 

Adogen has been shown to be an excellent phase-transfer catalyst for the per-carbonate oxidation of alcohols to the corresponding carbonyl compounds [1]. Generally, unsaturated alcohols are oxidized more readily than the saturated alcohols. The reaction is more effective when a catalytic amount of potassium dichromate is also added to the reaction mixture [ 1 ] comparable results have been obtained by the addition of catalytic amounts of pyridinium dichromate [2], The course of the corresponding oxidation of a-substituted benzylic alcohols is controlled by the nature of the a-substituent and the organic solvent. In addition to the expected ketones, cleavage of the a-substituent can occur with the formation of benzaldehyde, benzoic acid and benzoate esters. The cleavage products predominate when acetonitrile is used as the solvent [3]. 

Fluoro-oct-1-en-3-one (82) has been synthesized by allylic hydroxylation of vinyl fluoride (Scheme 31) [77,78], Oxidation of vinyl fluoride (83) using 0.5 equiv. of Se02 and 2 equiv. of ferf-butyl hydroperoxide with a catalytic amount of acetic acid followed by elimination formed to 2-fluoroalk-1-en-3-ols (84) in 32% overall yield for three steps. Subsequent pyridinium dichromate-oxidation of 84 yielded 2-fluoro-oct-1-en-3-one (83) in 81% (Scheme 31). 

The initial a-addition adduct from the reaction of methyl (S)-2-isocyano-4-methylpentanoate 232 and protected (S)-alaninal 233 further reacted with benzoic acid to furnish 234 as a diastereomeric mixture. The stereochemistry of the resulting benzoyl-protected alcohol was inconsequent since the latter functionality is oxidized during the course of the synthesis using pyridinium dichromate to afford the a-oxoamide in the final target. In general, however, in isocyanide MCRs the control of the newly created stereogenic center is problematic and separation of diastereomeric mixtures cannot be avoided. A recent report by Denmark and Fan on a catalytic asymmetric variant of this reaction therefore represents an interesting development [119]. 

Oxidation of alcohols. In the presence of catalytic amounts of pyridinium dichromate, this peroxide can oxidize primary and secondary alcohols to the corresponding carbonyl compounds in 70-100% yield. Reactions catalyzed by dichloro-tris(triphenylphosphinc)ruthenium are useful for highly selective oxidation of primary allylic and benzylic alcohols m the presence of secondary ones. 

Sharpless also found that this reaction works with only a catalytic amount of titanium-tartrate complex because the reaction products can be displaced from the metal centre by more of the two reagents. The catalytic version of the asymmetric epoxidation is well suited to industrial exploitation, and the American company J. T. Baker has employed it to make synthetic disparlure, the pheromone of the gypsy moth, by oxidation of the epoxy alcohol to an aldehyde with pyridinium dichromate (PDC) (p. 543), Wittig reaction (p. 689), and hydrogenation. 

SCHEME 7. Synthesis otturan tatty acid [21J. Keagents (i) lithium acetylide, tetrahy-drofuran (THF) (ii) H2O, HCOOH (iii) heptynyllithium (iv) aqueous f-butyl hydroperoxide, catalytic VO(acac)2 (v) HgCl2, H2SO4 (vi) pyridinium dichromate. 

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