Pyridinium dichromate applications

Mithramycin shows a completely P-linked chain of D-conflgurated saccharides. This requires a totally different approach for the synthesis which is also done by application of the DBE method. The previously obtained disaccharide 180 is P-glycosylated with the monosaccharide precursor 176 to give the trisaccharide 185. After reductive debromination (Bu3SnH), an acid deformylation deblocked the C-3" position which is oxidized with pyridinium dichromate. Nucleophilic attack at the carbonyl group by methyl lithium affords a 1 1.2 mixture of 186 and 187 none of which is the desired compound [93]. Obviously, the methyl branch is formed exclusively in the axial way. 

Pyridinium dichromate (14) is an isolable, stable orange solid that can be simply and safely prepared. PDC had been used previously, but it was Corey and coworkers who demonstra the wide applicability of this mild and selective oxidant in organic synthesis. PDC is very soluble in solvents such as DMF, water and DMSO, but sparingly soluble in chlorinated hydrocarbons and acetone. It is normally used either as a solution in DMF or as a suspension in dichloromethane (Table 12). 

This complex, formerly called pyridine perchromate and now finding application as a powerful and selective oxidant, is violently explosive when dry [1], Use while moist on the day of preparation and destroy any surplus with dilute alkali [2], Preparation and use of the reagent have been detailed further [3], The analogous complexes with aniline, piperidine and quinoline may be similarly hazardous [4], The damage caused by a 1 g sample of the pyridine complex exploding during desiccation on a warm day was extensive. Desiccation of the aniline complex had to be at ice temperature to avoid violent explosion [4]. Pyridinium chlorochromate is commercially available as a safer alternative oxidant of alcohols to aldehydes [5], See Chromium trioxide Pyridine Dipyridinium dichromate See Other AMMINECHROMIUM PEROXOCOMPLEXES... 

Due to the relative instability of silylated corticosteroids, alternatives have been developed by Her and Watron [52] and Courtheyn et al. [47]. For dexamethasone and structurally related analytes, an oxidation reaction with pyridinium chloro-chromate has proven to be useful in several laboratories. Later, the procedure was improved by changing the oxidizing reagent to dichromate and reducing the reaction time. Furthermore, the application of a hydrolysis step permitted triamcinolone acetonide to be included in the analytical procedure [46]. As described above, special attention should be given to the identification of dexamethasone and betamethasone because of their similar mass spectra and inadequate peak separation. 

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