Pyridinium Dichromate PDC

Pyridinium Dichromate. Problems caused by the acidity of PCC can be largely eliminated by using the more neutral reagent pyridinium dichromate (PDC), 36. Although first used by Coates and 

Corrigan it was not exploited synthetically until Corey prepared the reagent by addition of pyridine to neutral chromium trioxide solutions and used it for the oxidation of alcohols to aldehydes, ketones, and acids.xhe reagent is not acidic and the neutral conditions required for the oxidation are superior for the oxidation of allylic alcohols. In dichloromethane (nonaqueous workup) the oxidation is similar to that of PCC. Addition of catalytic amounts of pyridinium trifluoroacetate in dichloromethane significantly increases the rate of oxidation. Allylic alcohols are oxidized faster than aliphatic alcohols, making PDC the reagent of choice for this transformation. Cyclohexenol, for example, is oxidized 10 times faster than cyclohexanol with 

PDC in dichloromethane at 25°CJ Primary aliphatic alcohols are also oxidized, however, under very mild conditions.This oxidation occurs under essentially neutral conditions, as shown by the reaction of 37 with PDC to give aldehyde 38 in 92% yield, despite the presence of the acid sensitive enol ether.79 This reagent is very effective for many types of alcohols,. In Yokokawa s synthesis of (-)-hennoxazole A, alcohol 41 was converted to carboxylic acid 42 in 72% yield. 

When pyridine is added to a solution of chromium trioxide in water, it is possible to obtain a precipitate of the pyridinium salt of dichromic acid, that is pyridinium dichromate (PDC).11 

This oxidant is a bright-orange solid that is soluble in organic solvents, and very convenient to store and manipulate, because of its lack of hydro-philicity. Pyridinium dichromate (PDC), which is normally used in dichlor-omethane at room temperature, is a very efficient oxidant able to transform alcohols in aldehydes and ketones in high yield. The absence of water in the reaction media prevents the over-oxidation of aldehydes into carboxylic acids. 

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Conditions that do pennit the easy isolation of aldehydes in good yield by oxidation of primaiy alcohols employ vaiious Cr(VI) species as the oxidant in anhydrous media. Two such reagents ar e pyridinium chlorochromate (PCC), C5H5NH ClCi03, and pyridinium dichromate (PDC), (C5H5NH)2 Ci207 both are used in dichloromethane. 

Oxidation of primary alcohols to aldehydes (Section 15.10) Pyridinium dichromate (PDC) or pyridinium chloro-chromate (PCC) in anhydrous media such as dichloromethane oxidizes primary alcohols to aldehydes while avoiding overoxidation to carboxylic acids. 

The aldehyde function at C-85 in 25 is unmasked by oxidative hydrolysis of the thioacetal group (I2, NaHCOs) (98 % yield), and the resulting aldehyde 26 is coupled to Z-iodoolefin 10 by a NiCh/CrCH-mediated process to afford a ca. 3 2 mixture of diaste-reoisomeric allylic alcohols 27, epimeric at C-85 (90 % yield). The low stereoselectivity of this coupling reaction is, of course, inconsequential, since the next operation involves oxidation [pyridinium dichromate (PDC)] to the corresponding enone and. olefination with methylene triphenylphosphorane to furnish the desired diene system (70-75% overall yield from dithioacetal 9). Deprotection of the C-77 primary hydroxyl group by mild acid hydrolysis (PPTS, MeOH-ClHhCh), followed by Swem oxidation, then leads to the C77-C115 aldehyde 28 in excellent overall yield. 

Ring D inversion seems to be a crucial step in biogenetic transformations of protoberberines to related alkaloids such as rhoeadine, retroprotoberberine, spirobenzylisoquinoline, and indenobenzazepine alkaloids. 8,14-Cyclober-bin-13-ol 478 derived from berberine (15) was successively treated with ethyl chloroformate, silver nitrate, and pyridinium dichromate (PDC) in dimethyl-formamide to give the keto oxazolidinone 479 (Scheme 98). Heating of 479 with 10% aqueous sodium hydroxide in ethanol effected hydrolysis, retro-aldol reaction, cyclization, and dehydration to provide successfully the... 

A variety of oxidizing agents are available to prepare aldehydes from 1° alcohols such as pyridinium chlorochromate (PCC) and pyridinium dichromate (PDC). 

Towards the end of this section it may be worthwhile to point out some new reactions with high-valent metals and TBHP. The first is a pyridinium dichromate PDC-TBHP system134. Nonsubstituted or alkyl-substituted conjugated dienes, such as 1,3-cyclooctadiene (87) and others (also linear dienes), yield keto allyl peroxides 88 (equation 18), whereas phenyl-substituted dienes such as 1,4-diphenylbutadiene (89) gave diketo compounds, 90 (equation 19). In further research into a GIF-type system135 with iron and TBHP, limonene gave a mixture of products with carvone as the major product. The mechanism is thought to proceed initially by formation of a Fe(V)-carbon... 

Kinetic studies of the oxidation of some a-hydroxy acids with pyridinium dichromate (PDC) are consistent with a mechanism involving the loss of H2O from the pro-tonated substrate in the rate-determining step. The oxidation of 8-hydroxyquinoline (oxine) by PDC has been studied. The intermediacy of an acetochromate ion in the oxidation of some acetophenone oximes with PDC is suggested. 

Upon hydrogenation of 24 a 1,2-rearrangement of the epoxide occurred generating aldehyde 25 as a mixture of diastereoisomers. After reaction with methyl lithium, the diastereomeric alcohols 26 and 27 were separated and isolated in yields of 23% and 71%. While alcohol 26 as the minor diastereo-isomer could be oxidized with pyridinium dichromate (PDC) and methyle-nated to give the enantiomer of kelsoene (cnM), its diastereoisomer 27 with the inverse configuration at C-7 required a supplementary epimerization step with sodium methanolate. The enantiomerically pure ent- allowed for the determination of the absolute configuration of natural kelsoene (1) [9, 10]. The previously reported assignment based on NMR-correlation experiments [5] was corrected. 

General Procedure for Oxidation of Alcohols to Aldehydes and Ketones with Pyridinium Dichromate (PDC)... 

Chromium-based oxidants tend to react quicker with unsaturated alcohols, although the difference of oxidation speed with saturated alcohols is normally not sufficient for synthetic purposes. Nevertheless, the chromium-based reagent pyridinium dichromate (PDC) possesses a mildness and, therefore, a relative greater selectivity that allows its occasional employment for selective oxidations of allylic and benzylic alcohols.134... 

Cysteine proteases play key roles in the pathogenesis of a variety of disease states including osteoporosis [49], muscular dystrophy [50] and several CNS-related disorders [51]. A 100-member library of a-ketoamides 50 was generated via a two-step one-pot synthesis, in which the initial condensation was followed by a pyridinium dichromate (PDC) oxidation (Scheme 11.9). Yields were respectable, ranging from 53 to 75%. Note that a-ketoamides are potential reversible inhibitors with the ability to form hemi-thioacetals with the active thiol of cysteine residues. 

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