Pyridinium chlorochromate alcohols

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. 

Pyridinium chlorochromate. In this case, the alcohol that is cleaved is simultaneously oxidized to give a ketone. ... 

Secondary alcohols are oxidized easily and in high yield to give ketones. For large-scale oxidations, an inexpensive reagent such as Na2Cr207 aqueous acetic acid might be used. For a more sensitive or costly alcohol, however, pyridinium chlorochromate is often used because the reaction is milder and occurs at lower temperatures. 

Perhaps the most important reaction of alcohols is their oxidation to carbonyl compounds. Primary alcohols yield either aldehydes or carboxylic acids, secondary alcohols yield ketones, but tertiary alcohols are not normally oxidized. Pyridinium chlorochromate (PCC) in dichloromethane is often used for oxidizing primary alcohols to aldehydes and secondary alcohols to ketones. A solution of Cr03 in aqueous acid is frequently used for oxidizing primary alcohols to carboxylic acids and secondary alcohols to ketones. 

Draw the structure of the carbonyl compound(s) from which each of th< following alcohols might have been prepared, and show the products yen. would obtain by treatment of each alcohol with (i) Na metal, (ii) SOCl2, anc (iii) pyridinium chlorochromate. 

Q Primary alcohols can be oxidized to give aldehydes (Section 17.7). The reaction is often carried out using pyridinium chlorochromate (PCC) in dichloro-methane solvent at room temperature. 

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-... 

A biomimetic synthesis of benzo[c]phenanthridine alkaloids from a protoberberine via the equivalent of a hypothetical aldehyde enamine intermediate has been developed (130,131). The enamide 230 derived from berberine (15) was subjected to hydroboration-oxidation to give alcohol 231, oxidation of which with pyridinium chlorochromate afforded directly oxyche-lerythrine (232) instead of the expected aldehyde enamide 233. However, the formation of oxychelerythrine can be rationalized in terms of the intermediacy of 233 as shown in Scheme 41. An alternative and more efficient... 

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... 

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

After the known intermediate 79 (contaminated with ca. 6 % < /.v isomer) [39] was prepared from Hajos-Parrish ketone [40] 78, the tert-butyl ether was cleaved (quant.) and the ketone protected as the acetal (96 %). The secondary alcohol was oxidized by pyridinium chlorochromate (PCC) to provide ketone 80 in good yield (71 %) and after fractional crystallization afforded material absent of any m-hydrindane (Scheme 10.6). [NOTE All compounds shown in Schemes 10.6 and 10.7 are shown in the ent-configuration, as published]. The oxidation of protected hydrindane 80 under Saegusa-Ito conditions [41, 42] gave enone 81 (82 %), confirmed by X-ray crystallography. 

Historically the biotransformations of cyclic enones have been important, not least Leuenberger s transformation of the appropriate cyclohexenedione into the saturated ketone (15), a precursor for tocopherol1541. Similarly 2-methylcyclohex-2-enone is reduced by the microorganism Yamadazyma fari-nosa (also known as Pichia farinosa) to give a mixture of saturated alcohols and ketone pyridinium chlorochromate oxidation of this mixture afforded 3(R)-methylcyclohexanone (95% ee) in 67% yield1551. 

A better reagent for oxidation of primary alcohols to aldehydes in good yield is pyridinium chlorochromate (PCC), a complex of chromium trioxide with pyridine and HCl. 

Furfuryl alcohol is oxidized directly to 2,3-dideoxy-DL-pent-2-eno-pyranosid-4-nlose (325, R = H) by treatment with m-chloroperoxy-benzoie acid.236 A variety of substituted furfuryl alcohols have thus been converted into over 60 enediulose derivatives (345) in connection with studies of their antimicrobial activity.211 It was later found that pyridinium chlorochromate may be applied in this reaction, instead of a peroxy acid.237... 

Alcohols are the most important precursors in the synthesis of carbonyl compounds, being readily available. More complex alcohols are prepared by reaction of Grignard reagents with simpler carbonyl compounds. Ordinarily MnO and Cx OY in acid are used to oxidize 2° RjCHOH to RjCO. However, to oxidize 1° RCHjOH to RCHO without allowing the ready oxidation of RCHO to RCOOH, requires special reagents. These include (a) pyridinium chlorochromate (pcc),... 

A convenient reagent that selectively oxidizes primary alcohols to aldehyde is anhydrous pyridinium chlorochromate, abbreviated to PCC... 

Popular oxidation reactions of peptide alcohols such as the Parikh-Doering or Dess-Martin in addition to older oxidation reactions such as Collins, pyridinium chlorochromate, or Swern oxidation afford racemization free productsJ9121415 37-39 Oxidations using pyridinium dichromate results in racemization and low yields of product.[l3 Oxidation reactions have also been utilized in semisynthetic pathways of peptide aldehydes (1) peptide aldehydes are obtained through the enzymatic acylation of a peptide ester to an amino alcohol with subsequent oxidation of the peptide alcohol to afford the aldehyde, and (2) peptide aldehydes can also be obtained by direct enzymatic oxidation of the peptide alcohol by alcohol de-hydrogenaseJ40 41 ... 

General Procedure for Oxidation of Alcohols to Aldehydes and Ketones with Pyridinium Chlorochromate (PCC)237... 

Keywords alcohol, pyridinium chlorochromate, microwave oven, ketone, aldehyde... 

The oxidation of primary alcohols to aldehydes also suffers from the problem of overoxidation of the aldehyde to a carboxylic acid. Mild methods capable of stopping die oxidation at the aldehyde oxidation level are required if aldehydes are to be obtained. The most common and effective reagent for this purpose is pyridinium chlorochromate (PCC), produced by the reaction of pyridinium hydrochloride with chromium trioxide. This reagent is soluble in dichloromethane and smoothly oxidizes primary alcohols to aldehydes in high yields. Because of die mild, neutral reaction conditions and the use of stoichiomettic amounts of oxidant, the aldehyde product is not oxidized further. 

A novel one-pot tandem oxidation-cyclization-oxidation process was successfully applied in the transformation of unsaturated alcohols 250 [Eq. (5.315)].860 The intermediate aldehyde formed by oxidation with pyridinium chlorochromate (PCC) undergoes a carbonyl-ene cyclization followed by an additional oxidation to form 3-substituted piperidinones. 

The use of pyridinium-based chromium(vi) oxidising agents (cf. Section 5.7.1, p. 587) is illustrated by the use of the supported reagent, pyridinium chlorochromate-alumina, for the conversion of the cyclic allylic alcohol, carveol, into the corresponding oc,/ -unsaturated ketone, carvone125 (Expt 5.88). 

This method is of value when the alcohol is readily available from natural sources, or when it can be prepared, for example, by the reaction of an alkenyl-organometallic reagent with an aldehyde. An example of the former is the oxidation of the terpenoid alcohol carveol to carvone (Expt 5.88) using pyridinium chlorochromate-on-alumina reagent. 

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