Oxidation by pyridinium chlorochromate

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. 

Different reactivities of some substituted oxan-4-ols on oxidation by pyridinium chlorochromate (PCC) are rationalized on the basis of their conformational features, including twist conformations.2 A rate-determining carbon-carbon bond cleavage step in a glycol-PCC complex is proposed in the oxidation of butane-2,3-diol to acetaldehyde.3 Steroidal 6/i-hydroxy-4-en-3-onc was isolated as an intermediate in the oxidation of steroidal 5-en-3/i-ol with PCC.4... 

This methodology was applied to the synthesis of L-streptose [91] and methyl a-D-mycaroside [95 a], The difficulty of this photochemical oxidation originates from the sensitivity of pyruvates to hydrolysis. This approach seems limited to small quantities of substrates but does not need any separation technique as in oxidation by pyridinium chlorochromate [95 b]. Nethertheless, photolysis of pyruvates of partially protected derivatives of a-D-g/ucofuranose and P-D-fructofuranose in benzene yields the corresponding oxidized products in excellent yields [96]. 

Oxidations by pyridinium chlorochromate resemble those by dipyridine chromium(VI) oxide, both in scope and the mild conditions required. At room temperature, primary alcohols give aldehydes [604, 605], secondary alcohols afford ketones [605], allylic and benzylic methylene groups are oxidized to carbonyl groups [606, 6d7], enol ethers are converted into esters [608] or lactones [609], trimethylsilyl ethers of diphenols are transformed into quinones [610], and alkylboranes are converted into aldehydes (yll]. 

The bis(trimethylsilyl) ether of hydroquinone is oxidized by pyridinium chlorochromate to -benzoquinone (equation 342) [610]. 

Alcohols are oxidized by pyridinium chlorochromate (PCC) to the corresponding aldehydes or ketones. They are not further oxidized to the corresponding carboxylic adds because the reaction was done in organic solvents, not in water. If water existed, the carbonyls would form aldehyde hydrates or ketone hydrates, which are then oxidized to acids. 

In 2003, the first organocatalytic enantioselective inverse-electron-demand hetero-Diels-Alder reaction of p,Y-unsaturated-a-ketones 97 with aldehydes 98 promoted by a secondary amine catalyst (99) was accomplished by the j0rgensen group (41). After oxidation by pyridinium chlorochromate (PCC), various trans-lactones 100 were afforded with good yields and up to 94% ee (Scheme 38.27). The reaction is proposed to occur via an enamine intermediate generated from chiral secondary amine 99 and aldehydes 98 (transition state O). Notably, sUica gel is essential for regeneration of the chiral amine catalyst. 

What product would you obtain by reduction of digitoxigenin (Problem 27.39) with LiAl.H4 By oxidation with pyridinium chlorochromate ... 

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. 

D. Liu and C. A. Caperelli, A new synthesis of D-ribonolactone from D-ribose by pyridinium chlorochromate oxidation, Synthesis, (1991) 933-934. 

The secondary alcohol is readily oxidized with pyridinium chlorochromate (PCC)25 (65) which is commercially available or easily prepared by addition of pyridine to a solution of chromium(VI)-oxide in hydrochloric acid. 

Step 2 Oxidation with pyridinium chlorochromate (PCC) affords the aldehyde. Step 3 Wittig reaction provides the TM. (Note The ylide reagent is prepared by treating Ph3P with CD3I to obtain Ph3P+CD3 r and followed by deprotonation with w-BuLi.)... 

Further processing of 57 towards the ketone 60 is readily effected by highly regioselective tosylation of the primary hydroxyl group (66), hydride reduction 58 -> 59, and oxidation with pyridinium chlorochromate (PCC) on aluminum oxide to afford 60 in a yield of 70 % over the three steps (63). Due to the now practical accessibility of these furanoid building blocks supplementary modifications, that have already been performed, become preparatively relevant, e. g. the conversion of tosylate 58 into the 5,6-epoxide (66), C-extensions (63, 66), shortening of the carbon chain via periodation of 57 (63), and transformation of the respective products into acyclic derivatives by acid hydrolysis of the 1,2-0-isopropylidene group (63, 66). 

An alternative tt> the chromium trioxide-pyridine comidex is provided by pyridinium chlorochromate (PCC) and pyridinium dichromate (PDC). These reag ts, now ubiquitous for chromate-based oxidation of alcohols, overcome the hygroscofric nature of the chromium trioxide-i ridine complex and are prepared by a less hazardous procedure both are commercially available as are several other derivative reagents. 

This solid is obtained by dissolving CrOs in a minimum of water, adding pyridine, and collecting the precipitate. PDC is probably present in Sarett and Cornforth oxidants. Unlike pyridinium chlorochromate, it is nearly neutral. 

The reducing power of diborane has been blunted by forming its adduct with dimethyl sulfide. This adduct, Me2S BH3, is stable and commercially available and therefore more attractive as a hydride reagent than diborane itselfNevertheless, the adduct still reduces carboxylic acids to alcohols which are isolated as cyclic boroxins (2 Scheme 1). In a one-pot reaction, carboxylic acids can be reduced to boroxins and then oxidized with pyridinium chlorochromate to the required aldehyde (Scheme I)."... 

Because the reagent is slightly acidic, it cannot be used to oxidize acid-sensitive compounds [605. For easier isolation of the products, the complex may be formed in the presence of alumina. After the reagent is stirred with the alcohol at room temperature for 2 h, the aldehyde is isolated by filtration and evaporation of the filtrate [604. Another solid-support oxidant of pyridinium chlorochromate is prepared by treatment of cross-linked poly(vinylpyridine) with chromium trioxide and hydrochloric acid (equation 212) [612]. 

Alkylation of isovaleramide with 1,3-dichlorobut-2-ene yields (139) after methyl-ation acid-catalysed hydrolysis and internal aldol condensation gives ( )-piperitone. The value of piperitenone and isopiperitenone formation, probably via electrocyclic reaction of the pyrolytic acetic acid-elimination product from A - and A -isomers of (49), cannot be assessed in the absence of reaction yields. (S)-(-)-Pulegone is obtained in good yield from (- )-citronellol by oxidation with pyridinium chlorochromate followed by double-bond isomerization. Low-temperature reduction of ( —)-carvone to ( —)-cz5-carveol (140) and... 

Oxidations with pyridinium chlorochromate PCC and pyridinium dichromate PDC. 4 Oxidations with PCC and PDC of secondary hydroxyl groups of sugars and nucleosides is slow and incomplete. The reaction is markedly catalyzed by 3 A molecular sieves. Celite, alumina, and silica are not effective. CH2C12 is the most satisfactory solvent oxidations are slower in C1CH2CH2C1 and C6H5. The rate of oxidation increases in the order 5A<10A<4A<3 A. 

---