Pyridinium chlorochromate PCC , oxidation

The pyridinium chlorochromate (PCC) oxidations of pentaamine cobalt(III)-bound and unbound mandelic and lactic acids have been studied and found to proceed at similar rates.Free-energy relationships in the oxidation of aromatic anils by PCC have been studied. Solvent effects in the oxidation of methionine by PCC and pyridinium bromochromate (PBC) have been investigated the reaction leads to the formation of the corresponding sulfoxide and mechanisms have been proposed. The major product of the acid-catalysed oxidation of a range of diols by PBC is the hydroxyaldehyde. The reaction is first order with respect to the diol and exhibits a substantial primary kinetic isotope effect. Proposed acid-dependent and acid-independent mechanisms involve the rapid formation of a chromate ester in a pre-equilibrium step, followed by rate-determining hydride ion transfer via a cyclic intermediate. PBC oxidation of thio acids has been studied. ... 

Ketone rac-13 was transformed into the corresponding silylenolether and by Pd(II)-mediated Saegusa oxidation [14] into a, -unsaturated ketone rac-14. By alkylative enone transposition comprising methyl lithium addition and pyridinium chlorochromate (PCC) oxidation [15], rac-14 was finally converted into the racemic photo cycloaddition precursor rac-6. In conclusion, the bicyclic irradiation precursor rac-6 was synthesized in a straightforward manner from simple 1,5-cyclooctadiene (11) in nine steps and with an overall yield of 21%. 

The synthetic utility of a-silyl esters has been amply demonstrated by several examples. The basis for this chemistry is the observation that ester lithium enolates can be directly C-silylated with methyldiphenylchlorosilane, a reagent which avoids the more common O-silylation153. The a-silyl-y-valerolactone 80 was converted in two steps and high yield to racemic ancepsenolide by condensation of its lithium enolate with decane 1,10-dicarboxaldehyde followed by isomerization to the endocyclic double bonds of the natural product154 (equation 160). Treatment of the a-silyl-y-butyrolactone 81 or 80 with a Grignard reagent followed by pyridinium chlorochromate (PCC) oxidation provides 4-oxo aldehydes and 1,4-diketones, respectively155 (equation 161). 

The discovery route utilized the pyridinium chlorochromate (PCC) oxidation of 2-cyclohexylethanol in CH2CI2 in presence of molecular sieves. It is a simple process, as the aldehyde is simply isolated by filtration of the reaction mixture through silica gel. However, this process was proven to be difficult to scale up due to difficulties of filtration of the chromium salts. Furthermore, the environmental issues created by the large amount of toxic chromium salts make this process unsuitable for large-scale synthesis. Two other processes (Scheme 6.7) were therefore developed and tested to prepare the required 2-cyclohexyl acetaldehyde at the pilot-plant scale ... 

Pyridinium chlorochromate (PCC) oxidizes trialkylboranes (prepared from terminal olefins and diborane) directly to aldehydes in high yield, presumably via the corresponding borate ester (1) borate esters, prepared by two alternative routes from alcohols, are oxidized in high yield to aldehydes and ketones using the same reagent (Scheme 1). ... 

Tetronomycin (102), a structurally challenging 3-acyltetronic acid ionophore antibiotic, was isolated from a Streptomyces strain in the early 1980s [71]. The only total synthesis reported so far was accomplished in 1992 by Yoshii and coworkers (Scheme 1.14) [72]. By using as chiral building blocks the ethoxyethyl ether 98 and L-rhamnal diacetate 99, Yoshii reached aldehyde 100, a suitable precursor for the installation of the tetronic moiety. This was achieved by a reaction with the lithium anion of methyl tetronate 101 at —100°C that led to the target after pyridinium chlorochromate (PCC) oxidation and careful deprotection. 

Finally, Han, Huang, and Peng reported a multicomponent cascade reaction for the synthesis of the spirooxindole pyranone scaffold [38]. The reaction started with the addition of aliphatic aldehydes 15 to nitrostyrenes 64 catalyzed by chiral secondary amine catalyst I. Next, the resulting adduct reacted with A-benzyl isatin (54b) by a Henry-hemiacetal formation cascade followed by pyridinium chlorochromate (PCC) oxidation to afford the corresponding spirooxindoles 70 in good yields and stereoselectivities (Scheme 10.24). 

In 2003, Juhl and Jorgensen found that, after screening a series of pyrrolidine catalysts, catalyst 48 is again of great value for the inverse-electron-demand hetero-Diels-Alder reaction after pyridinium chlorochromate (PCC) oxidation, lactone products could be obtained as a single diastereomer in excellent enantioselectivity (Scheme 1.20) [255]. The proposed transition state model 58 indicates effective shielding of the Si-face of the enamine double bond by the diaryhnethyl substituent on the pyrrolidine ring of the catalyst. 

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. 

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. 

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-... 

The pyranocoumarin 105 can be prepared via a three-component Diels-Alder reaction between 4-hydroxycoumarin, ethyl vinyl ether and an a-dicarbonyl compound. Similarly to the above, upon treatment of 105 with sulfuric acid in THF, hydrolysis and rearrangement occur to give the furofurochromenone 106. The hemiacetal functionality in 106 may then be oxidized with pyridinium chlorochromate (PCC) to give the lactone 107 <2001EJ03711> (Scheme 28). 

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. 

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. 

Oxidation of the hydroxyl group in 186 with pyridinium chlorochromate (PCC) in CH2CI2 affords the aldehyde 197. The reduction of 197 back to 186 is possible in EtOH in the presence of TiCl4, whereas upon treatment of 197 with diisobutyl-aluminium hydride a competitive reaction with the fullerene core was observed. 

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),... 

These authors also described a three-step synthesis of 13Z-retinoic acid [56], The obtained hydroxydihydropyrane (66%) was oxidized either by Jones s reagent (CrC>3, water, H2SO4, 90%) or Corey s reagent (pyridinium chlorochromate (PCC), 65%). Finally, the dihydropyranone was transformed into retinoic acid (as a mixture of 9E, 13Z, and 9Z,13Z), by /BuOK, according to a known procedure [57], Fig. (26). 

Oxidative ring fission of furans using the commercially available reagent pyridinium chlorochromate (PCC) has been studied as well (80T661). Experimental evidence supports the preliminary formation of intermediate (87) formed by 1,4-electrophilic attack of chlorochromate anion upon the furan ring. This intermediate then breaks down by heterolytic cleavage of the Cr—O bonds to afford initially the cis enedione which isomerizes to the trans product. Treatment of (88) with sodium hydroxide in methanol effects ring closure with formation of the 4-methoxycyclopentenone (89 Scheme 22). 

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

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. 

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