With PCC

Bu3SnSMe, BF3-Et20, toluene, -20° 0°, 1.5 h H30, 70-97% yield. When treated with various electrophiles, the intermediate stannanes from this reaction form benzyl and MEM ethers, benzoates, and tosylates, and when treated with PCC, they form aldehydes." " ... 

PhB(OH)2, PhH or Pyr. A polymeric version of the phenyl boronate has been developed.Phenyl boronates are stable to the conditions of stannyla-tion and have been used for selective sulfation to produce monosulfated monosaccharides. Phenyl boronates were found to be stable to oxidation with pcc ... 

Enantiopure (7 )-3-alkylpiperidines (38, R = Me, Et) were obtained when perhydropyrido[2,l-Z)][l,3]benzoxazin-9-ones (37, R = H, Me) were treated first with an excess of AIH3, then with PCC, followed by a 2.5 N solution of KOH (99TL2421). Treatment of optically active perhydropyr-ido[2,l-Z)][l,3]benzoxazines 39 and 40 with LAH in the presence of AICI3 and DIBALH (if R = COOEt) yielded 3-substituted piperidines 41 (00TA2809). 

From intermediate 28, the construction of aldehyde 8 only requires a few straightforward steps. Thus, alkylation of the newly introduced C-3 secondary hydroxyl with methyl iodide, followed by hydrogenolysis of the C-5 benzyl ether, furnishes primary alcohol ( )-29. With a free primary hydroxyl group, compound ( )-29 provides a convenient opportunity for optical resolution at this stage. Indeed, separation of the equimolar mixture of diastereo-meric urethanes (carbamates) resulting from the action of (S)-(-)-a-methylbenzylisocyanate on ( )-29, followed by lithium aluminum hydride reduction of the separated urethanes, provides both enantiomers of 29 in optically active form. Oxidation of the levorotatory alcohol (-)-29 with PCC furnishes enantiomerically pure aldehyde 8 (88 % yield). 

The homology between 22 and 21 is obviously very close. After lithium aluminum hydride reduction of the ethoxycarbonyl function in 22, oxidation of the resultant primary alcohol with PCC furnishes aldehyde 34. Subjection of 34 to sequential carbonyl addition, oxidation, and deprotection reactions then provides ketone 21 (31% overall yield from (—)-33). By virtue of its symmetry, the dextrorotatory monobenzyl ether, (/ )-(+)-33, can also be converted to compound 21, with the same absolute configuration as that derived from (S)-(-)-33, by using a synthetic route that differs only slightly from the one already described. 

A two-step procedure was required for the preparation of a diverse set of pyrrole-3-carboxylic acid derivatives. The diketone 15 was prepared using a functional homologation of a 6-ketoester 14 with different aldehydes followed by oxidation with PCC. The Paal-Knorr reaction was carried out in AcOH in a sealed tube under microwave irradiation (180 °C, 5-10 min) to give differently substituted pyrroles with a COOMe group in position 3 (Scheme 5). This group was further transformed to expand the diversity of the products prepared with this method [32]. 

ANSWER We are starting with a primary alcohol, and we are oxidizing with PCC. This will give an aldehyde as a product (rather than oxidizing all the way up to a carboxylic acid) ... 

Catalytic hydrogenation of 146 and oxidation with PCC gave the 3-C-pro-panoyl derivative 147. Selective hydrolysis of the 5,6-0-isopropylidene group, followed by periodic acid oxidation, provided the aldehyde 148. 

O-acetylophiocarpine (381) with ethyl chloroformate afforded the C-8—N cleaved urethane 382 in quantitative yield. Sequential treatment of 382 with silver nitrate, PCC, sodium hydroxide, and p-toluenesulfonic acid in ethanol furnished acetal 384, which was reduced with lithium aluminum hydride followed by hydrolysis to afford the hemiacetal 385. Oxidation of 385 with PCC provided (+ )-a-hydrastine (369). Similar treatment of O-acetylepi-ophiocarpine (386) afforded ( )-/J-hydrastine (368) however, in this case, C—N bond cleavage of 386 with ethyl chloroformate proceeded without regioselectivity. 

Compound 25 (R2 = Me) could be transformed almost quantitatively into the 1-oxo derivative with PCC (see Scheme 7) <2001TA2883>. 

Intramolecular cyclizations could also be achieved by oxidation of 57 with PCC to 65 regioselective addition of an organometallic onto the 7(2)-carbonyl carbon of 65 was followed by treatment with acid to generate the iminium cation, and intramolecular trapping of the cation by an appropriate N-2 substituent (e.g., phenylethyl substituent) <2001TA2883> or C-4 substituent (e.g., benzyl group) <2002T6163>. 

One reason for the success of oxidation with PCC is that the oxidation can be carried out in a solvent such as CH2CI2, in which PCC is soluble. 

Regioselective oxygenation at the C-13 position of 233 with PCC and NaOAc yields enone 234, which can be stereoselectively reduced to the desired... 

RCH=CHZ —> RCHJCHODialkylchloroboranes, obtained as the major products of hydroboration of 1-alkenes with monochloroborane complexed with dimethyl sulfide, are oxidized by PCC to aldehydes (66-68% yield). Similar oxidation of dialkylchloroboranes derived from cyclic alkenes with PCC gives ketones in 70-85% yield. 

The alcohol epimers were oxidized with PCC to ketones (S)-16/(R)-16 in 71% combined yield. The separation of the diastereoisomers (S)-16 and... 

A) (23), was obtained in an improved yield using the modified literature procedure (28) starting from benzene diazonium chloride (1054) and hydroxymethylene-5-methylcyclohexanone (1055). A biomimetic coupling of l-hydroxy-3-methylcarba-zole (O-demethylmurrayafoline A) (23) by reaction with di-ferf-butyl peroxide l(t-BuO)2] afforded the dimer of O-demethylmurrayafoline A (204). Finally, oxidation of 204 with PCC afforded (+ )-bismurrayaquinone-A (215). The resolution of atropo-enantiomers was achieved by chiral HPLC using Chiracel OF. The assignment of the absolute configuration of the two enantiomers (S)-215 and (f )-215 was achieved by comparison of their theoretical and experimental circular dichroism (CD) spectra (166,167,661) (Scheme 5.164). 

A versatile method for the synthesis of perhydrofuropyrans begins with 2-chloromethyl-3-(2-methoxyethoxy)pro-pene, 79. Compound 79, in the presence of lithium powder and a catalytic amount of naphthalene, undergoes sequential reaction with two electrophiles, first a carbonyl compound and then an epoxide, to form methylidenic diols. Hydroboration-oxidation, followed by oxidation with PCC, affords perhydrofuropyrans (Scheme 15) <2000TL1661, 2001SL1197, 2003T5199>. 

Oxidations with pyridinium cUorochromate PCC and pyridinium dichromate PDCY 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 CICH2C H2CI and C6llf). The rate of oxidation increases in the order 5A< 10A<4A<3A. 

Knoevenagel reaction. Hydrogenation of the double bond, desilylation and oxidation of the released primary alcohol group to the aldehydic function with PCC in dichloromethane in the presence of molecular sieves, gives the branched chain L-riho-hepturonic acid derivative 31. Treatment with acetic anhydride and pyridine results in cyclization, and acetylation of the resulting alcohol affords the acetate 32 in 69% yield. 

Tertiary allylic alcohols form a chromate ester that, as it lacks a hydrogen on a to the alcohol, instead of suffering a normal oxidation to ketone rearranges to an enone. This transformation, which can be brought about by other chromium-based reagents, is normally carried out with PCC when it is purposefully sought at (see page 55). 

On occasions, an oxidation with PCC proceeds very quickly at the beginning of the reaction and slows down considerably as the reaction advances. This has been attributed to the formation of an acetal—catalyzed by the acidic nature of PCC—between the product and the starting alcohol.212... 

Alumina has been used in a similar manner. Normally, alumina is added to an aqueous solution of PCC in water, prepared by mixing chromium trioxide, hydrochloric acid (6N) and pyridine. Removal of water leads to the formation of alumina particles covered by PCC, described as PCC on alumina, which is commercially available. Alternatively, it has been described that best results are obtained when alumina and PCC are finely ground in a mortar.231 The alumina not only helps in the work-up by allowing an easy filtering of the chromium-containing by-products, but also accelerates the oxidation with PCC.229a... 

Occasionally alumina, working both as a solid support—used to facilitate the work-up—and as an accelerant, mixed with PCC is added, in a proportion of ca. 0.4-1.5 g of alumina per mmol of alcohol. Normally, PCC is deposited over the alumina.8... 

Functional Group and Protecting Group Sensitivity to Oxidation with PCC... 

Normally, alkenes do not interfere with the oxidation of alcohols with PCC. Although alkenes do react with PCC, this normally requires quite harsh conditions, and selective oxidations of alcohols are possible. 

Nevertheless, alkoxyalkenes, being very electron-rich olefins, do react quickly with PCC. This produces either the breakage of the carbon-carbon double bond yielding two carbonyl compounds,262 or the transformation of the alkoxyalkene into an ester or a lactone.263... 

Olefins, belonging to primary allylic alcohols and possessing a (cis) configuration, suffer isomerization to the (trans) compound during the oxidation of the alcohol to aldehyde with PCC.268 This isomerization is not avoided by the addition of sodium acetate as buffer.189... 

Although the PCC-mediated oxidative transposition of tertiary allylic alcohols is carried out under very mild conditions, normally it is possible to selectively oxidize a primary or secondary alcohol to aldehyde or ketone with PCC, without affecting a tertiary allylic alcohol present in the same molecule.273... 

The oxidation with PCC causes both, a normal oxidation of the primary alcohol and an oxidative transposition of the tertiary allylic alcohol. 

The authors of this book are not aware of any case, in which a primary allylic alcohol suffers an oxidative transposition with PCC. Such case would be most unlikely, because it would involve an equilibrating pair of allylic chromate ester, in which the less stable minor one would evolve to a carbonyl compound. 

Under oxidation with PCC, migration of alkenes into conjugation with aldehydes or ketones can be avoided by the addition of calcium carbonate (see page 47). 

As the oxidation of epoxides with PCC is relatively slow, it is possible to adjust the oxidation conditions so as to selectively transform an alcohol into an aldehyde or ketone in the presence of an epoxide.292... 

PCC very easily oxidizes lactols to lactones.293 However, at the time of writing, the scientific literature does not contain enough data to assess the relative ability of oxidation of lactols versus alcohols with PCC. 

Although certain cyclic acetals are transformed into lactones by PCC,295 sometimes with the help of some added AcOH 195b alcohols are routinely oxidized with PCC without affecting acetals in the same molecule.296... 

With respect to 1,4-diols, a similar behaviour is observed in 1,5-diols, in which one of the alcohols is a primary alcohol. That is, the treatment with PCC may result in the formation of a 8-lactone,300 although this does not happen when geometrical constrains prevent the formation of an intermediate lactol.301... 

As in the case of 1,4-diols, very often 1,5-diols are oxidized uneventfully with PCC, in spite of the potential formation of apparently stable lactols.302... 

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