Oxidation with PCC

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 ... 

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. 

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. 

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 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. 

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... 

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

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

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). 

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... 

It is important to note that the relative velocity of an uneventful oxidation of an alcohol with PCC versus a carbon-carbon bond breakage from a chromate ester, driven by the generation of a stable carbocation, is substantially substrate-dependent, and may change according to stereoelec-tronic factors, which may be difficult to predict. Thus, many alcohols are successfully oxidized to aldehydes and ketones, regardless of an apparently potential carbon-carbon bond breakage leading to stabilized carboca-tions.315 Consequently, failure to try an alcohol oxidation with PCC, because of fear of this side reaction is not recommended. 

Note. (1) 2-Methylcyclooctanone may be prepared by the procedures noted in earlier sections. Thus cyclooctanone may be converted into 1-methylcyclooctanol by reaction with methylmagnesium bromide (cf. Expt 5.40) dehydration then gives 1-methyl-cyclooct-l-ene (cf. Expt 5.12) hydroboration gives trans-2-methylcyclooctanol (cf. Expt 5.44) finally, oxidation with PCC yields 2-methylcyclooctanone (cf. Expt 5.76, or alternatively Expt 5.86). 

Iodotrimethylsilane may be the actual reagent. The products are useful for protection of quinones since they are reconverted to the parent quinones in 60-90% yield by oxidation with PCC in CH2C12 at 25°.2... 

The molecular formula, C4H10O, tells us that compound A is saturated. The fact that A can be oxidized with PCC to an aldehyde (note that B gives a positive Tollens test) tells us that A is a primary alcohol. We can draw only two structures for A that are consistent with these data ... 

The removal of the menthyl appendage in azetidines 124 has been effected by oxidation with PCC to 8-aminomenthone derivatives 125, which were treated with potassium hydroxide to enantiopure azetidine derivatives <2005JOC1408>. The latter compounds were isolated as fV-tosyl derivatives 126 by treatment with tosyl chloride in diisopropylethylamine (Scheme 26). 

In two cases the direct oxidation with PCC of the primary adduct 198 — presumably already contaminated with the corresponding y-lactol — has been tried and surprisingly the P-hydroxylated y-butyrolactones 201 are formed (Eq. 84) 96 b). 

The THP derivatives (130 X = H, Cl) have been deprotected and the resulting alcohols (131) oxidized with PCC yielding the chiral heterocycles (132) <92JOC1930>. Dehydration of a 1 1 mixture of the diastereoisomers (133) with phosphorus oxychloride gives the thiopyrano[4,3-c]thiopyran derivative (134) (48% yield) <84J0C5136>. 

Af-(Phenylsulfanylmethyl)oxazolidinones derived from camphor 494 can be lithiated with n-BuLi at —78°C to give the chiral formyllithium equivalent 478683 (Scheme 128). This intermediate added to aldehydes in good yields, but lower stereoselectivity than compound 477, to afford crystalline adducts, which allowed the isolation of the major diastereomer 495. Hydrolysis of these adducts gave a-hydroxy aldehydes, which can be oxidized with PCC to the corresponding a-hydroxy acids. 

Diketones or y-keto aldehydes (95) were prepared from y-acetoxy ketones by saponification of the acetate with aqueous sodium hydroxide, followed by oxidation with PCC (equation 23). 

Oxidation of trialkyl borates and of trialkoxyboroxines. Trialkyl borates, B(0R)3, prepared by reaction of primary and secondary alcohols with borane-dimethyl sulfide, are oxidized by PCC to aldehydes and ketones in good yield. This indirect oxidation of alcohols does not involve formation of water, which could be detrimental in some cases. Of even greater interest, carboxylic acids can be converted into aldehydes by reaction with borane-dimethyl sulfide to form a tri-alkoxyboroxine followed by oxidation with PCC (equation I). 

Aliphatic alcohols are converted to a symme-RCOOCH2R) by oxidation with PCC on aluminum without solvent. Oxone in aqueous methanol also converts aryl aldehydes to the corresponding ester. Ally lie alcohols are converted to conjugated esters with Mn02, NaCN in methanol-acetic acid. Primary alcohols are oxidized to the methyl ester with tri-chloroisocyanuric acid in methanol. This reagent also converts diols to lactones. 

A carbonyl transposition can be effected via the addition of a vinyl or an alkyl Grignard reagent to an a, 3-unsaturated ketone. Acid-catalyzed rearrangement of the resultant allylic alcohol during oxidation with PCC affords the transposed a,(3-unsaturated carbonyl substrate. This reaction represents a useful alternative when Wittig olefination of the ketone is problematic. 

The methodology for the conversion of epoxyphosphonates into allylic alcohols has been used in the preparation of diethyl 2-formylvinylphosphonate by basic ring opening of diethyl 2,3-epoxypropylphosphonate. When the latter is allowed to react with MeONa in MeOH at 0°C and then treated with an ion-exchange resin (Dowex SOW), diethyl ( )-3-hydroxy-l-propenylphosphonate is isolated in quantitative yield. " It is converted by room temperature oxidation with PCC in CIFCF into pure diethyl ( )-2-formylvinylphosphonate in 52% yield (Scheme 5.27). 

The remaining and chiral four-carbon building block (5 )-F was prepared from ethyl (S)-3-hydroxybutanoate by protection with TBS chloride, reduction with diisobutylaluminum hydride, and oxidation with PCC. Aldol reaction of E and (S )-F furnished G. The aldol G was oxidized with Dess-Martin periodinane, and the resulting diketone H was treated with acid to give I. Deprotection of I gave (S)-141, which was levorotatory. The absolute configuration of the naturally occurring (-)-neuchromenin was thus determined as S. 

For the synthesis of sulfobacin A (158) and flavocristamide A (160), TBS ether of (f )-3-hydroxy-15-methylhexadecanoic acid was necessary, which was synthesized from 10-bromo-l -decanol (A) as shown in Figure 6.20. Chain elongation of A under the Schlosser conditions gave B, which was oxidized with PCC to give aldehyde C. ( )- 3-Hydroxy ester D was prepared from C by treatment with ethyl acetate and LDA. The corresponding ( )-acid was acetylated with vinyl acetate in the presence of lipase PS to give enantiomerically pure (R)-hydroxy acid and the acetylated (S )-acid. The former was converted to its TBS ether E. 

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