Pyridinium chlorochromate allylic oxidation

Suitable mechanisms have been proposed following determination of the kinetic and activation parameters for oxidation of 2-naphthol and cyclic ketones by nicotinium dichromate some a-amino acids by tripropylammonium fluorochromate " distyryl ketone by quinaldinium fluorochromate methanol by benzyltriethylammonium chlorochromate catalysed by 1,10-phenanthroline substituted benzyl alcohols by tetraethylammonium bromochromate L-cysteine by pyridinium bromochromate lactic acid and 3,5-dimethyl-2,6-diaryl piperidin-4-one oximes by pyridinium chlorochromate allyl alcohol by IDC benzophenoxime by bispyridine silver(I) dichromate and alkyl phenyl sulfides by cetyltrimethylammonium dichromate. A non-linear Hammett plot obtained for the oxidation of substituted benzyl alcohols by IDC has been attributed to the operation of substituent effect on two steps of the proposed mechanism. " Kinetic and activation parameters for oxidation of o-toluidine and of A-methyl-2,6-diphenyl piperidin-4-one oxime and its 3-alkyl derivatives by sodium dichromate have been determined and suitable mechanisms have been suggested. Micellar catalysis in the 1,10-phenanthroline-promoted chromic acid oxidation of propanol... 

Xb was utilized in the elaboration of the hydrindan XIII and subsequently compound XIV. We expected that Xa could be converted to XIII in the same manner as was in the Sih synthesis. This was confirmed in practice (Scheme II). Oxidation of 569 mg (2.68 mmol) of Xa with pyridinium chlorochromate in methylene chloride furnished 532 mg (94%) of enone Ila (7,21). Treatment of 130 mg of Xa witlj a slight excess of Jones reagent (3,22) afforded 126 mg (98%) of Ila. Allylic brominatlon of Ila with a 20% excess of N-bromosuccinimide (NBS) in refluxing carbon tetrachloride provided Xlla in 98% yield. 

From a- or 0- substituted aldehydes or ketones by elimination reactions Benzeneselenenyl trichloride, 27 Methanesulfonyl chloride-4-Di-methylaminopyridine, 176 9-(Phenylseleno)-9-borabicyclo-[3.3.1]nonane, 245 By oxidation of allylic substrates Pyridinium chlorochromate-Benzo-triazole, 262 By other methods Alumina, 14... 

Although in principle naturally occurring (—)-galanthamine could have been prepared by an identical sequence of reactions commencing with D-tyrosine, an alternate route to 319, the enantiomer of 314, was developed. Thus, epimeriza-tion of the methyl ester group at C-6 of the A -trifluoroacetamide derived from 315 followed by oxidation of the allylic alcohol with pyridinium chlorochromate furnished 319 in 78% optical purity, albeit in low chemical yield. Since 319 could be converted to (-)-galanthamine (291) by the same sequence of reactions outlined for the transformation of 314 to (+)-galanthamine, its preparation may be considered to represent a formal total synthesis of 291 from L-tyrosine (163). 

Ac, acetyl AIBN, azobis(isobutanonitrile) All, allyl AR, aryl Bn, benzyl f-BOC, ferf-butoxycarbonyl Bu, Butyl Bz, benzoyl CAN, ceric ammonium nitrate Cbz, benzyloxycarbonyl m-CPBA, m-chloroperoxybenzoic acid DAST, diethylaminosulfur trifluoride DBU, l,8-diazabicyclo[5.4.0]undec-7-ene DCC, /V. /V - d i eye I oh e x y I c ar bo -diimide DCM, dichloromethyl DCMME, dichloromethyl methyl ether DDQ, 2,3-dichloro-5,6-dicyano-l,4-benzoquinone DEAD, diethyl azodicarboxylate l-(+)-DET, L-(+)-diethyl tartrate l-DIPT, L-diisopropyl tartrate d-DIPT, D-diisopropyl tartrate DMAP, 4-dimethylaminopyridine DME, 1,2-dimethoxyethane DMF, /V./V-dimethylformamide DMP, 2,2-dimethoxypropane Et, ethyl Im, imidazole KHMDS, potassium hexamethyldisilazane Me, methyl Me2SO, dimethyl sulfoxide MOM, methoxymethyl MOMC1, methoxymethyl chloride Ms, methylsulfonyl MS, molecular sieves NBS, N-bromosuccinimide NIS, /V-iodosuccinimide NMO, /V-methylmorpho-line N-oxide PCC, pyridinium chlorochromate Ph, phenyl PMB, / -methoxvbenzyl PPTs, pyridiniump-toluenesulfonate i-Pr, isopropyl Py, pyridine rt, room temperature TBAF, tetrabutylammonium fluoride TBS, ferf-butyl dimethylsilyl TBDMSC1, f-butylchlorodimethylsilane Tf, trifhioromethylsulfonyl Tf20, trifluoromethylsulfonic anhydride TFA, trifluoroacetic acid THF, tetrahydrofuran TMS, trimethylsilyl TPAP, tetra-n-propylammonium perruthenate / -TsOH. / -toluenesulfonic acid... 

When 125 was subjected to a two-step sequence, reductive cleavage of the 0-0 bond and subsequent oxidative dehydration, by treatment with zinc in AcOH, the allylic diol 135 and the 1,2-oxazocine 126, as minor product, were isolated. Oxidation of the diol 135 under several different reaction conditions (pyridinium chlorochromate (PCC), 2-iodoxybenzoic acid (IBX)/EtOH, IBX/DMSO, Pyr/SOj) gave 126 in high yield (68-92%), while with tetrapropylammonium perruthenate/Wmethylmorphaline Ar-oxidc (TPAP/NMO) as oxidizing agent 126 was the minor product and the a,/3-unsaturated 7-butyrolactone 136 was obtained in 68% yield (Scheme 28) <2005JOC6995>. 

The chroniium(VI) oxide-dipyridine complex also has beoi found to cause oxidative rearrangement of tertiary allylic alcohols to a,3-epoxy aldehydes and small amounts of a,3-unsaturated aldehydes (equation 6 and Table 3). This is potentially useful as a homologation sequence since the starting materials are readily available from vinyl metal addition to ketones. Use of pyridinium chlorochromate (PCC) for this transformation gives mosdy a,3 unsaturated aldehydes. 

Imidazolium dichromate is a selective oxidant for allylic and benzylic hydroxy groups. (Allylic alcohols are oxidized faster than benzylic alcohols.) The selectivity over saturate alcohols is similar to that of 4-(dimethylamino)pyridinium chlorochromate. DMF is recommended as the solvent for oxidations, since it appears that the choice of solvent is critical to obtaining high yields. This reagent has also been observed to cause some ( )/(Z)-isomerization during the oxidation of allylic alcohols. 

Pyridinium chlorochromate (PCC) Corey and Suggs prepared PCC by mixing CrOs with pyridine in HCl. PCC is used for the oxidation of primary and secondary alcohols in CH2CI2. This reagent is less efficient than Collins reagent for the oxidation of allyl alcohols. 

Pyridinium chlorochromate has been shown to be of particular value in the allylic oxidation of compounds containing an activated methylene group, such as 5,6-dihydropyrans (69a and 69b equations 31a and31b). 40... 

Cyclohexadienes. The last steps in a recent novel synthesis of /3-damascenone (3) from dimedone by way of 1 required oxidation of an allylic hydroxyl group and 1,4-elimination of acetic acid. Activated Mn02 proved to be superior to pyridinium chlorochromate (40% yield) and to NCS-S(CH3)2 (4,87-89), which caused elimination of the hydroxyl group. The last step was effected by DBU at reflux for 20 seconds in 79% yield. ... 

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]. 

Not only allylic alcohols but also homoallylic alcohols can be epoxi-dized. A peculiar transannular oxidation takes place when 1-methyl-4-cycloocten-l-ol is treated with the Fieser reagent, [CrOj-(CH3C0>20-CH3C00H], or with pyridinium chlorochromate. Two dia-stereomeric keto oxides are formed in different ratios (equation 281) [535],... 

The allyl alcohol in the seven-membered ring is oxidized to the corresponding a,P-unsaturated ketone as expected, whereas the allyl alcohol in the five-membered ring is oxidized to afford the transposed a,P-unsaturated ketone after [3,3]-sigmatropic rearrangement. This behavior is only observed with cyclic, tertiary allyl alcohols upon treatment with one equivalent of pyridinium chlorochromate. The desired oxidation product 34 is obtained in 90 % over two steps. 

Pyridinium chlorochromate oxidation of an allyl ether or benzyl ether gives the enone (CH2CI2, reflux, 84% yield). ... 

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