Pyridine chlorochromate

The acetonide 151 is available from a 4-substituted azetidinone in three steps and is converted into the 3,4-disubstituted azetidinone 152 by the action of pyridine chlorochromate in the presence of a catalytic amount of Jones reagent. The disubstituted /3-lactam is a suitable starting material for the synthesis of trinems <1996TL2467>. 

Oxyberberine (49), when further oxidized by pyridine chlorochromate and then treated with methanol, gives the methoxy-ketone (50 R = Me) the tetra-methoxy-analogue of (49) behaves similarly.132 The carbinolamine (50 R = H) is obtainable, together with the keto-acid (53), by the photolysis of oxidoberberine (51), in the presence of oxygen and rose bengal, to produce the peroxide (52), followed by treatment with pyridine hydrochloride a tetramethoxy-analogue of oxidoberberine, prepared by the reduction of norcoralyne with zinc and acetic acid, followed by oxidation with a peracid, behaves in the same way.133... 

Pyridine chlorochromate (PCC) is a gentler oxidizing agent. It also oxidizes secondary alcohols to ketones, but it stops the reaction at the aldehyde when it oxidizes primary alcohols. PCC must be used in an anhydrous solvent such as CH2CI2 because if water is present, the aldehyde will be further oxidized to a carboxylic acid. 

This complex, formerly called pyridine perchromate and now finding application as a powerful and selective oxidant, is violently explosive when dry [1], Use while moist on the day of preparation and destroy any surplus with dilute alkali [2], Preparation and use of the reagent have been detailed further [3], The analogous complexes with aniline, piperidine and quinoline may be similarly hazardous [4], The damage caused by a 1 g sample of the pyridine complex exploding during desiccation on a warm day was extensive. Desiccation of the aniline complex had to be at ice temperature to avoid violent explosion [4]. Pyridinium chlorochromate is commercially available as a safer alternative oxidant of alcohols to aldehydes [5], See Chromium trioxide Pyridine Dipyridinium dichromate See Other AMMINECHROMIUM PEROXOCOMPLEXES... 

AW, Acid-washed Choi, Cholesterol DMAP, 4-(Dimethylamino)pyridine DMF, N,/V-Dimethylformamide DMTr, Di(p-niethoxyphenyl)phenyl methyl GalNAc, N-Acetylgalactosamine, 2-acetamido-2-deoxy-D-galactose HMF, 5-Hydroxymethylfur-fural, 5-(hydroxymethyl)-2-furaldehyde INOC, Intramolecular nitrile oxide-alkene cycloaddition Lea, Lewisa Lex, Lewisx MOM, Methoxymethyl MP, p-Methoxyphe-nyl MS, Molecular sieves NIS, N-Iodosuccinimide PCC, Pyridinium chlorochromate PDC, Pyridinium dichromate PMA, Phosphomolybdic acid PMB, p-Methoxybenzyl ... 

The Collins/Sarett oxidation (chromium trioxide-pyridine complex), and Corey s PCC (pyridinium chlorochromate) and PDC (pyridinium dichromate) oxidations follow a similar pathway as the Jones oxidation. All these oxidants have a chromium (VI), normally yellow, which is reduced to Cr(IV), often green. 

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. 

All of the usual chromium-based oxidation reagents that have been used for the oxidation of cyclobutanols to cyclobutanones, for example, chromium(VI) oxide (Jones reagent),302 pyri-dinium chlorochromate,304 pyridinium dichromate,307 and chromium(YI) oxide/pyridine (Collins),303 are reported to do so without any serious problems. Alternatively, tetrapropylam-monium perruthenate in the presence of A-methylmorpholine A -oxide. oxalyl chloride in the presence of triethylamine in dimethyl sulfoxide (Swern),158,309,310 or phenyl dichlorophos-phate in the presence of triethylamine and dimethyl sulfoxide in dichloromethane (Pfitzner-Moffatt),308 can be used. The Pfitzner-Moffatt oxidation procedure is found to be more convenient than the Swern oxidation procedure, especially with respect to the strict temperature control that is necessary to achieve good yields in the latter, e.g. oxidation of 1 to give 2.308... 

Following solid-phase extraction, all extracts are adjusted in the pH range 7-7.5, and submitted to additional cleanup on an immunoaffinity column containing a mixture of dexamethasone- and prednisolone-specific gels. Column washing is performed with water, while elution of the analytes with 3 ml methanol/water (80 20). Aliquots of the eluates are submitted to oxidative reaction with pyridin-ium chlorochromate, and the oxidized corticosteroid derivatives are then analyzed by gas chromatography-mass spectrometry under the conditions shown in Table... 

The interaction of CrC>3 with hydrochloric acid, in the presence of water, results in an equilibrium, in which chlorocromic acid is present. Addition of pyridine results in the formation of a precipitate of the pyridinium salt of chlorocromic acid, the so-called pyridinium chlorochromate (PCC).12... 

Addition of one equivalent of CrC>3 (MW= 100.0) to 1.1 equivalents of hydrochloric acid (6 N) leads to a homogenous solution containing chlor-ochromic acid (CICrOiH). Slow addition of one equivalent of pyridine (MW= 79.1) to this solution, kept at 0°C, leads to the formation of pyridinium chlorochromate (PCC) that separates as yellow-orange crystals. Filtration through a sintered glass funnel, followed by drying in vacuum, allows the isolation of ca. 84% of pure PCC.12a... 

On the other hand, isoquinolinium (iQCC),385b p-methylpyridinium (y-PCC)390 and trimethylammonium (TMACC)391 chlorochromates closely resemble the oxidizing behaviour of PCC. p-Methylpyridinium chlorochro-mate has the distinctive advantage over PCC of containing p-methylpyridine that is less toxic than pyridine. 

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. 

The oxidation of 35 presents no problems as over-oxidation cannot occur. But aldehyde 36 could be oxidised to the corresponding carboxylic acid so care was needed. In fact PCC (pyri-dinium chlorochromate Cr03 and HC1 dissolved in pyridine) could be used for both.5... 

Chemicals AIBN CAN DIBAL DMAP DMSO HMPA LAH LDA mCPBA NBS NCS or PCC PDC Py TBAF TEMPO a.a -azobislisobutyronitrile) cerium(IV) ammonium nitrate diisobutylaluminium hydride 4-(dimethylamino)pyridine dimethyl sulfoxide hexamethylphosphoramide lithium aluminum hydride lithium diisopropylamide m-chloroperoxybenzoic acid A-bromosuccinimide A-chlorosuccinimide superoxide anion radical pyridinium chlorochromate pyridinium dichromate pyridine tetrabutylammonium fluoride 2,2,6,6-tetramethyl-l-piperidinyloxy free radical... 

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