Pyridinium chlorochromate oxidant

Historically the biotransformations of cyclic enones have been important, not least Leuenberger s transformation of the appropriate cyclohexenedione into the saturated ketone (15), a precursor for tocopherol1541. Similarly 2-methylcyclohex-2-enone is reduced by the microorganism Yamadazyma fari-nosa (also known as Pichia farinosa) to give a mixture of saturated alcohols and ketone pyridinium chlorochromate oxidation of this mixture afforded 3(R)-methylcyclohexanone (95% ee) in 67% yield1551. 

Keywords thiol, pyridinium chlorochromate, oxidative coupling, disulfide... 

D. Liu and C. A. Caperelli, A new synthesis of D-ribonolactone from D-ribose by pyridinium chlorochromate oxidation, Synthesis, (1991) 933-934. 

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

Zinc bismuthate Zn(Bi03)2, readily obtained as a light brown solid from sodium bismuthate and zinc chloride, has been successfully employed for the oxidation of alcohols, thiols, thioethers and oximes (Scheme 5.8) [92BCJ1131, 94SC489]. The yields are comparable with or better than those of the barium permanganate and pyridinium chlorochromate oxidations. 

A quantitative 1,4-chirality transfer is observed in the construction of the acyclic segment C-l to C-ll 11 of the antibiotic ionophore A-23187 (12, calcimycin)441. Both the wanted and unwanted stereoisomer 9 and 10, obtained from the alcohol 8 by pyridinium chlorochromate oxidation followed by Grignard reaction with vinylmagnesium bromide, can be rearranged by the ester enolate procedure simply by changing the reaction conditions to give the stereoisomer 11 with the correct configuration at C-10. 

Addition of thiophenol to cyclopropylallene 12, a less common method for the preparation of cist tram mixtures of divinylcyclopropanes, is used in the synthesis of karahanaenone850, an odoriferous constituent of Japanese hop and cypress oil. Karahanaenone is also prepared via a silyloxy-Cope rearrangement of 15 in 54% overall yield (starting with isobutyraldehyde and a mixture of cis- and /rpyridinium chlorochromate oxidation to give 14)851-852,... 

The reported synthesis for (5Z,9Z)-14-methylpentadeca-5,9-dienoic acid (14) started with commercially available 4-methylpentan-l-ol, which upon reaction with phosphorous tribromide afforded l-bromo-4-methylpentane [52], Commercially available pent-4-yn-l-ol was also protected as the tetrahydropyranyl ether as shown in Fig. (18). Formation of the lithium acetylide with n-BuLi in THF and subsequent addition of 1-bromo-4-methylpentane in hexamethylphosphoric acid triamide resulted in the isolation of the tetrahydropyranyl protected 9-methyldec-4-yn-l-ol. Hydrogenation of the alkyne with Lindlar s catalyst and quinoline in dry hexane afforded the cis hydropyranyl-protected 9-methyldec-4-en-l-ol. Deprotection of the alcohol with />-toluenesulfonic acid afforded (Z)-9-methyldec-4-en-l-ol. Pyridinium chlorochromate oxidation of the alcohol resulted in the isolation of the labile (Z)-9-methyldec-4-enal. Final Wittig reaction with (4-carboxybutyl) triphenylphosphonium bromide in THF/DMSO resulted in the desired (5Z,9Z)-14-methylpentadeca-5,9-dienoic acid (14). 

Two equivalents of pyridinium chlorochromate oxidize 5-bromo-2-furyl alcohols to the 4-hydroxy-butenolides (49) in 60—75% yield (three examples, R = simple alkyl). 

The scope of this asymmetric HDAR was evaluated under the optimized reaction conditions using o-benzoquinone diimide 268. Since the hemiaminal 270 was not stable enough for further analysis, PCC (pyridinium chlorochromate) oxidation was employed to produce the more stable quinoxalinones 271. As shown in Scheme 2.41, a variety of aldehydes 253 bearing simple linear or branched a-substituted alkyl groups were well tolerated and excellent enantioselectivities were generally obtained. 

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