Alcohols, primary with pyridinium dichromate

Primary alcohol groups can be exclusively oxidized to aldehyde groups with pyridinium dichromate [149,150] and to carboxyl groups with the 2,2,6,6-tetramethyl-1-piperidine oxoammonium ion (TEMPO) [151]. The aldehydes can then be reduced to primary alcohols by reaction with NaB H4 [150,152], giving radiolabeled H-starch and the carboxyl group can be inverted by the action of Azotobacter vinlandii poly- 8-D-marmuronic acid C-5-epimerase to give L-iduronic acid [153]. 

Primary and secondary hydroxyl groups can be mildly oxidized to carbonyl groups (aldehydes or ketones) by reaction with pyridinium dichromate. Primary tosyl groups can also be oxidized to aldehydes by reaction with DMSO in collidine. Primary hydroxyl groups can be mildly and selectively oxidized, in the presence of secondary alcohols, to carboxyl groups by reaction with 2,2,6,6-tetram-ethyl-1-piperidine oxoammonium ion (TEMPO) to form uronic acids. Uronic acid carboxyl groups can be reduced to primary alcohols by reaction with car-bodiimide and sodium borohydride. 

The aldehyde function at C-85 in 25 is unmasked by oxidative hydrolysis of the thioacetal group (I2, NaHCOs) (98 % yield), and the resulting aldehyde 26 is coupled to Z-iodoolefin 10 by a NiCh/CrCH-mediated process to afford a ca. 3 2 mixture of diaste-reoisomeric allylic alcohols 27, epimeric at C-85 (90 % yield). The low stereoselectivity of this coupling reaction is, of course, inconsequential, since the next operation involves oxidation [pyridinium dichromate (PDC)] to the corresponding enone and. olefination with methylene triphenylphosphorane to furnish the desired diene system (70-75% overall yield from dithioacetal 9). Deprotection of the C-77 primary hydroxyl group by mild acid hydrolysis (PPTS, MeOH-ClHhCh), followed by Swem oxidation, then leads to the C77-C115 aldehyde 28 in excellent overall yield. 

Oxidation of several primary aliphatic alcohols with potassium dichromate, pyri-dinium dichromate, quinolinium dichromate (QDC), imidazolium dichromate, nico-tinium dichromate, isonicotinium dichromate, pyridinium fluorochromate (PFC), quinolinium fluorochromate, imidazolium fluorochromate, pyridinium chlorochromate (PCC), quinolinium chlorochromate (QCC), and pyridinium bromochromate (PBC), in aqueous acetic acid and in the presence of perchloric acid, showed similar kinetics. The values of the reaction constants did not differ significantly, indicating operation of a common mechanism.1... 

Oxidation of alcohols is normally carried out with Cr(VI) reagents (Chapter 24) but these, like the Jones reagent (Na2Cr2C>7 in sulfuric acid), are usually acidic. Some pyridine complexes of Cr(Vl) compounds solve this problem by having the pyridinium ion (p Ta 5) as the only acid. The two most famous are PDC (Pyridinium DiChromate) and PCC (Pyridinium Chloro-Chromate). Pyridine forms a complex with CrO but this is liable to burst into flames. Treatment with HC1 gives PCC, which is much less dangerous. PCC is particularly useful in the oxidation of primary alcohols to aldehydes as overoxidation is avoided in the only slightly acidic conditions (Chapter 24). 

Bis(trimethylsilyl) peroxide, (CH3)3SiOOSi(CH3)3, is prepared from trimethylsilyl chloride, l,4-diaza[2,2,2]bicyclooctane, and Dabco s complex with 2 mol of hydrogen peroxide [127]. It is used alone [228] or in the presence of catalysts such as pyridinium dichromate [236] trimethylsilyl trifluoromethanesulfonate, CF3S03Si(CH3)3 [228, 237] or tris-(triphenylphosphine)ruthenium dichloride, [(C6H5)3P]3RuCl2 [236]. This reagent oxidizes primary alcohols to aldehydes (in preference to the oxidation of secondary alcohols to ketones [236]), ketones to esters or lactones Baeyer-Villiger reaction) [238], and nucleoside phosphites to phosphates [228]. All these oxidations require anhydrous conditions. 

Pyridinium dichromate in dichloromethane solution converts primary alcohols into aldehydes. In dimethylformamide at 25 °C, carboxylic acids are formed. Cyclohexylmethanol thus gives cyclohexanecarboxylic acid in 84% yield [603]. Oxidations of aliphatic alcohols with ten-butyl chromate yield mixtures of acids with aldehydes and esters [677]. 

Reaction of the C-0 and O-H Bonds Primary alcohols oxidize to carboxylic acids secondary alcohols oxidize to ketones with chromium trioxide or sodium dichromate. Tertiary alcohols do not oxidize under mild conditions. With pyridinium chlorochromate (PCC) the oxidation of primary alcohols can be stopped at aldehydes. 

---