Alcohols, secondary, oxidation with pyridinium dichromate

Mitsunobu reaction as well as by mesylation and subsequent base treatment failed, the secondary alcohol was inverted by oxidation with pyridinium dichromate and successive reduction with sodium borohydride. The inverted alcohol 454 was protected as an acetate and the acetonide was removed by acid treatment to enable conformational flexibility. Persilylation of triol 455 was succeeded by acetate cleavage with guanidine. Alcohol 456 was deprotonated to assist lactonization. Mild and short treatment with aqueous hydrogen fluoride allowed selective cleavage of the secondary silyl ether. Dehydration of the alcohol 457 was achieved by Tshugaejf vesLCtion. The final steps toward corianin (21) were deprotection of the tertiary alcohols of 458 and epoxidation with peracid. This alternative corianin synthesis needed 34 steps in 0.13% overall yield. 

The secondary alcohol was smoothly oxidized to a ketone in high yield with pyridinium dichromate in dimethylformamide (cf 36). The IR spectrum indicated the presence of a ketone group bonded to the a-carbon of a thiophene (absorption at 1660 cm 1). The 200 MHz XH NMR spectrum showed all the features expected of this structure (cf Figure 12) as did the mass spectrum (cf. 1). 

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. 

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. 

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. 

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