Alcohols oxidation with Collins* reagent

This is followed by hydrolysi.s of the ester moieties with potassium carbonate and reesterification of the carboxy moiety with diazomethane to produce intermediate 65. The solitary free alcoholic hydroxyl at C-9 is oxidized with Collins reagent and the silyl ether groups are removed with acetic acid to give enprostil (63) [15]. 

Collins reagent can transform tertiary allylic alcohols into rearranged enones,101 similar to PCC, which is routinely used for this purpose (see page 55). As this reaction is normally slower than the oxidation of primary and secondary alcohols, these can be oxidized with Collins reagent with no interference from tertiary allylic alcohols present in the same molecule.102... 

Deacetoxyhtion. In a synlhesis of I l-desoxy-IOz-hydroxyprostaglandins, Crabbe et al. oxidized the alcohol (1) with Collins reagent (dipyridine chromium(Vl) oxide, this volume) to give the diacetoxy aldehyde (2) treatment of this with pyridine for a few... 

Sharpless asymmetric epoxidation afforded epoxide 167 in 76% yield. Lindlar reduction, followed by oxidation with Collins reagent, yielded aldehyde 168. A Wittig reaction of aldehyde 168 with the ylide 169 gave the homoallylic alcohol 171 after deprotection of 170 under acidic conditions. Alcohol 171 was converted into the corresponding iodide and reacted with triphenylphosphine to provide phosphonium salt 172. 

Selenides are oxidized to selenoxides that normally suffer an in situ elimination.111 Amines are destroyed,112 although its protection as amides or carbamates prevents the reaction with Collins reagent. Lactols are very quickly oxidized to lactones,113 unless a very great steric hindrance is present.114 Tertiary lactols suffer oxidation via its opened hydroxyketone form.115 The oxidation of tertiary lactols may be slow, so that an alcohol can be selectively oxidized. 

Oxidation of a chiral alcohol. The (S -alcohol 1 is oxidized in high yield by DMSO-C5H5N SO3 to the aldehyde 2 with no more than 0.1% racemization. The same oxidation when effected with Collins reagent or with H2CrO4-Si02 (8, 110) proceeds with 5% and 22% racemization, respectively.4... 

Oxidation. This compound has been in the literature for some time, but was recognized as a useful oxidation reagent only recently by Corey and Suggs. It oxidizes primary and secondary alcohols in yields equal to or greater than those obtained with Collins reagent (4, 215-216) and has the advantage that a large excess is not necessary. The oxidations are usually conducted in methylene chloride at room temperature (1-2 hr.). With acid-sensitive substrates, the reaction can be buffered with sodium acetate as in the last example. 

Write out the product formed when allyl alcohol is oxidized with Collins s reagent and when benzyl alcohol is oxidized with Collins s reagent. 

The most widely used chromium(VI)-oxo reagents for synthetic use in organic chemistry are the Collins reagent, Cr03(py)2,273 and the Corey reagent, Cr03CrpyH+,297 with the latter being superior for the oxidation of primary alcohols to aldehydes.275... 

Most functional groups resist Collins oxidation, including the oxidation-sensitive sulfides106 and thioacetals.103 Although Collins reagent can transform alkenes into enones" and alkynes into inones,107 these reactions are slower than the oxidation of alcohols into aldehydes or ketones. Therefore, alcohols can be usually oxidized with no interference from alkenes108 or alkynes.109... 

Collins reagent is able to transform benzyl ethers into ketones and benzoates.110 Normally, this causes no interference with the oxidation of alcohols, because the oxidation of benzyl ethers demands more drastic conditions. 

Alternatively it is possible to oxidize a primary alcohol no further than to give the aldehyde. This is the domain of the Collins reagent, PCC, PDC, or activated dimethyl sulfoxide. The oxidation of primary alcohols with K2Cr207 in aqueous solution to nothing but the aldehyde, (i.e., without further oxidation to the carboxylic acid) is possible only if a volatile aldehyde results and is distilled off as it is formed. This is the only way to prevent the further oxidation of the aldehyde in the (aqueous) reaction mixture. Selective oxidations of primary alcohols to aldehydes with the Jones reagent succeed only for allylic and benzylic alcohols. Otherwise, the Jones reagent directly converts alcohols into carboxylic acids (see above). 

These olefins were transformed into unsaturated acetates by Woodward s protocol with iodine, silver acetate, and acetic acid. Saponification and subsequent oxidation of the liberated alcohols with Collin s reagent resulted in a 2 1 mixture of conjugated enones, 162 and 163, in mediocre yields (Scheme 16). The minor enone 163 was transformed into the cyanoketone 164 by 1,4-addition using... 

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. 

Oxidation of akohois. The oxidation of alcohols with the reagent is conducted in methylene chloride with a sixfold molar excess of oxidant. The oxidation usually proceeds to completion in 3 -13 min. at 23°. The reagent is particularly recommended for oxidation of primary alcohols to aldehydes in this case use of the Sarett reagent (1,145 -146 2,74-75) usually gives low yields. Thus 1-heptanol can be oxidized by the Collins reagent in methylene chloride to l-heptanal in 70-84% yield. 

A chromium(VI) oxidant that is applicable to oxidations of acid-sensitive substrates is the complex of chromium trioxide with two molecules of pyridine (Collins reagent). As described on pages 22 and 274, its preparation requires the portionwise addition of chromium trioxide to dry pyridine at 15-20 C (addition of pyridine to chromium oxide could cause ignition) [592, 595, 599]. Up to 6 mol of the complex is used to oxidize alcohols in dichloromethane solutions at 25 °C, and the reaction is finished in 5-15 min [595]. Alternatively, the oxidation can be carried out in pyridine cooled with an ice bath and is finished at room temperature within 15-22 h [592, 599]. 

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