Collins reagent oxidation

Coformycin, 2 -deoxy—see Pentostatin Colchicine synthesis, 1, 472 isothiazoles in, 6, 174 Collins reagent as oxidizing agent, 2, 170 Colour... 

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]. 

The chief drawbacks to using the Collins reagent are the nuisance involved in preparing pure dipyridine chromium VI) oxide,6 its hygroscopic nature5 and its propensity to enflame during preparation.2 3 5 The present method avoids these difficulties by simply preparing diehloromethane solutions of the complex directly.7 In... 

The Collins reagent in CH2CI2 oxidizes silylated primary alcohols in preference to the more hindered silylated secondary alcohols, as described for oxidation of the prostaglandin intermediate 2963 to the rather labile aldehyde 2964, which is immediately subjected to a Horner-Wittig-reaction to introduce the lower side chain [206] (Scheme 12.61). 

Aldehydes are further oxidized to carboxylic acids, RCOOH. To get aldehydes, milder reagents, such as the Jones (diluted chromic acid in acetone) or Collins reagent (a complex of CrOj with 2 mol of pyridine), are used. 

An alternative furan-based jasmone synthesis has been accomplished through oxidative cleavage of this heterocyclic ring (73AJC2671). Thus, the furan (85) was oxidized with Collins reagent to the enedione (86) and the conjugated double bond reduced with sodium dithionite. Base cyclization then gave pure (Z)-jasmone (Scheme 21). 

Treatment of 6-lithio-3,4-dihydro-2H-pyran (660) with the epoxy iodide (661) afforded the epoxydihydropyran (662). Compound (662) cyclized on exposure to basic A1203 to a single hydroxydihydropyran (663) which was oxidized with Collins reagent to the keto aldehyde (664) containing three contiguous asymmetric centers of fixed stereochemistry (Scheme 153). The metallated dihydropyrans have also found use in the construction of keto lactones (78JA7101). 

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... 

Collins reagent is used for the introduction of carbonyl groups at allylic positions." This transformation of alkenes into enones is much slower than the oxidation of alcohols, requiring a great excess of Cr03 2Py and prolonged reaction times. Consequently, alcohols can be oxidized to aldehydes and ketones by Collins reagent without interference from alkenes. 

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... 

Protecting groups, including very labile ones, withstand the action of Collins reagent. The very labile primary TMS ethers are transformed into the corresponding aldehydes.103 As secondary and tertiary TMS ethers resist the action of Collins reagent, a protocol involving per-silylation followed by Collins oxidation allows the selective oxidation of primary alcohols in the presence of secondary ones.104... 

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. 

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. 

Similar to Jones reagent, Collins reagent can produce a hydroxy directed epoxidation of allylic alcohols. This side-reaction only occurs in a limited number of allylic alcohols, most of them being oxidized uneventfully to the corresponding enones.117... 

Sometimes, alcohols can direct the oxidation of alkenes, resulting in highly stereoselective formation of tetrahydrofurans by the action of Collins reagent. Thus, 1,2-diols can form cyclic chromate esters that can intramole-cularly oxidize alkenes, positioned so as to allow the operation of five-membered cyclic transition states.119... 

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