Trityl ethers oxidation

Apart from the improvements to be geiined by attention to these factors, much better yields of disaccharides are obtained when acyclic alcoholic compounds eire used and in other modifications of usual procedures it has been recommended that mercuric oxide in the presence of small amounts of mercuric bromide acts efficiently as an acid acceptor and has the added advantage of low cost and photostability 28), German workers in particular have preferred to use trityl ethers rather than... 

The ready condensation of sucrose with excess triphenylmethyl chloride in pyridine to a tri-trityl ether is, however, more easily explained by XV or II, which have three, instead of two (c/. I), primary alcohol groups to react selectively in the condensation. Fleury and Courtois oxidized sucrose for twenty-four hours at 14° with an excess of suitably buffered periodic acid and found that three moles of the oxidant were consumed and one mole of formic acid was eliminated. This highly selective oxidant is known to cleave unsubstituted 1,2 glycols quantitatively to two carbonyl groups and to eliminate the center carbon atom... 

Ellis and Honeyman80 have raised strong doubts as to the validity of the structural determinations that have been applied to glycosylamines these doubts are based on the fact that such compounds are readily isomerized, as evidenced by their mutarotation. The formation of a trityl ether cannot be considered valid evidence for the presence of a primary hydroxyl group in the compound tritylated, and periodate oxidation, which is often excessive, may lead to faulty conclusions. 

The employment of trityl trifluoroborate is particularly interesting. This reagent is able to introduce trityl groups on both primary and secondary alcohols54 and to selectively oxidize secondary trityl ethers to ketones in the presence of primary trityl ethers.55 Thus, treatment of diols with trityl trifluoroborate leads to tritylation of both alcohols followed by oxidation of the secondary trityl ether, resulting in the formation of a ketone possessing a trityl-protected primary alcohol. A work-up by mild acidic hydrolysis provides the deprotection of the primary trityl ether and formation of a hydroxy ketone.54... 

Trityl trifluoroborate produces the tritylation of both alcohols and the regioselective oxidation of the resulting secondary trityl ether. The primary trityl ether is hydrolyzed on contact with silica gel during the work-up, resulting in the formation of an 80% yield of the... 

Carbonylat ion of 13 4-nitrophenyloxycarbonyl chloride (J 6) in pyridine] gave a good yield of the trityl ether-carbonate 14 which was de-tritylated by hydrogenolysis (2 4) to afford the 7,8-diol-4,5-carbonate 15 in practically quantitative yield. Compound 15.. by way of its 7,8-0 -dibutylstannylidene derivative (25> 26) was regioselectively 8-0-benzylated (dibutyltin oxide, benzene, reflux then benzyl bromide, dimethyl formamide, 95 ), in fair yield (J ). Finally, the aglycon U3 was obtained in high yield by sequential 7-Q-tert.-butyldimethylsilylation (27) (compound 17) and Zemplen saponification. 

Cleavage of acid-labile protective groups.1 The reaction of H202 (70%, FMC) and CI3CCOOH in CH2C12/(-butyl alcohol converts a dimethyl acetal (1) into a hydroperoxy methyl acetal (a), which can be isolated but which for convenience (and safety) is reduced to the aldehyde 2 in 80% overall yield. The same conditions can effect oxidative cleavage of tetrahydropyranyl and trityl ethers. 

Evidence for this suggestion was obtained by NMR spectroscopy of reacting mixtures which showed that triphenylmethane formation was almost eliminated and a trityl ether unit formed when the reaction systems contained propylene oxide. While this interpretation is consistent with most of the experimental work published, Kuntz and Melchior (25) also report that cocatalysis by propylene oxide can be observed when the latter is added several hours after the start of polymeriza-... 

Helferich. Burckhardt, Trityl Ethers of Carbohydrates, 3, 79—III Heyns, K., and Paulsen, H., Selective Catalytic Oxidation of Carbohydrates, Employing Platinum Catalysts, 17, 169-221... 

In carbohydrate trityl ethers and the partially acylated carbohydrate derivatives obtained from them, the free hydroxyl groups may be etheri-fied with other alcohols. This method has been used repeatedly, particularly for the preparation of partially methylated compounds. With the alkali resistant trityl ethers, any alkaline alkylation method may be used, for example, dimethyl sulfate and alkali, or methyl iodide and silver oxide. In choosing an alkylation method for partially acylated compounds, one must be selected which does not cause acyl migration or saponification. 

Triphenylcarbenium salts (PhgC X") selectively oxidize secondary t-butyl or triph-enylmethyl (trityl) ethers derived from alcohols. The oxidation proceeds via initial hydride abstraction followed by loss of the group on oxygen. The secondary-overprimary selectivity results from preferential formation of an oxocarbenium ion intermediate at the secondary center (R2 C-OTr is formed faster than RH C-OTr). 

Also the trityl group was applied for the masking of SH functions. The sulfides were prepared by the reaction of the thiol with trityl chloride (75% yield) or from trityl alcohol and the thiol in the presence of anhydrous TFA (85-90% yield). The cleavage of this group can be carried out under several conditions (Scheme 58). It is sensitive to acids (e.g. trifluoroacetic acid/ethanethiol 1 1) and to heavy metals. Thiocyanogen (SCN)2 oxidizes 5-trityl ethers to the disulfides and iodine converts 5-tritylcy -teine derivatives to cystine structures. 

Addition of lithium reagent derived from 355 to 382 produced a 1 1 mixture of adducts 363 in 75% yield. As this pro C-9 center will later be oxidized to the ketone, this mixture is of no consequence. LAH reduction of the propargyl alcohol followed by protection of the C-9 hydroxyl as the benzoate gave a 73% yield of the ( , )-diene 364. The primary and secondary alcohols, freed by cleavage of the acetonide, were protected as the trityl ether and benzoate, respectively, followed by desilylation to afford 365. The required two-carbon unit... 

Oxidation of alcohols. Trimethylsilyl ethers of secondary alcohols are oxidized in CH2CI2 to ketones in 95-1007o yield by trityl tetrafluoroborate. Oxidation of ethers of primary alcohols is too slow to be useful. Consequently selective oxidation of ethers of secondary alcohols is possible. Oxidation of bissilyl ethers is not clean, but use of bistrityl ethers or bis-r-butyl ethers is a useful method. The latter ethers are useful for selective oxidation of some 1,3-diols, because only... 

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