Use of trityl perchlorate

Although trityl perchlorate is used to accomplish the glycosidation of the C-8 hydroxyl in 44 with acetoxy glycoside 49, control experiments have demonstrated that no reaction takes place in the presence of 4 A molecular sieves or 2,6-di-terf-butylpyridine. This observation suggests that the actual catalyst is not trityl perchlorate, but perchloric acid. Consistent with this conclusion is the observation that catalytic amounts of a strong Brpnsted acid such as triflic or perchloric acid can catalyze the glycosidation of 44 with 49 in the absence of trityl perchlorate. 

Superior yields of ethers from aldehydes are obtained by the use of several other electrophilic species. The addition of 5 mol% of trityl perchlorate to a mixture of triethylsilane and 3-phenylpropanal in dichloromethane at 0° produces an 83% yield of bis-(3-phenylpropyl) ether within 10 minutes (Eq. 176),329 Reductive polycondensation of isophthalaldehyde occurs with two equivalents of triethylsilane in the presence of 10 mol% of trityl perchlorate to give 40-72% yields of polyether with average molecular weights ranging from 6,500 to 11,400 daltons (Eq. 177).337 Addition of one equivalent of an alkoxytrimethylsilane to the reaction mixture produces unsymmetrical ethers in good to excellent yields. Thus, a mixture of (ii)-cinnamaldehyde, 3-phenylpropoxytrimethylsilane, and triethylsilane in dichloromethane reacts under the influence of a catalytic amount of trityl perchlorate to give the unsymmetrical ether in 88% yield (Eq. 178).329... 

Recently Mukaiyama and coworkers introduced the use of trityl salts as efficient catalysts for the aldol reaction. Using a catalytic amount of trityl perchlorate (5 mol %) and t-butyldimethylsilyl enol ethers, the anti aldols were preferentially obtained (anti 73-84%) regardless of the double bond geometry. With trityl triflate (5 mol %) and dimethylphenylsilyl enol ethers, the syn isomers are produced predominantly (syn 63-79% Scheme 1). Several variations of the catalyst system have been developed. Trityl... 

The glycosidation reaction which depends upon the condensation of sugar 1,2-0-cyanoalkylidene derivatives and tritylated acceptors continues to attract the attention of Kotchetkov and colleagues. The rate-determining step of the condensation has been shown to be the reaction between the donor and the triphenylmethyl cation. The stereochemical outcome of the reaction is dependent upon the concentration of trityl perchlorate used as catalyst. A new mechanism for 1,2-cw-glycoside formation by this method was proposed. The nature of the O-protecting groups used in the acceptor also has an effect on the anomeric ratio produced. ... 

A convenient procedure for the tritylation of secondary hydroxy-groups of sugars is based on the use of triphenylmethylium perchlorate or tetrafluoroborate in... 

Tri-Q-acetyl-l,2-cyanomethylidene-a-D-glucopyranose condensed with methyl 3,4,6-tri-Q-acetyl-2-Q-trityl-a-D-glucopyranoside in the presence of trityl perchlorate gave 78% of 1,2-linked products with an o, -ratio of almost 1 30, and Helferich glucosylation of benzyl 2-acetylamido-4,6-Q-benzylidene-2-deoxy-a-D-gluco- or galacto-pyranoside has been used to obtain the j0-i- 3 linked disaccharides. 

The use of trimethylsilyl-based electrophilic catalysts with organosilicon hydrides also promotes the conversion of aldehydes into ethers and avoids the need to employ the potentially hazardous trityl perchlorate salt.314,334,338 One reagent pair that is particularly effective in the reductive conversion of aldehydes into symmetrical ethers is a catalytic amount of trimethylsilyl triflate combined with either trimethylsilane, triethylsilane, PMHS,334 or 1,1,3,3-tetramethyldisiloxane (TMDO, 64) as the reducing agent (Eq. 179).314 Either... 

A much more systematic approach involves the use of p-chlorides carrying non-participating groups at C-2 as is exemplified by the Wolfrom synthesis of p-isomaltose octa-acetate from 1,2,3,4-tetra-O-acetyl-p-D-glucopyranose and 3,4,6-tri-0-acetyl-2-0-nitro-p-D-glucopyranosyl chloride In this work silver carbonate was used as acid acceptor and soluble silver perchlorate was fmmd to exert valuable catalytic influence, but later the perchlorate itself was used in an application to a tiisacchar-ide s mthesis which incorporated the trityl ether modification 2 ). 

The same approach allows preparation of various pyrylium carboranes from the corresponding 4/f-pyran carboranes 174a,b and 175b by the action of acetyl perchlorate,245 perchloric acid,244 and triarylamine radical cation salts,244,245 as well as electrochemically.243 The oxidation of condensed 4H-pyran 345 with trityl perchlorate, 2,3,5,6-tetra-substituted 4f/-pyrans 431 and 153 with tropylium tetrafluoroborate or 153 with heterocyclic salt 393 led to useful preparations of pyrylium salts 394,330 395a,359 and 395b,360 respectively. 

The agents used for oxidative aromatization of 2/f-thiopyrans were trityl perchlorate,42-155,156 tetrafluoroborate,267 and iodide.267 Thus 3,5-diphen-yl-2//-thiopyran (222) was found to aromatize either on its own to thiopyrylium salts 396 or by S-methylation to l-methyl-3,5-diphenylthiabenzene (398) via intermediate 397,267 as shown in Scheme 18. 

The 1-benzothiopyrylium salts are best prepared from the easily accessible thiochroman-4-one [Eq. (35)].6,51,52,302-304 The common dehydrating agents include potassium hydrogen sulfate8 and phosphorus pentoxide51,302-304while sulfuryl chloride/perchloric acid51,302-304 and trityl perchlorate 6 have been useful as hydride extractors. Another method involves the cyclization of /1-ketosulfides (e.g., 81) or j8-arylthio-acroleins (82) with perchloric acid [Eq. (36)].305,306... 

This method has been used to prepare the 3-oxidothiopyrylium betaine (42) as its perchlorate salt by oxidation of thiopyranones (75JCS(Pl)2099) with trityl perchlorate (equation 114). 

In practice, extrapolations of p fR in water have usually used the older acidity function based method, for example, for trityl,61,62 benzhydryl,63 or cyclopropenyl (6) cations.66,67 These older data include studies of protonation of aromatic molecules, such as pKSi = —1.70 for the azulenium ion 3,59 and Kresge s extensive measurements of the protonation of hydroxy- and methoxy-substituted benzenes.68 Some of these data have been replotted as p fR or pKa against XQ with only minor changes in values.25,52 However, for more unstable carbocations such as 2,4,6-trimethylbenzyl, there is a long extrapolation from concentrated acid solutions to water and the discrepancy is greater use of an acidity function in this case gives pA 2° = —17.5,61 compared with —16.3 (and m = 1.8) based on X0. Indeed because of limitations to the acidity of concentrated solutions of perchloric or sulfuric acid pICs of more weakly nucleophilic carbocations are not accessible from equilibrium measurements in these media. 

A combination of trityl tetrafluoroborate or perchlorate with 2,4,6-tri(/err-butyl)-pyridine instead of pyridine enables an almost quantitative tritylation of the secondary hydroxyl group in l,2 5,6-di-0-isopropylidene-a-D-glucofuranose in 10 min at room temperature [321]. The more readily available 2,6-di(/m-butyl)-4-methylpyridine was used [322] in combination with trityl perchlorate for the selective tritylation of methyl 2,6-di-0-acetyl-a-D-galactopyranoside at the equatorial OH-3. Combination... 

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