Trityl perchlorate catalyst

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. 

The formation of ethers such as 1806 by EtsSiH 84b can also be catalyzed by trityl perchlorate to convert, e.g., benzaldehyde in 84% yield into dibenzyl ether 1817 [48]. The combination of methyl phenethyl ketone 1813 with O-silylated 3-phenyl-n-pro-panol 1818, in the presence of trityl perchlorate, leads to the mixed ether 1819 in 68% yield [48] (Scheme 12.15). Instead of trityl perchlorate, the combination of trityl chloride with MesSiH 84a or EtsSiH 84b and sodium tetrakis[3,5-bis-(trifluoro-methyl)phenyl]borane as catalyst reduces carbonyl groups to ethers or olefins [49]. Employing TMSOTf 20 as catalyst gives very high yields of ethers. Thus benzaldehyde reacts with O-silylated allyl alcohol or O-silylated cyclohexanol to give the... 

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... 

During the past decades, the scope of Lewis acid catalysts was expanded with several organic salts. The adjustment of optimal counter anion is of significant importance, while it predetermines the nature and intensity of catalytic Lewis acid activation of the reactive species. Discovered over 100 years ago and diversely spectroscopically and computationally investigated [131-133], carbocations stiU remain seldom represented in organocatalysis, contrary to analogous of silyl salts for example. The first reported application of a carbenium salt introduced the trityl perchlorate 51 (Scheme 49) as a catalyst in the Mukaiyama aldol-type reactions and Michael transformations (Scheme 50) [134-142]. 

The reactions proceeded efficiently under mild conditions in short time. The silyl enol ethers reacted with the activated acetals or aldehydes at -78 °C to give predominant erythro- or threo-products [136, 137] respectively. In the same manner, the aldol reaction of thioacetals, catalyzed by an equimolar amount of catalyst, resulted in <-ketosulfides [139] with high diastereoselectivity. In the course of this investigation, the interaction of silyl enol ethers with a,]3-unsaturated ketones, promoted by the trityl perchlorate, was shown to proceed regioselec-tively through 1,2- [141] or 1,4-addition [138]. The application of the trityl salt as a Lewis acid catalyst was spread to the synthesis of ]3-aminoesters [142] from the ketene silyl acetals and imines resulting in high stereoselective outcome. 

Homoallyl methyl ethers.4 Trityl perchlorate catalyzes a reaction of allyltri-methylsilanes with dimethyl acetals or ketals to form homoallyl methyl ethers in 60-90% yield. Diphenylboryl triflate is a somewhat less efficient catalyst. Example ... 

Simflar experiments were carried out a few years later by Kagiya et al. In this interesting study, a series of alkyl and aryl perchlorates were prepared in situ in bulk styrene and their relative activity as initiators assessed at 0°C rT)utyl perchlorate was found to be by far the most active catalyst and trityl perchlorate the least effective. No termination was detected in these processes. Other esters were also tried and their activity followed approximately the strength of the parent acid. Although these systems were too complicated to allow detafled mechanistic studies, mainly because the synthesis of the initiator proceeded alongside the polymerisation itself, they certaiidy provided furtha support for the pseudocationic theory, considering the very low polarity of the medium. 

For example, treatment of di-O-acetyl-L-rhamnal 26 with silylated pyrimidine or purine bases in the presence of a mild acidic catalyst (trityl perchlorate) afforded 2,3-unsaturated pyra-nosyl nucleosides, as a mixture of a,/3-anomers 27 (with the / -anomer predominating) [37]... 

Condensations. The reaction of silyl ketene acetals with aldehydes favors enals over saturated aldehydes, when promoted by BUjSnClO. The catalyst is obtained from reaction of trityl perchlorate and tributyltin hydride. 

Dehydrooenation 1,4-Benzoquinone. Chloranil. o-Chloranil. Copper chromite. Copper-Chromium oxide. Diethyl azodicarboxylate. 2,3-Dichloro-3,6-dicyano-I,4-benzoquinone. DIphenylpIcrylhydrazyl, N-Lithioethylenediamlne. Mercuric acetate. Nickel catalyst. Oleic add, Palladium. Perbenmic acid. Potassium l-butoxide. Pyrldinium hydrobromide per-bramlda, Balinlum. Selenium dioxide. Sodium borohydride. Sulfdr (sm 1,2-Naphthalic anhydridli preparation). Tetracyanoethylene. Thionyl chloride. Trityl perchlorate. 

With the usefulness of the chemistry described in the previous three schemes, it is important to note that similar reactions are applicable to furanose sugars. As shown in Scheme 2.3.22, Mukaiyama, et a/.,9 effected the illustrated reaction between allyltrimethylsilane and an acetate activated ribose derivative. Utilizing trityl perchlorate as a catalyst, the reaction proceeded in 90% yield with total a selectivity. Concurrently, a similar reaction was reported with a 1-methyl-1-acetoxy furanoside.54 This reaction, shown in Scheme 2.3.23, not only demonstrates that high stereoselectivity can be obtained with... 

Aldol-type condensation of enol silyl ethers and acetals or orthoesters can be accontplished by the use of trimethylsilyl trifluoromethanesulfonate (TMSOTf). In these reactions, TMSOTf acts as a true catalyst and is required in 1-S ntol %. The reactions show interesting chemoselectivity in that acetals are highly reactive receptors of enol silyl ethers but the parent aldehydes and ketones do not react under these conditions (Scheme 37). Similarly, trityl perchlorate is found to be an efficient catalyst to promote the reaction between enol silyl ethers and acetals. 

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. ... 

Mukaiyama-Michael Reactions. 1,4-Addition of ketene alkyl silyl acetals to a./S-unsaturated carbonyl compounds (eq 10) is promoted by a variety of Lewis acids (for example, TiCU, Ti(OR)4, SnCU, trityl perchlorate, lanthanide salts, Al-montmo-rillonite clay), or Lewis bases such as fluoride ion (eq 11), or quaternary ammonium carboxylates. Lanthanide salts are particularly effective catalysts, and in the case of ytterbium(III) tri-fluoromethanesulfonate, the catalyst can be recovered (eq 10). ... 

The aldol reaction of a variety of silyl enolates and acetals in the presence of a catalytic amount (1-10 mol%) of trityl perchlorate proceeded efficiently to afford /i-methoxy ketone in high yield. In the presence of trityl tetra-fluoroborate catalyst, the reaction of dithioacetal 62 vith silyl enol ethers affords /i-ethylthio ketones 63 (Eq. (28)) [36]. 

The most frequently employed acid catalyst is TfOH Trifluo-romethanesulfonic Acid, or triflic acid), but others that have been used include BF3 0Et2, TMSOTf, /7-TsOH, and TFA. Camphorsulfonic acid, pyridinium p-toluenesulfonate, and trityl perchlorate have also been investigated in conjunction with the more reactive 4-methoxybenzyl trichloroacetimidate. ... 

Acetylation Acetic anhydride. N-Acetoxyphthalimide. 2- and 3-Acetoxypyridine. Acetyl chloride. N-Acetylimidazole. Boron trifluoride. Catalysts (see Acetic anhydride). Ketene. Magnesium. Methyl oxocarbonium hexafluoroantimonate. Perchloric acid. Phenyl acetate. Pyridine. Sodium acetate. Tetraelhylammonium acetate. p-Toluenesulfonic acid. Tri-n-hexylethyl ammonium hydroxide. 2,4,6-Triisopropylbenzenesulfonyl chloride. Trityl-sodium. Zinc chloride. 

To inhibit the formation of the 3-substituted glycals, starting materials with allylic groups with enhanced leaving properties are recommended [81] or, otherwise, AT-trimethylsilylated bases can be used with catalysts such as trityl on lithium perchlorate [82]. 

Oxygen Nucleophile. Uchiro et al. have reported hydrolysis of a thioglycoside bond under very mild conditions using the combination of tetrabutylammonium periodate as oxidant, trityl tetrakis(pentafluorophenyl)borate as Lewis acid catalyst, and HMDO in anhydrous acetonitrile at 0 °C (eq 17). The reaction also proceeded well in the absence of HMDO, using aqueous tri-flic or perchloric acid as catalyst in place of the trityl derivative. The intersaccharidic bond of a disaccharide was found to be stable under these conditions, with no anomerization being observed. 

New catalysts have heen recently developed for promoting the aldol-type addition of acetate-derived silyl ketene acetals with high efficiency 10-methylacridinium perchlorate (5 mol %), cationic mono- and dinuclear iron complexes (5 mol %), t-hutyldimethylsilyl chloride-indium(III) chloride (10 mol %), [l,2-henzenediolato(2—)-0,0 ]oxotitanium(20 mol%), phos-phonium salts (7 mol%), and trityl salts (5-20 mol%). ... 

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