Tritylated acceptors

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. ... 

Kochetkov and coworkers have reported [154—156] an efficient approach for the synthesis of 1,2-cis pyranosides employing 1,2-trcms-glycosyl thiocyanates as glycosyl donors and tritylated sugar derivatives as glycosyl acceptors (Scheme 4.5). This... 

In this case, the 6-O-trityl group improved the a-sclcctivity of the glycosylation because of the steric effects. The trityl ether of the resulting product can act as an acceptor when it is glycosylated with a fully benzylated thioglycosyl donor using NIS... 

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... 

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 ). 

Triphenylmethyl (trityl) tetrafluoroborate, on the other hand, is sp2 hybridized but it is also extremely reactive toward electron donors. The acceptor orbital of the trityl cation is an unfilled 2p atomic orbital on the charged carbon. These carbon electrophiles are isolable compounds, but they are extremely reactive with any sort of electron donor (H2O vapor is a common culprit). 

Radical fragmentation of 2-nitrophenyl-azo-trityl resin was studied in the presence of various radical acceptor solvents to elucidate possible radical reaction pathways. When using benzene as solvent, only 2-nitro-bi-phenyl was formed as the product of radical substitution reaction (SNR) in 67% yield. Hydrogen-radical abstraction from the polymer backbone (e.g., from the benzylic units of polystyrene) was completely suppressed. When toluene was used as solvent, a mixture of the following products was obtained nitrobenzene, 4-methyl-2 -nitrobisphenyl, 2-methyl-2 -nitro-bisphenyl, and 3-methyl-2 -nitrobisphenyl (9 9 1 1). In the case of toluene, the nitro-aryl radicals undergo H-abstraction with radical substitution as a competing reaction pathway. These results indicate that H-abstraction... 

FIGURE 4A Examples of (a) glycosyl acceptors and (b) glycosyl donors used in trityl ether-assisted regioselective glycosylation. Tr, trityl. 

Michael reactions. This system promotes a Michael addition of silyl enol ethers to a,(3-unsaturated thiolesters, which are excellent Michael acceptors. Trityl salts are less effective, as are Lewis acids in combination with SnCl2. 

Pyridine is widely used as a reaction medium in acylation and related reactions involving, for example, acyl chlorides, anhydrides, sulfonyl chlorides, or chlorosilanes, and has the particular advantage that it can function both as a catalyst and acid acceptor. It is usually the solvent of choice for the trimethylsilylation of alditols and sugars,292-298 and for various derivatization reactions such as tritylation and sulfonylation.299-307 The tritylation reaction has been reviewed.306... 

Of the options explored by Paquette, those outiined in Schemes 15.8 and 15.9 were demonstrated to be entirely workable (Johnston and Paquette 1995). Two routes were developed to approach the acceptor glycoside 75, with that originating from commercially available 71 proving to be the more efficient. Benzyl protection of the more hindered secondary hydroxyl in this diol was realized via a three-step sequence involving highly regioselective f -tritylation in advance of reaction with sodium... 

Related to the above cyanoethylidene derivatives, thioorthoesters 231 also behave similarly, and can be activated with catalytic amounts of TrC104 to glycosylate trityl ethers [506,507]. By using the thioglycoside promoters NIS-TfOH [508] or I2 [254], glycosyl acceptors with free hydroxyls were glycosylated. 

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