Methylation direct selective

Acetates. Because of the significant interest in selective acetylation reactions of sucrose, the need for a convenient and unambiguous method of identification has been recognized (34,35). The position of an acetyl group in a partially acetylated sucrose derivative can be ascertained by comparison of its H-nmr acetyl methyl proton resonances after per-deuterioacetylation with those of the assigned octaacetate spectmm. The synthesis of partially acetylated sucroses has generally been achieved either by way of selectively protected derivatives such as trityl ethers and cychc acetals or by direct selective acetylation and deacetylation reactions. 

In contrast with the usual complete methylation, a selective methyla-tion of certain hydroxyl groups has been accomplished by direct reaction (as differing from methods requiring blocking groups and their subsequent removal) in a limited number of cases. Monomeric sugar derivatives will be considered first, to be followed by polysaccharides, since with the latter there are factors involved which introduce complications. 

Direct Process. Passing methyl chloride through a fluidized bed of copper and siUcon yields a mixture of chlorosilanes. The rate of methylchlorosilane (MCS) production and chemical selectivity, as determined by the ratio of dimethydichlorosilane to the other compounds formed, are significantly affected by trace elements in the catalyst bed very pure copper and siUcon gives poor yield and selectivity (22). 

Because much toluene is demethylated for use as benzene, considerable effort has been expended on developing processes in which toluene can be used in place of benzene to make directiy from toluene the same products that are derived from benzene. Such processes both save the cost of demethylation and utilize the methyl group already on toluene. Most of this effort has been directed toward manufacture of styrene. An alternative approach is the manufacture of i ra-methylstyrene by selective ethylation of toluene, followed by dehydrogenation. Resins from this monomer are expected to displace... 

In order to circumvent this problem, there has been significant activity directed toward the search for a less environmentally toxic and more selective oxidizing agent than chromium. For example, Hoechst has patented a process which uses organorhenium compounds. At a 75% conversion, a mixture of 86% of 2-methyl-l,4-naphthoquinone and 14% 6-methyl-l,4-naphthoquinone was obtained (60). Ceric sulfate (61) and electrochemistry (62,63) have also been used. 

Individual polyethers exhibit varying specificities for cations. Some polyethers have found appHcation as components in ion-selective electrodes for use in clinical medicine or in laboratory studies involving transport studies or measurement of transmembrane electrical potential (4). The methyl ester of monensin [28636-21 -7] i2ls been incorporated into a membrane sHde assembly used for the assay of semm sodium (see Biosensors) (5). Studies directed toward the design of a lithium selective electrode resulted in the synthesis of a derivative of monensin lactone that is highly specific for lithium (6). 

The formation of ethyl cyano(pentafluorophenyl)acetate illustrates the intermolecular nucleophilic displacement of fluoride ion from an aromatic ring by a stabilized carbanion. The reaction proceeds readily as a result of the activation imparted by the electron-withdrawing fluorine atoms. The selective hydrolysis of a cyano ester to a nitrile has been described. (Pentafluorophenyl)acetonitrile has also been prepared by cyanide displacement on (pentafluorophenyl)methyl halides. However, this direct displacement is always aecompanied by an undesirable side reaetion to yield 15-20% of 2,3-bis(pentafluoro-phenyl)propionitrile. 

Polypropylenes produced by metallocene catalysis became available in the late 1990s. One such process adopts a standard gas phase process using a metallocene catalyst such as rac.-dimethylsilyleneto (2-methyl-l-benz(e)indenyl)zirconium dichloride in conjunction with methylaluminoxane (MAO) as cocatalyst. The exact choice of catalyst determines the direction by which the monomer approaches and attaches itself to the growing chain. Thus whereas the isotactic material is normally preferred, it is also possible to select catalysts which yield syndiotactic material. Yet another form is the so-called hemi-isotactic polypropylene in which an isotactic unit alternates with a random configuration. 

Ill of Chart 3), as is shown in Fig. 4. This is of interest because both of these reactions involve direct attack on the ring and, in spite of the fact that they are of widely different types, they are both highly selective. The correlation, however, is limited to alkoxy, methyl, and halogeno groups, but its value would certainly be increased by extension to other substituent types. 

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