Selective synthesis isomer

The selective alkylation of toluene with methanol to produce -xylene as a predominant isomer can be achieved over shape-selective catalysts (99—101). With a modified ZSM-5 zeoHte catalyst, more than 99% -xylene in xylene isomers can be produced at 550°C. This -xylene concentration exceeds the equiHbrium concentration of 23% (99). The selective synthesis of -xylene using relatively low cost toluene is economically attractive however, this technology was not commercialized as of 1991. 

In the case of NH2OH with a sharp difference in the nucleophilicity of the two functions, the primary amino group reacts with the carbocation C-1 center. For example, the reaction of l-alkylaminoalk-l-en-3-ynes with hydroxylamine leads to selective synthesis of alkylisoxazoles (69ZOR1179). A preparative value of this method is evident because the use of dicarbonyl compounds as starting materials for the synthesis of alkylisoxazoles results in a mixture of isomers. 

An analogous reaction applied to l,l-dichloro-4-methyl-l,3-pentadiene would lead to a selective synthesis of permethric acid, of which the cis isomer was more desirable. It is interesting that this seemingly simple structural change provided... 

The reaction of styrene required tuning of the ligand nature to achieve acceptable selectivities the use of PPh2Cy (Cy=cyclohexyl) proved to be the right choice for the selective synthesis of the branched isomer 11a (Scheme 27)... 

It now became of interest to determine the biological activity of the individual isomers in AC 222,293 and this required an alternate regio-selective synthesis The synthesis for the m-toluate is shown in Scheme II. 

Aryl-1,4-dihydropyridines.2 These compounds are important calcium channel blockers or enhancers depending on the stereochemistry at C4. An enaritio-selective synthesis of the (S)-isomers (blockers) is based on addition of an aryl-lithium to achiral 3-dihydrooxazolyl-5-methoxycarbonylpyridine (2). Thus addition... 

Enethiols 61 are tautomerically stable isomers of thioketones 60 and, in contrast to enols, enethiols have been isolated and characterized. This method provides a route for the selective synthesis of enethiols by deprotonation of 60 and quenching with trimethylchlorosilane, since under these conditions no isomerization takes place. 

The application of molecular mechanics to enantio- and diastereo-selective synthesis is less straightforward, and publications in this area have started to appear only recently. In the case of the racemate separations described above, the isomer abundances of equilibrated solutions are taken to be related to the energy of all local minima. In contrast, in order to predict the enantiomeric excesses arising from chiral syntheses, the reaction mechanisms and the basic structure of relevant intermediates or transition states have to be known since their relative energies need to be computed in order to predict the expected enantiomeric excesses. 

The MFI class of channel zeolites, of which ZSM-5 is a member, are of enormous importance in the petrochemicals industry because of their shape-selective adsorption and transformation properties. The most well-known example is the selective synthesis and diffusion of p-xylene through ZSM-5, in preference to the o- and m-isomers. Calcined zeolites such as ZSM-5 are able to carry out remarkable transformations upon normally unreactive organic molecules because of super-acid sites that exist... 

The unsubstituted PhOions afford the ortho- ( 40%) and para- ( 20%) arylation products. By contrast, the selective synthesis of either the ortho or the para isomer can be performed when the t-Bu groups substitute two of the three possible coupling positions [20]. In the case of the 2,6-disubstituted phenoxide ion, the para-substituted compound is obtained (Scheme 10.22), and the t-Bu substituents can be easily removed later [20]. 

The 2,6- and 2,4-di-t-butyl phenoxide ions were shown to be more reactive than PhO ions. The reaction of these nucleophiles, mainly of 2,6-di-/-butylphenoxide ion (185), was studied with a considerable variety of compounds in order to achieve the synthesis of biaryls of importance in non-linear optics249,250. In these nucleophiles, the r-butyl groups substitute two of the possible coupling positions in order to perform a selective synthesis of either the o- or the /7-isomer. The substituents can be easily removed later251,252. 

On the other hand, stabilized ylides react with aldehydes almost exclusively via trans-oxaphosphetanes. Initially, a small portion of the cw-isomer may still be produced. However, all the heterocyclic material isomerizes very rapidly to the fnms-configured, four-membered ring through an especially pronounced stereochemical drift. Only after this point does the [2+2]-cycloreversion start. It leads to triphenylphosphine oxide and an acceptor-substituted fnms-configured olefin. This frara-selectivity can be used, for example, in the C2 extension of aldehydes to /ran.v-con figured aj8-unsaturated esters (Figure 9.11) or in the fnms-selective synthesis of polyenes such as /1-carotene (Figure 9.12). 

This paper deals with the selective synthesis of 2-acetyl-6-methoxynaphthalene, precursor of Naproxen, over zeolite catalysts and especially over HBEA zeolites. As has been previously observed3 8, acetylation of 2-methoxynaphthalene occurs preferentially at the kinetically controlled 1-position with formation of l-acetyI-2-methoxynaphthalene (I). The desired isomer, 2-acetyl-6-methoxynaphthalene (II) and the minor isomer, l-acetyl-7-methoxynaphthalene (HI), are the other primary products. However, it will be shown that in presence of 2MN, isomerization of I can occur allowing a selective production of II, the desired product the effect of the operating conditions (solvent, temperature) and of the acidity and porosity of the zeolite catalyst will be presented. 

However, the generation of only single isomer was achieved by using a shape-selective catalyst. The Cu(I) catalysts coordinated to chiral ligand, 3.47, 3.48 or 3.49, are used in the selective synthesis of sulfur ylides. 

This idea clearly was inspired by the successful L-dopa process of Monsanto [4]. At that time, little was known about the effects of the substituents at the C=C bond and the amide nitrogen. A selective synthesis of one of the three possible enamide isomers depicted in Fig. 3 looked difficult. 

An exciting prospect in synthetic organic electrochemistry is the selective synthesis of specific optical isomers by taking advantage of the asymmetry afforded by certain types of surface sites achieved with chemically modified electrode surfaces on substrates such as carbon. 

Important information about the stereochemistry of dehydration has been obtained by studying the transformation of cyclic alcohols many of these reactions have proved suitable for the selective synthesis of alkenes [2]. The dehydration of menthol (5) and neomenthol (6) illustrates the usefulness of such processes (Scheme 3 axial hydrogens participating in water loss to form the major menthene isomers are shown) [55]. The regioselectivity observed points to an anti elimination mechanism. Isomer 6, with a trans OH/H configuration in the most stable conformation, reacts faster than compound 5. 

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