Cyclohexane, transesterification

Excess diene and other volatiles were removed in vacuo and the supported cy-doadducts washed with cyclohexane. Transesterification with MeOH or EtOH... 

A monoacetate can be isolated by continuous extraction with organic solvents such as cyclohexane/CCI4. Monoacylation can also be achieved by ion exchange resin or acid-catalyzed transesterification. 

When 18-crown-6 was co-lyophilized with a-chymotrypsin, a 470-fold activation was seen over the free enzyme in the transesterification of APEE with 1-propanol in cyclohexane (Scheme 3.2) [96]. There was a low apparent specificity for the size and macrocyclic nature of the crown ether additives, suggesting that, during lyophilization, 18-crown-6 protects the overall native conformation and acts as a lyoprotectant. To examine this global effect, FTIR was used to examine the effect of crown ethers on the secondary structure of enzymes. In one study [98], subtilisin Carlsberg was shown to retain its secondary structure in 1,4-dioxane when lyophi-lized in a 1 1 ratio with 18-crown-6. In addition, examination of FTIR spectra from varying incubation temperatures indicated that an increase in crown ether content in the final enzyme preparation resulted in a decreased denaturation temperature in the solvent, indicating a more flexible protein structure. 

A more general scheme with both reactants adsorbed is that represented by scheme (g). It was considered probable in the liquid phase transesterification in a non-polar solvent (cyclohexane) [435] and it may correspond to all vapour phase esterifications and transesterifications where the rate equations from the kinetic analysis (see eqns. (27) and (29) suggested the involvement of dual- or triple-sites. 

Higher alkoxides, such as tetra(2-ethylhexyl) titanate, TYZOR TOT [1070-10-6]9 can be prepared by alcohol interchange (transesterification) in a solvent, such as benzene or cyclohexane, to form a volatile azeotrope with the displaced alcohol, or by a solvent-free process involving vacuum removal of the more volatile displaced alcohol. The affinity of an alcohol for titanium decreases in the order primary > secondary > tertiary, and branched > unbranched. Exchange processes are more convenient than direct synthesis of tetraalkoxide from TiCl an alcohol, and a base because a metal chloride need not be handled. However, in general, traces of impurities of mixed tetraalkyl titanates can result. 

With most of the R groups, the above reactions can be pushed to completion if the liberated isopropanol or acetate is removed azeotropically with a solvent such as benzene or cyclohexane. Alternatively, the reactions lead to the isolation of mixed alkoxides, many of which can be distilled without apparent disproportionation (6). For the replacement of isopropoxide by rcrr-butoxide groups, transesterification with rerf-butyl acetate is generally preferred (6) because of a larger difference in the boiling points of ferf-butyl and isopropyl acetate. 

Kodel polyester is formed by the transesterification of dimethyl terephthalate with 1,4-di(hydroxymethyl)cyclohexane. The stiff polyester chain causes the fiber to have a harsh feel that can be softened by blending it with wool or cotton. 

Based on a domino Knoevenagel/cne reaction, Tietze and Steinmetz developed a stereoselective solid-phase synthesis of cyclopentane and cyclohexane derivatives of type 326 and 327 using a Merrifield resin modiiled with a propandiol linker 320 as shown in Scheme 4.6.3. Subsequent reaction with monomethyl malonoyl chloride 321 afforded the polymer-bound malonate 322, which, in a two-component domino reaction was treated with unsaturated aldehydes 323 in the presence of a catalytic amount of piperidinium acetate and zinc chloride. Except for a-substituted aldehydes, the initial Knoevenagel condensation occurred without addition of dehydrating agents and the subsequent intramolecular ene reaction gave cyclopentane and cyclohexane derivatives 325 after cleavage firom the resin either by reduction or transesterification. 

A polyester from terephthalic acid and 1,2-dimethylol cyclohexane is produced mainly for use in fibers. This polymer is also formed from dimethyl terephthalate and the diol by a transesterification reaction. The material has the following structure ... 

The utihty of transesterification reactions has been extended considerably by the use of an inert solvent like cyclohexane (b.p. 80.8°C) which forms a convenient azeotrope with ethyl acetate (b.p. 72.8°C) and isopropyl acetate (b.p. 78.9°C). Using this modified technique, it is now possible to prepare mixed alkoxides also by taking the reactants in the desired stoichiometric ratios ... 

The fourth method is favourable in comparison with the previous one as it allows to obtain not only alkoxides but also trialkylsiloxy derivatives since in this case there is no need to use unstable trialkylsilanols. In other respects both these routes are similar. The transesterification is carried out in cyclohexane with continuous distilling of the formed isopropyl acetate as an azeotrope with cyclohexane. 

The list of cyclic alcohols that are substrates for enantioselective lipase-catalyzed transesterification is very long. Among these are many cyclohexane derivatives [219-226]. Scheme 48 presents only selected examples of this class of substrate. Note that the ethynyl cyclohexenol that is enzymatically [222] resolved is a chiral intermediate for the total synthesis of vitamin D. 

We previously mentioned the importance of determining the appropriate solvent for acyl transfer reactions (esterification and transesterification). Nevertheless, it is difficult to select a universal solvent for the esterification of (R,S) 2-arylpropionic acids. In fact, hydrophobic solvents such as cyclohexane [98,102], isooctane [97], or the mixtures isooctane/ChC or isooctane/toluene [100] are recommended for the highly hydrophobic substrates naproxen and ibuprofen (see Tables 6 and 7). On the contrary, moderately hydrophilic acids such as ketoprofen (Table 5 [92]) or flurbiprofen (Table 8 [111]) are better esterified in sUghtly hydrophilic solvents such as cffisopropyl ether, methylwobutyl ketone, or 1,4-dioxane. Iherefore, we can conclude that depending on the hydrophobicity of the substrate we must select the organic solvent in order to obtain the best catalytic performance. 

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