Esterification preferential

Examples of polyfunctional carboxylic acids esterified by this method are shown in Table I. Yields are uniformly high, with the exception of those cases (maleic and fumaric acids) where some of the product appears to be lost during work-up as a result of water solubility. Even with carboxylic acids containing a second functional group (e.g., amide, nitrile) which can readily react with the oxonium salt, the more nucleophilic carboxylate anion is preferentially alkylated. The examples described in detail above illustrate the esterification of an acid containing a labile acetoxy group, which would not survive other procedures such as the traditional Fischer esterification. 

Acylation of jS-D-glucose with azolides yields preferentially esterification in the 1-position of which for example, with R = C6H5 60% and with R=Ci7H35 71% yield are obtained. Azolides of sterically crowded acids [R = (C6H5)2CH or (CH3)3C] give poorer yields ( 28%).[193]... 

N.m.r. spectra support preferential protonation of the carbonyl oxygen of the acid (173) in the forward reaction (esterification), and... 

LCAT acts preferentially on lipids transported by HDL (so-called a-LCAT activity), but also on lipids transported by apoB-containing lipoproteins (so-called jS-LCAT activity) [58, 85]. In practice, LCAT activity is measured either as the activity required to esterify radioactive cholesterol that has been exogenously incorporated into native HDL or into artificial HDL-like particles (a-LCAT activity) or which has been equilibrated with endogenous lipoproteins of the plasma sample (cholesterol esterification rate, CER) [21, 58, 85]. Several variations of these assays have been reported, some of which are available as commercial test kits (e.g., Roar Biomedical, New York, USA). In addition, LCAT concentration can be determined by either laboratory-made tests or by a commercial ELISA kits [57]. However, the decrease in LCAT concentration is difficult to judge since it also decreases secondary to HDL deficiency due to causes other than LCAT deficiency. Plasma from patients with LCAT deficiency fails to esterify radioactive cholesterol provided by any substrate. By contrast, plasmas of patients with fish-eye disease show a near-normal cholesterol ester-fication rate but have a selective inability to esterify radioactive cholesterol provided to plasma with native HDL or reconstituted HDL (a-LCAT activity) [58, 85]. 

The first stage of the reaction involves preferential esterification of the primary hydroxyl groups with the anhydride to give... 

Anderson, Goodman, and Baker employed an O-(methoxycarbonyl) protecting group in an analogous manner for their synthesis of methyl 2,3-anhydro-D-ribofuranoside (XLV). Treatment of 1,2-0-isopropylidene-D-xylofuranose (XLI) with methyl chloroformate in pyridine gave, by preferential esterification of the least-hindered alcohol group (at C5), predominantly 1,2-0-isopropylidene-5-0-(methoxycarbonyl)-D-xylofuranose (XLII) a small proportion of the 3,5-di-0-(methoxycarbonyl) derivative was separated by fractional recrystallization. Subsequently, the C3-hydroxyl was esterified with tosyl chloride, and the resulting 3-0-tosyl ester XLIII... 

We next carried out selective esterification of two substrates in this reaction system. When a 1 1 mixture of lauric acid and acetic acid was esterified with dodecanol in the presence of DBSA under neat conditions at 40°C for 48 h, the laurate ester and the acetate ester were obtained in 63% and 35% yields, respectively (Table 13.7, entry 1). On the other hand, when the same reaction was conducted in water, the laurate ester was predominantly obtained in 81% yield, and the yield of the acetate was only 4% (entry 2). Similar selective esterification of lauric acid over acetic acid was also observed in the reaction of another alcohol (entry 4), Furthermore, even cyclohex-anecarboxylic acid, which is an a-disubstituted acid, was preferentially esterified in the presence of acetic acid (entries 5 and 6). These selectivities are attributed to the hydrophobic nature of lauric acid and cyclohexanecarboxylic acid as well as to the high hydrophilicity of acetic acid. These unique selectivities became possible by using water as a solvent. Selective esterification based on the difference in hydrophobicity was also attained in the reaction of two alcohols, one of which is hydrophobic and the other water-soluble. 

Philipp et al. [177,178] studied the esterification of cellulose dissolved in N2O4-DMF solution, which is known to form nitrite derivatives preferentially at the C6 position. Sulfation was shown to occur mainly at the 6-OH group presumably as a result of a transesterification reaction. Such a process, however, did not occur in phosphorylation, which appeared to take place directly on the 2-OH and 3-OH groups. 

Ueda et al. (37) have proposed magnesium oxide catalyst modified with a transition metal ion (M/MgO) for the vinylation of methyl propionate and acetonitrile. Acetonitrile is vinylated to acrylonitrile selectively (94% selectivity at about 10% conversion) over Cr/MgO catalysts at 350 C in the absence of oxygen. The selectivity for the vinylation of methyl propionate over Mn/MgO catalysts is not different from the value obtained with Ti -TSM in the presence of oxygen. The catalyst system, however, is not effective for the reaction of acetic acid. We conducted the reaction of acetonitrile and methanol over Ti -TSM in the presence of oxygen, and found that the vinylation does not take place but the hydrolysis to acetic acid and subsequent esterification with methanol into methyl acetate proceed preferentially. It is likely that Ti -TSM is an appropriate catalyst for the vinylation of carbonyl compounds and M/MgO is appropriate for the vinylation of nitriles. 

The reaction of small amounts of an alcohol with an NCA results in initiation of polymerization and even with large amounts of alcohol high yields of esters are not obtained, since the amine formed from esterification is nucleophilic and reacts preferentially with another NCA. Thus the preparation of esters from NCAs requires at least one equivalent of HCl. High yields of esters are obtained from the reaction of an alcohol and an NCA under anhydrous conditions and saturation with... 

In general, most of the high-separation factors reported for zeolite membranes are associated with pervaporation processes (see Section 10.5) or with vapor-separation applications where the permeated component is preferentially adsorbed. This has given rise to a variety of works in which the membranes have been used for equilibrium displacement by selective product permeation. The largest group probably corresponds to esterification processes, where hydrophilic zeolite membranes are employed to remove the product, water, replacing the extensively studied polymer membranes [187-192]. 

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