Transesterification preferential

Mixed phosphoric acid diesters via transesterification Preferential O-phosphorylation... 

The TSIL used in this study has an ester moiety in its structure, which enables it to react in enzyme-catalyzed transesterification reaction. In the first part of the cycle, one of the enantiomers of the racemic alcohol is preferentially transformed into an ester of the IL-coupled acid. The other, unreacted enantiomer of the alcohol is then extracted, while the newly formed IL ester is treated with an excess of ethanol in the presence of the same enzyme. This process is accompanied by the regeneration of TSIL in the active form. The main advantage of the presented... 

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. 

Fenvalerate[(RS)-a-cyano-3-phenoxybenzyl (RS)-2-(4-chlorophenyl)iaovalerate] consists of four optical isomers due to the presence of two chiral carbons in the acid and alcohol moieties. Of the four isomers, one specific isomer ([2R,CXS]) was preferentially metabolized in mammals including rats and mice to a cholesterol ester which was formed by condensation of the acid moity with cholesterol. This conjugate does not seem to be produced via three known endogenous biosynthesis routes of cholesterol esters, but via transesterification mediated by microsomal... 

In transesterification it is difficult to achieve high conversion because of the reversibility of the reaction. This drawback is overcome by employing enol esters such as vinyl and isopropenyl acetates. On transesterification these enol esters are converted to acetaldehyde and acetone, respectively, and removed from the equilibrium enabling irreversible acylation (Scheme 12.185) [307a, 338]. This procedure enables preferential acylation of primary alcohols to secondary alcohols and phenols. 

Much of the current interest in making simple derivatives of (+ )-castanospermine (239) can be traced to a seminal publication in 1989, which showed that the alkaloid s anti-HIV activity could be increased by as much as twenty times upon esterification (216). Positionally selective acylation procedures usually involve sequential protection, acylation, and deprotection steps e.g. the preparation of esters at the C-6 and C-7 (217) or the C-8 hydroxy groups (218). Also of interest are procedures that take advantage of enzyme-catalyzed transesterification with activated esters, e.g. the use of subtilisin for ester formation at C-1, pancreatic porcine lipase for preferential reaction at C-6 and C-7 (219-221), and cross-linked enzyme crystals (CLECs) of subtilisin for making the potentially valuable antitumor agent 1-0-butanoylcastanospermine (222). A cautionary note was sounded, however, when it was observed that 6-0-acyl castanospermine esters could equilibrate to a mixture of... 

Methyl esters of soya, rapeseed and palm oil fatty acids used as biodiesel (Chapters 7 and 8) are preferentially manufactured by transesterification." The acid catalyzed esterification and the alcoholysis of triacylglycerides apply also for the manufacture of higher alcohol monoesters as well as for fatty acid polyol esters. Due to their oily consistency and to their solubilizing properties fatty acid esters of low Mw alcohols are components in odorous substances like perfumes, and cosmetic and drug formulations such as ointments, creams or lotions. 

Actually, the H SO., reacts faster than the acetic anhydride and cellulose sulfete (CellOSOjH) is formed initially (20). Transesterification then occurs resulting in an acetyl substituent. Malm and Tanghe (21) showed sulphation occurs preferentially at the primary hydroxyl group. 

Preferential esterification. Startg. tricarboxylic acid and Amberlyst 15 in methanol stirred at room temp, for 10 h monomethyl ester. Y 80%. a,p-Unsatd. and ar. carboxyl groups were left unaffected the method failed with higher alcohols (/-PrOH /-BuOH). Under these mild conditions, a-epimerization, c/5-tra 5-rearrangement, and transesterification (of ethyl esters) are avoided, and keto and olefinic groups remain intact. F.e. inch methyl esters of bile acids s. M. Petrini et al., Synth. Commun. 18, 847-53 (1988). 

The lipase-catalysed resolution of alcohols or acids relies on the preferential reaction of one enantiomer (Scheme 4.3). The racemic alcohol (a) or add (b) can undergo transesterification with either an acylating agent or alcohol respectively. In this way, the products of the reaction can be easily separated by chromatography or re-crystallization. 

Sucrose contains eight hydroxyl groups three are primary (C-1, 6 and 6 ), and the remaining are secondary. In transesterification reactions, C-6 and 6 react preferentially. The selective substitution of hydroxyl groups in sucrose is difficult to achieve. 

The 3-(ferf-butyl)-4-hydroxyphenyl motif can likewise be incorporated in extended polyol assemblies. For example, Hostanox 03 ethane-1,2-diyl-bis(3,3-bis(3-(tert-butyl)-4-hydroxyphenyl)butanoate), a product of Clariant, is preferentially used as an antioxidant for plastics with high resistance against hydrolysis. It can be produced on a large scale by the reaction of p-tert-h xXy phenol with methyl acetoacetate in the presence of an alkyl mercaptan and final transesterification with ethylene glycol catalyzed by Bu2SnO (Scheme 2.72) [42]. 

Potassiumitert-butanol-molecular sieve Dicarboxylic acid monoesters via mixed dicarboxylic acid esters Preferential transesterification... 

Molecular sieves have been used in the selective transesterification of the diester (5), when the ejfo-ester reacts preferentially (Scheme 25). 

Sucrose was actylated at 0-1 preferentially by subtilisin-catatysed transesterification from the trifluoro- or trichloro- ethyl esters of acetic, propionic, octanoic, benzoic, and acrylic acid in DMF. Subsequent hydrolysis of the yoosidic bond with a-glucosidase gave D-fructose l-esters. ... 

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