Esterification primary positions

It is well known that the nucleophilic displacement reactions at tosylated polysaccharides are limited or at least mainly directed towards the primary positions . Therefore, our interest was focused on 6-0-tosyl starch samples with DStos 1- One suitable synthesis path is the protection of 0-2 and the subsequent tosylation. A useful protecting group may be the acetyl ester function. It was recently found that in contrast to conventional esterification processes of starch with acetic anhydride, which leads to a statistic distribution of the ester groups, an acetylation of starch dissolved in DMSO with acetic acid vinyl ester in the presence of sodium chloride yields 2-0-acetyl starch of varying DSac from 0.1 to 1.0. The functionalisation patterns of these new starch products were unambiguously proved by means of various NMR measurements including two dimensional methods . 

Monitoring the selectivity of sucrose esterifications is a real challenge. " For the purpose of preparing small analytical samples of sucrose esters with high selectivity at the primary positions, the sensitivity of 0-6 and 6 to steric hindrance can be exploited in Mitsunobu esterification conditions. Of course, this method is not acceptable for large-scale synthesis of commercial sucrose esters, but... 

DiisononylPhthalate andDiisodeeylPhthalate. These primary plasticizers are produced by esterification of 0x0 alcohols of carbon chain length nine and ten. The 0x0 alcohols are produced through the carbonylation of alkenes (olefins). The carbonylation process (eq. 3) adds a carbon unit to an alkene chain by reaction with carbon monoxide and hydrogen with heat, pressure, and catalyst. In this way a Cg alkene is carbonylated to yield a alcohol a alkene is carbonylated to produce a C q alcohol. Due to the distribution of the C=C double bond ia the alkene and the varyiag effectiveness of certain catalysts, the position of the added carbon atom can vary and an isomer distribution is generally created ia such a reaction the nature of this distribution depends on the reaction conditions. Consequendy these alcohols are termed iso-alcohols and the subsequent phthalates iso-phthalates, an unfortunate designation ia view of possible confusion with esters of isophthaUc acid. 

Another practical limitation of esterification reactions is steric hindrance. If either the acid or the alcohol participants possesses highly branched groups, the positions of equilibrium are less favorable and the rates of esterification are slow. In general, the ease of esterification for alcohols, ROH, by the mechanism described is primary R > secondary R > tertiary R with a given carboxylic acid. 

Reduction of 202a to the lactol and a subsequent Wittig reaction led to 203 in 52% yield. Protecting-group exchange and tosylation of the resulting primary alcohol provided the cyclization precursor 204 (76%). Base treatment then afforded an 87% yield of cyclopentanes 205 epimeric at the nitrile (an epimerizable position). Hydrolysis and esterification of either isomer led to the trans geminally substituted cyclopentane 206 in 82% yield. Reduction of 206 and protection of the ester produced benzyl ether 207, which was converted to alkyne 208 via a bromination-dehydrobromination sequence in 78% yield. 

The bile-salt-dependent lipase of pancreatic juice has many names such as cholesterol esterase, nonspecific lipase, the most rational being carboxyl ester lipase [27], In the case of water-insoluble substrates this enzyme has an absolute requirement for bile salts specifically having hydroxyl groups in the 3a and la positions [28.29]. The best documented role for this enzyme is to allow the absorption of dietary cholesterol, through hydrolysis of cholesterol esters in the lumen. The enzyme also catalyzes the esterification of cholesterol and a role for it has been proposed in cholesterol absorption [30]. In addition, a wide range of primary and secondary fatty acyl esters including glycerides, vitamin A and E esters are hydrolyzed by this enzyme. 

The hydroxylation of the terminal position of the ethyl group on ring B is a most remarkable modification among the polar groups introduced in NCCs. As noted above, the observed primary alcohol function represents a suitable function for further secondary conjugations with hydrophilic moieties (see Scheme 13), which possibly are required for the purpose of intra-organellar transport to the vacuoles (3, 98). Esterification and glucosylation (as first seen in 24a and 24b) (48, 94, 95) are reminiscent of many secondary plant metabolites (99) which are, like NCCs, deposited in the vacuoles (3, 98, 100). 

Esterification rates indicated that the hydroxyl groups are primary and secondary. The postulation of a primary hydroxyl limits its position to the side-chain 1-methyl group. 

Chemical modification of polymer-bound active ester groups is also subject to strong solvent effects. In copolyfAOTcp-styrere), both aminolysis and transesterification with primary alcohols are positively influenced by solvents in the order of dimethylformamide (DMF) > dioxan > diloroform > chlorobenzene > dimethylsulfoxide (DMSO). However, trans-esterification with phenols proceeds in dioxan, but not in DMF. The last-nan d solvent effect is probably related to inactivation of the phenolate ion in DMF, as observed ako for the acylation of polymer-bound phenolic groups by soluble trichlorophenyl esters [64]. 

The primary site of esterification of sucrose with fatty acid methyl esters is the sixth position on the glucose unit of the sucrose molecule. Commercial products contain, however, a mixture of sucrose mono-, di- and tri-esters. 

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