Primary esters

Alternatively, secondary and tertiary carboxylic acid methyl or ethyl esters react (6) with two equivalents of trimethylsilylmethy] lithium (prepared in pentane) to give /1-ketosilanesingood (80-96%) yield. Primary esters also give /3-ketosilanes, but in lower (45%) yield. 

In practice mixtures of primary and secondary esters are always obtained. A strong influence on the forming of primary esters is exerted by water in the reaction mixture which can come from traces of water in the alcohol as well as from P4O10. Long-chain alcohol also leads to a higher proportion of monoalkyl esters. 

These findings hold for the primary acetates. The behavior of the secondary esters is more complicated because these esters tend to hydrolyze more rapidly than primary esters, in general, the rearrangement occurred also. 

Lipase catalysed hydrolysis of racemic esters of the important chiron solketal , 1, 2-0-isopropyhdene glycerol, are not very stereospecific due to the fact that they are primary esters. Secondary esters usually show much higher -values. Table 2.1 shows -... 

From the Fischer rate study, it appears that primaiy ester-substituted radicals are not electrophilic but ambiphilic and the borderline between ambiphilic and electrophilic radicals is not at all clear. Consider our results68 (Scheme 16) on the atom transfer additions of ester-substituted radicals to alkynes (with the caution that it may be dangerous to compare yields in place of rate constants). The primary ester-substituted radical adds more efficiently to 1-heptyne but the tertiary ester-substituted radical prefers ethyl propiolate. 

LPL exhibits no fatty acid specificity during hydrolysis of mixed triglycerides but does have strong positional specificity (Morley and Kuksis, 1977). It acts on primary ester bonds with some preference for the sn-1 over the sn-3 position of triglycerides (Somerharju et al., 1978) and can hydrolyze 2-monoglycerides only after their conversion to the sn-1 or sn-3 isomers (Nilsson-Ehle et al., 1973). It shows phospholipase Ai activity on phosphatidyl choline (i.e., it hydrolyzes the primary ester bond at the sn-1 position). This contrasts with most phospholipases A, which exhibit A2 activitiy. 

Lipases can be divided into those that have a positional specificity and those that do not. The former preferentially hydrolyze the ester bonds of the primary ester positions. This results in the formation of mono- and diglycerides, as represented by the following reaction ... 

Methoxycarbonylation-methylation of esters. Reaction of a primary ester with dimethyl carbonate in the presence of sodium hydride results in a-methylation as well as methoxycarbonylation. 

Winstein showed that the solvolysis of crotylmercury(II) acetate under kineticalfy contrtdled conditions gives >99.5% of a-methylallyl acetate (equation 13)." Subsequent work indicated that both the solvolysis of cinnamylmercury(n) acetate and the mercuiy(II) acetate oxidation of allylbenzene give ca. 60% ciimamyl acetate (35) and 40% a-phenylallyl acetate (36 equation 14)." An equilibrium exists between (35) and (36) favoring the primary ester which constitutes >99.5% of the equilibrium mixture at 75 C. Oxidation of a range of bodi 1- and 2-alkenes under kinetically controlled conditions exclusively gave the secondary allylic esters. 

Lipases can be divided into two groups according to their positional specificity. Some lipases hydrolyze only the ester bonds in positions I and 3 of glycerol, i.e., the primary esters. This is true for pancreatic lipase (Table II). Other lipases, such as many microbial lipases, hydrolyze all three ester bonds. In this case, the primary esters are probably hydrolyzed faster than the secondary ester. 

The only known lipase that is nonspecific is the enzyme of a mold. Geotrichum candidum. It has a decided preference for fatty acids with a cis-A9 double bond such as oleic acid 14). There is no explanation for this specificity. The lipases of most other microorganisms are very similar to pancreatic lipase. For example, the lipase of the mold Rhizopus arrhizus is an exoenzyme which contains carbohydrates that are not essential for activity. It is not inhibited by DFP, it is specific for primary ester groups, and it does not distinguish between diflEerent fatty acids (15). Other microbial lipases are similar in these respects but may diflFer in their recognition of steric hindrance. The lipase of Stapyhlococcus aureus... 

The milk lipase that is activated by foaming and causes the rancidity of milk is a glycoprotein or a family of glycoproteins. It is inhibited by DFP and is specific for primary ester bonds (14), The physiological function of the lipase is mysterious since new-born animals already possess their own digestive lipases. Milk also contains a lipoprotein lipase which has the properties typical for such an enzyme it is sensitive to heparin and activated by serum proteins. This enzyme is probably serum lipoprotein lipase that has leaked into the milk (14). 

Sucrose polyesters have no primary ester bonds and are not digested. As a result, they remain in the oil phase and are not taken up and are excreted with the stools. In the small intestine, they have some effect on the partitioning of fat-soluble components between the emulsion and micellar phase and as a consequence on their absorption. Reduced absorption of fat-soluble vitamins can be avoided by enriching the sucrose polyesters with these vitamins. Their main use is related to the fact that they can replace usual food fats in many prepared foods but that they do not provide for calories. 

Group or atom substitutions for hydrogens at the C-1, C-a, and C-)S positions all effect reaction rates. However, the rates are most sensitive to substitutions at the C-a position. This is readily apparent from an examination of the rate coefiBcients in Table 1 (Column 3). With the exception of the 8-phenyl acetate esters, all primary esters have rate coefficients in the range of log k (600 °K) = —5.0 0.5. Note also that corrected activation energies are in the range of (primary) =... 

Selective deacylations of primary ester groups in the presence of secondary are also possible using enzymes (i.e., various lipases) [40]. Other lipases show selectivity for the anomeric position (Scheme 3.22). Also, as mentioned above, anomeric esters are more labile than other esters and can be removed selectively by mild base treatment. Furthermore, anomeric silyl ethers can be removed selectively on treatment with mild acid. 

One final example of acylation of alkynes by lactones forms part of the synthesis of neomethynolide by Yamaguchi. The Prelog-Djerassi lactone serves as the acylating agent (equation 49). The functionalized alkynide undergoes addition very selectively at the lactonic carbonyl group, despite the presence of a relatively unhindered primary ester. ... 

Benzoylation of 1,2-diol (90) by the conventional method (BzCl, Py) and subsequent silylation affords primary ester (92) in preference to the secondary ester (93) in a ratio of 96 4 (81% yield). A dramatic reversal of chemoselectivity is observed when (90) is first converted into dioxastannolane (91) followed by treatment with benzoyl chloride and work-up, giving (92) and (93) in a ratio of 5 95 (Scheme 36). ... 

It also follows that primary ester enolates should cyclize preferentially to secondary ester enolates, as is illustrated in the reaction of (19). The conformational requirement of the ring closure gives a single diastereomer (20a and 20b) from each diastereomeric ester (19a and 19b Scheme 21). 

The behaviour of polyalkylacrylates on degradation is strongly dependent on the nature of the ester group primary esters are different in... 

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