(2-Cyclopentenyl)acetic acid (96–9%)
764CHAPTER NINETEENCarboxylic Acids
Glycerol esters are hydrolysed to glycerol and the corresponding carboxylic acids (see Figure 1). The hydrolysis is catalysed by intestinal lipase (Tietz, 1986), which attacks the ester bonds at carbons 1 and 3. The ester bond at carbon 2 is more resistant to hydrolysis, possibly because of its stereochemistry and steric hindrance. The -monoglyceride can, however, spontaneously isomerise to the -form (3-acylglycerol), permitting further hydrolysis to yield glycerol.
Cyclopentene-4-carboxylic acid (6%)
In general, aliphatic esters of propylene glycol, lactic acid, and pyruvic acid are expected to be hydrolysed to their component alcohol and carboxylic acids. The hydrolysis is catalysed by classes of enzymes recognized as carboxylesterases or esterases (Heymann, 1980), the most important of which are the B-esterases, which, in mammals, predominate in hepatocytes (Heymann, 1980; Anders, 1989). The rates of hydrolysis follow first-order kinetics, with hydrolysis of the straight-chain esters occurring approximately 100 times more rapidly than that of branched-chain esters (Butterworth et al., 1975; Longland et al., 1977; Grundschober, 1977; Leegwater & van Straten, 1979).
In resting women who received intravenous injections of [2-14C]pyruvate, analysis of blood glucose 1 h later showed 96% conversion of pyruvic acid to glucose (Hostetler et al., 1969). When [2-14C]pyruvate was incubated with liver slices from fasted normal rats, 86% had been used after 90 min of incubation. Of the radiolabel associated with metabolized pyruvic acid, 23% was associated with glycogen and glucose, 16% with CO2, and 16% with lactic acid. In the presence of glycerol, the use of pyruvic acid was increased to 95%, accompanied by a decrease in conversion to glycogen (16%) and CO2 (8.6%) and an increase in the production of lactic acid (Teng et al., 1953).
Step (c) Synthesis of 5-Methyl-2-oxohexanoic Acid
Section 19.13Among the reactions of carboxylic acids, their conversion to acyl chlorides, primary alcohols, and esters were introduced in earlier chapters and were reviewed in Table 19.5.