Dehydration hydroxyacid

Treatment of a- or P hydroxyacids with sulfur tetrafluoride leads to conver Sion of the carboxylic group into the tiifluoromethyl group, but the hydroxyl group undergoes either fluorination, fluorosulfination, esterification, or dehydration to form esters, ethers, or alkenes The ratio of the products depends on P substitution [209, 210] (equations 103 and 104)... 

Saponification of the diester (47) gave the hydroxyacid (48) along with its dehydration product, the 7-azabicyclo[2.2.1]heptene (49). 

Problem 16.25 jS-Hydroxyacids (n = 1) readily undergo dehydration but do not yield a lactone. Give the strueture of the product and account for its formation. 

Mestres et al. [40] published a regioselective addition of a lithium trienediolate (generated from hexa-2,4-dienoic acid or dihydropyran-2-one) to p-ionone. Dehydration of the hydroxyacid, afforded a mixture of 9E/Z, 13E/Z retinoic acids which, isomerised in the presence of I2, led to all E retinoic acid in 35% and 30% yield, starting from dienic acid and pyranone, respectively, Fig. (16). 

A closely related E. coli protein is a 79-kDa multifunctional enzyme that catalyzes four different reactions of fatty acid oxidation (Chapter 17). The amino-terminal region contains the enoyl hydratase activity.32 A quite different enzyme catalyzes dehydration of thioesters of (3-hydroxyacids such as 3-hydroxydecanoyl-acyl carrier protein (see Eq. 21-2) to both form and isomerize enoyl-ACP derivatives during synthesis of unsaturated fatty acids by E. coli. Again, a glutamate side chain is the catalytic base but an imidazole group of histidine has also been implicated.33 This enzyme is inhibited irreversibly by the N-acetylcysteamine thioester of 3-decynoic acids (Eq. 13-8). This was one of the first enzyme-activated inhibitors to be studied.34... 

An example of such side reactions occurs during the attempted self-polymerization of Qt-hydroxyacids like lactic acid (5-1) which dehydrates readily at temperatures near 250"C. 

During aging of PHVB in sterile water at pH 7 and 60 °C, 2-butenoic acid (crotonic acid), 2-pentenoic acid, 3-hydroxybutyric acid, 3-hydroxyvaleric acid, 3-hydroxybutyrate dimer, 3-hydroxybutyrate-3-hydroxyvalerate dimer and 3-hydroxyvalerate dimer were formed [99]. The weight loss was, however, only 2% after 200 days at 60 °C. Monomers, oligomers and derivatives, produced by dehydration at the OH-terminus were identified after alkaline hydrolysis of PHB [110]. In accordance CZE showed that the accelerated hydrolysis of PHB leads to the formation of hydroxyacid oligomers and a series of peaks formed by a side reaction leading to a C=C bond at the noncarboxylic acid end [111]. Kinetics of the abiotic hydrolysis of PHB in acid and alkaline media were monitored by following the forma-... 

These aliphatic polyesters can be obtained by catalyzed dehydration of hydroxyacids and, more efficiently, by ring opening polymerization of the cyclic esters of hydroxyacids (equations (1) and (2)). Catalysts are generally used to facilitate the polymerization. Among the effective catalysts are Lewis acids in the form of metal salts of Sn, Zn, Ti, Al, and rare earth metals [17-23] alkali metal alkoxides and super-molecular complexes [20,24,25] and acids [26]. 

This type of reaction was first applied to the preparation of polyamides, as discussed above, for which it is somewhat more effective in forming high molecular weight polymers, but it has now been used for the synthesis of a wide variety of aromatic polyesters, either by the self-condensation of hydroxyacids or by the co-condensation of dicarboxylic acids and difunctional phenols.A fairly wide variety of phosphorous compounds can be used as reducing or dehydrating agents in these reactions, in addition to phosphines, including phosphites, chlorophosphates, phosphates, polyphosphates and phosphazenes. In most cases, lithium chloride is added and the reaction is run in either pyridine or an amide solvent system. The reaction has also been found to be catalyzed by tertiary amine salts. 

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