Esters of Hydroxy Acids

This method, analogous to the well-known Claisen condensation, can be illustrated by reference to the condensation of two molecules of ethyl benzoyloxyacetate (XLIX) under the influence of potassium  

There is obtained the hydroxytetronic acid L, the simplest analog of ascorbic acid containing the characteristic five-membered ring. Although this compound shows many of the reactions of an ascorbic acid it exhibits no antiscorbutic activity. If instead of using only ethyl benzoyl- 

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If silylation alone (0.2 ml of BSTFA + 0.05 ml of TMCS) without the preceding methoximation is carried out, TMS enol ether—TMS esters are produced from keto acids. Using the procedure described, methoxime-TMS esters of keto acids and TMS ether—TMS esters of hydroxy acids are produced. Unsubstituted acids give TMS esters. The procedure eliminates possible losses of the derivatives, which can be caused by, e.g., evaporation of the solvent between the esterification and the silylation steps, and is quantitative. SE-30, OV-17 and OV-22 can be used and retention data on these stationary phases have been reported for 15 acids [159]. An example of the separation of the derivatives of some acids prepared by this procedure is illustrated in Fig. 5.12. 

Enantiomers of carboxylic acids may sometimes be separated by GC as methyl esters, but special derivatives are mostly prepared for this purpose. Ackman et al. [188] resolved enantiomers of isoprenoid fatty acids after their conversion into L-menthyl esters. The acids under analysis were chlorinated by refluxing with distilled freshly prepared thionyl chloride and the chlorides produced were treated with L-menthol in the presence of pyridine under strictly anhydrous conditions. GC separation was carried out in a capillary column coated with butanediol succinate polyester. Annett and Stumpf [189] made use of L-menthyloxycarbonyl derivatives for the separation of enantiomers of methyl esters of hydroxy acids. The derivatization reagent, L-menthyl chloroformate, was prepared by the reaction of L-menthol with phosgene, with cooling with ice. Diastereoisomers of different hydroxy acids were thus separated on 1.5% OV-210. 

Some interesting results have recently been obtained in studies on elimination reactions of esters of hydroxy acids. The mechanism is not fully established, but probably is of the bimolecular type. An especially interesting observation is that sodium iodide promotes the removal of two vicinal sul-fonyloxy groups by a process of cfs-elimination a series of elimination reactions of this type is known in carbohydrate chemistry, but apparently does not yet include an example from which the stereochemistry of the reaction could be deduced. 

The lactones, the cyclic esters of hydroxy acids have been polymerised with various catalysts anionic catalysts, cationic catalysts and organometallic catalysts [28, 29, 39, 45, 54]. Five member lactone rings are unpolymerisable. Four, seven or eight member lactones are polymerisable [28, 29, 39, 45, 54]. 

A number of sugars have been alkylated by treatment with methyl iodide and dry silver oxide. The method is also applicable to the esters of hydroxy acids, to amides and to certain other classes of coftipounds. ... 

Lipids also influence the properties of starch. Like free amino acids, monoglycerides or fatty acid esters of hydroxy acids, lipids form inclusion compounds with amylose (cf. 4.4.4.14.3). Like di- and triglycerides, they also reduce the swelling capacity and solubility by inhibiting water diffusion. Therefore, both degreasing as well as lipid addition are of importance as physical modification methods of starches. 

Pyrrolidine derivatives of the complete series of methyl hydroxystearates and of their TMS ether derivatives have been prepared and their mass spectra described [936], The principal mode of fragmentation is enhanced and is again alpha to the carbon carrying the hydroxyl group, although the spectra of the 2-, 3- and 4-isomers are very different from the rest. Additional complications result when keto groups are present [932]. Picolinyl esters of hydroxy acids as the TMS derivatives also have very distinctive mass spectra [361]. 

Lactones are cyclic esters of hydroxy acids. Five- and stx-membered lactones commonly occur in nature. The lUPAC name of a lactone is constructed by adding lactone to the name of the related hydroxy acid. The common name is derived by changing the suffix -ic acid to —olactone. A Greek letter designates the position of the bridging oxygen atom that closes the lactone ring, a, P, y, etc. 

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