P-Hydroxycarboxylic acid esters

Series of racemic a- and p-hydroxycarboxylic acid esters (4 and 5, Scheme 4) could be separated into their enantiomers via coordination complex formation but in these cases the acidic salt, (DBTA)2Ca had to be used as resolving agent. [23, 24]... 

A suspension of Pd-C in ethyl acetate vigorously stirred under H2 until uptake of gas ceased, a soln. of /r 5-2-azidocyclohexanol and di- ert-butyl dicarbonate in the same solvent added, and the mixture stirred at room temp, under H2 for 19 h - product. Y 71%. Pre-saturation of the catalyst is important to prevent nitrile formation. F.e. incl. N-protected a-amino-p-hydroxycarboxylic acid esters and 2-acoxyamines, s. S. Saito et al.. Tetrahedron Letters 30, 837-8 (1989). 

Dialkylborinyl triflateslethyldiisopropylamine Stereospecific aldol-type condensation with a-alkoxycarboxylic acid esters under mild conditions 5> /f-a-Alkoxy-p-hydroxycarboxylic acid esters... 

Later, Dale-followed by Kricheldorf-observed the presence of carbon-carbon double bonds at the extremities of polymers formed in the so-performed polymerization of P-lactones [39, 40]. According to experimental results [41, 42], the initiation involved a nucleophilic attack at the carbonyl carbon atom of a monomer by the alkoxide anion of the initiator, cleaving the acyl-oxygen bond to yield the corresponding potassium alkoxide of the respective P-hydroxycarboxylic acid esters (Scheme 9.12). The acidic a-proton abstraction involved the formation of an unsaturated ester due to KOH elimination. Finally, polymerization occurred in the case involving P-lactone in excess, with the KOH acting as initiator. 

The scope of this quinine ester-catalyzed ketene addition reaction is promising. Optically pure a- and p-hydroxycarboxylic acids of either configuration can be prepared with facility. 

Transformation of the a- or P-hydroxycarboxylic acids into esters eliminates the acidic character of the racemate, thus one can avoid formation of the above mentioned unsoluble calcium salts. In the same time formation of a mixed calcium salt of DBTA and the racemic acid is impossible therefore coordination bonds should entirely replace it. 

Reaction with Lactones, Hydroxycarboxylic acid ester complexes of titanium are formed by reaction of a tetraalkyl titanate with a lactone, such as p-propiolactone, y-butyrolactone, or valerolactone (35). For example,... 

Cartoni et al. [88] studied perspective of the use as stationary phases of n-nonyl- -diketonates of metals such as beryllium (m.p. 53°C), aluminium (m.p. 40°C), nickel (m.p. 48°C) and zinc (liquid at room temperature). These stationary phases show selective retention of alcohols. The retention increases from tertiary to primary alcohols. Alcohols are retained strongly on the beryllium and zinc chelates, but the greatest retention occurs on the nickel chelate. The high retention is due to the fact that the alcohols produce complexes with jS-diketonates of the above metals. Similar results were obtained with the use of di-2-ethylhexyl phosphates with zirconium, cobalt and thorium as stationary phases [89]. 6i et al. [153] used optically active copper(II) complexes as stationary phases for the separation of a-hydroxycarboxylic acid ester enantiomers. Schurig and Weber [158] used manganese(ll)—bis (3-heptafiuorobutyryl-li -camphorate) as a selective stationary phase for the resolution of racemic cycUc ethers by complexation GC. Picker and Sievers [157] proposed lanthanide metal chelates as selective complexing sorbents for GC. Suspensions of complexes in the liquid phase can also be used as stationary phases. Pecsok and Vary [90], for example, showed that suspensions of metal phthalocyanines (e.g., of iron) in a silicone fluid are able to react with volatile ligands. They were used for the separation of hexane-cyclohexane-pentanone and pentane-water-methanol mixtures. 

J. Simon, H.-P. Muller, R. Koch, V. Muller, J. Engelhardt, K. Szablikowski, and W. Koch, Water-soluble biodegradable hydroxyalkyl cellulose-2-hydroxycarboxylic acid esters which can flocculate, US Patent 6 559 300, assigned to Wolff Walsrode AG (Walsrode, DE), May 6, 2003. 

For utility of optically active /3-hydroxy esters D. Seebach, S. Roggo, and J. Zimmermann, Biological-Chemical Preparation of 3-Hydroxycarboxylic Acids and Their Use in EPC-Syntheses, in W. Bartmann and K. B. Sharpless, eds., Stereochemistry of Organic and Bioorganic Transformations, p. 85, Verlag Chemie, Weinheim, 1987. 

Titanium chelates are formed from tetraalkyl titanates or halides and bi- or polydentate ligands. One of the functional groups is usually alcoholic or enolic hydroxyl, which interchanges with an alkoxy group, RO, on titanium to liberate ROH. If the second function is hydroxyl or carboxyl, it may react similarly. Diols and polyols, CC-hydroxycarboxylic acids and oxalic acid are all examples of this type. P-Keto esters, P-diketones, and alkanolamines are also excellent chelating ligands for titanium. 

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