Hydroxycarboxylic acids esters

A highly diastereofacial /7-selective aldol reaction by using ester derivatives of norephedrine as a chiral auxiliary has been recently reported by Kurosu and Lorca (Scheme 38).138 This practical and general method proceeds via initial ( )-selective substrate enolization and provides access to a broad range of optically active 2-alkyl-3-hydroxycarboxylic acid esters of type 91. 

This procedure was improved 123 for the synthesis of building units based on amino acids other than Gly, but with nonfunctionalized side chains (Table 6). To suppress the 3-elim-ination and racemization side reactions, triflates of a-hydroxycarboxylic acid esters 124 34 (L = OTf, Scheme 19) were used as substrates for the nucleophilic substitution. In order to prevent polyalkylation, the nucleophilic amine of to-BocNH- or co-tBu02C-alkylamines 33 were temporarily protected with the benzyl group. 115116 This protection also improved the yields and purity of Gly-based building units. In this case commercially available benzyl bromoacetate 34 (L=Br) was used as the substrate. In both cases the nucleophilic sub-... 

Toda, F., Sano, A., Nassimbeni, L. R., and Niven, M. L. (1991) Optical Resolution of Amino Acid and Hydroxycarboxylic Acid Esters by Complexation with Optically Active Host Compounds a Crystallographic Result, J. Chem. Soc., Perkin Trans. 2, 1971-1975. 

Key words optical resolution, diastereoisomeric complexes, 0,0 -dibenzoyltartaric acid, hydroxycarboxylic acid esters, hydroxycarboxylic acids, chiral phosphine oxides, racemic alcohols. 

Actually, coordination complexes of different metal salts of DBTA with hydroxycarboxylic acid esters, hydroxycarboxylic acids and alcohols as well as host-guest complexes of DBTA with chiral phosphine oxides and racemic alcohols can be prepared and used for separation of optical isomers. In the next subchapters theoretical and practical aspects of these recent resolution processes are summarised. 

Combination of the coordinating ability of metal ions for hydroxy group containing compounds with the before mentioned exceptional behaviours of DBTA in chiral recognation processes gave us a new possibility for direct resolution of a-alkoxycarboxylic acids, a-hydroxycarboxylic acid esters and series of racemic alcohols... 

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]... 

TABLE -1. Resolution of hydroxycarboxylic acid esters 4 and 5 with (DBTA)2Ca... 

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,... 

This procedure offers a convenient method for the esterification of carboxylic acids with alcohols2 and thiols2 under mild conditions. Its success depends on the high efficiency of 4-dialkylaminopyridines as nucleophilic catalysts 1n group transfer reactions. The esterification proceeds without the need of a preformed, activated carboxylic acid derivative, at room temperature, under nonacidic, mildly basic conditions. In addition to dichloromethane other aprotic solvents of comparable polarity such as diethyl ether, tetrahydrofuran, and acetonitrile can be used. The reaction can be applied to a wide variety of acids and alcohols, including polyols,2 6 a-hydroxycarboxylic acid esters,7 and even very acid labile... 

The reaction of a-hydroxycarboxylic acid esters 459 with two equivalents of carbodiimides in the presence of cupric chloride affords a mixture of an O-alkylisourea 460 and... 

Hydroxycarboxylic acid esters from cyclic ketones... 

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. 

Hydrogen chloride magnesium sulfate Hydroxycarboxylic acid esters from hydroxycarboxylic acids... 

Hydroxycarboxylic acid esters from ketones CO -> C(OH)-C-COOR... 

Oxo compds. from / -hydroxycarboxylic acid esters C(OH) C COOR CO... 

Metalalion of diazo compds. at low temp. a-Diazo-) -hydroxycarboxylic acid esters from 0X0 compds. 

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. 

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