Carboxylic hydroxycarboxylic

Besides simple alkyl-substituted sulfoxides, (a-chloroalkyl)sulfoxides have been used as reagents for diastereoselective addition reactions. Thus, a synthesis of enantiomerically pure 2-hydroxy carboxylates is based on the addition of (-)-l-[(l-chlorobutyl)sulfinyl]-4-methyl-benzene (10) to aldehydes433. The sulfoxide, optically pure with respect to the sulfoxide chirality but a mixture of diastereomers with respect to the a-sulfinyl carbon, can be readily deprotonated at — 55 °C. Subsequent addition to aldehydes afforded a mixture of the diastereomers 11A and 11B. Although the diastereoselectivity of the addition reaction is very low, the diastereomers are easily separated by flash chromatography. Thermal elimination of the sulfinyl group in refluxing xylene cleanly afforded the vinyl chlorides 12 A/12B in high chemical yield as a mixture of E- and Z-isomers. After ozonolysis in ethanol, followed by reductive workup, enantiomerically pure ethyl a-hydroxycarboxylates were obtained. 

Acetylsultam 15 is also used for stereoselective syntheses of a-unsubstituted /1-hydroxy-carboxylic acids. Thus, conversion of 15 into the silyl-A/O-ketene acetal 16 and subsequent titanium(IV) chloride mediated addition to aldehydes lead to the predominant formation of the diastereomers 17. After separation of the minor diastereomer by flash chromatography, alkaline hydrolysis delivers /f-hydroxycarboxylic acids 18, with liberation of the chiral auxiliary reagent 1919. 

No or little side reactions either with the unprotected hydroxy group present in the phenylamines, or with the unprotected hydroxyl group present in the carboxylic acids are observed. The catalytic amidation of unprotected a-hydroxycarboxylic acids also proceeds well under similar conditions. 

Besides the known activity of HRuX(PPh3)3 complexes where X is a carboxylate (/, p. 85), other complexes with X an a-hydroxycarboxylate and related bridged dimers [HRu(PPh3)3]2X have been found to effect alkene hydrogenation (124). 

Based on affinity and on distances from Mn(II) to and from Mn(II) to protons, measured in solution by nuclear magnetic resonance (NMR), Mn(II) is shown to be chelated by the a-hydroxycarboxylate group of lactate, but to coordinate only the carboxylate (not the a-carbonyl group) of pyruvate Tung etal, 1973 A. S. Mildvan, unpublished observations, 1991). 

The A -acyl derivatives of 4-substituted-3,4,5,6-tetrahydro-27/-l,3-oxazin-2-ones proved to behave as effective chiral auxiliaries in asymmetric enolate alkylations and aldol reactions, the stereoselectivities of which were found to be higher for 4-isopropyl than for 4-phenyl derivatives <2006OBC2753>. The transformations of 4-isopropyl-6,6-dimethyl-3-propa-noyl-3,4,5,6-tetrahydro-2/7-l,3-oxazin-2-one 251 to 252 or 253 proceeded with excellent diastereoselectivities (Scheme 47). 6,6-Dimethyl substitution within the oxazine ring facilitated exclusive exocyclic cleavage upon hydrolysis of the C-alkylated and the aldol products 252 and 253, to furnish a-substituted carboxylic acids 254 or a-methyl-/ -hydroxycarboxylic acids 256. 

Alkyl oxazoline-5-carboxylates 71, precursors of P-amino-a-hydroxycarboxylic acids, have been produced by iodocyclisation of alkyl 3-benzamidocatboxylates 70. The oxazolines can be resolved enzymatically <99SL1727>. The amides 72 are cyclised to N-aryloxazolium salts 73 by fluoroboric acid <99EJ0297>. 

Some aromatic hydroxycarboxylic acids, on treatment with sodium nitrite, lose carbon dioxide with the introduction of a nitroso group to replace the carboxylate group. 

Complexes with 2-hydroxycarboxylic acids, pyridine-2-carboxylate, and dicarboxylates are described in Sections II,C,6,c, II,C,4,f, and II,C,6,p, respectively. 

Similar polyacetals were prepared by BASF scientists from CO-aldehydic aliphatic carboxylic acids (189,190) and by the addition of poly(hydroxycarboxylic acid)s such as tartaric acid to divinyl ethers (191) as biodegradable detergent polymers. 

A few complexes of simple a-hydroxycarboxylates, which have known crystal structures, chelate to metal centres through ionized hydroxyl and carboxyl groups. This is the case for two complexes... 

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