O-Isopropylidene Tartaric Acid Derivatives

Various diesters (including dimethyl, diethyl, and diisopropyl) of L-( + )-tartaric acid la-c and the corresponding D-( — )-tartaric acid 2a-c, are commercially available, or can be easily prepared. 

Acid-catalyzed reaction of lb with acetone and simultaneous azeotropic removal of water provides diethyl (27 ,3i )-2,3-0-isopropylidenetartrate 3 in 82—83% yield after distillative purification [3]. However, it has been reported that upon application of this method in petroleum ether as solvent, significant racemate formation was observed [4]. The preferred method for the preparation of 3 involves a transketalization reaction of lb with 2,2-dimeth-oxypropane under acidic catalysis. The yield is nearly quantitative with no loss of optical integrity [5]. 

Alternatively, both ketalization and esterification can be accomplished simultaneously. The reaction of 2i, 3i -tartaric acid with 2,2-dimethoxypropane under acidic catalysis provides the corresponding dimethyl ester 4 in 85—92% yield [6]. 

A salient feature of 4 is its ability to undergo monosaponification to the monoacid 5 in good yield. The presence of free acid and ester groups in the same substrate allows independent functionalization of each chiral carbon. Chiral hydroxylated cyclopentanes are of general interest as building blocks for the synthesis of cyclopentanoid natural products. Conversion of 

Radical decarboxylation of A-hydroxy-2-thiopyridone esters in the presence of an olefin results in the formation of a carbon-carbon bond. The monoester 5 is not prone to jS-elim-ination, and the faces of the dioxolane ring are encumbered by the methyl ester, so radical trapping occurs preferentially from the side opposite the methyl ester. This results in overall retention of configuration at the reacting carbon. Irradiation of ester 11 in the presence of methyl acrylate affords the tmns-eAkQm 12 after appropriate isolation. Once the first carboxylic acid has been modified, the second can be reacted similarly, whereby the stereochemistry of the first substitution controls the stereochemistry of the second. In this way the resulting product is obtained with overall double retention [11]. 

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The diversity associated with silyl protecting groups as well as the chemical conditions available for their removal makes them attractive alternatives to benzyl protection of the hydroxy groups of either D- or L-tartaric acid derivatives. O-isopropylidene-L-threitol (37) is mono-protected with er -butyldimethylsilyl chloride to furnish 266, which is converted in three steps to the nitrile 267. Reduction with DIBAL and Wittig olefination followed by desilylation with fluoride and Swern oxidation of the resulting alcohol provides aldehyde 268, which reacts with methyl 10-(triphenylphosphorane)-9-oxo-decanoate (269) to afford enone 270. Reduction of 270 with subsequent preparative TLC and acetal hydrolysis furnishes (9R)-271 and (9 S)-272, both interesting unsaturated trihydroxy Cig fatty acid metabolites isolated from vegetables [91] (Scheme 62). 

The syntheses of several dialkyl xylarates from xylaric acid, and their conversion to 2,4-0-isopropylidene acetals and thence 3-O-methacryloyl derivatives have been described. " The copper(n) catalysed oxidation of tartaric acid by hexacyanoferrate in alkaline medium has been reported. ... 

A method for the electrochemical reduction of D-xylose to 2-deoxy-D-r/rr o-pentitol has been described. The homogeneous hydrogenation of sugars using tris-triphenylphosphine ruthenium chloride is improved in the presence of hydrogen chloride, which inhibits competitive decarbonylation of the sugar. L-(2,3)-Threitol is easily prepared from ( + )tartaric acid by lithium aluminium hydride reduction of the 2,3-0-isopropylidene derivative of diethyl tartrate, followed by acid hydrolysis of the resultant ketal. ... 

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