Tartaric acid, chiral

The answer is that Pasteur started with a 50 50 mixture of the two chiral tartaric acid enantiomers. Such a mixture is called a racemic (ray-see-mi c) mixture, or racemate, and is denoted either by the symbol ( ) or the prefix cl,I to indicate an equal mixture of dextrorotatory and levorotatory forms. Racemic mixtures show no optical rotation because the (+) rotation from one enantiomer exactly cancels the (-) rotation from the other. Through luck, Pasteur was able to separate, or resolve, racemic tartaric acid into its (-f) and (-) enantiomers. Unfortunately, the fractional crystallization technique he used doesn t work for most racemic mixtures, so other methods are needed. 

Dialkoxydialkyl stannanes cross-couple with derivatives of chiral tartaric acid, preserving the original chirality, as shown in reaction 32. The dialkylstannylene acetals produced in this reaction are useful reagents for synthesis of chiral compounds28611. 

The multi-step grafting strategy was applied, for instance, to line the surface of MCM-41 samples with chiral tartaric acid,311 which was linked onto an... 

CAB 2, R = H, derived from monoacyloxytartaric acid and diborane is also an excellent catalyst (20 mol %) for the Mukaiyama condensation of simple enol silyl ethers of achiral ketones with various aldehydes. The reactivity of aldol-type reactions can, furthermore, be improved, without reducing the enantioselectivity, by use of 10-20 mol % of 2, R = 3,5-(CF3)2C6H3, prepared from 3,5-bis(trifluoromethyl)phenyl-boronic acid and a chiral tartaric acid derivative. The enantioselectivity could also be improved, without reducing the chemical yield, by using 20 mol % 2, R = o-PhOCgH4, prepared from o-phenoxyphenylboronic acid and chiral tartaric acid derivative. The CAB 2-catalyzed aldol process enables the formation of adducts in a highly diastereo- and enantioselective manner (up to 99 % ee) under mild reaction conditions [47a,c]. These reactions are catalytic, and the chiral source is recoverable and re-usable (Eq. 62). 

The answer is that Pasteur started with a 50 50 mixture of the two chiral tartaric acid enantiomers. Such a mixture is called a racemic (ray-see-mic) mixture, or racemate, and is denoted either by the symbol ( ) or by... 

Figure 5. Liquid crystalline supramolecular polymers developed by Lehn, based on triple hydrogen bonds from chiral, tartaric acid based monomers (10) and from rigid monomers (11).

Recently, Li et al. [43a] also reported a successful result in this reaction. A chiral tartaric acid derivative grafted onto the surface of silica and separately in the mesopores of MCM-41 material (48 and 49, respectively, in Scheme 2.20) exhibited similar activity and enantioselectivity (86% ee) compared to the homogeneous Sharpless system (83% ee) in the AE reaction of allyl alcohol. Although the product could be separated easily from the catalyst by simple filtration, no data were available for the recyclability of the heterogeneous catalyst. 

Modification of finely divided Ni (Raney Ni) by working at 100°C in aqueous chiral tartaric acid and sodium bromide produces the best heterogeneous catalytic system for asymmetric hydrogenation of acetylacetone [to (2R,4R)- or (2/ ,45)-pentane-2,4-diol],... 

The high enantioselectivity of the enediol to (S)-benzoin transformation proves that the enediol neither tautoinerizes intramolecularly nor acts as a prolon source for the enediolate. Therefore, the chiral tartaric acid proton source must be responsible for the formation of (i )-benzoin. 

Fig. 25. H-bonded main-chain polymers incorporating recognition groups 1 and 2 linked to a flexible chiral tartaric acid spacer (35, 36) or to a rigid anthracene spacer (37, 38)...

Kita and coworkers developed a catalytic asymmetric oxidation using iodoxybenzene in a cationic reversed micellar system in the presence of chiral tartaric acid derivatives. Under these conditions, sulfides 824... 

In the case of a chiral molecule present at equilibrium as a racemic mixture, its symmetry number must be divided by two to account for the entropy of mixing of the two enantiomers. For example, chiral tartaric acid belongs to the C2 point group and contains a C2 rotation axis, thus for a pure enantiomer, CT = CText = 2, but for a mixture of ( )-tartaric acid one must consider CT — <7ex(/2 — 1. 

The separation of ( )-norephedrine was first performed by RLCCC using a solvent system composed of 1,2-dichloroethane and 0.5 M aqueous sodium hexafluor-ophosphate (pH 4) containing chiral tartaric acid ester... 

SIBX is a non-explosive formulation of the X -iodane 2-iodoxybenzoic acid (IBX) stabilized by benzoic acid. This reagent combination can be used as a suspension in various organic solvents to oxidize sulfides to sulfoxides. Most yields were comparable to those obtained using IBX or other iodanes such as PhlO and Phl02. The use of a chiral tartaric acid-based source in addition to SIBX gave asymmetric sulfoxide formation with moderate enantioselectivities [81]. 

Figure 4.4 PEG-supported chiral tartaric acid ester ligands for the Ti-catalyzed asymmetric epoxidation.

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