Lactic acid optical configuration

Acids of established configurations have been used to correlate the configurations of other oxygen-containing optically active compounds. Thus on the basis of lactic acid, the configuration of the simplest optically active alcohol, 2 butanol has been assigned as follows—... 

The ring-opening polymerization of dilactide (dimeric cyclic ester of lactic acid) allows the preparation of high molecular weight, optically active polyesters of lactic acid. The configuration of the asymmetric carbon atoms of the monomer is retained when the polymerization is initiated with SnCl4 or Et2Zn, for example ... 

Lactic acid (2-hydroxypropanoic acid) is the simplest 2-hydro)ycarbo)ylic acid with a chiral carbon atom and exists in two optically active stereoisomers, namely l and d enantiomers S and R in absolute configuration, respectively), as shown in Scheme 1.1. These l- and u-lactic acids are generally synthesized by fermentation using suitable micro-organisms. Racemic DL-lactic acid RS configuration) consisting of the equimolar mixture of D- and L-lactic acids shows characteristics different from those of the... 

Stereoisomers of lactic acid produced by lactic acid bacteria are useful for species identification. The optical configuration of lactic acid (Table 13.1) depends on the stereospecificity of the LDH. Some microorga-... 

If we convert (+)-lactic acid into its methyl ester, we can be reasonably certain that the ester will be related in configuration to the acid, because esterification should not affect the configuration about the chiral carbon atom. It happens that the methyl ester so obtained is levorotatory, so we know that (+)-lactic acid and (—)-methyl lactate have the same relative configuration at the asymmetric carbon, even if they possess opposite signs of optical rotation. However, we still do not know the absolute configuration that is, we are unable to tell which of the two possible configurations of lactic acid, 2a or 2b,... 

The absolute configuration of an enantiomer is determined only when the optical rotation of an enantiomer (+ or —) can be matched with its configuration (R or S). For example, the absolute configuration of lactic acid has been found to be R-(—) in that the R enantiomer is levorotatory. 

No relationship exists between the R and 5 prefixes that designate configuration and the (+) and (-) designations indicating optical rotation. For example, the S enantiomer of lactic acid is dex-trorotatoiy (+), whereas the S enantiomer of glyceraldehyde is levorotatory (-). 

Pasteur was unable to interpret his ideas in precise terms of molecular structure, because Kekul s theory of molecular structure, leading to structural formulae, had not been put forward at the time of Pasteur s experiments. With the advent of Kckul s theory it became possible, during the next few years, to examine the structural formulae of the tartaric and lactic acids, and of various other substances which were known to exhibit optical activity. As a result, it was found that the molecules of all these substances assumed an asymmetric form—or configuration, as it is usually called—when their flat Kekulean representations were converted into three-dimensional ones, accoiding to a very simple principle. This principle was embodied in the Theory of Molecular Configuration, which was advanced independently and almost simultaneously in 1874 by the French chemist Lc Bel, and the Dutch chemist, van t Hoff. 

Hydrolysis of L-lactide gives lactic acid with the same optical purity as hydrolysis of the polymer of L-lactide. This indicates that cationic polymerization of L-lactide proceeds with retention of configuration on both C atoms, i.e. by O-acyl cleavage 21,22) ... 

Like R and S, d and l indicate the configuration of an asymmetric carbon, but they do not indicate whether the compound rotates polarized light to the right (-h) or to the left (-) (Section 5.7). For example, o-glyceraldehyde is dextrorotatory, whereas D-lactic acid is levorotatory. In other words, optical rotation, like melting or boiling points, is a physical property of a compound, whereas R, S, d, and l are conventions humans use to indicate the configuration of a molecule. 

A modified Mitsunobu procedure in which 63 is first treated with the preformed complex 68 (prepared by reaction of triphenylphosphine and diisopropyl azodicarboxylate) and then cesium thioacetate leads to significant racemization [17]. However, if the free acid is reacted instead with an appropriate thioacid (rather than the ester and a cesium salt), optical yields improve significantly. Thus, thioacetylation of (S)-l can be accomplished by treating it with 68 followed by the addition of thioacetic acid in THF to provide in 48% yield (5)-2-(acet-ylthio)-2-phenylacetic acid (69) with 84% ee after recrystallization. The low yield is due in part to the unavoidable formation to the extent of at least 50% of a viscous, polymeric material. The reaction is complete in minutes, however, and proceeds with retention of configuration. Presumably this is a result of a double inversion mechanism that passes through an a-lactone. Interestingly, the corresponding reaction with lactic acid does occur with inversion [18]. 

The lactic acid produced by this reaction, however, is laevorotatory not dextrorotatory like the starting material, thus illustrating the above warning that there is no essential link between optical rotation and molecular configuration. 

Lactic acid is a compound that plays a key role in several biochemical processes. For instance, lactate is constantly produced and eliminated during normal metabolism and physical exercise. Lactic acid has been produced on an industrial scale since the end of the nineteenth century and is mainly used in the food industry to act, for example, as an acidity regulator, but also in cosmetics, pharmaceuticals and animal feed. It is, additionally, the monomeric precursor of PLA. It can be obtained either by carbohydrate fermentation or by common chemical synthesis. Also known as milk acid , lactic acid is the simplest hydroxyl acid with an asymmetric carbon atom and two optically active configurations, namely the L and D isomers (Fig. 21.2), which can be produced in bacterial systems, whereas mammalian organisms only produce the L isomer, which is easily assimilated during metabolism. 

This section gives a brief overview of PLA matrix (for further reading, readers should refer to Chapters 1,4 and 5). As with other commodity polymers, PLA actually refers to a large family of compounds that includes copolymers with other monomers. The monomer, that is, lactic acid (2-hydroxy propanoic acid) is the simplest hydroxy acid with an asymmetric carbon atom and exists in two optically active configurations (d and l). Generally, two major routes are followed for the synthesis of PLA, such as polycondensation... 

For example, (S)-lactic acid and (S)-sodium lactate both have an S configuration, but (S)-lactic acid is dextrorotatory whereas (S)-sodium lactate is levorotatory. When we know which direction an optically active compound rotates the plane of polarization, we can incorporate (+) or (-) into its name. 

As mentioned above, the basic building block for PLA is lactic acid, which exists in two optically active configurations L(+) lactic acid and D(-) lactic acid. Another intermediate monomer for PLA synthesis is a lactide and obtained by the depolymerization of low molecular weight PLA under reduced pressure to give a mixture of L-lactide, D-lactide, or meso-lactide as shown in Figure 13. 

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