Lactic acid, stereoisomers

Both L- and o-lactic acid stereoisomers are naturally occurring however, most of the lactic acid in nature is L-type or sometimes racemic. The fact that lactic acid that is produced in the human body is in the L-enantiomeric form and the interest in the biomedical applications of this polymer have led both research and production to concentrate on L-lactide or OL-lactide polymers [16-18]. The o-isomer does not have many applications, except for use in particular medicinal chemicals. 

Caimcross et al. [16] studied moisture sorption and transport in three different PLA films (high percentage l-lactide, a mixture of lactic acid stereoisomers, and a 50 50 blend of PLLA and PDLA) exposed to RH changes from 0 to approximately 25% at 40°C using a quartz crystal microbal-ance/heat conduction calorimeter (QCM/HCC). In addition, moisture transport, crystallization, and degradation in PLA... 

DUactide (5) exists as three stereoisomers, depending on the configurations of the lactic acid monomer used. The enantiomeric forms whereia the methyl groups are cis are formed from two identical lactic acid molecules, D- or L-, whereas the dilactide formed from a racemic mixture of lactic acid is the opticaUy iaactive meso form, with methyl groups trans. The physical properties of the enantiomeric dilactide differ from those of the meso form (6), as do the properties of the polymers and copolymers produced from the respective dilactide (23,24). 

Molecules that are not identical to their mirror images are kinds of stereoisomers called enantiomers (Greek encmtio, meaning "opposite"). Enantiomers are related to each other as a right hand is related to a left hand and result whenever a tetrahedral carbon is bonded to four different substituents (one need not be H). For example, lactic acid (2-hydroxypropanoic acid) exists as a pair of enantiomers because there are four different groups (—H, -OH, - CH3, -C02H) bonded to the central carbon atom. The enantiomers are called (-i-)-lactic acid and (-)-lactic acid. Both are found in sour milk, but only the (+) enantiomer occurs in muscle tissue. 

Molecules like lactic acid, alanine, and glyceraldehyde are relatively simple because each has only one chirality center and only two stereoisomers. The situation becomes more complex, however, with molecules that have more than one chirality center. As a general rule, a molecule with n chirality centers can have up to 2n stereoisomers (although it may have fewer, as we ll see shortly). Take the amino acid threonine (2-amino-3-hydroxybutanoic acid), for example. Since threonine has two chirality centers (C2 and C3), there are four possible stereoisomers, as shown in Figure 9.10. Check for yourself that the R,S configurations are correct. 

Lactide (LA), the cyclic diester of lactic acid, has two stereogenic centers and hence exists as three stereoisomers L-lactide (S,S), D-lactide (R,R), and meso-lactide (R,S). In addition, rac-lactide, a commercially available racemic mixture of the (R,R) and (S,S) forms, is also frequently studied. PLA may exhibit several stereoregular architectures (in addition to the non-stereoregular atactic form), namely isotactic, syndiotactic, and heterotactic (Scheme 15). The purely isotactic form may be readily prepared from the ROP of L-LA (or D-LA), assuming that epimerization does not occur during ring opening. The physical properties, and hence medical uses, of the different stereoisomers of PLA and their copolymers vary widely and the reader is directed to several recent reviews for more information.736 740-743... 

In general, the maximum number of optically active isomers is given by 2n where n represents the number of asymmetric carbon atoms. Thus for a compound where n = 1, as in lactic acid, there would be two stereoisomers, one the dextro and the other the laevo. For a compound with two asymmetric carbon atoms, there would be 22 = 4 stereoisomers. But if the two asymmetric carbon atoms carry exactly identical groups, as in tartaric acid, the number would be fewer than four and we know that it exists in three forms, the d the 1 and the meso. 

Investigations on the stereochemistry of chiral semiochemicals may be carried out by (gas) chromatographic separation of stereoisomers using chiral stationary phases, e.g. modified cyclodextrins [32]. Alter natively, formation of diastereomers (e.g. Mosher s ester or derivatives involving lactic acid etc.) may be followed by separation on conventional achiral stationary phases. Assignment of the absolute configuration of the natural product will again need comparison with an authentic (synthetic) reference sample. 

Due to its stereogenic center, lactic acid exists in two enantiomeric forms (d- and L-lactic acid), leading to three different lactide stereoisomers (d-, l- and mesolactide). Depending on the relative amounts of the different stereoisomers in the final polyester, the crystallinity of the resulting PLA is heavily influenced and this way the properties of the polymer can be adjusted to satisfy the needs of different applications [18-20]. 

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-... 

Table 13.1. Stereoisomers of Lactic Acid Produced by Various Microorganisms.

OPTICAL ISOMER- Either of two kinds of optically active three-dimensional isomers (stereoisomers). One kind is represented by mirror-image presence of one or more asymmetric carbon atoms in the compound (glyceraldehyde, lactic acid, sugars, tartaric add, amino acids). The other kind is exemplified by diastereoisomers, which are not mirror images. These occur in compounds having two or more asymmetric carbon atoms thus, such compounds have 2 optical isomers, where n is the number of asymmetric carbon atoms. 

What stereoisomers would result from reaction of ( j-lactic acid with phenylethylamine. and what is (he relationship between them ... 

Number of Stereo-isomers.—Plainly, with more than one asymmetric carbon atom in the molecule, we can have more than two stereoisomers, as we had in the case of lactic acid. When we speak of the number of stereo-isomers we do not include the racemic form as it... 

Molecules like lactic acid, alanine, and glyceraldehyde are relatively simple because each has only one chirality center and only two stereoisomers. The situation becomes more complex, however, with molecules that have] more than one chirality center. 

The basic constitutional unit of PLA is lactic acid. Lactic acid (2-hydroxy propionic acid) is an a-hydroxy acid with an asymmetric carbon atom and exists either as L(-t-) or D(-) stereoisomer, as shown in Figure 8.1. 

Figur 8.1 Stereoisomers of 2-hydroxypropionic acid (lactic acid).

Ethyl-2-methyl-l,6-dioxaspiro[4.5]decane is a constituent of the antiaggregative pheromone produced by several varieties of bees. All of the four thermodynamically stable stereoisomers of this spiroacetal have been synthesized using (5)-malic acid and (5)-lactic acid as the sources of chirality [15]. 

Monomers Figure3.8 shows a small selection of cyclic monomers suitable for ROP [43]. Additionally, three different stereoisomers of lactide exist as a consequence of the presence of two stereocenters per monomer unit, namely meso-, L- and d-lactide, see Fig. 3.9. Further, racemic mixture of L- and D-lactide are commercially available. While ROP of either pure l- and D-lactide enables synthesis of highly crystalline poly(L-lactic acid) or poly(D-lactic acid), ROP of rac- or wcj< -lactide with adequate catalysts allows the synthesis of stereoblock copolymers, heterotactic and syndiotactic poly(lactic acid). Notably, stereoregular PLAs display much lower rates of degradation than the amorphous atactic polymer. 

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