Lactic acid, configuration enantiomers

Figure 2-69. The two enantiomers of lactic acid assignment of R and S configurations to the enantiomers of lactic acid after ranking the four ligands attached to the chiral center according to the Cl P rules (OH > COjH > Me > H).

One further point needs to be mentioned—the matter of absolute configuration. How do we know that our assignments of R,S configuration are correct in an absolute, rather than a relative, sense Since we can t see the molecules themselves, how do we know that the R configuration belongs to the dextrorotatory enantiomer of lactic acid This difficult question was finally solved in 1951, when J. M. Bijvoet of the University of Utrecht reported an X-ray spectroscopic method for determining the absolute spatial arrangement of atoms in a molecule. Based on his results, we can say with certainty that the R,S conventions are correct. 

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 (-). 

This reaction is completely enantioselective. For example, reduction of pyruvic acid with NADH catalyzed by lactate dehydrogenase affords a single enantiomer of lactic acid with the S configuration. NADH reduces a variety of different carbonyl compounds in biological systems. The configuration of the product (/ or S) depends on the enzyme used to catalyze the process. 

Stereoselective dehalogenation of 2-haloaIkanoic acids has been demonstrated for a number of halidohydrolases (80-82). Figure 77 details the production of an L-haloacid intermediate used in the production of phen-oxypropionic acid herbicides. The R enantiomer of chloropropionic acid is selectively hydrolyzed to (S)-lactic acid due to an inversion of configuration that occurs during the hydrolysis (83). (S)-2-chloroproprionic acid is used as a chiral synthon to produce a number of (R)-phenoxypropionic acid herbicides, for example, Fusilade 2000 (ICI). 

The work of these chemists immediately provided absolute configurations for all those compounds whose relative configurations had been determined by relating them to (+)-glyceraldehyde. Thus, (—)-lactic acid has the configuration shown above. If (+)-glyceraldehyde had been the S enantiomer, (—)-lactic acid would have had the opposite configuration. 

Lactic acid bears an asymmetric carbon and is mainly produced by bacteria at nearly enantiopure (5) (l) configuration. The improvement of thermo-mechanical properties of PLA and more specifically of its heat resistance require the combination of pure P(l)LA and P(d)LA in so-called stereocomplexes.The production of the (R)-lactic acid was therefore studied, even if until now the strains producing this enantiomer are rarely reported and the production remains lower than for the (5) counterpart. ... 

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... 

As early as 1983, Francois et al. [10] reported their investigations on the configuration of PLA. They studied the C- H NMR spectra of a series of PLA stereocopolymers prepared by three different synthetic methods (1) ROP of l-, rac-, and mejo-LAs initiated by powdered Zn (2) copolymerization of L- and rac-LAs in different ratios and (3) condensation polymerization of rac-lactic acid. Here, rac- is the abbreviation of raceme that represents the mixture of two enantiomers, and meso- is the abbreviation of mesomer that represents the compound of two chiral carbon atoms with different stereochemical configurations. 

Racemic 1 and 2a were not resolved into their two enantiomers on a Cylodex B chiral stationary phase GC column (J W Scientific), and so the absolute configurations of insect-produced 1 and 2a were determined by derivatization with a chiral derivatizing reagent followed by GC analysis of the resulting diastereomers on an achiral column. Thus, racemic lavandulol, a sanq)le of (R)-(-)-lavandulol, and a hydrolyzed sample of the insect-produced conq)ound were derivatized with acetyl (5)-lactic acid chloride and pyridine in ether to form the acetyl lactate ester(s) (75). The diastereomeric derivatives from racemic lavandulol were resolved almost to baseline, with the derivative from (7 )-lavandulol (isolated from lavender oil) being the later eluting peak. 

L-lactic acid is a metabolic intermediate. It can be obtained at low cost from the fermentation of agricultural and food by-products containing carbohydrates such as corn and sugar [30-31]. Racemic lactic acid is produced by petrochemistry or racemisation of the L-enantiomer [32]. As mentioned before, the main advantage of this family is an outstanding versatility due to the chirality, and polymerisation and copolymerisation of various lactides. Figure 9.2 shows the formula of various lactides with links to derived homo- and stereocopolymers whose configurational structures (distribution of L- and D-lactyl units) are represented schematically. 

Fischer projection formulas are often used to depict compounds such as glyceraldehyde, lactic acid, and tartaric acid. Draw Fischer projections for both enantiomers of (a) glyceraldehyde, (b) tartaric acid, and (c) lactic acid, and specify the (R) or (S) configuration at each chirality center. [Note that in Fischer projection formulas the terminal carbon that is most highly oxidized is placed at the top of the formula (an aldehyde or carboxylic acid group in the specific examples here).]... 

Dilactide, the cylic diester of lactic acid, possesses two asymmetric carbons and exists in three configurations the two enantiomers D- and L- dilactide melting... 

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