Racemic lactic acid

HYDROXYPROPANOIC ACID 2-HYDROXYPROHONIC ACID tt-HYDROXYPROPIONIC ACID KYSELINAMLECNA /-LACTIC ACID MILCHSAURE MILK ACID ORDINARY LACTIC ACID RACEMIC LACTIC ACID... 

D enantiomer of poly (lactic acid) racemic mixture of D and L enantiomers of poly(lactic acid) poly(desaminotyrosyl-tyrosine hexyl ester)... 

Scheme 1.4 shows three lactides consisting of different stereoisomeric lactic acid units, l- and D-lactides consist of two l- and o-lactic acids, respectively, while meso-lactide consists of both d- and L-lactic acids. Racemic lactide (rac-lactide) is an equimolar mixture of d- and L-lactides. The melting points (Tm) of these lactides are compared in Table 1.3. Note that the is higher in rac-lactide and is lower in meso-lactide. 

Thermodynamic Parameters Crystalline l-( + )-Lactic Acid Racemic Lactic Acid Reference... 

The reaction of trioses takes place in a similar manner (Figure 4.50). D-Glyceraldehyde or 1,3-dihydroxyacetone dehydration yields 1,2-diulose methylglyoxal (also known as pyruvic acid aldehyde or pyruvaldehyde) and the intramolecular Cannizzaro reaction of methylglyoxal yields lactic acid (racemate). 

The enzyme is a single enantiomer of a chiral molecule and binds the coenzyme and substrate m such a way that hydride is transferred exclusively to the face of the carbonyl group that leads to (5) (+) lactic acid Reduction of pyruvic acid m the absence of an enzyme however say with sodium borohydride also gives lactic acid but as a racemic mixture containing equal quantities of the R and S enantiomers... 

Lactic acid is also the simplest hydroxy acid that is optically active. L (+)-Lactic acid [79-33-4] (1) occurs naturally ia blood and ia many fermentation products (7). The chemically produced lactic acid is a racemic mixture and some fermentations also produce the racemic mixture or an enantiomeric excess of D (—)-lactic acid [10326-41-7] (2) (8). 

Many of the physical properties are not affected by the optical composition, with the important exception of the melting poiat of the crystalline acid, which is estimated to be 52.7—52.8°C for either optically pure isomer, whereas the reported melting poiat of the racemic mixture ranges from 17 to 33°C (6). The boiling poiat of anhydrous lactic acid has been reported by several authors it was primarily obtained duriag fractionation of lactic acid from its self-esterification product, the dimer lactoyUactic acid [26811-96-1]. The difference between the boiling poiats of racemic and optically active isomers of lactic acid is probably very small (6). The uv spectra of lactic acid and dilactide [95-96-5] which is the cycHc anhydride from two lactic acid molecules, as expected show no chromophores at wavelengths above 250 nm, and lactic acid and dilactide have extinction coefficients of 28 and 111 at 215 nm and 225 nm, respectively (9,10). The iafrared spectra of lactic acid and its derivatives have been extensively studied and a summary is available (6). 

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). 

Most of the chiral membrane-assisted applications can be considered as a modality of liquid-liquid extraction, and will be discussed in the next section. However, it is worth mentioning here a device developed by Keurentjes et al., in which two miscible chiral liquids with opposing enantiomers of the chiral selector flow counter-currently through a column, separated by a nonmiscible liquid membrane [179]. In this case the selector molecules are located out of the liquid membrane and both enantiomers are needed. The system allows recovery of the two enantiomers of the racemic mixture to be separated. Thus, using dihexyltartrate and poly(lactic acid), the authors described the resolution of different drugs, such as norephedrine, salbu-tamol, terbutaline, ibuprofen or propranolol. 

To understand how this method of resolution works, let s see what happens when a racemic mixture of chiral acids, such as (+)- and (-)-lactic acids, reacts with an achiral amine base, such as methylamine, CH3NH2. Stereochemically, the situation is analogous to what happens when left and right hands (chiral) pick up a ball (achiral). Both left and right hands pick up the ball equally well, and the products—ball in right hand versus ball in left hand—are mirror images. In the same way, both ( H- and (-)-lactic acid react with methylamine equally... 

Figure 9.12 Reaction of racemic lactic acid with achiral methylamine leads to a racemic mixture of ammonium salts.

Figure 9.13 Reaction of racemic lactic acid with (RH-phenylethylamine yields a mixture of diastereomeric ammonium salts.

The racemic poly(DL-lactide) DL-PLA is less crystalline and lower uelting than the two stereoregular polymers, D-PLA and L-PLA. Further, the copolymers of lactide and glycolide are less crystalline than the two homopolymers of the two monomers. In addition, the lactic acid polymer, because of the methyl group, is more hydrophobic than the glycolide polymer. 

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... 

The method is based on the fact that certain bacteria, fungi, mould or yeast when allowed to grow in a racemic solution, assimilate or consume one of the enantiomers faster than the other. This is why the method is also known as selective assimilation or preferential decomposition. Thus Penicillium glaucum a species of green mould when allowed to grow in ammonium racemate solution consumes the d 0 tartaric acid and leaves the l form, but in a racemic lactic acid it assimilates the l form leaving behind the d form. 

Additionally, note that base hydrolysis of hyoscyamine gives ( )-tropic acid and tropine, with racemization preceding hydrolysis. Base hydrolysis of littorine gives optically pure phenyl-lactic acid, so we deduce that hydrolysis is a more favourable process than racemization. 

Racemic Lactic Acid Range Oil Range Oil Range Oil Raw Linseed Oil RC Plasticizer DBP RC Plasticizer DBP Realgar... 

During cheese production lactose is converted to lactic acid by starter lactic acid bacteria (LAB). Any unfermented lactose is converted to d- and L-lactate by nonstarter lactic acid bacteria (NSLAB) and racemization, respectively. Lactate can be oxidized by LAB in cheese to acetate, ethanol, formic acid, and carbon dioxide at a rate dependent on oxygen availability (McSweeney, 2004). Other pathways include conversion to propionate, acetate, water, and carbon dioxide by Propionibacterium spp. carbon dioxide and water by Penicillium spp. yeasts and butyric acid and hydrogen by Clostridium spp. The rate of lactose metabolism influences proteolysis and flavor formation (Creamer et al., 1985 Fox et al., 1990). 

Reduction of achiral precursors is often used to produce chiral products. The advantage of this approach is that the theoretical yield of product is 100% compared to the 50% theoretical maximum for the resolution of racemates. Cross-linked crystals of lactate dehydrogenase have been used to prepare L-lactic acid from pyruvic acid in an electrolytic cell. The LDH CLCs maintained constant... 

Lactic acid can be produced from a petrochemical route or from fermentation (6,7). The petrochemical route can only produce racemic mixtures of lactic acid, whereas fermentation canproduce optically pure isomer. D(-)-Lactic acid is toxic and must be limited in animal feeds (8), and an optically pure lactic acid is required to produce a specific PLA (9). In addition, fermentation utilizes renewable resources thatmake fermentationmore attractive than the petrochemical route. Extractive fermentation, which couples fermentation with on-line product removal, can eliminate end product inhibition and increase product yield, final product concentration, and reactor productivity. A number of extractive fermentation methods have been reported in the literature, including solvent extraction (10-12), precipitation (13), electrodialysis (14,15), adsorptionby ion-exchange resin (16-18), and an aqueous two-phase system (19-20). 

Monomers like glycolide or lactide are prepared by heating the corresponding acids under controlled conditions [53]. For example, lactide is prepared by heating lactic acid at 120 °C until water ceases to distill. The temperature is then increased to 140 °C and the pressure is reduced to 10 torn After heating for several hours at this temperature, the pressure is reduced further and the temperature increased until lactide begins to distill. Dilactide (3,6-dimethyl-l, 4-dioxan-2,5-dione) contains two asymmetric centers and therefore exists as L-lactide, D-lac-tide, meso-lactide, and the racemic mixture D,L-lactide and gives polymers with different properties. 

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