L enantiomers

Glyceraldehyde can be considered to be the simplest chiral carbohydrate It is an aldotriose and because it contains one chirality center exists in two stereoisomeric forms the D and l enantiomers Moving up the scale m complexity next come the aldotetroses Examining their structures illustrates the application of the Fischer system to compounds that contain more than one chirality center... 

The reaction is used for the chain extension of aldoses in the synthesis of new or unusual sugars In this case the starting material l arabinose is an abundant natural product and possesses the correct configurations at its three chirality centers for elaboration to the relatively rare l enantiomers of glucose and mannose After cyanohydrin formation the cyano groups are converted to aldehyde functions by hydrogenation m aqueous solution Under these conditions —C=N is reduced to —CH=NH and hydrolyzes rapidly to —CH=0 Use of a poisoned palladium on barium sulfate catalyst prevents further reduction to the alditols... 

Enzymatic hydrolysis is also used for the preparation of L-amino acids. Racemic D- and L-amino acids and their acyl-derivatives obtained chemically can be resolved enzymatically to yield their natural L-forms. Aminoacylases such as that from Pispergillus OTj e specifically hydrolyze L-enantiomers of acyl-DL-amino acids. The resulting L-amino acid can be separated readily from the unchanged acyl-D form which is racemized and subjected to further hydrolysis. Several L-amino acids, eg, methionine [63-68-3], phenylalanine [63-91-2], tryptophan [73-22-3], and valine [72-18-4] have been manufactured by this process in Japan and production costs have been reduced by 40% through the appHcation of immobilized cell technology (75). Cyclohexane chloride, which is a by-product in nylon manufacture, is chemically converted to DL-amino-S-caprolactam [105-60-2] (23) which is resolved and/or racemized to (24)... 

Depending on the stereoselectivity of the reaction, either the or the 5 configuration can generated at C-2 in the product. This corresponds to enantioselective synthesis of the d md L enantiomers of a-amino acids. Hydrogenation using chiral catalysts has been carefully investigated. The most effective catalysts for the reaction are ihodiiun... 

A mixed 4x4 peptide library consisting of 16 members was again prepared from the earlier-shown two families of building blocks 9-12 (all l enantiomers) and... 

Fig. 6-7. Asymmetry factor (AJ of the L-enantiomer versus sample load (A) and versus flow rate (B) on L-PA-imprinted polymers. Flow rate 1.0 ml min . Mobile phase MeCN/[potassium phosphate 0.05 M, pH 7] (7/3, v/v).

Finally, the data in Table 8-6 show the elution of the lead column. The eluent is H,0. The driving force for the elution in this case is the lack of C10 present to act as an anion in the binding of the ammonium perchlorate salt pair. The D-enantiomer versus L-enantiomer ratio in the elution is slightly greater than 6 1, as expected by the inherent selectivity of the ligand. For this separation system, LiClO is then added back to the eluent and the eluent is sent on as load to the next purification stage. 

The title compound is a key C6 building block. Several labs have prepared novel a-amino acids, biological probes and other interesting compounds using the D-diepoxide as a key intermediate.3 An efficient route to the L-enantiomer provides a pathway to compounds with the opposite configuration, one not readily available from commercial sources, and a valuable probe of stereochemistry in biological systems and reaction mechanism. 

In 1947, L-rhamnose was first recognized by Stacey as a constituent of Pneumococcus Type II specific polysaccharide. This finding was confirmed, in 1952, by Kabat et al. and in 1955 again by Stacey when 2,4- and 2,5-di-O-methyl-L-rhamnose were synthesized and the former was shown to be identical with a di-O-methylrhamnose, obtained by hydrolysis of the methylated polysaccharide. This result indicated a pyranose ring structure for the rhamnose units in the polysaccharide. Announcement of the identification of D-arabinofuranose as a constituent of a polysaccharide from M. tuberculosis aroused considerable interest. The L-enantiomer had been found extensively in polysaccharides, but reports of the natural occurrence of D-arabinose had been comparatively rare. To have available reference compounds for comparison with degradation products of polysaccharides, syntheses of derivatives (particularly methyl ethers) of both d- and L-arabinose were reported in 1947. 

In spite of the former widespread use of d and l to denote absolute configuration, the method is not without faults. The designation of a particular enantiomer as d or l can depend on the compounds to which it is related. Examples are known where an enantiomer can, by five or six steps, be related to a known d compound, and by five or six other steps, be related to the l enantiomer of the same compound. In a case of this sort, an arbitrary choice of d or l must be used. Because of this and other flaws, the DL system is no longer used, except for certain groups of compounds such as carbohydrates and amino acids. 

Five conformationally restricted analogs of creatine now have been prepared. These are D-N-amidinoproUne, 19 LN-amidinoproline, 20 l-carboxymethyl-2-iminoimidazolidine, 21 l-carboxymethyl-2-iminohexa-hydropyrimidine, 22 and D,L3-methyl-4-carboxy-2-iminoimidazolidine (the L enantiomer is depicted in structure 23). 

N-Benzoyl-Lalanine methyl ester is in turn about eight times more reactive than is its D enantiomer). The open-chain compounds may not bind to the enzyme in the same manner, however, as does the locked substrate. The conformation around the amido bond of the open-chain compounds, for example, can be transoid rather than cisoid (81). In addition, if equatorial 24 is considered to be the reactive conformer for both the Dand L enantiomers, and if the alanine methyl group is attracted to the hydrophobic aromatic binding subsite, then structures 34 and 38 would result. The L enantiomer of N-benzoyl-phenylalanine methyl ester 38 in this representation has approximately the same conformation as equatorial L-24. But attraction of the methyl of the D enantiomer to the location occupied by the methyl group of the L enantiomer causes the carbomethoxy group to move from the position it occupies in D-24. 

Since amines react more readily than alcohols in noncatalyzed reactions with anhydrides, the reaction is more difficult and initially required stoichiometric catalyst loadings [107], but could be performed in a catalytic sense with an O-acylated azlactone as acylating agent, which does not react with a benzylic amine at —50°C, but is capable of acylating the catalyst [108, 109]. Depending on the buUdness of the substrate, selectivities ranged from S = 11 to 27 (s = [ enantiomer l]/[ enantiomer 2])-... 

In order to verify if the colloidal catalytic system is still active after 24 h of reaction, several additions of the substrate rac-, were done on the reaction catalytic mixture. The data are collected in Table 2. Up to the third run, further conversion of (R)-l enantiomer of the substrate towards (5)-II alkylated product and accumulation of... 

C16H2207 l,5-Anhydro-2,3,4-tri-0-benzoylxylitol (ATBXYL10)114 PI Z = 2 Dx = 1.32 R = 0.053 for 3,536 intensities. The crystal structure contains centrosymmetrically related d and l enantiomers. The pyranoid conformation is an almost ideal (d) [1C4(l)]i withQ = 60 pm, 0=1°, with normal bond-lengths and valence-angles. The benzoyl groups are equatorial, with their planes approximately normal to the mean plane of the pyranoid ring. 

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