Threo form

Of the fonr possible optical isomers of chloramphenicol, only the o-threo form is active. This antibiotic selectively inhibits protein synthesis in bacterial ribosomes by binding to the 50S subunit in the region of the A site involving the 23 S rRNA. The normal binding of the aminoacyl-tRNA in the A site is affected by chloramphenicol in such a... 

Bardet, M. Robert, D. Lundquist, K. von Unge, S. Distribution of erythro and threo forms of different types of P-O-4 structures in aspen lignin by carbon-13 NMR using the 2D inadequate experiment. Magn. Reson. Chem. 1998, 36, 597-600. 

Various chiral lumazines produced from the parent pterins by an enzymatic reaction were separated using achiral-chiral multidimensional LC-LC by Klein et al. (1994). A Ci 8 column and a flavoprotein column were used in the reverse-phase mode to achieve the separation of the threo forms of the lumazines. The flavoprotein column was unable to resolve the erythro forms. 

Bipyridine is catalytically hydrogenated to 3,3 -bipiperidine, and the reduction may also be accomplished electrochemically. Erythro and threo forms of 3,3 -bipiperidine are produced by the latter method. 3,3 -Bipyridine is reduced with difficulty by means of tin and hydrochloric acid or sodium and alcohol. Some l,2,3,4,5,6-hexahydro-3,3 -bipyridine (also known as nicotidine) is produced, but several other partly reduced 3,3 -bipyridines are obtained as well. 3,3 -Bipyridine is reported to give a radical anion in tetrahydrofuran on reduction by sodium. Although there is some doubt about the authenticity of the starting material (see Section 3,4 -bipyridine is reported to be fully reduced to 3,4 -bipiperidine by catalytic hydrogenation and to l, 2, 3, 4, 5, 6 -hexahydro-3,4 -bipyridine by tin and hydrochloric acid. ... 

Similar procedures can be used to prepare AAbu (both E- and Z-isomers) from Thr derivatives. Srinivasan et al/891 found that (3-elimination of the Thr derivative, Ac-(2R,35)-Thr-OMe 37 (threo type), gave only the stable Z-isomer 38 upon O-tosylation and subsequent elimination by DABCO as a base (Scheme 14). The underlying mechanism for this reaction may be a traits E2-elimination. (25,3R)-2-Acetamido-3-chlorobutanoic add methyl ester (erythro) 39, derived from the Thr threo form by chlorination with inversion of configuration at the (3-carbon, yields predominantly the E-isomer 38 by brief treatment with DBU as a base. However, a prolonged reaction time and use of DABCO as a base causes a significant amount of isomerization to the Z-isomer. 

The conformations of the 2-oxo-tetrahydro-l,3-oxazines obtained from phosgene and the isomeric 3-amino-2,3-diphenylpropanols were used by Fodor et al. as an ingenious criterion for the configurational determination of the aminopropanols.13,14 The erythro form yielded an oxazinone that showed optical activity 4-8 times stronger than that from the threo form. 

For catalytic waves of hydrogen evolution in ammoniacal cobalt solutions, it has been observed (132) that ery/Aro-phenylcysteine gives a higher catalytic wave than the threo form (Fig. 28). These differences can be explained partly by differences in acid dissociation constants, and partly by variations in the stability constants of the cobalt-phenyl-cysteine complexes. 

Electrosynthesis is useful when the electrode reaction is stereo specific. For example, for a,a -dibromosuccinic acids, the threo form of both the free acid and its anions is reduced to fumaric acid. On the contrary, the erythro epimer is reduced to fumaric acid only in the undissociated form and as a dibasic anion. The univalent anion is at least partly reduced to maleic acid. Both threo and erythro epimers of dialkyl esters of dibro-mosuccinic acid are reduced, similarly to the undissociated free acids, to only the dialkyl ester of fumaric acid (134,135). 

There are three stereoisomers of 2,4-pentanediol. The meso form is achiral both threo forms are chiral. 

In parallel experiments, in which dihydrofrangulanine was hydrolyzed the hydroxyleucine which survived the hydrolysis occurred entirely in the rearranged threo form. 

Frequently the configuration of the hydroxyamino acid component of the peptide alkaloid could be deduced with the aid of NMR studies. The coupling constants, JaB, of several a-amino-j8-hydroxyamino acids as well as their A,iV-dimethyl derivatives and their methyl ethers were determined by Marchand et al. (34). They showed that only the N,N-dimethyl derivatives yielded JaB values of configurational significance and that the conversion of the hydroxyl to alkoxyl exerted virtually no influence. The Ja0 value of Ar,A-dimethyl-j3(p-tolyloxy)leucine in the threo form was 8.5 Hz whereas that in the erythro form was 2.5 Hz. Wenkert et al. studied frangulanine (10), discarine-A (6), and discarine-B (7) whose amide protons had been replaced by deuterium (22). The a- and jS-methine signals of hydroxyleucine occurred at 84,40 and 4.77, respectively, as a doublet with JaB = 8 Hz and as a double... 

Fig. 9.7 Effects of /3-phenyl-DL-serine, L-threonine and a-methyl-DL-serine on absorption (A) and CD (B) spectra of tryptophanase in 100 mM potassium phosphate buffer, pH 7.8, containing 2 mM dithiothreitol and 2mM EDTA. Protein concentration 1.8 mg/ml. Curve 1, unliganded enzyme. Curve 2, same as 1 + 15 mM /3-phenyl-ni -serine (threo form). Curve 3, same as 1 + 165 mM L-threonine. Curve 4, same as 1 + 500 mM a-methyl-DL-serine. (Reproduced with permission from Zakomirdina el al., Biochimie., 71, 548 549 (1989)).

It has been found68-70 that the reaction of tryptophanase with the inhibitory amino acids, /3-phenyl-DL-serine (threo form), L-threonine and D-alanine, is accompanied by a manifold increase in the reduced LD, i.e. in the ratio of LD to absorbance (AA / A) in the 420-425 nm band (Fig. 9.12 Table 9.2). This band belongs to the protonated internal PLP-lysine... 

Draw the structural formulae of both threo forms of the appetite depressant cathin and determine the absolute configuration of each isomer. 

Looker and Thatcher synthesized allophenylserine (DL-eryt/tro-P-phenylserine) in high yield by hydrogenation of methyl benzoyldiazoacetate over 5% Pd-C in 70% acetic acid (eq. 9.91).244 The hydrogenation was highly stereospecific, and the allophenylserine obtained did not reveal any of the diasteromeric threo form. The same reduction by a variety of chemical reducing agents led either to recovery of the starting... 

One case of a stereospecific enzyme reaction (in the chemical sense) has been described [57]. Chloroperoxidase adds the elements of hypochlorous acid across the double bond of propenylphosphonic acid. From the (Z)-isomer, 35, the product is the ( )-threo form of l-chloro-2-hydroxypropyl phosphonate, 36. On the other hand, from the ( )-isomer, 38, the product is the ( )-erythro form, 37. Since stereoisomerically different substrates give different stereoisomers as products, this enzymatic reaction is stereospecific in chemical terms. In biochemical terms the enzyme is almost a contradiction since the products are formed as racemates In a... 

Hauteville M, Lundquist K, von Unge S (1986) NMR studies of lignins 7 H NMR spectroscopic investigation of the distribution of erythro and threo forms of /J-O-4 structures in lignins Acta Chem Scand B40 31-35... 

Several derivatives obtained by reduction/acetylation of acidolysis monomers have been analyzed by high performance liquid chromatography (HPLC) (Kristersson and Lundquist unpubl. work 1983). The relative elution volumes for these compounds are listed in Table 6.1.1. Nonacetylated reduction products of acidolysis monomers have also been examined by HPLC (Kristersson and Lundquist unpubl. work 1983). From the results shown in Table 6.1.2, it is evident that good separation was achieved but, unfortunately, compounds 6a (threo form), 6b and 6c have very similar elution volumes. 

Results of stereochemical studies with various halohydrins confirm that the oxy-anion attacks the carbon from the opposite side, causing Walden inversion the erythro forms of 3-bromo-2-butanol, stilbene bromohydrin, and chloro malic acid give the trans oxide, and the corresponding threo forms lead to the cis oxides [146]. Furthermore, the frans-halohydrin of cyclohexene reacts 150 times as fast with OH- as the cis compound. Only the reaction of the trans compound yields an epoxide [147] in the reaction of the cis compound, an enol is formed first which then rearranges to the corresponding ketone [148, 149]. 

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