Erythro form

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. 

NMR-spectroscopic studies and a single crystal X-ray structure determination of the reduced Co / form of 100 revealed the presence of a bridging 2,3-dibromo-3-phenyl-propionato ligand (threo dl pair). The complex bearing the erythro form of 2,3-dibromo-3-phenyl-propionate is only produced in minor yields <3%. Therefore, the bromination of the encapsulated alkene is a highly diastereoselective syu-addition. This is rather... 

SCHEME 51. Erythro-forming transition state for the photooxygenation of phenyl-substituted chiral alkenes in solution... 

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. 

As outlined above, MeBmt contains three contiguous asymmetric carbon atoms and an (E)-trans double bond. The MeNH and OH groups are arranged in a threo configuration, whereas the OH and CH3 groups are oriented in the erythro form. 

Ruveda and co-workers have shown that the /5-hydroxyleucine, of which the aryl ether function in frangulanine is constructed, is present in the erythro-L-form (21). Dihydrofrangulanine was reduced with lithium in methylamine to an enol ether of /3-hydroxyleucine which on hydrolysis generated the free amino acid. It was shown that the hydroxyleucine in the hydrolysate is degraded by snake venom l-aminooxidase but not by pig kidney D-aminooxidase. Inasmuch as reo-hydroxyamino acids are attacked by neither enzyme it follows that the /3-hydroxyleucine, of hydrolytic origin, is the L-erythro 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... 

Cathodic reduction of nonconjugated steroidal ketones has been found to give the equatorial alcohols with a high degree of stereoselectivity and in very good yields 134 These reactions were run at -2.6 V in aqueous ethanol-tetrabutyl-ammonium bromide. a-Methyldesoxybenzoin gave only the erythro form of 1,2-diphenylpropanol-1 on reduction at mercury in 40 % ethanol at pH 8 (veronal buffer) at -1.85 V (SCE) 13S>. 

The third type of reactivity, using HTI in trimethyl orthoformate, involved migration of the aryl groups attached to the carbonyl moiety, resulting in a diastereoselective synthesis of methyl 2,3-diaryl-3-methoxypropanoates in their erythro form exclusively. 

Murakami et al. have utilized Mayer vesides to study aldolase-type reactions [48]. Formation of [i-phenylserinc from glydne and benzaldehyde proceeded effectively by cooperative catalysis of a hydrophobic pyridoxal derivative (47) and Zn(n) ions in the bilayer vesicle formed with 32. The threo isomer was dominantly produced over the erythro form. A marked enantioselectivity was observed in the co-veside of 32 and 35 in combination with 47 and Cu(ii) the ee for formation of (2S,3R)-P-phcnylscrinc over its enantiomeric (2R,3S)-isomer was 58%. Enantioselectivity also arose with another bilayer assembly, formed with 32, 35, and 37 in the presence of Cu(ii), where the (2R,3S) isomer was dominant over the (2S,3R) species in 13% ee. The opposite enantioselectivity performed by the second system, as compared with that for 47, might reflect a different stereochemical environment around the quinoid intermediate that allows the attack of benzaldehyde. 

Fig. 2. Molecular model of poly(THF diyl) (erythro form in trans-1,4-polybutadiene)551...

Even though it is commonly referred to as sphingosine, its proper chemical name is D-erythro-1,3-dihydroxy-4, 5-trans-octadecene. Other satisfactory names are rran.v-4-sphingosine, sphing-4-enine, or sphingenine in any event, all the naturally occurring sphingolipids contain only the D-erythro form. 

After the formation of the carbocation, two competing processes occur in the moderated systems racemization and Cl+ transfer. In the case of the meso compound, front attack leads to the thermodynamically stable erythro form, whereas in the dl system, the less stable threo diaste-reomer is formed, which readily leads to racemization. 

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... 

Benzylic acetate in erythro forms of fl-O-4 structures of syringyl ether type... 

Lundquist K, Stomberg R, von Unge S (1987) Stereochemical assignment of the threo and erythro forms of 2-(2,6-dimethoxyphenoxy)-l-(3,4-dimethoxyphenyl) 1,3 propanediol from X ray analyses of the synthetic intermediates (Z)-2-(2,6-dimethoxyphenoxy) 3 (3 4 dimethoxypheny )-2-propenoic acid and threo-2-(2,6 dimethoxyphenoxy) 3 (3 4-dimethoxyphenyl)-3-hydroxypropanoic acid Acta Chem Scand B41 499-510 Lundquist K, von Unge S (1986) NMR studies of lignins 8 Examination of pyridine-d, solutions of acetylated lignins from birch and spruce by H NMR spectroscopy Acta Chem Scand B40 791-797... 

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]. 

Examples of the use of H NMR spectroscopy for the characterization of lignin acetates in chloroform solutions have been described by Becker and Nimz (1974), Kirk and Chang (1975), Lundquist et al. (1979, 1980), Hemmingson (1983), Lapierre et al. (1982), Bardet et al. (1985), Hagstrom-Nasi and Sjostrom (1987). An example of the use of this technique for the elucidation of a specific structural problem is the analysis of the distribution of threo and erythro forms of P-O-A structures in lignins (Lundquist 1980, Hauteville et al. 1986, Tollier et al. 1986). It is noteworthy that the H NMR properties of lignin model compounds also have been the subject of theoretical treatments (Liptaj et al. 1980). 

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