Isomer, erythro

Sometimes the terms erythro and threo are used to specify fee relative configuration of two adjacent stereogenic centers. The terms are derived fom fee sugars erythrose and threose. The terms were originally defined such feat a Fischer projection formula in which two adjacent substituents were on the same side was fee erythro isomer and feat in whidi the substituents were on opposite sides was the threo isomer. 

Analogous reactions under free-radical conditions show a preference for the erythro isomer when X = Cl, however when X = tnfluoromethyl, the reaction shows essentially no stereoselectivity [S] (Table 3)... 

Drefahl and Horhold discussed the mechanism of N 0 acyl migration in A -benzoyl-l,2-diphenyl-3-aminopropanols. The migration does not seem to be possible in the erythro isomer as it would give an intermediate tetrahydro-1,3-oxazine with a bulky phenyl group in axial position. Consequently the erythro isomer is cyclized with inversion to form 2,5,6-triphenyl-5,6-dihydro-l,3-4 -oxazine... 

Of the four possible 5-deoxy-pent-4-enofuranoses, the D-erythro-isomer was of interest as a potential source of derivatives of L-lyxofuranose. For this purpose, a vinyl ether having the D-en/ hro-configuration has been prepared from derivatives of D-ribose. Condensation of D-ribose with acetone in the presence of methanol, cupric sulfate and sulfuric acid at 30°C., as described by Levene and Stiller(30) afforded a sirupy product consisting mainly of methyl 2,3-O-isopropylidene-D-ribofuranose (40). Treatment of a pyridine solution of the sirup with tosyl chloride... 

It was noted (23) that the NMR spectrum of compound 32 was identical with that published (34) for the third product (assigned structure 29) isolated from the reaction of tetra-O-acetyl-2-hydroxy-D-glucal with acetic anhydride and zinc chloride. The identity of the compounds was fully established and a revised structure proposed for this third product. In the presence of zinc chloride, therefore, epimerization can occur at an allylic site and the quasi-equatorial C-4 acetoxy group in the erythro isomers 27 and 28 can assume the favored quasi-axial orientation (24). 

The method described is successfully used for the alkylation and aralkylation of ethyl and /-butyl phenylacetate.3 The alkylation of ethyl phenylacetate with methyl iodide, M-butyl bromide, benzyl chloride, and a-phenylethyl chloride affords the corresponding pure monoalkylation products in 69%, 91%, 85%, and 70% (erythro isomer) yields, respectively. The alkylation of /-butyl phenylacetate with methyl iodide, M-butyl bromide, a-phenylethyl chloride, and /3-phenylethyl bromide gives the corresponding pure monoalkylated products in 83%, 86%, 72-73%, and 76% yields, respectively. 

Of course, the trans isomer will give the opposite results the threo pair if the addition is syn and the erythro pair if it is anti. The threo and erythro isomers have different physical properties. In the special case where Y=W (as in the addition of Br2), the erythro pair is a meso compound. In addition to triple-bond compounds of the type ACsCA, syn addition results in a cis alkene and anti addition in a trans alkene. By the definition given on page 166 addition to triple bonds cannot be stereospecific, though it can be, and often is, stereoselective. 

The unwanted erythro isomer of (11) crystallised out at this point, so the synthesis continues with the mother liquors. We must now protect the NH2 and both OH groups during the nitration - this also allows purification of crystalline threo (12). 

In practice, the anhydride gave a better Friedel-Crafts reaction. Note that the hindered Eiraine will cause no problems of over-react ion. Only the erythro isomer of (34) is biologically active so this must be separated from the threo which can be recycled by oxidation to (35),... 

Gourtieu, J. Deuterium NMR stereochemical analysis of threo-erythro isomers bearing remote stereogenic centres in racemic and non-racemic liquid crystalline solvents. Tetrahedron Asymmetry 2000, 11,1911-1918. 

Ralph, J. Landucci, L. L. Adducts of anthrahydroquinone and anthranol with lignin model quinone methides. 3. Independent synthesis of threo and erythro isomers. J. Org. Chem. 1983, 48, 3884-3889. 

Kibayashi and coworkers have used enantiometrically pure allylic silyl ethers obtained from amino acids in cycloaddition with nitrones (Eq. 8.49).71 Cyclic nitrone reacts with a chiral allyl ether to give selectively the exo and erythro isomer (de 90%). Optically active alkaloids containing a piperidine ring such as (+)-monomorine,71c (+)-coniine,71a and (-)-oncinotine71b have been prepared from the addition product. 

In acyclic secondary -allylic alcohols, epoxidation by the vanadium system shows opposite stereospecificity to that of peracid and molybdenum carbonyl-mediated epoxidation (see Scheme 6)22 The predominance of the erythro isomer in the former process is rationalized22 in terms of the energetically more favorable transition state (6, cf. 5) and in this context the mechanism has analogy in the epoxidation behavior of medium-ring cyclic allylic alcohols.23... 

The diastereoselectivity of the reduction of a-substiluted ketones has been the subject of much investigation. The reagent combination of trifluoroacetic acid and dimethylphenylsilane is an effective method for the synthesis of erythro isomers of 2-amino alcohols, 1,2-diols, and 3-hydroxyalkanoic acid derivatives.86,87,276,375 Quite often the selectivity for formation of the erythro isomer over the threo isomer of a given pair is >99 1. Examples where high erythro preference is found in the products are shown below (Eqs. 218-220).276 Similar but complementary results are obtained with R3SiH/TBAF, where the threo isomer product... 

The aldol condensation of benzaldehyde with the thioacetamide carbanion (RCHCSNRV), derived from the desilylation of the silyl-thioether with tetra-/i-buty-lammonium fluoride, is stereoselective at—80°C producing the erythro isomer of the p-hydroxy thioamide preferentially (Scheme 6.18, R = Me, erythro threo 95 5) via a conformationally mobile intermediate. However, when R is bulky, a greater amount of the threo isomer is formed. Predictably, as the reaction temperature is raised, so the stereoselectively decreases. This procedure contrasts with the corresponding condensation catalysed by titanium salts, where the complexed intermediate produces the threo isomer [47, 48],... 

At each stage, the location of the labelled material was followed by LSC of chromatographic fractions, with the dpm in steps (2) and (3) moving only with the fluorocitrate-containing fractions. In addition, capillary GC (30 m DB-5) was used to quantify citrate, isocitrate and fluorocitrate trimethyl esters and benzoates in each fraction. Finally, dpm were observed only in the erythro isomer of trimethyl 2-fluorocitrate benzoate. The progress of the purification is given in Table 2. Full details will be published separately(25). 

For the elimination of trimethylamine and water from the erythro- and threo-isomer of trimethyl-1,2-diphenylpropylammonium iodide 6 and 7 respectively, by treatment with sodium ethoxide, a stereospecific -elimination has been found to take place thus supporting a E2-mechanism. From the erythro-isom r 6 the Z-alkene 8 was obtained, while the tftreo-isomer 7 yielded the E-alkene 9 ... 

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