Erythro diastereomers

Organic chemists use an informal nomenclature system based on Eischer projections to distinguish between diastereomers. When the caibon chain is vertical and like substituents ar e on the sane side of the Eischer projection, the molecule is described as the erythro diastereomer. When like substituents are on opposite sides of the Eischer projection, the molecule is described as the three diastereomer. Thus, as seen in the... 

An additional important observation that (Z)-enolates exhibit erythro diastereoselection was made by Dubois and Fellmann (5b). Their investigation demonstrated that the magnesium enolate 24a (20°C, Et2 0) condensed with benzaldehyde under kinetic conditions to give exclusively the erythro diastereomer 25E (R3 = Ph, E T>95 5), and upon prolonged equilibration afforded the isomeric threo adduct (T E > 95 5) (eq. [ 15]). Heathcock has reported... 

One possible explanation for the diastereoface selection (AAGt -78°C 3 kcal/mol) observed for these chiral enolates is illustrated in Scheme 23. In the respective aldol transition states derived from conformers A and B leading to erythro diastereomers A and B, it may be assumed that developing imide resonance (118) will lock the chiral auxiliary in one of the in-plane conformations illustrated in products A or B. Based on an examination of models, it is projected that developing CHj R, allyhc strain steric interaction (37) disfavors that transition state leading to A. These steric considerations are largely attenuated in the transition state leading to the observed erythro adduct B. ... 

The only operative directing effect for chiral olefins withont heteroatoms at the stere-ogenic centre is 1,3-aUylic strain. In this case, a moderate preference for erythro diastere-oselectivity is observed (Scheme 37). For example, photooxidation of both 118 and 119 afforded mainly the erythro diastereomer (71/29 and 81/19, respectively). 

The reaction of methyl phenyldiazoacetate with N-Boc-piperidine (36) is a good illustration of the potential of this chemistry because it leads to the direct synthesis of f/ireo-methylphenidate (37) [27]. The most efficient rhodium car-boxylate catalyst for carrying out this transformation is Rh2(S-biDOSP)2 (2), which results in the formation of a 71 29 mixture of the readily separable threo and erythro diastereomers. The threo diastereomer 37 is produced in 52% isolated yield and 86% ee [Eq. (19)]. Other catalysts have also been explored for this reaction. Rh2(R-DOSP)4 gives only moderate stereoselectivity while Rh2(R-MEPY)4 gave the best diastereoselectivity in this reaction (94% de) [29]. 

Addition of bromine in dichloromethane or of thioacetic acid in ethanol onto the methylene group of 9-arylidene and 9-carboxymethylene moieties of 6,7,8,9-tetrahydro-4//-pyrido[l,2-a]pyrimidines 546 stereoselectively gave 9-substituted 6,7,8,9-tetrahydro derivatives 547 and 548, respectively, at ambient temperature (Scheme 34) (90JHC247). Addition was also stereoselective with respect to the C(9) and C(9)—C centers, giving the erythro diastereomers as the primary products, which may then undergo epimerization to the threo isomers. The structure and epimerization of the products were studied by H and l3C NMR spectroscopy and by molecular mechanics calculations. 

A 6-7 bicyclic, erythro diastereomer aminoalcohol precursor derived from 5,6,7,8-tetrahydro-9//-cyclohepta[h]pyridin-9-one was condensed with diethyl carbonate to afford the 6-7-5 tricyclic defused oxazol-2-one (114) (Equation (66)) <84JMC20>. 

For example, syn dihydroxylation of frans-crotonic acid gives the two enantiomers of the threo diastereomer of 2,3-dihydroxybutanoic acid. The same reaction with m-crotonic acid gives the erythro diastereomer of the product. 

The terms erythro and threo are used with dissymmetric molecules whose ends are different. The erythro diastereomer is the one with similar groups on the same side of the Fischer projection, and the threo diastereomer has similar groups on opposite sides of the Fischer projection. The terms meso and ( ) [or (4,/)] are preferred with symmetric molecules. 

Separate the isomers by flash column chromatography on silica gel (ethyl acetate then acetone as eluant). The first diastereomer eluted from the column is the erythro adduct, with further elution affording the corresponding threo isomer. The diastereomers are recrystallized separately from ethyl acetate to afford the erythro diastereomer (1/ S,2S/ )-2-diphenylphosphinoyl-1-phenylbutan-1-ol (1.05 g, 73%) and the threo diastereomer (1/ S,2/ S)-2-diphenylphosphinoyl-1-phenylbutan-1-ol (180 mg, 13%) as needles. 

Threo diastereoselectivity is consistent with a chelation-controlled (Cram cyclic model) organolithium addition (Figure 8a). Since five-membered chelation of lithium is tenuous, an alternative six-membered chelate involving the dimethylamino nitrogen atom of the thermodynamically less stable (Z)-hydrazone (in equilibrium with the ( )-isomer) cannot be discounted. The trityl ether (entry 4, Table 9) eliminates the chelation effect of the oxygen atom such that the erythro diastereomer predominates (via normal Felkin-Ahn addition) (Figure 8b). 

If isolated TF complex is utilized. Trepaied from (32), proper diene and Prt4gBr. 63 33 mixture of threo erythro diasteteomers. 93 3 mixture of threo.-erythro diastereomers. 90 10 mixture of threo erythro diastereomers. Prepared from (32) and 1-ethylallylmagnesium Immide m.p. 90.3-91.0 C. 

The deamination of 3-phenyl-2-butylamines,(J57), X=NH2, differs strongly from the solvolysis of the corresponding tosylates (Table 13)200. As pointed out previously, these reactions are conformationally controlled (Section 6.2). With the erythro diastereomer, the tram orientation of phenyl and amino (diazonium) groups is conformationally favored. A high yield of optically active erythro product results. With the threo diastereomer, phenyl participation is conformationally disfavored. Threo and erythro products are obtained in comparable quantities, and with partial race-mization, indicating the dominance of the kc pathway. 

In a later series of papers, Dubois and Fort studied the stereochemistry of the reaction of 2,2,5-tri-methylcyclopentanone with isovaleraldehyde (equation 94). in this study, it was shown that, whereas the erythro diastereomer is thermodynamically preferred, the threo isomer is formed more rapidly under certain conditions. 

Since the aldehyde and enolate both have enantiotopic faces, there are two relative ways they can approach one another. That is, the Si face of the aldehyde can become bonded to the face of the enolate (giving the erythro diastereomer) or the face of the aldehyde can be attacked by the face of the enolate (giving the threo diastereomer) ... 

Conpound 36 was isolated as amorphous. The spectral data of compound 36 and its acetate 36a suggested that the structure of 36 was close to those of conpound 35. Remarkable differences were seen in the H-NMR spectrum where the coupling constant between H-7 and H-8" was smaller (J=3.5 Hz) and the chemical shift of H-7 was in a higher field (6 4.96) than in compound 35 (J=8.6 Hz and 5 5.02, respectively) (Table 4.4). Thus, confound 36 was the erythro-diastereomer of compound 35. 

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