Threo isomers

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. 

Eor 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. Erom the -isomer... 

Both regio- and stereospecificiiy may be influenced by the catalyst and by alkali. Raney nickel opens ce>2,3-diphenylbul-2-ene epoxide with retention of configuration to give cr3 f/iro-2,3-diphenylbutan-2-ol, whereas palladium-on-carbon gives the inverted threo isomer. If a small amount of alkali is added to nickel-catalyzed reductions, nickel too gives the threo isomer (d5). 

Owing to the fully reversible equilibrium nature of the aldol addition process, enzymes with low diastereoselectivity will typically lead to a thermodynamically controlled mixture of erythro/threo-isomers that are difficult to separate. The thermodynamic origin of poor threo/erythro selectivity has most recently been turned to an asset by the design of a diastereoselective dynamic kinetic resolution process by coupling of L-ThrA and a diastereoselective L-tyrosine decarboxylase (Figure 10.47)... 

By this concept, a reversible enzymatic aldol reaction generates a mixture of l-threo/erythro aldol diastereomers (133) from which the i-threo isomer is preferentially decomposed by an irreversible decarboxylation to furnish aromatic aminoalcohol (R)-(134) vhth 78% ee in high yield. 

This indicated that retention had taken place. Note that both products are optically inactive and so caimot be told apart by differences in rotation. The meso and d/ dibromides have different boiling points and indexes of refraction and were identified by these properties. Even more convincing evidence was that either of the two threo isomers alone gave not just one of the enantiomeric dibromides, but the dl pair. The reason for this is that the intermediate present after the attack by the neighboring group (17) is symmetrical, so the external nucleophile Br can attack... 

Jackman, Hamilton, and Lawlor have studied the stereochemistry of the addition of [Co(CN)5D] to a,/3-unsaturated acids and identified the product of the addition of fumarate as the threo isomer, i.e., adduct formation has occurred by a stereospecific cis addition of Co—D across the double bond 94). 

Dideoxy-2-fluoro- 3-D-co r/zrc>-pentofuranosyl)adenine (877) was prepared from the 5 -(9-protected precursors (876) by treatment with DAST (CH2CI2 82% yield) or with BU4NF for the corresponding 2 -triflate. The corresponding threo isomer (879) was obtained by deoxygenation at... 

A study of the decomposition in basic media of the erythro- and threo-isomers of the /8-hydroxyphosphonate (169) showed that the first step in the phosphonate olefin synthesis is reversible and that the diastereo-isomers of (169) can also interconvert directly, presumably via the a-car-banion. 

In general, the Henry reaction proceeds in a non-selective way to give a mixture of anti (erythro) and syn (threo) isomers. Ab initio calculations on the Henry reaction suggest that free nitronate anions (not influenced by cations) react with aldehydes via transition states in which the nitro and carbonyl dipoles are antiperiplanar to each other. This kind of reaction yields anti-nitro alcohols. The Henry reaction between lithium nitronates and aldehydes is predicted to occur via cyclic transition states yielding syn-nitro alcohols as major products (Eq. 3.64).108... 

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 initially expected (75) cis-hydrometallation or olefin-insertion step with fumarate (R = C02Me) yields the threo isomer 8, which then undergoes the k2 step with retention to give racemic 1,2-dideuterosuccinate. Such retention is necessary to give the usually observed (7, p. 407) overall cis addition of H2 to olefinic bonds, but this study provided the first direct experimental proof, the difficulty being the scarcity of stable metal alkyl-hydride intermediates. The Cp2MoH2 complex also catalyzes hydrogenation of 1,3- or 1,4-dienes to monoenes (197). 

Erythro/threo Terms derived from carbohydrate nomenclature used to describe the relative configuration at adjacent stereocenters. Erythro refers to a configuration with identical or similar substituents on the same side of the vertical chain in Fischer projection. Conversely, a threo isomer has these substituents on opposite sides. These terms came from the nomenclature of two carbohydrate compounds, threose and erythrose (see Fig. 1-35). 

Boron triflates 45a and 45b are very useful chiral auxiliaries. Boron azaenolate derived from achiral35 and chiral36 oxazolines gives good stereoselectivity in the synthesis of acyclic aldol products, particularly for the rarely reached threo-isomers. By changing the chiral auxiliary, the stereochemistry of the reaction can be altered.37... 

Methyl oxetane-2-carboxylate derivatives (e.g., 284), obtained by ring contraction of aldonolactones, have been employed for the synthesis (279) of the nucleoside / -noroxetanocin [9-(/ -D-eryt/iro-oxetanosyl)adenine, 304] and its a-anomer via an a-chloride obtained by a modified Hunsdiecker reaction. Displacement of chloride by adenine and debenzylation gave 304. The threo isomer of304, /J-epinoroxetanocin (305), was likewise synthesized from D-lyxono-1,4-lactone. The oxetane nucleosides display potent antiviral activity against the human immunodeficiency virus (HIV). 

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