Erythro/threo

Alkylation of aldol type educts, e.g., /3-hydroxy esters, using LDA and alkyl halides leads stereoselectively to erythro substitution. The erythro threo ratio of the products is of the order of 95 5. Allylic and benzylic bromides can also be used. The allyl groups can later be ozonolysed to gjve aldehydes, and many interesting oligofunctional products with two adjacent chiral centres become available from chiral aldol type educts (G. Prater, 1984 D. Seebach, 1984 see also M. Nakatsuka, 1990, p. 5586). 

This chemical bond between the metal and the hydroxyl group of ahyl alcohol has an important effect on stereoselectivity. Asymmetric epoxidation is weU-known. The most stereoselective catalyst is Ti(OR) which is one of the early transition metal compounds and has no 0x0 group (28). Epoxidation of isopropylvinylcarbinol [4798-45-2] (1-isopropylaHyl alcohol) using a combined chiral catalyst of Ti(OR)4 and L-(+)-diethyl tartrate and (CH2)3COOH as the oxidant, stops at 50% conversion, and the erythro threo ratio of the product is 97 3. The reason for the reaction stopping at 50% conversion is that only one enantiomer can react and the unreacted enantiomer is recovered in optically pure form (28). 

Relative configuration (Section 7.5) Stereochemical conhgu-ration on a comparative, rather than an absolute, basis. Terms such as D, L, erythro, threo, a, and (3 describe relative conhguration. 

An important aspect is the control of the stereochemical outcome.During the course of the reaction two new chiral centers can be created and thus two diastereomeric pairs of enantiomers (syn/anti resp. erythro/threo pairs) may be obtained. 

Scheme 9. The threo 2,3-diol problem. The erythro/threo notation is based on Fischer projection formulas. For example, if threo 2,3-diol 28, shown here in a staggered zigzag conformation, was depicted in an eclipsed Fischer projection, the adjacent hydroxyls attached to carbons 2 and 3 would reside on opposite sides of the carbon chain. An alternative, perhaps less ambiguous, descriptor is the syn/anti notation.63...

Mesyloxy- -athylester 444 10-Oxo- -Kalium-Saiz 316 erythro/threo-9,10,18-Trihydroxy- 200 9,10,12-Trihydroxy- -3,5-dimethyl-pyrazolid 233... 

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)... 

The bromination products, dibromide in methylene chloride and methoxybromide in methanol, are a mixture of erythro- and threo-diastereoisomers, obtained in a ratio, Erythro/Threo = 70 / 30, which does not depend on the substituents or on the solvent. As expected, the reaction in the protic solvent is fiilly regioselective, i.e. methanol only traps the intermediate... 

Akiyama, T. Goto, H. Nawawi, D. S. Syafii, W. Matsumoto, Y. Meshitsuka, G. Erythro/threo ratio of P-O-4-structures as an important structural characteristic of lignin. Part 4 variation in the erythro/threo ratio in softwood and hardwood lignins and its relation to syringyl/guaiacyl ratio. Holzforschung 2005, 59, 276-281. 

The NMR spectrum of the threo + erythro mixture of methylphenyl(2-phenyl-propyl)tin hydride (51), another compound with two asymmetric atoms, shows two CH3Sn signals (Av = 6.6 Hz in benzene at 270 MHz) which remain aniso-chronous even in the presence of HMPA 22>. The analogous erythro + threo tri-organotin deuteride (52) shows two CH3Sn signals too (Av = 1.5 Hz at 60 MHz... 

Other systems that are highly stereoselective in the reduction of 2-substituted ketones include PMHS/Triton -B (erythro threo = 95 5),278 (TMSO SiH/ Triton -B (crythro threo = 95 5),278 and PhMe2SiH/TASF/HMPA (erythro threo = 93 7).320... 

Entry Nitrone Yield (%) erythro- cis threo- cis erythro- trans threo- trans erythro threo cis. trans... 

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). 

Substrate Catalyst Product 64 Yield (%) threo erythro threo-64 ee (%)... 

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],... 

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