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). 

A more eflicient and general synthetic procedure is the Masamune reaction of aldehydes with boron enolates of chiral a-silyloxy ketones. A double asymmetric induction generates two new chiral centres with enantioselectivities > 99%. It is again explained by a chair-like six-centre transition state. The repulsive interactions of the bulky cyclohexyl group with the vinylic hydrogen and the boron ligands dictate the approach of the enolate to the aldehyde (S. Masamune, 1981 A). The fi-hydroxy-x-methyl ketones obtained are pure threo products (threo = threose- or threonine-like Fischer formula also termed syn" = planar zig-zag chain with substituents on one side), and the reaction has successfully been applied to macrolide syntheses (S. Masamune, 1981 B). Optically pure threo (= syn") 8-hydroxy-a-methyl carboxylic acids are obtained by desilylation and periodate oxidation (S. Masamune, 1981 A). Chiral 0-((S)-trans-2,5-dimethyl-l-borolanyl) ketene thioketals giving pure erythro (= anti ) diastereomers have also been developed by S. Masamune (1986). 

Organic chemisfs use an informal nomenclafure sysfem based on Eischer projec lions fo dislmguish belween diasfereomers When fhe carbon chain is vertical and like subsliluenls are on fhe same side of fhe Eischer projecfion fhe molecule is described as fhe erythro diaslereomer When like subsliluenls are on opposite sides of fhe Eischer projecfion fhe molecule is described as fhe threo diaslereomer Thus as seen m fhe... 

Fischer projections of the stereoisomeric 2 3 dihydroxybutanoic acids compounds I and II are erythro stereoisomers and III and IV are threo... 

Draw Fischer projections or make molecular models of the four stereoisomeric 3 ammo 2 butanols and label each erythro or threo as appropriate... 

Stereoisomers that are not enantiomers are classified as diastereomers Each enantiomer of erythro 3 bromo 2 butanol is a diastereomer of each enantiomer of threo 3 bromo 2 butanol... 

Thiol (Section 15 13) Compound of the type RSH or ArSH Th reo (Section 7 11) Term applied to the relative configuration of two stereogenic centers within a molecule The threo stereoisomer has like substituents on opposite sides of a Fischer projection... 

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). 

Stmcture-activity and mechanism of action studies indicate that the requirements for chloramphenicol activity are the D-threo-configuration, the... 

Because the starting materials were optically active, the products were all pure enantiomers. Later, the synthetic scheme shown in Figure 5 was developed (22,45). Resolution of the racemic mixture was accompHshed at the penultimate stage and the optically active D-threo-amine (7) was converted to florfenicol (2). This synthetic process also resulted in the synthesis of thiamphenicol shown in Figure 6 using 1,1,2,3,3,3-hexafluoropropyl diethylamine (FPA) (46). More recently an improved method of synthesis of florfenicol has been developed (17). 

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