2R,3S isomer

The four stereoisomers of 2-amino-3-hydroxybutanoic acid can be grouped into two pairs of enantiomers. The 2R,3R stereoisomer is the mirror image of 25,35, and the 2R,35 stereoisomer is the mirror image of 25,3R. But what is the relationship between any two molecules that are not mirror images What, for example, is the relationship between the 2R,3R isomer and the 2R,3S isomer They are stereoisomers, yet they aren t enantiomers. To describe such a relationship, we need a new term—diastereomer. 

A series of 2-(4-chlorophenoxy)-3-oxoalkanoate were reduced by baker s yeast, and Kluyveromyces marxianus. Yeast reduction of ethyl 2-(4-chlorophenoxy)-3-oxo-3-phenylpropanoate (R=Ph) afforded enantiomerically pure ethyl 2R, 3S)-2-(4-chlorophenoxy)-3-hydroxy-3-phenylpropanoate out of the four possible stereoisomers in >99% de [29h]. Although baker s yeast reduction of butanoate (R = CFl3) was not selective (92% de), the use of K. marxianus afforded (2R, 3S)-isomer selectively [29i]. The products are intermediates for potential peroxisome prolifera-tor-activated receptor isoform a-agonists (Figure 8.39a). 

The enantioselective nitroaldol reaction of phe-nylalaninals 45 with nitromethane was also promoted with the N-anthracenylmethyl ammonium fluorides in the presence of potassium fluoride.1411 Interestingly, as shown in Scheme 16, the major product was the (2R,3S)-isomer 46a when N,N-dibenzyl-(S)-phenylalaninal and 12 (R=benzyl, X=F) were used while the (2S,3S)-isomer 46b was major when N-tert-butoxycarbonyl derivative 45b and 12 (R=allyl, X=Br) together with potassium fluoride were used. The nitroalcohols 46a and 46b were respectively converted to amprenavir 47a, a HIV protease inhibitor, and its diastereomer 47b. The... 

With two stereogenic centers, there are 22, or four, stereoisomers represented by this constitution. The one that is actually formed in this enzyme-catalyzed reaction is the 2R,3S isomer. 

Murakami et al. have utilized Mayer vesides to study aldolase-type reactions [48]. Formation of [i-phenylserinc from glydne and benzaldehyde proceeded effectively by cooperative catalysis of a hydrophobic pyridoxal derivative (47) and Zn(n) ions in the bilayer vesicle formed with 32. The threo isomer was dominantly produced over the erythro form. A marked enantioselectivity was observed in the co-veside of 32 and 35 in combination with 47 and Cu(ii) the ee for formation of (2S,3R)-P-phcnylscrinc over its enantiomeric (2R,3S)-isomer was 58%. Enantioselectivity also arose with another bilayer assembly, formed with 32, 35, and 37 in the presence of Cu(ii), where the (2R,3S) isomer was dominant over the (2S,3R) species in 13% ee. The opposite enantioselectivity performed by the second system, as compared with that for 47, might reflect a different stereochemical environment around the quinoid intermediate that allows the attack of benzaldehyde. 

Compounds that are achiral even though they have asymmetric carbon atoms are called meso compounds. The (2R,3S) isomer of 2,3-dibromobutane is a meso compound most meso compounds have this kind of symmetric structure, with two similar halves of the molecule having opposite configurations. In speaking of the two diastereomers of 2,3-dibromobutane, the symmetric one is called the meso diastereomer, and the chiral one is called the ( ) diastereomer, since one enantiomer is ( + ) and the other is (-). 

Full details have been given for the synthesis of a variety of a-diazo-esters (113) and of chiral oc-thio-esters (114). a,8-Epoxy-esters (115) can be obtained from the parent unsaturated esters either by using a new epoxidation reagent prepared from fluorine gas and aqueous acetonitrile or lithium t-butylhydroperoxide ( ). When this latter reagent is added to unsaturated esters derived from chiral alcohols, reasonable diastereoselection is observed but, as yet, not enough (up to 60%) to rival the Sharpless method. All four isomers of the useful epoxy-esters [(116) (2R,3S) isomer] have been obtained from the... 

Diastereomers were defined in the Introduction as stereoisomers that are not related as an object and its mirror image. Consider the four structures in Fig. 2.1. These structures exemplify the four possible combinations of two nonequivalent chiral centers in the stereoisomers of 2,3,4-trihydroxybutanal. The configurations at C-2 and C-3 are indicated according to the Cahn-Ingold-Prelog convention. Each structure is stereoisomeric with respect to any of the others. The 2R,3R and 2S,35 isomers are enantiomeric, as is the 25,3/ and 2R,3S pair. The 2R,3R isomer is diastereomeric with respect to the 2R,3S isomer, since they are stereoisomers but not enantiomers. The mirror image of 2R,3R is 25,35, and any object can have only one mirror image. 

E)-methyl crotonate to give, very largely, the (2R,3S)-isomer... 

In the enz3miatic hydrolysis of racemic 3-(4-methox5rphenyl)glycidic acid methyl ester in SCCO2 the initial rate of the 2S,3R) isomer was faster than the rate of the 2R,3S) isomer [65]. An immobilized lipase from Mucor miehei was used. The stereoisomeric excess of the (2R,3S) form reached 87% at 53% total conversion level. No effects of water content of the reaction mixture and initial concentration of the racemate on isomeric purity was foxmd. The reaction rate was faster in SCCO2 than in toluene/water. 

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