Epimers interconversion

The stereochemical isomerization or structural rearrangement resulting in the interconversion of epimers. For example, the conversion of jS-D-glucose to jS-D-galac-tose involves epimerization at the C-4 carbon atom, and the epimerization at the C-2 of jS-D-glucose results in the synthesis of jS-D-mannose. 

A conscientious investigation of the stereochemical features was undertaken, including Li and NMR studies of the lithium intermediates . The epimeric ratio is kinetically controlled in the deprotonation step by a high preference for the 1-pro-R-H in 398a, and the stereocentre is configurationally stable at —78°C. The ratio of l-endo-399 and its epimer (Li behind the plane) was found to be 96 4. The interconversion of the E/Z-isomers l-endo-399 and l-exo-399 at — 78°C is slower than the rate of alkylation with... 

The different conformations of indolo[2,3-a]quinolizidines play an important role in predicting the thermodynamically more stable epimer (see below). The indolo[2,3-a]quinolizidine system can exist in three main conformations one C/D trans ring juncture (conformation a) and two C/D cis ring junctures (conformations b and c). These are in equilibrium by nitrogen inversion and cw-decalin type ring interconversion. Ring C is considered to be in a half chair conformation and ring D in a chair conformation, Scheme (16) [40]. 

The network as depicted in the figure is a very simplified one. It does not incorporate epi-lactose (C-2 epimer of lactose, 4-0-p-D-galacto-pyranosyl-D-mannose) and further reactions of D-galactose. However, it does describe the major events the interconversion of lactose and lactulose, the formation of D-galactose by an elimination reaction and the simultaneous formation of acidic by-products from the glucose moiety. 

The interconversion of glucose and galactose occurs at the UDP-hexose level (fig. 16.2) with a 4-epimer-ase. The epimerase is unusual in that it has a covalently bound NAD +. What function do you propose for the NAD+, and what intermediate do you expect in the reaction ... 

The optically active square pyramidal complexes 39 and 41—49a and b epimerize in solution, as shown in Fig. 4 for 38a and b, some of them at lower, and some at higher temperatures5 f 63-65. The first-order decline in optical rotation yields overall rate constants k, which are the sum of the individual rate constants k and fc2 f°r the interconversion of both epimers into each other56,63-65 [Eq. (19)]. These first-order rate constants k for the approach to equilibrium were used for the calculation of the half-lives given in Table 1. A change in the solvent from DMF to toluene has only a small effect on the half-lives of the configurational stability63. ... 

Considerable effort continues to be expended on the synthesis of alkaloids of this group and on studying further their interconversion. However, an efficient synthesis of dihydroantirhine, uncontaminated by epimers, still eludes investigators. In the most recent synthetic study it was shown that hydrogenation of the acetate of the tetracyclic base (91a), prepared earlier,700 followed by deacetylation, affords a mixture of the C-20 epimeric 3-epidihydroantirhines (91b).706... 

Two diastereomeric sugars differing only in the configuration at a single asymmetric carbon atom. The epimeric carbon atom is usually specified, as in C4 epimers. If no epimeric carbon is specified, it is assumed to be C2. The interconversion of epimers is called epimerization. (pp. 1107, 1115)... 

When the inversion barriers are above ca. 23 kcal/mole, the isolation of epimers (invertomers) or optically active substances becomes feasible. The first-order rates for the unimolecular epimerization or racemization processes may then be measured at several temperatures by following the interconversion processes spectroscopically (NMR, UV, IR) or polari-metrically. The treatment of the data is straightforward 18>. 

Stereoselective toxicokinetics of pyrethroids was observed in rodents. Rats injected with a racemic dose of 0-cypermethrin had much lower amounts of the (+)-enantiomer compared to the (—)-enantiomer in plasma, heart, liver, kidney, and fat tissues. The authors suggested rapid interconversion of (+)-a5, l/f,35 -cypermethrin to its antipode ( )-a/f,15, 3R-cyper-methrin in plasma, but no reverse conversion of the ( )-enantiomer back to the (+)-enantiomer. This hypothesis was criticized [285] as implausible, as three separate epimer-ization reactions would be necessary for conversion of (+)-a5, l/f,35 -cypermethrin to (—)-a R, 15,3R-cypermethrin. However, the results do indicate significant enantioselectivity in the in vivo processing of cypermethrin by rats, but is not clear to what extent this enantioselectivity was from biotransformation or from tissue-specific redistribution. The latter was suggested by the data [84] consistent with the highly enantioselective screening of a-HCH by the rat blood-brain barrier [259]. 

All stereoisomers that are not enantiomers are termed diastereomers. For example, in a molecule containing two or more stereogenic centres any pair of stereoisomers that are not enantiomers are diastereomers. In other words, a pair of diastereomers share the same configuration at one or more, but not all, of their stereogenic centres and, unlike enantiomers, they have different physical properties. Two diastereomers that differ in configuration at only one of a number of stereogenic centres are called epimers, and their interconversion epimerization. 

Lutsig, M.J., Cadet, J., Boorstein, R.J., andTeebor, G.W. (1992) Synthesis of the diastereomers of thymidine glycol, determination of concentrations and rates of interconversion of their cis-trans epimers at equilibrium and demonstration of differential alkali lability within DNA. Nucleic Acids Res., 20, 4839-4845. 

The H-NMR spectra of salts of morphine and its congeners are complicated by the existence of protonated epimers, evident from signal duplication and first reported in a 600 MHz study carried out at low pH to slow the interconversion rate Figure 5.10 and [44]). The usual pronounced preference of N-methyl for the equatorial conformation in piperidine derivatives is reduced in the fused-ring system of morphine by non-bonded interactions with the fi-lO-... 

Mannose is found in the diet in small amounts. Tike galactose, it is an epimer of glucose, and mannose and glucose are interconverted by epimerization reactions. The interconversion can take place either at the level of fructose 6-phosphate to mannose 6-phosphate, or at the level of the derivatized sugars (see Fig. 30.8). 

Epimers are pairs of diastereoisomers differing in configuration at just one chiral centre. Epimerization is the interconversion of such diasteroisomers just as racemization is the interconversion of enantiomers. 

Several epimeric pairs of monosaccharides are produced by epimerization of nucleotide sugars (Luckner, 1990). For example, UDP-D-glucose-4-epimerase converts UDP-D-glu-cose to UDP-D-galactose. Enzymes that produce the other epimers are also known. A C-2 epimerase may be involved in the interconversion of D-glucose and D-mannose (Karr, 1976). 

Eplmers, eplmerization (Sections 18.3A and 22.8) Diastereo-mers that differ in configuration at only a single tetrahedral chirality center. Epimerization is the interconversion of epimers. 

Complex 66 is supposed to be present as a mixture of two diastereomers (epimers). Rapid interconversion between them relative to reductive elimination enables the dynamic kinetic resolution to take place. The striking performance of 2-iodo-A,A-diisopropylbenzamides was not mechanistically investigated but coordination of the amide carbonyl group to the Pd centre, forming a five-membered palladacycle, was suggested. 

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