Epimerization equilibrium

The characteristic ratio of atactic polylferf.-butyl vinyl ketone) is determined from light scattering and viscosimetry measurements, and at 300 K in benzene the dipole moment ratio and its temperature coefficient are measured. Calculations of Ca and Da based on a two-state RIS model, with parameters independently derived from a previously developed semiempirical potential energy surface and from epimerization equilibrium measurements for dimeric and trimeric oligomers, are in excellent agreement with the experimental results. The predicted temperature coefficient is positive but lower in magnitude than that observed. 

During the course of epimerization in a sufficiently alkaline medium, the nitronate absorption at Amax 250 nm remains virtually unchanged. However, on prolonged standing, the epimeric equilibrium mixtures undergo a slow, secondary, chemical change.161 This change is... 

Fig. 8.6 Exogonic acid, a free component of Brazhian jalap resin (not linked structurally to its resin glycosides) exists as epimeric equilibrium of two major stereoisomers shown structurally together with two minor stereoisomers (2R,5S,7R / ,Z and 2R,5RJR / Z,E not shown)...

This reaction is a clear example of the importance of tautomeric equilibrium studies in this series since, to the extent that the starting thiazolone does not epimerize in the medium, asymmetric induction may be expected in this reaction (453, 455). 

Penicillin sulfoxides can be epimerized by heat to afford thermal equilibrium mixtures of a- and /3-sulfoxides, the position of the equilibrium depending on the C(6) side chain (Scheme 5). Deuterium incorporation studies support a sulfenic acid, e.g. (18), as the intermediate in these transformations. This mechanism is also supported by the finding that when an a-sulfoxide epimerizes to a /3-sulfoxide there is a simultaneous epimerization at C(2) (71JCS(C)3540). With irradiation by UV light it is possible to convert a more thermodynamically stable /3-sulfoxide to the a-sulfoxide (69JA1530). 

The first, and to this writing still only case of a ketone a-cleavage-recombi-nation sequence in the steroid field was reported by Butenandt, who found that 17-ketones epimerize at C-13. Ultraviolet irradiation of either stereoisomer produces an equilibrium mixture in which the thermodynamically more favored 13a-compound cf. (15)] with cw-fusion of rings C and D predominates at room temperature. As ultraviolet absorption energies and intensities of the two isomeric ketones are practically identical, the equilibrium composition depends largely on the rate of the competing recombination process from (14). For further examples of the photoisomerization at C-13 of 17-ketosteroids, see ref. 8, 12, 15 and 43. 

In its present form, intermediate 12 is not a viable substrate for the crucial Dieckmann condensation it must undergo prior epimerization at C-16. When intermediate 12 is treated with sodium methoxide in hot methanol, enolization at C-16 occurs and an equilibrium is established between 12 and a diastereomeric substance, intermediate 11. Once formed, 11 can either revert back to 12 through the planar enolate form, or it can participate in a productive cyclization reaction to give a new six-membered ring. Under these conditions, the desired transformations take place with exceptional facility to give, after acidification of the reaction medium, enol ester 10. 

Commercial A -acetylneuraminic acid aldolase from Clostridium perfringens (NeuAcA EC 4.1.3.3) catalyzes the addition of pyruvate to A-acetyl-D-mannosamine. A number of sialic acid related carbohydrates are obtained with the natural substrate22"24 or via replacement by aldose derivatives containing modifications at positions C-2, -4, or -6 (Table 4)22,23,25 26. Generally, a high level of asymmetric induction is retained, with the exception of D-arabinose (epimeric at C-3) where stereorandom product formation occurs 25 2t The unfavorable equilibrium constant requires that the reaction must be driven forward by using an excess of one of the components in order to achieve satisfactory conversion (preferably 7-10 equivalents of pyruvate, for economic reasons). 

The formation of alkenes from thiirane dioxides may not be stereospecifically controlled in the presence of a sufficiently strong base and sufficiently acidic protons in the three-membered ring. Under such conditions (essentially those typical for the Ramberg Backlund reaction), epimerization via a carbanion intermediate produces an equilibrium mixture of thiirane dioxides19,99 and consequently a mixture of cis- and trans-alkenes. 

However, upon dissolution, an epimerization of the anions can occur in the presence of acidic counter-ions. This is particularly true for 16a-16d [39]. The nature of the solvent (MeOH, CHCI3) plays a crucial role on the kinetics of epimerization and the position of the resulting equilibrium. For anions made with a 2R, 3R) tartaric backbone, a A configuration is always preferred in MeOH the selectivity, obtained after a slow equilibration, being independent of the nature of the ester alkyl chain (diastereomeric ratio (d.r.) 3 1). However, in chloroform, the A diastereomer is rapidly obtained and the selectivity is best if the ester side chain is sterically demanding (d.r. 2 1 to 9 1 from 16a to 16d) (Scheme 16). 

C. 1,3,2-Oxazaphospholans.—The diastereoisomeric spirophosphoranes (25) and (26) obtained from ( - )-ephedrine and trisdimethylaminophosphine are in equilibrium in benzene solution, their relative proportions varying with temperature. That this equilibration, which requires epimerization at... 

Compound (S)Sn(S)c-(75) was obtained 27) by preferential crystallization of the less abundant and less soluble compound of the 40/60 equilibrium mixture of epimerizing (S)s (S)c and (R)s (S)c compounds. Since the two diastereomers have different... 

DMSO causes an epimerization by which both fractions are transformed in a few hours into an equilibrium mixture (48 52). 

For compound (79), the addition of pyridine or of DMSO does not cause any epimerization after a few hours. However, after three days, the (79)A (79)B = 54 46 mixture obtained via scheme 9 is tranformed into the 52 48 equilibrium mixture. 

Since transfer hydrogenation reactions of carbonyls are always equilibrium reactions, it is possible to perform both a reduction and an oxidation of a substrate simultaneously. In this way, these reactions can be utilized for both racemizations and epimerizations. 

One of the best-known systems involving epimerization is that of d-glucose. In aqueous solution the epimeric a- and 0-forms are in equilibrium, but each can be obtained by crystallization ethanol readily affords the pure a-form, whereas pure 0 may be obtained from pyridine. 

Dimethylcyclobutane, isomerization, 30 31 Dimethylcyclohexane conformational analysis of, 18 14 experimental equilibrium, 18 17 epimerization of, 25 136 vibrational spectra, 42 239 Dimethylcyclohexylbenzene, 42 432 Dimethylcyclopentanes aromatization, 30 54 isomerization, 30 34... 

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