Isomerization epimerization

Optical resolution of the dithiirane 1-oxides 2 and 3 was accomplished by HPLC equipped with a chiral column (97T12203). Absolute configurations of 2a and 2b were determined by X-ray crystallography. The stereospecific isomerization (epimerization) of 2a to 3b and 2b to 3a was observed during the resolution study. 

The chemo-, regie- and enantioselectivity of enzymes make them ideal asymmetric catalysts. Most important is the differential recognition of diastereotopic groups of chiral and prochiral substrates. The mild conditions under which most enzymes operate minimize isomerization, epimerization and racemization associated with many other chemical processes.. 8... 

EC 5 Isomerases Racemization, cis-trans isomerization, epimerization one-substrate reactions no co-factors required... 

The common reaction mechanisms of chemical degradation of pharmaceutical compounds include hydrolysis, oxidation, isomerization/epimerization, decarboxylation, rearrangement, dimerization/polymerization, photolysis, and reactions with excipients and salt forms. Examples are shown in Table 7.6 Interested readers should consult reference books on drug stability for more information on degradation pathways [13, 22]. 

Racemization, ds-frans-isomerization, epimerization Formation of C-0, C-S, C-N, C-C-bonds... 

Keywords Pentacoordinate Silicon / Zwitterionic A, Si-Silicates / Monocyclic 5i-Silicates / Chirality / Isomerization / Epimerization / NMR Spectroscopy... 

Third, there is the necessity to utilize the UDP ester to consummate the isomerization (epimerization) of a single hydroxyl group in the conversion of a derivative of D-glucose into one of D-galactose. It can be argued that this is because the epim-erase (EC 5.1.3.7) needed just that ester. 

Working backward, we notice that methyl rrani -d-isopropylcyclohexanecarboxylate has both large groups equatorial and is, therefore, more stable than the corresponding cis isomer. This stability of the trans isomer means that, if we were to synthesize the cis isomer or a mixture of both the cis and trans isomers, we could obtain the desired trans isomer by a base-catalyzed isomerization (epimerization) ... 

The stereochemistry of pyrazolines and pyrazolidines has already been discussed (Section 4.04.1.4.3). Optically active A - and A -pyrazolines have seldom been described (77JA2740, 79CJC360), but cis-trans isomeric pairs are common. The C-4 acid-catalyzed epimerization involves the mechanism shown in Scheme 38 (70TL3099), but in spite of some inconclusive arguments the C-5 epimerization has never been established with certainty. 

Pyridone also catalyzes epimerization of the anomeric position of the tetramethyl ether of glucose. The mechanism involves two double proton transfers. The first leads to a ring-opened intermediate, and the second results in ring closure to the isomerized product ... 

Ethynylation of 3j -hydroxy-16a-methyl-5a-androstan-17-one in a mixture of diethylene glycol dimethyl ether and diethylene glycol monoethyl ether in the presence of potassium hydroxide produces two isomeric 17-ethynyl derivatives. This result is not unexpected since molecular models suggest that the steric influence of the 13/ -methyl group is nearly offset by the 16a-methyl group. The presence of a 16a-acetoxy group in the estrone series also leads to the formation of epimeric 17-ethynyl compounds (61) and (62) on reaction with acetylenedimagnesium bromide. 

The first substance examined in the steroid field was 3j6-hydroxycholest-4-ene (1) and the epimeric 3a-alcohol (3). These compounds react stereospecifically in dry ether with the Simmons-Smith reagent to yield the isomeric cyclopropyl carbinols (2) and (4) in 90 % and 67 % yields, respectively. The rate of this reaction is about one fifth of that observed with simple cyclic car-binols. ... 

Oxidation of the hydroxy group in (10) to the ketone followed by isomerization affords the 10oc-methyl-A -3-ketone (11). In contrast, methylenation of 3)5-hydroxy-A ° -compounds proceeds in refluxing ether solution to give, after oxidation and acid rearrangement, the natural 10/5-methyl-A -3-keto steroids. With an epimeric mixture of 3fi- and 3a-A ° -alcohols only the )5-alcohol reacts under these imild conditions. ... 

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. 

This reaction converts ribulose-5-P to another ketose, namely, xylulose-5-P. This reaction also proceeds by an enediol intermediate, but involves an inversion at C-3 (Figure 23.31). In the reaction, an acidic proton located a- to a carbonyl carbon is removed to generate the enediolate, but the proton is added back to the same carbon from the opposite side. Note the distinction in nomenclature here. Interchange of groups on a single carbon is an epimerization, and interchange of groups between carbons is referred to as an isomerization. 

Intramolecular cycloadditions of 4/f-pyrido[l,2-n]pyrimidin-4-ones 235 (R = H, Me Ph) and MeNHOH HCl gave tetracyclic isoxazolo derivatives 237. In the case of 235 (R = Me) a minor epimer 238 was also isolated (00JCR(S)414). Similar reaction of 235 (R = H, Me, Ph) and sarcosine ethyl ester HCl afforded an isomeric mixture of epimeric tetracyclic pyrrolo derivatives 239 and 240. In the reaction of 235 (R = H) and PhCHjNHCHjCOOEt only one product 241 was obtained. 

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