Carbohydrates epimers

Thietanonium ions have been observed quite frequently either as intermediates or as isolated stable salts. Treatment of an activated carbohydrate epimer, a constituent of the antiobiotic lincomycin, with thietane led to the sulfonium salt 190. By nucleophilic attack with acetate ions the ring could be opened and the process mentioned above repeated so that compound 191 results. ... 

S. Passeron and E. Recondo, Dicyclohexylcarbodi-imide a new agent for carbohydrate epimer-ization and isomerization. Part I. Psicose formation, J. Chem. Soc., (1965) 813-81 R. Griinnagel and H. J. Haas, Darstellung von D-Tagatose, Ann., 721 (1969) 234-235. 

Antineoplastic Drugs. Cyclophosphamide (193) produces antineoplastic effects (see Chemotherapeutics, anticancer) via biochemical conversion to a highly reactive phosphoramide mustard (194) it is chiral owing to the tetrahedral phosphoms atom. The therapeutic index of the (3)-(-)-cyclophosphamide [50-18-0] (193) is twice that of the (+)-enantiomer due to increased antitumor activity the enantiomers are equally toxic (139). The effectiveness of the DNA intercalator dmgs adriamycin [57-22-7] (195) and daunomycin [20830-81-3] (196) is affected by changes in stereochemistry within the aglycon portions of these compounds. Inversion of the carbohydrate C-1 stereocenter provides compounds without activity. The carbohydrate C-4 epimer of adriamycin, epimbicin [56420-45-2] is as potent as its parent molecule, but is significandy less toxic (139). 

O-Isopropylidene derivatives of carbohydrates form structural isomers from carbohydrates which themselves are epimers. Since structural isomers often fragment differently whereas epimers do not, mass spectra of these derivatives may permit interpretation in terms of stereochemistry. Although molecular-ion peaks are not observed, the molecular weight can be determined readily from a relatively intense M-CH/ peak, resulting from loss of a methyl radical from a 1, 3-dioxolane ring (12). 

R. S. Bhatt, L. Hough, and A. C. Richardson, Transformation of lactose into its 3-epimer, 4-O-p-D-galactopyranosyl-D-allopyranose, Carbohydr. Res., 51 (1976) 272-275. 

With respect to five-membered lactones fused to hexopyranose units, some approaches have been reported so far and the exploitation of their synthetic potential has led to the access of new carbohydrate derivatives. Bicyclic derivatives of this type are key intermediates in the synthesis of the epimer at C-3 of the sugar moiety contained in miharamycins [212, 213]. The latter are antibiotics known to inhibit strongly Pyricularia oryzae, which produces the rice blast disease. These compounds are also considered to be a potential bioterrorism agent (Scheme 42). Hence, the 3,3-spiroepoxide 176 was converted into the 3-C-cyanomethyl derivative 177, the hydrolysis of which led to spontaneous cyclization in the presence of... 

Mannose, the C-2 epimer of glucose (see p. 84), is an important component of glycoproteins (see p. 164). Hexokinase phosphorylates mannose, producing mannose 6-phosphate, which, in turn, is (reversibly) isomerized to fructose 6-phosphate by phosphoman-nose isomerase. [Note There is little mannose in dietary carbohydrates. Most intracellular mannose is synthesized from fructose, or is preexisting mannose produced by the degradation of structural carbohydrates and salvaged by hexokinase.]... 

Intramolecular 1,3-dipolar additions of nitrones and nitrile oxides to carbohydrate alkene groups have met with success. Thus, treatment of the unsaturated heptose ether 68 (Scheme 17), which can be made following 1,3-dithianyl anion addition to C-l of 2,3,4-tri-0-benzyl-5,6-dideoxy-D-xy/o-hex-5-enose, with IV-methylhydroxylamine in refluxing methanol, affords the nitrone 69 that cyclizes to give the bicyclic isoxazolidine 70 (60% isolated) together with the epimer at the new asymmetric center carrying the methylene carbon atom (16% isolated) [35]. 

Classification of Carbohydrates 1102 23-3 Monosaccharides 1103 23-4 Erythro andThreo Diastereomers 1106 23-5 Epimers 1107... 

Schnupf, U. Willett, J. L. Momany, F. DFTMD studies of glucose and epimers Anomeric ratios, rotamer populations, and hydration energies, Carbohydr. Res. 2010, 345, 503-511. 

The Kiliani-Fischer synthesis lengthens a carbohydrate chain hy adding one carhon to the aldehyde end of an aldose, thus forming a new stereogenic center at C2 of the product. The product consists of epimers that differ only in their configuration about the one new stereogenic center. For example, the Kiliani-Fischer synthesis converts D-arabinose into a mixture of D-glucose and D-mannose. 

Carbohydrates that differ in their configuration around a specific carbon atom than the carbonyl carbon atom are called epimers. 

It has been mentioned (p. 13) that D-manno-o-tolo-heptitol is identical with natural voleraitol. If -one desires a supply of voleraitol, probably the best procedure at the present time is its synthetic production from D-mannose even though the D-manno-D-synthetic method appears to be more practicable than the isolation of the alcohol in quantity from known natural sources. The use of barium cyanide or the equivalent mixture of a barium salt and sodium cyanide, in place of hydrocyanic acid appears to favor the yield of the desired heptonic acid. On the other hand, the possibility of starting with o-altrose, which is now no longer a particularly rare sugar, appears of interest because the desired heptose (D-altro-D-mormo-heptose) should be the major product in that series if the generalizations that have been presented hold for altrose. This speculation has been offered because it seems to the writer that many carbohydrates that are now only laboratory curiosities, can be made at costs that are not prohibitive for the use of them in exploratory researches, especially in the bacteriological, physiological and biochemical fields. 

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