Methyl aldofuranosides

Calcium-selective electrodes have long been in use for the estimation of calcium concentrations - early applications included their use in complexometric titrations, especially of calcium in the presence of magnesium (42). Subsequently they have found use in a variety of systems, particularly for determining stability constants. Examples include determinations for ligands such as chloride, nitrate, acetate, and malonate (mal) (43), several diazacrown ethers (44,45), and methyl aldofuranosides (46). Other applications have included the estimation of Ca2+ levels in blood plasma (47) and in human hair (where the results compared satisfactorily with those from neutron activation analysis) (48). Ion-selective electrodes based on carboxylic polyether ionophores are mentioned in Section IV.B below. Though calcium-selective electrodes are convenient they are not particularly sensitive, and have slow response times. 

Methyl aldofuranosides are reluctant to complex Ca2+. In aqueous solution, at 298.2 K, K1 values range from 0.1 for methyl a-D-xylofuranoside to 1.4 for methyl a-D-ribofuranoside (46), though in methanol KA — 120 for the latter (655). 

Hudson s Rules of isorotation have been applied to furanosides and a correlation with known conformations of methyl aldofuranosides made. The molecular rotations and differences in molecular rotation were tabulated for all aldo-pentoses, aldohexoses, and hexuloses as their methyl furanosides, and hence their conformations were deduced. The jS-D-pentofuranosides were mainly in the T2 form with the 0-methyl group quasi-axial and the side-chain quasi-equatorial, whereas the a-D-pentofuranosides were either in the E form or a mixture of T forms. The optical rotations for all a-D-furanosides are negative, but for 3-d-furanosides they are all positive. ... 

In Table I, the more stable anomer is listed first. In each of the four cases, this anomer is the Irans-C 1 -C2-aldofuranoside, having a Irons relationship of the aglycon group to the hydroxyl group at C-2. Thus, the 0-d anomers of methyl D-ribofuranoside and methyl D-xylofuranoside are listed first, even though the interactions for the latter are equal. Later in this Chapter, the effect of this conformational stability will be discussed in relation to formation of products At present, it suffices to mention that there are two factors to be considered, the C-l and C-2 interactions, and the overall interactions (C-l and C-2, C-2 and C-3, C-3 and C-4). The... 

The uniqueness of the aldofuranosides, as compared to the aldopyrano-sides, was recognized initially through the difference in rates of hydrolysis by acid. This difference was confirmed by the use of methylation techniques the latter have, to a great extent, been supplanted by the periodate oxidation of Jackson and Hudson120 and by oxidation with lead tetraacetate. Hudson s rules of isorotation were also employed to a great extent. 

Tables XII to XV list the melting point and specific optical rotation of some methyl glycofuranosides, phenyl aldofuranosides, alkyl glycofur-anosides, and ethyl l-thioaldofuranosides.

Agars, 267, 272, 274, 279, 280, 282, 286 Albumins, of serum, 244 Alditols, 1-amino-l-deoxy, 41, 57 Aldofuranosides, hydrolysis of, 27, 42 Aldohexoses, tetra-O-methyl-, epimeri-zation of, 85 Aldol, 327... 

In their chemical properties, both these types of compound resemble to a certain degree the methyl /3-D-aldopyranosides or the methyl a- or /3-D-aldofuranosides. They do not have reducing properties, and are relatively stable in alkaline media, but are hydrolyzed by acids to the free hexoses. 

Likewise, the splitting of a methyl glycofuranoside (72), through (73) and (74), or through (76) and the cyclic ion (77), is possible. As found experimentally, the formation and hydrolysis of aldofuranosides occurs about 10-100 times faster than those of the corresponding aldopyrano-sides. On the basis of the theory of Brown and associates, this is easily understandable, if it is assumed that the reaction proceeds... 

The methyl furanosides of the uronic adds appear to be similar in properties to the neutral aldofuranosides. As yet no reliable evidence has been advanced for the existence of furanosiduronic acid groups in natural products. 

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