Conformation of pyranoid sugars

The symbols CA and CE, appended to the names of the structures shown, follow an improved system for indicating the principal conformations of pyranoid sugars and derivatives [H. S. Isbell and R. S. Tipson, J. Ret. Natl. Bur. Sid., 64A, 171 (I860)]. Through the use of symbols, a precise specification of all of the principal pyranoid conformers can be made. 

The two chairlike conformations of pyranoid sugars and their derivatives represent potential-energy minima as compared with other theoretically possible conformations. The idealized depictions given for the two possible chairlike forms of j8-D-glucopyranose show the axial and equatorial dispositions of the bonds to the various substituents. 

While the broad mission of the National Bureau of Standards was concerned with standard reference materials, Dr. Isbell centered the work of his laboratory on his long interest in the carbohydrates and on the use of physical methods in their characterization. Infrared spectroscopy had shown promise in providing structural and conformational information on carbohydrates and their derivatives, and Isbell invited Tipson to conduct detailed infrared studies on the extensive collection of carbohydrate samples maintained by Isbell. The series of publications that rapidly resulted furnished a basis for assigning conformations to pyranoid sugars and their derivatives. Although this work was later to be overshadowed by application of the much more powerful technique of nuclear magnetic resonance spectroscopy, the Isbell— Tipson work helped to define the molecular shapes involved and the terminology required for their description. 

The study of the conformations of cyclic sugars was developed more fully by Reeves in a series of papers, beginning in 1949, dealing with the formation of complexes of sugars and their derivatives in cuprammonia solution. Reeves was able to provide experimental evidence to indicate that some pyranoid sugars indeed adopt chair conformations in solution, and that, in such cases, one chair form usually appeared to preponderate over the other. Boat or skew conformations were considered possible if the nonbonded interactions between the substituents in the chair conformation became too large. " Reeves s investigations have formed the basis of much subsequent work in the conformational analysis of cyclic carbohydrate... 

The n.m.r. spectral method, as a tool for monitoring rate processes, can also, in suitable cases, be used for measurement of the rate of chair-chair conformational inversion of pyranoid sugar derivatives, and thus, of the energy barrier for ring inversion (see p. 91). 

Because the Reeves-Kelly scheme neglects several important non-bonded interactions, such as those between syn-diaxial hydroxyl groups and vicinal, gauche substituents, and because polar interactions are neglected altogether, the scheme may, at best, be considered to represent a qualitative attempt to predict the conformational behavior of pyranoid sugars in solution. 

The pyranoid monosaccharides provide a wide range of asymmetric molecules for study by the c.d. spectroscopist. However, these compounds are not without their difficulties. In aqueous solution, these compounds exist in a complex equilibrium involving the two possible chair conformers of the pyranoses, the furanoses, a and p anomers, and the acyclic form, as well as septanoses for aldohexoses and higher sugars. 

P2j Z = 2 D = 1.17 R = 0.080 for 3,888 intensities. This is aconfigu-rational analysis of the macrolide antibiotic 23672RP from Streptomyces chryeus. All three sugar residues are pyranoid the conformation of the a-ketose is CX, with Q = 56 pm, 0 = 9° that of the / -D-mycinose (6-deoxy-2,3-di-0-methyl-D-allose) is 4Clt with Q = 59 pm, 0 = 6° and that of the / -L-mycarose (2,6-dideoxy-3-C-methyl-L-riho-hexose) is 1C4, with Q = 53 pm, 0= 177°. The O-5-C-l-O-l-C glycosidic torsion-angles are —71, —87, —83°. The atomic coordinates reported in the paper refer to the opposite enantiomer. 

P. L. Durette and D. Horton, Conformational studies on pyranoid sugar derivatives by NMR spectroscopy. Correlations of observed proton-proton coupling constants with the generalized Karplus equation, Org. Magn. Reson., 3 (1971) 417-427. 

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