Pyranoid Sugars and Their Derivatives

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. 

The system of conformational terminology that has thus far been utilized most widely was introduced by Reeves, and is based on the second system mentioned, that is, it assigns symbols for specified ring-shapes, as shown. The symbols are defined as CJ and 1C for 

The Isbell-Tipson system of conformational nomenclature has been much less widely used in the literature than the Reeves system, al- 

In addition to the chair conformations, the pyranoid ring may also be formulated in a flexible cycle, where it is also free from bond-angle strain. The cycle includes six boat and six skew forms, each boat form being a maximum-energy transition-state between two skew forms (see Fig. 1). The high-energy transition state in passing 

For monocyclic sugars, non-bonded interactions render the skew, and, to a greater extent, the boat, forms less stable than the chair forms. The boat forms make an insignificant contribution to the conformational equilibrium of monocyclic pyranoid sugars at room 

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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 introduction of substituents and the effects of solvation profoundly influence the conformational equilibria of monocyclic pyranoid sugars and their derivatives, because of the changes in steric and electronic interactions that result. The conformations of numerous polysubstituted pyranoid sugars in various solvents have been investigated, mostly by application of the n.m.r. spectroscopic method. Six aldopentopyranose tetraacetates were examined in chloroform solution near room temperature they were considered to exist almost entirely in the CJ(d) conformation, except for the jS-D-ribo derivative, which appeared to contain substantial proportions of each chair form, and the -L.-arahino derivative which was estimated to contain mostly the CJ(l) conformation. 2-Deoxy-j8-D-e/T/thro-pentopyranose triacetate was considered to exist mainly in the 1C conformation. 

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... 

Tables V-LVII detail H and F shift and coupling information, and Tables LVIII to LXXI incorporate the C-n.m.r. data. The data within this compilation are arranged according to the following outline hexoses prior to pentoses, followed by anhydro sugars, sugar acids and lactones, amino sugars (and their synthetic, A -containing precursors), mono-, di-, and tri-deoxy sugars, branched derivatives, ketoses, polyfluorinated monosaccharides, and, finally, difluorinated amino sugars. Within this compilation, and even within each table, pyranoid derivatives are listed prior to their furanoid counterparts, hexoses prior to pentoses, functionalized prior to deoxy compounds the arrangement within each sub-table is made alphabetically.

A notable exception has been the application of i.r. spectroscopy by Isbell and Tipson to the analysis of the conformations of methyl aldopyranosides," acetylated methyl aldopyranosides," " and fully acetylated aldopyranoses in the solid state. Their approach was based on (J) the assumption that the pyranoid sugar derivatives predicted by Reeves to exist in only one of the chair conformations in solution would adopt that same conformation in the crystalline state, and (2) the observation that axially and equatorially attached groups at the anomeric carbon atom give rise to different vibrations. 

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