Conformations of Septanosides

The effect was first proposed by J. T. Edward, whose account of the historical background is published in ACS Symp. Ser., 1993, 539, 1. 

Streitweiser and P. H. Owens, Orbital and Electron Density Diagrams, Macmillan, New York, 1973, p. 120. 

Streitweiser, C. H. Heathcock and E. M. Kosower, Introduction to Organic Chemistry, Macmillan, New York, 4th edn, 1992. 

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Studies on conformations of septanosides in the solid-state, solution-phase, and computational methods are still limited, unlike in the case of naturally occurring furano-sides and pyranosides for which finer details of the conformations are well elucidated [34]. Being a larger ring system, septanosides are more flexible, leading to attenuation of more complex pseudorotational equilibria, with energy barriers among... 

FIGURE 134 Twist-chair ( TCj 4) conformation of septanosides 100-102 in the solid state. 

Conformation of septanosides in the solution phase is more diverse and complex, as compared with the solid-state structural determinations. The NMR technique of proton-proton coupling constants is used predominantly in the stractural determinations in solution phase. 

FIG. 11. Left Low-energy conformations of seven-membered rings. Right Different views of methyl a-D-g/ycero-D-ido-septanoside (198). Shown are wedge-and-dash, chair conformation, and two-labeled T conformers, illustrating how the T nomenclature works. 

Fig. 13. Conformational map of C-6-unsubstituted septanosides. Left A pseudorotational wheel of septanose conformations. Solid ovals mark conformations of P-septanosides, dashed ovals a-septanosides. Right Septanosides and the conformers that have been observed for them.

Fig. 17. Computed low-energy conformations of methyl septanosides. Left Methyl a-D-glycero-D-ido-septanoside (conf 36) Right Methyl P-D-g/yccro-D-gw/oseptanoside (conf 1).

N. V. Ganesh, S. Raghothama, R. Sonti,andN. Jayaraman, Ring expansion of oxyglycals. Synthesis and conformational analysis of septanoside-containing trisaccharides, J. Org. Chem., 75 (2010) 215-218. 

Figure 2.21 Some conformations of glucose-derived septanosides.

There have also been several analyses of septanoside derivatives, including that of 3-0-acetyl-l,2 4,5-di-0-isopropylidene-a-D-glucoseptanose (39). The seven-membered ring of this compound has a distorted chair conformation (torsion-angle range 9—89°), while one of the dioxolan rings has a distorted twist conformation (7—34°) and the other is an envelope (3—38°). The conformation of (39) may be contrasted with that of (40), in which... 

There are few instances wherein septanoside adopted a rC2 3 conformation. Figure 13.5 provides examples of septanoside derivatives with this conformation [50,51]. An analysis of the torsion angles C3—C4—C5—C6 of -49 and C4—C5—C6—06 of -32 of needle-type orthorhombic crystal of 103 showed that mean plane passed through C4, C5, and C6 atoms. Thus, the conformation was assigned as rC2 3, describing that 06 and Cl are above the plane whereas C2 and C3 below the plane [50]. 

The boat conformation of 105 was assigned on the basis of the matching of the observed torsion angles within the septanoside with that of oxepane... 

Figure 13.7 shows examples of septanosides, where the chair conformation was reported for septanoside derivatives, on the basis of and coupling constants. Specifically, the chair conformations were determined to be 2 through observation... 

The solution-state conformation of o of the bicyclic septanoside 123 and 124 was deduced from experimentally observed coupling constants (Figure 13.10). In both of the cases, the observed coupling constants > 0.8 Hz and 75 > 8.6 Hz... 

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