Glucopyranose boat form

All of the crystalline pyranoses thus far examined adopt a chair conformation. A boat conformation has not yet been found for crystalline monocyclic compounds of sugars. Fused-ring systems seem to be required for part of the molecule to adopt a boat form, as in sedo-heptulosan (5) (where a chair form is also a part of a boat form (fused to the boat form)27 and l,6-anhydro-/3-D-glucopyranose. 

Figure 11.7. Chair and Boat Forms of P - d -glucopyranose. The chair form is more stable because of less steric hindrance as the axial positions are occupied by hydrogen atoms.

A mathematical method has been described for determining, from independent variables that include the contribution of valence-angle deformation, the potential barrier for conformational inversion in j8-D-glucopyranose. The energy required for the conversion of a chair form to a boat form was calculated to be 13.5 kcal.mole , and the energy required for the reverse process, 6.5 kcal.mole . Ex-... 

C-1) form (Class B) of the D-glucopyranose precursor is much more widely encountered in plants than that via the alternative C4 (1-C) or skew-boat forms (Class C). As yet only one plant. Fuchsia sp. (Onagraceae), has been detected in which both classes of metabolite co-exist (44). 

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