Boat conformation glucose

The low reactivity of l,6-anhydro-2,3,4-tri-0-benzyl-/3-D-allopyra-nose is also of interest.106 This monomer has 1,3-interaction between substituents on C-2 and C-4 in the C4(d) conformation. However, when conversion into a boat conformation, as proposed, occurs on reaction with a propagating cation, eclipsed bonds develop at C-2, C-3, and C-4. It is, therefore, not surprising that the D-allo anhydride is less reactive than those of D-mannose, D-glucose, or D-galactose. The corresponding, D-altrose derivative has only one axial substituent in addition to the anhydro ring, and these are on opposite sides of the pyranose ring, and therefore do not interact it would be expected to be, and has proved to be, of very low polymerizability.106... 

Fig. 7. (a) Chair and boat conformations of pyranose rings (b) stable chair form of [3-D-glucose. 

D-(-f )-Glucose contains the six-membered, pyranose ring. Since the C—O—C bond angle (11 L) is very nearly equal to the tetrahedral angle (109.5°), the pyranose ring should be quite similar to the cyclohexane ring (Sec. 9.14). It should be puckered and, to minimize torsional and van der Waals strain, should exist in chair conformations in preference to twist-boat conformations. X-ray analysis shows this reasoning to be correct. 

Fig. 16.—Model of hypothetical cyclotriamylose with the D-glucose residues in the flexible ( boat ) conformation.

The polysaccharides are, after the proteins and the nucleic acids, the third important group of biological polymers. From the point of view of the quantum-mechanical calculations of their conformational problems, mention may be made in the first place of the work of Neely 13> who has studied by the Extended Hiickel Theory the relative conformational stabilities of a few of the different possible chair and boat conformations of D-glucose, II. Within the conformations studied, the chair one, known under the designation Cl, appears so far the most stable. Experimentally, it is apparently the most stable of all. Moreover, the (3-anomer comes out as 9 kcal/mole more stable than the a-one. Although this difference is far too large with respect to experiment, it is in the proper direction. 

This result implies that if aU-equatorial glucosyl donors were cmiveited into their all-axial counterparts, the reactivity could be dramatically increased. Based on the knowledge that steric congestion at the equatorial C-3 and C-4 positions causes conformational changes [71, 72], Bols and co-workers were able to exploit this phenomenon [70, 73, 74]. However, when TBS protection was applied to glucose derivative 72, the product 73 was found to exist in more of a skew-boat conformation [75] (Scheme 13) rather than the anticipated 4 conformation... 

The six-membered ring of pyranoses takes on the boat conformation. Possible conformers of oc- and / -D-glucose are given in Fig. 30. Several conformers of a... 

With only rare exceptions, hexapyranose molecules that could exist in boat conformations do not. They are found in chair conformations. Additionally, they normally are found in what is called the "Cj conformation, as opposed to the C4 conformation. As an example, a-D-glucose is shown (Structure 1) ... 

Figure 32 The a.-D-furanose forms of glucose and galactose with the 1,2- and 5,6-diols highlighted in red, as well as the twist boat conformation of the a-D-pyranose form of galactose with the 1,2- and 3,4-diols highlighted in red.

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