Pyranose ring boat conformation

For six-member pyranose rings, six conformations, P (planar), E (envelope), B (boat), S (skew, twisted, twist boat), H (half chair) and C (chair) are possible. The chair conformation C is by far the most stable of the six conformations. Two chair conformations are possible for pyranoses. To distinguish the two conformations, a reference plane is drawn through four atoms (e.g. C2, C3, C5 and 05) of the ring so that the C-atom with the lowest position niunber (i.e. Cl) lies out of the plane. A ring atom that lies above this reference plane is placed before the abbreviation of the ring conformation as a superscript. The one below the plane is written as a subscript following this letter as Ci (C4 above and Cl below) and C4 (Cl above and C4 below). 

In 1991, an important paper was published by Bock et a/.84 that described the steric and electronic effects on the formation of the dispiroketal dihexulose dianhydrides. The authors described the conformation of six dihexulose dianhydrides, as determined by X-ray crystallography or NMR spectroscopy. They concluded that these conformations are dictated by the anomeric and exo-anomeric effects. Thus, the dihexulose dianhydrides are disposed to adopt conformations that permit operation of these effects—even if this results in the dioxane ring having a boat conformation or all three substituents on one pyranose ring being axial. 

It has been shown that methyl thiocellotrioside (44d), tetraoside (44i) and pentaoside (44j) are potent inhibitors of endoglucanase I (EGI, family 7) and cellobiohydrolase II (CBHII, family 6) from Humicola insolens [41]. Furthermore, a crystal of EGI of Fusarium oxysporum was soaked in a solution of compound (44j), and a clear density for three sugar rings was seen in subsites -2, -1 and -hi. It has been found that the pyranose ring in subsite -1 adopts a boat conformation leading to a pseudo-axial orientation for the scissile bond (Fig. 2) [66]. 

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. 

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... 

In the pyranose ring, the true cis orientation is encountered in boat forms (for example, XIX) or in the half-chair forms (for example, XX) and again, as illustrated by the very rapid uptake of lead tetraacetate by methyl 2,6-anhydro-a-D-altropyranoside,84 which is known to have a boat conformation,... 

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

Fig. 2.—Different conformers of pyranose rings The 4C, chair (I) the B0 3 boat (II) two half-chairs (HI and IV), and a coplanar ring (V).

The pyranose ring conformations that are important in polysaccharides are the two chair conformations, designated and Q (Fig. 10) to indicate the disposition of atoms above and below the plane of the ring (in older notation the same conformations were denoted by Cl and 1C, respectively). Boat conformations probably have some existence in low proportions in disordered (random coil) polysaccharide chains. 

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. 

Moving clockwise around the pyranose ring, the next example is provided by the deamination of 4-amino-1,6-anhydro-4-deoxy-/3-n-mannose (LXXII), which gives 1,6 3,4-dianhydro- 3-D-talose (LXXIII). There is some evidence indicating that the parent compound 1,6-anhydro-/3-D-mannopyranose, which cannot adopt the usual chair conformation (Cl), favors the reverse chair conformation (1C) to the alternative boat form (SB) Thus, we can assume that the above derivative also favors the reverse chair conformation LXXII and that the anhydro sugar is formed through the mechanism involving axial and antiparallel substituents. 

The conformations of furanose and pyranose rings are described qualitatively by italicised letters T, E, C, H, S and B for twist, envelope, chair, halfchair, skew and boat, respectively T refers only to furanose rings and C, H, S and B only to pyranose rings, but E, describing conformations with just one ring atom out of the plane of the remainder, can refer to both pyranoses and furanoses. The particular conformation is specified by superscripts and subscripts referring to atoms above and below a defined plane thus (the... 

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