Pyranose rings chair conformations

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). 

Haworth formulas are satisfactory for representing configurational relationships in pyranose forms but are uninformative as to carbohydrate conformations X ray crystal lographic studies of a large number of carbohydrates reveal that the six membered pyra nose ring of D glucose adopts a chair conformation... 

The pyranose rings can adopt either of two different chair conformations called C, and C4. Pyranoses usually adopt a chair conformation that puts the majority of bulky groups in the equatorial position, so that steric interactions are minimized. The Ci(d) conformation and the ring numbering system are shown in formula 1. 

Monosaccharides have many structural variations that correspond to local minima that must be considered. Acyclic carbohydrates can rotate at each carbon, and each of the three staggered conformers is likely to correspond to a local minimum. The shapes of sugar rings also often vary. Furanose rings usually have two major local minima and a path of interconversion. Experimental evidence shows a clear preference for only one chair form for some pyranose rings, but others could exist in several conformers. For exanqple, the and conformers must all be considered as possible structures for L-iduronate, as discussed by Ragazzi et al. in this book. 

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 transition state for the substitution step requires that, for maximum stability, the incoming nucleophile be coplanar with the three-membered oxide ring, and, for an epoxide on a pyranose ring (for which a half-chair conformation is expected, by analogy with... 

In the application of this conformational rationale simultaneously to the structures of o-L- and jS-D-cardenolides, the a-L- and the jS-D-rhamno-sides (19) and (44) serve to illustrate the hypothesis that a structural correspondence can be obtained, which permits a satisfactory explanation for the fact that a-L and 0-d isomers have potencies which are, approximately, the same. On the basis of the structures shown, the aglycon component is equatorially disposed, both pyranose rings have the same chair form, and the order of substituents on the ring is the same. The only difference between (19) and (44) is a reversal of configuration of substituents on the pyranoid ring, which is tantamount merely to a vertical displacement of... 

Haworth structures are unambiguous in depicting configurations, but even they do not show the true spatial relationship of groups attached to rings. The normal angle between the bonds formed by the saturated carbon atoms (109°) causes the pyranose molecule to pucker into either a chairlike or boatlike conformation. For glucose and most other pyranoses the chair form (fig. 12.6c) predominates. However, we usually display pyranoses by the Haworth projection because it is easier to draw. 

The hemiacetal opens to give an intermediate containing a free aldehyde function. Cyclization of this intermediate can produce either the a or the /I configuration at this center. The axial and equatorial orientations of the anomeric hydroxyl can best be seen by drawing maltose with the pyranose rings in chair conformations. 

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. 

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