Furanose envelope forms

Jh-3. f 20.2 Hz in D2O), concluded that the furanose ring adopts the envelope form with C-3 endo, which differs greatly from the conformation of... 

Furanose rings, like pyranose rings, are not planar. They can be puckered so that four atoms are nearly coplanar and the fifth is about 0.5 A away from this plane (Figure 11.8). This conformation is called an envelope form because the structure resembles an opened envelope with the back flap raised. In the ribose moiety of most biomolecules, either C-2 or C-3 is out of the plane on the same side as C-5. These conformations are called C2-endo and C3-endo, respectively. 

For the furanose ring, which is adopted by some hexoses and pentoses, the five-member ring is only slightly puckered and exists in three forms, the more common envelope (E) and the less common twist (T). For the more stable envelope forms, and E3, the conformation is defined by the C3 atom that is above or below, the plane (described by C1,C2, C4 and 04). 

Very important furanose molecules (five-membered rings) are commonly foimd in nature. For example, D-ribose and D-deoxyribose are foimd as the building imits of nucleic acids, and fructose is a constituent of sucrose. These rings are not planar, either one (envelope form) or two (twist form) atoms are out of the plane containing the others. The different envelope and twist conformations... 

The five-membered furanose rings are also not planar. The furanose ring can have an envelope conformation, with four atoms coplanar and the fifth out of the plane (the latter can be any of the four carbons or the oxygen), or it can have a twist conformation in which three adjacent atoms are coplanar and the other two adjacent atoms are above and below the plane of the other three atoms. For any furanose ring, there are 10 possible envelope forms and 10 possible twist forms. The energies of the 20 possible forms are all about the same, and rapid interconversions can take place between them. 

The conformational dynamics of furanose rings may be described by the mechanisms of pseudorotation and inversion. The former mechanism describes a continuous pathway of interconversion between twenty idealized non-planar (envelope, twist) conformers (Figures 1 and 2) that does not involve the planar form (e.g., - E4 — ... 

Any one of the five atoms of the ring can be either above or below the plane defined by the other four in the envelope conformation. The energy barriers separating them are very low, and in cyclopentane or in proline all of the envelope conformations are freely interconvertible through intermediate skew forms.32 Furanose sugar rings are very flexible but the presence of the bulky substituents reduces the number of possible conformations.33-363 See Chapter 5 for further discussion. 

Furanose flve-membered rings are never planar. They adopt a puckered form for which a family of pseudorotational conformers [541] are possible. The main conformations are the envelope and twist forms denoted by atom names, Le., C(2 ) and C(3 ), and descriptors endo and exo. They refer to the displacement of these atoms from the mean plane through the other atoms of the five-membered ring, with reference to the exocyclic C(4 )-C(5 ) bond (Fig. 17.4). 

Conformations of the furanose sugar are shown in Figure 12.40. The furanose ring is generally found in one of two conformations an envelope [E) form with four atoms in a plane and the fifth out by as much as 0.5 A. and a twist (T) form with two adjacent atoms displaced on opposite sides of the plane through the other three atoms.Variations between these are common, and one conformation can readily, in the absence of steric constraints, be converted to another. If the displacement of an atom bound to the ring lies on the same side as the substituent C5, it is called endo if it lies on the other side, it is called exo. 

The five-membered furanose ring can adopt two main conformations the envelope (E), in which the reference plane is formed by four adjacent coplanar atoms with only one atom outside it (above or below the plane) and the twist (T), where the reference plane is formed by three adjacent coplanar atoms with the remaining two atoms placed on the opposite sides of it (Figure 2.26). Accordingly, there are ten different envelope conformations Ce, 1, % E2, E, 3, % 4, and 0) and ten different twist conformations (°Ti, Tq, T2, Ti, T2, Tq, and which... 

Furanose rings are usually flat or envelope-shaped but six-atom pyranose rings generally exist in chair rather than in boat form. The CF chair form (10.7a) is more stable than the alternative chair form (10.7b). 

Haworth projections are used extensively in the hterature, but here, to make use of our knowledge of conformation (Sections 4-3 and 4-4), the cychc forms of sugars will be presented mostly as envelope (for furanoses) or chair (for pyranoses) conformations (Figures 4-4 and 4-6). As in Haworth notation, the ether oxygen will be placed usually top right and the anomeric carbon at the right vertex of the envelope or chair. 

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