Conformations, anomeric effect envelope

The furanose ring of ribose or deoxyribose is flexible and can be interconverted smoothly among an infinite number of envelope (E) and skew or twist (T) conformations. See Chapter 4, Section A,2. However, there are limits set by steric and anomeric effects.27 62-64 Conformations are often described as in Fig. 5-10 by stating which atom in an envelope conformation lies mostly out of the plane of the other four atoms. If this atom lies above the ring, i.e., toward the base, the ring... 

Most furanoses prefer the envelope conformation and it appears that a quasi-equatorial exocyclic side chain and a quasi-axial C(l)—0(1) bond (anomeric effect) are equally important stabilising factors (Figure 1.8). 

Trioxolanes (molozonides) 232 are highly unstable species but because of their involvement in ozonolysis they have been the subject of many MO calculations. There is general agreement that the envelope conformation with 0(2) out of plane is the most stable. For the 4-halo derivatives 232 (R = F, Cl) the yy/z-conformation is calculated to be the most stable ( 2kcalmol-1) due to electrostatic repulsion and an anomeric effect <2002JPCA4745, CHEC-III(6.05.4.2)150>. 

From examination of the HNMR spectra of 5-alkoxy- and 5-hydroxy-A -l,2,3-triazo-lines, an envelope conformation of the molecule was deduced with the 5-substituent being in a pseudoaxi position at the flap and the 1-substituent being pseudoequatorial. This preferred conformation can originate a type of anomeric effect (74ACS(B)425). 5-Amino-l-(p-nitrophenyl)-A -l,2,3-triazolines have also been found to prefer the envelope conformation (74ACS(B)425>. 

During this cyclodehydration, inversion of configuration occurred at CT of 751 to give the /3-D-C-nucleoside 752 rather than its a-anomer because of (i) preference of the protonated bulky heterocycle to occupy the equatorial j8 position in the (envelope) conformation (753) to gain the stabilization of the reverse anomeric effect, and (ii) destabilization of the conformation of the alternative a-anomer (754) by the nonbonded CM-l,3-diaxial interaction between the C3 hydroxyl and the bulky heterocycle (80MI8) (Scheme 195). 

On the other hand, Ali and co-workers reported that the anomeric effect dictates the solution conformational behavior of several isoxazoli-dines, which exist in several envelope and half-chair puckered conformations (95SA(A)2279). As evidenced by NMR, nitrogen inversion converts the cis isomer (cis-4) into the trans isomer (trans- ), and the small coupling constants (0-1.5 Hz) between the C5-H proton and the C4r-H protons suggest a dihedral angle near 90°, which indicates that cis- and trans-4 adopt a conformation with a pseudoaxially oriented ethoxy group as a... 

Alkylation of the lactol HO-group also enhances the anomeric effect (Table 4.7). A reduction of the dielectric constant of the solvent (e. g., transition from water to methanol), resulting in an increase in the dipole-dipole interaction, also enhances the anomeric effect. Conformational isomers of fura-nose also occur since its ring is not planar. There are two basic conformations, the envelope (E) and the twist (T), which are the most stable in solu-... 

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