The analysis of X-ray data of 546 COCOC molecular fragments extracted from the Cambridge Crystallographic Data Base is associated with MO calculations on dimethoxypropane, conformational INDO map and MO optimization of structural parameters. This population study examines the complex monitoring, through electronic and steric control, of the anomeric effect, in endo plus exo anomeric situations. The two torsion angles of the COCOC fragment (C102C304C5) are shown to be the proper conformational mapping tool for 3D molecular investigation. Sixty-six percent of the acyclic fragments with an sp3 anomeric C3 carbon show structures having the C, symmetry. Anomeric effects of both oxygens exist in most fragments with a quaternary or a secondary C3 carbon, but the variation in torsion angle is very broad (4, = 60.11' with a standard deviation CT = 9 . 3 O ) . Conformational changes and anomeric effect discrepancies are greater when the central carbon is a tertiary one. For cyclic fragments with a tertiary C3 carbon, there is a wide dispersion of one of the torsion angles: 4~~~~ for the six-membered rings (&, , = 77', u = 19') and &"do for five-membered rings (&"do = 104O, u = 11.3'), while in contrast, the other torsion angle is close to 6n0 (or 180'). This is the result of a conformational strain hindering the characteristic orbital alignments typical of the endo and exo anomeric effects. The endo anomeric effect is found in about 56% of the pyranosides. For labile furanoses, the equatorial conformers are largely handicapped (76% of the alkoxy groups lie between the axial and bisecting positions with respect to the five-membered ring). No correlation was found between CO bond lengths or COC bond angle and torsion angles. In contrast the bond angle OCO at the anomeric center is very sensitive to conformational variation
Interconversion Mechanisms between Conformers of C2 SymmetryA. Crystallographic Pathways. In conformity with the structural correlation method," structures observed in crystals tend to concentrate in the lower areas of the potential energy surface characteristic of the substructure.Using the potential symmetry of the C O C O C moiety and without separating cyclic and acyclic fragments as in Figures 1 ( 2 5 ) Briggs, A. J.; Glenn, R.; Jones, P. G.; Kirby, A. J.; Ramaswamy, P. J . Am. Chem. SOC. 1984, 106, 6200.
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