CUMULATIVE ANOMERIC EFFECT - A THEORETICAL AND X-RAY-DIFFRACTION STUDY OF ORTHOCARBONATES

The combined effect of six anomeric pair interactions at a single carbon atom has been examined by X-ray diffraction and molecular mechanics. MM2 calculations on C(OR)4(R = H, Me, Ph) indicate two closely spaced minima with S4and D2dsymmetry. An interesting low-energy pathway resembling a three-dimensional domino has been computed for the interconversion of the S4conformer to the D2dform. Single-crystal X-ray structures determined for three C(OAr)4derivatives confirm that the energies of the two conformers do not differ significantly. Thus, the two crystallographically nonequivalent molecules in the unit cell of tetraphenoxymethane (4) as well as of tetrakis(3,5-dimethylphenoxy)methane (5) adopt conformations in which the central C(OC)4units have a near D2dsymmetry, while the corresponding fragment has a distorted S4symmetry in tetrakis(4-bromophenoxy)methane (6). The experimental C-O bond lengths and C-O-C and O-C-O angles are consistent with large anomeric interactions. © 1990, American Chemical Society. All rights reserved.