STRUCTURE AND CONFORMATION OF HETEROCYCLES .21. PROBING THE ANOMERIC EFFECT IN ORTHOESTERS - STRUCTURE, CONFORMATION, AND DYNAMIC BEHAVIOR OF A UNIQUE ORTHOOXALATE - 2,5,7,10,11,14-HEXAOXA[4.4.4]PROPELLANE

Geometrical parameters in the C(OC)3 moiety of a series of orthoesters in X-ray diffraction structures were retrieved from the Cambridge Structural Database (CSD) and analyzed in the context of stereoelectronic effects. The seven independent conformers of trimethyl orthoformate (3) were calculated both ab initio, using the 3-21G basis set, and with the MM2 force field suitably parametrized to include structural manifestations of the anomeric effect (MM2-AE). Selected orthoesters were calculated using this force field, and good agreement was obtained. The structure and conformation of the title compound HOP (4), a unique D3 orthooxalate, were investigated. The geometrical parameters of the anomeric moieties and the ring conformation were scrutinized in the crystal using X-ray diffraction analysis and by MM2-AE computation: very good agreement between the two methods was obtained. The kinetics of the ring-inversion (enantiomer interconversion) process of the system were interrogated in a variable temperature H-1-NMR spectroscopic study of HOP-d11, yielding DELTA-G(double dagger) = 11.8 kcal/mol. This result is discussed in the framework of a MM3 computational study of the minima and transition states encountered in the inversion process of HOP.