Conformational analysis of nine-membered cyclic acetals. Stereoelectronic effect in 2,4- and 3,5-benzodioxonine derivatives.

Conformations of the title compounds were examined using DFT calculations and NBO analysis in order to find the origins of their conformational preferences. The most stable conformations were TBC and TCBtype-1 for the 2,4- and 3,5-benzodioxonine derivatives, respectively. In both of these conformations the acetal moiety adopts the g+/-g+/- geometry. The NBO analysis yielded values of the stabilization energy associated with the stereoelectronic nO --> sigmaC-O* interactions that were highest for conformations other than the global minima. Conformers displaying the strongest interactions followed different patterns of atom arrangement within the acetal moiety, namely g+g-, and those in which one or both of the torsion angles within the C-O-C-O-C segment were close to 90 degrees . Steric repulsion caused by alkyl substituents at the anomeric carbon was found to influence the strength of the nO --> sigmaC-O* stabilization through modification of bond lengths and torsion angles. The adopted ground-state conformations result from accommodation of steric repulsions and stabilizing stereoelectronic interactions. It was shown that DFT calculations of conformational preferences of acetals together with GIAO prediction of 13C chemical shifts should be a useful methodology for studies on conformation and conformational equilibria of acetals in solution.