Molecular structure and conformations of eight-membered cyclic acetals with 1,4-diplanar fragments

Abstract

The molecular structures of nine eight-membered cyclic acetals containing two planar fragments in the ring, the derivatives of dibenzo- and dinaphtho[ d, g][1,3]dioxocines, have been studied by x-ray single crystal diffraction. Steric interactions were found to determine the conformation realized in the crystalline phase. Depending on the type of planar fragments in the cycle and the group bridging them, boat-chair (BC), distorted boat (DB), twist (T) or twist-boat (TB) conformations were observed. The BC conformation was found for derivatives of 6 H,12 H-dibenzo[ d, g][1,3]dioxocine containing methyl groups in the benzo fragments or one methyl substituent in the bridge methylene group. The T and TB conformations are realized for molecules with the carbonyl group bridging the planar fragments. 8 H,16 H-Dinaphtho[2,1- d,1′2′- g][1,3]dioxocine exhibits the first example of BC conformation among all eight-membered cyclic compounds containing two naphtho groups at the expense of large deviations of its geometrical parameters from their unstrained values. Other dinaphtho derivatives exist in the DB conformation. The molecular geometry and the possibility of weak intramolecular CH⋯O hydrogen bonding are discussed.