Abstract The structures of the title compounds bearing a five-, six-or seven-membered A ring have been investigated by uv and 1 H and 13 C nmr spectroscopy. The imine-enol-enamine (I-II-III) tautomerism of these compounds depends greatly on the ring size. A significant solvent-dependence is observed only for the five-membered-ring compounds 1 and 2, which in ethanolic solution exist predominantly in the imine form I, and in chloroform solution in ...