Nuclear magnetic resonance elucidation of ring-inversion processes in macrocyclic octaols

The conformational behaviour of the three macrocyclic octaols (4)–(6), obtained by acid-catalysed condensation of resorcinol with heptanal, is elucidated for the first time. Two of them, namely the diamond (5a) and the chair (6a) stereoisomers, undergo a ring-inversion conformational process in acetone to give the corresponding crown conformers (5b) and (6b). In DMSO or on addition of acetic acid to an acetone solution of diamond octaol (5), conformer (5a) is favoured. The presence of such equilibria and solvent effects are interpreted as an interplay between the tendency of the phenolic OH groups to form intramolecular hydrogen bonds and the alky chains to assume the endo position, avoiding steric repulsions and allowing self-aggregation.