Hydrogen bonding-assisted transformations of cyclic chalcones: E/Z-isomerization, self-association and unusual tautomerism

Condensation of 1- or 2-indanones and 1,3-indandiones with aromatic or heteroaromatic carbaldehydes gives products that demonstrate various transformations and effects of general interest for organic chemistry. In the present review, phenomena such as tautomerism, E/Z-isomerization, π-conjugation, hydrogen bonding, and homo- and heteroassociation are considered. The relative stability of various isomers of cyclic chalcones is shown to be determined by the intramolecular hydrogen bonding in their molecules and the formation of associates. The same effects also result in the formation of unusual, otherwise unstable tautomers, such as 2H-indazoles. The hydrogen bonding-assisted keto-enol and E/Z-isomerization are analyzed. A novel, specific type of conjugation for the studied compounds was proposed and termed ‘roundabout’ conjugation. The bibliography includes 101 references.