Crown-ether styryl dyes .15. Synthesis and two pathways of regio- and stereospecific cation-dependent [2+2]-autophotocycloaddition of chromogenic 15-crown-5-ether betaines of quinoline series

Styryl dyes (4a,b) containing a 15-crown-5 fragment and isomeric 2- and 4-quinolinium residues with an N-sulfopropyl substituent undergo [2+2] -autophotocycloaddition to give cyclobutane derivatives (9a,b) in acetonitrile only in the presence of Mg(ClO4)(2) or Ca(ClO4)(2). The stereospecificity of both pathways of photocycloaddition and its efficiency are explained by the preorganization of the supramolecular dimers derived from the trans-isomers of the dyes when they are bound into complexes with Mg and Ca cations.