Photochemical transformations of chalcone-podands into cyclobutane-containing benzocrown ethers

A method for the synthesis of cyclobutane-containing benzocrown ethers by the template photocycloaddition of the corresponding chalcone-podands in solutions was developed. It was shown by X-ray diffraction analysis, ¹H and ¹³C NMR, and UV spectroscopy that the intramolecular 2π+2π] photocycloaddition (PCA) is stereoselective with the predominant formation of structures of the β- and γ-truxinic type. The PCA rate was shown to depend on the size of the oxyethylene spacer and stereoorientation of the chalcone fragments in the podands. The selectivity of the PCA process can be controlled by a change in the concentration of the template (alkaline metal ions) and variation of the wavelength range of irradiation.