Toward the Synthesis of Sceptrin and Benzosceptrin: Solvent Effect in Stereo- and Regioselective [2+2] Photodimerization and Easy Access to the Fully Substituted Benzobutane

By simply switching solvents, two sets of conditions for the total regio- and stereoselective photodimerization of (E)-3-(imidazo[1,2-a]pyrimidin-2-yl)acrylic acid [(E)-7a] have been found. In acetonitrile in the presence of benzophenone as photosensitizer or in cyclohexane, the head-to-head cis-trans-cis isomer 8a was the only observable cycloadduct. In DMSO, the head-to-head trans-trans-trans isomer 10a was obtained as the unique cycloadduct. The diester 9a, derived from 8a, was cyclized to the challenging, fully substituted diimidazobenzobutane system 13 present in benzosceptrins.