Synthesis of Hexahydropyridazines by [4 + 2] Cycloaddition of Donor-Acceptor Cyclobutanes and -Diazenes.

The GaCl-catalyzed [4 + 2] cycloaddition between alkoxy- and aryl-activated donor-acceptor cyclobutane diesters and -diazene (4-phenyl-1,2,4-triazoline-3,5-dione, PTAD) is disclosed. The reaction provides hexahydropyridazine derivatives as single diastereomers in good to excellent yields in most cases. The structural assignment of the cycloadduct was unambiguously established by single-crystal X-ray diffraction.