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

The GaCl3-catalyzed [4 + 2] cycloaddition between alkoxy- and aryl-activated donor–acceptor cyclobutane diesters and cis-diazene 1a (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 3b was unambiguously established by single-crystal X-ray diffraction.