Acid-Mediated Electrocyclic Domino Transformations of 5,5-Disubstituted 1-Amino-1-azapenta-1,4-dien-3-ones into Dihydrospiroindenepyrazole and Dihydroindenodiazepine Derivatives

Trifluoromethyl-substituted 1-amino-1-azapenta-1,4-dien-3-ones 4, which are accessible in good yield from pyruvates 1 in a three-step procedure, undergo a cascade reaction involving inter alia two electrocyclizations upon treatment with a large excess of trifluoromethanesulfonic acid to give novel dihydrospiroindenepyrazole 5a−o and dihydroindenodiazepine 6a−j. We interpret this sequence of reactions on the basis of quantum chemical calculations as a dicationic cyclization of a pentadien-1-one (“superelectrophilic solvation”), where one of the double bonds is part of an aromatic ring and a subsequent rearrangement to form an (monocationic) iminium ion, which either cyclizes to give five-membered spiro ring systems (compounds 5) or tricyclic dihydroindenodiazepine derivatives 6. Hückel- and Möbius-type transition states of the electrocyclization reactions are discussed considering the results of NICS calculations. One 1-amino-1-penta-1,4-dien-3-one 4 and several dihydrospiroindenepyrazoles 5 and dihydroindenodiazepines 6 could be characterized by X-ray diffraction.