title: Acid-mediated electrocyclic domino transformations of 5,5-disubstituted 1-amino-1-azapenta-1,4-dien-3-ones into dihydrospiroindenepyrazole and dihydroindenodiazepine derivatives.
abstract: 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. Huckel- and Mobius-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.
authors.affiliation: Organisch-Chemisches Institut, Westfälische Wilhelms-Universität, Münster, Germany.
authors.forename: Nugzar
authors.initials: N
authors.lastname: Ghavtadze
authors.forename: Roland
authors.initials: R
authors.lastname: Fröhlich
authors.forename: Ernst-Ulrich
authors.initials: EU
authors.lastname: Würthwein