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2-Pyrazinyl (2) and 3-pyridazinylketone arylhydrazones (6) and their benzologues undergo a ring closure reaction to yield pyrazolo[3,4- b]pyrazines (4) and pyrazolo[4,3- c]pyridazines (7), respectively, in acceptable to good yields. The reaction was found to be accelerated by using acidic or basic conditions. Quantum chemical calculations suggest the key step of the mechanism to be a direct cyclization; analysis of aromaticity based on computed magnetic properties revealed its medium-dependent pericyclic or pseudopericyclic character. The cyclization reaction has also been extended for the synthesis of related ring systems ( 9, 12, 14).

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Benzothiazole derivatives R 0270Synthesis of Novel Pyrazolo[5,1-b]benzothiazoles: A New Pericyclic Pathway. -N-Amination of the benzylbenzothiazoles (I), followed by reaction of the N-amino salts (III) with ethyl orthoformate (IV) afford derivatives (V) of the pyrazolobenzothiazole ring system. The ring closure may proceed via deprotonation of the N-amino salt and subsequent electrocyclization to provide the tricyclic ring system. From a theoretical point of view, the pericyclic nature of the ring-closure step is of importance. -(TEKINER-GULBAS, B.; FILAK, L.; VASKO, G. A.; EGYED, O.; YALCIN, I.; AKI-SENER, E.; RIEDL, Z.; HAJOS*, G.; Heterocycles 75 (2008) 8, 2005-2012; Chem. Res. Cent., Hung. Acad. Sci., H-1025 Budapest, Hung.; Eng.) -H. Hoennerscheid 50-123