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■ On the Tautomerism of Cinnolin-4-ol, Cinnolin-4-thiol, and Cinnolin-4-amine

Wolfgang Holzer,* Gernot A. Eller, and Simeon Schönberger

*Department of Drug Synthesis, Faculty of Life Sciences, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria

Abstract

Detailed NMR spectroscopic investigations (¹H, ¹³C, ¹⁵N) with cinnolin-4-ol (1) reveal this compound to exist exclusively in the cinnolin-4(1H)-one form in deuteriodimethyl sulfoxide solution. This finding is confirmed by comparison of the NMR spectroscopic data of 1 with those of the corresponding ‘ixed’ O-methyl and N-methyl derivatives, i.e. 4-methoxycinnoline (3) and 1-methylcinnolin-4(1H)-one (5). Similarly, cinnoline-4-thiol (4) is present in the thione form in deuteriodimethyl sulfoxide, whereas cinnolin-4-amine (7) exists in the amino form. In addition, in-depth analyses of the NMR spectra of some simple 4-substituted cinnolines are provided.