Synthesis of substituted barbaralanes by electrochemical reduction of bridged 1,5-benzodiazepines

Abstract

Electrochemical reduction of bridged 1,5-benzodiazepines, condensation products of o-phenylenediamines with 4,6-dimethylbicyclo[3.3.1]nona-3,6-diene-2,8-dione, has been studied. Cathodic acylation of 4,6-dimethyl-5,2,8-ethanylylidene-5H-1,9-benzodiazacycloundecene (2) in the presence of acetic anhydride affords the substituted barbaralane 5,11-diacetyl-8,9-dimethyl-8,5a,10a-ethanylylidene-8H-cyclohepta[b]quinoxaline (6). Structures of the substituted barbaralanes have been established by ¹H n.m.r. analysis at +100 °C, at which temperature the complexities of conformational processes were avoided and the dominant valence isomers of the barbaralanes were recognised. At –2.35 V reduction proceeded in a single 2-electron wave. This behaviour is analogous to that of other Schiff bases and is contrasted with the electrochemical reduction of related heterocyclic systems.