Metal ions such as Co 2+, Mn 2+, Ni 2+, Zn 2+ and Ca 2+ have been found to catalyze the condensation of o-phenyleriediamine ( 1) with acetone, affording 2,3-dihydro-2,2,4-trimethyl-1H-1,5-benzodiazepine ( 2). In the presence of MnCl 2, 2 is oxidized catalytically by O 2 to the hitherto unknown 1,2-dihydro-2,2,4-trimethyl-3H-1,5-benzodiazepin-3-one, 5, (1,3,4,5-tetrahydro-4,4-dimethyl-2H-1,5-benzodiazepin-2-ylidene)methyl 2,3-dihydro-2,2-di-methyl-1H-1,5-benzodiazepin-4-yl ketone, 4 and to 5-formyl-1,3,4,5-tetrahydro-4,4-dimethyl-2H-1,5-benzodiazepin-2-one, 3. According to mechanistic studies, the reaction sequence leading to 4 involves an aldol condensation of 2 with 4-formyl-2,3-dihydro-2,2-dimethyl-1H-1,5-benzodiazepine, 6. In this reaction the expected product is not obtained, because instead of water elimination, the intermediate undergoes tautomerization to (1,3, 4,5-tetrahydro-4,4-dimethyl-2H-1,5-benzodiazepin-2-ylidine)methyl 2,3,4,5-tetrahydro-2, 2-dimethyl-1H-1,5-benzodiazepin-4-yl ketone, 10, which is followed by dehydrogenation with the Mn 2+/O 2 system, thus yielding 4.