[Reactions of 4,5-dihydro-4-oxo-1H-pyrido(3,2-b)indol-2-carboxylic acid ester].

The title compounds 5 were saponificated and decarboxylated, N- and O-alkylated; the ester group was reduced to the primary alcohol and than oxidized to the aldehyde. In the same manner the N- and O-methylated products 9 and 10 were derivatized. The tetrazoles 24 and 25 were synthesized from the aldehydes 18 and 19 via the aldoximes and nitriles. The aldehydes 18 and 19 reacted with beta-amino crotonic acid esters in the Hantzsch pyridine synthesis to mixtures, from which the 1,4-dihydropyridines (DHP) 26 and 27 as well as the ylidenes from methyl acetoacetate 31 and 32 and in the case of 18 the oxidation product 30 of a 1,2-DHP could be isolated. The pyridines 28 and 29 were obtained by chemical dehydrogenation of the 1,4-DHP 26 and 27. By anodic oxidation of 26 and 27 by means of difference pulse voltammetry not only an one step two-electronic dehydrogenation yielding the corresponding pyridines took place, but also a two step one-electronic oxidation of the vinylogous hydrazine structure to diiminium salts occurred.