Chemical and electrochemical reduction of pyrazino[2,3-g]quinoxalines and of their benzo and dibenzo derivatives; the structure of fluorindine and the formation of tetraanion

Abstract

The structure of fluorindine is established by nmr as the 5,14-dihydroquinoxalino[2,3-b]phenazine. The catalytic hydrogenation of 2,3-di(p-methoxyphenyl)pyrazino[2,3-b]phenazine 2a leads to the 6,11-dihydro derivative 4a. The electrochemical reduction in an hydroorganic medium furnishes 4a and then the 1,4,6,11-tetrahydro derivative 8a. In dry DMSO the voltammogram shows four monoelectronic reversible systems corresponding to the successive formation of a radical anion, dianion, radical trianion, and tetraanion. Thus 2a appears as a new example of the very restricted class of compounds leading to tetraanions upon electrochemical reduction. The catalytic hydrogenation of 2,7-diphenylpyrazino[2,3-g]quinoxaline 1a or the reaction of LiAlH4 with 1,2,7,8-tetramethylpyrazino[2,3-g]quinoxaline 1b leads to 1,2,3,4-tetrahydro compounds. The electrochemical reduction of 1a and 1b in hydroorganic medium leads successively to 1,4-dihydro and then to 1,4,6,9-tetrahydro compounds which undergo a further rearrangement. In dry DMSO 1a and 1b behave differently from 2a: one only observes two reversible monoelectronic systems.