Cleavage of 12-alkylbenzo[a]phenothiazines with lithium in tetrahydrofuran: a dealkylation–desulphurization reaction

A study of the cleavage of 12-methyl- and 12-ethyl-benzo[a]phenothiazine with lithium in tetrahydrofuran is reported. A novel dealkylation–desulphurization reaction was observed in the case of the 12-methyl derivative to yield benzo[a]carbazole, whereas the 12-ethyl derivative gave a complex mixture from which no identifiable material was isolated. However, t.l.c. indicated that the ethyl derivative had undergone dealkylation–desulphurization to a limited extent. The particle size of the lithium used appears to be critical in the cleavage. No cleavage was observed when lithium aluminium hydride or lithium aluminium hydride and lithium metal in tetrahydrofuran was employed.