Reduction of nitro- and nitroso-compounds by tervalent phosphorus reagents. Part VII. A new synthesis of phenothiazines involving a new molecular rearrangement

Abstract

Reductive cyclisation of aryl 2-nitroaryl sulphides by triethyl phosphite provides a new synthesis of phenothiazines (yields ca. 50–85%) which is superior to that involving the thermal decomposition of aryl 2-azidoaryl sulphides (yields ca. 30%). Both reactions proceed via a novel molecular rearrangement whereby, for example, 4-chlorophenyl 2-nitrophenyl sulphide gives 3-chlorophenothiazine, whereas 4-chloro-2-nitrophenyl phenyl sulphide gives 2-chlorophenothiazine. [4-²H]phenyl 2-nitrophenyl sulphide similarly gives [3-²H]phenothiazine, as shown by comparison of the e.s.r. spectrum of its cation-radical with those computer-simulated for [2-²H]- and [3-²H]-phenothiazine. A convenient N-ethylation of phenothiazines by triethyl phosphate is also reported.