Tetracyclic phenothiazines. VI. Attempted pummerer cyclization of 10‐(2‐Keto‐3‐methylsulfinylpropyl)phenothiazine

Abstract

AbstractThe title compound (3), a β‐ketosulfoxide prepared by a Claisen‐type condensation of ethyl 2‐(10‐phenothiazino)acetate (4) with dimsyl carbanion, was subjected to a variety of catalysts in attempts to achieve a Pummerer cyclization to 1,2‐dihydro‐3‐methylthio‐2‐oxo‐3H‐pyrido[3,2,1‐kl]phenothiazine (2). Although the Pummerer rearrangement product, 10‐(2‐oxo‐3‐hydroxy‐3‐methylthiopropyl)phenothiazine (5), could be obtained in excellent yield under mild conditions, neither it nor the β‐ketosulfoxide could be successfully cyclized under any of the conditions attempted. Instead, phenothiazine, 2‐hyroxy‐3‐(10‐phenothiazinyl)‐2‐propen‐1‐al (7), 3‐(10‐phenothiazinyl)‐1,1‐di(methylthio)propan‐2‐one (8), 3‐(10‐phenothiazinyl)‐1‐methylthiopropan‐2‐one (9) and 10‐phenothiazinylformamide (10) were variously obtained.