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

The 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‐3 H ‐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.