Computational study of base‐induced skeletal conversion via a spirocyclic intermediate in dibenzodithiocinium derivatives by ab initio MO calculations

Reaction of 6‐methyl‐12‐oxo‐5 H ,7 H ‐dibenzo[ b , g ][1,5]dithiocinium salt ( 1‐SO ) with methanolic KOH afforded a mixture of dibenzothiepin derivatives 2‐SO . In order to clarify the intermediates of the rearrangement, ab initio MO calculations with the HF/6–31G * basis set were performed on the reaction intermediates, the transition states and related compounds. The rearrangement was explained in terms of the usual [2,3]‐sigmatropic shift via a spirocyclic intermediate, followed by a 1,3‐shift of the sulfonyl group. However, a different type of rearrangement was observed in 6‐methyl‐5 H ,7 H ‐dibenzo[ b , g ][1,5]dithiocinium salt ( 1‐S ), giving an unexpected dibenzothiepin derivative 3‐S along with a small amount of a ring‐opening product 8 under the same reaction conditions. The formation of 3‐S and 8 was understandable by the assumption of a cationic intermediate resulting from heterolytic cleavage at the benzyl position. Copyright © 2003 John Wiley & Sons, Ltd.