Reinvestigation of base-induced skeletal conversion via a spirocyclic intermediate of dibenzodithiocinium derivatives and a computational study using the HF/6-31G* basis set

Abstract

Treatment of 6-methyl-12-oxo-5H,7H-dibenzo[b,g][1,5]dithiocinium salt (1a) with methanolic KOH afforded a mixture of dibenzothiepin derivative 5 in 66% yield. The rearrangement was explained in terms of usual [2,3]-sigmatropic shift via a spirocyclic intermediate. However, an unique rearrangement via an alternative spirocyclic intermediate was observed in 6-methyl-5H,7H-dibenzo[b,g][1,5]dithiocinium salt (2) to give an unexpected dibenzothiepin derivative 6 in 29% yield along with a small amount of a ring-opening product 7 in 5% yield under the same reaction conditions. In order to clarify the origin of the different behaviour between the sulfoxide derivative 1a and the sulfide derivative 2 in the rearrangements, 6-methyl-12-oxo-7,12-dihydro-5H-dibenzo[c,f]thiocinium salt (3) as the reference compound with an electron withdrawing group (carbonyl group) was prepared and treated under the same conditions to furnish a mixture of acid 8 and ester 9. It was considered that these products resulted from the ring-opening of a spirocyclic intermediate analogous to the intermediate from the sulfoxide derivative 1a. On the other hand, the rigid system, 10b-phenyl-10b,11-dihydro-6H-[1]benzothieno[2,1-a][2]benzothiophenium perchlorate (4) afforded the compound 10via the Stevens-type rearrangement. The ab initio MO calculations at HF/6-31G* basis set was performed on the possible reaction intermediates and products.