Condensed thiophen ring systems. Part III. A new synthesis of benzo[b]thiophen-2(3H)-and-3(2H)-ones and some reactions of benzo-[b]thiophen-2(3H)-one with dimethyl sulphate in the presence of base

Abstract

Benzo[b]thiophen-2(3H)-one (thio-oxindole)(72% overall yield) was prepared by treating 2,4,6-tri-(2-benzo[b]-thienyl)cyclotriboroxane with hydrogen peroxide. The boroxane was obtained when 2-benzo[b]thienyl-lithium was treated successively with n-butylborate and acid. 3-Methylbenzo[b]thiophen-2(3H)-one and benzo[b]-thiophen-3(2H)-one (thioindoxyl) were prepared similarly. The reactions of benzo[b]thiophen-2(3H)-one with dimethyl sulphate in the presence of base in various solvents were studied. Successive treatment of this compound with sodium hydride and dimethyl sulphate in hexamethylphosphoramide gave 2-methoxybenzo[b]thiophen (90% yield).