One-step synthesis of the erythrinane skeleton by acid-promoted double cyclization of N-(cyclohex-1-enyl)-N-[2-(3,4-dimethoxyphenyl)ethyl]-α-(methylsulphinyl)acetamide and its derivatives

Abstract

On being heated with toluene-p-sulphonic acid, N-(cyclohex-1-enyl)-N-[2-(3,4-dimethoxyphenyl)-ethyl]-α-(methylsulphinyl)acetamide (8) underwent double cyclization to give the erythrinane derivative (11), which was converted into the amide (13) by reduction with Raney nickel and into the enamide (14) by thermolysis of the corresponding sulphoxide. The double cyclization of the sulphoxide (18) gave a stereoisomeric mixture of the ether products (19a) and (19b). On treatment with Raney nickel, either (19a) or (19b) afforded a mixture of the alcohols (20a) and (20b), each of which was oxidized with chromium trioxide–pyridine to give the same ketone (21). The sulphoxide (25) also cyclized to afford two products (26) and (27), but in low yields, which were desulphurized with Raney nickel to give the erythrinones (28) and (29), respectively.