Alkene cyclisations of N-alkenyl-α-(methylsuiphinyl)acetamides and N-alkenyl-α-chloro-α-(methylthio)acetamides: novel synthesis of five-, six-, and seven-membered lactams

Abstract

On being treated with trifluoroacetic anhydride, N-isopropenyl-N-methyl-α-(methylsulphinyl)acetamide (4a) underwent cationic alkene cyclisation to give the six-membered lactams (6) and (7), whereas the N-but-2-enyl (4b) and N-(3-methylbut-2-enyl) derivatives (4c) afforded the five-membered lactams (8) and (9), and (10), respectively. A similar treatment of the N-prop-2-enyl derivative (4d) gave only the Pummerer rearrangement product (12), which, on further treatment with trifluoroacetic acid, afforded the five-membered lactams (13) and (14). On the other hand, N-but-2-enyl-α-chloro-N-methyl-α-(methylthio)acetamide (5b), when treated with stannic chloride or silica gel, or when heated without solvent, gave the lactam (8). Treatment of N-prop-2-enyl derivative (5d) with stannic chloride yielded the lactams (14) and (15). The cyclisations of N-alk-3-enyl derivatives (4e), (4f), and (5e) giving seven-membered lactams are also described.