Acid-promoted cyclization reactions of tetrahydroindolinones. Model studies for possible application in a synthesis of selaginoidine.

The synthesis of various substituted bicyclic lactams by an acid-induced Pictet-Spengler reaction of tetrahydroindolinones bearing tethered heteroaromatic rings is presented. The outcome of the cyclization depends on the position of the furan tether, tether length, nature of the tethered heteroaromatic ring, and the substituent group present on the 5-position of the tethered heteroaryl group. A one-pot procedure was developed to efficiently prepare tetrahydroindolinones containing tethered furan rings. In a typical example, the reaction of furanyl azide 26 with n-Bu3P delivered iminophosphorane 27, which was allowed to react with a 1-alkyl-(2-oxocyclohexyl)acetic acid to provide the desired furanyl-substituted tetrahydroindolinone system 29. Treatment of 29 with trifluoroacetic acid afforded the tetracyclic lactam skeleton 30 found in the alkaloid (+/-)-selaginoidine.