DIELS-ALDER REACTIONS OF DIHYDROPYRIDINONES - SYNTHETIC ENTRY TO THE MANZAMINE A TRICYCLIC CORE

For the construction of the tricyclic core of manzamine A (1), the Diels-Alder reactions of some dihydropyridinones were surveyed. The N-protecting group of the dihydropyridinone played an important role in achieving a successful Diels-Alder reaction. In view of its electron-withdrawing character as well as its thermal stability, the p-toluenesulfonyl protecting group was found to be best in our synthesis. An effective method for the preparation of 3-alkyldihydropyridinones via the Michael addition to dehydroalanine derivatives has also been devised. By the utilization of a high-pressure Diels-Alder reaction of the N-tosyl-3-alkyldihydropyridinone (17) with the Danishefsky diene, a facile construction of the central pyrroloisoquinoline skeleton (21) was successfully achieved.