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