α,β-Unsaturated Diazoketones as Building Blocks to Piperidine Alkaloids: Total Synthesis of (−)-Cassine

The total synthesis of (−)-cassine, a piperidine alkaloid, was accomplished in 10 steps, starting from N-Cbz-O-TBDPS-l-serinal. Key transformations include the preparation of a novel α,β-unsaturated α′-methyl diazoketone, followed by its cyclization into a highly functionalized dihydropyridine-3-one via a cis-selective intramolecular N–H insertion reaction. This strategy was further extended to other α′-alkylated α,β-unsaturated diazoketones, enabling the synthesis of new dihydropyridine-3-ones with potential applications in the total synthesis of cassine derivatives.