Total Syntheses of (+)‐Grandilodine C and (+)‐Lapidilectine B and Determination of their Absolute Stereochemistry

Enantioselective total syntheses of the Kopsia alkaloids (+)-grandilodine C and (+)-lapidilectine B were accomplished. A key intermediate, spirodiketone, was synthesized in 3 steps and converted into the chiral enone by enantioselective deprotonation followed by oxidation with up to 76 % ee. Lactone formation was achieved through stereoselective vinylation followed by allylation and ozonolysis. The total synthesis of (+)-grandilodine C was achieved by palladium-catalyzed intramolecular allylic amination and ring-closing metathesis to give 8- and 5-membered heterocycles, respectively. Selective reduction of a lactam carbonyl gave (+)-lapidilectine B. The absolute stereochemistry of both natural products was thereby confirmed. These syntheses enable the scalable preparation of the above alkaloids for biological studies.