Total Syntheses of (-)-Secologanin, (-)-5-Carboxystrictosidine, and (-)-Rubenine

The first enantioselective total syntheses of (-)-secologanin (1), (-)-5-carboxystrictosidine (2), and (-)-rubenine (3) were accomplished in 10, 9, and 14 steps, respectively. The key transformation in the synthesis of 1 was a sequential anti-selective organocatalytic Michael reaction/Fukuyama reduction/spontaneous cyclization to form an optically active dihydropyran ring. In addition, the secologanin tetraacetate (16), which is a potential key intermediate for the bioinspired divergent syntheses of monoterpenoid indole alkaloids, was prepared in gram-scale in seven steps. The total syntheses of 2 and 3, which are classified as glycosylated monoterpenoid indole alkaloids, were achieved through bioinspired transformations such as a diastereoselective Pictet-Spengler reaction, a site- and stereoselective epoxidation, and a site-selective epoxide ring-opening followed by a lactonization reaction.