Synthesis of Asperuloside Aglucon Silyl Ether and Garjasmine from (+)-Genipin via Gardenoside Aglucon Bis(silyl ether) as a Common Intermediate

Abstract Syntheses of C6-functionalized iridoids represented by asperuloside aglucon silyl ether (4) and garjasmine were accomplished from (+)-genipin by utilizing the gardenoside aglucon bis(silyl ether) (3) as a common intermediate. During the transformation of 3 into 4, the acid-catalyzed transposition reaction of the tertiary hydroxyl group was found to proceed from more hindered concave side to give C6-hydroxylated compound having the same stereochemistry as that of C6 in 4. This observed hydroxyl group transposition reaction was interesting since such migration of hydroxyl group in the proposed biosynthetic pathway of gardenoside from geniposide proceeded to the opposite direction.