Stereoselective synthesis of yohimbine alkaloids from secologanin

Abstract

Glucolysis of methyl secoxyloganin (3) at pH 7 has afforded, via a biogenetically patterned vinylogous aldol reaction, two key substituted cyclohexenes (6) from which the acetates of β-yohimbine (10a), yohimbine (10b), and their 19,20-dehydro derivatives (9) have been synthesised in addition to dihydrogambirtannine (11) induced to undergo selectively an alternative rearrangement leading to a carbocyclic product (6). Not only does this type of cyclisation represent the closest analogy yet for the in vivo formation of ring E in yohimbine alkaloids, but these can also readily be synthesised from (6).