THE ABSOLUTE STEREOCHEMISTRY OF AJUGAVENSINS, NEOCLERODANE DITERPENES FROM AJUGA-GENEVENSIS - A REVISION OF THE C-1 CONFIGURATION OF AJUGAVENSIN-A AND AJUGAVENSIN-B

Single-crystal X-ray diffraction analyses of ajugavensins A and B allowed us to establish a neo-clerodane absolute configuration for both compounds and a 2S stereochemistry for the 2-methylbutyric ester group of ajugavensin A. Moreover, application of Horeau's method to ajugavensin C defined as R the absolute configuration of its 1-beta alcohol function, thus also establishing a neo-clerodane stereochemistry for this diterpenoid. On the other hand, the X-ray analyses of ajugavensins A and B revealed that their substituents at C-1 possessed an axial beta-configuration with ring A in a twisted conformation. This result changes our previous assignments, which were based only on H-1 NMR data and the assumption that ring A had a chair conformation.