Configurational Study of Diastereoisomeric Royleanone Diterpenoids from Salvia concolor

Abstract

The known diterpenoids horminone (1) and taxoquinone (2) as a mixture, and pure 6,7-dehydroroyleanone (7) were isolated from the aerial parts of not yet studied Salvia concolor Lamb. Although 1 and 2 are known, the 1H Nuclear Magnetic Resonance (NMR) data of their acetyl derivatives 4 to 6 are only partially described. Moreover, the 13C NMR data assignments for 4 and 6 show some inconsistencies and reveal better agreement with those we obtained for diastereoisomeric 3 and 5. Since stereochemical aspects were in doubt, it was considered as mandatory to unambiguously determine the absolute configuration of 3 to 6 using vibrational circular dichroism spectroscopy, which then allowed the complete 1H and 13C NMR chemical shift assignments of 4 to 6.