Stereoisomeric Composition of Natural Myrtucommulone A

Myrtucommulone A (MC A) (1), isolated from Myrtus communis (myrtle), shows
the same pharmacological
activity for inhibition of inflammation and induction of apoptosis
as synthetic MC A, which consists of three stereoisomers, i.e., two
enantiomers and one meso form. This led to the question of whether
the natural MC A is a pure stereoisomer or a mixture of stereoisomers.
The specific rotation and electronic circular dichroism (ECD) data
of natural MC A (1) as well as of a pentacyclic derivative 4 revealed that naturally occurring MC A (1)
consists of the racemate and the meso form in a 1:1 ratio. A probable
precursor of MC A (1), nor-semimyrtucommulone (5), was also isolated from myrtle as a racemate. The absolute
configurations of the enantiomers of 1 and 5 were determined using a combination of experimental and quantum-chemical
calculated ECD spectra.