A trihomoabietane diterpenoid from Plectranthus grandidentatus and an unusual addition of acetone to the ortho-quinone system of cryptotanshinone

A new 9 alpha-(2-oxopropyl)abietane derivative (1) has been isolated from an acetone extract of Pleectranthus grandidentatus. Extraction of the plant material and analytical processes carried out in the absence of acetone also revealed the presence of 1 in the plant, thus suggesting that it is a natural product rather than an artifact. Attempts at obtaining Michael adducts between acetone and para-quinone abietane diterpenoids were unsuccessful, whereas treatment of the ortho-quinone cryptotanshinone (3) with acetone under basic conditions yielded compound 2. The structures of 1 and 2 were established by 1D and 2D NMR spectroscopic studies.