Structure elucidation of two new diterpenoids from Isodon phyllostachys: Phyllostacins A and B

Two new ent-kaurane-derived diterpene derivatives, phyllostacins A (1) and B (2), were isolated from the aerial parts of Isodon phyllostachys, together with two known compounds, irroratin A (3) and serrin B (4). Both 1 and 2 were found to be present as diastereoisomers. In the case of 1, the corresponding diastereoisomeric diacetates 5 and 6 were prepared and separated. The structures of the new compounds were elucidated by extensive 1D- and 2D-NMR spectroscopic methods, in combination with MS experiments. In (D₅)pyridine solution, the two epimers of 1 are present in equal amounts, but in CDCl ₃ or CD₃OD, the (S)-epimer predominates in the mixture of hemiacetals.