3,10-Dihydroxydielmentha-5,11-Diene-4,9-Dione. A Diterpenoid (Bismonoterpenoid) With a Novel Carbon Skeleton

3,10-Dihydroxydielmentha-5,11-diene-4,9-dione, a diterpenoid (C20H28O4) from Callitris macleayana heartwood, is shown by degradation, by both one- and two-dimensional high-field n.m.r. exploration, and by X-ray crystallographic analysis to have a unique carbon skeleton formally based upon the Diels-Alder self- dimerization of a p- menthadienone.