The crystal and molecular structure of methyl austrocorticinate (methyl 3-ethyl-l-hydroxy-6,8-dimethoxy-9,10 dioxoanthracene-2-carboxylate, C20H1807) has been determined. Crystals of the naturally derived material were found to be twinned, whereas the crystals of the totally synthetic material showed no evidence of twinning. The three six-membered rings and the two methoxy groups are almost coplanar, while the orientation of the methyl ester group is almost perpendicular to the ring plane. (5R ,6S,7R ,9R, 13S, 14R)-7-Chloro-7-cyano-4,5-epoxy-3,6-dimethoxy-N-methyl-6,14-ethenoisomorphinane, (1), C22H23C1N203, and 1-methoxy-5,5,6,6-tetracyanobicyclo[2.2.2]oct-2-ene, (2), C13HIoN40, which had not been convincingly characterized before by any other analytical method due to possible existence of several isomers, were obtained from the Diels-Alder reactions of (-)-thebaine [for compound (1)] and 1 methoxy-l,3-cyclohexadiene [for compound (2)]. The X-ray structural results provided reliable information on the stereochemical course of Diels-Alder reactions of (-)-thebaine and 1-methoxy-1,3-cyclohexadiene with a-chloroacrylnitrile and tetracyanoethene, respectively.