Compound 18 is a diketone that has carbonyl groups at C-3 and C-12. Independently of the C ring conformation (compounds 18 or 18c), the A ring is a rather distorted chair (Q = 0.490 D, q3 = -0.365 D, q2 = 0.320 D and 2 = 140º) in which C-3 is situated above the mean plane of the ring. Moreover, the f2 value (130º) is between the values (90º and 180º) appropriates for an inverted boat and twist boat.