On the constituents of Asclepiadaceous plants, a couple of Cynanchum sp. plants were investigated. We wish to report here the structures of the new compounds, atratogenin A(1) from C. atratum, cynajapogenin A(2) from C. japonicum, neocynapanogenin A(13) of the aglycone of neocynapanoside A(14) from C. paniculatum, and boerharigenin A(5) and B(6) from C. boerharifolium. Both atratogenin A(1) and cynajapogenin A(2) contain a 2,3 disubstituted furan ring which may be derived from the C-15 aldehyde and C-20 oxo group formed by oxidative cleavage at C-14,15 bond of hypothetical precursor such like 14β-hydroxy pregnenolone (4) which is isolated from C. paniculatum. The latter compoud(2) lacked C-18 methyl group. Neocynapanogenin A(13), an aglycone of 14, has an exo α,β unsaturated five membered lactone ring in addition to the nine membered lactone like glaucogenins type compound. Both 5 and 6 have the same substitution system in the ring D and defferent in that the former has benzoyloxygenated C-19 methyl. A plausible biogenetical pathway for pregnane type aglycones of the Cynanchum plants is given in the chart 1.
