Urceoloids A and B, Two C19 Steroids with a Rearranged Spirocyclic Carbon Skeleton from Urceola quintaretii

Urceoloids A (1) and B (2), two C19 steroids with a rearranged new carbon skeleton by featuring a very unique spiro[4.4]nona-3,6,8-triene system, and a biosynthetically related known steroid (3) were isolated from Urceola quintaretii. Their structures were completely established by a combined method. A plausible biosynthetic pathway for the new carbon skeleton represented by compounds 1 and 2 was proposed. Compounds 2 and 3 showed immunosuppressive activities.