Tuberostemospironine group

S. tuberosa provided most of the isolated Stemona alkaloids, followed by S. japonica, and S. sessilifolia. The phytochemical study of Croomia is restricted to C. heterosepala and C. japonica, from which only two Stemona alkaloids were isolated [croomine (29) and didehydrocroomine (34)], both from the tuberostemospironine group (III). Alkaloids from the stemoamide group (II) were found in eight of the twelve investigated Stemona species, while the alkaloids from the other groups were isolated from only two to four Stemona species (Table XXV). 

In this group, tuberostemospironine (28) is the only alkaloid which lacks the a-methyl-y-butyrolactone ring appended to C3 of the pyrrolidine ring A. Croomine (29), stemospironine (30), stemotinine (31), and didehydrocroomine (34) display an opposite stereochemistry at C9 to that found in tuberostemospironine (28), isostemotinine (32), and stemonidine (33). Curiously, stemotinine (31) and isostemotinine (32) have an oxygen bridge between C9a and C6. 

H-NMR Chemical Shifts (S, ppm) and Coupling Constants (/, Hz) for the Tuberostemospironine Alkaloid Group. 

---