Six pyrazole alkaloids of natural origin (isolated from Newbouldia laevis in DR Congo) that exhibit antimalarial activity-namely withasomnine, newbouldine, and their para-hydroxy and -methoxy derivatives-were investigated theoretically. The nitro derivatives of withasomnine and
The root bark of Newbouldia laevis afforded withasomnine, 4'-hydroxywithasomnine, 4'-methoxywithasomnine, newbouldine, 4'-hydroxynewbouldine, and 4'-methoxynewbouldine. 4'-Methoxywithasomnine and 4'-methoxynewbouldine are new natural products.