The synthesis of 4-[5-(4-phenoxyphenyl)-2H-pyrazol-3-yl] morpholine derivatives by Perozzo R et al [80] resulted in to the discovery of newer class of anti-trypanosomal compounds having stage specific action, as these compounds have shown moderate to very good activity against the blood stage of T. b. rhodesiense. The two compounds, 4-[3-(4-phenoxyphenyl)-1H-pyrazol-5-yl]morpholine (1.0µM) ( Figure 29a ) and 1-[3-(4-phenoxyphenyl)-1H-pyrazol-5-yl]piperazine (1.1 µM) (Figure 29b ) with a pyrazol ring, are the most potent anti-trypanosomals of the series and have same cytotoxicity (61.6 µM), indicating that the pyrazol ring is very important for anti-trypanosomal activity. The substitution of pyrazol ring with isoxazole derivative leads to a six fold reduction in activity as compared to the most potent compound. In addition, substitution with nitrophenyl or aminophenyl also results in strong reduction in activity. The phenoxy ring in the compounds is also important for activity as replacing it by an ethylene group results in nine fold reduction in efficacy. Further substitution with a nitro group or an amino group reduces potency up to 4-fold or 18-fold respectively. The stage specific action of these compounds is unknown. Further optimization of this class of compounds is required in order to lower the cytotoxicity profile of the compound. 