Siddiqui was the first scientist to bring the anthelmintic, antifungal, antibacterial, and antiviral constituents of the Neem tree to the attention of natural products chemists. In 1942, he extracted three bitter compounds from neem oil, which he named as nimbin, nimbinin, and nimbidin respectively. The process involved extracting the water-insoluble components with ether, petrol ether, ethyl acetate and dilute alcohol. The provisional naming was "nimbin" (sulphur-free crystalline product with melting point at 205 °C, empirical composition C7H10O2), "nimbinin" (with similar principle, melting at 192 °C), and "nimbidin" (cream-coloured containing amorphous sulphur, melting at 90–100 °C). Siddiqui identified "nimbidin" as the main active antibacterial ingredient, and the highest yielding bitter component in the neem oil. These compounds are stable and found in substantial quantities in the Neem. They also serve as natural insecticides.