"The Chemistry of Cannabis is Quite Complex...
...and the isolation and extraction of the active ingredient are difficult even today. The active agent in Cannabis is unique among psychoactive plant materials in that it contains no nitrogen and thus is not an alkaloid. Because Cannabis lacks nitrogen, the 19th century chemists who had been so successful in isolating the active agents from other plants were unable to identify its active component.
There are over 400 chemicals in marijuana, but only  of them are unique to the Cannabis plant -- these are called cannabinoids. One of them, delta-9-tetrahydrocannabinol (THC), was isolated and synthesized in 1964 and is clearly the most pharmacologically active."
Ray, Oakley Stern., and Charles Ksir. Drugs, Society, and Human Behavior. Boston: McGraw-Hill, 2004.