Chemical Synthesis

Chemical synthesis is the preparation of a compound, usually an organic compound, from easily available or inexpensive commercial chemicals. Compounds are prepared or synthesized by performing various chemical reactions using an inexpensive starting material and changing its molecular structure, by reactions with other chemicals. The best chemical syntheses are those that use cheap starting materials, require only a few steps, and have a good output of product based on the amounts of starting chemicals. The starting materials for organic synthesis can be simple compounds removed from oil and natural gas or more complex chemicals isolated in large amounts from plant and animal sources. The goal of chemical synthesis is to make a particular product that can be used commercially; for example as a drug, a fragrance, a polymer coating, a food or cloth dye, a herbicide, or some other commercial or industrial use. Compounds are also synthesized to test a chemical theory, to make a new or better chemical, or to confirm the structure of a material isolated from a natural source. Chemical synthesis can also be used to supplement the supply of a drug that is commonly isolated in small amounts from natural sources.

Chemical synthesis has played an important role in eradicating one of the major infectious diseases associated with the tropical regions of the world. Malaria is a disease that affects millions of people and is spread by mosquito bites. It causes a person to experience chills followed by sweating and intense fever, and in some cases can cause death. In about 1633, the Inca Indians told the Jesuit priests that the bark from the cinchona or quina-quina tree could be used to cure malaria. The cinchona tree is an evergreen tree that grows in the mountains of Peru. The healing properties of the bark were quickly introduced in Europe and used by physicians to treat the disease. The chemical responsible for the healing properties of the cinchona bark was isolated in 1820 and named quinine, after the quina-quina tree. By 1850, the demand for cinchona bark was so great that the trees in Peru were near extinction. To supplement the supply of quinine, plantations of cinchona trees were started in India, Java, and Ceylon, but by 1932, they were only able to supply 13% of the world's demand for the antimalarial drug. Chemical synthesis was used by German scientists from 1928 to 1933 to make thousands of new compounds that could be used to treat malaria and make up for the deficiency of natural quinine. They were able to identify two new antimalarial drugs and one of them, quinacrine, was used as a substitute for quinine until 1945. During World War II, the supply of cinchona bark to the Allied Forces was constantly interrupted and a new drug had to be found. British and American scientists began to use chemical synthesis to make compounds to be tested against the disease. Over 150 different laboratories cooperated in synthesizing 12,400 new substances by various and often long, involved chemical sequences. In just four years, they were able to identify the new antimalarial chloroquine and large quantities were quickly prepared by chemical synthesis for use by the Allied Forces in malarial regions. Today, there are more than half a dozen different drugs available to treat malaria and they are all prepared in large quantities by chemical synthesis from easily available chemicals.

Taxol is an anticancer drug that was isolated in the 1960s from the Pacific yew tree. In 1993, taxol was approved by the Food and Drug Administration (FDA) for treatment of ovarian cancer and is also active against various other cancers. The demand for this compound is expected to be very large, but only small amounts of the drug can be obtained from the yew bark, so rather than destroy all the Pacific yew trees in the world, chemists set out to use chemical synthesis to make the compound from more accessible substances. One chemical company found that they could convert 10-deacetylbaccatin III, a compound isolated from yew twigs and needles, into taxol by a series of chemical reactions. Furthermore, in 1994, two research groups at different universities devised a chemical synthesis to synthesize taxol from inexpensive starting materials.

Chemical synthesis can also be used to prove the chemical structure of a compound. In the early nineteenth century, the structure of a compound isolated from natural sources was deduced by chemical reactions that converted the original compound into substances of known, smaller molecular arrangements.

In 1979, chemical synthesis was used as a tool to determine the molecular structure of periplanone B, the sex excitant of the female American cockroach. In the Netherlands in 1974, C. J. Persons isolated 200 micrograms of periplanone B from the droppings of 75,000 virgin female cockroaches. He was able to deduce the gross chemical structure of the compound by modern analytical methods, but not its exact three dimensional structure. Without knowing the stereochemistry or three dimensional arrangement of the carbon atoms, larger quantities of the excitant could not be prepared and tested. In 1979, W. Clark Still at Columbia University in New York, set out to determine the structure of periplanone B. He noted that four compounds had to be made by chemical synthesis in order to determine the structure of the cockroach excitant. He chose an easily prepared starting material and by a series of chemical reactions was able to make three of the four compounds he needed to determine the chemical structure. One of the new substances matched all the analytical data from the natural material. When it was sent to the Netherlands for testing against the natural product isolated from cockroaches, it was found to be the active periplanone B.