Novel methods in synthesis of macrocyclic ethers and metal extractions

NOVEL METHODS IN SYNTHESIS OF MACROCYCLIC ETHERS AND METAL EXTRACTIONS SUMMARY The award of the 1987 Nobel Prize in chemistry to Charles Pedersen, Donald Cram, and Jean-Marie Lehn, marks an important turning point in the history of organic chemistry. For most of the 150 years between Wohler's synthesis of urea and the present time, the focus of organic chemists has been on molecules containing only covalently linked atoms. This Nobel Prize delineates that time in history when the focus changed from covalent to non-covalent interactions. Of course, this focus could only change as a result of the extensive, important, and often brilliant efforts of innumerable organic chemists to understand structures, shapes, and reaction mechanisms. During the post-war period of this century, we may view organic chemistry as having gone through two major phases. The period from the end of the second World War to the early 1970s has been regarded by many as the golden age of physical organic chemistry. During this time the basic mechanistic principles that control all organic reactions came into sharp focus for the first time. In the late 1960s, concurrent with the award of the Nobel Prizes to such giants of synthesis as Woodward and Barton, chemistry entered the golden age of natural product synthesis. At about the time that the fields of total synthesis and synthetic methods were in full force and vigor, a new era of chemistry was in its infancy. This new field was based on historical developments, as are all innovations, but it would find a new direction for science in terms of approach and thinking. There is, however, some important background that should be mentioned before we consider Pedersen's discovery of the crown ethers. One of the early workers was the great German chemist Lüttringhaus who was interested in preparing large molecules which might concatenate. His hope was to prepare structures which would interlock and have at least interesting and perhaps unusual properties. One of the substrates he chose for his studies was the resorcinol molecule. He reached this nucleophile with substituted diol derivatives. But unfortunately his compounds lacked sufficient donor groups to exhibit cation binding properties. Although the synthesis was interesting and the products were fascinating, the remarkable aspect of crown ethers and their relatives would eventually prove to be their ability to capture or include other species. It was Pedersen who recognized this ability. VI