W. Clark Still

William Clark Still (born 1946) is an American organic chemist. As a distinguished professor at Columbia University, Clark Still made significant contributions to the field of organic chemistry, particularly in the areas of natural product synthesis, reaction development, conformational analysis, macrocyclic stereocontrol, and computational chemistry. Still and coworkers also developed the purification technique known as flash column chromatography, which is widely used for the purification of organic compounds.

Major contributions

Total synthesis

Periplanone B

In 1979, Still reported the first total synthesis of periplanone B, the potent sex pheromone of the American cockroach. Although the structural connectivity of this compound had been established spectroscopically, Still's synthesis confirmed the relative stereochemical relationships present in this natural product. A key step in this synthesis makes use of an anionic oxy-Cope rearrangement to form the macrocyclic 10-membered ring found in periplanone B. In this transformation, the initial tertiary alcohol was treated with potassium hydride and 18-crown-6 at elevated temperature to induce the rearrangement, and the resulting enolate was trapped with chlorotrimethylsilane to form the macrocyclic silyl enol ether as a single diastereomer. This intermediate was then transformed into a key enone substrate over the course of five steps including Rubottom oxidation, TBS protection of the secondary alcohol, and formation of the exocyclic alkene via a selenoxide rearrangements.

These transformations set the stage for the introduction of both epoxide groups, which were formed with high levels of diastereoselectivity based on the principles of macrocyclic stereocontrol. The first epoxide was introduced via nucleophilic epoxidation of the enone using potassium hydride and tert-butylhydroperoxide; this led to the formation of a 4:1 mixture of diastereomeric epoxides, favoring the desired isomer in which attack had occurred from the peripheral face of the enone. At this stage, the ketone was transformed into the second epoxide group using the Johnson-Corey-Chaykovsky reaction; notably, the product was formed as a single diastereomer, again due to initial peripheral attack of the dimethylsulfonium methylide nucleophile on the ketone.