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December 10, 2001
Volume 79, Number 50
CENEAR 79 50 pp. 45-55
ISSN 0009-2347
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[Previous Story] [Next Story]CHEMISTRY HIGHLIGHTS 2001

ORGANIC CHEMISTRY. In the field of organic chemistry, 2001 saw significant new developments involving fluorous synthesis, ethane conformation, triplet carbenes, and the total synthesis of a major fish toxin.

FAVORITISM Goodman and Pophristic found new evidence that a quantum mechanical phenomenon called hyperconjugation causes ethane to adopt its preferred staggered conformation (top) instead of an eclipsed one (bottom). Blue = bonding orbitals, yellow = antibonding orbitals.
ADAPTED WITH PERMISSION FROM NATURE, © 2001
Researchers reported that fluorous mixture synthesis--in which starting materials are tagged with fluorinated labels of different length--offers potential time savings over current methods for synthesizing combinatorial libraries of small organic molecules. The fluorous labels make it possible to run reactions on different tagged starting materials in the same solution and yet easily separate and identify the reaction products. The technique thus makes it possible for chemists to synthesize a number of synthetic products at the same time, while using their favorite synthetic equipment and familiar solution-phase synthetic techniques. It was developed by Zhiyong Luo of Fluorous Technologies, Pittsburgh, and graduate student Qisheng Zhang, postdoc Yoji Oderaotoshi, and chemistry professor Dennis P. Curran at the University of Pittsburgh [Science, 291, 1766 (2001); C&EN, March 12, page 62].

In a physical organic study, new theoretical calculations by chemistry professor Lionel Goodman and graduate student Vojislava T. Pophristic of Rutgers University provided more definitive proof than ever before that hyperconjugation, a quantum-mechanical effect involving electron transfer between molecular orbitals, causes ethane to adopt its preferred staggered (nonoverlapping) conformation, instead of the alternative eclipsed conformation [Nature, 411, 565 (2001); C&EN, June 4, page 10]. The conventional view had been that steric effects, not hyperconjugation, accounted for ethane's staggered preference. The study could lead to a better understanding of related molecular phenomena, such as polymer rheology and protein folding.

In a serendipitous discovery that could lead to organic ferromagnetic materials, chemistry professor Hideo Tomioka, graduate student Eri Iwamoto, and coworkers at Mie University, Tsu, Japan, generated a triplet carbene derivative that is significantly more stable than previously observed triplet carbenes [Nature, 412, 626 (2001); C&EN, Aug. 13, page 11]. Triplet carbenes--which have two nonbonding electrons with parallel spin that occupy different orbitals--are notoriously transient and highly reactive organic radicals. The new species, with a half-life of 19 minutes in solution at room temperature, is the longest lived triplet carbene ever. "The previous most stable one, which we reported in 1999, has a half-life of about nine minutes," Tomioka told C&EN. Triplet carbenes are potentially useful as building blocks for organic ferromagnetic materials.

And after a 12-year effort, researchers at Tohoku University, Sendai, Japan, led by chemistry professor Masahiro Hirama, have achieved the total synthesis of an enormously complex organic structure, the caterpillar-shaped ciguatoxin CTX3C [Science, 294, 1904 (2001); C&EN, Dec. 3, page 9]. CTX3C has 30 stereogenic centers and 13 fused rings. Ciguatoxins can cause paralysis and death, but appreciable amounts have not been readily available for study. The new synthetic approach will help solve that problem.


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