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CIRCLE 31 ON READER SERVICE CARD September 19, 1983 C&EN
Greenberg's next problem was that the compound might be stabilized by aromaticity, but it certainly would be destabilized by the strain of three sp 2 -hybridized carbon atoms in a ring. His approach to the problem was to calculate a hypothetical heat of formation for unstrained, nonaromatic diphenylcyclopropenone, add an estimate for ring strain, and come up with an aromatic stabilization energy (or lack of it) by comparison with the experimental gas-phase heat of formation. He turned to methods for such calculations worked out by physical chemistry professor Sidney W. Benson at the University of Southern California, and extended by physical chemistry professor Joel F. Liebman at the University of Maryland, Baltimore County. These meth-
ods involved adding up atomic group contributions to heat of formation, and resulted in a value of 30.2 kcal per mole. To arrive at a strain energy to add to this hypothetical unstrained, nonaromatic diphenylcyclopropenone, Greenberg considered known strain energies of cyclopropane, cyclopropene, and methylenecyclopropane. This led him to an estimate of 67 kcal per mole. Adding this to the 30.2-kcal-per-mole value calculated earlier gave a heat of formation of 97.2 kcal per mole for a molecule with strain but no aromatic stabilization yet. Subtracting the experimental 74.9 kcal per mole from this led Greenberg to the conclusion that real cyclopropenone has an aromatic stabilization of 22 kcal per mole. D
Lemon odor helps identify male moth pheromone
Washington, D.C. * * * ***••*• "Courting with Cologne—a Male Moth Lures his Love" was the whimsical title at a press conference. "Moths in Love? It's Just Chemistry!" responded the headline over a story in the Washington Post the next day. Both refer to an intriguing research project described to the Division of Agricultural & Food Chemistry. Biochemist Wendell A. Roelofs of New York State Agricultural Experiment Station, Cornell University, Geneva, notes that the story "involves insect pheromones, plant hormones, fruit volatiles, and human perfumes"—and an exhibition of serendipity by a Japanese natural product chemist who likes tea with lemon. And the research indicates that an important commercial perfume ingredient in fact lacks any odor and receives its fragrance from an impurity. The work was carried out by Roelofs, flavor chemist Terry E. Acree, also of the Geneva laboratory, natural product chemist Ritsuo Nishida of Kyoto University in Japan, and entomologist Thomas C.
Baker, now at the University of California, Riverside. Roelofs points out that scientists studying insect pheromones have generally worked with female sex attractants—the chemicals that lure male insects from long distances to bring about mating. However, courtship in a major international insect pest, the Oriental fruit moth— Grapholitha molesta (Busck)—is unusual among Lepidoptera (moths and butterflies): After males are attracted to the vicinity of females by the female pheromone, the courtship behavior is not over. The male moths then propel their own short-
Compounds found in male fruit moth scent organs
Ethyl frans-cinnamate
COOChL Methyl jasmonate
(R)-(-)mellein
COOCH'3 Methyl 2-epijasmonate
Revolutionary Concepts in Automated range sex attractant over the females and with it lure females to start copulation. This attractant is pro duced by the male's hairpencils, groups of elongated hairlike scales bundled into special pouches at the end of the abdomen. The hairpencils produce what hu mans perceive as a pleasant, sweet herbal fragrance. Roelofs, Nishida, and Baker extracted hairpencils from some 5000 male moths, each moth bearing about 21 ng of pheromone. They identified one of the active components as ethyl trans-cinnamate. It is the only one of the fourcomponent pheromone compounds that by itself induces significant at traction and electroantennogram re sponse in female moths. An electro antennogram is made by hooking up an insect antenna to electrodes; changes in voltage show which chemical compounds stimulate the antennal receptors. However, greater attraction of fe males was produced when the cinnamate was blended with a mixture of other components that had the characteristic hairpencil smell. This mixture had no attractive activity without cinnamate. It was difficult to determine the identity of the other components, because of the small amount of avail able material, Nishida notes. At this point, however, serendipity took a role. Nishida, who is a tea drinker, noticed that the hairpencil aroma resembled that of the lemon slice in his tea. This led Nishida to bring in flavor chemist Acree to examine the components of lemon peel. The group found that methyl 2epijasmonate is present in both lem on peel and the moth hairpencils (it had never been reported before as present in lemon peel). They found that the male moth pheromone con sists of four compounds: ethyl transcinnamate, (R)-(—)-mellein, methyl jasmonate, and methyl 2-epijasmonate. The amount of the four com pounds in each moth is 0.5, 20, 0.3, and 0.01 ng, respectively. None of these compounds had been found previously in Lepidoptera. "Here we encountered a curious fact," remarks Nishida. Methyl jasmonate—originally extracted from jasmine plants—has long been believed to be one of the most fra-
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CIRCLE 5 ON READER SERVICE CARD September 19, 1983 C&EN
35
Science
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grant of aromas. Synthetic methyl jasmonate is a commercially important fragrance, a constituent of jasmine oil, and is known in the perfume industry as "the queen of aroma." However, astonishingly, Nishida and Acree found that pure methyl jasmonate has no odor at all. Instead, they find, the 2-epimer, which is methyl 2-epijasmonate, is odoriferous. The other isomers of methyl jasmonate, Acree says, are practically odorless—or have at least only a thousandth of the odor. Thus, the group concludes, commercial "methyl jasmonate's" fragrance actually is due to a minor component, methyl 2-epijasmonate, present as an impurity of less than 3% of the mixture. The finding should give the perfume industry a tool to measure the quality of its
methyl jasmonate, Acree notes, simply by measuring the concentration of methyl 2-epijasmonate in the mixture. The higher its level, the more powerful the fragrance, giving the potential for up to a 17-fold increase in odor activity. Perfume firms show interest in the finding, he notes. Nishida explains that it is not clear how the male moths obtain the fragrant compounds in their hairpencils. He hypothesizes that they receive immediate precursors from their fruit and nut host foods. This is supported by the observation that male moths reared on an artificial diet—rather than on their usual diet of small green apples—do not possess hairpencils with the characteristic herbal odor or the corresponding gas-liquid chromatographic peaks. Work on this continues. D
Drag molecule shows unusual tautomeric effect OCN —ft
\ - NCO
ELATE 160 p-phenylene diisocyanate
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CIRCLE 47 ON READER SERVICE CARD 36
September 19, 1983 C&EN
Washington, D£. * * * * * * * * * * From the very beginning, the experimental antitumor drug fredericamycin A has confounded and surprised researchers seeking to delineate its molecular structure. The discovery of its L-shaped spiro ring system was intriguing enough (C&EN, Aug. 16,1982, page 27). Now, investigators have come up with an explanation for an even more curious feature of the molecule: Half of it is invisible in proton and carbon-13 nuclear magnetic resonance spectra. The explanation put forth for this chemical quirk is rather bizarre in itself and may embroil the whole question in controversy. The case of the missing atoms was taken up by natural products chemists Ramesh C. Pandey and Renuka Misra, with the aid of NMR spectroscopist Bruce D. Hilton, all at the National Cancer Institute's Frederick Cancer Research Facility in Maryland. Pandey, who has since joined Abbott Laboratories in North Chicago, explains that they saw only 14 carbon atoms of fredericamycin A in the 13C NMR spectrum, but
they knew from mass spectral and other data that the molecule has 30 carbons. "We first thought that it is a symmetrical molecule," Pandey says. An x-ray crystal structure showed this not to be the case. The solution finally came to Misra, who had been struggling seemingly futilely with the puzzle. As a last resort, she added some trifluoroacetic acid to the dissolved compound in an NMR tube. The gambit worked. "All the missing carbons just came up," she marvels. Misra had added the acid in the hope of disrupting possible hydrogen bonding and molecular association that she thought might be at the root of the problem. But now the Frederick researchers have a more convincing explanation for the acid's effect. They think the three-ring portion of fredericamycin A that contains the quinone and hydroquinone functions displays extended conjugation, Pandey told the Microbial & Biochemical Technology Division. As a result, the molecule assumes several different tautomeric forms that are in equilibrium with each other. Thus, the many different lowintensity carbon signals from this mixture aren't seen because they are
