Of Men and Marigolds Counting the Quaterthienyls Daniel M. Perrine, S.J? Loyola College in Maryland, 4501 N. Charles St., Baltimore, Md 21210-2699 J. Richard VandeVelde, s.J.~ Department of Mathematics, Loyola University of Chicago, 6525 N. Sheridan Rd., Chicago, 11 60626-5385 Chemonumeric COnund~mS Recently, one of us (DMP) reported the first synthesis of a set of branched quaterthienyls (I),which are compounds containing four thiophene rings bonded as described below. In researching the literature on similar compounds, he was amazed that several distinguished chemists had incorrectly counted the total number of possible isomers. He enlisted the aid of a mathematician knowledgeable in combinatorics (JRV), and together they attempted a systematic counting of these isomers. The problem required some interesting mathematics, and we present the results here. Historically chemists were first interested in the synthesis of these compounds because they are related to the unusual natural product alpha-terthienyl. Al ha Terthienyl A bhiotoxic Insecticide in Marigolds Through most of December 1946,a group of Cal Tech students working under Laszlo Zechmeister (2)busily decapitated the fresh blossoms of over 10,000 common marigold plants, Tagetes erecta, L. Altogether, they collected 14.4 kg of bright-yellow petals. Using the recently developed technique of chromatography, they then isolated a scant 200 mg of the new blue-fluorescingcompound they were trying to identify, only traces of which had been detectible in smaller samplings from the previous year (3). (The substance is not limited to the blossoms, as these early workers believed, but is found throughout the plant.) The structure of the substance proved completely differentfrom the carotenoids found in their previous work.
In a surprising reversal of the more common sequence of events, nature seemed to be imitating artifice: The natural product was identical to a synthetic oligomer created in Dresden just four years earlier by Wilhelm Steinkopf (41, the "father of thiophene chemistry". He had named the substance a-terthienyl. Alpha-terthienyl, 1,indeed has a structure of seemingly artificial symmetry: It consists of three thiophene rings bonded to each other at their a positions. Starting with the sulfur atom as 1and counting around the ring, carbons 2 and 5 have traditionally been called the a positions (as shown on 21, while carbons 3 and 4 have been called the P positions (as shown on 3). 'Author to whom correspondence concerning chemistry should be addressed. 'Author to whom correspondence concerning mathematics should be addressed.
Alpha-terthienyl proved to be uniquely interesting for more than just its structure: Later work by Uhlenbroek and Bijloo (5)showed that it is a natural insecticide, killing nematodes and other organisms. Gommers (6)and others (7)showed that this activity occurs by a phototoxic mechanism whereby 1is excited to the triplet state by sunlight and then transfers its energy to ambient (ordinary) triplet oxygen (30.J. The short-lived and highly-energetic singlet oxygen that results ('02) is particularly lethal to small orgamsms. For instance, 1 is several times more effective than DDT in killing mosquito larvae. The isomers of 1and its hieher olieomers are also of considerable and erowine inter&: he; have been used as the repeating &its f& the construction of electrically conductive polymers (81. Counting the Terthienyls (Terthiophenes) With such techniques as NMR or X-ray crystallography unavailable to him in 1947, Zechmeister wondered whether "traces of isomeric terthienyls occur in the natural product" (2b).Isomeric terthienyls are the set of all other compounds that have three thiophene rings joined to each other by two single bonds, for example, 4 and 5.
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Nomenclature The currently acceptable IUPAC nomenclature for this class of ring assembly is not terthienyl but terthiophene (9).The number of like rings is indicated by a prefix: bi, ter, quater, or quinque. A set of numbers precedes the prefix to indicate the substitution pattern. Thus,
The title of a recent report by Wynberg illustrates the limitations of this systemof nomenclature: Volume 69 Number 9 September 1992
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