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Classical Methods in Structure Elucidation of Natural Products von Hoffmann, Reinhard W. (eBook)

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Classical Methods in Structure Elucidation of Natural Products

Organic chemistry as we know it today originated from a preoccupation with substances isolated from nature. In the period from 1860 to 1960, the main task was to elucidate their molecular structure by way of degradation and synthesis. In light of the limited experimental methods available and the lack of established reference compounds, this represented an unparalleled intellectual challenge.
This book makes use of twenty-five representative examples to retrace the great accomplishments made by the generation of chemists during this era. At the same time, it questions the reliability of the experimental results when judged by today's criteria, particularly since the structures for numerous natural products are stated as established facts in standard text books.
With each chapter devoted to one organic compound, the author combines results from historic experiments to trace a line of evidence that may follow the path put forward by the original contributors. However, in some cases the experimental facts have been combined to form another, hopefully shorter, line of evidence. As a result, readers are able to determine for themselves the 'facts behind the established structure assignments' of a number of important natural products.

Reinhard W. Hoffmann is Professor Emeriti of the University of Marburg. He studied chemistry at the Universitat Bonn and received his Dr. rer. nat. in 1958 under the direction of Prof. B. Helferich. He spent two years as postdoctoral fellow at The Pennsylvania State University with Prof. G. W. Brindley and moved then to the Universitat Heidelberg for another postdoctoral year with Prof. G. Wittig. In 1964 he obtained his habilitation and in 1967 he moved as lecturer to the Technische Hochschule Darmstadt and in 1970 to his present position as Professor of Organic Chemistry at the Universitat Marburg. His research interests are new methods in stereoselective C-C-bond formation, their application in natural products synthesis and the stereochemistry of reactive organometallic compounds. More recent research activities concern combinatorial approaches to new catalysts and conformation design.

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    Format: ePUB
    Kopierschutz: AdobeDRM
    Seitenzahl: 280
    Sprache: Englisch
    ISBN: 9783906390789
    Verlag: Wiley-VCH
    Größe: 24640 kBytes
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Classical Methods in Structure Elucidation of Natural Products

Hundred Years of Structure Elucidation

Those natural products, the structures of which appear in the textbooks, are the basic representatives that form the core of organic chemistry. Most of these structures have been elucidated in the period that present-day chemists consider the "Stone Age" of organic chemistry. All the more, chemists should be willing to question the validity of these structure assignments. How solid are the facts that support the structure assignment? How cogent are the connections of these facts to the final conclusion? Surprisingly, very little knowledge on the structure elucidation of those natural products prevails at present; reason enough to bring these achievements of the previous generations of chemists to light again.

Figure 1

Hence, this treatise deals with exemplary structures elucidated in the hundred years from 1860 to 1960. While the facts presented are historic, this is not a history of the structure elucidations. This would be much too detailed, as the structure elucidations of most of the products covered here were highly ramified with many culs-de-sac.

Rather, one should justify the limits 1860 and 1960. The year 1960 approximately marks the change from classical structure elucidation by degradation to the era in which structure elucidation is mainly based on spectroscopic evidences and X-ray crystallography. Since it is the emphasis of this treatise to address classical structure elucidation, efforts made after 1960 are only considered in exceptional cases.

The other limit, 1860, has to do with the notion of structure. Prior to the advent of structural theory [1], there was no conceptual framework to address the "structure" of a compound. This framework was provided by Kekulé (1857) [2], Couper (1858) [3], and Butlerov (1859) [4, 5]. The insight that the C-atom has four valencies and that C_C bonds form the backbone of organic compounds constituted the basis of the structural theory; it is the distinct connectivity between the atoms that defines the structure of an organic compound [5]. This theory made it for the first time comprehensible why (and how many) isomeric substances could exist for a given elemental composition of a compound.

Information Box 1

Of similar importance was the foundation of stereochemistry by LeBel [6] (1874) and van't Hoff [7] (1875), making it comprehensible why (and how many) "stereoisomeric" substances could exist for a given constitution of a compound.

Subsequently, the determination of such connectivities, that is, the structure of any compound of interest, became the predominant task of organic chemistry. Yet, this task constituted a challenge of unprecedented dimensions for the chemists at the end of the nineteenth century, because the structure of a new compound had to be related to that of structurally known compounds in, what one could call, a self-consistent network of information. Obviously, at the beginning, there were only a rather small number of structurally characterized compounds, which fortunately expanded rapidly with every decade of dedicated work. Obviously, in the course of time, it was increasingly easier to reach a known compound upon degradation of an unknown compound.

Figure 2

The situation was aggravated by the fact that the tools to elucidate structures of a given compound, that is, to establish the relation to other known compounds, were deplorably limited. Indeed, all that chemists had at their disposal were next to simple glassware, a balance, a Bunsen burner, and a few thermometers. All the more, one has to admire and respect the achievements of the chemists of that period.

Figure 3 Source: (a) ref. [16] (b) With kind permission of Dr. Timo Mappes, www.musoptin.com; (c) ref. [17].

The concept of structure could be attributed only to a "uniform" compound, that is, the

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