Natural Product Structures by X-Ray Methods

Paul, Iain Campbell (1962) Natural Product Structures by X-Ray Methods. PhD thesis, University of Glasgow.

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X-ray studies have been carried out on crystals of derivatives of naturally-occurring organic compounds. Three structures have been successfully determined in this way: the bitter principle, clerodin (C24H34O7), and the mould products, byssochlamic acid C18H20O6) and atrovenetin (C19H18O6). Information on the structure of clerodin was not extensive, when crystals of a bromo-gamma-lactone were provided by Professor Barton, The structure analysis was hindered by the proximity of the heavy atom to certain important structural features, which, as a consequence, proved difficult to locate. The picture of the structure, as shown by the three-dimensional Fourier maps, was clarified when it was recognised that the accepted molecular formula for clerodin was incorrect. The main remaining problem was the establishment of the correct absolute stereochemistry. The relative configuration of the molecule was found, despite some difficulties concerning the identification of the nature of the atoms in a three-membered ring, and by employing the anomalous scattering effect, the true absolute configuration of clerodin bromolactone, and hence of clerodin itself, was established. The crystal and molecular dimensions conformed to the normal pattern, although great accuracy in the location of atomic positions was not attempted. Professor Barton provided crystals of a p-bromo-phenylhydrazine derivative of the mould product bysso-chlamic acid. Very little was known of the structure of this compound. The major problem in this structure analysis proved to be the determination of the positions of the bromine atoms in the unit cell of the crystal. There were two bromine atoms in the asymmetric unit, and the derivative crystallised in the tetragonal system; these two facts resulted in a very complex Patterson function, which required a great deal of study before the heavy atom sites were located. Thereafter, the major problem was one of minimising the very large amount of computer time required for this complex analysis. The structure and relative stereochemistry of byssochlamic acid was established and it can be seen to occupy an important place in a series of compounds studied by Professor Barton. Some slightly anomalous molecular dimensions were found in this structure, although, once again, the limited accuracy of the analysis limits the conclusions that can be drawn. Unlike the other two compounds, a structure had been proposed for atrovenetin, although later work had cast some doubt on its complete validity. The solution of this structure by X-ray methods proved comparatively simple, although refinement was hindered by pseudo symmetry. The structure found differed in the manner of attachment of a five-membered ring to that published. From a crystallographic view-point, this structure is interesting on account of the very close similarity of two independent molecules in the crystal. These two molecules have crystallised in a manner which bears a close resemblance to the symmetry of a higher crystal class. The molecules are prevented from crystallising in this higher class because of the presence of an asymmetric centre. Despite inaccuracies in the analysis, some interesting conclusions can be drawn regarding the molecular dimensions. Three appendices contain an account of some work done on the crystal structures of 1:4-cyclohexanedione, dinitrogen tetroxide, and salts of furan tetracarboxylic acid. A fourth appendix contains some stereoscopic photographs of three-dimensional Fourier syntheses computed for clerodin bromolactone and atrovenetin trimethyl ether ferrichloride.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: Organic chemistry, Analytical chemistry
Date of Award: 1962
Depositing User: Enlighten Team
Unique ID: glathesis:1962-79434
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 05 Mar 2020 09:46
Last Modified: 05 Mar 2020 09:46

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