X-ray studies have been carried out on crystals of heavy-atom derivatives of naturally-occurring organic compounds and related photo-irradiation products. Four structures have been successfully solved: the alkaloid hunterburnine, the sesquiterpenoid santonin and its photo-irradiation product isophoto-a-santonic lactone, and the colouring material ergoflavin. Attempts to solve the structure of toxisterol A2, a photo-irradiation product of the steroid ergosterol, have proved fruitless. The elucidation of the structure of hunterburnine resulted from a straightforward application of the heavy atom technique to hunterburnine methiodide and the complete structure was assigned with certainty from the fifth three-dimensional Fourier synthesis. Subsequent refinement reduced the discrepancy, R, to 15.4% Hunterburnine represents a new class of indole alkaloid. The analysis of bromodihydroisophoto-a-santonic lactone acetate was undertaken to define the stereochemistry of the parent molecule. The analysis was hindered in the early stages by spurious symmetry which was eventually overcome with the aid of chemical knowledge of the structure. The structure has been refined and the final value of R is 12.9%. The analysis has established the relative stereochemistry of isophoto-alpha-antonic lactone. The configuration of the methyl group on the lactone ring of isophoto-alpha-santonic lactone is opposite to that generally accepted, Implying that the assignation of the stereochemistry of the corresponding methyl group in santonin itself was in error. Hence an analysis of the heavy-atom derivative 2-bromo-a-santonin was undertaken. In spite of spurious symmetry, the structure was solved and refined to give a discrepancy of 15.2%. The analysis has confirmed the revised configuration of the controversial methyl group in santonin. Inter- and intra-molecular dimensions in these three structures are in reasonable agreement with accepted values. The structure and stereochemistry of tetramethyl ergoflavin di-p-iodobenzoate have been established and refinement of the structure is in progress. Molecular dimensions have not been discussed, but the gross molecular structure is in agreement with chemical evidence with the exception of the assignment of hydroxyl and phenolic groupings. This project was studied in conjunction with A. T. McPhail and J. V. Silverton. The final chapter of this thesis describes an attempt to solve the structure of toxisterol A2 4-iodo-3-nitrobensoate which crystallises in the monoclinic system with space group P2 1. The heavy-atom positions have been located, but it has not been possible to allocate additional atomic sites from three-dimensional Fourier maps or a three-dimensional superimposed Patterson synthesis
