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STUDIES in NATURAL PRODUCT CHEMISTRY THESIS PRESENTED to the UNIVERSITY 0? for the DEGREE of Phd by JAMES PYOTT JOHNSTON

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STUDIES in NATURAL PRODUCT CHEMISTRY THESIS PRESENTED to the UNIVERSITY 0? for the DEGREE of Phd by JAMES PYOTT JOHNSTON STUDIES IN NATURAL PRODUCT CHEMISTRY THESIS PRESENTED TO THE UNIVERSITY 0? GLASGOW FOR THE DEGREE OF PhD BY JAMES PYOTT JOHNSTON 1969 ProQuest Number: 11011932 All rights reserved INFORMATION TO ALL USERS The quality of this reproduction is dependent upon the quality of the copy submitted. In the unlikely event that the author did not send a com plete manuscript and there are missing pages, these will be noted. Also, if material had to be removed, a note will indicate the deletion. uest ProQuest 11011932 Published by ProQuest LLC(2018). Copyright of the Dissertation is held by the Author. All rights reserved. This work is protected against unauthorized copying under Title 17, United States C ode Microform Edition © ProQuest LLC. ProQuest LLC. 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106- 1346 ACKTCOV/LEDGl'IEHTS • I should like, to record my sincere gratitude to the many people who have assisted me in this work during the past three years* I am particularly grateful to my supervisor, Dr* K.H* Overton, for his expert guidance and sustained interest and to Dr* G. Ferguson, who supervised the X-ray crystallographic studies* The analyses- and spectra quoted throughout are the result of the careful work carried out by the technical staff in this department and I should like, to acknowledge their important contribution* Special thanks are due to Roberta for perseverence in typing the manuscript* Thesa investigations were, performed during the tenure of a Carnegie Scholarship and I wish to express my gratitude to this body for its financial assistance and to Professor R*A. Raphael for* his support in obtaining this award* CQIIffj&TTS. ’’The Atisane - Aconane Interconversion”. Chapter 1:- Introduction ........... 1 Discussion lb Restricted Rotation in Amides ***** 60 General Experimental ........ 6b Experlmental................ ***** 66 Structures Chapter 2:- X-ray Analysis of a Key Intermediate Crystal Data * . * * ............. 113 Crystallographic Measurements • • • • • lib Structure Determination ••••».* 115 Refinement * . * ................... 116 Discussion .............. 117 Conformational Analyses »»»•••* 120 References . 12^ SECTION TtfO. ’’The Isopimarane - Cassane Interconversion’1. Introduction • •»«•••»»•••»• 132 Discussion .................. lbl Exp or?, mental • ♦*-* • • •• • • • • • • I63 References .»••»»»»»••»»»• 18b SECTION 1. THE ATI SANE. - ACONANE INTSHCOHVEESIOIF* SUMMARY The first section of this thesis is concerned with a biogenetic- type approach to the synthesis of the aconitine-lycoctonine group 6f diterpene alkaloids. The synthetic precursor, atisine, was transformed in an eleven step sequence into a keto-tosylate. This material under­ went a novel, stereospecific pyrolytic rearrangement to give a key intermediate in the proposed atisane-aconane biogenesis, whose constit­ ution and stereochemistry were confirmed by anx-ray crystallographic analysis, conducted on a heavy atom derivative. Our efforts to convert this intermediate into the desired aconitine- lycoctonine skeleton, a task which had already been accomplished in principle by other workers, met with limited success. N.M.R. studies on some acetamides, obtained in the foregoing syn­ thesis, revealed an interesting example of restricted rotation around 0 the ^ bond. Variable temperature work enabled a crude barrier to \ rotation to be extracted. The second section of the thesis, also in the realm of diterpenes, concerns the synthesis of the cassane skeleton, in a biogenetically- patterned fashion, from isopimaric acid. The route from isopimaric acid to an important intermediate enone is described. Despite numerous attempts,we could not induce the Wagner-Meerwein rearrangement in this enone,which would have resulted in the desired cassane skeleton. 1 CHAPTER 1. INTRODUCTION. 1 For almost a century, the diterpene alkaloids of the Garrva. Ac on i turn and Delphinium species have provided a formidable challenge to the ingenuity of the organic chemist* The early stimulus undoubtedly resulted from the pharma­ cological properties of the plants containing these substances and extracts of Aconitum, for example, have been used to relieve a wide variety of ailments* Unfortunately, the extremely toxic nature of some of the Delphinium alkaloids, their lethal dose for man being in the order of a milligram, has seriously limited their applications in modern medicine. Further stimulus: arose when it rapidly became evident that these alkaloids were structurally very complex and. were a potential source of fascinating mechanistic and degradative problems* One of the early difficulties in the study of the diterpene alkaloids, which was common to many other branches of natural product chemistry, was in the aquisition of pure materials for chemical degradation. The closely related alkaloids frequently formed mixtures which were not easily separable by the techniques available at that time and for this reason much of the early work is now of historical rather than 2 . chemical interest. In fact, soma of these results were so confusing; and contradictory that they seriously impeded, rather than aided, later work. Structure elucidation was further hampered by the plethora of products, which resulted from the application of classical degradation techniques to molecules which were heavily substituted with oxygen functionality.. Until the early 1950*s, success was limited to. the recognition of the relative disposition of various oxygen substituents, mainly due to oxidation and pyrolysis experiments* Unfortunately, these experiments were less clear cut as regards defining the carbon skeleton and as far as the more complex alkaloids were concerned, dehydrogenation was- also of little value and at one stage positively mis­ leading • The real breakthroughs in the chemistry of these alkaloids came in the mid 1950 *s, with the. development and widespread use of new and very powerful physical tools. These compounds, in particular, proved to be excellent substrates for X-ray crystallography because of the relatively easy introduction of a heavy atom and the key to the interpretation of the wealth of chemical information accumulated throughout the years was found in the brilliant. X-ray work of Przybylska, 2 which established the structure and later the absolute 3 configuration of des.(oxymethylene)-lycoctonine(l). The X- ray method was also instrumental in elucidating the structure cf the. simpler alkaloid, veatchine(2) via the azaphenan- threne dehydrogenation product(3)* Using the lycoctonine 6 skeleton as a basis, the structure of delpheline was deduced and by a combination of X-ray and chemical correlation methods the structures of a great variety of other diterpene alkaloids rapidly emerged* The diterpene alkaloids may be divided into two broad categories; those with a Cjq skeleton and those with a Cjg skeleton* The Cjqcompounds are relatively simple, non-toxic amino alcohols and they may be distinguished chemically from the C,, type in that they revert to phenanthrene on selenium or palladium dehydrogenation* These compounds are not extensively oxygenated, containing usually two or three hydroxyls and occasionally an acetate or benzoate ester and, in contrast to the C^gvariety, their structures were, in the main, assigned from chemical studies* Furthermore, the alkaloids all have counterparts in the non-basic tetracyclic diterpenoids, from which they may be formally derived by interposition of a nitrogen residue, typically an ethylamine or p-amino-ethanol function, -between C(19) and C(20)* In fact, the. main subgroups, of which there are two, within the C2Q classification are. based on the non-nitrogenous skeletons* Atisine(h) is- the parent compound of the first subgroup, whose members have, been isolated from both Aconitum and Deluhinium species* Their biogenetic precursor may be (-) atisirene,in which rings C and D constitute, a bicycla.7- [2,2,2] octane system* Although isolated by Broughton in 1877» the structure of atisine(k) was only conclusively M established in 195^ by Wiesner, following the excellent 9 groundwork of Jacobs and his collaborators. In recent years, some Aconitum species indigenous to Japan and India have yielded several interesting alkaloids based on an atisine skeleton, but possessing an additional, ring fusion between C(lV) and C(20), Hetisine(5), whose heptacyclic , 10 nature; was discerned by X-ray crystallography and the 11 recently isolated spiradine-D(6) exemplify these compounds. The majority of the hetisine-type alkaloids also contain a N-C(6) bond. The garrya C20 subgroup, isolated from Garrya species and typified by veatchine(7)> are formally derived from the (-)- kaurena skeleton, with the characteristic bicyclo [3 ,2 ,l]- octane. residue. Wiesner noted the very close chemical similarity between veatchine and atisine, as elucidated by Jacobs, and proposed the structure (7) for veatchine. This postulate has since been rigorously proved. In parallel with the atisines, there is a group of modified garrya diterpene alkaloids, of which lucidusculine, whose structure (8) was 12 determined by X-rays, is representative. These compounds form a link between the garrya and atisine bases in that, although they were isolated from aconitum species, they possess kauranoid skeletons. There is a small group of four such alkaloids, all containing an N-ethyl group, three oxygen substituents, sited as in lucidusculine. and certainly the; most striking feature, a bond between C(7) and C(20). The second main category of diterpene alkaloids,
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