STUDIES RELATED TO THE BIOSYNTHESIS OF THE MORPHINE AND ERYTHRINA ALKALOIDS A thesis presented by RONALD JAMES in partial fulfilment of the requirements for the degree of DOCTOR OF PHILOSOPHY Imperial College, S. W. 7 . November, 1966 11 . TO MY MOTHER ABSTRACT The stereochemistry and biosynthesis of the morphine alkaloids is reviewed, along with the structure, stereochemistry, and attendant biogenetic theories of the Erythrina alkaloids, as the essential plinth on which the present work is based. The absolute configuration at C -9 of the morphine alkaloids has been determined by direct correlation with a benzyl- isoquinoline, The stereochemistry of the epimeric salutaridinols has been defined by degrading them to optically active glyceric acids, and the method extended to obtain the stereochemistry of the alkaloid nudaurine. The structure of erythratine has been revised in the light of biogenetic theory, and evidence provided to confirm this structure and to define its stereochemistry. The structural elucidation of three other Erythrina alkaloids has been completed. Possible precursors of the Erythrina alkaloids have been fed to Erythrina plants and a modified theory for the biosynthesis of these alkaloids is suggested based on the results from these tracer studies. iv . ACKNOWLEDGMENTS I wish to express my sincere gratitude to Professor D.H.R. Barton for the privilege of working under his inspiring guidance, and also to Dr. G.W. Kirby for very many helpful and friendly discussions during the course of this work. I have to thank also Dr. D.W. Turner who obtained all the 100 Mc. /sec. n.m.r. spectral data, Dr. E.S. Waight and his staff for mass spectral measurements, Mr. Young who grew the plants, and Mr. D.R. Aldrich and his staff for all kinds of technical assistance. My colleagues in the department have given me help, advice, and friendship, all of which were much appreciated. Finally I am deeply indebted to my mother, for without her sacrifices over many years this thesis would not have been written. v. CONTENTS page Abstract Acknowledgments iv• REVIEW Stereochemistry of the morphine alkaloids 1 Biosynthesis of the opium alkaloids 14 Structure and stereochemistry of the Erythrina alkaloids 38 Biogenesis of the Erythrina alkaloids 49 DISCUSSION Absolute configuration at C-9 of the morphine alkaloids 57 Stereochemistry of the salutaridinols 62 Stereochemistry of nudaurine 72 Structures of some Erythrina alkaloids 75 Biosynthesis of the Erythrina alkaloids 89 EXPERIMENTAL 103 REFERENCES 129 1. REVIEW The stereochemistry of the morphine alkaloids In 1805 the "vegetable alkali"1 morphine (1; R=H) became the first member of a large group of naturally occurring substances, 2 later grouped together as the alkaloids, to be isolated pure . The elucidation of its structure was a spur and a building block in the progress of organic chemistry from that time until the structure 3 4 proposed by Sir Robert Robinson was finally verified in 1950 . 5 The way was then clear for Stork to deduce the relative stereochemistry of morphine and codeine (1; R=Me) from the experimental evidence already available at the time. His arguments, which rest on the known steric requirements of certain reactions, are outlined below, and are followed by some more recent evidence which supports his conclusions. Morphine and codeine have five asymmetric centres but the presence of an ethanamine bridge limits the number of possible isomers to sixteen (eight racemates) instead of thirty-two. Stork first related the hydrogen at C-6 to the C-5 oxygen function. He assumed that the C-6 hydrogen was above the plane of the ring in codeine (2; R=}1). Isocodeine is then represented by (3; R=H). Of the two derived methyl ethers (2, 3; R=Me), the isocodeine ether is recovered unchanged from treatment with methoxide, whilst the 6 codeine epimer is isomerised to the phenolic enol ether (4) . Z. RO MeO 1VIe0 Mule RO H (1) nn (z) ./..CDMe HO (5) (4) The recovery of the isocodeine ether shows that no epimerization takes place under the conditions used, and since the f3 elimination requires the existence of a trans relationship between the hydrogen at C -6 and the oxygen at C-5, the configuration of codeine can be expanded to (5). Confirmation of the cis relationship between the adjacent oxygen functions is found in the greater rate of the lead tetra-acetate cleavage of tetrahydromorphitetrol (7), compared with 7 that of tetrahydro-a-isomorphitetrol (8) . These glycols are ? prepared from dihydrocodeine (6) and dihydroisocodeine, respectively, by the sequence shown for the codeine series (6) —> (7). 3. COOMe e HO HO HO (6) LiA1H 4 HO (8) HO- (7) Ozonolysis of the substituted catechol cleaves the most oxygenated double bond and subsequent hydrogenation reduces only one of the remaining double bonds, the tetra-substituted one being stable to the conditions used. Lithium aluminium hydride reduction of the resulting ester -lactone affords the morphitetrol (7). Since cis glycols are known to be cleaved faster than trans, by lead tetra-acetate, the two oxygen functions at C-5, C-6, must be cis. Next, the ethanamine bridge was related to the C-5 hydrogen by a consideration of the relative stabilities of cis and tram fused 6,5 ring systems. Dihydrocodeinone (10) is of the hexahydro- l-indanone (9) type. If it had a trans junction of the ether ring and 4. and the C ring, then it would be expected to isomerise in base to a v3 cis junction, which is known to be the more stable form of (9) , 9 whereas in fact dihydrocodeinone is stable in base . Further, since dihydrocodeine (6) is the sole product of catalytic reduction of both 10 dihydrocodeinone (10) and codeine (5) , these must all have the same stereochemistry at this junction. HCr , ' (10) (6) HO (5) If the ring junction is cis, it follows that C-15 and the bridge oxygen must be trans, and this assignment is supported by a number of concerted eliminations of the ethanamine bridge with cleavage of the oxygen bridge, by nucleophilic attack at C-15. For example, when p-methyl-morphimethine (11), a Hofmann degradation product, is 11 heated with sodium ethoxide, morphenol (12) is formed . OEt 1 + CH CH 2— 2 NMez (12) 5 • These eliminations are most satisfactorily explained if a trans relationship exists between C-15 and the bridge oxygen. Stork believed that he had further evidence for this trans junction in the ease of generation of 1-bromodihydrocodeinone (13) by alkali 12 treatment of 1,5-dibromodihydrothebainone (14) . OHO 80 % In fact, Fieser and Fieser relied heavily on this argument in their 13 earlier deduction of the morphine sterochemistry , but later work 14 by Gates has shown that the bromine is at C-7 and is equatorial, and not at C-5 and axial, and he suggests that the reaction must proceed through the enol (15). Bile° 0 > (13) IC HO HO' (Br (15) (16) 6. Nevertheless on the other evidence the formula for codeine can be expanded to (16). The remaining centre at C-14 was assigned on the evidence from catalytic hydrogenation of thebaine (17) which gives dihydrocodeine methyl ether (18) and not 15 the isocodeine series (19) . /vie° w MeO w MeO H2 / , .. a NMe N/vie MeO H (19) (17) (18) It is argued that hydrogen must add to the diene either from the same side as the ethanarnine bridge, in which case (18) results, or from the opposite side to give (19). Hence the configuration at C-14 must be the same as that at C-6, and since the configuration at C-6 is known then the complete relative stereochemistry of morphine (20; R=H) and codeine (20; R=IvIe) is deduced. e HO (zo) 16,17 Recent work by Rapoport has provided additional chemical evidence for Stork's assignments, while two complete 18 19, 20 syntheses support, and X-ray studies confirm the stereo- chemistry as shown (20). 7. Rapoport showed that exhaustive methylation of dihydro- isocodeine (21) leads to a small yield of the cyclic ether 6-codiran (22), whereas none is formed from dihydrocodeine (6) under the same 16 conditions . Me Me MeO (6) Me Me N 3 HO 0 (21) (21A) (22) Formation of this cyclic ether requires a cis relationship between the C -6 hydroxyl and the ethanaxnine bridge. If this is the case in isocodeine then it must be trans in the codeine series. In another paper apoport17 challenges the evidence on which Stork assigned the C-I4 stereochemistry, on the grounds that there are several known cases where hydrogenation of the 8-14 double bond produced the unnatural epirneric configuration at C-14, and further, that if the hydrogenation of thebaine is interrupted before it is complete, neopine methyl ether (23) can be isolated, which may in turn be reduced, but not necessarily from the same side as that from which the 6-7 double bond was reduced. Me Me Me Me (17) 8. He then arrived at the same stereochemistry at C-14 in the following 17 way . Thebenone (24) is a degradation product of thebaine (17) having the same stereochemistry as morphine at C-14. MeO Me• Me i) Hof. H2/Pd/C H2/Pd/BaSO 4 > H AcOH iii) Hof.> (24) The C ring was further degraded by way of the dio2dminothebenone (25) formed by condensation at both activated methylene groups. A neat double application of a variant of the Beckmann rearrangement, followed by partial hydrolysis gave the acid amide (26), which gave a cyclic imide (27) on ring closure. MeC (24) RONO> Ts .C1 ®OBut PY• (27) H • The C-14 epimeric compound prepared from epi-thebenone in the same way did not ring close under the same conditions.