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Studies Related to Tue Synthesis of Tetracycline

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Studies Related to Tue Synthesis of Tetracycline STUDIES RELATED TO TUE SYNTHESIS OF TETRACYCLINE a thesis presented by • MOSHE NATHAN ROSENFELD in partial fulfilment of the requirements for the award of the degree of DOCTOR OF PHILOSOPHY OF THE UNIVERSITY OF LONDON WHIFFEN LABORATORY, CHEMISTRY DEPARTMENT, IMPERIAL COLLEGE, LONDON SW7 2AY. AUGUST, 1976, nwp,To N1M nwyaw nnl , evnlw xln 'Into nn .ynyn nnn win 12a TInl nIn1710 ell 'n7 win vin nT MR1 'IDIOT nal r21 .1313m'7n 'In 172N (1—n I n Onp) That which hath been, is that which shall be , And that which hath been done,is that which shall be done, And there is nothing new under the sun . Is There a thing, whereof it is said:'see this is new' , It hath been already in the ages, which were before us . (Liber Ecclesiastae 1,9-11) 3. ACKNOWLEDGEMENTS I thank Professor Sir Derek Barton, F.R.S., for the opportunity of working with him and for his encouragement, guidance and tolerance throughout the course of this work. My colleagues in the Whiffen Laboratory, especially Dr. S.V.Ley„ are warmly thanked for their assistance and friendship at all times. Mr. K.I.Jones and his staff are thanked for their excellent analytical service, Mrs. Lee for the mass spectra, and Dr. L.Phillips and his team for the 13C n.m.r. spectra. Technical assistance from Mr. R.Carter, Mr. A.Ellis, and Mr. T. Adey was greatly appreciated, as was the kindness and co-operation of Mrs. Day in the 'Organic Stores'. I wish to express my thanks to the Wellcome Trust for the award of a Fellowship for the period of this research. Mosche N. Rosenfeld , Whiffen Laboratory, July 1976. 4. ABSTRACT Reviews of hydroxylation procedures for phenols and synthetic uses of organo-selenium compounds are presented. The nature and chemical properties of diphenylseleninic anhydride are discussed. Diphenylseleninic anhydride has been used to oxidise simple and tetracycline model phenols; the products being hydroxydien- ones (e.g. 2-Carboxymethy1-3,4-dihydroxy-4„5-dimethylcyclohexa-2„5- dienone (A) ), quinones (e.g. 2-Carboxymethy1-5„6-dimethyl-3-hydroxy- benzoquinone (B) ), and phenylseleno-substituted species (e.g. Methyl 2„6-dihydroxy-314-dimethy1-5-phenylselenobenzoate (C) ). SePh OH OH 1 CO2Me OH OH (A ) (B) (C) Prior formation of the phenolate anion, generally increased the yield of o-hydroxylation. The various mechanistic aspects of these transformations are discussed. Over•oxidation products (e.g. 3-t-butyl-5-phenylseleno-l„2-benzo- quinone (D) ) were sometimes encounted„ if excess diphenylseleninic an- hydride was used, or by inverse addition procedures. The use of hexamethyldisilyl amide to generate the phenolate anions prior to treatment with diphenylseleninic anhydride, lead to the formation of novel selenoimines (e.g. 4„6-Dimethyl-1,2-benzoqui- none monophenylseleno-2-imine (E) ). The structures and mechanism of formation are presented. 5. PhSe NSe Ph (D) (E) The tetracycline ring A model phenol series was extended with the synthesis of cyclic boronates (e.g. 2-Carbomethoxy-516-dimethy1-4- hydroxybenzene-1,2-phenylboronate (F) ), diethylazo dicarboxylate and urazole derivatives (e.g. Methyl 5-diethylhydrazodicarboxy1-2,6-dihy- droxy-3,4-dimethylbenzoate (G) and Methyl 2,6-dihydroxy-3,4-dimethy1- 5-(4-phenylurazole)-benzoate (H) ), and hydroxylation with diphenyl- seleninic anhydride was attempted. EtO2C CO2Et I I N--NH 0 OH OH OEt CO2Me CO2Me CO2Me OH OH OH OH (F) (G) (H) (I) A synthesis of a number of model cathylate phenols (e.g. Methyl 3,4-dimethy1-2-hydroxy-6-(ethylcarbonate)-benzoate (I) ) and their conversion to hydroxycyclohexadienones, using diphenylseleninic an- hydride, has been developed. The synthesis of diphenyltellurinic anhydride and phenyltellurinyl chloride is presented, and reactions with phenols have been investigated. 6. CONTENTS Page Acknowledgements 3 Abstract . 4 Introduction 7 Review Hydroxylation of Phenols 9 Organo-selenium Oxidations 20 References 36 Discussion i ) Discovery and Synthesis of Diphenylsele- ninic Anhydride 42 ii ) '12a' Hydroxylation 47 iii) T-4, Functionality 67 iv ) Synthesis of Diphenyltellurinic Anhydride 86 Appendix 13C -nmr studies of model cathylates 92 Experimental 94- Discussion and Experimental References 133 7. INTRODUCTION The most important antibiotics in modern therapy were discovered between 1940 and 1960. Since then, relatively few antibiotics of tho- roughly novel structure and action have come into use. The advance- ments during the last decade have been achieved mainly by chemical modifications of the classical antibiotics . 1 The first tetracycline was aureomycin (1), isolated by Duggar in 1948 from streptomyces aurefaciens, followed by terramycin (2) , which was obtained by Finlay et al.2 in 1950 from streptomyces ri- mosus. Structural elucidation was achieved by Woodward and a research team at Chas-Pfizer and Co.5 . 7 6 4 8 9 10 11 12 1 R2 I Cl H II H OH 8. The chemistry of tetracyclines has been the subject of extensive 4 reviews and a number of Ph.D. theses at Imperial College5 . (The numbering system, which will be used throughout this thesis with reference to the linear naphthacene skeleton, is shown in (1) ). 9. HYDROXYLATION OF PHENOLS. The synthetic route to the tetracyclines employed at Imperial College necessitates, at some stage, the intruduction of the l2a- hydroxyl group into the aromatic ring A of a tetracyclic intermediate. OH COR~ OH o o OH o OH o A large number of oxidation studies on model phenols have been underta- ken with the objective of creating ortho-hydroxydienones. A stmmary of' this work follows. i.) Lead Tetra-acetate. 6 In 1950, Wessely et ale examined lead tetra-acetate in acetic acid as a potential reagent to convert phenols into quinol acetates. It was shown, that para-substituted phenols form, in alcoholic solution, the co­ responding para-quinol ethers7 . Ortho-quinol derivatives could only be obtained from an ortho-substituted phenol. If, however, an ortho- and para- position can be attacked, the acetoxy group usually enters preferentially at the ortho-position. This position remains the reactive centre even when substituted with bulky groups such as methyl, ethyl, isopropyl, n-propyl or sec-butyl. However, a t-butyl substituent drastically reduces attack at 8 the ortho Position . Phenols with an uno~cupied ortho position giv~ ortho- quinone diacetates (III and IV) ,: in addition to, or instead of, tl-., quinol acetates (I and II)9 • Those phenols having a free para positicD:!a'.1. give 10. p-quinones as minor or in some cases major products OH 0 0 4- R OAc R (I) OH 0 1 0 (Iv) 2,4,6-Trimethylphenol (mesitol), being a diortho aswell as para - substituted phenol, affords upon treatment with lead tetra-acetate, 2- acetoxy-2,4,6-trimethyl cyclohexadienone (V) . (V) Similarly 2„4-dimethylphenol was converted to the corresponding dienone, which was isolated as its dimer. The accepted mechanism for the Wessely acetoxylation involves an in- 11 tramolecular reaction . 11. product Pb(0Ac)2 'Intramolecular electrophilic mechanism for the Wessely acetoxylation' Although the Wessely acetoxylation does not show a general preference for reaction at an ortho carbon with some substrates (such as mesitol), the proportion of ortho attack exceeds statistical predictions. The preference cannot be attributed to obvious steric or electronic factors in terms of 12 intermolecular substitution . OH 0 e +0Ac prod. R Pb(0Ac)3 R tR -- co pboA02 Me 'Electrophilic mechanism for Wessely acetoxylation,involving plumbylation followed by intramolecular substitution '. In summary„the evidence suggests, acetoxylation proceeds via inter- molecular electrophilic attack, involving initial formation of a lead triacetate intermediate. Although there is no evidence clearly favouring initial C-plumbylation or direct acetoxylation of phenols by lead tetra- acetate, there is no doubt, that the phenolic substrate is attacked elec- 12. 13 trophilically rather than either nucleophilically or homolytically . ii.) Sodium Periodate. dl A er showed that 2,4-dimehylphenol with sodium periodate in aqueous or dilute acetic acid solution gave the corresponding o-quinol as its dimer, together with its Diels-Alder adduct with 3,5-dimethyl- o-benzoquinone. OH OH 0 Similar results were obtained with 2,6-dimethylphenol15 . However, the para-quinol could be isolated as a minor component in the oxidation of 2„4-dimethylphenol and 2,4,6-trimethylpheno114' 16 . Adler et al. pro- posed a mechanism for the oxidation based on studies of the periodate- 17 induced demethylation of catechol monoethers in 018-labelled water . In the reaction it could not be determined, whether the attack by sol- vent HOX takes place on the iodate ester by SN 2' process, or whether the ester undergoes loss of iodate and hydroxylation by an S 1 process. N An intramolecular ortho-attack via a cyclic diester is possible, but 10 thought unlikely, as pars:.- tack is equally rapid . 13. HO /0 HOX /I=0 0`1.0e XO HOX -H20 In conclusion, aqueous sodium periodate oxidises phenols, depending on the substitution pattern, to o- and E-quinones, aswell as o- and 1D- quinols. The solvent was shown to participate in the reaction, since H2180 gave labelled o- and 2.-quinones. Recently it was shown, that the use of periodic acid, H5106, gave less pronouced para oxidation of phe- 18 nols . iii.) Acylperoxide and Peracetic Acid. There is some confusion in the earlier literature on the reaction 19 between phenols and diacyl peroxides . As a result of work carried out 20 in these laboratories clarification of this oxidation was achieved. Thus treatment of sodium mesitate with benzoyl peroxide in ether at -20. yields the ortho-dienone (VI) as the major product. A certain amount of the para-dienone can be obtained upon heating (VI) to 120.. The rearran- 21 gement was explained in terms of a thermal sigmatropic (3,3) suprafacial migration of the acyloxy group along the periphery of the cyclodienone 14.
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