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Heterocyclic Compounds with Bridgehead Nitrogen Atoms

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Heterocyclic Compounds with Bridgehead Nitrogen Atoms HETEROCYCLIC COMPOUNDS WITH BRIDGEHEAD NITROGEN ATOMS by DAVID FARQtAR B.Sc. Thesis presented for the Degree of Doctor of Philosophy University of Edinburgh January 1969 TO MY PARENTS ACKNOWLEDGEMENTS I wish to record my sincere thanks to Dr. Derek Leaver for the constant advice and encouragement he has so freely given throughout the course of this work. I also thank the University of Edinburgh for the provision of laboratory facilities and the Science Research Council for the provision of a maintenance grant. INDEX Page SUMMARY • . • . I INTRODUCTION Object of research . 1 The cyclazines . .. ... . .. . .. .6 DISCUSSION Attempts to synthesise cyc1[333TJazines by selective cleava e of the l,2-bond of cyclopenta[c 7 d]cycl3.3.3]azines 2+ Attempts to expand the five-membered ring of cyclopenta[c,d]cycl[3.3.3]azines 27 Quinolizines, quinolizones and quinolizinium salts . ... . •1 30 Attempts to synthesise methylenequinolizines 36 Synthesis of methylenequinolizines . 43 Synthesis of cycl[3.3.3]azines . 62 Chemical reactions of cycl[3.3.3]azines . 100 Synthesis of cycl[3.3.2]azin-1-one and some simple derivatives of the cycl[3.3.2]azinium system . 119 EXPERIMENTAL Attempts to effect selective fission of the 1,2-bond of cyclopenta[c,djcycl[3.3.3]azines. 127 Attempts to expand the five-membered ring of cyclopenta[c,dJcycl[3.3.3]azines . ,. 128 Page EXPERIMENTAL (Continued) Attempted synthesis of methylenequinoliziries . 131 Synthesis of niethylenequinolizines . 135 Attempted synthesis of cycl[3.3.3]azines . 152 Synthesis of cycl[3.3.3]azines . 155 Chemical reactions of cycl[3.3.3] azines 1)Hydrogenation . .. .. 176 Diels-Alder addition . 178 Electrophilic substitution . 185 Synthesis of cycl[3.3.2jazin -1 -one and some simple derivatives of the cyclL3.3. 2]azinium system . 194 Nuclear magnetic resonance spectral data . 198-208 BIBLIOGRAPHY , . • . • , . 209 1. -I - SUMMARY The object of this research was to synthesise cycl[3.3.3]azine - a 12i-i' peripheral-conjugated member of the cyclazine series of heterocyclic compounds - and to investigate its chemical properties. Three approaches to the ring-system have been investigated. Attempts were made to effect selective fission of the 1,2-bond of cyclopenta[c,dJcycl[3.3.3]azines using a variety of ...'double-bond' reagents. Decomposition product was formed from these reactions which suggested that the reaction intermediates (possibly cyc13.3.3Jazines) were more susceptible to attack by the reagents than the initial cyclazines. A, previously projected synthetic route from diethyl 6-methylquinolizin-+-y1idenema1onate has been attempted. The latter compound, together with methyl 6-methr1quinolizinJ+- ylidenecyanoacetate has been synthesised but attempts to convert them into cyclazines by base-catalysed intramolecular cyclisation has led, in most instances, to the formation of decomposition product. A reported synthesis of diethyl quinolizin-+-ylidenemalonate is shown to be erroneous. Reaction has been effected between electrophilic acetylenes and several alkyl quinolizin-+--ylideneacetates. The latter compounds were prepared by selective hydrolysis and decarboxylatlon _II_ of one of the ester groups of dialkyl quinolizin--i-- ylidenemalonates. A series of cyclazine derivatives has been prepared and proof of the ring-system has been obtained. Structures are proposed for the reaction intermediates. The parent cyclazine has been prepared by pyrolysis of the 1, 3-di--t-butoxycarbonyl derivative. Although stable under nitrogen it decomposes within minutes on exposure to air. N.m.r. evidence indicates that the system is polyolefinic in character. The proton resonances are amongst the highest observed to date for protons joined to trigonal carbon. This is regarded as evidence of a paramagnetic ring-current in the peripheral, non-aromatic, 127Y-electron system. O-Orientated electron-withdrawing substituents exert a stabilising influence on the ring-system and, rather remarkably, cause an increase in the electron density at the second Cc-position of the ring bearing the substituent. An explanation of this phenomonen has been advanced. Chemical confirmatory evidence for the polyolefinic character of the system has been obtained by cycloadditiori reactions conducted on the 1 93 -diethoxycarbonyl derivative. Substitution reactions are also possible with electrophilic reagents and are regarded as analogous to those of a typical enamine rather than as evidence of aromaticity. cycl[3.392]azln_l_one and its 2-ethoxycarbonyl derivative has been prepared by pyrolysis of di ethya/quinoli z in _)+_ylidenemal oria t e. The former has been converted Into simple derivatives of the hitherto unknown cycl[3.3.2]azinium system. 0 INTRO DUCT ION -1 - OBJECT OF RESEARCh The. continuing Investigations directed towards a fundamental unders-tanding of the concept of aromaticity have centred during the past decade on the study of peripherally-conjugated macrocyclic hydrocarbons. The intense activity current in this field has resulted from the potential utilisation of these compounds in investigating the validity of Huckel's ( 1+n + 2)Tr'-electron rule.' The latter predicts that planar monocyclic conjugated systems incorporating a closed shell of (4n + 2) 11' -electrons should possess an inherent electronic stability. Systems formally constituted of alternating single and double- bonds have been termed annulenes the ring size being indicated by a number in parenthesis - . These compounds; may be further categorised according to their ability to satisfy the Huckel criteria for aromaticity. Thus, planar (+n+ 2)T1' peripherally- conjugated systems are potentially capable of exhibiting aromatic properties whereas: their 4nTY analogues are expected to be essentially olefinic In character. The interpretation of the term "'aromatic" has long been a source of contradictory and controversial opinion. According to the classical concept, a compound was considered aromatic if It exhibited benzene-like stability and chemical behaviour. However, this definition Is no longer considered valid, the latter criteria depending on the free-energy change between the ground- state of the molecule and the transition-state for the chemical reaction• involved -2- The modern definition considers a compound to be aromatic if there is a measurable degree of cyclic delocalisation of the Di' -electron system in the ground-state of the molecule. A consequence of such is that - carbon - carbon bonds show little alternation in length and the molecule as a whole possesses a lower energy content than would be expected from classical considerations. Delocalisation in (n + 2)11' -electron systems further results in the ability of the molecule to sustain a diamagnetic ring-current in an applied magnetic-f ield' 5 In the case of aromatic annulenes. this: current shields the inner protons and deshields the outer. 5 This property can be readily investigated by nuclear magnetic resonance spectroscopy which (together with diamagnetic susceptibility measurements) provides the most convenient means, at present, of determining whether a • compound is aromatic or not. • . Until comparatively recently the only known fully-conjugated monocyclic systems were benzene ([6] annulene) and cyclooctatetraene ([8] annulene) higher vinylogues having resisted classical methods7of synthesis. However, the discovery8 -11 in 1956 of simple methods for the generation of large-ring hydrocarbons containing 193 -diacetylenic units and their subsequent proto- tropic rearrangement to fully-conjugated cyclic products 12has resulted in prolific researches by the Sondheimer group, culminating to datein the synthesis13 of an entire series of annulenes. Investigations 13of the physical characteristics of • these compounds have conclusively substantiated the generality of the Huckel 11' -electron rule. However, thethe pursuit of annulene chemistry cannot, within itself, be considered a wholly integral method for the evaluation of the unique chemical characteristics associated with (+n + 2)11'- electron delocalisation. An inherent defect in the approach arises from molecular distortion consequent on steric interaction of inwardly-directed skeletal protons. while El olannillene (i) satisfies the numerical Huckel electron-requirement, realisation of its potential aromatic property appears, at present, to be inhibited by the failure. of the molecule to attain planar ring- geometry and hence cyclic delocalisation of the peripheral fl'-electrons. Similar structural considerations apply to the. 1+nll' non-Huckel vinylogue [2] annulene. III. ' (1) (2) The importance of the. steric factor is clearly demonstrated by the apparent chemical instability of these lower ring- vinylogues. Thus, [10] annulene ( 1) has; resisted synthetic endeavour l6i8and remains a hypothetical molecule whil [12] annulene (2) is still probably unknown 19-20 The [lçjliuckel vinylogue cyclotetradeca- heptaene (3) has, however, been synthesised 21and shown to be capable of existence 22in two conformationally isomeric forms (A)and (B) (A) (8) (3) A preliminary X-ray investigation 3 coupled with a low- temperature (-60 °c.) n.m.r. study2has shown at least one of these conformers (A) to be aromatic in character in spite of it apparent non-planar structure. The molecule, however, is very unstableundergo.ing complete decomposition when exposed to light and the atmosphere for a period of one day. The investigation of the chemical and physical behaviour of annulenes of intermediate ring-size thus presents considerable theoretical
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