Tobacco Documents Online

RECENT STUDIES IN NICOTINE CHEMISTRY Jeffrey I. Seeman Philip Morris Research Center P. O . Box 26583 Richmond, VA 23261 -~~° .

ABSTRACT The synthesis in our laboratories of a wide variety of nicotine analogues 471 ~~ihas served as the basis for a range of chemical and pharmacological investiga- . _ . . . . _ , . tions. tions. We have examined the effect of substituents on nicotine's structure, conformation, and chemical reactivity. A chemical model for nicotinic activity has been develo i ~~'' ped by a thorough evaluation of the iodomethylation of these ~ ! analogues thereby simultaneously quantitatiing the relative and absolute , , I "I ~ nitrogen nucleophilicity of these Implicationd extensions of nicotinoids.ns a t~fn~Y ~,-this work will be discussed.

At a meeting entitled "Chemistry of Tobacco and Tobacco Smoke", it certainly seems appropriate to discuss the ch i l d h i l em ca an p vs ca wrooerties ~ of tobacco alkaloids and related compounds. Nicotine (1) is undoubtedly the _ .~.. . most well known tobacco , . ., _ ., , . constituent to the lay person, if not the only constit- known known to the tobacco consumer and nonconsumer alike. `''This alkaloid may well have of France.2'5 Nicotine was first isolated in 1828, before the isolation Nicotine has a long and splendid history (see Table I). It was named after Jean Nicot, a French ambassador to Portugal, who in the mid-sixteenth century is said to have introduced tobacco seed and leaf to the royal courts such other important alkaloids as codeine, atropine, papavarine and physost- igmine.s The correct structure of 1 was proposed by Pinner in 1893 and it N was first synthesized by Pictet in 1895.5 nineteenth century is replete with reference example, the famous August KekulE reported 12 The chemical literature of the to nicotine's reactivity. For 1853, some twelve years before he proposed the structure of benzene.

Nicotine clearly has had a life of its own, independent of its inherent relationship with tobacco and' tobacco products. There have been many '.' interesting chemical results which originated during studies on nicotine and ;related compounds. In perhaps a larger sense, nicotine has played' a central roie in tne investigation or mammaiian neuropnysioiogy ana neurocnemistry. major classification of the autonomic nervous system is the designation of the cholinergic synapses and neuroeffector junctions as either nicotinic or muscarinic, based on which of these two alkaloids (nicotine or muscarine) is activating.13 Nicotine continues to be an important "scientific tool in physiolog- 2 ical' and' pharmacological studies of the nervous system". Importantly, the very concept of receptors was postulated irn 1905 by Langley as part of his 2 .research on nicotine pharmacology. While nicotine was once used in the treatment of various human diseases, it no longer has any therapeutic applica- tions in man, though there remains some veterinary uses for the material. Probably its major utility outside the tobacco industry is as an insecticide and a fumigant, where there remain specific applications for this natural and biodegradabfe compound.2 e.~, L ~ : , ~ ,,b _ (% ' When we beg an our work in the area of nicotine chemistry and pharma- ~~' cology, we were quite impressed with the amount of information already available on these topics. Indeed, we initially wondered whether there were interesting unanswered challenges in the area. We intend to indicate in this report that our encounters in the field of nicotine chemistry have been stimulating, thought-provoking and thoroughly engaging. We shall, first give a modest overview of the current state-of-the-art in nicotine pharmacology. Second, to place our work .in perspective, we will present a particular postulate in nicotine structure-activity relationships (SAR), which we will examine and evaluate in the course of the presentation

• . .~ ~ ,. of our results. Third, we wiJ'I discuss the conformational properties of the nicotine analogues of pertinence to this study. We willi then discuss chemical reaction modelling as it relates to pharmacological activity. Finally, we will ~ ~ ~'^ ~F~`1*~ '.,, . ' .... .. ~. r put together our physical and chemical evidence and suggest an important feacture in nicotine analogue SAR. 13 14 2 ' , Il. NICOTINE PHARMACOLOGY: AN OVERVI'EW Nicotine has numerous direct and indirect pharmacological activities i .~_ which are d'escribed in literally thousands of research reports and' numerous review articles and' books. nervous systems (PNS and' CNS, respectively) as well as on the cardio- :t`._- '. vascular system, the gastrointestinal tract, and the endocrine glands. Its mechanisms of CN!S action are less well understood' than its PNS actions.2'13,1i4 sympathetic ganglia which results in vasoconstriction and increases in blood pressure.. Nicotine also stimulates the central nervious system, though its variety of organs, and it is this property combined' with its stimulation, of activity is particularly complex because it can have both stimulant and depres- sant phases of action. It acts at cholinergic synapses of autonomic gaglia and' at neuromuscular junctions, first stimulating and then depressing (or blocking) activity. Nicotine can cause the release of catecholamines irn a Many of the classical' pharmacological tests for nicotinic activity relate to its PNS properties, e.g., contraction of the guinea pig ileum and frog, rectus muscle, which when combined' with the appropriate blocking or antagonism studies results in information, regarding nicotinic activity. Other tests, e.g., contraction of rabbit aortic strips, are more complex in that nicotine may induce the release of adrenergiic mediators which in turn are responsible for the observed pharmacological events. Still other tests, e.g., blood

pressure, may simultaneously incorporate the actions of a variety of inech- .s anisms involving both the PNS and CNS. Lastly, in the derivation, of SAR, one is interested in determining relative toxicity (LD50) of the compounds under investigation, ~~ k'... 2,1'3,14 systems. certainly represent the effects of numerous One cannot quantify or even describe the CNS properties of compounds in the same fashion that PN!S activity is characterized. Behavioral pharmacol- 10 ogy has become a major tool in the evaluation of CNS properties of nicotine and many other compounds of medicinal interest. In this field', the emphasis is on evaluating the effects of the compound on a variety of behavioral be predicted. : tasks, the end result being a behavioral profile from which CNS actions can The actions of nicotine are mediated through its biinding, and subsequent interactions at nicotinic receptors located throughout an organism. It is evident that all nicotinic receptors need not be identical. For example, hexamethonium acts as an antagonist at nicotinic cholinergic ganglionic sites while decamethonium is inactive at the ganglia; the converse antagonistic activity for hexamethonium and' decamethonium is observed at the neuro- and these 14 muscular . junction. Much work has been done in recent years in the isolation, id'entiification, and characterization of nicotinic receptors, and these particularly successful with regard to the peripheral nicotinic chol'inergic receptor from the electric organi of the Torpedo fish, and electric eel.is A wide range of receptor biinding! studies has been performed, both with purified receptors and with inhomogeneous mixtures, e.g., rat brain homogenate.8 An important and crucial issue in the development of erties. SAR is the relationship between receptor binding studies and in vivo prop-

Until recently, one of the serious deficiencies in the development of SAR for nicotine has been the lack of suitable analogues, particularly those rncorporating the nicotine (or other tobacco alkalbid) ring system. For examplle, very interesting and thorough classical pharmacological studies were 16 ~ and their collab- reported some fifteen years ago by Barlow and Haglid~ "` orators in which a large number of aminoal'kylpyridines 2 were examined. A CCH2 ) N ~ R1 n R 2 To achieve our goal of developing a SAR for the tobacco alkaloids, we 1. To synthesize a variety of nicotine analogues which incorporate . have defined the following subobjectives: different electronic, steric, and stereoelectroniic features. 2. To determine a wide range of physical and' chemical data for these analogues which would (hopefully) correlate with pharmacological results. modest series of nicotine analogues was prepared' by Yamamoto as part of his work on nicotinoids as insecticides. Of course, the tobacco alkaloids them= selves are structurally related to nicotine and have been the subject of many pharmacolbgicat tests.

3. To choose for our syntheses specific analogues which can aid in other aspects of our work, e.g., in potential nicotine receptor isola- tion tiion studies. III. THE EXAMINATION OF A N!ICOTINE SAR HYPOTHESIS: THE HAGLID .In 1967, Frank Haglid~ reported that 4!-methylnicotine (3) was pharmacol- _ PROPOSAL ogically inert in a number of tests. He suggested that the inactivity of 3- was due to its inability to adopt specific molecular conformations necessary for receptor binding.18 Of course, this is only one of a number of possible , . _ explanations for low activity, e.g., an alternative possibility is steric bulk around the pyridine ring. We decided to examine Hagliid''s hypothesis in t light of our own interest in this area, and we prepared a series of pyridine substituted nicotinoids in addition to 4-methyfnicotine: 2-methyl, 5-methyl,, and 6-methylnicotine 19-21 - CH3 N' I U I N ~ , 1 N rt-I, CH3 ' . `- ',3 CH3 4 VV Figure I illustrates the activities of these compounds and nicotine with regard to three tests: LDso, guinea pig ileum, and~ rat blood pressure.22-23 In all cases, 5-methylnicotine and 6-methylnicotine were equiactive, within an order of magnitude, to nicotine while both 2-methyllnicotine and 4-methylni-

~cotine were significantly less active than nicotine. These results confirm group at C4. But consistency is insufficient proof when we are dealing with so few compounds. We will further illustrate our work in the area of nicotine SAR by examining the Haglid postulate in terms of the conformational and' chemical properties of these and other analogues, with, a goal of further one would, anticipate that a methyl substituent at nicotine's C2 would cause the same across ring interactions with the pyrrolidine moiety as a methyl' Haglid's pharmacological data and are consistent with his hypothesis,18 since defining the limits and value of this particular SAR. Specification of nicotine's conformation requires definition of three stereochemical features: the orientation of the pyrrolidline N'-methyil group relative to the pyrid'ine ring (cis as in 7, trans as in 8); the conformation of the five membered pyrrol,idine ring; and the orientation of the pyridine ring relative to the pyrrolidine ring, conveniently described by the dihedral angle IV. THE CONFORMATION OF NICOTINE =t(H2,C2,C3,C2) (c.f., 7). By conformation we mean "any one of the infinite number of momentary arrangements of the atoms in space that result from rotation about single bonds. "24 If we strictly keep to this definition, it may be argued, and' in fact has been argued by this author, that the orientational properties of the N'-methyl group in nicotine is a configurational question, not a conformational one.25 This is a subtle distinction about which there remains some debate among practicing stereochemists, and we will not concern ourselves about it in this context.

8 CH3 K H ~ .-- Ar N 0 CH3 There is one important consideration which must not be overlooked simply because relevant experimental and theoretical information is quite difficult to obtain: moleculies do not exist in a finite number of conforma- tions but should be represented by conformational energy maps which ill- lustrate potential energy wells and potential energy barriers between and around these wells. Clearly, for most molecules, these representations would have to be multidimensional giveni the large number of degrees of freedom involved. However, one can often focus attention on particular structural' variables, often dihedral angles, to glean conformational information. To further complicate the issue, consider that the physical, chemical, and! pharmacological properties of molecules are somehow dependent on its confor- mations -- in multidimensional space!26-27 ..,._ A. The Orientation of the N"-Methyl Group In the x-ray analyses of both nicotine diihydroiodide28 and niicotine- saticylic acid complex (1 :1),29 the N'-methyl group was found to be trans to the pyridine ring, as in 8, However, this result does not necessarily reflect solution or gas phase conformational propensities, since it is well known that crystal' lattice forces are not excellent models for alternate environments. Furthermore, the crysalliine samples were undoubtedly prepared under condi-