Synthetic Methods for the Preparation of Conformationally Restricted Analogues of Nicotine

In the context of naturally occurring nitrogen heterocycles, nicotine is a chiral alkaloid present in tobacco plants, which can target and stimulate nicotinic acetylcholine receptors (nAChRs), a class of ligand-gated ion channels commonly located throughout the human brain. Due to its well-known toxicity for humans, there is considerable interest in the development of synthetic analogues; in particular, conformationally restricted analogues of nicotine have emerged as promising drug molecules for selective nAChR-targeting ligands. In the present mini-review, we will describe the synthesis of the conformationally restricted analogues of nicotine involving one or more catalytic processes. In particular, we will follow a systematic approach as a function of the heteroarene structure, considering: (a) 2,3-annulated tricyclic derivatives; (b) 3,4-annulated tricyclic derivatives; (c) tetracyclic derivatives; and (d) other polycyclic derivatives. For each of them we will also consider, when carried out, biological studies on their activity for specific nAChR subunits.

Dealkenylative Alkenylation: Formal σ-Bond Metathesis of Olefins

The dealkenylative alkenylation of alkene C(sp3 )-C(sp2 ) bonds has been an unexplored area for C-C bond formation. Herein 64 examples of β-alkylated styrene derivatives, synthesized through the reactions of readily accessible feedstock olefins with β-nitrostyrenes by ozone/FeII -mediated radical substitutions, are reported. These reactions proceed with good efficiencies and high stereoselectivities under mild reaction conditions and tolerate an array of functional groups. Also demonstrated is the applicability of the strategy through several synthetic transformations of the products, as well as the syntheses of the natural product iso-moracin and the drug (E)-metanicotine.

Synthesis and characterization of Schiff base of nicotinic hydrazide as antibacterial agent along with in vivo wound healing activities and atomic force microscopic study of bacterial cell wall affected by synthesized compound

The present work reports the synthesis of Schiff base series of nicotinic hydrazide (C-1-C-5) and it's antibacterial and wound healing evaluation. The synthetic molecules were characterized with different spectroscopic techniques and explored for their antibacterial potential. The objective of this work was to explore antimicrobial agent using two types of microorganisms, one Gram-positive (S. aureus ATCC 9144) and one Gram-negative (E. coli ATCC 10536). C-2, C-4 and C-5 potentially inhibit bacterial growth (p<0.001). Atomic force microscopy (AFM) imaging was obtained to get high-resolution images of the effect of treated drugs on the bacterial morphology. The images obtained also revealed the antibacterial effects of potent molecule. The magnified pictures captured under AFM suggest significantly damaged cell surface and disturbed morphology. The compounds were further analyzed for in vivo wound healing potential on mice. The compound C-2, C-4 and C-5 heal the wounds comparatively in less time duration as compared to control group (p<0.001). Compound C-1 and C-3 took more time to heal the wound as compare to compound C-2, C-4 and C-5. The re-epithelialization process of wound in animals group treated with potent compound was highly significant (p<0.001) and faster than control. Results of this study suggest that the compounds C-2, C-4 andC-5 possess pronounced antibacterial and wound healing potential and need to be further evaluated for mechanism of action.

Synthesis of Novel Nicotinic Ligands with Multimodal Action: Targeting Acetylcholine α4β2, Dopamine and Serotonin Transporters

Nicotinic acetylcholine receptors (nAChRs), serotonin transporters (SERT) and dopamine transporters (DAT) represent targets for the development of novel nicotinic derivatives acting as multiligands associated with different health conditions, such as depressive, anxiety and addiction disorders. In the present work, a series of functionalized esters structurally related to acetylcholine and nicotine were synthesized and pharmacologically assayed with respect to these targets. The synthesized compounds were studied in radioligand binding assays at α4β2 nAChR, h-SERT and h-DAT. SERT experiments showed not radioligand [³H]-paroxetine displacement, but rather an increase in the radioligand binding percentage at the central binding site was observed. Compound 20 showed K_i values of 1.008 ± 0.230 μM for h-DAT and 0.031 ± 0.006 μM for α4β2 nAChR, and [³H]-paroxetine binding of 191.50% in h-SERT displacement studies, being the only compound displaying triple affinity. Compound 21 displayed K_i values of 0.113 ± 0.037 μM for α4β2 nAChR and 0.075 ± 0.009 μM for h-DAT acting as a dual ligand. Molecular docking studies on homology models of α4β2 nAChR, h-DAT and h-SERT suggested potential interactions among the compounds and agonist binding site at the α4/β2 subunit interfaces of α4β2 nAChR, central binding site of h-DAT and allosteric modulator effect in h-SERT.

Chalcogen analogues of nicotine lactam studied by NMR, FTIR, DFT and X-ray methods

The selenoanalogue of nicotine has been synthesized and characterized by spectroscopic and X-ray diffraction methods. The crystals of selenonicotine are isomorphic with the thionicotine homologue and consist of molecules engaged in columnar π⋯π stacking interactions between antiparallely arranged pyridine moieties. These interactions, absent in other crystals containing nicotine fragments, seem to be induced by the presence of a lactam group. The molecular structures in the vacuum of the oxo-, thio- and selenonicotine homologues have been calculated by the DFT method and compared with the available X-ray data. The delocalized structure of thionicotine is stabilized by intramolecular C-H⋯S hydrogen bond, which becomes weaker in the partial zwitterionic resonance structure of selenonicotine in favor of multiple C-H⋯Se intermolecular hydrogen-bonds. The calculated data allow a complete assignment of vibration modes in the solid state FTIR spectra. The (1)H and (13)C NMR chemical shifts were calculated by the GIAO method with B3LYP/6-311G(3df) level. A comparison between experimental and calculated theoretical results indicates that the density functional B3LYP method provided satisfactory results for predicting FTIR, (1)H, (13)C NMR spectra properties.

Prodrug-Oriented Molecular Design of Neonicotinoids: Preparation of Imidacloprid-Related 5,5-Dimethoxy-1,3-diazacyclohexane Derivatives and Their Insecticidal Activity

Prodrug-oriented molecular design was attempted for the potent acyclic neonicotinoid insecticide, clothianidin (1-(2-chloro-5-thiazolylmethyl)-3-methyl-2-nitroguanidine). Molecules bearing a CH2COCH2 bridge linking the 1,3-NH ends of clothianidin or their acetals would possibly be hydrolyzed, regenerating the mother compounds. This strategy was used to prepare seven acetals of clothianidin-based molecules that combined 2-chloro-5-thiazolylmethyl, 6-chloro-3-pyridylmethyl or 3-tetrahydrofurfuryl with a nitroimine, cyanoimine or nitromethylene group. The key intermediate, 1,3-diamino-2,2-dimethoxypropane, was prepared from the dihydroxyacetone dimer in four steps. A selected acetal showed a characteristic nerve-impulse pattern for neonicotinoids on an excised American cockroach ganglion, although the neuroblocking activity was fairly low. Some acetals were highly insecticidal against the pea aphid at 0.8-20 ppm 7 days after a spray treatment, this being in a contrast to their far weaker activity by injection into American cockroaches. The biological results suggest that the intrinsic insecticidal activities of the acetals are weak, but would exhibit enhanced activity if hydrolyzed in an external environment.

Synthesis and haemolytic activity of novel salts made of nicotine alkaloids and bile acids

A series of novel salts made of nicotine alkaloids and bile acids were synthesized and their haemolytic activity was examined in vitro using human erythrocytes. All compounds were characterized by spectroscopic methods. The novel salts show membrane-perturbing properties inducing the erythrocyte shape alterations and haemolysis in dose-dependent manner. Nicotine decreases the membrane interacting potential of bile acids in the novel compounds. The presence of sulfur or selenium atom in the nicotine molecule affects the haemolytic activity of its novel salts depending on the hydrophobicity of bile acids.

[Design, synthesis and evaluation of bis-nicotine derivatives as inhibitors of cholinesterases and beta-amyloid aggregation]

A novel series of bis-nicotine derivatives (3a-3i) were designed, synthesized and evaluated as bivalent anti-Alzheimer's disease agents. The pharmacological results indicated that compounds 3e-3i inhibited both acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) in the micromolar range (IC50, 2.28-117.86 micromol x L(-1) for AChE and 1.67-125 micromol x L(-1) for BChE), which was at the same potency as rivastigmine. A Lineweaver-Burk plot and molecular modeling study showed that these derivatives targeted both the catalytic active site (CAS) and the peripheral anionic site (PAS) of AChE. Besides, these compounds could significantly inhibit the self-induced Abeta aggregation with inhibition activity (11.85%-62.14%) at the concentration of 20 micromol x L(-1).

Design, multicomponent synthesis, and bioactivities of novel neonicotinoid analogues with 1,4-dihydropyridine scaffold

Novel neonicotinoid analogues bearing a 1,4-dihydropridine scaffold were designed and synthesized by multicomponent reactions (MCRs) to enhance pi-pi stacking. The synthesized compounds were identified by (1)H NMR, (13)C NMR, high-resolution mass spectroscopy, and elemental analysis. Bioassay tests showed that some of them exhibited high insecticidal activities against pea aphid ( Aphis craccivora ).

Divalent and oxabridged neonicotinoids constructed by dialdehydes and nitromethylene analogues of imidacloprid: design, synthesis, crystal structure, and insecticidal activities

A series of divalent and oxabridged neonicotinoids were synthesized by reactions of nitromethylene analogues of imidacloprid and dialdehydes, and their structures were confirmed by (1)H NMR, (13)C NMR, high-resolution mass spectroscopy, and X-ray diffraction analysis. The bioassays indicated that some of them were endowed with excellent insecticidal activities against cowpea aphid ( Aphis craccivora ), armyworm ( Pseudaletia separata Walker), and brown planthopper ( Nilaparvata lugens ). Divalent neonicotinoid 6 and oxabridged 8a had higher activities than imidacloprid against cowpea aphids and armyworm; furthermore, the activity of 8a was 40.4-fold higher than that of imidacloprid against imidacloprid-resistant brown planthopper.

Highly enantioselective borane reduction of heteroaryl and heterocyclic ketoxime ethers catalyzed by novel spiroborate ester derived from diphenylvalinol: application to the synthesis of nicotine analogues

An asymmetric synthesis for the preparation of nonracemic amines bearing heterocyclic and heteroaromatic rings is described. A variety of important enantiopure thionyl and arylalkyl primary amines were afforded by the borane-mediated enantioselective reduction of O-benzyl ketoximes using 10% of catalyst 10 derived from ( S)-diphenylvalinol and ethylene glycol with excellent enantioselectivity, in up to 99% ee. The optimal condition for the first asymmetric reduction of 3- and 4-pyridyl-derived O-benzyl ketoxime ethers was achieved using 30% of catalytic loading in dioxane at 10 degrees C. ( S)- N-ethylnornicotine ( 3) was also successfully synthesized from the TIPS-protected ( S)-2-amino-2-pyridylethanol in 97% ee.

Enantioselective Synthesis of Nicotinic Receptor Probe 7,8-Difluoro-1,2,3,4,5,6- hexahydro-1,5-methano-3-benzazocine

[Structure: see text] The development of a concise enantioselective synthesis of nicotinic alkaloid 1 is presented. The route features the synthesis and use of a "stable" aliphatic triflate 21 in an alkylation step to generate Heck precursor 24 and an enantioselective cyclization to establish a compound with the key [3.2.1]-bicyclic core, 29.

Regioselective C-2 and C-6 substitution of (S)-nicotine and nicotine derivatives

[reaction: see text] Regioselective deprotonations of (S)-nicotine and derivatives at the C-2 and C-6 positions of the pyridine ring were performed in good to excellent yields. These methodologies allow the direct introduction of a plethora of functional groups onto the pyridine ring of nicotine.

Synthesis and pharmacological evaluation of novel 9- and 10-substituted cytisine derivatives. Nicotinic ligands of enhanced subtype selectivity

We report the synthesis and pharmacological properties of several cytisine derivatives. Among them, two 10-substituted derivatives showed much higher selectivities for the alpha4beta2 nAChR subtype in binding assays than cytisine. The 9-vinyl derivative was found to have a very similar agonist activity profile to that of cytisine.

Synthesis of Nicotine Derivatives via Reductive Disilylation of (S)-Nicotine

[reaction: see text] A variety of novel nicotine derivatives were prepared via reductive disilylation of (S)-nicotine. Treatment of nicotine with lithium powder and chlorotrimethylsilane afforded 1,4-bis(trimethylsilyl)-1,4-dihydronicotine in high yield. Addition of various carbonyl electrophiles and a catalytic amount of TBAF provided either C-5 substituted nicotines or 1,4-dihydronicotine derivatives.

A general procedure for the enantioselective synthesis of the minor tobacco alkaloids nornicotine, anabasine, and anatabine

The minor tobacco alkaloids nornicotine, anabasine, and anatabine from Nicotiana tobacum are known to possess nicotinic receptor agonist activity, although they are relatively less potent than S-(-)-nicotine, the principal tobacco alkaloid. Previous pharmacological investigations and structure-activity studies have been limited owing to the lack of availability of the optically pure forms of these minor alkaloids. We now report a 2-step synthetic procedure for the enantioselective synthesis of the optical isomers of nornicotine and anabasine, and a modified procedure for the synthesis of anatabine enantiomers. These procedures involve initial formation of the chiral ketimine resulting from the condensation of either 1R, 2R, 5R-(+)- or 1S, 2S, 5S-(-)-2-hydroxy-3-pinanone with 3-(aminomethyl)pyridine followed by enantioselective C-alkylation with an appropriate halogenoalkane or halogenoalkene species, N-deprotection, and base-catalyzed intramolecular ring closure, to form the appropriate, chirally pure minor tobacco alkaloid. Using this approach, the R-(+)- and S-(-)-enantiomers of the above minor tobacco alkaloids were obtained in good overall chemical yield and excellent enantomeric excess.

Synthesis of C-4 Substituted Nicotine Derivatives via an N-Acylpyridinium Salt of (S)-Nicotine

[reaction: see text] A variety of novel nicotine derivatives were prepared from (S)-nicotine via a two-step sequence. Addition of a cuprate reagent to an N-acylpyridinium salt of nicotine, followed by aromatization with elemental sulfur, afforded C-4 substituted nicotines in moderate to high yield. Using this method, 4-(dimethylphenylsilyl)nicotine was prepared and oxidized to afford (S)-4-hydroxynicotine.

6-(2-Phenylethyl)nicotine: A novel nicotinic cholinergic receptor ligand

6-(2-Phenylethyl)nicotine (1b; K(i)=15 nM) was unexpectedly found to bind at alpha4beta2 nicotinic cholinergic (nACh) receptors. Although this compound failed to produce nicotine-like agonist action in several functional assays, 1b antagonized the antinociceptive effects of nicotine (mouse tail-flick assay) in a dose-dependent fashion when administered via an intrathecal route.

Development of a novel prolonged-release nicotine transdermal patch

A transdermal patch for delivering nicotine for periods of 12-48h was designed. An inclusion complex formed between the nicotine and beta-cyclodextrine (beta-CD) was used in drug depot. The usefulness of a specially cross-linked polyvinyl alcohol (cross-PVA) membrane was investigated as a rate controlling membrane. The influence of carbopol polymers, type C-934P and C-940 and propylene glycol on transdermal permeation of nicotine through the rat skin was investigated. The results indicated a maximum flux of 42 microgcm(-2)h(-1) after 48 h from the patches made from C-934P when the propylene glycol concentration was 15% and the nicotine-beta-CD mole ratio in the inclusion complex was 3:1. These nicotine transdermal patches can be fabricated to obtain a controlled release, zero order systems.

Coaxing a pyridine nucleus to give up its aromaticity: synthesis and pharmacological characterization of novel conformationally restricted analogues of nicotine and anabasine

A series of novel nicotine and anabasine related conformationally restricted compounds including those with pi-bonds in the connecting tether were synthesized following the hitherto unprecedented phenylsulfanyl group assisted generation of pyridine o-quinodimethane intermediates and their trapping by an intramolecular Diels-Alder reaction. Pharmacological characterization of some of these analogues at activating alpha3beta4 nAChRs was investigated, and constrained anabasine analogues 35 and 43 as well as constrained nicotine analogue 42 were found to exhibit moderately potent nicotinic agonist activity. Of special note is the fact that the pyrrolidinic nitrogen in these compounds is bound to a carbomethoxy group and, therefore, is not free to be protonated unlike all the known analogues of nicotine and anabasine, specifically designed as nAChRs agonists/antagonists. The structure-activity relationship studies indicate that when pi-cation interaction is absent, the position of chlorine atom in the pyridine ring and steric bulk at the connecting tether between the pyridine and pyrrolidine ring of the constrained nicotinic ligands are important descriptors for their binding affinity at alpha4beta2 and alpha3beta4 nAChRs as well as the subtype selectivity issue. These findings are likely to improve our understanding of the structural requirements for selectivity, which, at present, is probably the most important goal in the field of nicotinic ligands.

Synthesis of pyrido[2,1-c][1,2,4]triazine,1,2,4 triazolo[4,3-a] pyridine and 2-(substituted-pyrazolyl)nicotinonitrile and their effect on Biomphalaria alexandrina snail enzymes

The reaction of 2-hydrazino-4-(4-methoxyphenyl)-6-pheny-nicotinonitrile 3 with halo compounds yielded 4a-c,5,6. Heating 3 with carbon disulphide gave 7-(4-methoxyphenyl)-5-phenyl-3-thioxo-2,3-dihydro [1,2,4-] triazolo [4,3-a] pyridine-8-carboxylic amide 7. The behaviour of 3 towards some alpha,beta-unsaturated nitriles ,ethoxymethylene and ketene dithioacetal derivatives has been investigated, affording 9a-c,11a-c,13a-c,16a,b respectively. The activity of compounds 4a,5,6 and 7 have been investigated as molluscicidal.

Studies on intramolecular Diels-Alder reactions of furo[3,4-c]pyridines in the synthesis of conformationally restricted analogues of nicotine and anabasine

En route to conformationally restricted analogues of nicotine and anabasine, a novel synthetic route to bridged anabasines is described that hinges on a domino intramolecular [4 + 2]-cycloaddition/ring opening-elimination sequence of 3-amino-substituted furo[3,4-c]pyridines. Extension of this route to bridged nicotines, however, proved abortive, even when the dienophile tether is activated by a p-tolylsulfonyl group or when the diene moiety is activated by an electron-releasing methoxy substituent. A detailed density functional theoretical study (B3LYP/6-31+G) was undertaken to provide insight into the factors that facilitate an intramolecular Diels-Alder reaction in the former case.

Binding of nicotine and homoazanicotine analogues at neuronal nicotinic acetylcholinergic (nACh) receptors

A total of 20 substituted analogues of nicotine (1a) and homoazanicotine (3a) were examined in order to determine whether or not they might bind in a similar manner at alpha4beta2 nicotinic acetylcholinergic (nACh) receptors. It was found that parallel structural changes in the two series resulted in parallel shifts in affinity. Evidence suggests that the two series are binding in a comparable fashion.

A facile synthesis of cis-1-methyl-1,2,3,3a,4,8b- hexahydropyrrolo[3,2-f]pyrindine, an annulated nicotine analog

[reaction: see text] The title compound, 2, has been synthesized in 45% overall yield in six steps from 3-bromopyridine. The hexahydropyrrolo[3,2-f]pyrindine skeleton was constructed from key intermediate 5, via intramolecular azomethine ylide-alkene [3 + 2] cycloaddition. The present work constitutes a general method for rapid assembly of other related tricyclic nicotine analogues.

Homoazanicotine: a structure-affinity study for nicotinic acetylcholine (nACh) receptor binding

We have recently identified 3-[(1-methyl)-4,5-dihydro-1H-imidazol-2-yl)methyl]pyridine (homoazanicotine, 8) as a novel nicotinic acetylcholinergic (nACh) receptor ligand. In the present investigation, after we determined that 8 binds selectively at nicotinic (K(i) = 7.8 nM) vs muscarinic (K(i) > 10,000 nM) acetylcholinergic receptors, we examined its structure-affinity relationships for nACh receptor binding. The features investigated included the influence of (i) the composition of connector that separates the two rings, (ii) the N-methyl group, (iii) the ring opening of the imidazoline ring, (iv) the pyridine nitrogen atom, and (v) the aromatization of the imidazoline ring on nACh receptor affinity. As with nicotine, the parent structure seems optimal and most structural changes reduce nACh receptor affinity. Also, as with nicotine analogues, alteration of the spacer group influences affinity in a manner that is somewhat different than that seen with the parent structure.

(−)6-n-Propylnicotine antagonizes the antinociceptive effects of (−)Nicotine

Several 6-alkyl analogues of nicotine were examined in radioligand binding and in vivo functional assays. Although (-)6-ethylnicotine (3) binds with high affinity at nACh receptors (Ki=5.6 nM) and produces nicotine-like actions, its n-propyl homologue (-)4 (Ki=22 nM) failed to produce such effects. In fact, (-)4 antagonized the antinociceptive effects of (-)nicotine in the tail-flick assay in mice, but not the spontaneous activity or discriminative stimulus effects of (-)nicotine. Compound (-)4 appears to selectively antagonize only one of the three effects examined and is an interesting cholinergic agent for subsequent investigation.

Conformationally constrained nicotines: polycyclic, bridged, and spiro-annulated analogues as novel ligands for the nicotinic acetylcholine receptor

A set of novel nicotine-related, conformationally constrained compounds, including tetracyclic, bridged (4), and tricyclic, spiro-annulated (5) structures, were synthesized in a straightforward manner and optically resolved in a convenient fashion with (+)- and (-)-O,O'-di-p-toluoyltartaric acids. Absolute configurations were determined by X-ray crystallography. These compounds were evaluated for their ability to displace [(3)H]cytisine in a rat forebrain preparation and compared to (-)-nicotine. Three substances emerged with high affinity in the low nanomolar range. Moreover, one of these compounds ((+)-5b) showed not only high binding affinity (K(i) = 4.79 nM) but also significant enantioselectivity over its antipode (K(i) = 148 nM), supporting the hypothesis that conformational restraint can lead to high-affinity ligands, which are stereochemically discriminated by the nicotinic acetylcholine receptor and may feature optimum locations of the active sites of the pharmacophore.

Synthesis of (+/-)-methyl-(1-aryl-4-pyridin-3-yl-but-3-enyl)-amines

trans-Metanicotine, a subtype (alpha4beta2)-selective ligand for neuronal nicotinic acetylcholine receptor, is under clinical phase for Alzheimer's disease. An efficient synthetic route for (+/-)-methyl-(1-aryl-4-pyridin-3-yl-but-3-enyl)-amines, derivatives of trans-metanicotine, was explored. Allylation reaction of aryl aldimines with allylmagnesium bromide in THF gave (+/-)-methyl-(1-aryl-but-3-enyl)-amines. Protection of the amines with the Boc group and following Heck reaction of the N-Boc amines with 3-bromopyridine gave (+/-)-methyl-(1-aryl-4-pyridin-3-yl-but-3-enyl)-carbamic acid tert-butyl esters. Deprotection of the N-Boc group in aqueous 1N-HCI solution gave the titled amines in good yields. Thus, trans-metanicotine analogues modified at the a-position of the methylamino group with aryl groups were obtained in 5 steps.

Enantiospecific synthesis of annulated nicotine analogues from D-glutamic acid. 7-Azabicyclo[2.2.1]heptano[2.3-c]pyridines

The conformationally restricted nicotinoid (1S,4S)-7-methyl-7-azabicyclo[2.2.1]heptano[2,3-c]pyridine dihydrochloride has been prepared enantiospecifically from D-glutamic acid. The method involved a lithium cis-2,6-dimethylpiperidide-mediated intramolecular anionic cyclization of (2S,5R)-N-(tert-butyloxycarbonyl)-5-[3-(4-N-chloropyridinyl]proline methyl ester in tandem with a standard decarboxylation sequence. Reductive amination afforded the desired N-methylated [2.2.1]bicyclonicotinoid. Cyclization of the corresponding iodopyridinylproline methyl ester, obtained via ultrasound-facilitated chloro-iodo exchange, was also effected.

Synthesis of two conformationally constrained analogues of the minor tobacco alkaloid anabasine

The anabasine analogues spiro[4-azaindan-1,2'-piperidine] (7) and spiro[6-azaindan-1,2'-piperidine] (8) have been prepared. A series of palladium-catalyzed reactions, where an intramolecular cyclization constituted a key reaction, were utilized for the preparation of the two target compounds.

A concise synthetic pathway for trans-metanicotine analogues

A convenient pathway for synthesis of trans-metanicotine analogues was developed. trans-Metanicotine, a subtype (alpha4beta2)-selective ligand for neuronal nicotinic acetylcholine receptor, is under clinical phase for Alzheimer's disease. Zn-mediated allylation of allyl bromide and acetaldehyde followed by Heck reaction with 3-bromopyridine gave 5-pyridin-3-yl-pent-4-en-3-ol (2). Tosylation of 5-pyridin-3-yl-pent-4-en-3-ol followed by substitution reaction with methylamine in sealed tube gave methyl-(1-methyl-4-pyridin-3-yl-but-3-enyl)-amine (4) in good yields. Thus, trans-metanicotine analogues modified at the alpha-position of the methylamino group with various functional groups can be obtained in 4 steps.

A novel enantioselective synthesis of (S)-(-)- and (R)-(+)-nornicotine via alkylation of a chiral 2-hydroxy-3-pinanone ketimine template

An asymmetric synthesis of the optically pure isomers of the minor tobacco alkaloid and CNS nicotine metabolite, nornicotine, has been achieved with moderately high optical purity. The synthetic pathway involves alkylation of a chiral ketimine, prepared from either 1R,2R,5R-(+)- or 1S,2S,5S-(-)-2-hydroxy-3-pinanone and 3-(aminomethyl)pyridine with 3-bromopropan-1-ol. After cleavage of the respective C-alkylated ketimines with NH2OH.HCl, and treatment of the resulting amino alcohols with HBr, followed by base-catalyzed intramolecular ring closure, (S)-(-)-nornicotine and (R)-(+)-nornicotine were obtained with ee values of 91% and 81%, respectively.

Conformationally restricted analogues of nicotine and anabasine

A series of conformationally restricted analogues of nicotine has been synthesized and evaluated as agonists of neuronal acetylcholine receptors. Compound 2 (SIB-1663), which selectively activated human recombinant alpha 2 beta 4 and alpha 4 beta 4 nAChRs, was shown to be active in animal models of Parkinson's disease and pain.

Synthesis and characterization of radioiodinated (S)-5-iodonicotine: a new ligand for potential imaging of brain nicotinic cholinergic receptors by single photon emission computed tomography

(S)-5-Iodonicotine (4a), an (S)-nicotine analog iodinated at the 5-position of the pyridine ring, was synthesized and evaluated as a potential radiopharmaceutical for investigating brain nicotine receptors by single photon emission computerized tomography (SPECT). [125I]-(S)-Iodonicotine ([125I]-4a) was synthesized by the iododestannylation reaction under no-carrier-added conditions and purified by high-performance liquid chromatography (HPLC). The binding affinity of 4a for brain nicotine receptors was measured in terms of displacement of [3H]cytisine from binding sites in rat cortical membranes. The binding data revealed that the affinity of 4a was the same as that of (S)-nicotine and 80-fold higher than that of the (R)-enantiomer (4b). Biodistribution studies in mice disclosed that the brain uptake of [125I]-4a was rapid and profound. Regional cerebral distribution studies in rats by autoradiography disclosed that the accumulation of [125I]-4a was dense in the thalamus, intermediate in the cortex and striatum, and less marked in the cerebellum. Furthermore, the administration of (S)-nicotine reduced the uptake of [125I]-4a in the thalamus and resulted in a nearly identical level of radioactivity in the cerebellum. [125I]-(R)-5-Iodonicotine ([125I]-4b) showed more rapid washout from the brain and a less extensive regional cerebral distribution than the (S)-enantiomer ([125I]-4a). Thus, 4a bound to brain nicotine receptor in vivo, and therefore iodine-123-labeled 4a may be a potential radioligand for use in vivo cerebral nicotinic receptor studies by SPECT.

Synthesis and evaluation of nicotine analogs as neuronal nicotinic acetylcholine receptor ligands

A series of 3'-, 4'-, and 5'-substituted nicotine analogs have been synthesized and evaluated as ligands of the neuronal nicotinic acetylcholine receptor. The compounds prepared were found to have binding affinities ranging from 4 to 3500 nM. The results indicate that only a small substituent or functionality is well tolerated at the C4' position of nicotine and that binding affinity is affected by both steric and electronic properties of the substituent. On the other hand, the C3' and C5' positions seem to be the more sensitive toward bulky substituents. The best compound, 4'-methylnicotine, is nearly equipotent to nicotine. It possesses the most favorable binding affinity.

5-Isothiocyanonicotine: a high-affinity irreversible ligand for brain nicotinic receptors

A newly synthesized affinity ligand, (R,S)-5-isothiocyanonicotine (ISCN-N) was found to inhibit irreversibly the binding of [3H]methylcarbamylcholine (a specific nicotinic receptor ligand) to brain membranes. Plots of percent inhibition versus ligand concentration yielded an IC50 of 7 x 10(-8) M for SCN-N and Ki values of 6 x 10(-9) and 2 x 10(-9) M for (R,S)-5-aminonicotine and (S)-nicotine, respectively. The IC50 value for irreversible inhibition of [3H]methylcarbamylcholine by SCN-N was 2 x 10(-7) M. The affinity ligand irreversibly inhibited brain nicotinic receptors in vivo in a dose-dependent manner, the inhibition being 49% at a dose of 20 mumol/kg. Behavioral studies in mice revealed that SCN-N had less than one-fifth the potency of nicotine in producing muscle weakness and seizures, whereas 5-aminonicotine was without significant behavioral effects at doses up to 20 mumol/kg.

Identification of radiolabeled metabolites of nicotine in rat bile. Synthesis of S-(-)-nicotine N-glucuronide and direct separation of nicotine-derived conjugates using high-performance liquid chromatography

Four metabolites of nicotine, including two glucuronides, have been separated by high-performance liquid chromatography. This separation was applied to identification of biliary metabolites of radiolabeled nicotine by radiometric detection. S-(-)-Nicotine N-glucuronide was synthesized and used as a standard in method development.

Synthesis, optical resolution, absolute configuration, and preliminary pharmacology of (+)- and (-)-cis-2,3,3a,4,5,9b-hexahydro-1-methyl-1H- pyrrolo-[3,2-h]isoquinoline, a structural analog of nicotine

Title compound, 8, has been synthesized from isoquinolinone, 1 (an improved preparation for which is presented) and separated into its antipodes with D- and L-di-p-toluoyltartaric acids. These antipodes and the racemic precursor have been evaluated (and found active) in two in vivo systems for their effects. The most potent of the three, (+)-8, has an ED50 of 7.13 mumol/kg for inhibition of spontaneous activity and 7.45 mumol/kg for antinociception compared to 4.44 and 4.81 mumol/kg, respectively, for (S)-(-)-nicotine. Compounds (-)-8 and 7 are about one-fourth as potent. Isomer (+)-8 has the 3aR,9bS configuration, the latter corresponding to (S)-(-)-nicotine as determined by X-ray crystallography. However, (+)-8 failed to compete for [3H]-nicotine binding, and its pharmacological effects were not blocked by mecamylamine. These bridged nicotine analogs either are binding to an as-yet-unidentified nicotinic receptor or they represent a novel class of non-nicotinic analgesics.

Blood-brain barrier penetration abolished by N-methyl quaternization of nicotine

The present study determined the effect of organically quaternizing either of the two tertiary nitrogen sites of nicotine to assess the in vivo effects of the permanently ionized states of the synthesized N-[14C]methylnicotines on brain uptake in rat after intracarotid injection. Male Sprague-Dawley rats were used to measure the brain uptake index (BUI) by single-pass clearance in brain after rapid injection at pH 7.4 into the left common carotid artery (expressed as a percentage) relative to simultaneously injected 3HOH. The BUI of [14C]mannitol, a control for the method background, was measured to be 2.6 +/- 0.6. At physiological pH, in striking contrast to the [pyrrolidine-2-14C]nicotine BUI of 120 +/- 3, the N-[14C]-methylnicotines had a BUI of 3.0 +/- 0.6, which was not significantly different from the method background and which indicated abolition of blood-brain barrier penetration of nicotine with the sensitivity of the BUI method.

Synthesis of (S)-N-[methyl-11C]nicotine and its regional distribution in the mouse brain: a potential tracer for visualization of brain nicotinic receptors by positron emission tomography

A nicotine agonist, 11C-labeled (S)-nicotine, was synthesized by N-methylation of (S)-nornicotine with [11C]-methyl iodide in dimethylformamide-dimethylsulfoxide in order to study nicotinic receptors in the human brain by positron emission tomography. The radiochemical yield of this N-methylation reaction was more than 90% within 5 min. After purification by high performance liquid chromatography the radiochemical purity of the product was more than 99% and the specific radioactivity was 7.4-11.1 GBq/mumol. The regional distribution of (S)-[11C]nicotine in the mouse brain after intravenous injection was compared with that of (R)-[11C]nicotine. After injection of (S)-[11C]nicotine, the regional uptake of radioactivity was in the following order: cortex greater than thalamu approximately hippocampus greater than striatum greater than hypothalamus greater than cerebellum. Moreover, (S)-[11C]nicotine was displaced from the brain by unlabeled (S)-nicotine, but unlabeled (R)-nicotine caused no change in uptake. In contrast, (R)-[11C]nicotine showed a lower brain uptake and lesser regional differences in radioactivity.

Biotransformation of primary nicotine metabolites: metabolism of R-(+)-[3H-N'-CH3; 14C-N-CH3] N-methylnicotinium acetate--the use of double isotope studies to determine the in-vivo stability of the N-methyl groups of N-methylnicotinium ion

The in-vivo metabolism of R-(+)-[3H-N'-CH3; 14C-N-CH3]-N-methylnicotinium acetate (NMN) was studied in the guinea-pig to determine the in-vivo stability of the N-methyl and N'-methyl groups of this primary nicotine metabolite. The results showed that N-demethylation does not occur. However, losses of 34 and 36%, respectively, of the 3H label in the N'-CH3 group of urinary NMN and the secondary metabolite, N-methyl-N'-oxonicotinium ion (NMNO), were observed. These results suggest that biotransformation of NMN may involve either an initial N'-demethylation step to N-methylnornicotinium ion (NMNor) followed by N'-methylation back to NMN, or the formation of an N'-methylene iminium species, which may be reductively converted back to NMN.

Isolation and characterization of N-methyl-N'-oxonicotinium ion, a new urinary metabolite of R-(+)-nicotine in the guinea pig

Biotransformation of both R-(+)-nicotine and R-(+)-N-methyl-nicotinium acetate in male Hartley guinea pigs affords a new quaternary amine metabolite, which was isolated and purified from urine by preparative HPLC. The structural analysis of the metabolite was carried out using UV spectrophotometry, direct thermospray mass spectrometry, and Fourier transform 1H-NMR spectroscopy. The structure of the metabolite was confirmed by synthesis and shown to be a mixture of the cis-1'S,2'R-, and trans-1'R,2'R-diastereomers of N-methyl-N'-oxonicotinium ion, formed in the ratio 1.6:1.0, respectively. S-(-)-Nicotine under similar conditions does not undergo this biotransformation.

Synthesis of 4,4-ditritio-(+)-nicotine: comparative binding and distribution studies with natural enantiomer

The preparation of 4,4-ditritio-(+)-nicotine (Vb) (specific activity 10.3 Ci/mmole)from (+)-nicotine (Ib) via (-) 4,4-dibromocotinine (IIIb) is described. Although Ib is 10-30 times less potent than (-)-nicotine (Ia) in the CNS, its binding affinity for the crude mitochondrial or nuclear fraction of whole rat brain is only three times less than that of Ia. However, distribution studies showed that the maximum brain levels of (-)-[3H] nicotine are nearly twice those of (+)-[3H]-nicotine following administration of a 2-micrograms/kg dose. Binding affinity and disposition of the stereoisomers account for a portion of the pharmacological stereospecificity of nicotine.

Metabolic beta-hydroxylation and N-oxidation of N'-nitrosonornicotine

3'-Hydroxy-N'-nitrosonornicotine (2), 4'-hydroxy-N'-nitrosonornicotine (3), N'-nitrosonornicotine 1-N-oxide (4) were synthesized and identified as metabolites in the F-344 rat of the tobacco-specific carcinogen N'-nitrosonornicotine (1). For the synthesis of 2, myosmine (5) was converted to 3'-bromomyosmine (6). Displacement by acetate and hydrolysis gave 3'-hydroxymyosmine (7), which was reduced and nitrosated to give 2. 4'-Hydroxymyosmine (13), the precursor to 3, was prepared by ammonolysis of 1,2-epoxy-4-(N-morpholino)-4-(3-pyridyl)-4-cyanobutane (10). N'-Nitrosonornicotine 1-N-oxide (4) was prepared by m-chloroperbenzoic acid oxidation of 1. When 1 was incubated with liver microsomes from Aroclor-pretreated F-344 rats, trace amounts of 2 and 3 were produced and 4 was a major metabolite. The urine from rats treated with N'-nitrosonornicotine-2'-14C contained only trace amounts of 2 and 3, whereas 4 accounted for 6.7-9.4% of the dose.

Synthesis and preliminary binding studies of 4,4-ditritio-(-)-nicotine of high specific activity

4,4-Ditritio-(-)-nicotine (5) of high specific activity (4.7 Ci/mmol) has been synthesized from (-)-nicotine via the readily prepared 4,4-dibromocotinine (3). Scatchard analysis of the binding of 5 to the crude mitochondrial fraction of whole rat brain revealed a Ka of 4.7 X 10(6) M-1 and 13 fmol of binding sites/mg of protein.

Origin in tobacco smoke of N'-nitrosonornicotine, a tobacco-specific carcinogen

To evaluate risk factors and to approach methods of reduction of the carcinogenic potential of cigarette smoke, the transfer rate of N-nitrosonornicotine in a popular U.S. blended cigarette into mainstream smoke was quantitatively determined. The mean transfer rate was 11.3%; thus approximately 46% of the tobacco-specific carcinogen in the smoke came from the tobacco, and the remainder was synthesized during smoking.

Nicotine-11C: synthesis and distribution kinetics in animals

In order to study "in vivo" the distribution kinetics of (-)-nicotine in animals, this molecule was labeled with carbon-11. Two synthesis methods are described. Immediately after intravenous administration of (-)-nicotine-methyl-11C in rabbits the gamma-camera shows a strong radioactivity build-up in the brain and kidneys. The biological interest of this carbon-11 labeled molecule is discussed.