Synthesis, Nicotinic Acetylcholine Receptor Binding, and Pharmacological Properties of 3’- (Substituted phenyl) Deschloroepibatidine Analogs

Abstract

A series of 3’-(substituted phenyl)deschloroepibatidine analogs (5a–j) were synthesized. The α4β2* and α7 nicotinic acetylcholine receptor (nAChR) binding properties and functional activity in the tail-flick, hot-plate, locomotor, and body temperature tests in mice of 5a–j were compared to those of the nAChR agonist, nicotine (1), epibatidine (4), and deschloroepibatidine (13) the partial agonist, varenicline (3) and the antagonist 2’-fluoro-3’-(substituted phenyl)deschloroepibatidine analogs (7a–j). Unlike epibatidine and deschloroepibatidine, which are potent agonists in the tail-flick test, 5a–k show no or very low antinociceptive activity in the tail-flick or hot-plate test. However, they are potent antagonists in nicotine-induced antinociception in the tail-flick test, but weaker than the corresponding 2’-fluoro-3’-(substituted phenyl)deschloroepibatidines.

1. Introduction

The use of tobacco products is believed to be in large part due to addiction to nicotine (1), which is one of the most abused reinforcing agents. It is estimated that there are four million smoking-related deaths annually from diseases such lung cancer, chronic obstructive pulmonary disease (COPD) and cardiovascular disease.¹ Consequently, there is great interest in the development of pharmacotherapies for aiding people to stop smoking.² Present FDA approved drugs for treating smoking cessation include nicotine (1) replacement medication in the form of gum, patch, lozenge, sublingual tablet, nasal spray, and vapor inhaler, the antidepressant bupropion (2) and the α4β2* nAChR partial agonist varenicline (3).^2-4

During the last few years, we have conducted structure activity studies (SAR) using the potent nAChR agonist epibatidine (4) as a lead structure to identify pharmacophores for the nAChR. The studies have identified nAChR agonists more potent than epibatidine as well as analogs that showed pure antagonist or partial agonist activity.^5-12 These studies showed that introduction of a substituted phenyl group at the 3’-position on the pyridine ring of epibatidine exerted a profound influence on both receptor binding (recognition) and receptor activation. Interestingly, substitution of different groups of the 2’-position distinguished between agonist and antagonist properties. In this study, we report the comparison of the nAChR binding, and pharmacological properties of the 3’-(substituted phenyl)deschloroepibatidine analogs (5a–j) to those of the nAChR agonist nicotine (1), epibatidine (4), and deschloroepibatidine (6), the partial agonist varenicline (3), and the corresponding 2’-fluoro-3’-(substituted phenyl)deschloroepibatidine analogs (7a–j). The analogs 5a–j showed α4β2* nAChR binding affinity as well as nAChR agonist and antagonist activity in mouse antinociceptive, hypothermia, and spontaneous activity test more like the partial agonist varenicline (3) than the nAChR agonist nicotine (1), epibatidine (4), and deschoroepibatdine (6a). Like the 2’-fluoro-3’-(substituted phenyl)deschloroepibatidine analogs 7a–j, most of the 3-(substituted phenyl)deschloroepibatidine analogs 5a-j were antagonist of nicotine-induced antinociception in the tail-flick test.

2. Chemistry

The synthesis of 5a–h is shown in Scheme 1. Palladium acetate catalyzed coupling of tert-butoxycarbonyl-3’-bromodeschloroepibatidine (8)¹⁰ with the appropriately substituted phenylboronic acid in dimethoxyethane (DME) in the presence of tri-(o-tolyl)phosphine and sodium carbonate gave the tert-butoxycarbonyl-protected 3’-(substituted phenyl)deschloroepibatdine analogs (9a-g). Reduction of the 4-nitrophenyl intermediate 9g with iron powder in hydrochloric acid gave the 4-aminophenyl compound 10. Treatment of 9a–g and 10 with trifluoroacetic acid in methylene chloride removed the protecting tert-butoxycarbonyl group and afforded the desired 3-(substituted phenyl)deschloroepibatidine analogs 5a-h.

Scheme 1.

Reagents: (a) Pd(OAc)₂, P(o-tolyl)₃, Na₂CO₃, (X,Y)C₆H₄B(OH)₂ DME; (b)CF₃CO₂H; (c)Fe, HCl(H₂O).

The 3’-(3-aminophenyl)- and 3’-(3-methoxyphenyl)deschloroepibatidine analogs 5i and 5j, respectively, were synthesized as outline in Scheme 2. Palladium acetate catalyzed addition of 3-nitrophenylboronic acid or 3-methoxyphenylboronic acid to 7-tert-butoxycarbonyl-2-exo-2-(2-amino-3-bromo-8-pyridinyl)-7-azabicyclo[2.2.1]heptane (11)¹⁰ provided the tert-butoxycarbonyl-protected 3’-(3-nitrophenyl)- and 3’-(3-methoxyphenyl)deschloroepibatidine 12a and 12b, respectively. Diazotization of 12a and 12b using sodium nitrite in hydrochloric acid yielded 3’-(3-methoxyphenyl)epibatidine 13a and 13b, respectively. Catalytic hydrogenation of 13a and 13b using 10% palladium on carbon catalyst in methanol yielded the desired 5i and 5j, respectively.

Scheme 2.

Reagents: (a) Pd(OAc)₂, P(o-tolyl)₃, Na₂CO₃, DME, H₂O, 3-NO₂C₆H₄B(OH)₂ or CH₃OC₆H₄B(OH)₂; (b) NaNO₂, HCl;(c)10% Pd/C, CH₃OH.

3. Biology

The K_i values for the inhibition of [³H]epibatidine binding at the 〈4®2* nAChR in male rat cerebral cortex for compound 5a–j are compared to values for nicotine (1), epibatidine (4), varenicline (3) and 2’-fluoro-3’-(substituted phenyl)deschloroepibatidine analogs (7a–j) (Table 1). The binding assays were conducted and the K_i values calculated as previously described.⁸ Compounds (10 μM) were also evaluated for inhibition of binding to 〈7 nAChR using [¹²⁵I) iodoMLA as previously reported.⁸

Table 1.

Comparison of epibatidine and varenicline radioligand binding and antinociception data of 3’-(substituted phenyl)deschloroepibatidine analogs to standard nAChR agonist, partial agonist, and antagonist

Compd a	X	Y	α4β2* [3H]Epibatidine (Ki, nM)b	α7 [125I]iodo MLA (Ki, nM)b	ED 50 mg/kg Tail-Flickc	ED 50 mg/kg Hot-Platec	ED 50 mg/kg Hypothermiac	ED 50 mg/kg Spontaneous Activity c	AD 50 (μg/kg)c
									Tail-Flick	Hot-Plate	Body Temperature
Epibatidine (4)			0.026 ± 0.002	198 ± 4	0.006	0.004	0.004	0.001
6ad			0.020 ± 0.001		0.002
6bd			0.027 ± 0.001
Nicotine (1)			1.5 ± 0.3	670 ± 33	1.3	0.65	1.0	0.5
Varenicline (3)			0.12 ± 0.02	32.5 ± 1.3	11% @ 10	10% @ 10	2.8	2.1	0.2	470	0% @ 10,000
5a	H	H	0.50 ± 0.03	7% @ 50 nM	6% @ 10	30% @ 10	1.5 (1.2–3.3)	0.5 (0.13–2)	300 (100–3000)	10% @ 1000	0% @ 1000
5b	Cl	H	0.21 ± 0.03	>2000	1% @ 10	11% @ 10	2% @ 10	10.5 (8.2–15.5)	0% @ 10	0% @ 10	5% @ 1000
5c	H	Cl	0.17 ± 0.021	>2000	1% @ 20	10% @ 20	0% @ 20	3.8 (0.36–41)	0.01 (0.001–0.1)	0% @ 1000	0% @ 1000
5d	F	H	0.15 ± 0.003	>2000	3% @ 10	13% @ 10	-1.6% @ 10	3.0 (2.2–3.9)	13 (2–70)	10% @ 100	0% @ 1000
5e	H	F	0.20 ± 0.04	>2000	2% @ 10	8% @ 10	0% @ 10	5 (3.2–7.9)	3.1 (0.07–1)	3000 (2500–3600)	0% @ 10
5f	NO2	H	0.34 ± 0.005	>2000	1% @ 10	26% @ 10	2.9 (2.1–3.5)	0.66 (0.1–3.9)	47(0.7–33)	10% @ 100	0% @ 100
5g	H	NO2	0.17 ± 0.009	>2000	5% @ 20	21% @ 20	37.6 (30–45)	4.7 (0.25–8.5)	2.5 (0.4–18)	11,000 (900– 13,300)	0% @ 2000
5h	NH2	H	0.34 ± 0.05	>2000	6% @ 10	10% @ 10	0% @ 10	15% @ 10	1 (0.1–4)	45% @ 2000	0% @ 20
5i	H	NH2	1.16 ± 0.14	>2000	6.3 (3.8–10.3)	5.4 (3.2–9.1)	3.8 (2.8–4.8)	1.5 (0.5–4.5)	NT	NT
5j	H	CH3O	0.43 ± 0.05	>2000	1% @ 10	20% @ 10	0% @ 10	2.9 (0.8–10)	0% @ 10	0% @ 10	0% @ 10
7ae	H	H	0.24	>2000	3% @ 15	4% @ 15	-0.5 °C @ 10	4.7	500	1200
7be	Cl	H	0.044		2% @ 10	7% @ 10	0% @ 10	5% @ 10	0.3	260	0% @ 1000
7ce	F	H	0.073		3% @ 10	14% @ 10	0% @ 10	15% @ 10	12	450	0% @ 1000
7de	H	F	0.029		2% @ 10	15% @ 10	10% @ 10	2	0.5	230	0% @ 100
7ee	H	F	0.087		3.5	3.3	1.8	0.36	5	2% @ 1000	0% @ 1000
7fe	NO2	H	0.009		5% @ 10	10% @ 10	3.5	0.22	3	120	0% @ 1000
7ge	H	NO2	0.053		3% @ 10	20% @ 10	0% @ 10	6.5	0.5	130	5% @ 500
7he	NH2	H	0.095		2% @ 10	10% @ 10	0% @ 10	6	5	1800	0% @ 5000
7ie	H	NH2	0.16		1% @ 10	4% @ 10	0% @ 10	8.5	9	820	5% @ 5000
7je	H	CH3O	0.12		0% @ 10	8% @ 10	0% @ 10	6	8	2000	0% @ 10

^a)All epibatidine analogs with the exception of 5b and 5d were tested as hydrochloride salts and all were racemates.

^b)Data represent means ± SE from at least three independent experimentals.

^c)Results are provided as ED₅₀ or AD₅₀ values (± confidence limits) or as a percent effect at the individual dose.

^d)Taken from reference ⁹.

^e)Data take from reference ¹¹.

The above compounds were evaluated in two acute pain models, the tail-flick and the hot-plate tests, and the results are listed in the Table 1.¹³ In the tail-flick method of D’Amour and Smith,¹⁴ the tail is exposed to a heat lamp and the amount of time taken for the animal to move (flick) its tail away from the heat is recorded. A control response (2–4 s) was determined for each mouse before treatment, and a test latency was determined after drug administration. The method used for the hot-plate test is a modification of those described by Eddy and Leimbach¹⁵ and Atwell and Jacobson.¹⁶ Mice were placed into a 10-cm wide glass cylinder on a hot plate (Thermojust Apparatus) maintained at 55.0 °C. Two control latencies at least 10 min apart were determined for each mouse. The normal latency (reaction time) was 8–12 s. The reaction time was scored when the animal jumped or licked its paws. The mice were tested 5 min after sc injections of nicotinic ligands for the dose-response determination. Antinociceptive response was calculated as percentage of maximum possible effect (% MPE, where % MPE = [(test–control)/(maximum latency–control) × 100]).

To measure the effect of analogs on spontaneous activity, mice were placed into individual Omnitech photocell activity cages (28 cm × 16.5 cm) 5 min after sc administration of either 0.9% saline or the epibatidine analog. Interruptions of the photocell beams (two banks of eight cells each) were then recorded for the next 10 min. Data were expressed as number of photocell interruptions. Rectal temperature was measured by a thermistor probe (inserted 24 mm) and digital thermometer (Yellow Springs Instrument Co., Yellow Springs, OH). Readings were taken just before and at different times after the sc injection of either saline or epibatidine analogs. The difference in rectal temperature before and after treatment was calculated for each mouse. The ambient temperature of the laboratory varied from 21 to 24 °C from day to day.

For the antagonist experiments, mice were pretreated sc with either saline or epibatidine analogs 10 min before nicotine. Nicotine was administered at a dose of 2.5 mg/kg, sc (an ED₈₄ dose), and mice were tested 5 min later. ED₅₀ and AD₅₀ values with 95% confidence limits were determined.

4. Results and Discussion

The desired target compounds 5a–j could be synthesized by a procedure similar to that used to prepare the previously reported 2’-fluoro-3’-(substituted phenyl)deschloroepibatidine analogs (7a–k).¹¹ The key step in both syntheses is palladium acetate catalyzed Suzuki^17,18 cross coupling of a 3-bromopyridine starting material 8 and 11 with the appropriately substituted phenylboronic acids.

Binding affinities for the 3’-(3- and 4-substituted phenyl)deschloroepibatidine analogs 5a–j at α4β2* and α7 nAChRs along with data for nicotine (1), epibatidine (4), deschloroepibatidine (6a), 2’-fluorodeschloroepibatidine (6b), varenicline (3), and the corresponding 2’-fluoro-3’-(substituted phenyl)deschloroepibatidine analogs 7a–j are listed in Table 1. 3’-Phenyldeschloroepibatidine has a K_i = 0.5 nM for the α4β2* nAChR. Substitution of the 3’-phenyl group with a 3’- or 4’-position electron withdrawing or releasing substituent had only small effects on binding affinity. The K_is varied between 0.15 and 0.43 nM for 5b–i. The 3’-(4-fluorophenyl) and 3’-(3-chlorophenyl) analogs 5d and 5c with K_is = 0.15 and 0.17 nM,respectively, possessed the highest affinity and are almost identical to the K_i for varenicline (k_i = 0.12 nM), but are much lower than the K_is of 0.026 and 0.020 nM for epibatidine (4) and deschloroepibatidine (6). All compounds have >2000 nM affinity for the α7 nAChR compared to a K_i = 32.5 nM for varenicline. In every case, the K_i value of the deschloroepibatidine analogs 5a–j were larger (weaker affinity) than the K_i value for the corresponding 2’-fluoro-(substituted phenyl)-deschloroepibatidine analogs 7a–j. Since deschloroepibatidine (6a) and 2’-fluorodeschloroepibatidine (6b) have almost identical K_i values (Table 1) for α4β2* AChR binding, the difference between the K_i values of 5a–j and 7a–j are apparently due to different types of effects of the 3-(substituted phenyl) groups in each series.

The 3’-(substituted phenyl)deschloroepibatidine analogs 5a–j were also evaluated for their in vivo nAChR properties in mice and compared to similar properties of varenicline and the 2-fluoro-(substituted phenyl)-deschloroepibatidine analogs 7a–j (Table 1). With the exception of the 3’-(3-aminophenyl) analog 5i, which has an ED₅₀ = 6.3 mg/kg, no compound possessed antinociceptive activity in the tail-flick or hot-plate test. This contrasts sharply with descloroepibatidine (6), which has an ED₅₀ = 0.002 mg/kg in this test, but is very similar to the results obtained with the 2-fluoro-(substituted phenyl)-deschloroepibatidine analogs 7a–j. Compounds 5a, 5f, and 5i showed weak activity in the hypothermia test (ED₅₀ = 0.5 to 3.8 mg/kg) similar to that of varenicline (ED₅₀ = 2.8 mg/kg). All compounds with the exception of 5b and 5h showed activity in the spontaneous activity test similar to varenicline; ED₅₀ = 0.5 to 5 mg/kg compared to 2.1 mg/kg for varenicline. Even though there was not a one-to-one correlation between the analogs 5a–j and 7a–j, most of the 2-fluoro-(substituted phenyl)-deschloroepibatidine analogs (7a–j) showed similar potency in this test.

All analogs with the exception of the 3’-(3-methoxyphenyl) analog 5j were potent antagonists in the tail-flick test. The 3-substituted phenyl analogs 5c, 5e, and 5g were more potent antagonists than the 4-substituted analogs 5c, 5d, and 5f. The 3’-(3-chlorophenyl) analog 5c with and AD₅₀ = 0.01 μg/kg was the most potent analog. The 3’-(3-fluorophenyl) and 3’-(3-nitrophenyl) analogs 5e and 5g with AD₅₀s = 3000 and 11,000 μg/kg, respectively, were weak antagonists in the hot-plate test; the other compounds were inactive in this test. No compound antagonized nicotine-induced hypothermia.

For comparison, varenicline¹⁹ blocked nicotine-induced antinociception in the tail-flick and hot-plate tests with ED₅₀s of 0.2 and 470 μg/kg, respectively. Interestingly, compound 5c was almost 20 times more potent in blocking nicotine’s effects on the tail-flick test than varenicline, suggesting that 5c is a more potent antagonist or partial agonist. However, our in vivo models cannot discriminate between these two properties. Interestingly, compared to varenicline, the differential in vivo blockade potency in the different tests, suggest a better in vivo and possibly in vitro selectivity at nAChRs. While there was a lack of a good correlation, the 2-fluoro-(substituted phenyl)-deschloroepibatidine analogs 7a–j had potencies as antagonist in the tail-flick similar to those of 5a–j. In contrast, 7a–j tended to be more potent antagonist in the hot-plate test than 5a–j.

In summary, the addition of a 3’-phenyl or 3’-(substituted phenyl) group to deschloroepibatidine (6) provided compounds 5a–j, which showed binding affinity at the α4β2* nAChR similar to that of nicotine (1) and varenicline (3), but considerably less than that of deschloroepibatidine (6), epibatidine (4), and 2’-fluoro-3’-(substituted phenyl)deschloroepibatidine analogs 7a–j. In contrast to varenicline (3), the analogs 5a–k had no affinity for the α7 nAChR. Unlike deschloroepibatidine (6), which is a potent agonist in the tail-flick test, 5a–h and 5i show no antinociceptive activity in the tail-flick or hot-plate test. Similar to varenicline and 2’-fluoro-3’-(substituted phenyl)deschloroepibatidine analogs 7a–j are modest activators of locomotor activity and are potent antagonist of nicotine-induced antinociception in the tail-flick test. However, unlike varenicline (3) and the 2-fluoro-(substituted phenyl)-deschloroepibatidine analogs 7a–j, which had good potency as antagonist in the hot-plate test, the 3-phenyldeschloroepibatidine analogs 5a–j had weak or no activity in this test. Additionally, the 3-substituted phenyl analogs exhibited greater antagonistic activity than the corresponding 4-substituted phenyl analogs.

5. Experimental

Melting points were determined on a Mel-temp (Laboratory Devices, Inc.) capillary tube apparatus. NMR spectra were recorded on a Bruker Avance 300 or AMX 500 Spectrometer using tetramethylsilane as internal standard. Thin layer chromatography was carried out on Whatman silica gel 60 plates. Visualization was accomplished under UV or in an iodine chamber. Microanalysis was carried out by Atlantic Microlab, Inc. Flash chromatography was carried out using silica gel 60 (230–400 mesh) using various solvents combined with a solvent mixture of 80% chloroform, 18% methanol, and 2% concentration ammonium hydroxide (CMA80).

The [ ³H]epibatidine was purchased from Perkin Elmer Inc., (Boston, MA). The [¹²⁵]iodo-MLA was synthesized as previously reported.

7-tert-Butoxycarbonyl-2-exo-[3-phenyl-5-pyridinyl)]-7-azabicyclo[2.2.1]heptane (9a)

To a resealab1e reaction tube under nitrogen was added 8¹⁰ (300 mg, 0.85 mmol), Pd(OAc)₂ (16 mg, 0.071 mmol), P(o-tolyl)₃ (43 mg, 0.2 mmol), sodium carbonate (190 mg, 1.8 mmo1), phenylboronic acid (171 mg, 1.4 mmol), degassed water (1 mL), and DME (10 mL). The reaction mixture was heated at 80 °C overnight, cooled and added to a saturated sodium bicarbonate solution. The mixture was extracted with CH₂C1₂, 4 × 50 mL. The extracts were dried (Na₂SO₄)and concentrated. The resulting residue was purified by column chromatography on silica gel, eluting with hexane-ethyl acetate (3:1) to yield 0.28 g, (95%) of 9a as an oil. ¹H NMR (CDCl₃) δ (ppm): 1.42 (s, 9H), 1.53-1.66 (m, 2H), 1.87-1.93 (br s, 3H), 2.00-2.07 (m, 1H, CH₂), 2.93-2.98(m, 1H, CH), 4.28 (br s, 1H), 4.41 (br s, 1H), 7.36-7.48 (m, 3H), 7.57-7.59 (d, 2H), 7.87 (s, 1H), 8.48 (s, 1H), 8.70 (s, 1H).

3-Phenyldeschloroepibatidine (5a) Dihydrochloride

To a stirred solution of 9a (280 mg, 0.797 mmol) in methylene chloride (5 mL) at 0 °C was added trifluroacetic acid (5 mL). After stirring at 25 °C for 2 h, the reaction mixture was poured into 150 mL of a solution of concentrated NH₄OH-H₂O (1:1). The mixture was extracted with CH₂Cl₂ (4 × 50 mL), dried (Na₂SO₄) and concentrated. The residue was purified by silica gel column chromatography,eluting with CMA80-ethyl acetate (1:3) to yield 180 mg, (90%) of 5a .¹H NMR (CDCl₃) δ 1.51-1.76 (m, 5H), 1.92-2.00 (m, 1H, CH₂), 2.85-2.90 (m, 1H, CH), 3.65 (br s, 1H), 3.81 (br s, 1H),7.35-7.48 (m, 3H), 7.57-7.60 (d, 2H), 7.91 (s, 1H), 8.50 (s, 1H), 8.66 (s, 1H). ¹³C NMR (CDCl₃) δ (ppm) (I7-C): 30.464, 31.747, 40.692, 45.767, 56.873, 63.169, 127.625(2C), 128.297, 129.345(2C), 133.457, 136.703, 138.539, 142.274, 146.329, 148.302.

The free base was dissolved in CH₃OH and excess 2M ethereal HCl was added. The solvents were removed and the resulting solid was recrystallized from a CH₃OH and Et₂O mixture. The dihydrochloride salt had mp 170–175 °C, anal (C₁₇H₂₀Cl₂N₂•1.5H₂O) C, H, N.

7-tert-Butoxycarbonyl-2-exo-[3-(4-chlorophenyl)-5-pyridinyl)]-7-azabicyclo[2.2.1]heptane (9b)

Using a procedure similar to that described for 9a, compound 8 (400 mg, 1.13 mmol) was coupled to 292 mg (1.86 mmol) of 4-chlorophenylboronic acid to give 0.40 g (97%) of 9b as a white solid; mp 119–120 °C. ¹H NMR (CDCl₃,) δ 1.42 (s, 9H), 1.53-1.66 (m, 2H), 1.86-1.93 (m, 3H), 2.00-2.08 (m, 1H), 2.93-2.98 (m, 1H), 4.27 (br, s, 1H), 4.41 (br, s, 1H), 7.43 (d, J = 10.5 Hz, 2H), 7.51 (d, J = 10.5, 2H), 7.83 (t, J = 2.1 Hz, 1H), 8.48 (s, J = 2.1 Hz, 1H), 8.65 (s, J = 2.1 Hz, 1H); ¹³C NMR (CDCl₃) δ 155.3, 148.5, 146.4, 136.8, 135.6, 134.6, 132.8, 129.5, 128.8, 80.1, 62.2, 56.3, 45.9, 40.8, 30.2, 29.2, 28.7 ESI-MS (m/z): 385.5 (M+1);Anal (C₂₂H₂₅ClN₂O₂) C, H, N.

3-(4-Chlorophenyl)deschloroepibatidine (5b)

To a solution of 9b (400 mg, 1.04 mmol) in CH₂Cl₂ (7 mL) at 0 °C was added trifluoroacetic acid (7 mL). After 2 h at 25 °C, the reaction mixture was poured into 200 mL of concentrated NH₄OH-H₂O (1:1), extracted with CH₂Cl₂ (5 x 70 mL), dried (Na₂SO₄) and concentrated. The residue was purified by silica gel chromatography eluting with EtOAc-MeOH (2:1 to 2:3) to give 184 mg (62 %) of 5b as a crystalline solid; mp 118–119 °C.¹H NMR (CDCl₃) δ 1.50-1.76 (m, 5H), 1.92-2.00 (m, 1H), 2.84-2.89 (m, 1H), 3.65 (br, s, 1H), 3.82 (br, s, 1H), 7.04 (d, J = 8.04 Hz, 2H), 7.52 (d, J = 7.52, 2H), 7.90 (t, J = 2.1 Hz, 1H), 8.52 (d, J = 2.1 Hz, 1H), 8.63 (d, J = 2.l Hz, 1H); ¹³C NMR (CDCl₃) δ 30.6, 31.9, 40.8, 45.7, 56.9, 63.2, 128.9, 129.5, 133.3, 134.5, 135.6, 137.1, 142.5, 146.1, 148.7; ESI-MS (m/z):285.5 (M+1); Anal (C₁₇H₁₇ClN₂) C, H, N.

7-tert-Butoxycarbonyl-2-exo-[3-(3-chlorophenyl)-5-pyridinyl)]-7-azabicyclo(2.2.1]heptane (9c)

Using a procedure similar to that described for 9a, compound 8 (600 mg, 1.70 mmol) was coupled to 440 mg (2.80 mmol) of 3-chlorophenylboronic acid to give 0.67 g (>100%) of 9c as a noncrystalline solid.¹H NMR (CDCl₃) δ 1.34 (s, 9H), 1.39-1.57 (m, 2H), 1.78-1.82 (m, 3H), 1.91-1.98 (m, 1H), 2.84-2.89 (m, 1H), 4.20 (br, s, 1H), 4.32 (br, s, 1H), 7.227.46 (m, 3H), 7.46 (s, br, 1H), 7.76 (s, br, 1H), 8.40 (s, br, 1H), 8.56 (s, br, 1H; ¹³C NMR (CDCl₃) δ 28.6, 29.1, 30.1, 40.7, 45.7, 56.2, 62.2, 80.0, 125.6, 127.5, 128.3, 130.6, 133.0, 135.2, 135.3, 140.0, 141.6, 146.2, 148.5, 155.2.

3’-(3-Chlorophenyl)deschloroepibatidine (5c) Dihydrochloride

To a stirred solution of 9c (600 mg, 1.56 mmol) in CH₂Cl₂ (10 mL) at 0 °C was added trifluoroacetic acid (10 mL). After 2 h at 25 °C, the reaction mixture was poured into 300 mL of concentrated NH₄OH-H₂O (1:1) extracted with CH₂Cl₂ (5 × 80 mL), dried (Na₂SO₄) and concentrated. The residue was purified by silica gel chromatography eluting with EtOAc-MeOH (2:1 to 4:3) to yield 430 mg, (98 %) of 5c. ¹H NMR (CDCl₃) δ 1.47-1.75 (m, 5H), 1.89-1.96 (m, 1H), 2.02-2.05 (m, 1H), 2.81-2.86 (m, 1H), 3.62 (br, s, 1H), 3.80 (br, s, 1H), 7.29-7.45 (m, 3H), 7.54 (s, br, 1H), 7.91 (s, br, 1H), 8.52 (5, br, 1H), 8.61 (5, br, 1H); ¹³C NMR (CDCl₃) δ 30.5, 31.8, 40.5, 45.6, 56.8, 63.1, 125.8, 127.6, 128.3, 130.6, 133.4, 135.2, 135.3, 140.3, 142.4, 146.1, 148.9.

The dihydrochloride salt had mp 227–230 °C; Anal (C₁₇H₁₉C_l3N₂•1.5H₂O) C, H, N.

7-tert-Butoxycarbonyl-2-exo-[3-(4-fluorophenyl)-5-pyridinyl)]-7-azabicyclo[2.2.1]heptane (9d)

Using a procedure similar to that described for 9a, compound 8 (1.0 g, 0.0028 mol) was coupled to 650 mg (4.67 mmol) of 4-fluorophenylboronic acid to give 1.04 g (97%) of 9d as a noncrystalline solid. ¹H NMR (CDCl₃) δ 1.43 (5, 9H), 1.58-1.64 (m, 2H), 1.87-1.93 (m, 3H), 2.00-2.08 (m, 1H), 2.96-2.98 (m, 1H), 4.29 (br, s, 1H), 4.42 (br, s, 1H), 7.14 (m, 2H), 7.55 (m, 2H), 7.84 (m, 1H), 8.05 (br, s, 1H), 8.65 (br, s, 1H).

3’-(4-Fluorophenyl)deschloroepibatidine (5d)

To a stirred solution of 9d (1.38 g, 0.0038 mmol) in CH₂Cl₂ (23 mL) at 0 °C was added trifluoroacetic acid (23 mL). After 2 h, the reaction mixture was poured into 500 mL of concentrated NH₄OH-H₂O (1:1) extracted with CH₂Cl₂ (5 × 100 mL), dried (Na₂SO₄), and concentrated. The residue was purified by silica gel chromatography eluting with EtOAc-MeOH (2:1 to 2:3) to yield 0.81 g, (81 %) of 5d as a crystalline solid; mp 115–116 °C. ¹H NMR (CDCl₃) δ 1.46-1.76 (m, 5H), 1.92-1.99 (m, 1H), 2.84-2.89 (m, 1H), 3.65 (br, s, 1H), 3.82 (br, s, 1H), 7.16 (t, J = 9.6 Hz, 2H), 7.55 (m, 2H), 7.89 (m, 1H), 8.50 (br, s, 1H), 8.62 (br, s, 1H); ¹³C NMR (CDCl₃) δ 30.6, 31.9, 40.8, 45.7, 56.9, 63.2, 116.3 (d, J_CF = 21.6 Hz), 129.3 (d, J_CF = 8.1 Hz) 133.2, 134.7, 135.8, 142.4, 146.2, 148.4, 163.0 (d, J_CF = 236.3) ; ESI-MS (m/z); 269.1 (M+l); Anal (C₁₇H₁₇FN₂) C, H, N.

7-tert-Butoxycarbonyl-2-exo-[3-(3-fluorophenyl)-5-pyridinyl)]-7-azabicyclo[2.2.1]heptane (9e)

Using a procedure similar to that described for 9a, compound 8 (600 mg, 1.70 mmol) was coupled to 392 mg (2.80 mmol) of 3-fluorophenylboronic acid to give 0.63 g (96%) of 9e as a noncrystalline solid. ¹H NMR (CDCl₃) δ 1.44 (s, 9H), 1.49-1.67 (m, 2H), 1.87-1.94 (m, 3H), 2.01-2.08 (m, 1H), 2.94-2.99 (m, 1H), 4.29 (br, s, 1H), 4.42 (br, s, 1H), 7.07 (m, 1H), 7.26-7.46 (m, 3H), 7.87 (m, 1H), 8.50 (br, s, 1H), 8.68 (br, s, 1H); ¹³C NMR (CDCl₃) δ 28.3, 28.8, 29.8, 40.4, 45.4, 55.9, 61.9, 79.7, 114.4 (m), 122.8, 130.6, 132.7, 135.1, 140.2, 141.3, 145.9, 148.2, 154.9, 161.6, 164.8.

3’-(3-Fluorophenyl)deschloroepibatidine (5e) Dihydrochloride

To a solution of 9e (600 mg, 1.63 mmol) in CH₂Cl₂ (27 mL) at 0 °C was added trifluoroacetic acid (27 mL). After 2 h, the reaction mixture was poured into 300 mL of NH₄OH-H₂O (1:1), extracted with CH₂Cl₂ (5 × 80 mL), dried (Na₂SO)₄, and concentrated. The residue was purified by silica gel chromatography eluting with (EtOAc-MeOH) (2:1 to 1:1) to give 0.39 g (90%) of 5e. ¹H NMR (CDCl₃) δ 1.48-1.61 (m, 5H), 1.68-1.76 (m, 1H, CH₂), 1.90-1.97 (m, 1H, CH), 2.82-2.87 (m, 1H), 3.63 (br, s,1H), 3.80 (br, s, 1H), 7.02-7.08 (m, 1H), 7.26-7.30 (m, 1H), 7.34-7.44 (m, 2H), 7.93 (m, 1H), 8.53 (br, s, 1H), 8.63 (br, s, 1H); ¹³C NMR (CDCl₃) δ 30.2, 31.4, 40.4, 42.3, 56.5, 62.8, 113.9 (d, J_CF = 22.4 Hz), 114.7 (d, J_CF = 21.1 Hz), 122.9 (d, J_CF = 2.9 Hz), 131.0 (d, J_CF = 8.2 Hz), 133.0, 135.1, 140.0 (d, J_CF = 9.1 Hz), 138.3, 148.4, 148.1, 163.2 (d, J_CF = 246.2 Hz).

The dihydrochloride salt had mp 165–167 °C; Anal (C₁₇H₁₉Cl₂N₂•1.5H₂O) C, H, N.

7-tert-Butoxycarbonyl-2-exo-[3-(4-nitrophenyl)-5-pyridinyl)]-7-azabicyclo[2.2.1]heptane (9f)

Using a procedure similar to that described for 9a, compound 8 (400 mg, 2.40 mmol) was coupled to 312 mg (1.87 mmol) of 4-nitrophenylboronic acid to give 0.40 g (90%) of 9f as a solid. ¹H NMR (CDCl₃) × 1.43 (s, 9H), 1.58-1.75 (m, 2H), 1.89-1.94 (m, 3H), 2.04-2.11 (m, 1H), 2.97-3.01 (m, 1H), 4.29 (br, s, 1H), 4.42 (br, s, 1H), 7.75 (d, J = 8.7 Hz, 2H), 7.93 (br, s, 1H), 8.33 (d, J = 8.4 Hz, 2H), 8.56 (br, s, 1H), 8.72 (br, s, 1H); ¹³C NMR (CDCl₃) δ 28.6, 29.2, 30.0, 40.7, 45.7, 56.3, 62.1, 80.0, 124.5, 128.2, 133.1, 134.3, 141.8, 144.7, 146.4, 147.8, 149.6, 155.2.

3’-(4-Nitrophenyl)deschloroepibatidine (5f) Dihydrochloride

To a stirred solution of 9f (390 mg, 0.99 mmol) in CH₂Cl₂ (6 mL) at 0 °C was added trifluoroacetic acid (6 mL). After 2 h,the reaction mixture was poured into 150 mL of concentrated NH₄OH-H₂O (1:1) extracted with CH₂Cl₂ (5 × 70 mL), dried (Na₂SO₄), and concentrated. The residue was purified by silica gel chromatography eluting with EtOAc-MeOH (2:1 to 2:3) to yield 0.25 g (77%) of 5f. ¹H NMR (CDCl₃) δ 1.52-1.86 (m, 5H), 1.95-2.02 (m, 1H), 2.88-2.93 (m, 1H), 3.61 (br, s, 1H), 3.84 (br, s, 1H), 7.78 (d, J = 8.7 Hz, 2H), 8.07 (br, s, 1H), 8.30 (d, J = 8.7, 2H), 8.61 (s, 1H), 8.70 (s, 1H); ¹³C NMR (CDCl₃) δ 28.6, 29.9, 38.8, 43.5, 54.8, 61.2, 122.5, 126.3, 131.7, 132.3, 140.9, 143.0, 144.1, 145.7, 147.8.

The dihydrochloride salt had mp 215–216 °C; Anal (C₁₇H₁₉Cl₂N₃O₂) C, H, N.

7-tert-Butoxycarbonyl-2-exo-[3-(3-nitrophenyl)-5-pyridinyl)]-7-azabicyclo[2.2.1]heptane (9g)

Using a procedure similar to that described for 9a, compound 8 (600 mg, 1.70 mmol) was coupled to 382 mg (3.60 mmol) of 3-nitrophenylboronic acid to give 0.53 g (79%) of 9g as a oil. ¹H NMR (CDCl₃) δ 1.44 (s, 9H), 1.49-1.71 (m, 2H), 1.90-1.95 (m, 3H), 2.04-2.13 (m, 1H), 3.00-3.05 (m, 1H), 4.32 (br, s, 1H), 4.44 (br, s, 1H), 7.67 (t, J = 7.8 Hz, 1H), 7.94 (br, s, 2H), 8.23-8.26 (m, 1H), 8.44 (s, br, 1H), 8.58 (s, br, 1H), 8.73 (s, br 1H); ¹³C NMR (CDCl₃,) δ 28.3, 28.8, 29.8, 40.4, 45.5, 56.1, 61.9, 79.8, 121.9, 122.7, 130.1, 132.8, 133.1, 134.1, 139.8, 141.5, 146.0, 148.8, 149.0, 155.0.

3’-(3-Nitrophenyl)deschloroepibatidine (5g) Dihydrochloride

To a stirred solution of 9g (0.47 mg, 1.19 mmol) in CH₂Cl₂ (8 mL) at 0 °C was added trifluoroacetic acid (8 mL). After 2 h,the reaction mixture was poured into 300 mL of concentrated NH₄OH-H₂O (1:1), extracted with CH₂Cl₂ (5 × 50 mL), dried (Na₂SO₄), and concentrated. The residue was purified by silica gel chromatography eluting with EtOAc-MeOH (2:1 to 1:1) to yield 0.25g (71%) of 5g as an oil. ¹H NMR (CDCl₃) δ 1.91-2.64 (m, 6H), 3.29-3.34 (m, 2H), 3.77-3.82 (m, 1H), 4.42-3.32 (m, 1H), 4.72-4.89 (m, 1H), 7.85-7.90 (m, 1H), 8.31-8.34 (m, 1H), 8.42-8.45 (m, 1H), 8.78-8.79 (m, 1H), 9.00 (br, 2, 2H), 9.23 (br, s, 1H); ¹³C NMR (CDCl₃) δ 25.6, 27.6, 35.8, 42.8, 59.2, 62.4, 122.6, 124.5, 130.8, 133.9, 135.7, 137.0, 139.1, 140.3, 142.1, 143.4, 149.3.

The dihydrochloride salt had mp 248–250 °C; Anal (C₁₇H₁₉Cl₂N₃O₂) C, H, N.

7-tert-Butoxycarbonyl-2-exo-[3-(4-aminophenyl)-5-pyridinyl)]-7-azabicyclo[2.2.1]heptane (10)

To a stirred solution of 9g (0.34 g, 0.86 mmol) in EtOH (3 mL), water (0.6 mL), and conc. HCl (0.1 mL), excess Fe powder was added in one portion over 1 min. After heating at 90 °C for 1 h, the reaction mixture was poured into 100 mL of NH₄OH-H₂O (1:1) solution, extracted with CH₂Cl₂ (5 × 50 mL), dried (Na₂SO₄), and concentrated. The residue was purified by silica gel chromatography eluting with hexanes-EtOAc (2:1 to 1:2) to yield 0.25 g (80%) of 10 as a solid. ¹H NMR (CDCl₃) δ 1.42 (s, 9H), 1.51-1.64 (m, 2H), 1.85-1.91 (m, 3H), 1.97-2.04 (m, 1H), 2.89-2.93 (m, 1H), 3.82 (br, s, 2H), 4.27 (br, s, 1H), 4.40 (br, s, 1H), 6.74 (d, J = 8.4 Hz, 2H), 7.38 (d, J = 8.4 Hz, 2H), 7.79 (br, s, 1H), 8.38 (br, s, 1H), 8.63 (br, s, 1H); ¹³C NMR (CDCl₃) δ 28.7, 29.2, 30.3, 40.7, 45.9, 56.3, 62.3, 80.0, 115.8, 128.0, 128.4, 132.1, 136.7, 141.2, 145.9, 146.9, 147.2, 155.3.

3-(4-Aminophenyl)deschloroepibatidine (5h) Trihydrochloride

To a stirred solution of 10 (240 mg, 0.66 mmol) in CH₂Cl₂ (4 mL) at 0 °C was added trifluoroacetic acid (4 mL). After 2h, the reaction mixture was poured into 100 mL of concentrated NH₄OH-H₂O (1:1) solution, extracted with CH₂Cl₂ (5 × 50 mL), dried (Na₂SO₄), and concentrated. The residue was purified by silica gel chromatography eluting with EtOAc-MeOH (2:1 to 1:1) to yield 0.17 g (95 %) of 5h. ¹H NMR (CDCl₃) δ 1.48-1.78 (m, 5H), 1.92-1.99 (m, 1H), 2.85-2.89 (m, 1H), 3.65 (br, s, 1H), 3.80 (br, s, 1H), 6.77 (d, J = 8.4 Hz, 2H), 7.40 (d, J = 8.4 Hz, 2H), 7.81 (s, 1H), 8.41 (s, 1H)), 8.61 (s, 1H); ¹³C NMR (CDCl₃) δ 30.4, 31.7, 40.6, 45.9, 56.9, 63.2, 115.8, 128.3, 128.5, 132.5, 136.7, 142.1, 145.7, 147.1, 147.2.

The trihydrochloride salt had mp 238–240 °C; Anal (C₁₇H₂₁Cl₃N₃•1.5H₂O) C, H, N.

7-tert-Butoxycarbonyl-2-exo-[5’-(3’-(3-nitrophenyl)-2’-aminopyridinyl)]-7-azabicyclo[2.2.1]heptane (12a)

To a resealable reaction tube under nitrogen was added 11 (600 mg, 1.6 mmol), Pd(OAc)₂ (29.1 mg, 0.129 mmol), P(o-tolyl)₃ (78.5 mg, 0.258 mmol), sodium carbonate (348 mg, 3.28 mmol), 3-nitrophenylboronic acid (411 mg, 2.47 mmol), degassed water (1.65 mL) and DME (8.2 mL). The reaction mixture was heated at 80°C overnight, cooled and poured into saturated sodium bicarbonate solution and extracted with ethyl acetate (3 × 20). The organic combination was dried (Na₂SO₄) and concentrated. The resulting residue was purified by silica gel column chromatography, eluting with hexane-ethyl acetate (1:2) to yield 0.56 g, (85%) of 12a as an oil. ¹H NMR (CDCl₃) δ 1.39 (s, 9H), 1.49-1.62 (m, 2H), 1.76-1.85 (br s, 3H), 1.94- 2.01 (m, 1H), 2.79-2.84 (m, 1H), 4.16 (br s, 1H), 4.36 (br s, 1H), 4.56 (br s, 1H), 7.40 (s, 1H), 7.61-7.66 (t, 1H), 7.81-7.84 (d, 1H), 7.99 (s, 1H), 8.21-8.24 (d, 1H), 8.33 (s, 1H)

3-(3-Nitrophenyl)epibatidine (13a)

To a solution of 12a (158 mg, 0.192 mmol) in concentrated hydrochloric acid (2 ml) was added sodium nitrite 473 mg (6.86 mmol) in portions over 30 min and stirring continued for 1 h at 0 °C, then to room temperature for additional 3 h. The mixture was poured into 100 mL solution of concentrated NH₄OH-H₂O (1:1), extracted with CHCl₃ (3 × 20 mL), dried (Na₂SO₄), and concentrated. The residue was purified by silica gel column chromatography, eluting with CMA80-ethyl acetate (1:3) to yield 80 mg (63%) of 13a as an oil. ¹H NMR (CDCl₃) δ 1.54-1.71 (m, 5H), 1.92-2.02 (dd, 1H), 2.80-2.85 (dd, 1H), 3.62 (br s, 1H), 3.81 (br s, 1H), 7.64-7.67 (t, 1H), 7.80-7.87 (m, 2H), 8.27-8.37 (m, 3H).

3’-(3-Aminophenyl)deschloroepibatidine (5i) Trihydrochloride

Compound 13a, (70 mg, 0.211 mmol), 10% Pd/C (110 mg), and methanol (20 m1) were placed into a Fisher-Porter tube under nitrogen. The tube was evacuated, refilled with hydrogen gas (50 psi). The reaction was allowed to shake for 7 h, filtered through a celite pad and the solvent was removed. The resulting residue was purified by flash chromatography eluting with CMA80-ethyl acetate (1:1) to yield 45 mg (80%) of 5i. ¹H NMR (CD₃OD) δ 1.50-1.68 (m, 5H), 1.91-1.98 (dd, 1H), 2.89-2.94 (dd, 1H), 3.57 (br s, 1H), 3.66 (br s, 1H), 6.64-6.67 (d, 1H), 6.84-6.90 (m, 2H), 7.07-7.12 (t, 1H), 7.85 (s, 1H), 8.31 (s, 1H), 8.44 (s, 1H); ¹³C NMR (CD₃OD) δ 30.2, 32.1, 41.3, 46.8, 58.3, 63.9, 115.2, 116.8, 118.2, 131.3, 135.2, 139.2, 139.9, 143.6, 146.3, 148.2, 150.1.

The trihydrochloride salt had mp 209 °C (dec); Anal (C₁₇H₂₂Cl₃N₃•1.25H ₂O) C, H, N.

7-tert-Butoxycarbonyl-2-exo-[2-amino-3-(3-methoxyphenyl)-5-pyridinyl]-7-azabicyclo[2.2.1]heptane (12b)

Using a procedure similar to that described for 12a, (997 mg, 2.7 mmol) of 11 was coupled to 3-methoxyphenylboronic acid (649 mg, 4.3 mmol) to give 959 mg (90%) of 12b as a colorless oil. ¹H NMR (CDCl₃) δ 1.38 (s, 9H), 1.45-1.65 (m, 3H), 1.7-20 (m, 3H), 2.78 (dd, J = 4.9, 7.7 Hz, 1H), 3.82 (s, 3H), 4.16 (s, 1H), 4.34 (s, 1H), 4.66 (s, 2H), 6.85-7.07 (m, 3H), 7.28-7.40 (m, 2H), 7.92 (s, 1H); ¹³C NMR (CDCl₃) δ 28.1 (3C), 28.6, 29.7, 40.1, 44.8, 55.1, 55.7, 62.1, 79.3, 113.1, 114.1, 120.8, 121.4, 129.9, 131.5, 136.5, 139.5, 145.5, 154.3, 154.9, 159.8.

3’-(3-Methoxyphenyl)epibatidine (13b)

Using a procedure similar to that described for 13a, (167 mg, 0.422 mmol) of 12b was converted to 83 mg (63%) of 13b as a colorless oil. ¹H NMR (CDCl₃) δ 1.4-1.8 (m, 4H), 1.8-2.05 (m, 2H), 2.81 (dd, J = 5.0, 8.8 Hz, 1H), 3.62 (br s,1H), 3,78 (br s, 1H), 3.84 (s, 3H), 6.8-7.1 (m, 3H), 7,31 (t, J = 7.9 Hz, 1H), 7.76 (s, 1H), 8.34 (s,1H); ¹³C NMR (CDCl₃) δ 30.0, 31.3, 40.2, 44.5, 55.3, 56.4, 62.7, 113.5, 115.1, 121.7, 129.2, 136.1, 138.5, 139.0, 141.2, 146.9, 147.4, 159.2.

3-(3-Methoxyphenyl)deschloroepibatidine (5j) Dihydrochloride

Dechlorohydrogenation of 13b (80 mg, 0.254 mmol) using conditions similar to that described for 12b yielded 28 mg (39%) of 5j as a colorless oil. ¹H NMR (CDCl₃) δ 1.45-1.80 (m, 4H), 1.86 (s, 1H), 1.94 (dd, J = 8.9, 12.3 Hz, 1H), 2.87 (dd, J = 5.1, 8.7 Hz, 1H), 3.65 (br s, 1H), 3.80 (br s, 1H), 3.86 (s, 3H), 6.9-7.5 (m, 4H), 7.88 (s, 1H), 8.50 (s, 1H), 8.64 (s, 1H); ¹³C NMR (CDCl ₃) δ 30.1, 31.3, 40.3, 45.5, 55.4, 56.5, 62.8, 113.2, 113.2, 119.7 (2C), 130.0, 133.1, 136.2, 139.7, 141.8, 146.0, 148.0

The dihydrochloride salt had mp 242 °C (dec); Anal (C₁₈H₂₂Cl₂N₂O•H₂O) C, H, N.

Supplementary Material

01. Supporting Information Available.

Elemental analysis data. This material is available free of charge via the internet at http://pubs.acs.org.