Total Synthesis of (±)-Quinagolide: A Potent D2 Receptor Agonist for the Treatment of Hyperprolactinemia.

A potent dopamine (D2) receptor agonist (±)-quinagolide, which is used for the treatment of hyperprolactinemia, was synthesized using the ring closing metathesis (RCM) approach from meta-hydroxybenzaldehyde as the starting material. The key features of this synthesis are pyrolytic elimination, late-stage expedient synthesis of functionalized trans-fused tetrahydropyridine-3-carboxylates from olefin 6, via conjugate addition-elimination upon acetate 11, followed by RCM and phenyliodine bis(trifluoroacetate) (PIFA)-mediated Hofmann rearrangement of piperidine-3-carboxamide, which enables the synthesis of 3-aminopiperidine skeleton of quinagolide. For the total synthesis of natural products such as ergot alkaloids, late-stage synthesis of functionalized trans-fused tetrahydropyridine-3-carboxylates using RCM and PIFA-mediated Hofmann rearrangement of piperidine-3-carboxamide, which allows quick access to the synthetically challenging 3-aminopiperidine skeleton, are the main achievements of the present work.

Introduction

Prolactinoma is a benign tumor (n class="Disease">adenoma) of the pituitary gland, which produces hormone prolactin, and a condition that arises because of elevated prolactin levels in blood is defined as hyperprolactinemia. Pharmacological causes such as use of certain medications for the treatment of various diseases and physiological causes such as pregnancy and stress are the main factors behind the elevated level of prolactin. Hyperprolactinemia may also be the result of disease of other organs such as the liver, kidneys, ovaries, and thyroid. The most common symptoms of hyperprolactinemia are hypogonadism, infertility, and erectile dysfunction in men and galactorrhea and disruptions in the normal menstrual period in women. Although hyperprolactinemia is not considered as a life-threatening disease, it causes severe effects on the life of patients and often leads to multiple life-threatening diseases.[1] For the treatment of hyperprolactinemia, drugs such as bromocriptine (1), cabergoline (2), and quinagolide (3) are used as medications (see Figure ).[2]

Figure 1

Available hyperprolactinemia medications bromocriptine (1), cabergoline (2), and quinagolide (3).

Out of these medications available in the market, bromocriptine (1) and n class="Chemical">cabergoline (2) have serious side effects, whereas quinagolide (3) which is newly introduced by Ferring Pharmaceuticals under the trade name NORPROLAC is considered as a first-line therapy in the treatment of hyperprolactinemia.[1b]

The potent dopamine (D2) receptor agonist n class="Chemical">quinagolide is developed by combining structural features of both ergot and apomorphine alkaloids.[3] Quinagolide was first synthesized in racemic form, and subsequently, its biological activity was reported by Nordmann et al.[3] Later in the year 2000, scalable synthesis of quinagolide intermediate was reported by Bänziger et al.[4] Though the dopaminomimetic activity is completely associated with the (−) enantiomer of quinagolide,[3b] in the present days, it is sold in its racemic form as hydrochloride salt.

With the aim of developing a practical route to this potent dopamine agonist, synthetic studies toward quinagolide were initiated in our group. Toward this, recently (2018), we reported an efficient synthesis of quinagolide, which features a Claisen rearrangement, PPTS-catalyzed one-pot acetal deprotection and intramolecular diastereoselective Henry reaction, and ceric ammonium nitrate-mediated regioselective azidoalkoxylation of enol ether as the key steps.[5] In that context, our group effectively utilized ring closing metathesis (RCM) as an important synthetic tool in the total synthesis of natural products.[6] Herein, we report our alternative synthetic approach for the total synthesis of quinagolide using RCM as the key reaction for the construction of 3-aminopiperidine skeleton of quinagolide.

Over the last 3 decades, RCM is found to be an important tool for the synthesis of numerous piperidine and pyrrolidine alkaloids.[7] The synthesis of alkaloids using RCM generally requires either N-protecting groups (especially electron withdrawing) or Lewis acid, so that lone pair of basic amine does not adversely interfere in the reaction through chelating with the catalyst. Furthermore, synthesis of functionalized 3-substituted tetrahydropyridine scaffolds which are an integral part of many biologically active natural products such as ergot alkaloids using RCM is considered as a challenging task.[8,9]

To the best of our knowledge, in literature, there are only a few reports in which late-stage synthesis of functionalized tetracyclic 3-substituted tetrahydropyridine scaffolds using RCM is docun class="Species">mented (Figure ).[10,11]

Figure 2

Late-stage synthesis of functionalized tri or tetracyclic 3-substituted tetrahydropyridines using RCM approach.

After the seminal work by Martin and co-workers for the synthesis of functionalized tetracyclic tetrahydropyridine scaffolds using RCM,[12] the late-stage synthesis of tetracyclic 3-substituted tetrahydropyridine for the total synthesis of (+)-isolysergol was reported by the same group in the year 2010 (eq 1 in Figure ).[10] Also, recently (2017), Jia and co-workers reported late-stage synthesis of tetracyclic 3-substituted tetrahydropyridine for the total synthesis of (−)-agroclavine and (−)-elymoclavine using RCM (eq 2 in Figure ).[11] Here, in the case of first synthesis, preparation of functionalized tetrahydropyridine scaffolds using RCM of a basic amine containing electron-rich olefins was done using Schrock’s catalyst, whereas in the later synthesis, Zhan 1-B catalyst was used for the construction of functionalized tetrahydropyridine scaffolds using RCM of an electron-deficient amine containing electron-rich olefins. Reportedly, in both of these elegant syntheses, Grubbs I, II and Grubbs–Hoveyda catalysts did not work. In that context, late-stage synthesis of tricyclic trans-fused tetrahydropyridine-3-carboxylates using RCM could be more challenging, as the basic amine and the electron-deficient olefin are the main deciding factors.[7] Here, we thought that construction of tetrahydropyridine-3-carboxylate using RCM can fulfill the requirements for the rapid assembly of 3-aminopiperidine core of quinagolide.

Accordingly, retrosynthetic plan for (±)-quinagolide is shown in Scheme . It was thought that quinagolide could be accessed from an inexpensive and commercially available starting material meta-hydroxybenzaldehyde.

Scheme 1

Key Retrosynthetic Disconnections for Quinagolide

As per the retrosynthetic plan, quinagolide 3 could be obtained from tricyclic tetrahydropyridine-3-carboxylate 5 using phenyliodine bis(trifluoroacetate) (PIFA)-mediated Hofmann rearrangement on the corresponding carboxamide 4 followed by sulfonation and demethylation. Tricyclic trans-fused tetrahydropyridine-3-carboxylate 5 could be obtained from olefin 6 upon conjugate addition–elimination with required acetate followed by RCM. Olefin 6 could be obtained from ester 7 by reduction to alcohol, corresponding sulfoxide synthesis and pyrolytic elimination as key steps. Ester 7 in turn could be accessed from meta-hydroxybenzaldehyde using PPTS-catalyzed one-pot acetal deprotection followed by diastereoselective Henry reaction as a key step, which was reported earlier by our group.[5]

Results and Discussion

Synthesis commenced with the goal of conversion of n class="Chemical">ester functionality in compound 7 into corresponding olefin 6 through pyrolytic elimination (Scheme ).

Scheme 2

Synthesis of Olefin 6 from Ester 7 by Pyrolytic Elimination

To this end, ester 7 was reduced to the corresponding alcohol 8 using NaBH4–LiCl in tetrahydrofuran (THF)–EtOH (1:1) under reflux condition in 68% yield. Alcohol 8 was then converted into sulfide 9 by the combined action of diphenyl disulfide and tributyl phosphine in 95% yield.[13] Oxidation of the sulfide 9 to the corresponding sulfoxide 10 was performed using NaIO4 in 95% yield.[14] Pyrolytic elimination of sulfoxide 10 under refluxing xylene condition afforded olefin 6 in 98% yield.[15] Here, pyrolytic elimination was found to be a clean and high yielding reaction, and it is a good alternative reaction sequence for the construction of a vinyl group over other reduction–elimination sequence in terms of overall yield and cost effectiveness of the process.

After successful synthesis of olefin 6, the next task was the synthesis of diene for RCM (Scheme ). At this stage, few conditions for the N-alkylation using methyl 2-(bromomethyl)acrylate and acetate 11 were attempted, but all of them failed to provide the desired product. In most of the cases, the starting material was recovered. Therefore, it was decided to remove the N-Boc protecting group and then attempt alkylation. Accordingly, olefin 6 was treated with TFA (10 equiv) in CH2Cl2 for 5 h and the corresponding amine was treated with acetate 11 in CH2Cl2 at room temperature by applying the protocol described by Ramachandran et al.[16] To our delight, our crude amine upon conjugate addition–elimination with acetate 11 smoothly provided the desired N-alkylated product 12 in 80% yield over two steps. Here, it was observed that the concentration of the reaction mixture affects the rate of the reaction and 0.05 M concentration gives the optimum yield. In the next step, N-propylation was carried out using propyl iodide and K2CO3 in acetonitrile under reflux condition for 15 h to afford the corresponding diene 13 in 70% yield.

Scheme 3

Synthesis of Tricyclic Core of Quinagolide Using RCM

With the diene 13 in hand, we were in comfortable position to evaluate the crucial RCM reaction for the synthesis of trans-fused tricyclic skeleton. Though it is reported in the literature that, for the synthesis of tetracyclic skeleton using RCM, Grubbs catalysts did not work,[10,11] to our delight, the RCM reaction of diene 13 using Grubbs’ II generation catalyst and p-toluenesulfonic acid (PTSA) (1.1 equiv) in toluene at 80 °C for 14 h smoothly provided the required tricyclic core 5 of the quinagolide in 93% yield. It is worth mentioning that the solvent has great influence on the rate of reaction. When the reaction was carried out in CH2Cl2 as the solvent under reflux condition for 24 h, less than 30% conversion was observed along with the recovery of the starting material, while toluene turned out to be the best solvent for metathesis. Also, it was observed that the reaction was completely homogeneous in toluene at 80 °C and precipitation of ammonium salt was not observed when formed.

After the crucial intermediate in hand, the next aim was the synthesis of amino side chain of quinagolide. To this end, reduction of the double bond of 5 was performed using Pd/C under hydrogenation condition to afford the compound 14 in 87% yield. The stereochemistry of newly generated center was confirmed by comparison of the spectrum of compound 14 prepared by a different route.[4,17] Furthermore, epimerization at ester center of 14 was performed using a known protocol to afford piperidine-3-carboxylate 15 in 85% yield.[4] Compound 15 showed identical 1H and 13C NMR spectra, which were consistent with the data reported by Bänziger et al.[4]

Toward the completion of the total synthesis of quinagolide, the next aim was the construction of the side chain. To this end, ester 15 was converted into the corresponding carboxamide 4 using NH3/MeOH and CaCl2 in 90% yield (Scheme ).[18] Carboxamide 4 was subjected for PIFA-mediated Hofmann rearrangement to obtain the corresponding amine 16 in 82% yield.[19] Amine 16 was then sulfonated using diethylsulfamoyl chloride to obtain the compound 17 in 71% yield. In the last step, demethylation was performed using AlCl3–EtSH in 66% yield to complete the total synthesis of quinagolide 3.[20] All the spectral data of quinagolide 3 were in complete agreement with the reported data.[3a,5]

Scheme 4

Completion of the Total Synthesis

Conclusions

To conclude, total synthesis of (±)-quinagolide was performed using the RCM approach. Pyrolytic elimination, late-stage synthesis of functionalized trans-fused tetrahydropyridine-3-carboxylate from olefin 6 upon conjugate addition–elimination with acetate 11 followed by RCM and PIFA-mediated Hofmann rearrangement of piperidine-3-carboxamide, which enables the synthesis of 3-aminopiperidine skeleton of quinagolide, are key features of this synthesis. The present synthetic route allows late-stage synthesis of trans-fused tetrahydropyridine-3-carboxylates using RCM reaction, which could serve as an important example in the context of the total synthesis of natural products having this structural motif such as ergot alkaloids, and PIFA-mediated Hofmann rearrangement of piperidine-3-carboxamide allowing quick access to the 3-aminopiperidine skeleton, which poses a challenge to synthetic chemists, are the main achievements of the present work. Our efforts are currently directed toward the enantioselective total synthesis of quinagolide and will be reported in due course.

Experimental Procedures

General

All reactions were carried out in oven-dried glassware under a positive pressure of argon or nitrogen unless otherwise mentioned with magnetic stirring. Air-sensitive reagents and solutions were transferred via syringe or cannula and introduced to the apparatus via rubber septa. All reagents, starting materials, and solvents were obtained from commercial suppliers and used as such without further purification. Reactions were monitored by thin-layer chromatography with 0.25 mm precoated silica gel plates (60 F254). Visualization was accomplished with either UV light, iodine adsorbed on silica gel, or by immersion in ethanolic solution of phosphomolybdic acid, p-anisaldehyde, 2,4-dinitrophenylhydrazine, KMnO4, or ninhydrin solution followed by heating with a heat gun for ∼15 s. Melting points are uncorrected. The 1H NMR spectra were recorded on Bruker AV 200, 400, and 500 MHz NMR spectrometers using the solvent residue signal as an internal standard [1H NMR: CDCl3 (7.27), DMSO-d6 (2.50); 13C NMR: CDCl3 (77.00), DMSO-d6 (39.51)]. The 13C NMR spectra were recorded on 200 NMR (50 MHz), 400 NMR (100 MHz), and 500 NMR (125 MHz) spectrometers. High-resolution mass spectrometry (HRMS) [electrospray ionization (ESI)] spectra were recorded on an Orbitrap (quadrupole plus ion trap) and time-of-flight mass analyzer. The IR spectra were recorded on a Fourier transform infrared spectrometer. Column chromatographic separations were carried out on silica gel (60–120 mesh and 230–400 mesh).

rac-tert-Butyl((2R,3S)-3-(2-hydroxyethyl)-5-methoxy-1,2,3,4-tetrahydronaphthalen-2-yl)carbamate (8)

To a stirred solution of ester 7 (1 g, 2.75 mmol, 1 equiv) in EtOH–THF (1:1, 50 mL), NaBH4 (0.416 g, 11.0 mmol, 4 equiv) followed by LiCl (0.467 g, 11.0 mmol, 4 equiv) were added at room temperature. The resulting suspension was refluxed for 16 h. After completion of the reaction, the solvent was evaporated under reduced pressure and the residue was treated with 1 N HCl (20 mL) and extracted with EtOAc (3 × 100 mL). The organic layer was separated, washed with saturated NaHCO3 and brine, dried over anhydrous Na2SO4, and filtered, and the solvent was evaporated under reduced pressure. The obtained residue was purified by silica gel (230–400 mesh) column chromatography using EtOAc–petroleum ether (PE) (30:70) to yield alcohol 8 (0.6 g, 68%) as a thick colorless liquid. Rf: 0.5 (EtOAc–PE = 50:50); IR (CHCl3): νmax 3435, 1702, 1587, 1215, 771 cm–1; 1H NMR (CDCl3, 200 MHz): δ 7.10 (t, J = 8.1 Hz, 1H), 6.68 (d, J = 8.1 Hz, 2H), 4.63 (br s, 1H), 3.91–3.65 (m, 6H), 3.18–2.87 (m, 2H), 2.73–2.32 (m, 2H), 2.15–1.70 (m, 4H), 1.45 (s, 9H); 13C NMR (CDCl3, 50 MHz): δ 157.1, 155.7, 135.3, 126.4, 123.7, 121.1, 107.2, 79.3, 60.5, 55.2, 49.6, 35.2, 34.8, 28.4 (3C), 26.8; HRMS (ESI) m/z: calcd for C18H28O4N, [M + H]+, 322.2013; found, 322.2014.

rac-tert-Butyl((2R,3S)-5-methoxy-3-(2-(phenylthio)ethyl)-1,2,3,4-tetrahydronaphthalen-2-yl)carbamate (9)

To a stirred solution of alcohol 8 (0.6 g, 1.86 mmol, 1 equiv) and n class="Chemical">diphenyl disulfide (1.224 g, 5.60 mmol, 3 equiv) in THF (15 mL), tri-n-butylphosphine (1.38 mL, 5.60 mmol, 3 equiv) was added and the reaction mixture was stirred for 24 h at room temperature. After completion of the reaction, the solvent was evaporated under reduced pressure and the obtained residue was purified by silica gel (230–400 mesh) column chromatography using EtOAc–PE (10:90) to yield the sulfide 9 (0.735 g, 95% yield) as a white solid. Rf: 0.6 (EtOAc–PE = 10:90); mp 98–99 °C; IR (CHCl3): νmax 3435, 1698, 1504, 771 cm–1; 1H NMR (CDCl3, 200 MHz): δ 7.33–7.00 (m, 6H), 6.63 (d, J = 8.0 Hz, 2H), 4.45 (d, J = 8.3 Hz, 1H), 3.77 (s, 3H), 3.75–3.61 (m, 1H), 3.18–2.82 (m, 4H), 2.66–2.18 (m, 2H), 2.06–1.75 (m, 2H), 1.64–1.50 (m, 1H), 1.36 (s, 9H); 13C NMR (CDCl3, 50 MHz): δ 157.1, 155.5, 136.5, 135.3, 129.1 (2C), 128.9 (2C), 126.5, 125.8, 123.4, 121.1, 107.2, 79.2, 55.2, 49.6, 37.3, 34.9, 31.9, 31.2, 28.4 (3C), 26.7; HRMS (ESI) m/z: calcd for C24H32O3NS [M + H]+, 414.2097; found, 414.2100.

rac-tert-Butyl((2R,3S)-5-methoxy-3-(2-(phenylsulfinyl)ethyl)-1,2,3,4-tetrahydronaphthalen-2-yl)carbamate (10)

To a stirred solution of sulfide 9 (0.7 g, 1.69 mmol, 1 equiv) in n class="Chemical">MeOH–H2O (9:1, 120 mL), NaIO4 (0.399 g, 1.86 mmol, 1.1 equiv) was added at 0 °C and the reaction mixture was stirred at room temperature for 12 h. The crude reaction mixture was diluted with CH2Cl2 (100 mL) and H2O (100 mL), the organic layer was separated, and the aqueous phase was extracted with CH2Cl2 (3 × 50 mL). The combined organic layer was washed with saturated NaHCO3 and brine, dried over anhydrous Na2SO4, and filtered, and the solvent was evaporated under reduced pressure. The obtained residue was purified by silica gel (230–400 mesh) column chromatography using EtOAc–PE (40:60) to yield the sulfoxide 10 (0.69 g, 95%) as a thick colorless liquid. Rf: 0.3 (EtOAc–PE = 40:60); IR (CHCl3): νmax 3298, 1698, 1526, 1168, 752 cm–1; 1H NMR (CDCl3, 200 MHz): (mixture of diastereomers) δ 7.69–7.57 (m, 2H), 7.57–7.45 (m, 3H), 7.10 (t, J = 8.0 Hz, 1H), 6.68 (d, J = 8.0 Hz, 1H), 6.64 (d, J = 8.0 Hz, 1H), 4.83–4.49 (m, 1H), 3.80 (s, 1.5H), 3.79 (s, 1.5H), 3.77–3.63 (m, 1H), 3.15–2.75 (m, 4H), 2.72–2.50 (m, 1H), 2.44–2.25 (m, 1H), 2.14–1.79 (m, 3H), 1.45 (s, 9H); 13C NMR (CDCl3, 50 MHz): (mixture of diastereomers) δ 157.0, 155.4, 143.9, 143.6, 135.2, 135.1, 130.9, 129.2, 126.5, 124.0, 123.96, 123.1, 123.0, 121.0, 107.2, 79.4, 55.1, 54.7, 54.3, 49.7, 49.2, 37.8, 37.3, 35.2, 35.0, 28.4 (3C), 27.1, 26.9, 25.3, 24.6; HRMS (ESI) m/z: calcd for C24H32O4NS [M + H]+, 430.2047; found, 430.2047.

rac-tert-Butyl((2R,3S)-5-methoxy-3-vinyl-1,2,3,4-tetrahydronaphthalen-2-yl)carbamate (6)

To a stirred solution of sulfoxide 10 (0.67 g, 1.56 mmol, 1 equiv) in xylene (40 mL), sodium bicarbonate (0.262 g, 3.12 mmol, 2 equiv) was added, and the resulting mixture was refluxed for 15 h. After completion of the reaction, the reaction mixture was cooled to room temperature, and then water (20 mL) was added and extracted with EtOAc (3 × 50 mL). The combined organic layer was dried over anhydrous Na2SO4, filtered and concentrated under reduced pressure. The obtained residue was purified by silica gel (230–400 mesh) column chromatography using EtOAc–PE (10:90) to yield the olefin 6 (0.465 g, 98%) as a white solid. Rf: 0.5 (EtOAc–PE = 10:90); mp 73–75 °C; IR (CHCl3): νmax 3438, 1706, 1040, 770 cm–1; 1H NMR (CDCl3, 200 MHz): δ 7.11 (t, J = 8.0 Hz, 1H), 6.71 (d, J = 8.0 Hz, 1H), 6.67 (d, J = 8.0 Hz, 1H), 5.93–5.72 (m, 1H), 5.23–5.05 (m, 2H), 4.56 (br s, 1H), 3.82 (s, 3H), 3.74 (br s, 1H), 3.24 (dd, J = 5.0, 16.4 Hz, 1H), 2.98 (dd, J = 5.4, 17.4 Hz, 1H), 2.73–2.32 (m, 3H), 1.46 (s, 9H); 13C NMR (CDCl3, 50 MHz): δ 157.0, 155.5, 140.0, 135.6, 126.4, 123.5, 121.1, 116.2, 107.2, 79.2, 55.2, 49.7, 44.2, 35.4, 29.6, 28.4 (3C); HRMS (ESI) m/z: calcd for C18H26O3N [M + H]+, 304.1907; found, 304.1906.

Methyl 2-(Acetoxymethyl)acrylate (11)

Acetate 11 was prepared according to the previously reported literature procedure, whose spectroscopic data matched with the one reported in the literature.[21]

rac-Methyl 2-((((2R,3S)-5-Methoxy-3-vinyl-1,2,3,4-tetrahydronaphthalen-2-yl)amino)methyl)acrylate (12)

To a stirred solution of olefin 6 (0.45 g, 1.48 mmol, 1 equiv) in n class="Chemical">CH2Cl2 (15 mL), trifluoroacetic acid (1.13 mL, 14.8 mmol, 10 equiv) was added at 0 °C. After stirring for 5 h at room temperature, the mixture was treated with saturated NaHCO3 and extracted with CH2Cl2 (3 × 50 mL). The combined organic layer was dried over anhydrous Na2SO4, filtered, and concentrated under reduced pressure to yield crude amine, which was used in the next reaction without further purification.

To a stirred solution of the above crude amine in n class="Chemical">CH2Cl2 (10 mL), acetate 11 (0.235 g, 1.48 mmol, 1 equiv) was added and the mixture was stirred for 12 h at room temperature. After completion of the reaction, the solvent was evaporated under reduced pressure and the obtained residue was purified by silica gel (230–400 mesh) column chromatography using EtOAc–PE (30:70) to yield the N-alkylated compound 12 (0.36 g, 80% yield) as a colorless liquid. Rf: 0.2 (EtOAc–PE = 30:70); IR (CHCl3): νmax 3435, 1702, 1587, 1215, 771 cm–1; 1H NMR (CDCl3, 200 MHz): δ 7.11 (t, J = 7.8 Hz, 1H), 6.67 (d, J = 7.8 Hz, 1H), 6.73 (d, J = 7.8 Hz, 1H), 6.26 (s, 1H), 5.74 (s, 1H), 5.66 (dd, J = 8.4, 9.8 Hz, 1H), 5.36–5.08 (m, 2H), 3.81 (s, 3H), 3.78 (s, 3H), 3.67 (d, J = 14.8 Hz, 1H), 3.47 (d, J = 14.8 Hz, 1H), 3.22–2.87 (m, 2H), 2.79–2.24 (m, 5H); 13C NMR (CDCl3, 50 MHz): δ 167.0, 157.0, 140.5, 138.4, 136.2, 126.3 (2C), 123.9, 121.1, 117.3, 107.1, 55.2, 54.6, 51.8, 47.6, 44.8, 35.2, 29.0; HRMS (ESI) m/z: calcd for C18H24O3N [M + H]+, 302.1751; found, 302.1738.

rac-Methyl 2-((((2R,3S)-5-Methoxy-3-vinyl-1,2,3,4-tetrahydronaphthalen-2-yl) (propyl)amino)methyl)acrylate (13)

To a stirred solution of compound 12 (0.35 g, 1.16 mmol, 1 equiv) in dry acetonitrile (15 mL), pulverized n class="Chemical">K2CO3 (0.482 g, 3.48 mmol, 3 equiv) was added and the reaction mixture was heated to 50 °C. After stirring for 20 min at that temperature, n-propyl iodide (0.34 mL, 3.48 mmol, 3 equiv) was added and the reaction mixture was refluxed for next 15 h. The reaction mixture was then cooled to room temperature, filtered, and concentrated under reduced pressure. The residue thus obtained was extracted with EtOAc (3 × 50 mL). The combined organic layer was dried over anhydrous Na2SO4, filtered, and concentrated under reduced pressure. The obtained residue was purified by silica gel (230–400 mesh) column chromatography using EtOAc–PE (10:90) to yield the diene 13 (0.28 g, 70%) as a colorless liquid. Rf: 0.6 (EtOAc–PE = 10:90); IR (CHCl3): νmax 3019, 1710, 1601, 1216, 769 cm–1; 1H NMR (CDCl3, 200 MHz): δ 7.10 (t, J = 8.0 Hz, 1H), 6.72 (d, J = 8.0 Hz, 1H), 6.66 (d, J = 8.0 Hz, 1H), 6.20 (br s, 1H), 6.11–5.81 (m, 2H), 5.15–4.92 (m, 2H), 3.81 (s, 3H), 3.75 (s, 3H), 3.56–3.24 (m, 2H), 3.07–2.72 (m, 4H), 2.68–2.31 (m, 4H), 1.53–1.35 (m, 2H), 0.85 (t, J = 7.4 Hz, 3H); 13C NMR (CDCl3, 50 MHz): δ 167.8, 157.0, 142.4, 139.2, 137.8, 126.2, 125.6, 124.5, 121.2, 113.7, 106.9, 59.5, 55.2, 51.9, 51.6, 50.8, 42.5, 30.8, 29.3, 21.9, 11.8; HRMS (ESI) m/z: calcd for C21H30O3N [M + H]+, 344.2220; found, 344.2227.

rac-Methyl-(4aS,10aR)-6-methoxy-1-propyl-1,2,4a,5,10,10a-hexahydrobenzo[g]quinoline-3-carboxylate (5)

To a degassed solution of the diene 13 (0.25 g, 0.73 mmol, 1 equiv) in n class="Chemical">toluene (100 mL), PTSA (152 mg, 0.80 mmol, 1.1 equiv) was added and the reaction mixture was heated to 50 °C for 30 min. Then, Grubbs’ II catalyst (10 mol %) was added in three equal parts at the intervals of 2 h and the reaction mixture was stirred for 14 h at 80 °C. After completion of the reaction, it was cooled to room temperature, treated with saturated Na2CO3 solution, and the resulting mixture was extracted with EtOAc (3 × 30 mL). The combined organic layer was washed with brine, dried over anhydrous Na2SO4, and filtered, and the solvent was evaporated under reduced pressure. The obtained residue was purified by silica gel (230–400 mesh) column chromatography using EtOAc–PE (10:90) to the yield tricyclic compound 5 (214 mg, 93%) as a colorless liquid. Rf: 0.5 (EtOAc–PE = 30:70); IR (CHCl3): νmax 1714, 1620, 1175, 756 cm–1; 1H NMR (CDCl3, 200 MHz): δ 7.14 (t, J = 8.0 Hz, 1H), 6.95 (br s, 1H), 6.78 (d, J = 8.0 Hz, 1H), 6.69 (d, J = 8.0 Hz, 1H), 3.83 (s, 3H), 3.77 (s, 3H), 3.71 (s, 1H), 3.27–3.00 (m, 3H), 2.98–2.68 (m, 2H), 2.56–2.14 (m, 4H), 1.69–1.53 (m, 2H), 0.95 (t, J = 7.3 Hz, 3H); 13C NMR (CDCl3, 50 MHz): δ 166.4, 157.1, 140.7, 136.5, 128.2, 126.6, 124.3, 121.6, 107.2, 58.6, 55.3, 53.9, 51.6, 50.4, 38.5, 34.4, 28.9, 18.6, 12.0; HRMS (ESI) m/z: calcd for C19H26O3N [M + H]+, 316.1907; found, 316.1903.

rac-Methyl-(3R,4aR,10aR)-6-methoxy-1-propyl-1,2,3,4,4a,5,10,10a-octahydrobenzo[g]quinoline-3-carboxylate (14)

To a stirred solution of compound 5 (200 mg, 0.63 mmol, 1 equiv) in dry MeOH (10 mL), Pd/C (20 mg) was added and the reaction mixture was stirred under H2 atmosphere at 60 psi pressure for 6 h. The reaction mixture was then filtered through Celite, washed thoroughly with EtOAc (3 × 50 mL), and concentrated under reduced pressure. The obtained residue was purified by silica gel (230–400 mesh) column chromatography using EtOAc–PE (15:85) to yield the pure compound 14 (175 mg, 87% yield) as a pale yellow liquid. Rf: 0.4 (EtOAc–PE = 20:80); IR (CHCl3): νmax 3019, 1728, 1253, 771 cm–1; 1H NMR (CDCl3, 400 MHz): δ 7.11 (t, J = 7.8 Hz, 1H), 6.74 (d, J = 7.8 Hz, 1H), 6.67 (d, J = 7.8 Hz, 1H), 3.82 (s, 3H), 3.70 (s, 3H), 3.27–3.12 (m, 2H), 2.98 (dd, J = 4.9, 17.6 Hz, 1H), 2.82–2.52 (m, 4H), 2.39 (t, J = 11.5 Hz, 1H), 2.28–2.14 (m, 3H), 1.74–1.61 (m, 1H), 1.60–1.46 (m, 2H), 1.37–1.28 (m, 1H), 0.91 (t, J = 7.3 Hz, 3H); 13C NMR (CDCl3, 50 MHz): δ 174.5, 156.7, 136.3, 126.3, 124.2, 121.2, 107.0, 60.2, 55.2, 55.1, 54.2, 51.7, 41.6, 36.7, 34.7, 34.6, 30.5, 17.5, 11.9; HRMS (ESI) m/z: calcd for C19H28O3N [M + H]+, 318.2064; found, 318.2065.

rac-Methyl-(3S,4aR,10aR)-6-methoxy-1-propyl-1,2,3,4,4a,5,10,10a-octahydrobenzo[g]quinoline-3-carboxylate (15)

To a cooled (−78 °C) solution of diisopropylamine (0.22 mL, 1.56 mmol, 3.1 equiv) in anhydrous n class="Chemical">THF (5 mL), n-BuLi (1.6 M in hexane, 0.95 mL, 1.51 mmol, 3 equiv) was added dropwise and the reaction mixture was allowed to reach to 0 °C and stirred for 30 min. It was again cooled to −40 °C and a solution of compound 14 (160 mg, 0.50 mmol, 1 equiv) in anhydrous THF (5 mL) was added dropwise. After stirring for 1 h at −40 °C, trimethylsilyl chloride (0.135 mL, 1.05 mmol, 2.1 equiv) was added dropwise and the reaction mixture was stirred for another 1 h at −40 °C. After completion of the reaction, the reaction mixture was poured on the ice-cold solution of 1 N HCl (5 mL), followed by the addition of 1 N Na2CO3 (10 mL) solution. The reaction mixture was then extracted with EtOAc (3 × 50 mL). The combined organic layer was washed with brine, dried over anhydrous Na2SO4, and filtered, and the solvent was evaporated under reduced pressure. The obtained residue was purified by silica gel (230–400 mesh) column chromatography using EtOAc–PE (10:90) to yield the N-propyl ester 15 (136 mg, 85%) as a white solid. Rf: 0.5 (EtOAc–PE = 20:80); mp 96–98 °C; IR (CHCl3): νmax 1718, 1263, 775 cm–1; 1H NMR (CDCl3, 200 MHz): δ 7.09 (t, J = 7.8 Hz, 1H), 6.72 (d, J = 7.8 Hz, 1H), 6.66 (d, J = 7.8 Hz, 1H), 3.82 (s, 3H), 3.71 (s, 3H), 3.47 (d, J = 11.5 Hz, 1H), 3.13 (dd, J = 4.9, 16.0 Hz, 1H), 2.98 (dd, J = 4.6, 17.1 Hz, 1H), 2.85–2.52 (m, 3H), 2.47–2.28 (m, 3H), 2.26–2.01 (m, 2H), 1.98–1.73 (m, 1H), 1.58–1.40 (m, 2H), 1.38–1.20 (m, 1H), 0.90 (t, J = 7.3 Hz, 3H); 13C NMR (CDCl3, 50 MHz): δ 174.4, 156.7, 136.7, 126.1, 124.6, 121.3, 106.8, 61.5, 55.2, 54.5, 53.6, 51.6, 39.9, 34.8, 34.2, 32.3, 30.7, 18.2, 11.8; HRMS (ESI) m/z: calcd for C19H28O3N [M + H]+, 318.2064; found, 318.2065.

rac-(3S,4aR,10aR)-6-Methoxy-1-propyl-1,2,3,4,4a,5,10,10a-octahydrobenzo[g]quinoline-3-carboxamide (4)

To a stirred solution of compound 15 (120 mg, 0.38 mmol, 1 equiv) in MeOH (5 mL) in a sealed tube, n class="Chemical">CaCl2 (88 mg, 0.79 mmol, 2.1 equiv) was added followed by NH3 in MeOH (1:1, 5 mL) and the reaction mixture was stirred at 80 °C for 24 h. The solvents were then evaporated under reduced pressure, the obtained residue was treated with saturated solution of NaHCO3 and extracted with EtOAc (3 × 50 mL). The combined organic layer was washed with brine, dried over anhydrous Na2SO4, and filtered, and the solvent was evaporated under reduced pressure. The obtained residue was purified by silica gel (230–400 mesh) column chromatography using MeOH–EtOAc (1:99) to yield the amide 4 (103 mg, 90%) as a white solid. Rf: 0.3 (MeOH–EtOAc = 1:99); mp 179–181 °C; IR (CHCl3): νmax 3465, 1665, 1216, 758 cm–1; 1H NMR (CDCl3, 200 MHz): δ 8.63 (d, J = 4.3 Hz, 1H), 7.10 (t, J = 8.0 Hz, 1H), 6.72 (d, J = 8.0 Hz, 1H), 6.65 (d, J = 8.0 Hz, 1H), 6.02 (d, J = 4.3 Hz, 1H), 3.80 (s, 3H), 3.34–3.07 (m, 2H), 3.03–2.53 (m, 4H), 2.53–2.01 (m, 5H), 2.00–1.70 (m, 1H), 1.70–1.34 (m, 3H), 0.93 (t, J = 7.3 Hz, 3H); 13C NMR (CDCl3, 50 MHz): δ 178.5, 156.6, 135.9, 126.2, 124.2, 121.0, 107.0, 61.2, 55.1, 54.8, 53.3, 41.0, 35.0, 34.0, 33.8, 30.4, 18.3, 12.0; HRMS (ESI) m/z: calcd for C18H27O2N2 [M + H]+, 303.2067; found, 303.2069.

rac-(3S,4aS,10aR)-6-Methoxy-1-propyl-1,2,3,4,4a,5,10,10a-octahydrobenzo[g]quinolin-3-amine (16)

To a stirred solution of PIFA (0.171 g, 0.39 mmol, 1.5 equiv) in acetonitrile–water (deionized) (10 mL, 1:1 v/v) in a round-bottomed flask covered with aluminum foil, amide 4 (80 mg, 0.26 mmol, 1 equiv) was added at room temperature. Stirring was continued at room temperature for 12 h. The reaction mixture was then concentrated under reduced pressure, the obtained residue was dissolved in water (10 mL), acidified with concentrated HCl (3 mL), and extracted with Et2O. The ether layer was washed with 10% aq HCl, the combined aqueous layers were basified with NaHCO3, extracted with EtOAc (3 × 50 mL), and concentrated under reduced pressure. The obtained residue was purified by silica gel (230–400 mesh) column chromatography using MeOH–CHCl3 (10:90) to yield the pure compound 16 (60 mg, 82% yield) as a thick yellow liquid. Rf: 0.2 (MeOH–CHCl3 = 10:90); IR (CHCl3): νmax 3362, 2958, 1588, 1253, 1076, 766 cm–1; 1H NMR (CDCl3, 500 MHz): δ 7.08 (t, J = 7.8 Hz, 1H), 6.71 (d, J = 7.8 Hz, 1H), 6.64 (d, J = 7.8 Hz, 1H), 5.03 (br s, 2H), 3.79 (s, 3H), 3.37 (br s, 1H), 3.11 (dd, J = 4.8, 16.2 Hz, 1H), 3.01 (d, J = 11.8 Hz, 1H), 2.89 (dd, J = 4.8, 17.4 Hz, 1H), 2.75–2.64 (m, 2H), 2.56–2.44 (m, 2H), 2.24 (dt, J = 5.0, 10.5 Hz, 1H), 2.15–2.04 (m, 2H), 1.95–1.81 (m, 1H), 1.52–1.33 (m, 3H), 0.87 (t, J = 7.4 Hz, 3H); 13C NMR (CDCl3, 125 MHz): δ 156.7, 136.4, 126.2, 124.4, 121.3, 106.9, 60.9, 56.6, 55.2, 54.4, 46.2, 37.0, 34.6, 32.1, 30.5, 17.5, 11.9; HRMS (ESI) m/z: calcd for C17H27N2O [M + H]+, 275.2118; found, 275.2120.

rac-N,N-Diethyl-N′-(3S,4aS,10aR)-6-methoxy-1-propyl-1,2,3,4,4a,5,10,10a-octahydrobenzo[g]quinolin-3-sulfamide (17)

To a stirred solution of amine 16 (30 mg, 0.11 mmol, 1 equiv) in n class="Chemical">CHCl3 (5 mL), triethylamine (0.08 mL, 0.55 mmol, 5 equiv) and diethylsulfamoyl chloride (47 mg, 0.27 mmol, 2.5 equiv) were added and the mixture was stirred for 12 h at 50 °C. Ice and 1 N NaHCO3 (2 mL) were added and stirred continuously for 10 min at room temperature. The mixture was extracted with CH2Cl2 (3 × 10 mL). The combined organic layer was washed with brine, dried over anhydrous Na2SO4, and filtered, and the solvent was evaporated under reduced pressure. The obtained residue was purified by silica gel (230–400 mesh) column chromatography using MeOH–CH2Cl2 (1:99) to yield the pure product 17 (32 mg, 71% yield) as a white solid. Rf: 0.5 (MeOH–CH2Cl2 = 1:99); mp 86–88 °C; IR (CHCl3): νmax 3383, 3019, 1215, 1105, 757 cm–1; 1H NMR (CDCl3, 400 MHz): δ 7.10 (t, J = 7.9 Hz, 1H), 6.71 (d, J = 7.9 Hz, 1H), 6.66 (d, J = 7.9 Hz, 1H), 3.80 (s, 3H), 3.68 (br s, 1H), 3.28 (q, J = 6.7 Hz, 4H), 3.14 (dd, J = 3.7, 15.3 Hz, 2H), 2.95 (d, J = 14.0 Hz, 2H), 2.86 (br s, 1H), 2.67 (br s, 2H), 2.48 (br s, 1H), 2.23–2.03 (m, 3H), 1.57 (br s, 2H), 1.39 (t, J = 11.6 Hz, 1H), 1.18 (t, J = 7.0 Hz, 6H), 0.94 (t, J = 7.0 Hz, 3H); 13C NMR (CDCl3, 100 MHz): δ 156.6, 135.2, 126.5, 123.7, 121.1, 107.1, 61.7, 56.5, 55.2, 54.3, 47.9, 41.7 (2C), 36.5, 33.4, 31.9, 30.2, 17.3, 13.7 (2C), 11.6; HRMS (ESI) m/z: calcd for C21H36N3O3S [M + H]+, 410.2472; found, 410.2477.

rac-N,N-Diethyl-N′-(3S,4aS,10aR)-6-hydroxy-1-propyl-1,2,3,4,4a,5,10,10a-octahydrobenzo[g]quinolin-3-sulfamide (3) Hydrochloride

To a stirred solution of ethanethiol (1 mL) in dry n class="Chemical">CH2Cl2 (1 mL), aluminum chloride (49 mg, 0.36 mmol, 5 equiv) was added at 0 °C. The resulting solution was warmed to room temperature and compound 17 (30 mg, 0.07 mmol, 1 equiv) was added under stirring. After being stirred for 12 h, the reaction mixture was poured into water, basified with 1 N NaHCO3, and extracted with CH2Cl2 (3 × 5 mL). The combined organic layer was washed with brine (5 mL), dried over anhydrous Na2SO4, filtered, and concentrated under reduced pressure. The obtained residue was passed through a short bed of silica gel (230–400 mesh) column using MeOH–CHCl3 (10:90) and solvent was evaporated under reduced pressure. The obtained product was dissolved in CH2Cl2 (2 mL) and precipitated by addition of HCl in Et2O. The precipitated product was filtered and washed with CH2Cl2 to yield pure quinagolide 3 (19 mg, 66%) as hydrochloride. mp 228–232 °C (lit. mp 234–236 °C);[3a]1H NMR (DMSO-d6, 400 MHz): δ 10.08 (br s, 1H), 9.52 (br s, 1H), 7.72 (br s, 1H), 6.95 (t, J = 8.0 Hz, 1H), 6.64 (d, J = 8.0 Hz, 1H), 6.60 (d, J = 8.0 Hz, 1H), 3.66 (br s, 1H), 3.32–3.07 (m, 11H), 2.86 (d, J = 14.6 Hz, 1H), 2.22–2.12 (m, 2H), 1.98 (d, J = 12.2 Hz, 1H), 1.68 (br s, 3H), 1.10 (t, J = 8.0 Hz, 6H), 0.95 (t, J = 8.0 Hz, 3H); 13C NMR (DMSO-d6, 100 MHz): δ 154.4, 133.9, 126.7, 121.1, 119.3, 112.1, 62.1, 54.2, 53.5, 46.5, 41.2 (2C), 33.6, 29.8, 29.77, 29.7, 15.9, 13.7 (2C), 11.0; HRMS (ESI) m/z: calcd for C20H34N3O3S [M]+, 396.2315; found, 396.2320.