Synthesis and evaluation of the structural elements in alkylated tetrahydroisoquinolines for binding to CNS receptors

Abstract

Diseases of the CNS are often complex and involve multiple receptor systems and thus, the treatment options for these diseases must focus on targeting the multiple receptors implicated in the various disorders. Schizophrenia and depression are examples of such diseases and their pharmacotherapy thus depends on agents which target multiple receptors including the dopamine, serotonin and even cholinergic receptors at the same time. In our previous campaign to find multi-receptor ligands, we have identified the benzothiazole 1a as an initial lead molecule. In the current work, we have expanded the structure affinity relationship (SAFIR) of 1a resulting in the identification of a partially restrained butyrophenone 3j as a potent and selective dual 5-HT_1A and 5-HT₇ receptor ligand. It is expected that compound 3j may serve as a new lead for further development in our search for newer and novel ligands with the potential to treat diseases of CNS origin.

Graphical abstract

1. Introduction

Over the years, it has become evident that pharmacotherapy of major CNS diseases such as depression, bipolar disorder, schizophrenia and anxiety disorders rely on drugs which target multiple CNS receptors simultaneously.^{1, 2} For instance, the superior efficacy and improved side effect profiles of atypical antipsychotics such as lurasidone, ziprasidone and aripiprazole, have been attributed to their broad spectrum of activities involving dopaminergic, serotonergic and even cholinergic neurotransmission.³ In the same way, antidepressants such as vilazodone, that target the reuptake of serotonin (5-HT) along with the 5-HT_1A receptor are known to be fast acting, efficacious and tolerable.^{4, 5} However, a more defined combination of pharmacological activities at these and other targets is desirable for such agents to offer optimum therapeutic benefits in treating diseases of CNS origin.

It is now well established that targeting the D₂-like receptors (D₂, D₃ and D₄, antagonists), 5-HT_1A (agonists), 5-HT_2A (antagonists) and 5-HT₇ (antagonists) are desirable features in the pharmacotherapy for schizophrenia.^{6, 7} On the other hand, antidepressants may benefit from targeting the serotonin transporter (SERT), along with 5-HT_1AR (agonist) and 5-HT₇R (antagonist) for an improved profile.^8-10 And now with the introduction of the D₂R partial agonist and functionally selective aripiprazole as a well-tolerated and effective antipsychotic, the drug development paradigm for schizophrenia has significantly shifted in a new and exciting direction.¹¹ The caveat for multiple receptor targeting has been that it may also lead to off-target activities that may culminate in unforeseen side effects. Therefore, as part of our drug design strategy, there is also a focus on evaluating synthetic compounds at culprit receptors including the 5-HT_2B, receptors associated with valvular heart disease,¹² and the 5-HT_2C and H₁ weight gain and sedation side effects.^13-14

N-alkylated tetrahydroisoquinolines have been at the center of discussion recently as key ligands for certain CNS receptors associated with major brain disorders.^15-17 We have previously reported that the tetrahydroisoquinoline (THIQ) moiety, appropriately substituted with arylalkyl groups such as benzothiazole alkyl groups or halobutyrophenones could produce agents that provide differential binding profiles at clinically relevant CNS receptors including serotonin (5-HT) and dopamine (DA) receptor subtypes.^{18, 19} In this manuscript, we further explore changes in three segments (A, B and C) of compound 1a (Figure 1) in order to understand the structure-affinity relationships (SAFIRs) associated with this lead molecule.

Figure 1.

Structures of the benzothiazole hit compounds depicting the various segments of modification.

2. Chemistry

Compounds 1a and 1c were previously reported.¹⁸ Compound 1b was prepared by coupling 2-(3-chloropropyl)benzo[d]thiazole (A) previously reported by us^18,20-21 (Figure 2) to 1,2,3,4-tetrahydro-isoquinoline (THIQ) under the general alkylation condition B (4.1.2) that used K₂CO₃ as the base, KI as a catalyst and either acetonitrile (CH₃CN) or dimethoxyethane (DME) as the refluxing solvent. Alkylating agent C was synthesized according to a modified literature method²² outlined in Scheme 1 and was reacted with THIQ to afford compound 1d (Scheme 2). Compound 1e was prepared in a similar manner as 1b except that 8-chloro-1,2,3,4-tetrahydroisoquinoline was used in place of THIQ, and B was the alkylating agent. The alkylating agent D was obtained using a similar approach as was used to obtain alkylating agents A and B and was reacted with the various amines (THIQ, aromatic substituted THIQs and decahydroisoquinoline) as depicted in Scheme 2 to afford the corresponding compounds 1f-i.

Figure 2.

Alkylating agents used in the syntheses of compounds in group 1 (1a – i).

Scheme 1.

Synthesis of alkylating agent C. Reagents and conditions: 5N HCl, reflux.

Scheme 2.

Synthesis of compounds in Group 1. Reagents and conditions: i) K₂CO₃ (Et₃N for 1d), KI, DME, CH₃CN, or DMF (for 1d), reflux or rt (for 1d), 12-18h.; ii) ethereal HCl or HBr.

To synthesize the indene 2a and the 1,2-dihydronaphthalene analog, 2b in group 2, the alkylating agents 14 and 15 were prepared following a four-step procedure (Scheme 3). First, the commercially available indanone 6 and the α-tetralone 7 were separately refluxed with glyoxylic acid in an aqueous acid in a cross-aldol condensation reaction to produce the α,β-unsaturated ketones 8 and 9 respectively. The α,β-unsaturated keto function in 8 and 9 was then reduced using a palladium-carbon catalyzed hydrogenation reaction to afford the corresponding keto-acids 10 and 11 which were subsequently converted to the alcohols 12 and 13 under reductive conditions using LiAlH₄. The primary hydroxyl group was converted to an iodo group under Appel reaction conditions.²³ Interestingly, the Appel reaction also led to the generation of a styrene-like double bond seen in intermediates 14 and 15 (Scheme 3).

Scheme 3.

Synthesis of the indene and dialin derivatives of THIQ. Reagents and conditions: i) glyoxylic acid, H₂SO₄-H₂O (1:4), dioxane, reflux; ii) Pd/C (H₂), 40 psi, 48h; iii) LiAlH₄, toluene/ether, reflux ; iv) PPh₃, I_2, imidazole, DCM; v) K₂CO₃, KI, CH₃CN, reflux.

The bis-p-chlorobenzene alkylating agent 19 used to prepare compound 2c was serendipitously isolated in a previous attempt to form intermediate 17 from a reaction of the commercially available Grignard reagent 16 and 4-chlorobutyryl chloride (Scheme 4). A plausible mechanism to explain the formation of this product is that the intended product 17 underwent further 1,2 addition of the Grignard reagent (4-chlorophenyl)magnesium chloride (16) to the carbonyl function to generate the tertiary alcohol 18 that dehydrated in the presence of MgCl₂ acting as a Lewis acid (catalyst) to produce 19. The isolated alkylating agent 19 was then coupled to THIQ under the general alkylation condition B to afford 2c (Scheme 4). In Scheme 5, the dimethylglutarimide analogs 2d and 2e were obtained through a simple two-step reaction. Commercially available dimethylglutarimide 20 was N-alkylated using dibromobutane and the resulting alkylbromide 21 coupled separately to THIQ and isoindoline to afford compounds 2d and 2e respectively (Scheme 5).

Scheme 4.

Synthesis of bis-p-chlorophenyl analog of THIQ. Reagents and conditions: i) 4-chlorobutyryl chloride, dry THF, rt; ii) THIQ, K₂CO₃, KI, DME; iii) ethereal HBr.

Scheme 5.

Synthesis of dimethylglutarimide analogs of THIQ. Reagents and conditions: i) 1,4-dibromobutane, CH₃CN, reflux; ii) amine (THIQ for 2d, and isoindoline for 2e), K₂CO₃, CH₃CN, reflux; iii) ethereal HCl.

Compounds 3c-f were obtained via a one-step N-alkylation with the commercially available 4-chloro-1-(4-chlorophenyl)butane-1-one,.22 of the various amines (tetrahydroquinoline and tetrahydrobenzazepines) as depicted in Scheme 6. To prepare compound 3g, oxindole 23 was acylated under Friedel-Crafts acylation conditions to obtain the ketone 24 which was subsequently used to react with THIQ to obtain compound 3g (Scheme 7). Preparation of compounds 3c-g and 3i utilized microwave heating (general alkylation method A) that led to reduced reaction time (up to 60 min) and higher yields compared to conventional heating (general alkylation method B) (24-48 h). Compounds 3h-j were prepared by N-alkylating THIQ using the indanone alkylating agents 25-27 previously reported by our lab (Scheme 8).^{20, 21}

Scheme 6.

Synthesis of p-chlorobutyrophenone analogs. Reagents and conditions: i) appropriate amine, K₂CO₃, KI, DME, MW; ii) ethereal HCl.

Scheme 7.

Synthesis of the oxindole analog of THIQ. Reagents and conditions: i) 4-chlorobutyryl chloride, AlCl₃, CS₂, 0°-rt; ii) THIQ, K₂CO₃, KI, DME, MW.

Scheme 8.

Synthesis of indanone analogs of THIQ. Reagents and conditions: i) THIQ, K₂CO₃, KI, toluene, MW (for 3i) or reflux (for 3h and 3j); ii) ethereal HCl for 3i and 3j.

3. Results and discussion

2-(4-(3,4-Dihydroisoquinolin-2(1H)-yl)butyl)benzo[d]thiazole (compound 1a) was previously reported¹⁸ and serves as a lead compound for modification to better understand the structure affinity relationship (SAFIR) associated with binding to key CNS receptors. The binding affinity data of compound 1a at the D₂-like receptors (Ki : D₂ = 167 nM, D₃ = 8.7 nM and D₄ = 67 nM), 5-HT_1A (Ki = 10 nM) and the 5-HT₇ receptor (Ki = 22 nM) are reported in Table 1. To this end, we embarked on an SAFIR study to explore the effect of structural changes in the benzothiazole moiety (segment A), the alkyl linker (segment B), and the tetrahydroisoquinoline moiety (segment C) of 1a on binding affinity at the various CNS receptors. This study led to the generation of three structural types of compounds, the benzothiazoles, cycloalkyl/cycloalkylamines and butyrophenone analogs classified as group 1, 2, and 3 agents respectively, which will be the focus of this discussion.

Table 1.

Group 1 analogs and their binding affinity constants at clinically relevant CNS receptors

Compd	Structure	Ki (nM) (pKi±SEM)
		D2	D3	D4	5-HT1A	5-HT2A	5-HT7	5-HT2B	5-HT2C	H1
1a*		167 (6.78±0.09)	8.7 (8.5±0.1)	67.0 (7.17±0.05)	10.0 (8±0.03)	1681 (5.77±0.06)	22.0 (7.66±0.06)	18.0 (7.74±0.05)	910 (6.04±0.04)	583 (6.23±0.08)
1b		MTA	80.0 (7.1±0.10)	41.0 (7.39±0.04)	95.0 (7.02±0.03)	2051 (5.69±0.06)	37.0 (7.43±0.07)	18.0 (7.74±0.04)	1182 (5.93±0.05)	822 (6.09±0.08)
1c*		990 (6±0.04)	259 (6.59±0.06)	141 6.85±0.07	111 (6.95±0.07)	>10,000	211 (6.68±0.08)	135 (6.87±0.06)	MTA	762 (6.12±0.08)
1d		915.0 6.09±0.09	2176 5.66±0.07	127.0 6.9±0.07	65.0 7.19±0.07	>10K	91.0 7.04±0.04	MTA	>10K	290 ± 40
1e		272 (6.57±0.08)	31.0 (7.52±0.04)	129 (6.89±0.08)	29.0 (7.53±0.08)	258 (6.6±0.10)	45.0 (7.35±0.04)	239 (6.62±0.08)	675 (6.17±0.09)	293 (6.53±0.08)
1f		138 (6.86±0.08)	447 (6.35±0.07)	572 (6.24±0.09)	15.0 (7.84±0.07)	702 (6.2±0.10)	25.0 (7.6±0.09)	16.0 (7.81±0.07)	759 (6.12±0.08)	247 (6.61±0.06)
1g		853 (6.07±0.08)	91.0 (7.04±0.05)	279 (6.55±0.08)	42.0 (7.38±0.08)	483 (6.3±0.1)	144 (6.84±0.04)	327 (6.49±0.09)	820 (6.09±0.09)	560 (6.25±0.07)
1h		1087 (5.96±0.08)	526 (6.28±0.08)	37.0 (7.43±0.06)	11.0 (7.93±0.06)	1144 (5.9±0.10)	44.0 (7.36±0.07)	64.0 (7.19±0.09)	501 (6.3±0.07	268 (6.57±0.06)
1i		418 (6.38±0.09)	1083 (5.97±0.07)	272 (6.57±0.07)	9.9 (8±0.05)	667 (6.2±0.1)	67.0 (7.17±0.06)	7.0 (8.15±0.08)	329 (6.48±0.08)	327 (6.49±0.08)

MTA = Missed 50% of threshold inhibition.

^*Binding affinity data from ¹⁸

Shortening the butyl linker in segment B of 1a to a three-carbon chain (1b), led to a lower affinity for all the DA and 5-HT receptors investigated except at the D₄ receptor where a moderate increase in affinity was observed. Replacing the THIQ ring in 1a with isoindoline (1c) also resulted in decreased binding affinity to all the receptors under consideration. Compound 1d was prepared to explore the effect of replacing benzothiazole moiety (segment A) of compound 1b with benzimidazole on binding affinity. The binding data suggest that the benzothiazole was preferred at all the receptors evaluated. Thus, the data on compounds 1a -d suggests a benzothiazole with a 4-carbon spacer attached to THIQ is preferred for the DA and 5-HT receptors explored.

Compound 1e, with an 8-chloro substitution on segment C of 1a did not result in significant changes in binding affinities suggesting substitution on the THIQ ring is tolerated at least at the 8-position. Similarly, a 5-chloro substitution on the benzothiazole moiety (1f) was tolerated at all the receptors except for the D₃ and D₄ receptors. However, substituting the same substituents simultaneously on the benzothiazole and the THIQ moieties (1g) resulted in diminished affinities for all the receptors. Compound 1h was synthesized to explore the need for the aromatic ring in THIQ for binding to the receptors under consideration. While the D₂ and D₃ receptors suffered significant reductions in affinity, the 5-HT_1A, 5-HT_2A, 5-HT_2C and 5-HT₇ were essentially unaffected. Similarly, introducing the 9,10-dimethoxy group on 1f to form 1i, ill-affected binding affinity to the D₂-like receptors but not the serotonin receptors under consideration. It is worth noting that none of the benzothiazole analogs displayed high affinity Ki values for the "culprit" receptors H₁ and 5-HT_2C which have been associated with metabolic and sedative side effects.^12-14 On the other hand, there is high affinity binding and significant variability in the binding to the 5-HT_2B receptors with a range of 7.0 – 327 nM.

To further evaluate the structural requirements for segment A/B binding affinity, we synthesized four analogs, 2a – d and the binding affinity constants are reported in Table 2. Compounds 2a and 2b can be viewed as partially restricted butyl spacers using cyclopentene and cyclohexene rings. Both compounds displayed diminished binding affinity for the D₂R and showed no definitive trends at other clinically relevant receptors. Compound 2c, 2d and 2e generally showed no significant binding affinities for any of the receptors under consideration.

Table 2.

Compounds in group 2 and their binding affinity constants at relevant CNS receptors

Compd	Structure	Ki(nM)* (pfi ± SEM)
		D2	D3	D4	5-HT1A	5-HT2A	5-HT7	5-HT2B	5-HT2C	H1
2a		MTA	88.0 (7.05±0.09)	194 (6.71±0.04)	1,109 (5.96 ±0.08)	1,867 (5.73±0.04)	132 (6.88±0.07)	149 (6.83±0.07)	MTA	MTA
2b		5,647 (5.25±0.06)	184 (6.73±0.04)	140 (6.85±0.05)	138 (6.86 ±0.04)	975 (6.01 ±0.09)	65.0 (7.18 ±0.07)	126 (6.9±0.04)	2,903 (5.54 ±0.07)	1437 (5.84 ±0.07)
2c		MTA	150 (6.8 ±0.1)	MTA	MTA	MTA	497 (6.3±0.07)	MTA	2,028 (5.69±0.05)	998 (6 ± 0.07)
2d		MTA	1,645 (5.8 ± 0.1)	MTA	MTA	MTA	MTA	MTA	MTA	MTA
2e		2,933 (5.53±0.09)	>10,000	>10,000	339 (6.47±0.07)	>10,000	739 (8.2 ± 0.1)	531 (6.28±0.07)	305 (6.53±0.07)	3,867 (5.41±0.07)

MTA = Missed 50% of threshold inhibition.

Compounds with the butyrophenone alkyl group (group 3) are analogs of either compound 3a or 3b previously reported by us.¹⁹ Inspired by the promising binding affinity profiles of these butyrophenones at the relevant CNS receptors (Table 3), we embarked on an exploration to further understand the SAFIR of these compounds. To begin with, we sought to understand the role of the position of the nitrogen in segment C. The THIQ moiety in 3b was replaced with tetrahydroquinoline and 2,3,4,5-tetrahydro-1H-benzo[b]azepine to produce compounds 3c and 3d respectively, resulting in the formation of aromatic nitrogen atoms in both analogs. Significantly, this change resulted in no apparent receptor binding affinity at the selected CNS receptors. This observation will lead one to speculate that an aliphatic nitrogen atom with a higher pKa is more desirable in these compounds for binding to the receptors. Alternatively, the positioning of the nitrogen atom proximal to the phenyl ring may have prevented optimal interaction with the complementary functional group at the receptors. Compounds 3e and 3f were synthesized to further explore the above trains of thought. Thus, moving the nitrogen away from a direct interaction with the phenyl ring resulted in a minor improvement in the binding potencies at the DA and 5-HT receptor subtypes when compared to 3c and 3d but fell way short of the original affinities seen with 3b. Replacing the 4-chlorophenyl group with an oxindole bicyclic moiety to form 3g produced no significant improvements in binding affinity at the various receptors.

Table 3.

Compounds in group 3 and their binding affinity at clinically relevant CNS receptors

Comp d	Structure	Ki(iM) (pKi ± SEM)
		D2	D3	D4	5-HT1A	5-HT2A	5-HT7	5-HT2B	5-HT2C	H1
3a**		49.0 ±3.0	72.0 ±5.0	2.3 ±0.2	19.5	21.0	381	519	>10,000	86.3±7.3
3b**		126 (6.9±0.06)	17.0 (7.77±0.04)	86.0 (7.07±0.04)	8.2 (8.09±0.07)	MTA	3.6 (8.45±0.07)	232 (6.63±0.07)	2,976 (5.53±0.06)	597 (6.22±0.05)
3c		MTA	MTA	MTA	MTA	MTA	MTA	MTA	>10,000	MTA
3d		MTA	MTA	MTA	MTA	MTA	MTA	MTA	MTA	MTA
3e		3,578 (5.45±0.08)	342 (6.47±0.04)	442 (6.35±0.04)	1,712 (5.77±0.07)	120 (6.92±0.03)	257 (6.59±0.05)	501 (6.3±0.05)	348 (6.46±0.04)	3,224 (5.5±0.1)
3f		1,170 (5.93±0.08	106 (6.97±0.04)	82 (7.09±0.04)	214 (6.67±0.06)	281 (6.55±0.03)	138 (6.68±0.05)	549 (6.26±0.05)	611 (6.21±0.07)	1,087 (6±0.3)
3g		5,399 (5.27±0.09)	66 (7.18±0.04)	3,335 (5.48±0.04)	122 (6.91±0.07)	1,022 (5.99±0.03)	127 (6.9±0.05)	483 (6.32±0.05)	4,234 (5.37±0.09)	MTA
3h		750 (6.12±0.07)	223 (6.65±0.04)	251 (6.6±0.06)	19.0 (7.73±0.04)	1,204 (5.92±0.09)	1.6 (8.8±0.06)	294	MTA	ND
3i		2,583.5	866.0	1,102	48.3	391.3	265	391	1,184 (5.93±0.06)	24 (7.6±0.1)
3j		946 (6.02±0.08)	783 (6.11±0.07)	50 (7.3±0.06)	16 (7.81±0.06)	748 (6.1±0.1)	0.5 (9.33±0.06)	109 (6.96±0.07)	128 (6.89±0.08)	553 (6.26±0.06)
	aSB269970	ND	ND	ND	<5	<5	1.3 8.9±0.1	5	<5	ND
	bCompound 18	ND	ND	ND	219±11	ND	7±2	ND	ND	ND
	cCompound 3p	ND	ND	ND	70±12	ND	4.5±1	ND	ND	ND

MTA = Missed 50% of threshold inhibition, ND = Not determined.

^*Ki values without the associated SEM, are within 20% of the mean value.

^**Binding affinity data from reference 19.

^aBinding affinity data from reference 28.

^bBinding affinity data from reference 29;

^cBinding affinity data from reference 30

Compounds 3h-3j were the partially restrained analogs of the butyrophenone analogs 3a or 3b. Restricting the keto group into an indanone led to some rather interesting observations as reported in Table 3. First, compound 3h, the restrained analog of 3a, records over 15-fold decrease in potency at the D₂R (Ki = 750 nM), compared to 3a (Ki = 49 nM), retained activity at the 5-HT_1A (Ki = 19 nM) and an awe-inspiring low nanomolar binding affinity at the 5-HT₇ receptor (Ki = 1.6 nM). This is of biological significance because of the paucity of selective dual 5-HT_1A and 5-HT₇ receptor ligands in the literature.²⁴ Also, a growing body of knowledge suggests that the 5-HT₇ receptor controls normal circadian rhythm, sleep, mood, memory and learning, and cognition^25-27 and may therefore serve as a plausible target for treating neuropsychiatric disorders such as schizophrenia and mood disorders. Having obtained 3h as a possible lead, we investigated the effect of removing the fluoro group in 3h to produce compound 3i. This resulted in over 165-fold decrease in binding affinity at the 5-HT₇ receptor which suggested that a halo-phenyl moiety may be required for this 5-HT₇ affinity. Further confirmation of this thought was observed by replacing the fluoro atom with a chloro atom to form compound 3j with a sub-nanomolar binding affinity constant (Ki = 0.5 nM) at the 5-HT₇ receptor. Compound 3j compares favorably with the binding affinity of SB269970 (pKi = 1.3 nM), the selective 5-HT₇ antagonist, albeit it has a dual binding affinity profile.²⁸ Similarly, when compared to the most potent analogs from modifications to the lead compound, UCM-5600²⁹ and a recently reported arylindole series,1-(naphthyl)indole derivative 3p,³⁰ compound 3j is about 9 to14-fold more potent at the 5-HT₇ receptor and over 4 to 10-fold higher affinity at the 5-HT_1A receptor. Overall, the binding affinity constants of the indanone analogs fared poorly at the D₂-like receptors as well as the culprit 5-HT receptors.

In conclusion, beginning with compound 1a as an initial lead molecule of the benzothiazole series, we have obtained several alkylated THIQ analogs with potent and desirable multi-receptor binding features especially at the 5-HT_1A and 5-HT₇ receptors. The 5-chloro-indanone analog 3j, displaying low nanomolar and a sub-nanomolar affinity values at the 5-HT_1A and the 5-HT₇ receptors respectively, is an addition to the rather scarce group of dual 5-HT_1A and 5-HT₇ receptor selective ligands in the literature that can be used to probe the role of these receptors in treating the affective and cognitive diseases of CNS origin.

4. Experimental

Melting points of final compounds were determined on a Gallenkamp (UK) apparatus and are reported as uncorrected. All NMR spectra were obtained on a Varian 300 MHz Mercury Spectrometer and the free induction decay (FID) data were processed using Mestrelab’s Mnova NMR software (version 8.1) to obtain the reported NMR data. Elemental analyses were carried out by Atlantic Microlab, Inc., Norcross, GA, and are within 0.4% of theory unless otherwise stated. Flash chromatography was performed using CombiFlash^® with Davisil grade 634 silica gel. Starting materials were obtained from Sigma–Aldrich or Matrix scientific and were used without further purification. All microwave assisted syntheses (MW) were carried out using a Biotage Initiator^®.

4.1.1 General alkylation procedure A

A mixture of alkylating agent (1 equiv), appropriate amine (1.1 equiv) K₂CO₃ (1.1 equiv), and KI (catalytic) in DME or CH₃CN (10 mL) was placed in a 20 mL microwave vial (for MW) with a stirrer and tightly sealed. The mixture was subjected to microwave (MW) heating at 120 °C for 60 mins. The resulting crude mixture was directly purified on silica gel by flash chromatography (gradient up to 70% EtOAc in hexanes) to afford the final compounds. The free base where necessary, was converted to the HCl or HBr salt and crystallized out of a mixture of MeOH-Et₂O.

4.1.2 General alkylation procedure B

A mixture of alkylating agent (1 equiv), appropriate amine (1.1 equiv) K₂CO₃ (1.1 equiv), and KI (catalytic) in DME or CH₃CN (50 mL) was placed in a round bottomed flask with a stirrer was heated to reflux on a heating plate for 24-28 h. The reaction was monitored by TLC for product formation. After reaction was complete, the resulting crude mixture was directly purified on silica gel by flash chromatography (gradient up to 70% EtOAc in hexanes) to afford the final compounds. The free base where necessary, was converted to the HCl or HBr salt and crystallized out of a mixture of MeOH-Et₂O.

4.2. Synthesis of Compounds

4.2.1. 2-(3-(3,4-Dihydroisoquinolin-2(1H)-yl)propyl)benzo[d]thiazole hydrobromide, 1b

Previously reported alkylating agent 2-(3-chloropropyl)benzo[d]thiazole¹⁸ was reacted with THIQ under the general alkylation conditions B (4.1.2) described above to produce the hygroscopic compound 1b as the HBr salt in 20% yield. ¹H NMR (DMSO-d₆): 9.96 (brs, 1H), 8.08 (d, J = 7.5 Hz, 1H), 7.93 (d, J = 7.5 Hz, 1H), 7.50 (t, J = 7.5 Hz, 1H), 7.42 (t, J = 7.5 Hz, 1H), 7.20-7.30 (m, 4H), 4.61 (d, J = 13.8 Hz, 1H), 4.34 (dd, J = 7.8, 15.6 Hz, 1H), 3.74-3.79 (m, 1H), 3.34-3.42 (m, 3H), 3.23-3..28 (m, 2H), 3.02-3.18 (m, 2H), 2.30-2.40 (m, 2H). Anal. calcd for C₁₉H₂₂Br₂N₂S: C 48.53, H 4.72, N 5.96. Found: C 48.61, H 4.72, N 5.89.

4.2.2. 2-(3-Chloro-propyl)-1H-benzoimidazole, D

To a mixture of 1,2-diaminobenzene,4 (0.5g, 4.6 mmol) and 4-chlorobutanoic acid,5 (0.86 g, 7 mmol) in a schlenk tube was added 5N HCl solution (25 mL) and heated to boil for 5 h. The reaction mixture was then cooled and added to water (25 mL). The precipitate obtained was filtered and vacuum dried to give a white solid (1.25 g, 56%) which was used in the next step without further purification.

4.2.3. 2-(3-(1H-benzo[d]imidazol-2-yl)propyl)-1,2,3,4-tetrahydroisoquinoline, 1d

A mixture of 2-(3-chloro-propyl)-1H-benzoimidazole, D, (1.2 g, 6.15 mmol), THIQ (1.2 g, 9.0 mmol), KI (100 mg) and Et₃N (4 mL, 28.5 mmol) in DMF (5 mL) was stirred for 56 h at room temperature (rt). The mixture was diluted with EtOAc (200 mL), washed with brine (3 × 50 mL). The organic layer was dried over Na₂SO₄, and filtered. The filtrate was concentrated in vacuo to dry and followed by column chromatography on silica gel to afford 2-[3-(1H-benzoimidazol-2-yl)-propyl]-1,2,3,4-tetrahydro-isoquinoline, 1d as the HCl salt (0.32 g ,14%) and crystallized from MeOH-Et₂O mixture. Mp: 234-235 °C; ¹H NMR (DMSO-d₆): 7.77 (m, 2H), 7.50 (m, 2H), 7.22 (m, 4H), 4.45 (brs, 2H), 3.52 (brs, 2H), 3.34 (m, 4H), 3.15 (brs, 2H), 2.52 (m, 2H). Anal calcd for C₁₉H₂₃Cl₂N₃•0.3H₂O: C 61.72, H 6.27, N 11.37. Found: C 61.40, H 6.48, N 11.35.

4.2.4. 2-(4-(8-Chloro-3,4-dihydroisoquinolin-2(1H)-yl)butyl)benzo[d]thiazole hydrochloride, 1e

Using the previously reported alkylating agent 2-(4-chlorobutyl)-benzo[d]thiazole,¹⁸ the amine 8-chloro-1,2,3,4-tetrahydroisoquinoline was N-alkylated under the general alkylation reaction condition B, described above to produce compound 1e as a white HCl salt in 45% yield. Mp: 192-194 °C. ¹HNMR (300 MHz, DMSO-d₆): δ 11.01 (brs, 1H), 8.19 (d, J = 7.8 Hz, 1H), 8.07 (d, J = 7.8 Hz,1H), 7.62-7.54 (m, 3H), 7.45 (t, J = 6.9 Hz, 1H), 7.35 (d, J = 7.8 Hz, 1H), 5.05-4.95 (m, 1H), 4.69 (d, J = 1.5 Hz, 1H), 4.47-4.40 (m, 1H), 3.89-3.85 (m, 1H), 3.48-3.32 (m, 4H), 3.22 (t, J = 5.1 Hz, 2H), 2.05 (s, 4H). Anal. calcd for; C₂₀H₂₂Cl₂N₂S•0.9.H₂O: C 58.65, H; 5.41, N; 6.84, Found; C; 58.43, H; 5.81, N; 6.36.

4.2.5. Alkylating agent, 5-Chloro-2-(4-chlorobutyl)benzo[d]thiazole, C

Using similar cyclization reaction procedure previously described by us,¹⁸ 2-amino-4-chlorobenzenethiol was reacted with 5-chloropentanoyl chloride in toluene at rt to afford alkylating agent C (see ref¹⁸ for details). ¹H NMR (300 MHz, CDCl₃): δ 7.85 (d, J = 2.1 Hz, 1H), 7.75 (d, J = 8.1 Hz, 1H), 7.34(dd, J = 1.8 Hz, 6.6 Hz, 1H), 3.59 (t, J = 6.6 Hz, 2H), 3.15 (t, J = 7.2 Hz, 2H), 2.08-2.01 (m, 2H), 1.96-1.89(m, 2H).

4.2.6. 5-Chloro-2-(4-(3,4-dihydroisoquinolin-2(1H)-yl)butyl)benzo[d]thiazole hydrochloride, 1f

Alkylating agent C was reacted with THIQ under the general alkylation condition B described above to afford compound 1f as the HCl salt in 75% yield. Mp: 229-231 °C, ¹H NMR (300 MHz, DMSO-d₆): δ 10.68 (s, 1H), 8.10 (d, J = 8.4 Hz, 1H), 8.00 (d, J = 1.5 Hz,1H), 7.47-7.44 (dd, J = 2.1, 8.4 Hz, 1H), 7.27-7.17 (m, 4H), 4.52-4.47 9d, J = 15.6 Hz, 1H), 4.28-4.21 (dd, J = 7.8, 15.3 Hz, 1H), 3.70-3.64 (m, 1H), 3.37 (s, 2H), 3.24-3.16 (m, 4H), 3.02-2.97 (m, 1H), 1.96-1.89 (m, 4H). Anal. calcd for C₂₀H₂₂Cl₂N₂S: C 61.07, H 5.64, N 7.12. Found: C 60.89, H 5.65, N 6.94.

4.2.7. 5-Chloro-2-(4-(5-chloro-3,4-dihydroisoquinolin-2(1H)-yl)butyl)benzo[d]thiazole hydrochloride, 1g

Using method B of the general alkylation reaction condition, 8-chloro-1,2,3,4-tetrahydroisoquinoline was N-alkylated with alkylating agent C to afford compound 1g as a white solid HCl salt in 73% yield. Mp: 218-220°C, ¹H NMR (300 MHz, DMSO-d₆): δ 11.70 (brs, 1H), 8.08 (d, J = 8.7 Hz, 1H,), 7.99 (d, J = 2.1 Hz, 1H), 7.47-7.42, (m, 2H), 7.30 (t, J = 7.8 Hz, 1H), 7.19 (d, J = 7.8Hz, 1H), 4.55-4.5 (m, 1H), 4.32-4.24 (m, 1H), 4.02-3.97 (m, 2H), 3.36-3.18 (m, 4H), 3.06 (t, J = 5.1Hz, 2H), 1.89 (s, 4H). Anal. calcd for C₂₀H₂₁Cl₃N₂S•0.24.H₂O: C 55.58, H 4.90, N 6.48. Found: C 55.57, H 5.10, N 6.19.

4.2.8. 5-Chloro-2-(4-(octahydroisoquinolin-2(1H)-yl)butyl)benzo[d]thiazole hydrochloride, 1h

General alkylation method B was used. Alkylating agent C was reacted with decahydroisoquinoline to afford compound 1h in 85% yield. Mp: 141-143°C. ¹H NMR (300 MHz, DMSO-d₆): δ (9.93 (s, 1H), 8.11-8.08 (d, J = 8.4 Hz, 1H), 8.00 (d, J = 1.8 Hz, 1H), 7.47-7.44 (dd, J = 2.1, 11.4 Hz, 1H) 3.50-3.30 (m, 4H), 3.25 (d, J = 10.2 Hz 1H), 3.18-3.13 (t, J = 7.2 Hz, 2H), 3.06-3.300 (m, 2H), 2.90-2.80 (m, 1H), 1.86-1.78 (m, 4H), 1.70-1.66 (m, 2H), 1.60-1.43 (m, 4H), 1.24-1.18 (t, J = 10.2 Hz, 2H), 1.00-0.87 (t, J = 12.0 Hz, 2H). Anal. calcd for C₂₀H₂₈Cl₂N₂S•0.95.H₂O: C 57.67, H 6.78, N 6.73. Found: C 57.66, H 7.08, N 6.51.

4.2.9. 5-Chloro-2-(4-(6,7-dimethoxy-3,4-dihydroisoquinolin-2(1H)-yl)butyl)benzo[d]thiazole hydrochloride, 1i.

Compound 1i in its HCl salt form was prepared similarly to 1h above using 6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline as the amine in a 79% yield. Mp: 180-182°C, ¹H NMR (300 MHz, DMSO-d₆): δ 10.60 (s, 1H), 8.09 (d, J = 9.0 Hz, 1H), 8.00 (d, J = 1.8 Hz, 1H), 7.47-7.44 (dd, J = 2.1, 8.4 Hz, 1H), 6.78 (d, J = 8.4Hz, 2H), 3.80-3.74 (m, 5H), 3.30-3.16 (m, 5H), 1.90-1.86 (m, 4H). Anal. calcd for C₂₀H₂₈Cl₂N₂S•0.32EtOAc: C 54.91, H 5.45, N 5.82. Found: C 54.76, H 5.84, N 5.54.

4.2.10. (E)-2-(1-oxo-1H-inden-2(3H)-ylidene)acetic acid, 8

A mixture of 1-indanone, 6 (3 g, 22.7 mmol), glyoxylic acid (50% aqueous solution, 5.9 g, 54.5 mmol), and conc. H₂SO₄ (0.74 mL) in dioxane (5 mL) were stirred at refluxing temperature for 12 h. The mixture was cooled, the product filtered off, washed with water and dried to give the acid (E)-2-(1-oxo-1H-inden-2(3H)-ylidene) acetic acid (3.68 g, 86.2%) as a white solid. Mp: 202–204 °C (lit. mp 205–206 °C), ¹H NMR (300 MHz, DMSO-d₆): δ 12.00 (brs, 1H), 7.73–7.80 (m, 2H), 7.68 (d, J = 7.7 Hz, 1H), 7.49 (t, J = 7.9 Hz, 1H), 6.55 (t, J = 2.4 Hz, 1H), 4.08 (d, J = 1.8, 2H).

4.2.11. (E)-2-(1-oxo-3,4-dihydronaphthalen-2(1H)-ylidene)acetic acid, 9

Intermediate 9 was prepared similarly to 8 above using α-tetralone instead of 1-indanone (Scheme 3). The crude product obtained after filtration was used for the next step without further purification. Yield (3.5 g, 84%). ¹H NMR (300 MHz, DMSO-d₆): 12.96 (brs, 1H), 7.95-7.92 (m, 1H), 7.83-7.57(m, 1H), 7.43-7.38(m, 2H), 6.65-6.64(m, 1H), 3.31-3.27(m, 2H), 2.98(t, J = 6.6 Hz, 2H).

4.2.12. 2-(1-oxo-2,3-dihydro-1H-inden-2-yl)acetic acid, 10

(E)-2-(1-oxo-1H-inden-2(3H)-ylidene) acetic acid, 8 (10 g, 53 mmol) in MeOH (45 mL) and dioxane (150 mL) with Pd/C (10%, 1 g) was stirred under H₂ (40 psi) for 48 h. The mixture was filtered through celite and the solvent evaporated to give 2-(1-oxo-2,3-dihydro-1H-inden-2-yl)acetic acid (7) as an off-white solid. Mp 85–88°C, ¹H NMR (300 MHz, DMSO-d₆): δ 8.47 (br s, 1 H, enol OH ), 7.08–7.18 (m, 4 H, H-4, H-5, H-6, H-7), 2.99–3.06 (m, 2 H, H-1, H-3), 2.69–2.74 (m, 1 H, H-2), 2.53–2.60 (m, 2 H, H-1, H-3), 2.48 (d, J = 7.4 Hz, 2 H, CH₂CO₂).

4.2.13. 2-(1-hydroxy-3,4-dihydronaphthalen-2-yl)acetic acid, 11

Intermediate 11 was prepared similarly to 10 above using (E)-2-(1-oxo-3,4-dihydronaphthalen-2(1H)-ylidene)acetic acid(9) as the precursor (Scheme 3). ¹H NMR (300 MHz, DMSO-d₆): δ 12.13 (brs, 1H), 7.85(d, J = 8.1Hz, 1H), 7.56-7.5(m, 1H), 7.35-7.31(m, 2H), 3.13-3.02(m, 1H), 2.95(m, 2H), 2.74-2.66(m, 1H), 2.44-2.37(m, 1H), 2.17-2.09(m, 1H), 2.0-1.85(m, 1H).

4.2.14. 2-(2-hydroxyethyl)-2,3-dihydro-1H-inden-1-ol, 12

A solution of 2-(3-Hydroxy-1H-inden-2-yl)acetic acid (3.4 g, 19.8 mmol) in dry THF (100 mL) was added dropwise to a suspension of LiAlH₄ (1.5 g, 39.6 mmol) in dry THF (50 mL) at 0 °C and the resulting mixture was stirred at refluxing temperature for 12 h. EtOAc was added to quench excess LiAlH₄ and then aqueous HCl solution (10%, 50 mL) was added and the organic fraction separated. The aqueous solution was extracted with EtOAc (3 × 50 mL), and the combined organic fraction dried and the solvent evaporated to give alcohol 2-(2-hydroxyethyl)-2,3-dihydro-1H-inden-1-ol (2.18 g) as a yellow oil which was used for the next step without further purification. ¹H NMR (300 MHz, CDCl₃): δ 7.43-7.35(m, 1H), 7.26-7.18(m, 3H), 4.9(br d, J = 6.6 Hz, 1H), 3.91-3.73(m, 2H), 3.4(s, 1H), 3.1-2.92(m, 1H), 2.6-2.46(m, 1H), 2.29-2.2(m, 1H), 1.95-1.86(m, 2H).

4.2.15. 2-(2-hydroxyethyl)-1,2,3,4-tetrahydronaphthalen-1-ol, 13

Synthesis of intermediate 13 followed the same procedure as 12 above and was used for the next step without further purification.

4.2.16. 2-(2-iodoethyl)-1H-indene, 14

A solution of triphenylphosphine (5.28 g, 20.2 mmol) and imidazole (1.37 gm, 20.2 mmol) in CH₂Cl₂ was cooled to 0 °C, and iodine (5.09 g, 20.15 mmol) was added. The mixture was stirred for 30 min and then a CH₂Cl₂ solution (20 mL) of the crude 2-(2-hydroxyethyl)-1H-inden-3-ol, 12 (2.18 gm, ~13.43 mmol) obtained above was added in a dropwise manner. The reaction mixture was stirred for 12 h at rt, filtered, the organic layer washed with H₂O and then by aqueous sodium thiosulfate (50 mL), H₂O 950 mL) and brine 950 mL). The organic layer was dried over sodium sulfate, excess solvent removed under reduced pressure and the residue purified on combiflash column using EtOAc/hexane (1:9) as eluent to afford 2-(2-iodoethyl)-1H-indene (14) as a brown solid. ¹H NMR (300 MHz, CDCl₃): δ 7.4(d, J = 7.5 Hz, 1H), 7.32(d, J = 7.5 Hz, 1H), 7.26-7.21(m, 1H), 7.17-7.11(m, 1H), 6.62(s,1H), 3.4-3.35(m, 4H), 3.11-3.07(m, 2H)

4.2.17. 3-(2-iodoethyl)-1,2-dihydronaphthalene, 15

Under the same Appel reaction conditions described for 14 above, the alkylating agent 15 was prepared in 26% yield using 2-(2-hydroxyethyl)-1,2,3,4-tetrahydronaphthalen-1-ol,13 as the precursor. ¹H NMR (300 MHz, CDCl₃): δ 7.15-7.1(m, 3H), 7.03-7.0(m, 1H), 6.28(s, 1H), 3.31(t, J = 7.8 Hz, 2H), 2.86-2.75(m, 4H), 2.27(t, J = 8.4, 2H).

4.2.18. 2-(2-(1H-inden-2-yl)ethyl)-1,2,3,4-tetrahydroisoquinoline, 2a

Using method B, the alkylating agent 2-(2-iodoethyl)-1H-indene (11) was used to alkylate THIQ (section 4.1.2) to afford compound 2a as a white solid in 29% yield. Mp: 87-89 °C, ¹H NMR (300 MHz, CDCl₃): δ 7.38 (d, 1H, J = 7.2 Hz), 7.29-7.19 (m, 2H), 7.15-7.02 (m, 5H), 6.59 (s, 1H), 3.74 (s, 2H), 3.38 (s, 2H), 2.95 (t, J = 6.0 Hz, 2H), 2.83 (s, 6H). Anal. calc for C₂₀H₂₁N: C 87.23, H 7.69, N 5.09; Found: C 86.97, H 7.74, N 4.99.

4.2.19. 2-(2-(3,4-Dihydronaphthalen-2-yl)ethyl)-1,2,3,4-tetrahydroisoquinoline, 2b

Using the alkylating agent 12, THIQ was alkylated under the general alkylation method B to produce compound 2b as a hygroscopic solid in 35% yield, ¹H NMR (300 MHz, CDCl₃): δ 7.18-6.96 (m, 8H), 6.29(s, 1H), 3.7(s, 2H), 2.93(t, J = 6 Hz, 2H), 2.86-2.77(m, 4H), 2.76-2.68(m, 2H), 2.51(t, J = 8.7 Hz, 2H), 2.31(t, J = 8.1 Hz, 2H). ¹³C NMR (75 MHz, CDCl₃): δ 140.18, 134.74, 134.39, 134.23, 128.66, 127.18, 126.6, 126.42, 126.2, 126.12, 125.6, 125.38, 123.4, 56.92, 56.16, 51.02, 35.41, 29.14, 28.15, 27.58. Anal. calcd. for C₂₁H₂₃N : C 87.15, H 8.01, N 4.84 ; Found: 87.04, 7.96, 4.78.

4.2.20. 4,4'-(4-Chlorobut-1-ene-1,1-diyl)bis(chlorobenzene), 19

To a solution of 4-chlorobutyryl chloride (5 mL, 44 mmol) in dry THF (50 mL) was added dropwise to a solution of 4-chlorophenylmagnesium bromide (100 mL, 1.0 M in Et₂O, 100 mmol) at −5 °C in 1 hr. After addition was complete, the reaction mixture was stirred at rt overnight, and then quenched with saturated NH₄Cl solution followed by extraction with EtOAc (400 mL). The organic layer was separated and washed with brine (2 × 200 mL), then dried with anhydrous Na₂SO₄ and filtered. The filtrate was concentrated in vacuo, and the residue purified directly on silica gel using flash chromatography to give the pure product, 4,4'-(4-chlorobut-1-ene-1,1-diyl)bis(chlorobenzene) (19), 9.3 g, yield 68%. ¹H NMR (300MHz, CDCl₃): 7.36 (d, J = 6.0 Hz, 2H), 7.24 (d, J = 6.6 Hz, 2H), 7.12 (d, J = 6.0 Hz, 2H), 7.09 (d, J = 6.0 Hz, 2H), 6.10 (t, J = 7.2 Hz, 1H), 3.57 (t, J = 6.6 Hz, 2H), 2.56 (m, 2H).

4.2.21. 2-(4,4-Bis(4-chlorophenyl)but-3-en-1-yl)-1,2,3,4-tetrahydroisoquinineol hydrobromide, 2c

Compound 2c was prepared by reacting the alkylation agent 4,4'-(4-chlorobut-1-ene-1,1-diyl)bis(chlorobenzene) (19) and THIQ under the general alkylation B conditions to afford 2c in 56% yield. Mp 215-216 °C ,¹H NMR (DMSO-d₆): 9.69 (brs,1H), 7.51 (d, J = 8.1 Hz, 2H), 7.38 (d, J = 8.1 Hz, 2H), 7.15-7.28 (m, 8H), 6.18 (t, J = 7.5 Hz, 1H), 4.46-4.50 (m, 1H), 4.23-4.31 (m ,1H), 3.61-3.66 (m, 1H), 3.30-3.38 (m, 3H), 3.04-3.09 (m, 2H), 2.53-2.58 (m, 2H). Anal. calcd for C₂₅H₂₄BrCl₂N: C 61.37, H 4.94, N 2.86. Found: C 61.33, H 5.05, N 2.95.

4.2.22. 1-(4-bromobutyl)-4,4-dimethylpiperidine-2,6-dione, 21

A mixture of 4,4-dimethylpiperidine-2,6-dione, 20 (0.93 g, 5 mmol) and 1,4-dibromobutane (5.4 g, 25 mmol) was stirred under reflux in dry CH₃CN (20 mL) for 12 h. The reaction mixture was allowed to cool to room temperature and the excess solvent was removed under reduced pressure. The crude product obtained was directly purified on flash column chromatography (silica gel, ethyl acetate/light petroleum 1:3) to afford 1-(4-bromobutyl)-4,4-dimethylpiperidine-2,6-dione, (21) as a colorless oil which was used in the next stage without further purification. ¹H NMR (300 MHz, CDCl₃): δ 3.8(t, J = 7.2, 2H), 3.42(t, J = 6.6 Hz, 2H), 2.51(s, 4H), 1.91-1.82 (m, 2H), 1.73-1.62(m, 2H), 1.08(s, 6H).

4.2.23. 1-(4-(3,4-Dihydroisoquinolin-2(1H)-yl)butyl)-4,4-dimethylpiperidine-2,6-dione hydrochloride, 2d

Under the general alkylation method B described above, the alkylating agent 21 was reacted with THIQ to afford compound 2d as a highly hydroscopic HCl salt in 76% yield. ¹HNMR (300 MHz, DMSO-d₆): δ 9.82 (brs, 1H), 7.66-7.60 (m, 1H), 7.47-7.38 (m, 2H), 7.15(dd, J = 5.4, 10.8 Hz,1H), 3.59 (t, J = 7.4 Hz, 2H), 3.40 (s, 2H), 2.97 (s, 2H), 2.50 (s, 8H), 1.70-1.60 (m, 2H), 1.35 (t, J = 7.5 Hz, 2H), 9.95 (s, 6H). Anal. calcd for C₂₀H₂₉ClN₂O₂. 0.15 EtOAc: C; 63.53, H; 7.73, N; 7.41. Found: C; 63.41, H; 8.10, N; 7.02.

4.2.24. 1-(4-(Isoindolin-2-yl)butyl)-4,4-dimethylpiperidine-2,6-dione, 2e

Using isoindoline as the amine, compound 2e was prepared in its free base form similarly to compound 2d above in 23% yield. Mp: 84-85 °C, ¹HNMR (300 MHz, CDCl₃): δ 7.18 (s, 4H), 3.83 (s, 4H), 3.81-3.75 (t, J = 3.9 Hz, 2H), 2.76-2.71 (t, J = 6.9 Hz, 2H), 2.49 (s, 4H), 1.62-1.58 (t, J = 7.7 Hz, 4H), 1.06 (s, 6H). Anal calcd for C₁₉H₂₆N₂O₂•0.15 H₂O: C 71.96, H 8.26, N 8.83. Found: C 71.93, H 8.25, N 8.81.

4.2.25. Synthesis of compounds 3c-f

In general, compounds 3c-f were synthesized following the general alkylation method A described above (section 4.1.1) using the common alkylating agent 4-chloro-1-(4-chlorophenyl)butan-1-one (22) to obtain the respective final compounds as HCl salts, except for 3c which was obtained as a free base (Scheme 6).

4.2.25.1. 1-(4-Chlorophenyl)-4-(3,4-dihydroquinolin-1(2H)-yl)butan-1-one, 3c

Using 1,2,3,4-tetrahydroquinoline as the amine and 4-chloro-1-(4-chlorophenyl)butan-1-one(22) as the alkylating agent, compound 3c was produced as a white crystalline solid in 33% yield. Mp: 192-193 °C.¹H NMR (300 MHz, DMSO-d₆) δ 7.97 (d, J = 8.3 Hz, 2H), 7.59 (d, J = 8.2 Hz, 2H), 7.07 – 6.91 (m, 2H), 6.85 (d, 1H), 6.66 (d, 1H), 3.38 – 3.25 (m, 4H), 3.11 (t, J = 6.9 Hz, 2H), 2.71 (t, J = 6.2 Hz, 2H), 1.97 – 1.83 (m, 4H). ¹³C NMR (75MHz, DMSO-d₆) δ 198.54, 145.25, 139.46, 135.16, 129.44, 129.22, 128.92, 127.15, 122.34, 115.64, 110.65, 50.64, 49.49, 35.66, 28.15, 22.22, 20.87. Anal. calcd for C₁₉H₂₀ClNO: C, 72.72; H, 6.42; N, 4.46. Found: C, 72.48; H, 6.30; N, 4.35.

4.2.25.2. 1-(4-Chlorophenyl)-4-(2,3,4,5-tetrahydro-1H-benzo[b]azepin-1-yl)butan-1-one hydrochloride, 3d

Using 2,3,4,5-tetrahydro-1H-benzo[b]azepine as the amine and reacting it with 22, compound 3d was obtained as a white crystalline HCl salt. Yield: 35%, mp:192-193 °C. ¹H NMR (300 MHz, Methanol-d₄) δ 7.95 (d, J = 8.1 Hz, 2H), 7.68 (d, J = 7.3 Hz, 1H), 7.54 – 7.35 (m, 5H), 4.86 (t, J = 2.6 Hz, 2H), 4.03 – 3.44 (m, 4H), 3.32 – 3.02 (m, 4H), 2.36 – 1.79 (m, 4H). ¹³C NMR (75 MHz, CD₃OD) δ 197.63, 139.28, 138.80, 136.67, 134.94, 133.19, 130.46, 129.43, 128.59, 127.72, 123.75, 56.70, 52.39, 34.45, 33.51, 25.45, 23.68, 19.47. Anal. calcd for C₂₀H₂₃Cl₂NO: C, 65.94; H, 6.36; N, 3.84. Found: C, 65.66; H, 6.41; N, 3.74.

4.2.25.3. 1-(4-Chlorophenyl)-4-(4,5-dihydro-1H-benzo[c]azepin-2(3H)-yl)butan-1-one hydrochloride, 3e

Intermediate 22 was used to alkylate the amine 2,3,4,5-tetrahydro-1H-benzo[c]azepine to obtain compound 3e as a white solid crystal. Yield: 52%, mp: 201-202 °C. ¹H NMR (300 MHz, DMSO-d₆) δ 11.24 (s, 1H), 7.93 (d, J = 8.8 Hz, 2H), 7.56 (dd, J = 8.5 Hz, 2H), 7.42 (d, J = 7.3 Hz, 1H), 7.34 – 7.21 (m, 3H), 4.56 (d, J = 14.1 Hz, 1H), 4.38 (dd, J = 5.1, 14.1 Hz, 1H), 3.52 – 3.42 (m, 2H), 3.35 (s, 2H), 3.12 (td, J = 2.4, 6.9 Hz, 2H), 2.88 (t, J = 18.8 Hz, 2H), 2.02 (q, J = 7.6 Hz, 2H), 1.94 – 1.84 (m, 2H). ¹³C NMR (75 MHz, DMSO-d₆) δ 198.15, 143.47, 138.59, 135.44, 132.21, 130.48, 130.25, 129.93, 129.62, 129.24, 127.10, 56.85, 55.95, 35.66, 33.38, 22.40, 18.39. Anal. calcd for C₂₀H₂₃Cl₂NO: C, 65.94; H, 6.36; N, 3.84. Found: C, 65.67; H, 6.44; N, 3.72.

4.2.25.4. 1-(4-Chlorophenyl)-4-(4,5-dihydro-1H-benzo[d]azepin-3(2H)-yl)butan-1-one hydrochloride, 3f

Amine 2,3,4,5-tetrahydro-1H-benzo[d]azepine was reacted with 22 to produce compound 3f as a white solid crystal. Yield: 59%, mp: 240-242 °C. ¹H NMR (300 MHz, DMSO-d₆) δ 11.31 (s, 1H), 7.98 (dd, J = 8.3 Hz, 2H), 7.59 (dd, J = 8.4 Hz, 2H), 7.22 – 7.15 (m, 4H), 3.72 – 3.59 (m, 2H), 3.53 – 3.41 (m, 2H), 3.18 (dt, J = 6.6, 14.4 Hz, 4H), 2.95 (dt, J = 6.8, 16.9 Hz, 4H), 2.09 (q, J = 7.6 Hz, 2H).¹³C NMR (75 MHz, DMSO-d₆) δ 198.18, 139.80, 138.62, 135.52, 130.28, 129.53, 129.29, 127.40, 56.86, 53.70, 35.87, 31.16, 18.34. Anal. calcd for C₂₀H₂₃Cl₂NO: C, 65.94; H, 6.36; N, 3.84. Found: C, 65.83; H, 6.44; N, 3.90.

4.2.26. 5-(4-Chlorobutanoyl)indolin-2-one, 24

A modified acylation reaction described by Lackey et al.³¹ was followed to access intermediate 3b. Briefly, to a dry 100 mL round-bottomed flask equipped with a stirrer was added 5 g (37.5 mmol) of AlCl₃, 30 mL of carbon disulfide (CS₂), and 2.5 mL (22.5 mmol) of 4-chlorobutyryl chloride at 0 °C with stirring. To the mixture obtained was added 2 g (15 mmol) of oxindole (23) in a portionwise manner over 20 minutes. After the addition was completed, the reaction mixture was allowed to warm to rt and stirred overnight to produce a red precipitate. The content was dumped into a beaker containing 100g of ice with 5 mL conc. HCl and stirred thoroughly. The brick red precipitate obtained was dissolved in methanol and loaded onto silica column and subsequently separated by combiflash (gradient elution up to 50% EtOAc in hexanes) to afford 2.6 g (73%) of 5-(4-chlorobutanoyl)indolin-2-one, 24. ¹H NMR (300 MHz, DMSO-d₆) δ 10.75 (s, 1H), 7.84 (dd, J = 1.9, 8.2 Hz, 1H), 7.78 (s, 1H), 6.88 (d, J = 8.2 Hz, 1H), 3.68 (t, J = 6.7 Hz, 2H), 3.52 (s, 2H), 3.07 (t, J = 7.1 Hz, 2H), 2.03 (q, J = 6.9 Hz, 2H).¹³C NMR δ 197.83, 177.19, 148.82, 130.53, 129.41, 126.55, 124.52, 109.19, 45.39, 35.95, 35.16, 27.48.

4.2.27. 5-(4-(3,4-Dihydroisoquinolin-2(1H)-yl)butanoyl)indolin-2-one, 3g

Using the alkylating agent 24, THIQ was alkylated under the general alkylation method A condition to afford compound 3g as a free base in 59% yield. Mp: 166-168 °C. ¹H NMR (300 MHz, CDCl₃) δ 9.23 (s, 1H),7.84 (d, J = 4.7 Hz, 1H), 7.87 (d, J = 8.2 Hz, 1H), 7.16 – 6.96 (m, 4H), 6.85 (s, 1H), 3.61 (s, 2H), 3.51 (s, 2H), 3.02 (t, J = 7.2 Hz, 2H), 2.86 (t, J = 5.9 Hz, 2H), 2.75 (t, J = 5.9 Hz, 2H), 2.59 (t, J = 7.1 Hz, 2H), 2.04 (q, J = 7.2 Hz, 2H). ¹³C NMR (75MHz, CDCl₃) δ 199.01, 177.77, 146.77, 134.74, 134.31, 131.90, 129.38, 128.60, 126.55, 126.11, 125.57, 125.34, 124.63, 109.16, 57.34, 55.97, 50.78, 35.97, 35.90, 29.01, 21.92. Anal. Calcd for C₂₁H₂₂N₂O₂: C, 75.42; H, 6.63; N, 8.38; Found: C, 75.12; H, 6.74; N, 8.16.

4.2.28. 2-(2-(3,4-Dihydroisoquinolin-2(1H)-yl)ethyl)-5-fluoro-2,3-dihydro-1H-inden-1-one, 3h

Using the previously reported alkylating agent 2-(2-chloroethyl)-5-fluoro-2,3-dihydro-1H-inden-1-one (25),^{20, 21} THIQ was alkylated under alkylation method B to afford compound 3h in 23% yield. Mp 240-241 °C; ¹H NMR (DMSO-d₆): 7.73 (dd, J = 5.8, 8.4 Hz, 1H), 7.45 (d, J = 9.0 Hz, 1H), 7.24 (m, 5H), 4.52 (m, 2H), 4.28 (m, 2H), 3.68 (m, 1H), 3.34 (m, 6H), 2.99 (m,1H), 2.89 (m, 2H), 2.31 (m, 1H), 1.99 (m, 1H). Anal. calcd for C₂₀H₂₁ClFNO: C 69.46, H 6.12, N 4.05; Found: C 69.18, H 6.08, N 4.60.

4.2.29. 2-(2-(3,4-Dihydroisoquinolin-2(1H)-yl)ethyl)-2,3-dihydro-1H-inden-1-one hydrochloride, 3i

Under the general alkylation method B, the previously described alkylating agent 2-(2-chloroethyl)-2,3-dihydro-1H-inden-1-one (26)²⁰ was coupled to THIQ to afford compound 3i as the HCl salt in 65% yield. Mp: 201-203 °C. ¹H NMR (300 MHz, DMSO-d₆) δ 11.18 (s, 1H), 7.74 – 7.63 (m, 2H), 7.60 (d, J = 7.6 Hz, 1H), 7.48 – 7.40 (m, 1H), 7.29 – 7.15 (m, 4H), 4.49 (s, 1H), 4.31 (s, 1H), 3.68 (s, 1H), 3.40 (t, J = 8.4 Hz, 3H), 3.01 (s, 1H), 2.92 (d, J = 4.1 Hz, 1H), 2.89 – 2.77 (m, 2H), 2.41 – 2.28 (m, 1H), 1.99 (d, J = 9.8 Hz, 1H). ¹³C NMR (75 MHz, DMSO-d₆) δ 207.35, 154.03, 136.27, 135.65, 131.92, 128.97, 128.07, 128.00, 127.38, 127.04, 123.76, 53.85, 52.05, 49.10, 44.65, 32.59, 25.42. Anal. Calcd for C₂₀H₂₂ClNO·0.2H₂O: C, 72.47; H, 6.81; N, 4.23. Found: C, 72.55; H, 6.56; N, 4.29.

4.2.30. 5-Chloro-2-(2-(3,4-dihydroisoquinolin-2(1H)-yl)ethyl)-2,3-dihydro-1H-inden-1-one hydrochloride, 3j.

The previously reported alkylating agent 27²⁰ was reacted with THIQ under the general alkylation method B condition to afford compound 3j as the HCl salt in 41% yield. Mp 239-240 °C, ¹H NMR (DMSO-d₆): 11.10 9 (brs,1H), 7.72 (s,1H), 7.67 (d, J = 8.1 Hz, 1H), 7.49 (d, J = 8.1 Hz, 1H), 7.17-7.27 (m, 4H), 4.51 (d, J = 12.6 Hz, 1H), 4.25-4.32 (m,1H), 3.65-3.2 (m,1H), 3.23-3.42 (m, 5H), 2.92-3.02 (m,1H), 2.86-2.92 (m, 2H), 2.29-2.37 (m,1H), 1.95-2.05 (m,1H).Anal. calcd for C₂₀H₂₁Cl₂NO: C 66.30, H 5.84, N 3.87. Found: C 66.29, H 5.94, N 3.93.

5. Receptor binding studies

Binding affinities reported in Tables 1-3 were conducted by the National Institute of Mental Health Psychoactive Drug Screening Program (NIMH-PDSP). Details of the methods and radioligands used for the binding assays were previously reported.³²