Synthesis and Characterization of 1,4-Benzodioxane-6-Carboxylic Acid Amide Analogs

Nabil Idris¹, Alan J. Anderson², Oladapo Bakare^1*

¹Department of Chemistry, Howard University, Washington DC, USA.
²Department of Natural Sciences, Bowie State University, Bowie, USA.

DOI: 10.4236/ijoc.2022.123012                

Abstract

A library of new 1,4-benzodioxane-6-carboxylic acid amide analogs was designed and synthesized. These analogs were obtained in six steps from gallic acid. Firstly, esterification of the commercially available gallic acid in methanol in the presence of sulfuric acid afforded methyl 3,4,5-trihydroxybenzoate (9) in satisfactory yield. The ester 9 was then reacted with an excess of 1,2-dibromoethane in the presence of K₂CO₃ in acetone to furnish the 6,8-disubstituted-1,4-benzodioxane (10) in 45% yield. The reaction of 10 with various mercaptans gave the sulfide derivative 11, 12, and 13 in moderate yield. Subsequent hydrolysis of the methyl ester in 13 followed by conversion to the acid chloride and reaction of the acid chloride intermediate with different commercially available primary and secondary amines gave the amide analogs 18 - 32 with an average yield of 43%. Conversion of the sulfide group in Compound 23 to Sulfoxide 33 or Sulfone 34 was accomplished by reaction with either 30% H₂O₂/TeO₂ or 30% H₂O₂, respectively. The structures of the synthesized compounds were characterized using FTIR, ¹H-NMR, ¹³C-NMR, and high-resolution ESI-MS.

Keywords

Gallic Acid, 1, 4-Benzodioxane Derivatives, Prostate Cancer, Anticancer Drugs

1. Introduction

The 1,4-benzodioxane is an important scaffold in some biologically useful natural products including some lignin and neolignan natural products such as Silybin (1) and Purpurenol (2) (Figure 1). Hence, the 1,4-bezodioxane has not only

Figure 1. Chemical structure of Silybin (1), Purpurenol (2) and Doxazosin (3).

received attention for its synthesis, but more importantly, it has received significant attention in medicinal and pharmaceutical research. Some 1,4-benzodioxanes have demonstrated hepatoprotective, antioxidant, cytotoxic, and antimicrobial activities in addition to properties that suggest therapeutic applications in areas such as antidepressant, antihyperglycemic, and potential for the treatment of other diseases [1] - [7].

The 1,4-benzodioxane moiety is a significant part of doxazosin (3, marketed as CARDURA in the United States, Figure 1), a drug used in the treatment of hypertension and benign prostatic hyperplasia (BPH) [6] [7].

Various 1,4-benzodioxane derivatives have been shown to display notable anti-inflammatory and anticancer activities [3] [8] - [13]. For example, Vazquez, et al demonstrated that 2,3-dihydro-1,4-benzodioxin analog bearing an acetic acid substituent displayed anti-inflammatory activity. A study of the structure-activity relationships demonstrated that the relative position of the acetic acid group for optimum anti-inflammatory activities was at position-6 (4, Figure 2). The regioisomer with the acetic acid substituent at position-2 (Compound 5, Figure 2), showed mediocre anti-inflammatory activity [3] [4].

Of particular interest is the discovery of the 1,4-benzodioxane bisamide chemical probe, CCT251236 (6, Figure 3), which was reported as an HSF1 pathway inhibitor that showed growth inhibitory activities in the human ovarian carcinoma xenograft model [12]. The 1,4-benzodioxane moiety was found to be critical to the growth inhibitory activities of this compound and attempts to replace this moiety resulted in compounds with reduced or loss of growth inhibitory activity. Another benzodioxane derivative (7, Figure 3) was recently reported as a potent inhibitor of the physiologically important p38α MAPK pathway [13]. The p38α MAPK pathway is important in a number of physiological processes including cancer and chronic graft-versus-host disease which can present a risk factor in various types of transplants including transplants for certain cancer patients. Both

Figure 2. Chemical structure of (2,3-dihydrobenzo [1,4] dioxin-6-yl) acetic acid (4) and (2,3-dihydrobenzo [1,4] dioxin-2yl) acetic acid (5).

Figure 3. Chemical structure of 1,4-benzodioxane bisamides^12,13 (6) and (7).

benzodioxane derivatives, CCT251236 (6) and Compound 7, possess a benzamide moiety at position-6 of the 1,4-benzodioxane template.

In our continuous efforts to develop novel small organic molecules for studies on Metastatic Castration-Resistant Prostate Cancer (MCRPC), we designed some 1,4-benzodioxane derivatives with a benzamide moiety at position-6. This paper presents the synthesis and characterization of new 1,4-benzodioxane-6-carboxylic acid amide analogs from gallic acid as a readily available starting material as shown in Figure 4.

2. Results and Discussion

In our synthesis of the new 1,4-benzodioxane-6-carboxylic acid amide analogs, we found gallic acid to be an attractive, inexpensive, and readily available starting material that is perfectly suited with a carboxylic acid group to create the 6-substituted 1,4-benzodioxane with the potential to further functionalize position-8 of the benzodioxane scaffold for wider SAR studies. Consequently, the 1,4-benzodioxane derivatives were synthesized in six steps by chemical transformation of gallic acid as outlined in Figures 5-8.

Firstly, the Fischer esterification of the commercially available gallic acid in methanol with sulfuric acid readily afforded methyl 3,4,5-trihydroxybenzoate (9) in good yield [14]. The subsequent reaction of ester 9 with 1,2-dibromoethane in the presence of K₂CO₃ in acetone gave the 1,4-benzodioxane ring yielding methyl 8-(2-bromoethoxy)-2,3-dihydrobenzo[b][1,4]dioxine-6-carboxylate (10). The formation of Compound 10 was confirmed by the study of its ¹H- and ¹³C-NMR spectra. Furthermore, the molecular ion region of the mass spectrum (EI) confirmed the presence of the M⁺ and M + 2 peaks in about 1:1 ratio typical for compounds containing one bromine atom. To incorporate diversity at position-8

Figure 4. Chemical transformation of gallic acid to 1,4-benzodioxane moiety.

Figure 5. Schematic diagram for stepwise conversion of gallic acid to 8-alkoxy-1,4-benzodioxane-6-carboxylic acids.

Figure 6. Conversion of carboxylic acid analog 16 to 1,4-benzodioxane-6-carboxylic acid amide derivatives (18 - 31).

Figure 7. Reaction of emetine dihydrochloride with 1,4-benzodioxane-6-carbonyl chloride 17 to form an emetine-benzodioxane conjugate 32.

Figure 8. Oxidation of sulfide moiety in Compound 23 to form Sulfoxide 33 and Sulfone 34.

for SAR studies, we elected to incorporate a sulfur-containing group in position-8 based on other work in our group as well as the biological importance of sulfur-containing compounds [15] [16] [17]. Hence, benzodioxane derivative 10 was subsequently reacted with selected mercaptans to afford the three thia-analogs 11, 12 and 13. To accomplish our goal to synthesize a small library of the 1,4-benzodioxane-6-carboxylic acid amide analogs for initial bioassay screening, one of these, Compound 13, was reacted further as outlined in Figures 5-7. Incorporation of benzamide group at position-6 of Compound 13 was achieved by initial base-induced hydrolysis of the ester group in 13 to furnish the carboxylic acid 16 (Figure 5) followed by the conversion of 16 to the acid chloride 17 using oxalyl chloride, and the reaction of acid chloride 17 with various primary and secondary amines to obtain the 1,4-benzodioxane-6-carboxylic acid amide analogs 18 to 32 (as shown in Figure 6 and in Table 1) including the emetine derivative 32 (Figure 7). The relatively low yield of Compound 28 is attributed to the loss of substantial amount of this compound during several trituration steps because of difficulties in purification.

Table 1. Set of R (aliphatic and aromatic) groups in 6-substituted 1,4-benzodioxane analogs (18 - 31).

Upon purification by trituration or column chromatography on silica gel, the pure products were characterized by FTIR, ¹H-NMR, and ¹³C NMR spectroscopy as well as by high-resolution ESI-MS. Fourier transform infrared spectra of 1,4-benzodioxane analogs (18 to 32) is characterized by the presence of the amide C=O stretching vibration between 1626 and 1676 cm⁻¹, and the N-H stretching between 3275 - 3346 cm⁻¹ for Compounds 18 to 28. The ¹H-NMR spectra of the 1,4-benzodioxane-6-carboxylic acid amide analogs 18 to 32 displayed the proton chemical shift values of the two methylene groups sandwiched between the oxygen atoms in the 1,4-benzodioxane ring as multiplets between 4.25 and 4.30 ppm (for two protons each) consistent with literature values [9], while the protons of the methylene protons adjacent to the ether oxygen at position-8 appeared as a triplet at about 4.2 ppm. The methylene groups adjacent to the sulfur atom appeared as triplets at about 2.6 and 2.9 ppm [17]. In the C-13 NMR spectra of Compounds 18 to 32, the amide carbonyl carbon appeared at 164 ppm, while the sp³ carbons at positions 2 and 3 of the 1,4-benzodioxane ring could be seen at 64 ppm. On the other hand, the sp³ carbon atom adjacent to the ether oxygen at position-8 occurred at 69 ppm. All of Compounds 18 to 32 were studied by high-resolution ESI-MS to obtain exact masses consistent with each compound.

In order to study the effect of the oxidation state of the sulfur atom on the biological activities of one of these 1,4-benzodioxane analogs, Compound 23 was further converted to the Sulfoxide 33 and Sulfone 34 derivatives. The reaction of Compound 23 with excess 30% H₂O₂ afforded the Sulfone 34 in 74% yield, while the reaction of 23 with 30% H₂O₂ in the presence of TeO₂ furnished the Sulfoxide 33 in 64% yield without over oxidation to the sulfone (Figure 8) [18]. The spectra data of Compounds 33 and 34 including the high-resolution ESI-MS studies were consistent with the formation of sulfoxide and sulfone, respectively.

3. Conclusion

In this paper, we designed and synthesized a library of 1,4-benzodioxane-6-carboxylic acid amide analogs via a six-step reaction sequence. All the synthesized compounds were characterized by infrared spectroscopy, nuclear magnetic resonance spectroscopy, and high-resolution electrospray ionization mass spectrometry to confirm the structure of the compounds.

4. Experimental Section

General. Infrared (IR) spectra were obtained on a Perkin Elmer PE 100 spectrometer with an Attenuated Total Reflectance (ATR) window. ¹H- and ¹³C-NMRs were performed on the Bruker Avance 400 MHz spectrometer. All compounds were analyzed in deuterated chloroform (CDCl₃), deuterated dimethyl sulfoxide (DMSO-d6) or deuterated methanol (MeOD-d4). The chemical shifts are in δ units (ppm) from TMS (0.00 ppm), CDCl₃ (7.26 ppm), (CD₃)₂SO (2.52 ppm) or CD₃OD (3.31) as the internal standard for ¹H-NMR; and CDCl₃ (77.00 ppm), (CD₃)₂SO (39.50 ppm) or CD₃OD (49.00) for ¹³C-NMR. The apparent resonance multiplicity is described as s (singlet), br s (broad singlet), d (doublet), dd (doublet of doublets), t (triplet), q (quartet) and m (multiplet). High-resolution electrospray ionization mass spectrometry was recorded on Agilent Technologies 6224 TOF LCMS. The mass spectra showed signals corresponding to M + H, 2M + H, M + Na and 2M + Na. Additionally, the chlorinated and brominated derivatives showed corresponding M + 2 + H peaks. Thin layer chromatography (TLC) analysis was carried out on 5 × 20 cm plate coated with silica gel GF254 type 60 (25 - 250 mesh).

Methyl 3,4,5-trihydroxybenzoate (9)

To a stirred solution of gallic acid (508 mg, 2.99 mmol) in MeOH (20 mL), conc. H₂SO₄ (0.18 mL, 3.4 mmol) was added. The reaction mixture was refluxed for 2 h, and then evaporated. The residue was taken up in water and extracted with ethyl acetate three times. The combined organic phase was washed with brine, dried over anhydrous MgSO₄, filtered, and concentrated in vacuo to afford the methyl gallate as a white solid (495 mg, 90%). Mp 202˚C - 204˚C. IR (cm⁻¹): 3460, 3301, 3085, 3017, 2954, 1691, 1615, 1540, 1470, 1456, 1436, 1383, 1308, 1247, 1193, 1097, 1034, 1002, 917, 892, 866, 806, 765, 746, 733. ¹H NMR (400 MHz, MeOD) δ 3.80 (3H, s), 4.88 (3H, s), 7.03 (2H, s). ¹³C NMR (100 MHz, MeOD) δ 52.3 (1C), 110.0 (2C), 121.4 (1C), 139.7 (1C), 146.5 (2C), 169.0 (1C). MS (EI, M⁺) m/z 184.1 (100%)

Methyl 8-(2-bromoethoxy)-2,3-dihydrobenzo[b][1,4]dioxine-6-carboxylate (10)

To a stirred solution of methyl gallate (1.0532 g, 5.72 mmol) and potassium carbonate K₂CO₃ (3.7977 g, 27.48 mmol) in acetone (45 mL), 1,2-dibromoethane (2.40 mL, 27.85 mmol) in 5 mL acetone was added drop wise at RT. The reaction mixture was reflux for 18 hrs. The resulting mixture was cooled, filtered and the filtrate was evaporated. The residue was taken up in 25 mL CH₂Cl₂, washed with water (2 × 25 mL) and an aqueous NaOH (2N) solution (2 × 25 mL), and then water (2 × 25 mL) and brine (25 mL). The organic layer was dried over anhydrous MgSO₄, filtered and concentrated in vacuo to obtain a white solid (0.85 g, 47%). Mp 105˚C - 107˚C. IR (cm⁻¹): 2996, 2948, 2882, 1698, 1592, 1506, 1433, 1389, 1375, 1348, 1323, 1271, 1246, 1201, 1185, 1171, 1126, 1071, 1053, 1036, 1002, 977, 935, 903, 883, 872, 832, 786, 765, 726, 696. ¹H NMR (400 MHz, CDCl₃) δ 3.61 (2H, t, J = 6.4 Hz), 3.80 (3H, s), 4.20 - 4.22 (2H, m), 4.28 - 4.32 (4H, m), 7.11 (1H, d, J = 1.2 Hz), 7.22 (1H, d, J = 1.2 Hz). ¹³C NMR (100 MHz, CDCl₃) δ 28.5 (1C), 52.1 (1C), 63.9 (1C), 64.7 (1C), 68.9 (1C), 107.4 (1C), 113.0 (1C), 122.1 (1C), 137.9 (1C), 143.8 (1C), 146.9 (1C), 166.4 (1C). ESI-TOF-MS m/z: 317.0018/319.0001, ([C₁₂H₁₃^79/81BrO₅ + H]⁺ calcd. 317.0025/319.0005); 338.9852/340.9813, ([C₁₂H₁₃^79/81BrO₅ + Na]⁺ calcd. 338.9838/340.9824).

Methyl 8-(2-((2-hydroxyethyl)thio)ethoxy)-2,3-dihydrobenzo[b][1,4]dio-xine-6-carboxylate (11)

To a stirred solution of bromo derivative (10) (313 mg, 0.99 mmol) in N,N-dimethylformamide (4.0 mL) was added 2-mercaptoethanol (0.1 mL, 1.3 mmol) and potassium carbonate (268 mg, 1.93 mmol). The reaction was heated at 60˚C for 24 h, and then cooled to room temperature. The reaction was diluted with ethyl acetate (20 mL), and washed with water (5 × 20 mL), brine (20 mL), dried over anhydrous magnesium sulfate, and concentrated under vacuum to give a white paste material. The crude product was purified by column chromatography over silica gel using gradient elution. Hexane was employed to elute nonpolar impurities followed by elution with 20% to 35% of EtOAc in hexane to obtain the desired product (164 mg, 53%). IR (cm⁻¹): 3444, 2930, 2870, 1712, 1591, 1505, 1433, 1317, 1242, 1204, 1040, 999, 885, 764. ¹HNMR (400 MHz, CDCl₃): δ 2.85 (2H, t, J = 5.6 Hz), 2.99 (2H, t, J = 6.8 Hz), 3.80 (2H, t, J = 5.6 Hz), 3.88 (3H, s), 4.25 (2H, t, J = 6.8 Hz), 4.26 - 4.28 (2H, m), 4.35 - 4.38 (2H, m), 7.19 (1H, d, J = 1.6 Hz), 7.28 (1H, d, J = 2 Hz). ¹³C NMR (100 MHz, CDCl₃): δ 30.3 (1C), 35.7 (1C), 52.1 (1C), 60.9 (1C), 63.9 (1C), 64.7 (1C), 68.9 (1C), 106.7 (1C), 112.7 (1C), 122.1 (1C), 137.7 (1C), 143.8 (1C), 147.4 (1C), 166.6 (1C). ESI-TOF-MS m/z: 337.0662, ([C₁₄H₁₈O₆S + Na] ⁺ calcd. 337.0722).

Methyl 8-(2-(butylthio)ethoxy)-2,3-dihydrobenzo[b][1,4]dioxine-6-carb-oxylate (13)

To a stirred solution of bromo derivative (10) (0.6512 g, 2.05 mmol) in N,N-dimethylformamide (5.0 mL) was added butanethiol (0.3 mL, 2.67 mmol) and potassium carbonate (0.4824 g, 3.49 mmol). The reaction was heated at 60˚C for 24 h, and then cooled to room temperature. The reaction was diluted with ethyl acetate (20 mL), and washed with water (5 × 20 mL), brine (20 mL), dried over anhydrous magnesium sulfate, and concentrated under vacuum to give oily residue. The crude product was purified by column chromatography over silica gel using gradient elution. Hexane was employed to elute nonpolar impurities followed by elution with 20% to 35% of EtOAc in hexane to obtain the desired product (355 mg, 53%). IR (cm⁻¹): 2922, 1712, 1592, 1506, 1431, 1388, 1373, 1342, 1314, 1203, 1130, 1065, 1049, 1022, 1002, 964, 881, 759, 730, 699. ¹H NMR (400 MHz, DMSO-d6): δ 0.88 (3H, t, J = 7.2 Hz), 1.32 - 1.41 (2H, m), 1.45 - 1.56 (2H, m), 2.63 (2H, t, J = 7.20 Hz), 2.85 (2H, t, J = 6.8 Hz), 3.8 (3H, s), 4.15 (2H, t, J = 6.4 Hz), 4.25 - 4.32 (4H, m), 7.09 (1H, d, J = 2 Hz), 7.10 (1H, d, J = 2 Hz). ¹³C NMR (100 MHz, CDCl₃) δ 13.5 (1C), 21.3 (1C), 30.0 (1C), 31.2, (1C), 31.3 (1C), 52.0 (1C), 63.8 (1C), 64.2 (1C), 68.7 (1C), 106.2 (1C), 111.3 (1C), 121.1 (1C), 137.7 (1C), 143.6 (1C), 147.5 (1C), 165.7 (1C). ESI-TOF-MS m/z: 327.1262, ([C₁₆H₂₂O₅S + H]⁺ calcd. 327.1266).

8-(2-((2-hydroxyethyl)thio)ethoxy)-2,3-dihydrobenzo[b][1,4]dioxine-6-carboxylic acid (14)

A solution of 11 (126.1 mg, 0.40 mmol) and NaOH (2N, 10 mL) in methanol (10 mL) was refluxed for 8 h. The reaction mixture was concentrated under vacuum and the resulting mixture acidified with conc. HCl to give a white precipitate which was filtered under suction, washed with water and allowed to dry in a vacuum oven to obtain a white solid (77.6 mg, 64%). Mp: 148˚C - 150˚C. IR (cm⁻¹): 3500 - 2400, 3414, 2959, 2883, 2621, 1682, 1591, 1507, 1434, 1393, 1352, 1315, 1263, 1211, 1160, 1123, 1101, 1040, 986, 963, 919, 883, 846, 820, 760, 731, 705, 689. ¹H NMR (400 MHz, DMSO-d6): δ 2.68 (2H, t, J = 6.8 Hz), 2.89 (2H, t, J = 6.4 Hz), 3.56 (2H, q, J = 5.6 and 12.4 Hz), 4.14 (2H, t, J = 6.8 Hz), 4.25 - 4.26 (2H, m), 4.28 - 4.30 (2H, m), 4.79 (1H, t, J = 5.2 Hz), 7.07 (1H, d, J = 1.6 Hz), 7.10 (1H, d, J = 1.6 Hz), 12.76 (1H, br s). ¹³C NMR (100 MHz, CDCl₃): δ 29.6 (1C), 30.4 (1C), 61.0 (1C), 63.8 (1C), 64.2 (1C), 68.6 (1C), 106.5 (1C), 111.5 (1C), 122.3 (1C), 137.4 (1C), 143.5 (1C), 147.4 (1C), 166.8 (1C). ESI-TOF-MS m/z: 323.0564, ([C₁₃H₁₆O₆S + Na]⁺ calcd. 323.0565).

8-(2-((6-hydroxyhexyl)thio)ethoxy)-2,3-dihydrobenzo[b][1,4]dioxine-6-carboxylic acid (15)

To a stirred solution of bromo derivative (10) (1.1596 g, 3.66 mmol) in N,N-dimethylformamide (4.0 mL) was added 6-mercaptohexanol (0.65 mL, 4.75 mmol) and potassium carbonate (859.1 mg, 6.22 mmol). The reaction was heated at 60˚C for 24 h, and then cooled to room temperature. The reaction was diluted with ethyl acetate (20 mL), and washed with water (5 × 20 mL), brine (20 mL), dried over anhydrous magnesium sulfate, and concentrated under vacuum to give a white paste material (12). Since purification was difficult at this step, the residual white paste (492.3 mg, 1.5661 mmol) and NaOH (2N, 15 mL) in methanol (15 mL) was refluxed for 8 h. The reaction mixture was concentrated under vacuum and the resulting mixture acidified with conc. HCl to give a white precipitate which was filtered under suction, washed with water and allowed to dry in a vacuum oven to obtain Compound 15 as a white solid (392.0 mg, 83% yield). Mp: 138˚C - 140˚C. IR (cm⁻¹): 3500 - 2400, 3435, 2930, 2855, 1680, 1590, 1507, 1463, 1432, 1383, 1364, 1347, 1310, 1260, 1243, 1204, 1120, 1042, 988, 966, 924, 887, 875, 766, 731, 701, 687. ¹H NMR (400 MHz, DMSO-d6): δ 1.26 - 1.64 (8H, m), 2.62 (2H, t, J = 7.2 Hz), 2.69 (1H, t, 7.2 Hz), 2.85 (2H, t, J = 6.4 Hz), 3.37 (2H, t, J = 6.4), 4.138 (2H, t, J = 6.4), 4.25 - 4.30 (4H, m), 7.07 (1H, J = 2Hz), 7.09 (1H, J = 1.6 Hz), 12.78 (1H); ¹³C NMR (100 MHz, CDCl₃): δ 25.1 (1C), 28.2 (1C), 29.3 (1C), 30.0 (1C), 31.6 (1C), 32.5 (1C), 60.6 (1C), 63.8 (1C), 64.2 (1C), 68.6 (1C), 106.4 (1C), 111.5 (1C), 122.3 (1C), 137.4 (1C), 143.4 (1C), 147.3 (1C), 166.8 (1C). ESI-TOF-MS m/z: 379.1191, ([C₁₇H₂₄O₆S + Na]⁺ calcd. 379.1191).

8-(2-butylsulfanyl-ethoxy)-2,3-dihydro-benzo[1,4]dioxine-6-carboxylic acid (16)

A solution of 13 (321.5 mg, 0.9850 mmol) and NaOH (2N, 10 mL) in methanol (10 mL) was stirred and refluxed for 8 h. The reaction mixture was concentrated under vacuum and the resulting mixture acidified with conc. HCl to give a white precipitate which was filtered under suction, washed with water and allowed to dry in a vacuum oven to obtain a white solid (232.1 mg, 75.4%). Mp: 122˚C - 123˚C. IR (cm⁻¹): 3400 - 2400, 2954, 2927, 2859, 2580, 1674, 1591, 1509, 1434, 1422, 1389, 1372, 1350, 1317, 1269, 1249, 1215, 1125, 1058, 1045, 970, 948, 919, 885, 868, 778, 763, 732, 711, 695. ¹H NMR (400 MHz, CDCl₃) δ 0.93 (3H, t, J =7.2 Hz), 1.38 - 1.47 (2H, m), 1.57 - 1.65 (2H, m), 2.64 (2H, t, J = 7.2 Hz), 2.95 (2H, t, J = 7.2 Hz), 4.23 (2H, t, J = 6.8 Hz), 4.28 (2H, s), 4.38 (2H, s), 7.26 (1H, s), 7.36 (1H, s). ¹³C NMR (100 MHz, CDCl₃) δ 13.7 (1C), 22.0 (1C), 30.5 (1C), 31.8 (1C), 32.3 (1C), 63.9 (1C), 64.8 (1C), 68.9 (1C), 107.2 (1C), 113.3 (1C), 113.3 (1C), 121.0 (1C), 138.5 (1C), 143.8 (1C), 147.6 (1C), 171.2 (1C). TOF-MS m/z: 335.0922, ([C₁₅H₂₀O₅S + Na]⁺ calcd. 335.0929).

Formation of 1,4-benzodioxane-6-carboxylic acid amide analogs (18) to (31)

To a stirred solution of 16 (200 mg, 0.64 mmol) and N,N-dimethylformamide (2 drops) in anhydrous dichloromethane (15 mL) at 0˚C - 5˚C, oxalyl chloride (0.11 mL, 1.31 mmol) was added. The reaction mixture was refluxed for 1 h under nitrogen. The resulting mixture was concentrated under vacuum. This resulted in yellow oil, 17, which was used for the next step without further purification.

To a stirred solution of the amine in anhydrous CH₂Cl₂ (10 mL) was added a solution of acid chloride (17) in CH₂Cl₂ dropwise at 0˚C - 5˚C. The reaction mixture was stirred for two hours at 0˚C, and at room temperature for overnight. CH₂Cl₂ (15 mL) was then added to the reaction mixture and the resulting mixture washed with H₂O (25 mL), HCl (1N, 2 × 25 mL), H₂O (25 mL), and brine (25 mL). The organic layer was dried over anhydrous MgSO₄ followed by solvent removal in-vacuo. The residue was triturated with hexane to obtain the product or purified by column chromatography on silica gel using 20% EtOAc in Hexane for 21, 40% EtOAc in Hexane for 29 - 30, and 5% MeOH in DCM for 31.

8-(2-(butylthio)ethoxy)-N-phenyl-2,3-dihydrobenzo[b][1,4]dioxine-6-carboxamide (18)

(166 mg, 47%). Mp: 68˚C - 70˚C. IR (cm⁻¹): 3276, 2956, 2928, 2872, 1642, 1588, 1528, 1500, 1463, 1439, 1385, 1367, 1346, 1320, 1274, 1225, 1202, 1120, 1035, 925, 888, 870, 800, 750, 726, 690. ¹H NMR (400 MHz, CDCl₃): δ 0.92 (3H, t, J = 7.6 Hz), 1.37 - 1.46 (2H, m), 1.56 - 1.63 (2H, m), 2.62 (2H, t, J = 7.2 Hz), 2.94 (2H, t, J = 7.2 Hz), 4.21 (2H, t, J = 7.2 Hz), 4.27 - 4.29 (2H, m), 4.34 - 4.36 (2H, m), 7.01 (1H, d, J = 2.0 Hz), 7.11 - 7.15 (2H, m), 7.36 (2H, t, J = 7.6 Hz), 7.62 (2H, d, J = 8.0 Hz), 7.84 (1H, s). ¹³C NMR (100 MHz, CDCl₃) δ 13.6 (1C), 21.9 (1C), 30.6 (1C), 31.8, (1C), 32.3 (1C), 64.1 (1C), 64.6 (1C), 68.9 (1C), 105.2 (1C), 108.9 (1C), 120.1 (2C), 124.4 (1C), 127.0 (1C), 129.0 (2C), 136.6 (1C), 138.0 (1C), 143.8 (1C), 148.1 (1C), 165.0 (1C). ESI-TOF-MS m/z: 388.1574, ([C₂₁H₂₅NO₄S + H]⁺ calcd. 388.1583).

8-(2-(butylthio)ethoxy)-N-(4-methoxyphenyl)-2,3-dihydrobenzo[b][1,4]dioxine-6-carboxamide (19)

(134 mg, 47%). Mp: 136˚C - 138˚C. IR (cm⁻¹): 3282, 2954, 2926, 2870, 1634, 1588, 1512, 1500, 1463, 1427, 1411, 1349, 1318, 1278, 1228, 1204, 1181, 1117, 1060, 1043, 1030, 976, 929, 890, 827, 811, 757, 725, 672. ¹H NMR (400 MHz, CDCl₃): δ 0.92 (3H, t, J = 7.2 Hz), 1.37 - 1.46 (2H, m), 1.56 - 1.63 (2H, m), 2.62 (2H, t, J = 7.6 Hz), 2.94 (2H, t, J = 7.2 Hz), 3.80 (3H, s), 4.22 (2H, t, J = 7.2 Hz), 4.27 - 4.29 (2H, m), 4.33 - 4.36 (2H, m) 6.89 (2H, d, J = 9.2 Hz), 6.99 (1H, d, J = 2.0 Hz), 7.11 (1H, d, J = 2.0 Hz), 7.51 (2H, d, J = 8.8 Hz), 7.69 (1H, s). ¹³C NMR (100 MHz, CDCl₃) δ 13.6 (1C), 22.0 (1C), 30.7 (1C), 31.8, (1C), 32.3 (1C), 55.5 (1C), 64.1 (1C), 64.6 (1C), 69.0 (1C), 105.3 (1C), 108.9 (1C), 114.2 (2C), 122.0 (2C), 127.1 (1C), 131.1 (1C), 136.6 (1C), 143.8 (1C), 148.1 (1C), 156.5 (1C), 164.9 (1C). ESI-TOF-MS m/z: 418.1678, ([C₂₂H₂₇NO₅S + H]⁺ calcd. 418.1688).

8-(2-(butylthio)ethoxy)-N-(4-chlorophenyl)-2,3-dihydrobenzo[b][1,4]dioxine-6-carboxamide (20)

(108 mg, 37%). Mp: 105˚C - 107˚C. IR (cm⁻¹): 3275, 2977, 2956, 2926, 2870, 1643, 1588, 1525, 1527, 1501, 1458, 1428, 1398, 1362, 1323, 1266, 1239, 1221, 1198, 1122, 1067, 1046, 1031, 1008, 887, 849, 818, 791, 735, 703, 682. ¹H NMR (400 MHz, CDCl₃): δ 0.92 (3H, t, J = 7.3 Hz), 1.37 - 1.46 (2H, m), 1.56 - 1.63 (2H, m), 2.62 (2H, t, J = 7.2 Hz), 2.95 (2H, t, J = 7.2 Hz), 4.22 (2H, t, J = 6.8 Hz), 4.29 - 4.30 (2H, m), 4.35 - 4.36 (2H, m) 7.00 (1H, s), 7.10 (1H, s), 7.32 (2H, d, J = 8.8 Hz), 7.58 (2H, d, J = 8.8 Hz), 7.78 (1H, s). ¹³C NMR (100 MHz, CDCl₃) δ 13.7 (1C), 22.0 (1C), 30.7 (1C), 31.8 (1C), 32.3 (1C), 64.1, (1C), 64.7 (1C), 69.0 (1C), 105.2 (1C), 108.9 (1C), 121.3 (2C), 126.6 (1C), 129.1 (2C), 129.4 (1C), 136.6 (1C), 136.8 (1C), 143.9 (1C), 148.2 (1C), 164.9 (1C). ESI-TOF-MS m/z: 422.1137, ([C₂₁H₂₄ClNO₄S + H]⁺ calcd. 422.1193).

8-(2-(butylthio)ethoxy)-N-(4-(trifluoromethyl)phenyl)-2,3-dihydrobenzo[b][1,4]dioxine-6-carboxamide (21)

(218 mg, 80%). Mp: 176˚C - 178˚C. IR (cm⁻¹): 3286, 2957, 2928, 2872, 1653, 1590, 1525, 1502, 1463, 1430, 1406, 1385, 1351, 1321, 1245, 1265, 1225, 1202, 1155, 1114, 1065, 1037, 1018, 891, 839, 757, 742, 699, 675. ¹H NMR (400 MHz, CDCl₃): δ 0.92 (3H, t, J = 7.3 Hz), 1.37 - 1.46 (2H, m), 1.56 - 1.64 (2H, m), 2.63 (2H, t, J = 7.4 Hz), 2.96 (2H, t, J = 7.1 Hz), 4.24 (2H, t, J = 7.2 Hz), 4.29 - 4.32 (2H, m), 4.36 - 4.38 (2H, m) 7.01 (1H, d, J = 2.0 Hz), 7.11 (1H, d, J = 2.0 Hz), 7.62 (2H, d, J = 8.6 Hz), 7.76 (2H, d, J = 8.4 Hz), 7.86 (1H, s). ¹³C NMR (100 MHz, CDCl₃) δ 13.7 (1C), 14.2 (1C), 22.0 (1C), 29.7 (1C), 30.7 (1C), 31.8, (1C), 32.4 (1C), 50.9 (1C), 60.4 (1C), 64.1 (1C), 64.7 (1C), 69.0 (1C), 105.3 (1C), 109.1 (1C), 119.6 (1C), 126.3 - 126.4 (2C), 137.1 (1C), 141.1 (1C), 143.9 (1C), 148.2 (1C), 165.1 (1C). ESI-TOF-MS m/z: 456.1456, ([C₂₂H₂₄F₃NO₄S + H]⁺ calcd. 456.1456).

8-(2-(butylthio)ethoxy)-N-(2-chlorobenzyl)-2,3-dihydrobenzo[b][1,4]dioxine-6-carboxamide (22)

(107 mg, 35%). Mp: 88˚C - 90˚C. IR (cm⁻¹): 3273, 2956, 2928, 2872, 1636, 1589, 1538, 1497, 1463, 1442, 1427, 1383, 1366, 1345, 1316, 1272, 1247, 1210, 1119, 1069, 1040, 933, 910, 887, 853, 812, 747, 727, 679. ¹H NMR (400 MHz, CDCl₃): δ 0.91 (3H, t, J = 7.3 Hz), 1.36 - 1.45 (2H, m), 1.55 - 1.62 (2H, m), 2.61 (2H, t, J = 7.4 Hz), 2.93 (2H, t, J = 7.2 Hz), 4.20 (2H, t, J = 7.2 Hz), 4.25 - 4.27 (2H, m), 4.31 - 4.34 (2H, m), 4.69 (2H, d, J = 6.0 Hz), 6.50 (1H, t, J = 5.4 Hz), 6.89 (1H, d, J = 1.9 Hz), 7.06 (1H, d, J = 1.9 Hz), 7.23 - 7.26 (2H, m), 7.37 - 7.39 (1H, m), 7.44 - 7.46 (1H, m). ¹³C NMR (100 MHz, CDCl₃) δ 13.7 (1C), 22.0 (1C), 30.6 (1C), 31.8, (1C), 32.3 (1C), 42.1 (1C), 64.1 (1C), 64.6 (1C), 68.9 (1C), 105.2 (1C), 108.8 (1C), 126.3 (1C), 127.1 (1C), 129.0 (1C), 129.6 (1C), 130.4 (1C), 133.7 (1C), 135.6 (1C), 136.4 (1C), 143.7 (1C), 148.0 (1C), 166.6 (1C). ESI-TOF-MS m/z: 436.1341, ([C₂₂H₂₆ClNO₄S + H]⁺ calcd. 436.1349).

8-(2-(butylthio)ethoxy)-N-(4-fluorobenzyl)-2,3-dihydrobenzo[b][1,4]dioxine-6-carboxamide (23)

(126.1 mg, 41%). Mp: 121˚C - 122˚C. IR (cm⁻¹): 3264, 2961, 2930, 2873, 1629, 1589, 1538, 1496, 1460, 1427, 1385, 1367, 1344, 1318, 1273, 1210, 1156, 1116, 1069, 1043, 1016, 998, 958, 909, 886, 853, 822, 810, 726, 697, 681. ¹H NMR (400 MHz, CDCl₃): δ 0.92 (3H, t, J = 7.3 Hz), 1.36 - 1.46 (2H, m), 1.55 - 1.63 (2H, m), 2.61 (2H, t, J = 7.4 Hz), 2.94 (2H, t, J = 7.2 Hz), 4.21 (2H, t, J = 7.2 Hz), 4.25 - 4.28 (2H, m), 4.31 - 4.34 (2H, m), 4.58 (2H, d, J = 5.7 Hz), 6.29 (1H, dist t), 6.89 (1H, d, J = 2 Hz), 7.00 - 7.05 (2H, m), 7.06 (1H, d, J = 2 Hz), 7.29 - 7.33 (2H, m). ¹³C NMR (100 MHz, CDCl₃) δ 13.7 (1C), 22.0 (1C), 30.6 (1C), 31.8, (1C), 32.3 (1C), 43.4 (1C), 64.1 (1C), 64.6 (1C), 68.9 (1C), 105.2 (1C), 108.8 (1C), 115.5 (1C), 115.7 (1C), 126.3 (1C), 129.5 (1C), 129.6 (1C), 134.1 (1C), 136.5 (1C), 143.8 (1C), 148.0 (1C), 162.3 (1C, JC-F = 245 Hz), 166.6 (1C). ESI-TOF-MS m/z: 420.1617, ([C₂₂H₂₆FNO₄S + H]⁺ calcd. 420.1645).

8-(2-(butylthio)ethoxy)-N-(4-methoxybenzyl)-2,3-dihydrobenzo[b][1,4]dioxine-6-carboxamide (24)

(139 mg, 46%). Mp: 120˚C - 121˚C. IR (cm⁻¹): 3283, 2956, 2927, 2875, 1634, 1612, 1590, 1537, 1501, 1463, 1428, 1389, 1349, 1310, 1273, 1250, 1211, 1174, 1127, 1066, 1037, 963, 930, 910, 885, 819, 769, 718, 680. ¹H NMR (400 MHz, CDCl₃): δ 0.92 (3H, t, J = 7.2 Hz), 1.36 - 1.46 (2H, m), 1.55 - 1.61 (2H, m), 2.61 (2H, t, J = 7.2 Hz), 2.93 (2H, t, J = 7.2 Hz), 3.80 (3H, s), 4.21 (2H, t, J = 7.2 Hz), 4.22 - 4.26 (2H, m), 4.32 - 4.34 (2H, m) 4.54 (2H, d, J = 5.2 Hz), 6.21 (1H, dist. t), 6.87 - 6.89 (3H, m), 7.06 (1H, d, J = 2.0 Hz), 7.26 - 7.28 (2H, m). ¹³C NMR (100 MHz, CDCl₃) δ 13.6 (1C), 22.0 (1C), 30.6 (1C), 31.8 (1C), 32.3 (1C), 43.7, (1C), 55.3 (1C), 64.1 (1C), 64.6 (1C), 68.9 (1C), 105.2 (1C), 108.8 (1C), 114.2 (2C), 126.5 (1C), 129.3 (1C), 130.3 (1C), 131.5 (1C), 136.4 (1C), 143.7 (1C), 147.9 (1C), 159.1 (1C), 166.5 (1C). ESI-TOF-MS m/z: 432.1859, ([C₂₃H₂₉NO₅S + H]⁺ calcd. 432.1845).

8-(2-(butylthio)ethoxy)-N-(4-chlorobenzyl)-2,3-dihydrobenzo[b][1,4]dioxine-6-carboxamide (25)

(120.7 mg, 41%). Mp: 115˚C - 117˚C. IR (cm⁻¹): 3275, 2956, 2928, 2870, 1633, 1590, 1538, 1493, 1463, 1427, 1406, 1388, 1345, 1317, 1272, 1228, 1211, 1122, 1090, 1067, 1043, 1015, 958, 886, 855, 800, 781, 726, 681. ¹H NMR (400 MHz, CDCl₃): δ 0.92 (3H, t, J = 7.2 Hz), 1.37 - 1.46 (2H, m), 1.56 - 1.63 (2H, m), 2.62 (2H, t, J = 7.6 Hz), 2.94 (2H, t, J = 7.2 Hz), 4.21 (2H, t, J = 7.2 Hz), 4.25 - 4.27 (2H, m), 4.33 - 4.35 (2H, m) 4.58 (2H, d, J = 6.0 Hz), 6.89 (1H, d, J = 2.0 Hz), 7.06 (1H, d, J = 2.0 Hz), 7.28 - 7.29 (2H, m), 7.31 (2H, t, J = 2.0 Hz), 7.32 - 7.34 (1H, dist. t). ¹³C NMR (100 MHz, CDCl₃) δ 13.7 (1C), 22.0 (1C), 30.7 (1C), 31.8 (1C), 32.3 (1C), 43.4, (1C), 64.1 (1C), 64.6 (1C), 69.0 (1C), 105.2 (1C), 108.8 (1C), 126.2 (1C), 128.9 (2C), 129.2 (2C), 133.4 (1C), 136.6 (C), 136.8 (1C), 143.8 (1C), 148.0 (1C), 166.7 (1C). ESI-TOF-MS m/z: 436.1349, ([C₂₂H₂₆ClNO₄S + H]⁺ calcd. 436.1349).

8-(2-(butylthio)ethoxy)-N-hexyl-2,3-dihydrobenzo[b][1,4]dioxine-6-carboxamide (26)

(108 mg, 38%). Mp: 98˚C - 100˚C. IR (cm⁻¹): 3286, 2955, 2926, 2858, 1631, 1589, 1539, 1499, 1463, 1426, 1381, 1367, 1346, 1316, 1273, 1251, 1209, 1117, 1068, 1042, 960, 927, 889, 854, 767, 724, 679. ¹H NMR (400 MHz, CDCl₃): δ 0.87 - 0.94 (6H, m), 1.25 - 1.46 (8H, m), 1.56 - 1.63 (4H, m), 2.62 (2H, t, J = 7.3 Hz), 2.94 (2H, dist. t), 3.38 - 3.43 (2H, q, J = 6.9 Hz), 4.21 (2H, t, J = 7.2), 4.25 - 4.28 (2H, m), 4.32 - 4.34 (2H, m), 5.99 (1H, dist. t), 6.86 (1H, d, J = 1.9 Hz), 7.03 (1H, d, J = 1.8 Hz). ¹³C NMR (100 MHz, CDCl₃) δ 13.7 (1C), 14.0 (1C), 22.0 (1C), 22.5, (1C), 26.6 (1C), 29.6 (1C), 30.6 (1C), 31.5 (1C), 31.8 (1C), 32.3 (1C), 40.1 (1C), 64.1 (1C), 64.6 (1C), 68.9 (1C), 136.8 (1C), 105.2 (1C), 108.6 (1C), 127.0 (1C), 136.2 (1C), 143.7 (1C), 147.9 (1C), 166.7 (1C). ESI-TOF-MS m/z: 396.2206, ([C₂₁H₃₃NO₄S + H]⁺ calcd. 396.2209).

8-(2-(butylthio)ethoxy)-N-(pyridin-2-ylmethyl)-2,3-dihydrobenzo[b][1,4]dioxine-6-carboxamide (27)

(121 mg, 30%). Mp: 142˚C - 143˚C. IR (cm⁻¹): 3259, 2951, 2926, 2872, 2574, 1640, 1621, 1588, 1538, 1495, 1465, 1426, 383, 1345, 1318, 1250, 1227, 1207, 1120, 1072, 1040, 1003, 960, 885, 760, 727. ¹H NMR (400 MHz, CDCl₃): δ 0.90 (3H, t, J = 7.3 Hz), 1.35 - 1.44 (2H, m), 1.53 - 1.61 (2H, m), 2.62 (2H, t, J = 7.3 Hz), 2.93 (2H, t, J = 7.0 Hz), 4.21 - 4.28 (4H, m), 4.29 - 4.32 (2H, m), 4.97 (2H, d, J = 6.1 Hz) 7.21 - 7.22 (2H, m), 7.74 (1H, t, J = 6.6 Hz), 7.98 (1H, d, J = 7.9 Hz), 8.27 - 8.31 (1H, m), 8.66 (1H, d, J = 5.2 Hz), 8.78 (1H, t, J = 5.7 Hz). ¹³C NMR (100 MHz, CDCl₃) δ 13.7 (1C), 22.0 (1C), 30.7 (1C), 31.8, (1C), 32.2 (1C), 40.4 (1C), 63.9 (1C), 64.6 (1C), 69.0 (1C), 105.0 (1C), 110.3 (1C), 124.7 (1C), 125.0 (1C), 127.1 (1C), 136.8 (1C), 141.6 (1C), 143.9 (1C), 144.8 (1C), 147.8 (1C), 154.4 (1C), 167.1 (1C). ESI-TOF-MS m/z: 403.1672, ([C₂₁H₂₆N₂O₄S + H]⁺ calcd. 403.1692).

N-((1H-benzo[d]imidazol-2-yl)methyl)-8-(2-(butylthio)ethoxy)-2,3-dihydrobenzo[b][1,4]dioxine-6-carboxamide (28)

(58.8 mg, 19.4%). Mp: 216˚C - 218˚C. IR (cm⁻¹): 3346, 3185, 2926, 2870, 1642, 1587, 1548, 1498, 1459, 1370, 1343, 1320, 1273, 1252, 1227, 1211, 1117, 1067, 1041, 1003, 968, 936, 884, 866, 842, 767, 736. ¹H NMR (400 MHz, CDCl₃): δ 0.89 (3H, t, J = 7.6 Hz), 1.33 - 1.41 (2H, m), 1.49 - 1.56 (2H, m), 2.49 (2H, t, J = 7.2 Hz), 2.83 (2H, t, J = 7.2 Hz), 4.07 (2H, t, J = 6.8 Hz), 4.20 (2H, br s), 4.30 (2H, br s), 4.78 (2H, d, J = 4.8 Hz) 7.20 - 7.26 (5H, m), 7.55 (2H, br s), 9.15 (1H, dist. t). ¹³C NMR (100 MHz, CDCl₃) δ 13.7 (1C), 21.9 (1C), 30.6 (1C), 31.8, (1C), 32.2 (1C), 38.3 (1C), 64.0 (1C), 64.6 (1C), 68.7 (1C), 105.0 (1C), 109.9 (1C), 123.0 (1C), 125.0 (1C), 136.7 (1C), 143.8 (1C), 147.8 (1C), 152.5 (1C), 168.6 (1C). ESI-TOF-MS m/z: 442.1788, ([C₂₃H₂₇N₃O₄S + H]⁺ calcd. 442.1801).

8-(2-(butylthio)ethoxy)-N,N-bis(2-methoxyethyl)-2,3-dihydrobenzo[b][1,4]dioxine-6-carboxamide (29)

(89.1 mg, 30%). IR (cm⁻¹): 2946, 2927, 2873, 1628, 1588, 1509, 1455, 1424, 1380, 1365, 1338, 1304, 1272, 1246, 1211, 1193, 1114, 1056, 1043, 1015, 9640, 925, 888, 863, 766, 742. ¹H NMR (400 MHz, CDCl₃): δ 0.91 (3H, t, J = 7.3 Hz), 1.36 - 1.45 (2H, m), 1.55 - 1.63 (2H, m), 2.61 (2H, t, J = 7.4 Hz), 2.93 (2H, t, J = 7.3 Hz), 3.34 (6H, s), 3.50 - 3.63 (8H, m), 4.16 (2H, t, J = 7.9 Hz), 4.24 - 4.27 (2H, m), 4.31 - 4.33 (2H, m), 6.65 (1H, d, J = 1.8 Hz), 6.67 (1H, d, J = 1.8 Hz). ¹³C NMR (100 MHz, CDCl₃) δ 13.6 (1C), 21.9 (1C), 30.6 (1C), 31.8, (1C), 32.3 (1C), 58.9 (1C), 64.2 (1C), 64.5 (1C), 68.8 (1C), 70.7 (1C), 105.2 (1C), 109.6 (1C), 128.4 (1C), 134.4 (1C), 143.8 (1C), 147.6 (1C), 171.6 (1C). ESI-TOF-MS m/z: 428.2104, ([C₂₁H₃₃NO₆S + H]⁺ calcd. 428.2107).

8-(2-(butylthio)ethoxy)-2,3-dihydrobenzo[b][1,4]dioxin-6-yl)(morpholino)methanone (30)

(61.5 mg, 25%). IR (cm⁻¹): 2956, 2927, 2857, 1633, 1613, 1589, 1509, 1455, 1421, 1382, 1367, 1342, 1307, 1273, 1249, 1212, 1122, 1066, 1045, 1028, 970, 931, 888, 862, 820, 758, 741, 692. ¹H NMR (400 MHz, CDCl₃): δ 0.92 (3H, t, J = 7.9 Hz), 1.38 - 1.45 (2H, m), 1.55 - 1.63 (2H, m), 2.61 (2H, t, J = 7.2 Hz), 2.93 (2H, t, J = 7.2 Hz), 3.62 - 3.68 (8H, m), 4.16 (2H, t, J = 7.2 Hz), 4.25 - 4.27 (2H, m), 4.31 - 4.33 (2H, m) 6.58 (1H, d, J = 1.9 Hz), 6.60 (1H, d, J = 1.9 Hz). ¹³C NMR (100 MHz, CDCl₃) δ 13.8 (1C), 22.1 (1C), 24.1 (1C), 29.8 (1C), 30.8, (1C), 32.0 (1C),32.5 (1C), 64.3 (1C), 64.7 (1C), 67.0 (1C), 69.2 (1C), 105.4 (1C), 109.7 (1C), 127.2 (1C), 135.2 (1C), 144.0 (1C), 148.2 (1C), 170.1 (1C). ESI-TOF-MS m/z: 382.1692, ([C₁₉H₂₇NO₅S + H]⁺ calcd. 382.1688).

[8-(2-butylsulfanyl-ethoxy)-2,3-dihydro-benzo[1,4]dioxin-6-yl]-piperidin-1-yl-methanone (31)

(81 mg, 31%). IR (cm⁻¹): 2927, 2857, 2577, 1676, 1631, 1590, 1508, 1432, 1372, 1349, 1321, 1269, 1249, 1212, 1115, 1058, 1044, 948, 885, 868, 763, 732, 711, 695, 667. ¹H NMR (400 MHz, CDCl₃) δ 0.91 (3H, t, J = 7.2 Hz), 1.25 - 1.68 (10H, m), 2.63 (2H, t, J = 7.2 Hz), 2.95 (2H, t, J = 7.6 Hz), 4.22 (2H, t, J = 7.2 Hz), 4.27 - 4.29 (2H, m), 4.36 - 4.38 (2H, m), 7.25 (1H, d, J = 2 Hz), 7.35 (1H, d, J = 2 Hz). ¹³C NMR (100 MHz, CDCl₃) δ 13.7 (1C), 22.0 (2C), 24.4 (1C), 26.1 (1C), 30.5 (1C), 30.6 (1C), 31.8 (1C), 32.3 (2C), 63.9 (1C), 64.8 (1C), 68.8 (1C), 107.1 (1C), 113.3 (1C), 121.1 (1C), 138.4 (1C), 143.7 (1C), 147.6 (1C), 170.8 (1C). ESI-TOF-MS m/z: 380.1892, ([C₂₀H₂₉NO₄S + H]⁺ calcd. 380.1896).

[8-(2-butylsulfanyl-ethoxy)-2,3-dihydro-benzo[1,4]dioxin-6-yl]-[1-(3-ethyl-9,10-dimethoxy-1,3,4,6,7,11b-hexahydro-2H-pyrido[2,1-a]isoquinolin-2-ylmethyl)-6,7-dimethoxy-3,4-dihydro-1H-isoquinolin-2-yl]-methanone (32)

To a stirred solution of 16 (225 mg, 0.68 mmol) and Ν,Ν-dimethylformamide (2 drops) in anhydrous dichloromethane (15 mL) at 0 - 5˚C, oxalyl chloride (0.11 mL, 1.31 mmol) was added. The reaction mixture was refluxed for 1 h under nitrogen. The resulting mixture was concentrated under vacuum. This resulted in yellow oil, 17, which was used for the next step without further purification.

To a stirred solution of triethylamine (0.1 mL) in CH₂Cl₂ (10 mL) at room temperature was added emetine dihydrochloride (200 mg, 0.36 mmol). After all the emetine was completely dissolved, a solution of acid chloride 17 in CH₂Cl₂ (5 mL) was added dropwise at 0˚C - 5˚C. The reaction mixture was stirred at room temperature for 24 h. CH₂Cl₂ (15 mL) was then added to the mixture and the resulting mixture washed with distilled water (2 × 25 mL), and brine (25 mL). The organic layer was dried over anhydrous MgSO₄ followed by solvent removal in-vacuo. The residue was applied onto a silica gel column and eluted with 10% methanol in ethyl acetate. The yellow solid obtained was further dried in a vacuum oven to obtain a powdery yellow solid (95 mg, 33%).

Mp: 115˚C - 117˚C. IR (cm⁻¹): 2929, 2865, 2825, 2749, 1610, 1626, 1588, 1509, 1458, 1425, 1364, 1339, 1252, 1203, 1120, 1030, 928, 888, 859, 766. ¹H NMR (400 MHz, CDCl₃) δ 0.89 - 0.95 (6H, m), 1.31 - 1.48 (4H, m), 1.50 - 1.54 (1H, m), 1.56 - 1.66 (3H, m), 1.68 - 2.36 (1H, m), 2.56 - 2.65 (3H, m), 2.67 - 2.89 (3H, m), 2.91 - 2.99 (3H, m), 2.99 - 3.46 (3H, m), 3.49 - 3.71 (3H, m), 3.79 - 3.89 (9H, m), 3.92 - 4.11 (2H, m), 4.13 - 4.40 (11H, m), 4.46 - 4.54 (1H, m), 6.54 - 6.59 (4H, m), 7.23 (1H, d, J = 2Hz), 7.43 (1H, d, J = 2.4); ¹³C NMR (100 MHz, CDCl₃) δ 13.7, 22.9, 29.1, 30.7, 31.8, 32.4, 40.5, 48.7, 52.4, 55.8 - 56.2 (4C), 64.2, 64.5, 104.6, 108.8, 110.0, 111.2, 111.4, 124.4, 128.7, 134.7, 170.2; ESI-TOF-MS m/z: 775.3985, ([C₄₄H₅₈N₂O₈S + H]⁺ calcd. 775.3992).

Transformation of the sulfide to sulfoxide or sulfone analogs

8-(2-(Butylsulfinyl)ethoxy)-N-(4-fluorobenzyl)-2,3-dihydrobenzo[b][1,4]dioxine-6-carboxamide (33)

Hydrogen peroxide (30 wt%, 0.08 mL) was added dropwise to a mixture of 23 (150 mg, 0.36 mmol) and tellurium dioxide (6.00 mg, 0.036 mmol) in MeOH: CH₂Cl₂ (1:1, 8 mL). The mixture was stirred at room temperature for 12 h. The solvent was evaporated in vacuo, and water (20 mL) was added to the reaction mixture and then extracted with CH₂Cl₂ (2 × 20 mL). The organic layer was washed with brine, dried over anhydrous MgSO₄ and concentrated under reduced pressure. The residue was triturated with hexane to obtain 33 as a light yellow solid (99 mg, 64%).

Mp: 141˚C - 143˚C. IR (cm⁻¹): 3287, 2954, 2924, 2872, 1632, 1606, 1589, 1537, 1510, 1500, 1466, 1423, 1392, 1367, 1348, 1311, 1274, 1249, 1230, 1209, 1160, 1125, 1097, 1067, 1027, 982, 972, 911, 887, 858, 834, 820, 773, 760, 725, 702, 678. ¹H NMR (400 MHz, CDCl₃): δ 0.95 (3H, t, J = 7.2 Hz), 1.38 - 1.53 (2H, m), 1.68 - 1.76 (2H, m), 2.78 (2H, t, J = 8 Hz), 2.96 - 3.02 (1H, m), 3.13 - 3.20 (1H, m), 4.22 - 4.23 (2H, m), 4.26 - 4.27 (2H, m), 4.40 - 4.42 (2H, m), 4.51 (2H, d, J = 5.6 Hz), 6.96 (2H, t, J = 8.8), 7.08 (1H, d, J = 1.6 Hz), 7.10 (1H, d, J = 2 Hz), 7.26 - 7.30 (2H, m), 7.43 (1H, t, J = 6 Hz). ¹³C NMR (100 MHz, CDCl₃) δ 13.5 (1C), 21.9 (1C), 24.5 (1C), 43.1, (1C), 51.2 (1C), 52.5 (1C), 61.7 (1C), 63.9 (1C), 64.4 (1C), 105.3 (1C), 110.1 (1C), 115.1 (1C), 115.3 (1C), 126.2 (1C), 129.3 (1C), 129.4 (1C), 134.4 (1C), 136.3 (1C), 143.7 (1C), 146.9 (1C), 161.9 (1C, d, JC-F= 244), 166.5 (1C). ESI-TOF-MS m/z: 436.1584, ([C₂₂H₂₆FNO₅S + H]⁺ calcd. 436.1594).

8-(2-(Butylsulfonyl)ethoxy)-N-(4-fluorobenzyl)-2,3-dihydrobenzo[b][1,4]dioxine-6-carboxamide (34)

To a stirred solution of 23 (150 mg, 0.36 mmol) in acetic acid (5 mL), hydrogen peroxide (2 mL, 30 wt%) was added in one portion and the resulting mixture was stirred at room temperature for 20 h. The mixture was then poured onto crushed ice resulting in the formation of a white precipitate. The precipitate was separated by filtration, washed with cold water and dried to provide 34 as a white solid (120 mg, 74%).

Mp: 166˚C - 168˚C. IR (cm⁻¹): 3407, 2956, 2938, 2875, 1651, 1633, 1588, 1546, 1501, 1464, 1428, 1388, 1344, 1314, 1286, 1269, 1212, 1157, 1117, 1070, 1036, 1015, 972, 930, 909, 886, 858, 821, 759, 724, 663. ¹H NMR (400 MHz, CDCl₃): δ 0.97 (3H, t, J = 7.6 Hz), 1.42 - 1.52 (2H, m), 1.83 - 1.90 (2H, m), 3.21 - 3.25 (2H, m), 3.41 (2H, t, J = 5.2 Hz), 4.26 - 4.29 (4H, m), 4.44 (2H, t, J = 5.2 Hz), 4.56 (2H, d, J = 5.6 Hz), 6.50 (1H, dist t), 6.97 (1H, d, J = 2 Hz), 7.02 (2H, t, J = 8.8 Hz), 7.05 (1H, d, J = 1.6 Hz), 7.28 - 7.32 (2H, m). ¹³C NMR (100 MHz, CDCl₃) δ 13.7 (1C), 22.0 (1C), 30.6 (1C), 31.8, (1C), 32.3 (1C), 43.4 (1C), 64.1 (1C), 64.6 (1C), 68.9 (1C), 105.2 (1C), 108.8 (1C), 115.5 (1C), 115.7 (1C), 126.3 (1C), 129.5 (1C), 129.6 (1C), 134.0 (1C), 136.5 (1C), 143.8 (1C), 148.0 (1C), 162.2 (1C, d, JC-F = 245 Hz), 166.6 (1C). ESI-TOF-MS m/z: 452.1556, ([C₂₂H₂₆FNO₆S + H]⁺ calcd. 452.1543).

Acknowledgements

We gratefully acknowledge the MRI grant number CHE-1126533 from the National Science Foundation.

Conflicts of Interest

The authors declare no conflicts of interest regarding the publication of this paper.

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