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Synthesis of 2-Aminobenzothiazolomethyl Naphthols Using l-Valine Organocatalyst: An Efficient, Versatile and Biodegradable Catalyst

  • Jaggi LalEmail author
  • Shiwani Singh
  • Pooja Rani
Original Article
  • 19 Downloads

Abstract

A simple, practical and scalable approach for the synthesis of 2-aminobenzothiazolomethyl naphthols using one-flask three-component reaction of 2-aminobenzothiazole, substituted aromatic aldehydes and β-naphthol in water is reported. The reaction is catalyzed by l-valine as non-toxic, green, easily available, cheap, sustainable and biodegradable catalyst. The product formation takes shorter reaction time with good to excellent yield. The method is demonstrated on multi gram level. Easy workup procedure, short reaction time and most importantly environmentally benign are the most outstanding advantages of this procedure.

Keywords

l-Valine 2-Aminobenzothiazolomethyl naphthols Green synthesis Scalable synthesis 

1 Introduction

In organic chemistry, multi-component synthesis (MCS) has emerged as an important protocol to build a complex molecule in a single step with a diverse range of complexity which can easily be achieved from easily available starting materials [1]. In MCS several old bonds are broken and new bonds are formed in a single operation and offer valuable advantages like operational simplicity, facile automation, convergence, reduction in the number of workup, extraction and purification procedures and hence minimize the waste products, rendering the green organic conversions [2]. MCS often reduces reaction duration, giving higher yield of the synthesized compounds than multi-step syntheses and therefore can reduce the energy and man power [3].

Now a days, to minimize hazardous materials which are harmful to environment and the atom economy in the use of starting materials, the target of science and technology has been shifting towards more environment friendly, sustainable resources and catalyst [4, 5]. Thus, l-valine may be an attractive candidate (Fig. 1) in the search for such type of biodegradable and non-toxic acid catalyst for the synthesis of 2-aminobenzothiazolomethyl naphthols. It is easily available and inexpensive catalyst. The 2-aminobenzothiazolomethyl naphthols have found to posses broad range of biological activities, such as antiviral [6], antitumor [7], antibacterial [8], antifungal [9], topoisomerase II inhibitory [10], anticonvulsant [11] as well as anti-inflammatory [12]. Synthesis of 2-aminobenzothiazolomethyl naphthols [13] has been reported by Shaabani et al. with some remarkable drawbacks like large quantity of catalyst at higher temperature. Kumar et al. [14] have also synthesized the target compounds with some drawbacks like higher temperature, long reaction time, no recyclability and comparatively high mol% of the catalyst.
Fig. 1

Chemical structure and catalytic features of catalyst used

In literature, several methods for the synthesis of 2-aminobenzothiazolomethyl naphthols have been reported using microwave irradiations [15], graphite-supported perchloric acid (HClO4-C) [16], NBS [17], heteropoly acids [18], ionic liquids [19], fumaric acid [20], agar [21], citric acid [22] and sphalerite [23]. Apart from the trichloroisocyanuric acid [24], NaHSO4·H2O [25], Wells–Dawson heteropoly acid [26], multi-SO3H functionalized ionic liquid [27], Fe3O4@SiO2-ZrCl2-MNPs [28], phosphate fertilizers [29], magnetic nanocatalyst [30], maltose [31], grindstone [32] and zinc oxide micelles as a recoverable and reusable catalyst [33] have also been used to catalyzed such type of reaction.

To the best of our knowledge, very few reports are available in literature on the use of l-valine alone as catalyst in organic synthesis [34], but various compounds derived from l-valine are used as organocatalyst [35, 36, 37, 38]. Herein, we disclose a new protocol for one-flask three-component reaction, which starts from easily available 2-aminobenzothiazole, substituted aromatic aldehydes and β-naphthol; afforded 4a–n (Scheme 1) using l-valine catalyst in water.
Scheme 1

Synthesis of 2-aminobenzothiazolomethyl naphthols using l-valine as catalyst

2 Experimental

The reagents used in the reaction were obtained commercially from Alfa Aesar, Merck, Rankem, Qualikems and Sigma Aldrich; and used as received. All the synthesized derivatives were identified by comparing their melting points and spectral data with those of the authentic samples. Melting point of all the synthesized compounds was determined on electrical melting point apparatus in an open capillary and was uncorrected. IR spectra of the synthesized compounds were recorded on A2 technology in terms of frequency of absorption (cm−1). Mass spectra (EIMS) were recorded on Egilent ION TRAP 6310 mass spectrometer. 1H NMR spectra were recorded on BRUKER AVANCE II 400 NMR Spectrometer using tetramethylsilane as an internal standard at 20–22 °C in DMSO d6 solvent. Silica pre-coated Merck alumina TLC plates (0.5 mm) were used.

2.1 Synthesis of 2-Aminobenzothiazolomethyl Naphthols

A 10 mL dry reaction vial was charged with 2-aminobenzothiazole 1 (0.33 mmol), substituted aromatic aldehydes 2 (0.33 mmol), β-naphthol 3 (0.33 mmol) and l-valine catalyst (10 mol%) in 2 mL water was heated at 70 °C for about 1–2 h. The reaction was monitored by TLC using ethyl acetate/hexane (30:70) as eluent. After completion, the reaction mixture was cooled to room temperature, and stirred the contents after adding ethanol. The residual product was recrystallized from hot ethanol to give the pure product 4a–n (Scheme 1).

2.2 Gram Scaled-Up Synthesis of 2-Aminobenzothiazolomethyl Naphthols (4a)

A 50 mL dry round bottom flask was charged with 2-aminobenzothiazole 1 (33.29 mmol, 5 g), benzaldehyde 2 (36.62 mmol, 3.89 g), β-naphthol 3 (36.62 mmol, 5.28 g) and l-valine catalyst (10 mol%, 600 mg) in 10 mL water was heated at 70 °C for about 1–2 h. The reaction was monitored by TLC using ethyl acetate/hexane (30:70). After completion, the reaction mixture was cooled to room temperature and stirred the contents after adding ethanol. The residual product was recrystallized from hot ethanol to give the product 4a (Scheme 2).
Scheme 2

Gram scaled up synthesis of 2-aminobenzothiazolomethyl naphthol using l-valine

2.3 Competitive Experiment

A 10 mL reaction vial was charged with 2-aminobenzothiazole 1 (0.33 mmol), β-naphthol 3 (0.33 mmol), p-methoxybenzaldehyde 2 (0.33 mmol), p-nitrobenzaldehyde 2 (0.33 mmol) and l-valine catalyst (10 mol%) in 2 mL water was heated at 70 °C for about 2–3 h. The reaction was monitored by TLC using ethyl acetate/hexane (30:70). After completion, the reaction mixture was cooled to room temperature and stirred the contents after adding ethanol. The residual product was recrystallized from hot ethanol to give the products 4f and 5m in 66% and 21% yields respectively (Scheme 3).
Scheme 3

Competitive experiment

2.4 Characterization Data of Some Selected Compounds

2.4.1 1-(Benzo[d]thiazol-2-ylamino)(phenyl) methyl)naphthalene-2-ol (4a)

White solid, IR λmax, cm−1: 3501, 3386, 1594, 1546, 1512, 1451; 1H NMR (DMSO-d6, 400 MHz): δ = 6.95–7.92 (16 H, m, 15 H arom, 1H-CH), 8.64 (1H, s, NH), 10.12 (1H, s, OH); 13C NMR (DMSO-d6, 100 MHz): δ = 53.34, 117.02, 116.52, 117.80, 121.47, 121.82, 121.25, 122.57, 124.22, 125.03, 126.81, 127.31, 129.57, 128.31, 131.57, 131.13, 141.24, 152.94, 151.17, 167.31; EIMS m/z: Calcd for C24H18N2OS 382.4810, found 383.4813 [M + 1]+.

2.4.2 1-(Benzo[d]thiazol-2-ylamino)(p-tolyl)methyl)methyl)naphthalene-2-ol (4b)

White solid, IR (λmax, cm−1): 3609, 3007, 2922, 1625, 1510, 1267 cm−1; 1H NMR (DMSO-d6, 400 MHz): δ = 2.23 (3H, s, CH3), 6.95–7.39 (15H, m, 14H arom and 1H-CH), 8.66 (1H, s, NH), 10.14 (1H, s, OH); 13C NMR (DMSO-d6, 100 MHz): δ = 20.60, 53.33, 118.00, 118.66, 118.92, 120.51, 120.85, 122.27, 125.26, 126.03, 128.34, 128.50, 128.60, 129.22, 130.55, 132.13, 135.17, 139.13, 151.97, 153.20, 166.39; EIMS m/z: Calcd for C25H20N2OS 396.5080, found 397.5082 [M + 1]+.

2.4.3 1-(Benzo[d]thiazol-2-ylamino)(2,4-dichloro phenyl)naphthalene-2-ol (4c)

White solid, IR (λmax, cm−1): 3381, 1599, 1542, 1515, 1449 cm−1; 1H NMR (DMSO-d6, 400 MHz): δ = 6.96–7.93 (14H, m, 13 H arom, 1H-CH), 8.67 (1H, s, NH), 10.11 (1H, s, OH); 13C NMR (DMSO-d6, 100 MHz): δ = 53.46, 118.22, 118.87, 118.98, 120.49, 120.84, 122.27, 123.58, 125.23, 126.04, 127.83, 128.34, 128.56, 129.28, 130.59, 132.12, 142.25, 151.96, 153.19, 166.37; EIMS m/z: Calcd for C24H16N2OS 451.3650, found 452.3651, [M + 1]+.

2.4.4 1-(Benzo[d]thiazol-2-ylamino)(4-chloro phenyl)naphthalene-2-ol (4d)

White powder, IR (λmax, cm−1): 3602, 3304, 1627, 1267, 1122 cm−1; 1H NMR (DMSO-d6, 400 MHz): δ = 6.67–7.78 (15H, m, 14H arom, 1H-CH), 8.57 (1H, s, NH); 13C NMR (DMSO-d6, 100 MHz): δ = 53.48, 117.71, 118.13, 118.96, 120.95, 121.12, 122.43, 122.47, 125.81, 126.82, 127.30, 127.78, 128.22, 129.54, 130.32, 134.52, 138.16, 151.61, 153.28, 166.89; EIMS m/z: Calcd for EIMS m/z: Calcd for C24H17ClN2OS 416.9220, found 417.9221 [M + 1]+.

2.4.5 1-(Benzo[d]thiazol-2-ylamino)(4-methoxy phenyl)methyl)naphthalene-2-ol (4f)

Off white solid, IR (λmax, cm−1): 3612, 3513, 3014, 2920, 1623, 1509, 1276; 1H NMR (DMSO-d6, 400 MHz): δ = 3.67 (3H, s, OCH3), 6.68–7.96 (15H, m, 14H arom, 1H-CH), 8.53 (1H, s, NH), 10.52 (1H, s, OH); 13C NMR (DMSO-d6, 100 MHz): δ = 54.83, 67.45, 118.14, 119.01, 119.13, 120.56, 122.51, 125.51, 126.24, 127.40, 128.45, 128.76, 129.36, 130.21, 132.14, 133.54, 132.10, 142.23, 151.66, 153.36, 166.82; EIMS m/z: Calcd for C25H20N2O2S 412.5070, found 413.5069 [M + 1]+.

2.4.6 1-(Benzo[d]thiazol-2-ylamino)(4-hydroxy-3-methoxyphenyl) methyl) naphthalene-2-ol (4h)

Off white solid, IR (λmax, cm−1): 3512, 3380, 1597, 1541, 1517, 1448 cm−1; 1H NMR (DMSO-d6, 400 MHz): δ = 3.58 (3 H, s, OCH3) 6.67–7.91 (14 H, m, 13 H arom, 1H-CH), 8.81 (1 H, s, NH), 10.13 (1 H, s, OH); 13C NMR (DMSO-d6, 100 MHz): d = 54.36, 56.55, 116.11, 118.74, 119.42, 119.60, 119.83, 121.81, 121.94, 123.26, 126.41, 126.97, 129.42, 129.53, 130.26, 131.21, 133.11, 133.65, 146.13, 148.23, 152.46, 154.04, 167.21; EIMS m/z: Calcd for C25H20N2O3S 428.5050, found 429.5052 [M + 1]+.

2.4.7 1-(Benzo[d]thiazol-2-ylamino)(2-hydroxy phenyl)methyl) naphthalene-2-ol (4i)

Off white solid, IR (λmax, cm−1): 3612, 3302, 2943, 1628, 1506, 1284, 961–811 cm−1; 1H NMR (DMSOd6, 400 MHz): δ = 6.68–7.92 (15H, m, 14H arom, 1H-CH), 8.67 (1H, s, NH), 10.02 (1H, s, OH); 13C NMR (DMSO-d6, 100 MHz): δ = 51.84, 116.73, 117.75, 118.81, 119.43, 120.62, 121.13, 121.22, 122.51, 125.52, 126.32, 128.21, 128.96, 129.82, 128.53, 129.26, 132.14, 136.35, 150.83, 152.63, 154.46, 166.52; EIMS m/z: Calcd for C24H18N2O2S 398.4800, found 399.4802 [M + 1]+.

2.4.8 1-(Benzo[d]thiazol-2-ylamino)(4-nitrophenyl)methyl)naphthalene-2-ol (4m)

White powdered solid, IR (λmax, cm−1): 3503, 3346, 2930, 1628, 1512, 1265, 961–814 cm−1; 1H NMR (DMSO-d6, 400 MHz): δ = 6.72–7.31 (15H, m, 14H arom, 1H-CH), 8.64 (1H, s, NH), 10.02 (1H, s, OH); 13C NMR (DMSO-d6, 100 MHz): δ = 53.41, 117.83, 118.32, 118.56, 119.57, 120.44, 122.51, 123.26, 125.22, 127.32, 125.34, 126.62, 127.34, 128.55, 128.97, 129.91, 145.96, 150.66, 153.36, 166.68; Calcd for C24H17N3O3S 427.4780, found 428.4782 [M + 1]+.

2.4.9 1-(Benzo[d]thiazol-2-ylamino)(3-nitrophenyl)naphthalene-2-ol (4n)

White powdered solid, IR (λmax, cm−1): 3335, 1627, 1597, 1531, 1452; 1H NMR (DMSO-d6, 400 MHz) δ = 7.05–7.88 (15H, m, 14H arom, 1H-CH), 8.90 (s, 1H, NH), 10.12 (s, 1H, OH); 13C NMR (DMSO-d6, 100 MHz): δ = 54.38, 116.12, 118.72, 119.41, 119.62, 119.82, 121.84, 121.91, 123.28, 126.43, 126.99, 129.41, 129.55, 130.27, 131.23, 133.12, 146.14, 152.48, 154.06, 167.22; Calcd for C24H17N3O3S 427.4780, found 428.4781 [M + 1]+.

The spectral data are in good agreement with the literature data.

3 Results and Discussion

We initiated the study with the reaction of 2-aminobenzothiazole, benzaldehyde and β-naphthol using l-valine as a green and sustainable catalyst in methanol with 46% yield in 3 h. The synthesized product was recrystallized by hot ethanol and characterized by 1H-NMR, 13C-NMR, IR and Mass spectral studies. After confirmation of the desired product; various solvents viz. methanol, ethanol, propanol, butanol, water and a mixture of water with ethanol/methanol have been screened for finding best solvent for the reaction (Table 1, entries 1–8). This shows that the yield of the product increases with the polarity of the solvent, water gives best results 96%, (Table 1, entry 8), hence, water was chosen as best solvent for the reaction.
Table 1

Optimization of solvent system

Entry

Solvent

Catalyst

Catalyst (mol%)

Time (h)

Yield (%)a

1

Methanol

l-Valine

10

3

46

2

Ethanol

l-Valine

10

3

39

3

Propanol

l-Valine

10

3

31

4

Butanol

l-Valine

10

3

28

5

Methanol/water (20:80 v/v)

l-Valine

10

3

69

6

Methanol/water (10:90 v/v)

l-Valine

10

3

76

7

Ethanol/water (10:90 v/v)

l-Valine

10

3

61

8

Water

l-Valine

10

2

96

9

Solvent-free

l-Valine

10

3

17

Reaction conditions: 2-aminobenzothiazole (0.33 mmol), benzaldehyde (0.33 mmol), β-naphthol (0.33 mmol) and catalyst (10 mol%) in 1 mL water was heated at 70 °C for about 2–3 h

aIsolated yield

After studying the effect of solvents, extended the work to solvent-free conditions and observed that, in absence of solvent, product is formed only in 17% yield in 3 h. (Table 1, entry 9).

Within this perspective, we performed our reaction with various amino acids such as alanine, glycine, arginine, glutamine, cystine, leucine, l-proline, serine and tyrosine (Table 2). The screening results revealed that l-valine is superior catalyst and gives maximum yield (96%) as compared to other amino acids, because it provides optimum acidity required for carrying out the reaction.
Table 2

Optimization of the catalyst

Entry

Catalyst

Solvent

Catalyst (mol%)

Time (h)

Yield (%)a

1

Alanine

Water

10

3

18

2

Glycine

Water

10

4

29

3

Arginine

Water

10

4

37

4

Glutamine

Water

10

3

31

5

Cystine

Water

10

4

59

6

Leucine

Water

10

3

62

7

l-Proline

Water

10

3

81

8

l-Valine

Water

10

2

96

9

Serine

Water

10

3

45

10

Tyrosine

Water

10

3

23

Reaction conditions: 2-aminobenzothiazole (0.33 mmol), benzaldehyde (0.33 mmol), β-naphthol (0.33 mmol) and catalyst (10 mol%) in 2 mL water was heated at 70 °C for about 2–3 h

aIsolated yield

After optimizing the solvent system and catalyst, the reaction was carried out at various temperature ranges viz. 50, 60, 65, 70, 75, 80, 85 and 90 °C for temperature optimization, when temperature increases from 70 to 80 °C there is no change in the product yield, on further increase in temperature from 80 to 90 °C the yield of the product decreases, therefore, the optimum temperature for the reaction was 70 °C (Fig. 2).
Fig. 2

Effect of temperature on the product yield

In order to find out the optimum reaction time, a model reaction between 2-aminobenzothiazole, benzaldehyde and β-naphthol in presence of l-valine in water was performed at various time intervals viz. 1, 2, 3, 4 and 5 h and concluded that, there is a sharp increase in the yield of the product at 2 h, the yield of the product reaches its maximum level and no further enhancement was observed, if a longer reaction time 3–5 h was applied, hence 2 h was selected as an optimum reaction time for the reaction (Table 3, entry 1–5).
Table 3

Optimization of time for the reaction

Entry

Time (h)

Yield (%)a

1

1

61

2

2

96

3

3

96

4

4

96

5

5

96

Reaction conditions: 2-aminobenzothiazole (0.33 mmol), benzaldehyde (0.33 mmol), β-naphthol (0.33 mmol) and catalyst (10 mol%) in 1 mL water was heated at 70 °C for about 1–5 h

aIsolated yield

Next, we examined the amount of catalyst on the course of the reaction with a choice of various amounts 5 mol%, 10 mol%, 15 mol%, 20 mol% and 25 mol% (Fig. 3). It was observed that 10 mol% gives best results with 96% yield in water.
Fig. 3

Catalyst amount experiment

With the optimized reaction conditions established above for the reaction, we have designed and synthesized fourteen derivatives in good to excellent yields ranging from 89 to 96% under standard reaction conditions. It is evident from the variations of the product yields (4a–n) that the presence of an electron-donating group in the reacting aldehyde facilitates the reaction, a somewhat relatively lower yield for electron-withdrawing groups. In case of gram scale experiment similar results were obtained. The physico-chemical data of the synthesized derivatives have been given in Table 4.
Table 4

Physico chemical data of synthesized 2-aminobenzothiazolomethyl naphthols

Entry

R

Time (h.)/yield (%)a

M.P. (°C)

Rf valuec

Elemental analysis (%) found/calculated

Found

Reportedb

C

H

N

4a

-H

2/96

203–204

204–205 (11)

0.72

75.71/75.37

4.72/4.75

7.29/7.32

4b

-4-CH3

3/93

181–182

182–183 (11)

0.65

75.76/75.73

5.12/5.08

7.04/7.07

4c

-2,4-Cl2

3/89

205–206

206–207 (17)

0.45

63.88/63.86

3.52/3.57

6.19/6.21

4d

-4-Cl

3/91

208–209

209–210 (11)

0.56

69.12/69.14

4.13/4.11

6.74/6.72

4e

-3-Cl

3/90

192–193

192–194 (12)

0.57

69.12/69.14

4.09/4.11

6.70/6.72

4f

-4-OCH3

2/94

173–174

175–176 (11)

0.76

72.76/72.79

4.87/4.89

6.80/6.79

4g

-3-OCH3

2/95

184–185

184–186 (12)

0.65

72.80/72.79

4.86/4.89

6.78/6.79

4h

-4-OH,3-OCH3

2/94

193–194

194–195 (17)

0.78

72.78/72.79

4.88/4.89

6.77/6.79

4i

-2-OH

3/93

162–163

163–165 (12)

0.76

72.33/72.34

4.52/4.55

7.02/7.03

4j

-4-F

3/90

175–176

176–178 (12)

0.65

71.92/71.98

4.25/4.28

7.06/7.00

4k

-3-Br

3/92

203–204

202–204 (12)

0.67

62.46/62.48

3.73/3.71

6.05/6.07

4l

-4-Br

3/91

199–200

200–202 (25)

0.74

62.46/62.48

3.73/3.71

6.04/6.07

4m

-4-NO2

3/89

188–189

189–191 (12)

0.73

67.41/67.43

4.04/4.01

9.82/9.83

4n

-3-NO2

3/89

197–198

198–199 (11)

0.77

67.44/67.43

4.02/4.01

9.81/9.83

Reaction conditions: 2-aminobenzothiazole (0.33 mmol), benzaldehyde (0.33 mmol), β-naphthol (0.33 mmol) and l-valine catalyst (10 mol%) in 2 mL water was heated at 70 °C for about 2–3 h

aIsolated yield

bReported in literature

cRf value (Ethyl acetate: hexane 30:70 v/v)

Encouraged by these results, we performed competitive experiment (Scheme 3) for examining whether any change in the starting material would cause variation in the product yields. When 2-aminobenzothiazole 1 and β-naphthol 3 was allowed to react with an equimolar mixture of p-methoxybenzaldehyde and p-nitrobenzaldehyde 2, products 4f and 4m were obtained in 66% and 21% yields respectively. The experiment revealed that when the substrates bearing electron-donating and electron-withdrawing groups were allowed to react individually, they provided better yields as compared to a substrate bearing electron-withdrawing group. However in the mixture, substrates with electron-donating group not only reacted faster, but also resulted in higher yields of the product as compared to substrates having electron-withdrawing group.

3.1 Plausible Catalytic Cycle

An uncertain catalytic mechanistic pathway for the reaction has been illustrated in Scheme 4. In the acidic catalyst, l-valine releases a proton which activates the carbonyl group of the aldehyde, the latter reacts with β-naphthol to give the intermediate, then the catalysts activate the ortho carbonyl naphthol which will subsequently react in situ with 2-aminobenzothiazole to give the desired product.
Scheme 4

Plausible catalytic cycle for the synthesis of 2-aminobenzothiazolomethyl naphthols

4 Conclusions

In conclusion, we have developed a new l-valine catalyzed simple, efficient and practical one-flask method for the synthesis of 2-aminobenzothiazolomethyl naphthols from 2-aminobenzothiazole, substituted aromatic aldehydes and 2-naphthol. The product formation takes shorter duration with good to excellent yield. Ease of work-up, use of eco-friendly solvent, mild reaction conditions, wide substrate tolerance and short reaction duration make the method more advantageous over reported procedures. Furthermore, the present method is readily amenable to large scale synthesis of pharmaceutically relevant heterocyclic scaffolds.

Notes

Acknowledgements

We are grateful to Dr. K.N. Modi University, Newai, Rajasthan for financial support.

Compliance with Ethical Standards

Conflict of interest

Authors have no conflict of interest.

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Copyright information

© The Tunisian Chemical Society and Springer Nature Switzerland AG 2019

Authors and Affiliations

  1. 1.Department of ChemistryDr. K.N. Modi UniversityNewaiIndia
  2. 2.Department of ChemistryMalaviya National Institute of TechnologyJaipurIndia

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