K2S2O8-Mediated Hydroxyalkylation of Benzothiazoles with Alcohols in Aqueous Solution.

The K2S2O8-mediated hydroxyalkylation of 2H-benzothiazoles with aliphatic alcohols in aqueous solution was described. The mild and convenient protocol generated a series of hydroxyalkylated benzothiazoles in moderate to good yields. Besides, benzimidazole and ethers were also compatible in this reaction, leading to corresponding C2 ether-substituted heteroarenes.

Introduction

C2-Alkylated benzothiazoles are widely present in pharmaceutical compounds,[1] agricultural chemicals,[2] and organic functional materials.[3] The incorporation of a hydroxyl group into the benzylic position of benzothiazole derivatives has been applied to synthesize various pharmaceuticals such as muscarinic antagonist,[4] CaS receptor allosteric modulators,[5] and antifungal agents[6] (Figure ).

Figure 1

Pharmaceuticals containing a benzylic hydroxyl in benzothiazoles.

According to the literature studies, most of the traditional routes on synthesizing hydroxyalkylated benzothiazoles mainly focused on addition of aldehydes or ketones including Li/Mg,[7]n-BuLi,[8]t-BuLi,[9] and t-BuOLi[10] catalyzed metalation of 2H-benzothiazoles as well as onium amide bases, t-Bu-P4,[11] P5F,[12] tetramethylammonium fluoride,[13] catalyzed deprotonation of 2H-benzothiazoles before addition of aldehydes or ketones. In addition, the [Rh(COD)Binapine]BF4-catalyzed direct hydrogenation of 2-carbonyl benzothiazoles was reported.[14] Generally, the above routes suffered from difficulty for the preparation of the starting materials, harsh reaction conditions, complex procedures, and expensive organocatalysts or transition-metal catalysts. Subsequently, photochemical methods about oxidative cross-coupling of alcohols with different heteroarenes were reported by Opatz[15] (Scheme A) and Lei[16] (Scheme B). Both methodologies were easy for preparing hydroxyalkylated heteroarenes but suffered from long reaction times. On the other hand, the hydroxyalkylation of heterocycles with alcohols under heating conditions was occasionally reported. In 2011, Wang et al. reported the hydroxyalkylation of benzothiazoles with alcohols in the presence of tert-butyl hydroperoxide (TBHP) at 120 °C (Scheme C).[17] In 2017, Kianmehr group published hydroxyalkylation reactions of thiophenes and pyridines with alcohols using ditert-butyl peroxide (DTBP) as the oxidant at 130 °C (Scheme D).[18] Next year, DTBP-mediated C2-hydroxyalkylation reactions of chromones with alcohols at 140 °C were developed by Yu et al (Scheme E).[19] Despite the significant advances of these reactions, they still suffered from certain limitations such as relatively high reaction temperature, anhydrous reaction systems, as well as the limited structural diversity of organic peroxides oxidants, which are not easy to be removed in the post-treatment process and can easily cause explosion in the industrial production. Accordingly, we envisioned whether the use of inorganic oxidants would facilitate the development of an advantageous strategy with mild conditions and operational simplicity.[20] In the light of the reported reference,[21] K2S2O8 could provide the sulfate radical anion (SO4•–) under mild conditions, which was regarded as a very strong one-electron oxidant with a redox potential of 2.5–3.1 V. Along this line, we herein report a mild and convenient direct hydroxyalkylation reaction of benzothiazoles with a variety of alcohols by using K2S2O8 as the oxidant in H2O at 65 °C.

Scheme 1

Hydroxyalkylation Reactions of Heterocycles with Alcohols

Results and Discussion

Initially, we examined this hydroxyalkylation reaction using benzothiazole (1a) as a model substrate and 2-propanol (2a) as a coupling partner. The results were outlined in Table . First, the reaction proceeded by smoothly employing K2S2O8 (4 equiv) as the oxidant in a mixture of alcohol and water at 45 °C. The desired product 3ad was achieved in 35% yield within 6 h (Table , entry 1). Subsequently, different catalysts were then examined, such as Na2S2O8, (NH4)2S2O8, DTBP, TBHP, tert-butyl peroxybenzoate (TBPB), 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), and benzoyl peroxide (BPO) (Table , entries 2–8), but no further improvement of the yield was obtained. Among them, those reactions without persulfates did not give the desired product (Table , entries 4–8). Inspiringly, the product yield increased to 85% when the reaction temperature increased to 65 °C (Table , entry 10). We further tested the amount of K2S2O8, but no improvement was observed (Table , entries 12–13). In addition, the results showed that the product could be obtained in comparable yields as the amount of solvents was reduced to 4 mL (2-propanol/H2O = 2 mL:2 mL) or the reaction time was shortened to 3 h (Table , entries 14–15). Finally, the control experiments demonstrated that K2S2O8 and H2O were necessary for successful coupling, while O2 had no effect on the reaction (Table , entries 16–18). Notably, the procedure was applicable to 10 mmol scale (1.353 g), and the product 3ad was isolated in 72% (1.390 g) yield under the optimized reaction conditions (Scheme ).

Table 1

Optimization of Reaction Conditionsa

entry	oxidant (equiv)	solvent (v/v, mL)	temperature (°C)	yield (%)
1	K2S2O8 (4)	2a/H2O (4:4)	45	35
2	Na2S2O8 (4)	2a/H2O (4:4)	45	9
3	(NH4)2S2O8 (4)	2a/H2O (4:4)	45	21
4	DTBP (4)	2a/H2O (4:4)	45	n.r.
5	TBHP (4)	2a/H2O (4:4)	45	n.r.
6	TBPB (4)	2a/H2O (4:4)	45	n.r.
7	DDQ (4)	2a/H2O (4:4)	45	n.r.
8	BPO (4)	2a/H2O (4:4)	45	n.r.
9	K2S2O8 (4)	2a/H2O (4:4)	55	60
10	K2S2O8 (4)	2a/H2O (4:4)	65	85
11	K2S2O8 (4)	2a/H2O (4:4)	75	58
12	K2S2O8 (3)	2a/H2O (4:4)	65	61
13	K2S2O8 (5)	2a/H2O (4:4)	65	84
14	K2S2O8 (4)	2a/H2O (2:2)	65	85
15b	K2S2O8(4)	2a/H2O(2:2)	65	83
16c	K2S2O8 (4)	2a/H2O (2:2)	65	85
17	K2S2O8 (4)	2a (4)	65	n.r.
18		2a/H2O (2:2)	65	n.r.

Reaction conditions: benzothiazole (1a, 0.5 mmol), 6 h under aerobic condition.

For 3 h.

N2 atmosphere.

Scheme 2

Upscaling of the Reaction to the Gram Level

Having optimized the reaction conditions, we investigated the scope of the alcohols (2) in the direct hydroxyalkylation with benzothiazole (1a) and the results are shown in Table . As expected, a series of aliphatic alcohols, such as methanol, ethanol, propanol, 2-propanol, butanol, 2-methyl-1-propanol, 2-butanol, pentanol, 3-methyl-1-butanol, and 2-pentanol, were all fine hydroxyalkyl sources, generating the corresponding hydroxyalkylated benzothiazoles in moderate to good yields (Table , 3aa–3aj). The results showed that along with the increase of the carbon chain, the yields of the corresponding products decreased gradually. It can also be seen that the yields of benzothiazoles with secondary alcohols were generally higher than benzothiazoles with primary alcohols containing the same number of carbon atoms (Table , 3ac, 3ae, and 3ah compared to 3ad, 3ag, and 3aj, respectively). In addition, the direct alkylation of benzothiazole with cyclopentanol and ethylene glycol also provided the corresponding products 3ak (71%) and 3al (73%), respectively. Unfortunately, when allyl alcohol, benzyl alcohol, or phenyl ethanol was used as the hydroxyalkyl source, no desired cross-coupling products were observed (Table , 3am and 3an), which might be due to the combination of the steric and electronic effects of the conjugated system. Interestingly, while diethylene glycol was used as the hydroxyalkyl source, only 3ao was obtained in 56% yield (Table , 3ao), which revealed that hydroxyl carbon was preferred to α-carbon of ether to construct the C–C bond.

Table 2

Substrate Scope of Alcoholsa

Reaction conditions: benzothiazole (1a, 0.5 mmol), alcohols (2, 2.0 mL), K2S2O8 (4.0 equiv), H2O (2.0 mL), 65 °C, and 3–6 h under aerobic condition.

To further demonstrate the applicability of this reaction, we examined the substrate scope of substituted benzothiazoles. As shown in Table , moderate to good yields were obtained when different substituted benzothiazoles and alcohols were utilized (Table , 3ba–3bj). It was noteworthy that the yields of benzothiazoles with electron-donating groups were obviously higher than benzothiazoles with electron-withdrawing groups. Benzothiazoles with electron-donating groups (−OMe) gave the target products in 75–88% yields (Table , 3ba, 3bc, 3be, 3bf, 3bh, and 3bj), while benzothiazoles with electron-withdrawing groups (−Cl) offered the target products in 55–63% yields (Table , 3bb, 3bd, 3bg, and 3bi) and 6-nitrobenzothiazole only gave the trace amount of the product (Table , 3bk). In particular, the reactions of 6-methoxy-benzothiazole and 7-methoxy-benzothiazole with 2-propanol produced the coupling products in similar yields (Table , 3be and 3bf), which seemed that the reactions were not sensitive to the steric hindrance caused by benzene substitutions.

Table 3

Substrate Scope of 2H-Benzothiazolesa

Reaction conditions: benzothiazoles (1, 0.5 mmol), alcohols (2, 2.0 mL), K2S2O8 (4.0 equiv), H2O (2.0 mL), 65 °C, and 3–6 h under aerobic condition.

To explore the applicability of the present hydroxyalkylation of heteroarenes with ethers, we studied benzothiazole with tetrahydrofuran, 1,4-dioxane, 1,3-dioxolane, and ethylene glycol dimethyl ether. As can be seen from Table , the alkylation products 5a–e were obtained in satisfactory yields under the optimized conditions. Furthermore, the reaction of benzimidazole with 1,4-dioxane also proceed smoothly. Among them, the yield of 5d was 2.5 times that of 5d′, which shows a tendency to activate C–H bonds with weaker bond dissociation energy. In addition, the reaction of benzothiazole with ethylene glycol dimethyl ether afforded a regioisomeric mixture with a ratio of 1.2/1.0 in favor of the external product in 85% combined yield (Table , 5e and 5e′). It may be caused by the difference of the stability of the free-radical intermediate and the steric hindrance of the α-position sp3 C–H in ethylene glycol dimethyl ether.

Table 4

Further Applicationa

Reaction conditions: azole (1, 0.5 mmol), ethers (2, 2.0 mL), K2S2O8 (4.0 equiv), H2O (2.0 mL), 65 °C, and 6 h under aerobic condition.

In order to provide some insights into the intermolecular competition, a series of experiments were carried out. The results demonstrated that the electron density in the phenyl rings of benzothiazoles strongly affected the reaction rate (Scheme A). Furthermore, compared to primary alcohols as the hydroxyl source, the yields of benzothiazole with secondary alcohols were obviously higher (Scheme B–D). For example, the reaction of benzothiazole with a mixture of 2-propanol and propanol provided corresponding products with a ratio of 2.1/1.0 in 68% combined yield (Scheme B), the reaction of benzothiazole with a mixture of 2-butanol and butanol gave corresponding products with a ratio of 3.6/1.0 in 46% combined yield (Scheme C), and the reaction with a mixture of 2-pentanol and 3-methyl-1-butanol produced corresponding products with a ratio of 3.0/1.0 in 44% combined yield (Scheme D). We speculated that primary alcohols could be oxidized to aldehydes and the formed products could be further oxidized to ketones in the presence of the oxidant,[22] while the products with secondary alcohols could avoid the latter situation. Thus, we concluded that the secondary alcohols were more suitable for the reaction than primary alcohols.

Scheme 3

Intermolecular Competition Experiments

As shown in Scheme , when 2.0 equiv of 3,5-ditert-butyl-4-hydroxytoluene was added to the standard reaction, the product yield was significantly reduced from 81 to 35%, which could indicate that this transformation involved radical intermediates. More importantly, we observed the adducts resulting from the sequestration of the a-hydroxyl radical and 2-benzothiazole radical by BHT (Scheme , 6a and 6b), which revealed that the reaction should proceed through a radical pathway.[23]

Scheme 4

Radical Trapping Experiment

According to the radical trapping experiment, both hydroxyl radical and benzothiazole radical were trapped by radical scavenger; therefore, cross-coupling between the radicals would occur in the reaction. In addition, benzothiazole would be attacked by other radicals,[24] and thus, two plausible routes were speculated in Scheme . First, homolytic cleavage of K2S2O8 generated sulfate radical anions (SO4•–),[25] which abstracted hydrogens from benzothiazole 1a and 2-propanol 2d affording the benzothiazole radical 1aa and hydroxyl radical 2da, respectively.[18,19] Then, cross-coupling reaction took place between the two radicals producing the desired product 3ad.[17] On the other hand, hydroxyl radical 2da would attack the 2-position of benzothiazole 1a to form radical cation intermediates 1ab, which were deprotonated by the SO4•– to give the products 3ad.[24]

Scheme 5

Proposed Mechanism

Conclusions

In conclusion, we have developed an efficient K2S2O8-mediated hydroxyalkylation reaction of benzothiazoles with alcohols in aqueous solution providing a range of hydroxyalkylated benzothiazoles in moderate to good yields. In addition, benzimidazole and ethers were also compatible in this reaction, leading to corresponding C2 ether-substituted heteroarenes. This procedure took advantage of the solubility of inorganic salts in water to lead to an easier and safer post-treatment process. The reaction worked well in aqueous condition and under a lower reaction temperature than before. Accordingly, we believe that the protocol not only contributes to the realization of “safer” and “milder” synthesis but also offers opportunities for different approaches in chemical synthesis.

Experimental Section

General Information

All reagents were purchased from commercial sources and used without further purification. All reactions were monitored by thin-layer chromatography, which was performed on precoated aluminum sheets of silica gel 60 (F254). The products were purified by flash column chromatography on silica gel (200–300 mesh). Melting points were uncorrected. 1H NMR spectra were recorded at 500 MHz in CDCl3 or DMSO-d6, and 13C NMR spectra were recorded at 125 MHz in CDCl3 or DMSO-d6; copies of their 1H NMR and 13C NMR spectra are provided; all chemical shifts are given in ppm. All new products were further characterized by high-resolution mass spectrometry (HRMS) obtained on a time-of-flight liquid chromatograph/mass spectrometer equipped with an electrospray ionization (ESI) source.

General Procedure

To a 25 mL test tube, a mixture of benzothiazoles 1 (0.5 mmol), alcohols (ethers) 2 (3) (2 mL), K2S2O8 (4.0 equiv, 2.0 mmol, 540 mg), and H2O (2.0 mL) was stirred at 65 °C in open air. The reaction system was stirred for 3–6 h until the reaction was completed (monitored by TLC). The solvent was then removed under vacuum, and the residue was extracted with ethyl acetate (30 mL) and washed with saturated NaHCO3 solution. The organic phase was then dried over anhydrous Na2SO4, filtered, and concentrated in vacuum. The resulting residue was purified by column chromatography on silica gel, eluting with petroleum ether/ethyl acetate (1:1–10:1, v/v) to afford the products 3 and 5.

Benzo[d]thiazol-2-ylmethanol (3aa)[16,17]

Yellow solid; yield 67% (40.9 mg); mp 94–95 °C; 1H NMR (500 MHz, CDCl3): δ 8.00 (d, J = 8.0 Hz, 1H), 7.91 (d, J = 8.0 Hz, 1H), 7.53–7.47 (m, 1H), 7.44–7.38 (m, 1H), 5.10 (s, 2H), 3.20 (br, 1H); 13C{1H} NMR (125 MHz, CDCl3): δ 172.1, 152.9, 134.8, 126.2, 125.2, 122.9, 121.9, 62.8.

1-(Benzo[d]thiazol-2-yl)ethan-1-ol (3ab)[17]

Yellow solid; yield 81% (53.6 mg); mp 55–56 °C; 1H NMR (500 MHz, CDCl3): δ 7.98 (d, J = 8.5 Hz, 1H), 7.89 (d, J = 8.0 Hz, 1H), 7.48 (ddd, J = 8.5, 7.5, 1.5 Hz, 1H), 7.41–7.37 (m, 1H), 5.27 (q, J = 6.5 Hz, 1H), 3.62 (br, 1H), 1.72 (d, J = 6.5 Hz, 3H); 13C{1H} NMR (125 MHz, DMSO): δ 179.55, 153.17, 134.40, 125.91, 124.67, 122.36, 122.23, 67.09, 23.85.

1-(Benzo[d]thiazol-2-yl)propan-1-ol (3ac)[9a,17]

White solid; yield 65% (46.4 mg); mp 102–103; 1H NMR (500 MHz, CDCl3): δ 7.99 (d, J = 8.0 Hz, 1H), 7.89 (d, J = 8.0 Hz, 1H), 7.55–7.43 (m, 1H), 7.43–7.33 (m, 1H), 5.06 (dd, J = 7.5, 5.0 Hz, 1H): δ 3.59 (br, 1H), 2.10 (m, 1H), 2.02–1.92 (m, 1H), 1.07 (t, J = 7.5 Hz, 1H); 13C{1H} NMR (125 MHz, CDCl3): δ 176.2, 152.8, 134.8, 126.1, 125.0, 122.8, 121.8, 73.4, 31.1, 9.4.

2-(Benzo[d]thiazol-2-yl)propan-2-ol (3ad)[17]

White solid; yield 85% (60.7 mg); mp 81–82 °C; 1H NMR (500 MHz, CDCl3): δ 8.00 (d, J = 8.0 Hz, 1H), 7.89 (d, J = 8.0 Hz, 1H), 7.48 (t, J = 7.5 Hz, 1H), 7.38 (t, J = 7.0 Hz, 1H), 3.35 (br, 1H), 1.77 (s, 6H); 13C{1H} NMR (125 MHz, CDCl3): δ 180.1, 153.1, 135.3, 126.0, 124.9, 122.8, 121.8, 73.6, 30.8.

1-(Benzo[d]thiazol-2-yl)butan-1-ol (3ae)[17]

Yellow solid; yield 74% (56.7 mg); mp 82–83 °C; 1H NMR (500 MHz, CDCl3): δ 7.97 (d, J = 8.0 Hz, 1H), 7.88 (d, J = 8.0 Hz, 1H), 7.50–7.44 (m, 1H), 7.40–7.35 (m, 1H), 5.12 (dd, J = 8.0, 4.5 Hz, 1H), 3.41 (br, 1H), 2.05–1.88 (m, 2H), 1.63–1.46 (m, 2H), 0.98 (t, J = 7.5 Hz, 3H); 13C{1H} NMR (125 MHz, CDCl3): δ 176.8, 152.6, 134.7, 126.1, 125.0, 122.8, 121.8, 72.0, 40.2, 18.5, 13.8.

1-(Benzo[d]thiazol-2-yl)-2-methylpropan-1-ol (3af)[9b]

White oil; yield 73% (55.9 mg); 1H NMR (500 MHz, CDCl3): δ 8.01 (d, J = 8.0 Hz, 1H), 7.91 (d, J = 8.0 Hz, 1H), 7.53–7.47 (m, 1H), 7.43–7.38 (m, 1H), 4.94 (d, J = 4.5 Hz, 1H), 3.06 (br, 1H), 2.35–2.25 (m, 1H), 1.10 (d, J = 7.0 Hz, 3H), 0.99 (d, J = 7.0 Hz, 3H); 13C{1H} NMR (125 MHz, CDCl3): δ 175.5, 152.3, 134.8, 126.1, 125.1, 122.8, 121.8, 35.3, 19.1, 16.3.

2-(Benzo[d]thiazol-2-yl)butan-2-ol (3ag)

Yellow oil; yield 80% (82.8 mg); 1H NMR (500 MHz, CDCl3): δ 8.01 (d, J = 8.0 Hz, 1H), 7.90 (d, J = 8.0 Hz, 1H), 7.51–7.45 (m, 1H), 7.42–7.35 (m, 1H), 3.27 (s, 1H), 2.12–1.97 (m, 2H), 1.72 (s, 3H), 0.94 (t, J = 7.5 Hz, 3H); 13C{1H} NMR (126 MHz, CDCl3): δ 179.36, 152.97, 135.46, 125.98, 124.85, 122.87, 121.76, 76.07, 36.29, 29.14, 8.00; HRMS (ESI): calcd C11H13NOS [M + H]+, 208.0791; found, 208.0800.

1-(Benzo[d]thiazol-2-yl)-2-methylpropan-1-ol (3ah)[17]

Yellow oil; yield 68% (55.6 mg); 1H NMR (500 MHz, CDCl3): δ 8.02 (d, J = 8.0 Hz, 1H), 7.91 (d, J = 7.5 Hz, 1H), 7.54–7.47 (m, 1H), 7.44–7.38 (m, 1H), 5.14 (dd, J = 8.0, 4.5 Hz, 1H), 3.15 (br, 1H), 2.12–2.00 (m, 1H), 2.00–1.90 (m, 1H), 1.57–1.44 (m, 2H), 1.44–1.33 (m, 2H), 0.93 (t, J = 7.5 Hz, 3H); 13C{1H} NMR (125 MHz, CDCl3): δ 176.7, 152.2, 134.6, 126.3, 125.2, 122.7, 121.9, 72.3, 37.9, 27.2, 22.5, 14.0.

1-(Benzo[d]thiazol-2-yl)-3-methylbutan-1-ol (3ai)

Yellow oil; yield 67% (54.8 mg); 1H NMR (500 MHz, CDCl3): δ 8.05 (dd, J = 8.0, 0.5 Hz, 1H), 7.98–7.91 (m, 1H), 7.48 (ddd, J = 8.5, 7.0, 1.0 Hz, 1H), 7.42–7.37 (m, 1H), 4.42 (t, J = 5.0 Hz, 1H), 3.42–3.37 (m, 2H), 2.01–1.95 (m, 2H), 1.45 (s, 6H); 13C{1H} NMR (125 MHz, CDCl3): δ 180.5, 152.6, 134.4, 126.0, 124.8, 122.4, 122.1, 57.5, 45.9, 40.2, 28.6; HRMS (ESI): calcd C12H16NOS [M + H]+, 222.0947; found, 222.0954.

2-(Benzo[d]thiazol-2-yl)pentan-2-ol (3aj)

Yellow oil; yield 75% (82.9 mg); 1H NMR (500 MHz, CDCl3): δ 8.00 (d, J = 8.0 Hz, 1H), 7.93–7.87 (m, 1H), 7.52–7.45 (m, 1H), 7.42–7.35 (m, 1H), 3.16 (s, 1H), 2.05–1.93 (m, 2H), 1.72 (s, 3H), 1.61–1.41 (m, 2H), 0.92 (t, J = 7.5 Hz, 3H); 13C{1H} NMR (126 MHz, CDCl3): δ 179.54, 152.98, 135.47, 126.01, 124.87, 122.89, 121.77, 75.91, 45.77, 29.60, 17.03, 14.24; HRMS (ESI): calcd C12H16NOS [M + H]+, 222.0947; found, 222.0953.

1-(Benzo[d]thiazol-2-yl)cyclopentan-1-ol (3ak)[17]

Yellow solid; yield 71% (57.5 mg); mp 82–83 °C; 1H NMR (500 MHz, CDCl3): δ 7.98 (d, J = 8.5 Hz, 1H), 7.88 (d, J = 8.0 Hz, 1H), 7.51–7.43 (m, 1H), 7.40–7.34 (m, 1H), 3.23 (s, 1H), 2.33 (ddd, J = 15.0, 8.5, 1.5 Hz, 2H), 2.14–1.93 (m, 7H); 13C{1H} NMR (125 MHz, CDCl3): δ 179.3, 153.2, 135.3, 126.0, 124.8, 122.8, 121.7, 84.0, 42.7, 24.2.

1-(Benzo[d]thiazol-2-yl)ethane-1,2-diol (3al)[17,26]

Yellow solid; yield 73% (52.7 mg); mp 144–145 °C; 1H NMR (500 MHz, DMSO-d6): δ 8.08 (dd, J = 8.0, 0.5 Hz, 1H), 7.95 (d, J = 8.0 Hz, 1H), 7.53–7.46 (m, 1H), 7.44–7.38 (m, 1H), 6.41 (d, J = 5.0 Hz, 1H), 5.01 (t, J = 6.0 Hz, 1H), 4.92 (ddd, J = 6.5, 5.0, 4.0 Hz, 1H), 3.84 (ddd, J = 10.0, 5.5, 4.0 Hz, 1H), 3.67 (dt, J = 11.5, 6.5 Hz, 1H); 13C{1H} NMR (125 MHz, DMSO-d6): δ 176.3, 153.1, 134.4, 125.9, 124.7, 122.4, 122.2, 72.6, 65.9.

1-(Benzo[d]thiazol-2-yl)-2-(2-hydroxyethoxy)ethan-1-ol (3ao)

Yellow oil; yield 56% (49.5 mg); 1H NMR (500 MHz, CDCl3): δ 8.09 (d, J = 7.5 Hz, 1H), 7.97 (d, J = 8.0 Hz, 1H), 7.53–7.47 (m, 1H), 7.46–7.40 (m, 1H), 4.82 (dd, J = 6.0, 4.0 Hz, 1H), 3.83 (dd, J = 11.5, 4.0 Hz, 1H), 3.74 (dd, J = 11.5, 6.0 Hz, 1H), 3.67–3.61 (m, 2H), 3.60–3.56 (m, 2H), 1.22 (s, 1H); 13C{1H} NMR (125 MHz, CDCl3): δ 173.0, 152.9, 134.6, 126.0, 125.1, 122.7, 122.4, 81.2, 72.0, 64.5, 60.3; HRMS (ESI): calcd C11H13NNaO3S [M + Na]+, 262.0508; found, 262.0514.

1-(6-Methoxybenzo[d]thiazol-2-yl)ethan-1-ol (3ba)

Yellow solid; yield 85% (65.7 mg); mp 91–92 °C; 1H NMR (500 MHz, CDCl3): δ 7.86 (d, J = 9.0 Hz, 1H), 7.34 (d, J = 2.5 Hz, 1H), 7.08 (dd, J = 9.0, 2.5 Hz, 1H), 5.22 (q, J = 6.5 Hz, 1H), 3.88 (s, 3H), 3.34 (br, 1H), 1.70 (d, J = 6.5Hz, 3H); 13C{1H} NMR (125 MHz, CDCl3): δ 173.9, 157.6, 147.3, 136.3, 123.4, 115.5, 104.3, 68.5, 55.9, 24.0; HRMS (ESI): calcd C10H11NNaO2S [M + Na]+, 232.0403; found, 232.0410.

1-(5-Chlorobenzo[d]thiazol-2-yl)ethan-1-ol (3bb)

Yellow solid; yield 61% (48.1 mg); mp 109–110 °C; 1H NMR (500 MHz, CDCl3): δ 7.96 (d, J = 2.0 Hz, 1H), 7.80 (d, J = 8.5 Hz, 1H), 7.37 (dd, J = 8.5, 2.0 Hz, 1H), 5.25 (q, J = 6.5 Hz, 1H), 2.81 (br, 1H), 1.72 (d, J = 6.5 Hz, 3H); 13C{1H} NMR (125 MHz, CDCl3): δ 178.9, 153.8, 133.2, 132.2, 125.6, 122.67, 68.6, 24.0; HRMS (ESI): calcd C9H9ClNOS [M + H]+, 214.0088; found, 214.0094.

1-(6-Methoxybenzo[d]thiazol-2-yl)propan-1-ol (3bc)

Yellow solid; yield 81% (66.8 mg); mp 116–117 °C; 1H NMR (500 MHz, CDCl3): δ 7.85 (d, J = 9.0 Hz, 1H), 7.32 (d, J = 2.5 Hz, 1H), 7.06 (dd, J = 9.0, 2.5 Hz, 1H), 5.01 (dd, J = 7.0, 5.0 Hz, 1H), 3.88 (s, 3H), 3.31 (br, 1H), 2.12–2.01 (m, 1H), 2.01–1.90 (m, 1H), 1.06 (t, J = 7.5 Hz, 3H); 13C{1H} NMR (125 MHz, CDCl3): δ 173.4, 157.6, 147.2, 136.1, 123.3, 115.4, 104.3, 73.3, 55.8, 31.1, 9.4; HRMS (ESI): calcd C11H13NNaO2S [M + Na]+, 246.0559; found, 246.0566.

1-(5-Chlorobenzo[d]thiazol-2-yl)propan-1-ol (3bd)

Yellow oil; yield 55% (46.2 mg); 1H NMR (500 MHz, CDCl3): δ 7.99 (d, J = 2.0 Hz, 1H), 7.82 (d, J = 8.5 Hz, 1H), 7.38 (dd, J = 8.5, 2.0 Hz, 1H), 5.07 (dd, J = 7.0, 4.5 Hz, 1H), 2.53 (br, 1H), 2.16–2.07 (m, 1H), 1.97 (tt, J = 14.5, 7.3 Hz, 1H), 1.08 (t, J = 7.5 Hz, 3H); 13C{1H} NMR (125 MHz, CDCl3): δ 178.1, 153.6, 133.1, 132.2, 125.6, 122.7, 73.4, 31.1, 9.3, 0.01; HRMS (ESI): calcd C10H11ClNOS [M + H]+, 228.0244; found, 228.0249.

2-(6-Methoxybenzo[d]thiazol-2-yl)propan-2-ol (3be)

Yellow oil; yield 88% (72.6 mg); 1H NMR (500 MHz, CDCl3): δ 7.84 (d, J = 9.0 Hz, 1H), 7.30 (d, J = 2.5 Hz, 1H), 7.05 (dd, J = 9.0, 2.5 Hz, 1H), 3.86 (s, 3H), 3.54 (br, 1H), 1.74 (s, 6H); 13C{1H} NMR (125 MHz, CDCl3): δ 177.4, 157.5, 147.5, 136.6, 123.3, 115.3, 104.3, 73.4, 55.8, 30.8; HRMS (ESI): calcd C11H13NNaO2S [M + Na]+, 246.0559; found, 246.0568.

2-(7-Methoxybenzo[d]thiazol-2-yl)propan-2-ol (3bf)

Yellow oil; yield 85% (70.1 mg); 1H NMR (500 MHz, CDCl3): δ 7.64–7.59 (m, 1H), 7.42 (t, J = 8.1 Hz, 1H), 6.82 (d, J = 8.0 Hz, 1H), 3.99 (s, 3H), 3.37 (s, 1H), 1.76 (s, 6H); 13C{1H} NMR (125 MHz, CDCl3): δ 180.6, 154.7, 154.4, 127.0, 124.0, 115.5, 104.9, 73.5, 55.9, 30.9; HRMS (ESI): calcd C11H13NNaO2S [M + Na]+, 246.0559; found, 246.0567.

2-(5-Chlorobenzo[d]thiazol-2-yl)propan-2-ol (3bg)

White solid; yield 63% (52.9 mg); mp 133–134 °C; 1H NMR (500 MHz, CDCl3): δ 7.96 (d, J = 1.5 Hz, 1H), 7.78 (d, J = 8.5 Hz, 1H), 7.35 (dd, J = 8.5, 2.0 Hz, 1H), 3.28 (br, 1H), 1.75 (s, 6H); 13C{1H} NMR (125 MHz, CDCl3): δ 182.3, 154.1, 133.6, 132.1, 125.4, 122.7, 73.8, 30.8; HRMS (ESI): calcd C10H11ClNOS [M + H]+, 228.0244; found, 228.0247.

1-(6-Methoxybenzo[d]thiazol-2-yl)-2-methylpropan-1-ol (3bh)

Yellow oil; yield 75% (65.8 mg); 1H NMR (500 MHz, CDCl3): δ 7.86 (d, J = 9.0 Hz, 1H), 7.33 (d, J = 2.5 Hz, 1H), 7.08 (dd, J = 9.0, 2.5 Hz, 1H), 3.88 (s, 3H), 3.83 (s, 2H), 2.91 (br, 1H), 1.48 (s, 6H); 13C{1H} NMR (125 MHz, CDCl3): δ 177.6, 157.6, 146.7, 135.3, 123.1, 115.4, 104.2, 71.4, 55.8, 42.9, 26.2; HRMS (ESI): calcd C12H16NO2S [M + H]+, 238.0896; found, 238.0903.

1-(5-Chlorobenzo[d]thiazol-2-yl)-2-methylpropan-1-ol (3bi)

Yellow solid; yield 59% (52.6 mg); mp 63–64 °C; 1H NMR (500 MHz, CDCl3): δ 7.97 (d, J = 2.0 Hz, 1H), 7.79 (d, J = 8.5 Hz, 1H), 7.37 (dd, J = 8.5, 2.0 Hz, 1H), 3.84 (s, 2H), 2.99 (br, 1H), 1.50 (s, 6H); 13C{1H} NMR (125 MHz, CDCl3): δ 182.2, 153.3, 132.2, 125.5, 122.6, 122.3, 71.2, 43.3, 26.2; HRMS (ESI): calcd C10H11ClNOS [M + H]+, 228.0244; found, 228.0245.

1-(6-Methoxybenzo[d]thiazol-2-yl)butan-1-ol (3bj)

Brown solid; yield 82% (71.9 mg); mp 78–79 °C; 1H NMR (500 MHz, CDCl3): δ 7.85 (d, J = 9.0 Hz, 1H), 7.32 (d, J = 2.5 Hz, 1H), 7.07 (dd, J = 9.0, 2.5 Hz, 1H), 5.07 (dd, J = 8.0, 4.5 Hz, 1H), 3.88 (s, 3H), 3.12 (br, 1H), 2.03–1.83 (m, 2H), 1.63–1.46 (m, 2H), 0.98 (t, J = 7.5 Hz, 3H); 13C{1H} NMR (125 MHz, CDCl3): δ 173.9, 157.6, 147.0, 136.1, 123.2, 115.5, 104.3, 72.0, 55.8, 40.1, 18.5, 13.8; HRMS (ESI): calcd C12H15NNaO2S [M + Na]+, 260.0716; found, 260.0721.

2-(Tetrahydrofuran-2-yl)benzo[d]thiazole (5a)[17,27]

Colorless oil; yield 59% (44.8 mg); 1H NMR (500 MHz, DMSO-d6): δ 8.08 (dd, J = 8.0, 0.5 Hz, 1H), 7.96 (d, J = 8.0 Hz, 1H), 7.55–7.47 (m, 1H), 7.45–7.38 (m, 1H), 5.30 (dd, J = 8.0, 5.5 Hz, 1H), 4.02 (td, J = 7.5, 5.5 Hz, 1H), 3.91 (dd, J = 15.0, 7.0 Hz, 1H), 2.45 (ddd, J = 15.0, 12.5, 8.0 Hz, 1H), 2.17–2.09 (m, 1H), 2.03–1.88 (m, 2H); 13C{1H} NMR (125 MHz, DMSO-d6): δ 176.0, 153.2, 134.2, 126.1, 124.9, 122.5, 122.3, 77.9, 68.7, 32.8, 25.2.

2-(1,4-Dioxan-2-yl)benzo[d]thiazole (5b)[17,27]

Yellow solid; yield 67% (54.8 mg); mp 73–74 °C; 1H NMR (500 MHz, DMSO-d6): δ 8.12 (dd, J = 8.0, 0.5 Hz, 1H), 8.00 (d, J = 8.0 Hz, 1H), 7.52 (ddd, J = 8.0, 7.0, 1.0 Hz, 1H), 7.45 (td, J = 8.0, 1.0 Hz, 1H), 5.07 (dd, J = 9.5, 3.0 Hz, 1H), 4.18 (dd, J = 11.5, 3.0 Hz, 1H), 4.01–3.94 (m, 1H), 3.92–3.85 (m, 1H), 3.79 (d, J = 12.0 Hz, 1H), 3.68–3.57 (m, 2H); 13C{1H} NMR (125 MHz, DMSO-d6): δ 169.4, 152.5, 134.0, 126.2, 125.2, 122.7, 122.3, 74.4, 69.4, 66.3, 65.7.

2-(1,4-Dioxan-2-yl)-1H-benzo[d]imidazole (5c)

White solid; yield 55% (55.8 mg); mp 208–209 °C; 1H NMR (500 MHz, DMSO): δ 7.53 (s, 2H), 7.24–7.12 (m, 2H), 4.88 (dd, J = 9.5, 3.0 Hz, 1H), 4.09 (dd, J = 11.5, 3.0 Hz, 1H), 3.95–3.89 (m, 1H), 3.88–3.81 (m, 1H), 3.79 (d, J = 12.0 Hz, 1H), 3.73 (dd, J = 11.5, 9.5 Hz, 1H), 3.63 (ddd, J = 12.0, 11.0, 3.0 Hz, 1H); 13C{1H} NMR (151 MHz, DMSO): δ 151.44, 122.25, 72.09, 69.25, 66.53, 66.26, 60.23, 21.22, 14.54; HRMS (ESI): calcd C10H11ClNOS [M + H]+, 205.0972; found, 205.0979.

2-(1,3-Dioxolan-2-yl)benzo[d]thiazole (5d)[27]

Yellow oil; yield 49% (37.7 mg); 1H NMR (500 MHz, CDCl3): δ 8.09 (d, J = 8.5 Hz, 1H), 7.93 (d, J = 8.0 Hz, 1H), 7.55–7.48 (m, 1H), 7.47–7.41 (m, 1H), 6.26 (s, 1H), 4.26–4.11 (m, 4H); 13C{1H} NMR (125 MHz, CDCl3): δ 169.2, 153.3, 135.0, 126.3, 125.7, 123.9, 122.0, 100.6, 65.8.

2-(1,3-Dioxolan-4-yl)benzo[d]thiazole (5d′)[27]

Yellow oil; yield 20% (15.4 mg); 1H NMR (500 MHz, CDCl3): δ 8.00 (d, J = 3.0 Hz, 1H), 7.95–7.88 (m, 1H), 7.53–7.47 (m, 1H), 7.45–7.38 (m, 1H), 5.47 (dd, J = 7.0, 4.5 Hz, 1H), 5.32 (s, 1H), 5.13 (s, 1H), 4.39 (m, 1H), 4.23 (m, 1H); 13C{1H} NMR (125 MHz, CDCl3): δ 172.4, 153.5, 134.8, 126.2, 125.3, 123.1, 121.9, 96.4, 75.2, 71.0.

2-(1,2-Dimethoxyethyl)benzo[d]thiazole (5e)[17]

Yellow oil; yield 38% (31.4 mg); 1H NMR (500 MHz, CDCl3): δ 8.03 (d, J = 8.5 Hz, 1H), 7.93 (d, J = 8.0 Hz, 1H), 7.53–7.47 (m, 1H), 7.44–7.39 (m, 1H), 4.86 (dd, J = 6.5, 3.5 Hz, 1H), 3.83 (qd, J = 10.5, 5.0 Hz, 2H), 3.55 (s, 3H), 3.45 (s, 3H); 13C{1H} NMR (125 MHz, CDCl3): δ 171.5, 153.1, 135.1, 126.0, 125.2, 123.1, 121.9, 81.1, 75.3, 59.5, 58.6.

2-((2-Methoxyethoxy)methyl)benzo[d]thiazole (5e′)[17]

Yellow oil; yield 47% (38.8 mg); 1H NMR (500 MHz, CDCl3): δ 8.01 (d, J = 8.0 Hz, 1H), 7.91 (d, J = 7.5 Hz, 1H), 7.52–7.45 (m, 1H), 7.44–7.36 (m, 1H), 4.99 (s, 2H), 3.83–3.79 (m, 2H), 3.68–3.60 (m, 2H), 3.43 (s, 3H); 13C{1H} NMR (125 MHz, CDCl3): δ 170.3, 153.1, 135.1, 126.0, 125.1, 123.0, 121.8, 71.8, 70.8, 70.6, 59.2.