HETEROCYCLES

Paper | Regular issue | Vol. 89, No. 11, 2014, pp. 2545-2553
Received, 28th August, 2014, Accepted, 1st October, 2014, Published online, 8th October, 2014.
DOI: 10.3987/COM-14-13077

■ The Reaction of 2-((Trimethylsilyl)methyl)-2H-tetrazoles with Aldehydes and Ketones in the Presence of 1,8-Diazabicyclo[5.4.0]undec-7-ene

Hideaki Umemoto,* Takuya Onaka, Yasuyoshi Miki, Akira Nakamura, and Tomohiro Maegawa*

Business Development Department, FUJIMOTO CHEMICALS Co., Ltd., 1-2-38 Kinrakuji-cho, Amagasaki, Japan

Abstract

2-((Trimethylsilyl)methyl)-2H-tetrazoles were treated with aldehydes and ketones in the presence of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) to give 2-(2-hydroxyethyl)-2H-tetrazoles. This simple procedure offers a valuable strategy for the preparation of 2-(hydroxyethyl)-2H-tetrazoles.

INTRODUCTION
Tetrazoles and their derivatives constitute an interesting subclass of heterocycles with a variety of applications.1 Tetrazole derivatives have been shown to play an important role in biochemistry and medicinal chemistry.2 Recently, 2-alkyltetrazole derivatives were shown to have interesting biological properties such as antimicrobial and anticonvulsant activities.3
(Trimethylsilyl)acetonitrile4 and ethyl (trimethylsilyl)acetate5 are widely used building blocks for easily introducing alkyl, alkylidene, and acyl groups at the α-position of silicon atoms. In the case of 2-((trimethylsilyl)methyl)-2H-tetrazoles, a few examples have been reported.6 Ogata and co-workers6a reported the reaction of 1-((trimethylsilyl)methyl)-1H-tetrazole with benzophenone using t-BuOK gave the rearranged product, 5-(diphenylhydroxymethyl)-1-methyl-1H-tetrazole, instead of 1-(hydroxyethyl)-tetrazole. The reaction of 2-((trimethylsilyl)methyl)-2H-tetrazoles with aldehydes using t-BuLi was reported and they afforded 2-alkenyltetrazoles as Peterson olefination products.6b,c The alkylation of 1,5- and 2,5-substituted tetrazoles without TMS group was also reported,7 which afforded the corresponding alkylated product, but they used t-BuLi as a base. This report describes the reactivity of 2-((trimethylsilyl)methyl)-2H-tetrazoles with aldehydes and ketones using DBU as a simple synthetic procedure of 2-(hydroxyethyl)-2H-tetrazoles.

RESULTS AND DISCUSSION
Initially, 5-phenyl-2-((trimethylsilyl)methyl)-2H-tetrazole (1a), which can be purchased from Kanto Chemical Co., Ltd. or synthesized from 5-phenyltetrazole and (chloromethyl)trimethylsilane by Ogata’s procedure,6a and benzaldehyde were selected as reaction substrates, and the resulting products were studied under various reaction conditions (Table 1). The treatment of 1a with benzaldehyde in the presence of tetrabutylammonium fluoride (TBAF) in THF gave the desired product 2a8 in low yield (23%) and a desilylated product 39 with a 60% yield (entry 1). The reaction was repeated in the presence of KOAc, which has been used in reactions of (trimethylsilyl)acetonitrile with benzaldehyde,4e to afford 2a in low yield (17%) with recovery of 1a (entry 2). Because the electron-withdrawing character of the tetrazole ring is weaker than that of cyanide, stronger bases were evaluated. Compound 1a hardly reacted with benzaldehyde in the presence of Li2CO3, and 1a was recovered (entry 3). The reaction of 1a with benzaldehyde using Na2CO3 did not reach completion (entry 4), but the reaction using K2CO3 afforded 2a in 76% yield and 3 in 19% yield (entry 5). However, when the same reaction was carried out with NaOH, the yield of 2a decreased significantly (62%, entry 6). The use of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) as an organic base gave 2a and 3 in 87% and 13% yields, respectively (entry 7). In the use of catalytic amount of DBU (0.1 equiv), 2a also was obtained with increase of 3 (entry 8).

Compound 1a did not react with benzaldehyde in the presence of other organic bases such as pyridine, Et3N, and N,N-diisopropylethylamine (DIPEA) (entries 9–11). The reaction proceeded well in N-methylpyrrolidone (NMP), DMSO, DMF, MeCN, and THF as a solvent (Table 2, entries 1–5). In the use of MeOH and t-BuOH, desilylated product 3 was obtained as a main product (entries 6, 7).

The results for the reaction of 1a with aldehydes and ketones in the presence of DBU under optimized conditions are summarized in Table 3. Aromatic aldehydes having electron-donating or -withdrawing groups reacted smoothly to provide the desired products 2b–2f in good yields (64–88%) (entries 2–6). The reaction of 2-methoxybenzaldehyde afforded the product 2g in low yield (32%) (entry 7). Reaction with cinnamaldehyde also gave the desired product 2h (entry 8). Aliphatic aldehydes also provided the desired products in good yields (79–82%) (entries 9, 10). Conversely, reactions with ketones proceeded but the yields of desired products (2k–2m) were low (18–43%), and the desilylated product 3 was obtained as a side product (entries 11–13).
Next, the reactions of various 2-((trimethylsilyl)methyl)-2H-tetrazoles (1b–1d) with benzaldehyde were examined. Although the reaction of 5-amino-2-((trimethylsilyl)methyl)-2H-tetrazole (1b) resulted in a low yield (Table 4, entry 1), reactions with 5-methyl-2-((trimethylsilyl)methyl)-2H-tetrazole (1c) and 2-((trimethylsilyl)methyl)-2H-tetrazole (1d) gave the corresponding adducts in good yields (entries 2, 3). In a similar manner, 1-(2-chlorophenyl)-2-(2H-tetrazol-2-yl)ethanol3a (4d), which is intermediate of investigational drug with anticonvulsant activity, was synthesized with a 79% yield by the reaction of 1d with 2-chlorobenzaldehyde (entry 4).

In conclusion, this report demonstrates that 2-((trimethylsilyl)methyl)-2H-tetrazoles may be used as building blocks for 2-alkyltetrazoles by reactions with aldehydes and ketones. This simple synthetic procedure provides a valuable means of preparing a wide variety of 2-(hydroxyethyl)-2H-tetrazoles.

EXPERIMENTAL
Melting points were uncorrected. IR spectra were recorded using an attenuated total reflectance measurement. 1H-NMR spectra were recorded at 400 MHz and 13C-NMR spectra were recorded at 100 MHz with tetramethylsilane as an internal standard. High resolution mass spectra were recorded on a time-of-flight instrument using electrospray ionization method. Column chromatography was performed with silica gel 60N (40–100 µm, spherical, neutral). All materials and solvents were purchased and used without further purification.
Typical Procedure for Preparation of Products 2, 3, and 4.
1-Phenyl-2-(5-phenyl-2H-tetrazol-2-yl)ethanol8 (2a) and 2-methyl-5-phenyl-2H-tetrazole9 (3)
DBU (0.50 mmol) was added to the mixture of 5-phenyl-2-((trimethylsilyl)methyl)-2H-tetrazole (1a) (0.50 mmol) and benzaldehyde (0.55 mmol) in dry NMP (0.5 mL). The mixture was stirred at 35 °C for 19 h. Hydrochloric acid (1.0 mol/L) was added to the reaction mixture and the mixture was stirred at room temperature for 0.5 h. Water was added to the reaction mixture and the mixture was extracted with AcOEt. The organic layer was washed with water, dried over Na2SO4, and concentrated under reduced pressure. The residue was purified by column chromatography on silica gel to afford the 2a (87%) and 3 (13%).
2a: white solid; mp 97–98 °C (n-hexane-CHCl3, lit.8 100.5–102 °C); IR (ATR) 3234 cm-1; 1H-NMR (CDCl3) δ 3.02 (1H, d, J = 3.6 Hz, OH), 4.86 (1H, d, J = 5.2 Hz, N-CH2-), 4.87 (1H, d, J = 7.2 Hz, N-CH2-), 5.35-5.45 (1H, m, -CH-OH), 7.37-7.57 (8H, m, aromatic protons), 8.12-8.20 (2H, m, aromatic protons); 13C-NMR (CDCl3) δ 59.9, 72.3, 125.9, 126.8, 126.9, 128.6, 128.8, 128.8, 130.4, 139.4, 165.0; HRMS (ESI) calcd for C15H15N4O [M+H]+ 267.1246, found 267.1246.
3: colorless crystals; mp 51 °C (n-hexane); 1H-NMR (CDCl3) δ 4.41 (3H, s, CH3), 7.44-7.53 (3H, m, aromatic protons), 8.12-8.19 (2H, m, aromatic protons); 13C-NMR (CDCl3) δ 39.5, 126.8, 127.4, 128.9, 130.3, 165.3; HRMS (ESI) calcd for C8H9N4 [M+H]+ 161.0827, found 161.0826.
Methyl 4-(1-hydroxy-2-(5-phenyl-2H-tetrazol-2-yl)ethyl)benzoate (2b):
White solid; mp 110–111 °C (n-hexane-CHCl3); IR (ATR) 3356, 1712 cm-1; 1H-NMR (CDCl3) δ 3.15-3.70 (1H, bs, OH), 3.93 (3H, s, CH3), 4.86 (1H, d, J = 6.8 Hz, N-CH2-), 4.86 (1H, d, J = 5.2 Hz, N-CH2-), 5.46 (1H, dd, J = 5.6, 6.8 Hz, -CH-OH), 7.47-7.52 (3H, m, aromatic protons), 7.55 (2H, d, J = 8.0 Hz, aromatic protons), 8.07 (2H, d, J = 8.0 Hz, aromatic protons), 8.10-8.15 (2H, m, aromatic protons); 13C-NMR (CDCl3) δ 52.3, 59.6, 72.1, 126.0, 126.9, 129.0, 129.6, 130.2, 130.5, 130.6, 144.1, 165.3, 166.6; HRMS (ESI) calcd for C17H17N4O3 [M+H]+ 325.1301, found 325.1308.
1-(4-Bromophenyl)-2-(5-phenyl-2H-tetrazol-2-yl)ethanol (2c):
Colorless crystals; mp 131–132 °C (EtOH); IR (ATR) 3307 cm-1; 1H-NMR (CDCl3) δ 3.12 (1H, d, J = 4.0 Hz, OH), 4.83 (1H, d, J = 7.2 Hz, N-CH2-), 4.83 (1H, d, J = 5.6 Hz, N-CH2-), 5.32-5.42 (1H, m, -CH-OH), 7.32-7.38 (2H, m, aromatic protons), 7.48-7.57 (5H, m, aromatic protons), 8.12-8.18 (2H, m, aromatic protons); 13C-NMR (CDCl3) δ 59.7, 71.8, 126.9, 127.0, 127.4, 129.0, 129.2, 130.6, 134.6, 137.7, 165.4; HRMS (ESI) calcd for C15H14N4OBr [M+H]+ 345.0351, found: 345.0356.
1-(4-Chlorophenyl)-2-(5-phenyl-2H-tetrazol-2-yl)ethanol3b (2d):
Colorless crystals; mp 129–130 °C (EtOH, lit.3b 120–121 °C); IR (ATR) 3303 cm-1; 1H-NMR (CDCl3) δ 3.12 (1H, bs, OH), 4.83 (1H, d, J = 6.8 Hz, N-CH2-), 4.84 (1H, d, J = 5.2 Hz, N-CH2-), 5.38 (1H, t, J = 5.8 Hz, -CH-OH), 7.37-7.44 (4H, m, aromatic protons), 7.48-7.53 (3H, m, aromatic protons), 8.12-8.18 (2H, m, aromatic protons); 13C-NMR (CDCl3) δ 59.6, 71.9, 122.8, 126.9, 127.0, 127.7, 129.0, 130.6, 132.1, 138.2, 165.4; HRMS (ESI) calcd for C15H14N4OCl [M+H]+ 301.0856, found: 301.0859.
1-(2-Chlorophenyl)-2-(5-phenyl-2H-tetrazol-2-yl)ethanol (2e):
Colorless crystals; mp 89–90 °C (EtOH); IR (ATR) 3248 cm-1; 1H-NMR (CDCl3) δ 3.33 (1H, d, J = 4.0 Hz, OH), 4.77 (1H, dd, J = 8.8, 14.0 Hz, N-CH2-), 5.01 (1H, dd, J = 2.4, 13.6 Hz, N-CH2-), 5.74-5.80 (1H, m, -CH-OH), 7.28-7.38 (2H, m, aromatic protons), 7.41 (1H, dd, J = 1.6, 7.6 Hz, aromatic proton), 7.47-7.53 (3H, m, aromatic protons), 7.68 (1H, dd, J = 1.6, 7.6 Hz, aromatic proton), 8.12-8.18 (2H, m, aromatic protons); 13C-NMR (CDCl3) δ 58.1, 69.3, 126.9, 127.1, 127.4, 127.5, 128.9, 129.7, 129.7, 130.5, 131.8, 136.7, 165.3; HRMS (ESI) calcd for C15H14N4OCl [M+H]+ 301.0856, found: 301.0859.
1-(4-Methoxyphenyl)-2-(5-phenyl-2H-tetrazol-2-yl)ethanol6b (2f):
White solid; mp 84–85 °C (n-hexane-CHCl3); IR (ATR) 3367 cm-1; 1H-NMR (CDCl3) δ 2.98 (1H, d, J = 3.2 Hz, OH), 3.82 (3H, s, CH3), 4.77-4.88 (2H, m, N-CH2-), 5.31-5.38 (1H, m, -CH-OH), 6.93 (2H, d, J = 8.8 Hz, aromatic protons), 7.39 (2H, d, J = 8.8 Hz, aromatic protons), 7.47-7.53 (3H, m, aromatic protons), 8.12-8.18 (2H, m, aromatic protons); 13C-NMR (CDCl3) δ 55.4, 59.9, 72.1, 114.3, 126.9, 127.2, 127.3, 128.9, 130.5, 131.4, 159.9, 165.2; HRMS (ESI) calcd for C16H17N4O2 [M+H]+ 297.1352, found: 297.1362.
1-(2-Methoxyphenyl)-2-(5-phenyl-2H-tetrazol-2-yl)ethanol (2g):
A colorless oil; IR (ATR) 3369 cm-1; 1H-NMR (CDCl3) δ 3.35 (1H, d, J = 6.8 Hz, OH), 3.91 (3H, s, CH3), 4.87-4.99 (2H, m, N-CH2-), 5.50-5.57 (1H, m, -CH-OH), 6.94 (1H, d, J = 8.0 Hz, aromatic proton), 7.00 (1H, dt, J = 0.1, 7.4 Hz, aromatic proton), 7.33 (1H, dt, J = 0.1, 8.0 Hz, aromatic proton), 7.43 (1H, dt, J = 0.2, 7.6 Hz, aromatic proton), 7.47-7.53 (3H, m, aromatic protons), 8.12-8.18 (2H, m, aromatic protons); 13C-NMR (CDCl3) δ 55.4, 58.6, 69.6, 110.5, 121.1, 126.9, 127.1, 127.3, 127.3, 128.9, 129.6, 130.3, 156.4, 165.1; HRMS (ESI) calcd for C16H17N4O2 [M+H]+ 297.1352, found: 297.1363.
(E)-4-Phenyl-1-(5-phenyl-2H-tetrazol-2-yl)but-3-en-2-ol (2h):
Colorless crystals; mp 136–137 °C (EtOH); IR (ATR) 3425 cm-1; 1H-NMR (CDCl3) δ 2.85 (1H, d, J = 4.0 Hz, OH), 4.76-4.88 (2H, m, N-CH2-), 4.96-5.03 (1H, m, -CH-OH), 6.26 (1H, dd, J = 6.4, 16.0 Hz, -CH=CH-Ph), 6.79 (1H, d, J = 16.0 Hz, -CH=CH-Ph), 7.26-7.41 (5H, m, aromatic protons), 7.46-7.52 (3H, m, aromatic protons), 8.12-8.18 (2H, m, aromatic protons); 13C-NMR (CDCl3) δ 58.4, 71.0, 126.3, 126.7, 126.9, 127.1, 128.3, 128.7, 128.9, 130.5, 133.3, 135.8, 165.3; HRMS (ESI) calcd for C17H17N4O [M+H]+ 293.1402, found: 293.1399.
1-(5-Phenyl-2H-tetrazol-2-yl)pentan-2-ol (2i):
A colorless oil; IR (ATR) 3381 cm-1; 1H-NMR (CDCl3) δ 0.98 (3H, t, J = 7.0 Hz, CH3), 1.44-1.64 (4H, m, CH3-CH2-CH2-), 2.67 (1H, d, J = 5.2 Hz, OH), 4.22-4.32 (1H, m, -CH-OH), 4.63 (1H, dd, J = 8.0, 14.0 Hz, N-CH2-), 4.73 (1H, dd, J = 3.0, 14.0 Hz, N-CH2-), 7.47-7.54 (3H, m, aromatic protons), 8.12-8.18 (2H, m, aromatic protons); 13C-NMR (CDCl3) δ 13.9, 18.6, 36.3, 58.8, 69.8, 126.9, 127.2, 128.9, 130.5, 165.2; HRMS (ESI) calcd for C12H17N4O [M+H]+ 233.1402, found: 233.1386.
3-Methyl-1-(5-phenyl-2H-tetrazol-2-yl)butan-2-ol (2j):
A colorless oil; IR (ATR) 3381 cm-1; 1H-NMR (CDCl3) δ 1.07 (6H, d, J = 6.8 Hz, CH3), 1.82 (1H, oct, J = 6.5 Hz, (CH3)2CH-), 2.64 (1H, d, J = 4.8 Hz, OH), 3.98-4.07 (1H, m, -CH-OH), 4.67 (1H, dd, J = 8.4, 14.0 Hz, N-CH2-), 4.77 (1H, dd, J = 3.2, 14.0 Hz, N-CH2-), 7.44-7.54 (3H, m, aromatic protons), 8.10-8.19 (2H, m, aromatic protons); 13C-NMR (CDCl3) δ 17.5, 18.7, 31.8, 57.0, 74.7, 126.9, 127.2, 128.9, 130.5, 165.2; HRMS (ESI) calcd for C12H17N4O [M+H]+ 233.1402, found: 233.1395.
2-Methyl-1-(5-phenyl-2H-tetrazol-2-yl)butan-2-ol (2k):
A colorless oil; IR (ATR) 3430 cm-1; 1H-NMR (CDCl3) δ 1.02 (3H, t, J = 7.6 Hz, CH2CH3), 1.22 (3H, s, CH3), 1.53 (2H, q, J = 7.6 Hz, CH2CH3), 2.83 (1H, s, OH), 4.66 (1H, d, J = 14.0 Hz, N-CH2-), 4.71 (1H, d, J = 14.0 Hz, N-CH2-), 7.45-7.54 (3H, m, aromatic protons), 8.11-8.21 (2H, m, aromatic protons); 13C-NMR (CDCl3) δ 9.0, 23.9, 32.3, 61.7, 72.5, 126.9, 127.1, 128.9, 130.5, 165.2; HRMS (ESI) calcd for C12H17N4O [M+H]+ 233.1402, found: 233.1400.
2-Phenyl-1-(5-phenyl-2H-tetrazol-2-yl)propan-2-ol (2l):
A colorless oil; IR (ATR) 3419 cm-1; 1H-NMR (CDCl3) δ 1.59 (3H, s, CH3), 3.70 (1H, s, OH), 4.90 (1H, d, J = 14.0 Hz, N-CH2-), 5.00 (1H, d, J = 14.0 Hz, N-CH2-), 7.25-7.38 (3H, m, aromatic protons), 7.46-7.54 (5H, m, aromatic protons), 8.08-8.15 (2H, m, aromatic protons); 13C-NMR (CDCl3) δ 27.4, 63.1, 74.1, 124.8, 126.9, 127.8, 128.6, 128.9, 130.6, 143.3, 165.1; HRMS (ESI) calcd for C16H17N4O [M+H]+ 281.1402, found: 281.1394.
1,1-Diphenyl-2-(5-phenyl-2H-tetrazol-2-yl)ethanol (2m):
Colorless needles; mp 149–150 °C (EtOH); IR (ATR) 3472 cm-1; 1H-NMR (CDCl3) δ 4.84 (1H, s, OH), 5.39 (2H, s, N-CH2-), 4.71 (1H, d, J = 14.0 Hz, N-CH2-), 7.22-7.35 (6H, m, aromatic protons), 7.44-7.54 (7H, m, aromatic protons), 8.03-8.10 (2H, m, aromatic protons); 13C-NMR (CDCl3) δ 61.5, 77.9, 126.1, 126.8, 126.9, 127.9, 128.5, 128.9, 130.6, 142.5, 164.8; HRMS (ESI) calcd for C21H19N4O [M+H]+ 343.1559, found: 343.1546.
2-(5-Amino-2H-tetrazol-2-yl)-1-phenylethanol (4a):
Colorless crystals; mp 141–142 °C (CHCl3); IR (ATR) 3436, 3305 cm-1; 1H-NMR (DMSO-d6) δ 4.42-4.54 (2H, m, N-CH2-), 5.05-5.09 (1H, m, -CH-OH), 5.78 (1H, d, J = 5.2 Hz, OH), 5.98 (2H, s, NH2), 7.26-7.40 (5H, m, aromatic protons); 13C-NMR (DMSO-d6) δ 58.8, 70.7, 126.0, 127.6, 128.2, 141.7, 166.8; HRMS (ESI) calcd for C9H12N5O [M+H]+ 206.1042, found: 206.1043.
2-(5-Methyl-2H-tetrazol-2-yl)-1-phenylethanol (4b):
White solid; mp 61 °C (n-hexane-CHCl3); IR (ATR) 3351 cm-1; 1H-NMR (CDCl3) δ 2.56 (3H, s, CH3), 3.01 (1H, d, J = 3.6 Hz, OH), 4.76 (1H, d, J = 6.4 Hz, N-CH2-), 4.76 (1H, d, J = 5.6 Hz, N-CH2-), 5.28-5.33 (1H, m, -CH-OH), 7.33-7.47 (5H, m, aromatic protons); 13C-NMR (CDCl3) δ 10.9, 59.5, 72.5, 125.9, 128.7, 128.9, 139.3, 163.1; HRMS (ESI) calcd for C10H13N4O [M+H]+ 205.1089, found: 205.1077.
1-Phenyl-2-(2H-tetrazol-2-yl)ethanol (4c):
A colorless oil; IR (ATR) 3410 cm-1; 1H-NMR (CDCl3) δ 2.90 (1H, d, J = 3.6 Hz, OH), 4.84 (1H, dd, J = 4.4, 13.8 Hz, N-CH2-), 4.89 (1H, dd, J = 8.0 13.8 Hz, N-CH2-), 5.32-5.40 (1H, m, -CH-OH), 7.34-7.48 (5H, m, aromatic protons), 8.55 (1H, s, H-5); 13C-NMR (CDCl3) δ 59.7, 72.5, 125.9, 128.9, 129.0, 139.2, 152.9; HRMS (ESI) calcd for C9H11N4O [M+H]+ 191.0933, found: 191.0932.
1-(2-Chlorophenyl)-2-(2H-tetrazol-2-yl)ethanol3a (4d):
Colorless crystals; mp 71–72 °C (n-hexane-CHCl3); IR (ATR) 3346 cm-1; 1H-NMR (CDCl3) δ 3.24 (1H, d, J = 4.0 Hz, OH), 4.79 (1H, dd, J = 8.4, 14.0 Hz, N-CH2-), 4.99 (1H, dd, J = 2.6, 13.8 Hz, N-CH2-), 5.70-5.76 (1H, m, -CH-OH), 7.27-7.43 (2H, m, aromatic protons), 7.41 (1H, dd, J = 1.6, 7.6 Hz, aromatic proton), 7.63 (1H, dd, J = 2.0, 7.6 Hz, aromatic proton), 8.55 (1H, s, H-5); 13C-NMR (CDCl3) δ 58.0, 69.2, 127.3, 127.5, 129.7, 129.8, 131.8, 136.6, 152.9; HRMS (ESI) calcd for C9H10N4OCl [M+H]+ 225.0543, found: 225.0527.

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