An efficient one-pot synthesis of N,N′-disubstituted ureas and carbamates from N-acylbenzotriazoles

Abstract

A facile and high-yielding one-pot synthesis of carbamates and N,N′-disubstituted symmetrical ureas from N-acylbenzotriazoles has been devised. It is believed that, the intermediate acyl-azide undergo Curtius rearrangement and in different solvents gives different products i.e. carbamates in alcohols and N,N′-disubstituted symmetrical urea in THF.

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Introduction

Functionalized urea and carbamate motifs are essential structural elements of many biologically active compounds.¹ Also, their derivatives play a vital role in the research of pharmaceuticals and organic chemistry.² Generally, synthesis of substituted urea involves the reaction of suitable amines with urea,³ phosgenation⁴ and reductive or oxidative carbonylation of amines.⁵ But these well-established protocols include some drawbacks, such as, insufficiency for symmetric ureas, involvement of highly toxic reagents and longer reaction time. During past few years, a number of papers have reported synthesis of urea and carbamate derivatives via metal catalyst.⁶ Katritzky et al. have synthesized the symmetric ureas^7a,b where the benzotriazole moiety acts as a leaving group and is substituted by amines. This methodology has been further explored in a number of ways by many research groups and contributed thousands of pharmaceutical compounds.⁸ However, a better yielding, non-toxic, mild and practical approach for synthetic, pharmaceutical as well as industrial significance is still under investigation.

In this manuscript, we have introduced N-acylbenzotriazole as a suitable reagent for the preparation of ureas and carbamates via Curtius rearrangement.⁹ Good leaving tendency of benzotriazole moiety has been used in present work to afford ureas and carbamates without adding amine (Scheme 1).

Scheme 1 Comparative illustration of previous and present work.

Certainly, an N-acyl azide intermediate is formed, which on hydrolysis generates amines in situ. Acyl-azide, on heating, undergoes Curtius rearrangement which leads to formation of corresponding isocyanates. These isocyanates, on reaction with variety of amines (in situ generated by hydrolysis of isocyanates) and/or alcohols, give corresponding ureas and carbamates.

Results and discussion

Our strategy began with the synthesis of core compound N-acyl benzotriazoles by the reaction of corresponding aromatic/aliphatic acids with benzotriazole using SOCl₂ reagent in dichloromethane, following the sound known process reported in literature.¹⁰ The one-pot reaction procedure completed within two hours and the crude mass was easily purified by flash column chromatography. The corresponding N-acylbenzotriazoles 2a–q (Table 1) were characterized by IR, mass and NMR spectroscopic studies.

Table 1 Synthesis of N-acylbenzotriazoles (ArCOBt) 2a–q from acids^a

a Molar ratios: carboxylic acids (1a–q) (1.0 equiv.), SOCl₂ (1.2 equiv.), benzotriazole (3.25 equiv.). Yields reported after purification by column chromatography (SiO₂).

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The acylated benzotriazole derivatives play a key role in large number of reactions and can be used to synthesize a variety of compounds.¹¹ Formerly, we have also synthesized amide by the help of acylated benzotriazole derivatives in excellent yields.¹² Our present work is focussed on further exploration of benzotriazole methodology for Curtius rearrangement, through which synthesis of carbamates and symmetric ureas can be achieved. N-acyl benzotriazole, when refluxed with sodium azide in presence of homogeneous medium of water and protic organic solvent, gives carbamates via isocyanate intermediate, generated in situ when N-acylazides were heated. Similar protocol, when applied in water and organic aprotic solvent system, affords symmetric ureas.

In our proto-type reaction, a mixture of (1H-benzo[d][1,2,3]triazol-1-yl)(phenyl)methanone 2a (2.0 mmol) & sodium azide (3.0 mmol) was refluxed in water–ethanol (1 : 1 ratio mixture) for 3 hours (Scheme 2).

Scheme 2 Synthesis of carbamate (3a) and symmetric ureas (6a).

Among the three spots appeared on TLC (10% ethyl acetate/n-hexane) of the reaction mass, first spot (R_f = 0.7) was identified as ester, second spot (R_f = 0.5) was characterized as carbamate and the third spot (R_f = 0.4) was symmetric urea. The ratio of the three products i.e. ester, carbamate and urea, was found to be 1.35 : 1.61 : 1 respectively and these compounds have been successfully isolated in pure form after column chromatography (SiO₂) using gradient of EtOAc/n-hexane. The formation of carbamate 3a and urea 6a, without externally added amines suggests that Curtius rearrangement must be involved here which gives amines by the rearrangement of acyl azides. Further, in search of appropriate solvent system to obtain optimum yield of our products of interest, i.e. urea and carbamate, we performed the reaction in different type of organic solvents with water in different compositions (Table 2).

Table 2 Optimization of reaction conditions

Entry^a	Solvent system^b	Ratios	Yield of 3a^c (%)	Yield of 6a^c (%)
a Molar ratio: 1H-benzo[d][1,2,3]triazol-1-yl)(phenyl)methanone 2a (2.0 mmol) & sodium azide (6.0 mmol).b Binary solvents.c Yields reported after purification by column chromatography (SiO2).
1	Ethanol/water	50/50	37	23
2	Ethanol/water	85/15	64	17
3	Ethanol/water	90/10	65	7
4	Ethanol/water	95/05	67	5
5	Ethanol/water	100/0	20	0
6	Ethanol/water	15/85	29	20
7	i-PrOH/water	50/50	41	26
8	THF/water	50/50	0	67
9	THF/water	15/85	0	18
10	THF/water	70/30	0	25
11	THF/water	85/15	0	81
12	t-BuOH/water	50/50	0	43
13	Toluene/water	50/50	0	53
14	Benzene/water	50/50	0	54
15	DCM/water	50/50	0	Trace
16	Acetone/water	50/50	0	57
17	Ethyl acetate/water	50/50	0	55

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It was observed that carbamates were formed preferentially when the binary solvent system was a mixture of nucleophilic-protic solvent (generally alcohols) and water (Table 2; entry 1–7). Greater composition of nucleophilic alcohols ensures the capture of intermediate isocyanate leading to formation of carbamate as the major product (Table 2; entry 3 & 4). A 19 : 1 ratio of alcohol and water was found to be the most reliable solvent system. However, symmetric urea was the major product, when the binary solvent includes a non-nucleophilic-aprotic organic solvent along with water (Table 2; entry 8–17). The carbamate formation was inhibited possibly due to the capture of intermediate isocyanate with aniline in absence of nucleophilic alcohols. The essentiality of water is for the solubility of sodium azide as well as for Curtius rearrangement. Further, to find out optimum ratios of different solvents for the most favourable solvent system to obtain better yields of urea and carbamate separately, the same proto-type reaction was carried out in different solvent ratios as depicted in Table 2 & Fig. 1.

Fig. 1 Optimization for carbamate.

After the optimization of solvent system, we investigated the reaction for generalization by varying the alcohol and acylated benzotriazole derivatives. Experimentally, it was found that primary and secondary alcohols (for example, methanol, ethanol, 1-propanol, 1-butanol, 1-pentanol, and isopropyl alcohol) give good yields of carbamates. It was noticed that reaction with tertiary alcohols lead to the formation of symmetric urea as the major product. For example, reaction of compound 2a with sodium azide in presence of t-butanol/water (in 1 : 1) ratio gave symmetric urea 6a. A library of 20 compounds has been prepared which contains both aliphatic (saturated/unsaturated) and aromatic carbamates in moderate to good yields (Table 3). The yield of carbamate 3l, formed with methanol/water as solvent, is fairly low since methanol undergoes substitution relatively at higher rate than rearrangement. Benzotriazole derivatives of phenyl acetic acid 2j and 2p afforded carbamates 3i, 3s and 3t in good yields. Reaction of bezotriazole derivative of isovaleric acid 2q under similar reaction condition did not produce carbamate product even in trace amount. Compounds have been characterized on the basis of their NMR, mass and IR spectra.

Table 3 Synthesis of carbamates (3a–t) from acylbenzotriazoles (ArCOBt) (2a–q) by NaN₃ induced Curtius rearrangement^a

a Molar ratios: acyl benzotriazoles (1.0 equiv.), NaN₃ (3.0 equiv.), alcohols : water (19 : 1). Yields reported after purification by column chromatography.

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To enhance the generality and utility of this methodology, we further explored the reaction for the synthesis of intramolecular carbamates (Scheme 3). The reaction of compound 2o under above mentioned conditions affords cyclic carbamate 4 via intramolecular cyclization. The intermediate isocyanate formed by Curtius rearrangement must have been captured by hydroxyl at ortho to the carbonyl. The structure of compound 4 has been characterized by NMR, mass and IR spectra. Also, the structure of compound 4 was established by single crystal X-ray analysis. The crystallographic and instrumental details for compound 4 have been summarized in Table 4.

Scheme 3 Preparation of carbamate 4 from corresponding acyl benzotriazole (2o). Molar ratio: acyl benzotriazole (1.0 equiv.), NaN₃ (3 equiv.), alcohols/THF : water (8 : 2). Yields reported after purification by column chromatography (SiO₂).

Table 4 Crystallographic refinement data^a for compound 4

a For details, see ESI file (CIF) enclosed with manuscript.
Property	4
Mol. formula	C7H5NO2
Formula weight	135.12
Crystal system	Orthorhombic
Space group	P212121
a (Å)	4.4436(14)
b (Å)	6.641(2)
c (Å)	20.957(6)
α, β, γ (^o)	90, 90, 90
V (Å^3)	618.5(3)
Z	4
Density (calc.)	1.451
F(000)	264
μ (mm^−1)	0.108
Crystal size [mm]	0.14 × 0.15 × 0.22
Temperature (K)	293
Radiation (MoKα)	0.71073
θ min–max [^o]	3.22, 24.94
h, k, l	−5 : 5; −6 : 89; −16 : 27
Tot., uniq. data, R(int)	3269, 1471, 0.0267
Obs. data [I > 2.0 σ(I)]	1322
Nref, Npar	1528, 91
R1, wR2, S	0.0386, 0.0969, 1.061
Min–max resd. dens. [e Å^−3]	−0.258, 0.133
CCDC	1482294

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During our investigation, it was found that in intramolecular reaction, single and same product is formed with almost 100% conversion, regardless the solvent system we used. However the results are optimum with aprotic polar solvents. Interestingly, when we used thiols instead of alcohol in the reaction, thiocarbamates were obtained in moderate yields (Scheme 4).

Scheme 4 Preparation of thiocarbamates 5a and 5b from corresponding acyl benzotriazole (2a, 2l). Yields reported after purification by column chromatography (SiO₂).

Contrary to carbamates, urea was preferably formed in aprotic organic solvent–water system and THF/water medium was found the best in this regards. Further optimization of solvent-system composition and temperature for the reaction revealed that THF/water in 85 : 15 ratios at 90 °C gave most significant results. We explored this altered reaction path by changing functionality of N-acyl benzotriazole and a series containing seven different symmetric ureas (Table 5) has been developed.

Table 5 Synthesis of symmetric urea using THF/water system^a

a Molar ratios: acyl benzotriazoles (1.0 equiv.), NaN₃ (3.0 equiv.), THF : water (85 : 15). Yields reported after purification by column chromatography (SiO₂).

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The structures of compounds 6a–g have been characterized by NMR, mass and IR spectra. Also, the structure of compound 6a was established by single crystal X-ray analysis (see ESI CIF file for details†).

Mechanistic consideration

A possible mechanistic way to explicate the product formation and rearrangement is given in mechanistic consideration (Scheme 5). The mechanism certainly involves Curtius rearrangement as the key step of reaction which gives amines by the rearrangement of acyl azides.

Scheme 5 Plausible mechanism involving Curtius rearrangement.

At first, intermediate acyl azide A, formed by attack of azide ion on carbonyl carbon to replace the benzotriazole moiety in acyl benzotriazole, undergoes Curtius rearrangement to form isocyanate intermediate B with the consequent loss of molecular nitrogen (N₂). Finally, the intermediate isocyanate is immediately captured by nucleophilic alcoholic solvent to afford carbamate product. Alternatively, isocyanate B generates aniline via carbamic acid, which undergoes decarboxylation to afford anilines. This aniline, on reaction with intermediate isocyanate B, generates symmetric urea.

Combination with click chemistry

With escalating importance of click-chemistry¹³ as an efficient tool for development of biologically and pharmaceutically relevant compounds, the significance of the methodologies which can easily and effectively afford starting materials for click reaction (i.e. azides and alkynes) has also increased. After successfully employing our methodology in generation of carbamates with a terminal alkyne functionality, we extended this work for cascade synthesis of 1,4-substituted 1,2,3-triazolyl carbohydrate derivatives via click chemistry (Scheme 6).

Scheme 6 Click-inspired synthesis of novel triazoyl glycoconjugate carbamates (8a–c). Molar ratios: acyl benzotriazoles (1.0 equiv.), NaN₃ (6.0 equiv.), ^tBuOH–water mixture (8 : 2), sugar azide (1.1 equiv.), CuSO₄·H₂O (0.02 mol%) and NaAsc (0.2 mol%). Method-I: one-pot sequential reaction; method-II: successive reaction. Yields reported after purification by column chromatography (SiO₂).

The reaction involved heating N-acyl benzotriazole with NaN₃ in presence of water and propargyl alcohol (as reactant as well as solvent) followed by removal of excess propargyl alcohol by evaporation and addition of sugar azide along with CuSO₄ and sodium ascorbate in ^tBuOH–water mixture (8 : 2). Though the reaction moved in the way we expected, but the desired triazolyl sugar derivatives were formed with very low yields (8–13%). On the other hand conventional click reaction of developed alkyne-functionalized carbamates (3q and 3r) with sugar azide resulted in excellent yields (88–94%) of triazolyl products. The low yield in cascade approach may be due to some side reactions involved in this process. We are trying to find out the exact causes behind it to improve this reaction for better results.

Conclusions

In summary, we have developed a one-pot, low-toxic and efficient method for synthesis of ureas and carbamates from N-acyl benzotriazole. Through this synthetic route, we have successfully achieved good yields under mild reaction conditions in aqueous medium without using catalyst and with simple purification methods. Furthermore, we have efficiently utilized this methodology for generation of terminal alkyne armed carbamates which were clicked with sugar azides to afford novel triazolyl sugar derivatives. A cascade approach was also attempted for synthesis of triazolyl sugar derivatives by employing this methodology along with click-chemistry, but results were not very pleasing. Work on improvement of cascade approach is ongoing in our laboratory.

Experimental

General remarks

All reagents and solvents were of pure analytical grade. Thin-layer chromatography (TLC) was performed on 60 F254 silica gel, pre-coated on aluminium plates and revealed with either a UV lamp (λ_max = 254 nm) or a specific colour reagent (Dragendorff reagent or iodine vapour) or by spraying with methanolic H₂SO₄ solution and subsequent charring by heating at 100 °C. Solvents were evaporated under reduced pressure at temperature <50 °C. Column chromatography was carried out on silica gel (230–400 mesh, E Merck). Distilled n-hexane and ethyl acetate were used for the column chromatography. ¹H and ¹³C NMR were recorded at 300 and 75 MHz, respectively. Chemical shifts are given in ppm downfield from internal TMS; J values in Hz. Mass spectra were recorded using electron spray ionization mass spectrometry (ESI-MS). Elemental analyses were performed on a Perkin-Elmer 2400 C, H, N analyzer and values were found to be within ±0.5% of the calculated values. Infrared spectra recorded as Nujol mulls in KBr plates.

Single-crystal X-ray data of compound 6a were collected on Xcalibur Eos (Oxford) CCD-Diffractometer using graphite monochromated MoKα radiation (λ = 0.71073 Å). The data integration and reduction were processed with CrysAlis Pro software.¹⁴ Data of compound 4 was collected on Bruker SMART CCD-Diffractometer using graphite monochromated MoKα radiation (λ = 0.71073 Å). The structures were solved by the direct method and then refined on F² by the full matrix least-squares technique with the SHELX-97 set of software¹⁵ using the WinGX (version 1.80.05) program package.¹⁶ All non-hydrogen atoms were refined anisotropically and hydrogen atoms were treated as riding atoms using SHELX default parameters. Molecular structures have been drawn using ORTEP software. Further information on the crystal structure (excluding structure factors) has been given in CIF file, Tables S1 & S2 and Fig. S1 & S2 (ESI†) and also deposited in the Cambridge Crystallographic Data Centre as supplementary publication numbers 1482293 (6a) and 1482294 (4).

Procedure for crystallization of compound 6a and 4

For crystallization a mixture of ethyl acetate and hexane (2 : 8) has been used and kept in dark place at temperature 25 °C. The single crystal appeared after three days was isolated in its initial state of growth.

Typical experimental procedure for synthesis of N-acylbenzotriazoles

Compound 1 (1.0 g, 7.34 mmol) was added to a RB flask containing dichloromethane (15.0 mL) equipped with freshly prepared CaCl₂ guard tube and temperature was maintained 0–5 °C. Thionyl chloride (0.6 mL, 8.27 mmol) was added dropwise with vigorous stirring and constant cooling. The reaction mixture was allowed to stir for 15 min then added 1H-1,2,3-benzotriazole (3.06 g, 25.69 mmol) in fraction after complete addition stirred the reaction mixture 2–3 hour at room temperature. After completion of reaction (monitored by TLC), the excess SOCl₂ was quenched with ice maintaining the temperature at 0 °C. Extracted with CH₂Cl₂, washed with 10% Na₂CO₃, water, and brine solution, the organic layer was dried over anhydrous Na₂SO₄, concentrated till dry under reduced pressure. Further, purification using flash column chromatography using gradient mixtures of ethyl acetate and n-hexane afforded product 2a–q in pure.

Physical data of developed compounds (2a–q)

(1H-1,2,3-Benzotriazole-1-yl)-phenyl-methanone (2a)¹⁷. White crystalline solid, 1.73 g, yield 95%; R_f = 0.6 (10% ethyl acetate/n-hexane); mp = 112–113 °C (lit. mp = 112 °C); ¹H NMR (300 MHz, CDCl₃): δ 8.40 (d, J = 8.1 Hz, 1H), 8.23–8.16 (m, 3H), 7.73–7.67 (m, 2H), 7.61–7.53 (m, 3H); ¹³C NMR (75 MHz, CDCl₃): δ 166.7, 145.7, 133.6, 132.3, 131.7, 131.6, 131.4, 130.3, 128.4, 126.3, 120.1 and 114.7 ppm; MS: m/z 224 [M + H]⁺; anal. calcd for C₁₃H₉N₃O: C, 69.95; H, 4.06; N, 18.82. Found: C, 69.78; H, 4.09; N, 18.7.

(1H-Benzo[d][1,2,3]triazol-1-yl)(p-tolyl)methanone (2b)¹⁷. White crystalline solid, 1.5 g, yield 87%; R_f = 0.7 (10% ethyl acetate/n-hexane); mp = 123–124 °C (lit. mp = 123 °C); ¹H NMR (300 MHz, CDCl₃): δ 8.38 (d, J = 8.1 Hz, 1H), 8.17–8.12 (m, 3H), 7.69 (t, J = 7.5 Hz, 1H), 7.53 (t, J = 7.5 Hz, 1H), 7.38 (d, J = 7.2 Hz, 2H), 2.48 (s, 3H); ¹³C NMR (75 MHz, CDCl₃): δ 166.5, 145.6, 144.8, 131.8, 130.2, 129.1, 128.5, 126.1, 120.0, 114.7 and 21.7 ppm; MS: m/z 238 [M + H]⁺; anal. calcd for C₁₄H₁₁N₃O: C, 70.87; H, 4.67; N, 17.71. Found: C, 70.63; H, 4.78; N, 17.89.

(1H-Benzo[d][1,2,3]triazol-1-yl)(m-tolyl)methanone (2c)¹⁸. White crystalline solid, 1.4 g, yield 83%; R_f = 0.8 (10% ethyl acetate/n-hexane); mp = 207–209 °C (lit. mp = 205 °C); ¹H NMR (300 MHz, CDCl₃): δ 8.37 (d, J = 8.1 Hz, 1H), 8.16 (d, J = 8.4 Hz, 1H), 7.99 (bs, 2H), 7.69 (t, J = 7.5 Hz, 1H), 7.56–7.43 (m, 3H), 2.47 (s, 3H); ¹³C NMR (75 MHz, CDCl₃): δ 166.8, 145.7, 138.2, 134.4, 132.3, 132.0, 131.3, 130.2, 128.8, 128.2, 126.2, 120.1, 114.7 and 21.3 ppm; MS: m/z 238 [M + H]⁺; anal. calcd for C₁₄H₁₁N₃O: C, 70.87; H, 4.67; N, 17.71. Found: C, 70.53; H, 4.71; N, 17.95.

(1H-Benzo[d][1,2,3]triazol-1-yl)(o-tolyl)methanone (2d). White crystalline solid, 1.5 g, yield 88%; R_f = 0.8 (10% ethyl acetate/n-hexane); ¹H NMR (300 MHz, CDCl₃): δ 8.39 (d, J = 8.1 Hz, 1H), 8.15 (d, J = 8.4 Hz, 1H), 7.72–7.61 (m, 2H), 7.56–7.47 (m, 2H), 7.34 (t, J = 8.1 Hz, 3H), 2.43 (s, 3H); ¹³C NMR (75 MHz, CDCl₃): δ 168.1, 146.0, 137.8, 132.2, 131.7, 131.6, 130.9, 130.3, 129.9, 126.3, 125.3, 120.1, 114.5 and 19.9 ppm; MS: m/z 238 [M + H]⁺.

(1H-Benzo[d][1,2,3]triazol-1-yl)(4-chlorophenyl)methanone (2e)¹⁹. White crystalline solid, 1.2 g, yield 71%; R_f = 0.5 (10% ethyl acetate/n-hexane); mp = 137–138 °C (lit. mp = 138 °C); ¹H NMR (300 MHz, CDCl₃): δ 8.38 (d, J = 8.1 Hz, 1H), 8.25–8.16 (m, 3H), 7.72 (t, J = 7.5 Hz, 1H), 7.60–7.55 (m, 3H); ¹³C NMR (75 MHz, CDCl₃): δ 165.6, 145.7, 140.4, 133.1, 130.5, 129.7, 128.8, 126.4, 120.2 and 114.8 ppm; MS: m/z 258 [M + H]⁺.

(1H-Benzo[d][1,2,3]triazol-1-yl)(3-(trifluoromethyl)phenyl)methanone (2f). White crystalline solid, 0.817 g, yield 53%; R_f = 0.7 (10% ethyl acetate/n-hexane); mp = 52–53 °C; IR (KBr): ν_max (cm⁻¹) 3230, 2920, 1699, 1594, 1485, 1452, 1390, 1330, 1302, 1265, 1071, 958, 946, 802, 759, 749, 693, 677; ¹H NMR (300 MHz, CDCl₃): δ 8.48 (s, 1H), 8.42 (d, J = 7.5 Hz, 1H), 8.35 (d, J = 8.1 Hz, 1H), 8.15 (d, J = 8.4, 1H), 7.92 (d, J = 7.5 Hz, 1H), 7.74–7.68 (m, 2H), 7.55 (t, J = 7.5 Hz, 1H); ¹³C NMR (75 MHz, CDCl₃): δ 165.2, 145.7, 134.8, 132.2, 132.0130.6, 130.0, 129.9, 129.0, 128.5, 128.4, 126.5, 120.2 and 114.6 ppm; MS: m/z 292 [M + H]⁺; anal. calcd for C₁₄H₈F₃N₃O: C, 57.74; H, 2.77; N, 14.43. Found: C, 57.91; H, 2.87; N, 14.29.

(1H-Benzo[d][1,2,3]triazol-1-yl)(3,5-dichlorophenyl)methanone (2g). White crystalline solid, 1.2 g, yield 79%; R_f = 0.7 (10% ethyl acetate/n-hexane × 2); mp = 148–152 °C; IR (KBr): ν_max (cm⁻¹) 3068, 2333, 1708, 1564, 1484, 1452, 1420, 1379, 1322, 1291, 1240, 1155, 969, 877, 773, 664; ¹H NMR (300 MHz, CDCl₃): δ 8.35 (d, J = 8.4 Hz, 1H), 8.18 (d, J = 8.1 Hz, 1H), 8.11 (s, 2H), 7.73 (t, J = 7.5 Hz, 1H), 7.66 (d, J = 1.8 Hz, 1H), 7.57 (t, J = 7.5 Hz, 1H); ¹³C NMR (75 MHz, CDCl₃): δ 164.1, 145.7, 135.3, 134.0, 133.4, 132.0, 130.8, 129.9, 126.8, 120.4 and 114.7 ppm; MS: m/z 292 [M + H]⁺; anal. calcd for C₁₃H₇Cl₂N₃O: C, 53.45; H, 2.42; N, 14.38. Found: C, 53.71; H, 2.31; N, 14.27.

(1H-Benzo[d][1,2,3]triazol-1-yl)(3-methoxyphenyl)methanone (2h). White crystalline solid, 1.2 g, yield 72%; R_f = 0.7 (10% ethyl acetate/n-hexane); mp = 80–84 °C; IR (KBr): ν_max (cm⁻¹) 3105, 3087, 3019, 2963, 2837, 1943, 1704, 1585, 1486, 1449, 1367, 951, 746 cm⁻¹; MS: m/z 254 [M + H]⁺; ¹H NMR (300 MHz, CDCl₃): δ 8.34 (d, J = 8.1 Hz, 1H), 8.13 (d, J = 8.1 Hz, 1H), 7.79 (d, J = 7.5 Hz, 1H), 7.64–7.70 (m, 2H), 7.54–7.43 (m, 2H), 7.21 (d, J = 6.3 Hz, 1H), 3.87 (s, 3H); ¹³C NMR (75 MHz, CDCl₃): δ 166.3, 159.2, 145.5, 132.4, 132.2, 130.2, 129.3, 126.1, 124.1, 119.9, 116.0, 114.6, 114.5 and 55.4 ppm.

(1H-Benzo[d][1,2,3]triazol-1-yl)(3-phenoxyphenyl)methanone (2i). White crystalline solid, 1.19 g, yield 81%; R_f = 0.6 (13% ethyl acetate/n-hexane); mp = 52–53 °C; ¹H NMR (300 MHz, CDCl₃): δ 8.31 (d, J = 8.1 Hz, 1H), 8.12 (d, J = 8.4 Hz, 1H), 7.91 (d, J = 7.5 Hz, 1H), 7.81 (s, 1H), 7.67–7.62 (m, 1H), 7.52 (t, J = 7.8 Hz, 2H), 7.37–7.29 (m, 3H), 7.14–7.05 (m, 3H); ¹³C NMR (75 MHz, CDCl₃): δ 165.8, 157.2, 156.1, 145.5, 132.8, 132.0, 130.2, 129.8, 129.6, 126.2, 126.1, 123.9, 123.5, 121.1, 120.0, 119.1 and 114.5 ppm; MS: m/z 316 [M + H]⁺; anal. calcd for C₁₉H₁₃N₃O₂: C, 72.37; H, 4.16; N, 13.33. Found: C, 72.67; H, 4.09; N, 13.23.

1-(1H-Benzo[d][1,2,3]triazol-1-yl)-2-phenylethanone (2j)¹⁷. White solid, 1.5 g, yield 87%; R_f = 0.7 (5% ethyl acetate/n-hexane); mp = 63–64 °C (lit. mp = 65–66 °C); MS: m/z 238 [M + H]⁺; ¹H NMR (300 MHz, CDCl₃): δ 8.24 (d, J = 8.1 Hz, 1H), 8.11 (d, J = 8.4 Hz, 1H), 7.61 (t, J = 7.5 Hz, 1H), 7.50–7.44 (m, 3H), 7.38–7.29 (m, 3H), 4.72 (s, 2H); ¹³C NMR (75 MHz, CDCl₃): δ 170.2, 146.2, 132.4, 131.1, 130.4 (2C), 129.7 (2C), 128.7, 127.5, 126.2, 120.1, 114.4 and 41.9 ppm.

(E)-1-(1H-Benzo[d][1,2,3]triazol-1-yl)-3-phenylprop-2-en-1-one (2k). White solid, 1.0 g, yield 62%; R_f = 0.7 (5% ethyl acetate/n-hexane); mp = 114–118 °C; MS: m/z 250 [M + H]⁺; ¹H NMR (300 MHz, CDCl₃): δ 8.37 (d, J = 8.1 Hz, 1H), 8.11 (bs, 3H), 7.70–7.61 (m, 3H), 7.51–7.43 (m, 4H); ¹³C NMR (75 MHz, CDCl₃): δ 163.7, 148.5, 146.2, 133.9, 131.3, 130.1, 128.9, 128.8, 126.0, 120.0, 115.9 and 114.6 ppm.

(1H-Benzo[d][1,2,3]triazol-1-yl)(4-bromophenyl)methanone (2l)²⁰. Pale yellow solid, 1.2 g, yield 83%; R_f = 0.5 (10% ethyl acetate/n-hexane); mp = 140–142 °C (lit. mp = 142–143 °C); IR (KBr): ν_max (cm⁻¹) 3117, 3092, 1706, 1588, 1482, 1449, 1377, 1226, 943, 888, 750; ¹H NMR (300 MHz, CDCl₃): δ 8.36 (d, J = 8.4 Hz, 1H), 8.17–8.09 (m, 3H), 7.72–7.67 (m, 3H), 7.54 (t, J = 7.8 Hz, 1H); ¹³C NMR (75 MHz, CDCl₃): δ 165.6, 145.6, 133.1, 132.1, 131.7, 130.5, 130.1, 129.0, 126.4, 120.2 and 114.7 ppm; MS: m/z 302 [M + H]⁺.

(1H-Benzo[d][1,2,3]triazol-1-yl)(2-chlorophenyl)methanone (2m)²¹. White crystalline solid, 1.3 g, yield 81%; R_f = 0.5 (10% ethyl acetate/n-hexane); mp = 81–83 °C; IR (KBr): ν_max (cm⁻¹) 3112, 3062, 1723, 1588, 1484, 1449, 1361, 936, 738; ¹H NMR (300 MHz, CDCl₃): δ 8.38 (d, J = 7.8 Hz, 1H), 8.13 (d, J = 7.5 Hz, 1H), 7.72–7.63 (m, 2H), 7.56–7.51 (m, 2H), 7.43 (br, 1H); ¹³C NMR (75 MHz, CDCl₃): δ 165.6, 146.1, 132.7, 132.4, 132.1, 131.1, 130.5, 130.0, 126.5, 120.2 and 114.3 ppm; MS: m/z 258 [M + H]⁺.

(1H-Benzo[d][1,2,3]triazol-1-yl)(2-methoxyphenyl)methanone (2n)²². Colourless solid, 1.48 g, yield 89%; R_f = 0.6 (10% ethyl acetate/n-hexane); mp = 95–96 °C (lit. mp = 97 °C); ¹H NMR (300 MHz, CDCl₃): δ 8.36 (d, J = 8.1 Hz, 1H), 8.09 (d, J = 8.1 Hz, 1H), 7.67–7.59 (m, 2H), 7.56–7.45 (m, 2H), 7.11–7.01 (m, 2H), 3.73 (s, 3H); ¹³C NMR (75 MHz, CDCl₃): δ 166.9, 157.8, 146.0, 133.5, 131.4, 130.3, 130.2, 126.2, 122.7, 120.4, 120.0, 114.4, 111.7 and 55.8 ppm; MS: m/z 254 [M + H]⁺.

(1H-Benzo[d][1,2,3]triazol-1-yl)(2-hydroxyphenyl)methanone (2o)²³. Pale yellow solid, 1.6 g, yield 92%; R_f = 0.7 (15% ethyl acetate/n-hexane); mp = 115–116 °C (lit. mp = 116 °C); ¹H NMR (300 MHz, CDCl₃): ¹H NMR (300 MHz, CDCl₃): δ 8.61 (d, J = 8.1 Hz, 1H), 8.33 (d, J = 8.1 Hz, 1H), 8.19 (d, J = 8.1 Hz, 1H), 7.92–7.89 (m, 1H), 7.72 (t, J = 7.5 Hz, 1H), 7.69–7.46 (m, 2H), 7.15–7.01 (m, 2H); ¹³C NMR (75 MHz, CDCl₃): δ 169.0, 163.4, 145.2, 136.9, 133.6, 132.2, 130.4, 126.3, 120.1, 119.4, 118.2 and 114.7 ppm; MS: m/z 240 [M + H]⁺.

1-(1H-benzo[d][1,2,3]triazol-1-yl)-2-(4-chlorophenyl)ethanone (2p). White solid, yield 83%; 1.65 g, R_f = 0.7 (5% ethyl acetate/n-hexane); mp = 80–82 °C; MS: m/z 272 [M + H]⁺; IR (KBr): ν_max cm⁻¹ 2926, 1732, 1688, 1483, 1453, 1400, 1325, 1121, 1089, 1074, 980, 806, 772, 751; ¹H NMR (300 MHz, CDCl₃): δ 8.24 (d, J = 8.4 Hz, 1H), 8.12 (d, J = 8.4 Hz, 1H), 7.64 (dd, J = 7.2, 0.9 Hz, 1H), 7.50 (m, 1H), 7.40 (d, J = 8.4 Hz, 2H), 7.33 (d, J = 8.4 Hz, 2H), 4.69 (s, 2H); ¹³C NMR (75 MHz, CDCl₃): δ 169.8, 146.3, 133.7, 131.2 (2C), 131.1, 130.9, 130.6, 129.0 (2C), 126.4, 120.2, 114.4 and 41.3 ppm.

1-(1H-Benzo[d][1,2,3]triazol-1-yl)-3-methylbutan-1-one (2q). Oil, yield 71%; R_f = 0.6 (15% ethyl acetate/n-hexane); MS: m/z 204 [M + H]⁺; ¹H NMR (300 MHz, CDCl₃): δ 8.25 (d, J = 7.2 Hz, 1H), 8.07 (d, J = 8.1 Hz, 1H), 7.61 (t, J = 7.5 Hz, 1H), 7.46 (t, J = 7.5 Hz, 1H), 3.30 (d, J = 6.9 Hz, 2H), 2.49–2.40 (m, 1H), 1.11 (d, 6H); ¹³C NMR (75 MHz, CDCl₃): δ 171.6, 145.8, 130.8, 129.9, 125.7, 119.7, 114.1, 43.8, 25.3 and 22.2 (2C) ppm.

Typical experimental procedure for the synthesis of carbamates (3a–t)

To a stirring solution of compound 2 (1.0 equiv.) in mixture of alcohol/water (19 : 1) was added sodium azide (3.0 equiv.) in portions. The reaction was stirred under heating at 90–100 °C for 4 hours. After completion of the reaction (monitored by TLC), the reaction mixture was concentrated in vacuo. The crude reaction mixture was purified directly using flash column chromatography to afford carbamate derivative (3a–t). Purified compounds were recrystalised using CHCl₃/n-hexane.

Physical data of developed compounds (3a–t)

Ethylphenylcarbamate (3a)²⁴. White crystalline solid, 0.49 g, yield 67%; R_f = 0.6 (15% ethyl acetate/n-hexane); mp = 50–51 °C; ¹H NMR (300 MHz, CDCl₃): δ 7.30 (d, J = 7.8 Hz, 2H), 7.19 (t, J = 6.9 Hz, 2H), 6.95 (t, J = 7.2 Hz, 1H), 6.78 (bs, 1H), 4.13 (q, J = 6.9 Hz, 2H), 1.20 (t, J = 6.9 Hz, 3H); ¹³C NMR (75 MHz, CDCl₃): δ 153.8, 138.0, 129.0, 123.3, 118.8, 61.2 and 14.5 ppm; MS: m/z 166 [M + H]⁺; anal. calcd for C₉H₁₁NO₂: C, 65.44; H, 6.71; N, 8.48. Found: C, 65.27; H, 6.88; N, 8.46.

Ethyl-p-tolylcarbamate (3b)²⁵. Oil, 0.43 g, yield 57%; R_f = 0.6, (15% ethyl acetate/n-hexane); ¹H NMR (300 MHz, CDCl₃): δ 7.17 (d, J = 8.1 Hz, 2H), 6.99 (d, J = 8.1 Hz, 2H), 6.65 (bs, 1H), 4.12 (q, J = 6.9 Hz, 2H), 2.20 (s, 3H), 1.20 (t, J = 6.9 Hz, 3H); ¹³C NMR (75 MHz, CDCl₃): δ 153.7, 135.3, 132.7, 129.4, 118.8, 61.0, 20.6 and 14.4 ppm; MS: m/z 180 [M + H]⁺.

Ethyl-m-tolylcarbamate (3c)²⁶. Oil, 0.51 g, yield 68%; R_f = 0.6, (15% ethyl acetate/n-hexane); IR (KBr): ν_max (cm⁻¹) 3320, 2980, 2928, 1735, 1717, 1614, 1596, 1542, 1492, 1445, 1227, 1070, 778, 690; ¹H NMR (300 MHz, CDCl₃): δ 7.22–7.10 (m, 3H), 6.98 (bs, 1H), 6.83 (d, J = 6.9 Hz, 1H), 4.20 (q, J = 6.9, 2H), 2.27 (s, 3H), 1.26 (t, J = 6.9, 3H); ¹³C NMR (75 MHz, CDCl₃): δ 153.7, 138.5, 137.9, 128.5, 123.7, 119.2, 115.7, 60.7, 21.0 and 14.1 ppm; MS: m/z 180 [M + H]⁺.

Ethyl-o-tolylcarbamate (3d)²⁷. Oil, 0.41 g, yield 55%; R_f = 0.5, (10% ethyl acetate/n-hexane); IR (KBr): ν_max (cm⁻¹) 3328, 3025, 2977, 2933, 2868, 1736, 1719, 1591, 1542, 1459, 1302, 1218, 1063, 753; ¹H NMR (300 MHz, CDCl₃): δ 7.76 (d, J = 5.7 Hz, 1H), 7.24–7.13 (m, 2H), 7.10 (t, J = 7.5 Hz), 6.54 (bs, 1H), 4.22 (q, J = 6.9, 2H), 2.24 (s, 3H), 1.30 (t, J = 6.9 Hz, 3H); ¹³C NMR (75 MHz, CDCl₃): δ 153.9, 135.7, 130.1, 127.9, 126.5, 123.9, 121.4, 60.9, 17.3 and 14.3 ppm; MS: m/z 180 [M + H]⁺.

Ethyl(3-(trifluoromethyl)phenyl)carbamate (3e). Oil, 0.26 g, yield 33%; R_f = 0.45, (10% ethyl acetate/n-hexane); ¹H NMR (300 MHz, CDCl₃): δ 7.70 (s, 1H), 7.55 (d, J = 8.1 Hz, 1H), 7.43–7.38 (m, 1H), 7.32–7.26 (m, 1H), 6.75 (bs, 1H), 4.24 (q, J = 6.9 Hz, 2H), 1.37 (t, J = 6.9, 3H); ¹³C NMR (75 MHz, CDCl₃): δ 153.2, 151.9, 146.5, 138.6, 138.5, 129.4, 121.4, 119.8, 115.2, 61.4 and 14.3 ppm; MS: m/z 234 [M + H]⁺; anal. calcd for C₁₀H₁₀NO₂F₃: C, 51.51; H, 4.32; N, 6.01. Found: C, 51.32; H, 4.48; N, 5.97.

Ethyl(3,5-dichlorophenyl)carbamate (3f). Oil, 0.39 g, yield 46%; R_f = 0.5, (15% ethyl acetate/n-hexane); ¹H NMR (300 MHz, CDCl₃): δ 7.36 (s, 2H), 7.06 (s, 1H), 6.67 (bs, 1H), 4.25 (q, J = 6.9 Hz, 2H), 1.33 (t, J = 6.9 Hz, 3H); ¹³C NMR (75 MHz, CDCl₃): δ 153.0, 139.9, 135.3, 123.2, 116.7, 61.7 and 14.4 ppm; MS: m/z 234 [M + H]⁺; anal. calcd for C₉H₉NO₂Cl₂: C, 46.18; H, 3.88; N, 5.98. Found: C, 45.97; H, 3.91; N, 5.89.

Ethyl(3-methoxyphenyl)carbamate (3g)²⁸. Oil, 0.39 g, yield 51%; R_f = 0.5 (15% ethyl acetate/n-hexane); ¹H NMR (300 MHz, CDCl₃): δ 7.16–7.12 (m, 3H), 6.90 (d, J = 7.8 Hz, 1H), 6.58 (d, J = 8.1 Hz, 1H), 4.20 (q, J = 6.9 Hz, 2H), 3.73 (s, 3H), 1.26 (t, J = 6.9 Hz, 3H); ¹³C NMR (75 MHz, CDCl₃): δ 160.0, 153.6, 139.2, 129.4, 110.8, 108.9, 104.3, 61.1, 55.0 and 14.3 ppm; MS: m/z 196 [M + H]⁺.

Ethyl(3-phenoxyphenyl)carbamate (3h). Oil, 0.50 g yield 62%; R_f = 0.6, (15% ethyl acetate/n-hexane); ¹H NMR (300 MHz, CDCl₃): δ 7.19 (t, J = 7.8 Hz, 2H), 7.12–7.06 (m, 1H), 7.00–6.96 (m, 3H), 6.90–6.84 (m, 3H), 6.56 (d, J = 7.5 Hz, 1H), 4.06 (q, J = 7.2 Hz, 2H), 1.14 (t, J = 7.2 Hz, 3H); ¹³C NMR (75 MHz, CDCl₃): δ 157.8, 156.8, 153.4, 139.4, 129.8, 129.6, 123.2, 118.9, 113.3, 109.2, 61.1 and 14.3 ppm; MS: m/z 258 [M + H]⁺; anal. calcd for C₁₅H₁₅NO₃: C, 70.02; H, 5.88; N, 5.44. Found: C, 70.27; H, 6.01; N, 5.41.

Ethyl benzylcarbamate (3i)²⁹. Oil, 0.42 g, yield 56%; R_f = 0.5, (15% ethyl acetate/n-hexane); ¹H NMR (300 MHz, CDCl₃): δ 7.37–7.26 (m, 5H), 4.98 (bs, 1H), 4.35 (d, J = 4.8 Hz, 2H), 4.14 (q, J = 7.2 Hz, 2H), 1.24 (t, J = 7.2 Hz, 3H); ¹³C NMR (75 MHz, CDCl₃): δ 156.6, 138.5, 128.6, 127.4, 120.0, 60.9, 44.9 and 14.6 ppm; MS: m/z 180 [M + H]⁺.

(E)-Ethyl styrylcarbamate (3j)³⁰. Oil, 0.23 g; yield 30%; R_f = 0.5, (20% ethyl acetate/n-hexane); ¹H NMR (300 MHz, CDCl₃): δ 7.26–7.15 (m, 6H), 6.81 (s, 1H), 5.95 (d, J = 14.4 Hz, 1H), 4.21–4.19 (m, 2H), 1.28 (bs, 3H); ¹³C NMR (75 MHz, CDCl₃): δ 153.7, 136.3, 128.5, 126.1, 125.2, 124.1, 110.5, 61.5 and 14.4 ppm; MS: m/z 192 [M + H]⁺.

Ethyl(4-bromophenyl)carbamate (3k)³¹. White solid, 0.57 g yield 71%; R_f = 0.5, (20% ethyl acetate/n-hexane); mp = 58–62 °C (lit. mp = 85 °C); ¹H NMR (300 MHz, CDCl₃): δ 7.39 (d, J = 9.0 Hz, 2H), 7.27 (d, J = 9.0 Hz, 2H), 6.67 (bs, 1H), 4.21 (q, J = 6.9 Hz, 2H), 1.30 (t, J = 7.2 Hz, 3H); ¹³C NMR (75 MHz, CDCl₃): δ 153.4, 137.0, 131.9, 131.5, 120.1, 115.7, 61.4 and 14.4 ppm; MS: m/z 244 [M + H]⁺.

Methyl(4-bromophenyl)carbamate (3l)²⁵. White solid, 0.09 g, yield 13%; R_f = 0.45 (15% ethyl acetate/n-hexane); mp = 94–96 °C; IR (KBr): ν_max (cm⁻¹) 3345, 2948, 1704, 1600, 1547, 1488, 1397, 1312, 1240, 1075, 825; ¹H NMR (300 MHz, CDCl₃): δ 7.40 (d, J = 8.7 Hz, 2H), 7.27 (d, J = 8.4 Hz, 2H), 6.74 (bs, 1H), 3.77 (s, 3H); ¹³C NMR (75 MHz, CDCl₃): δ 153.8, 136.9, 131.9, 120.2, 115.9 and 52.4 ppm; MS: m/z 230 [M + H]⁺.

Propyl(4-bromophenyl)carbamate (3m). White solid, 0.56 g yield 66%; R_f = 0.5, (15% ethyl acetate/n-hexane); mp = 62–64 °C; ¹H NMR (300 MHz, CDCl₃): δ 7.40 (d, J = 9.0 Hz, 2H), 7.27 (d, J = 9.3 Hz, 2H), 6.62 (bs, 1H), 4.12 (t, J = 6.6 Hz, 2H), 1.69 (q, J = 7.2 Hz, 2H), 0.97 (t, J = 7.5 Hz, 3H); ¹³C NMR (75 MHz, CDCl₃): δ 153.4, 137.1, 131.9, 120.1, 115.8, 67.0, 22.2 and 10.3 ppm; MS: m/z 258 [M + H]⁺; anal. calcd for C₁₀H₁₂NO₂Br: C, 46.53; H, 4.69; N, 5.43. Found: C, 46.37; H, 4.76; N, 5.35.

Isopropyl(4-bromophenyl)carbamate (3n)³². White crystalline solid, 0.52 g, yield 61%; R_f = 0.5 (20% ethyl acetate/n-hexane); mp = 107–108 °C (lit. mp = 105 °C); ¹H; IR (KBr): ν_max (cm⁻¹) 3353, 2978, 2933, 1692, 1590, 1530, 1399, 1236, 1110, 826, 774; ¹H NMR (300 MHz, CDCl₃): δ 7.39 (d, J = 8.7 Hz, 2H), 7.27 (d, J = 8.7 Hz, 2H), 6.64 (bs, 1H), 5.04–4.96 (m, 1H), 1.34–1.27 (m, 6H); ¹³C NMR (75 MHz, CDCl₃): δ 153.0, 137.2, 131.8, 120.1, 115.6, 69.0 and 22.0 ppm; MS: m/z 258 [M + H]⁺.

Butyl(4-bromophenyl)carbamate (3o)³². White crystalline solid, 0.69 g yield 77%; R_f = 0.5 (10% ethyl acetate/n-hexane); mp = 56–58 °C; ¹H NMR (300 MHz, CDCl₃): δ 7.39 (d, J = 8.7 Hz, 2H), 7.28 (d, J = 8.7 Hz, 2H), 6.71 (bs, 1H), 4.15 (t, J = 6.6 Hz, 2H), 1.64 (t, J = 7.5 Hz, 2H), 1.40 (q, J = 7.5 Hz, 2H), 0.94 (t, J = 7.5 Hz, 3H); ¹³C NMR (75 MHz, CDCl₃): δ 153.5, 137.1, 131.9, 120.1, 115.7, 65.3, 30.8, 19.0 and 13.6 ppm; MS: m/z 272 [M + H]⁺.

Pentyl(4-bromophenyl)carbamate (3p). White solid, 0.76 g yield 81%; R_f = 0.4, (10% ethyl acetate/n-hexane); mp = 58–60 °C; ¹H NMR (300 MHz, CDCl₃): δ 7.37 (d, J = 8.7 Hz, 2H), 7.27 (d, J = 8.7 Hz, 2H), 6.94 (bs, 1H), 4.13 (t, J = 6.6 Hz, 2H), 1.64 (t, J = 6.6 Hz, 2H), 1.32 (m, 4H), 0.90 (t, J = 6.6 Hz, 3H); ¹³C NMR (75 MHz, CDCl₃): δ 153.6, 137.1, 131.7, 120.2, 115.6, 65.5, 28.4, 27.8, 22.2 and 13.8 ppm; MS: m/z 286 [M + H]⁺; anal. calcd for C₁₂H₁₆NO₂Br: C, 50.37; H, 5.64; N, 4.89. Found: C, 50.54; H, 5.47; N, 4.73.

Prop-2-yn-1-yl(4-bromophenyl)carbamate (3q)³³. White crystalline solid, 0.36 g yield 43%; R_f = 0.5, (10% ethyl acetate/n-hexane); mp = 142–143 °C (lit. mp = 143–144 °C); ¹H NMR (300 MHz, CDCl₃): δ 7.42 (d, J = 8.7 Hz, 2H), 7.28 (d, J = 8.7 Hz, 2H), 6.71 (bs, 1H), 4.77 (s, 2H), 2.51 (s, 1H); ¹³C NMR (75 MHz, CDCl₃): δ 152.2, 136.5, 132.0, 120.3, 116.4, 77.4, 75.2 and 52.9 ppm; MS: m/z 254 [M + H]⁺.

Prop-2-yn-1-yl phenylcarbamate (3r)³⁴. White crystalline solid, 0.33 g yield 41%; R_f = 0.6, (15% ethyl acetate/n-hexane); mp = 59–61 °C (lit. mp = 62–63 °C); ¹H NMR (300 MHz, CDCl₃): δ 7.39–7.24 (m, 5H), 7.07 (t, J = 7.5 Hz, 1H), 6.78 (m, 1H), 4.78 (s, 2H), 2.50 (s, 1H); ¹³C NMR (75 MHz, CDCl₃): δ 152.4, 137.3, 129.0, 123.7, 118.8, 77.8, 75.0 and 52.7 ppm; MS: m/z 176 [M + H]⁺.

Butyl benzylcarbamate (3s)³⁵. Oil, 0.45 g yield 51%; R_f = 0.5, (10% ethyl acetate/n-hexane); MS: m/z 208 [M + H]⁺; ¹H NMR (500 MHz, CDCl₃): δ 7.24–7.17 (m, 5H), 5.05 (bs, 1H), 4.26 (d, J = 5.5 Hz, 2H), 4.00–3.98 (m, 2H), 1.50–1.47 (m, 2H) 1.29–1.26 (m, 2H), 0.83 (t, J = 7.5 Hz, 3H); ¹³C NMR (125 MHz, CDCl₃): δ 156.7, 138.5, 128.5, 127.4, 127.2, 64.7, 44.8, 30.9, 18.9 and 13.6 ppm.

Ethyl(4-chlorobenzyl)carbamate (3t). White solid, 0.46 g yield 59%; R_f = 0.4, (10% ethyl acetate/n-hexane); mp = 55–58 °C (lit. mp = 62 °C); MS: m/z 214 [M + H]⁺; ¹H NMR (500 MHz, CDCl₃): δ 7.29 (d, J = 7.5 Hz, 2H), 7.21 (d, J = 8.0 Hz, 2H), 5.06 (bs, 1H), 4.31 (d, J = 5.5 Hz, 2H), 4.16–4.11 (m, 2H), 1.25–1.23 (m, 3H); ¹³C NMR (125 MHz, CDCl₃): δ 156.6, 137.1, 133.1, 128.7, 61.0, 44.2 and 14.5 ppm.

Benzo[d]oxazol-2(3H)-one (4). Crystalline solid, 0.54 g yield 97%; R_f = 0.5, (15% ethyl acetate/n-hexane); mp = 134–138 °C (lit. mp = 138 °C); ¹H NMR (300 MHz, DMSO-d⁶): δ 9.86 (bs, 1H), 7.18–7.04 (m, 4H); ¹³C NMR (75 MHz, DMSO-d⁶): δ 156.3, 143.8, 129.4, 124.1, 122.6, 110.2 and 110.1 ppm; MS: m/z 136 [M + H]⁺; anal. calcd for C₇H₅NO₂: C, 62.22; H, 3.73; N, 10.37. Found: C, 62.45; H, 5.26; N, 10.11.

Typical experimental procedure for the synthesis of thiocarbamates (5a & 5l)

To a stirring solution of compound 2a & 2l (1.0 equiv.) in mixture of thiol/water (9 : 1) was added sodium azide (3.0 equiv.) in portions. The reaction was stirred at 100 °C for 4 hours. After completion of the reaction (monitored by TLC), the reaction mixture was concentrated in vacuo. The crude reaction mixture was purified directly using flash column chromatography to afford thiocarbamate derivative (5a & 5b).

S-Furfuryl N-(4-bromophenyl)thiocarbamate (5a). Brown oil, 0.38 g yield 37%; R_f = 0.5, (10% ethyl acetate/n-hexane); MS: m/z 312 [M + H]⁺; ¹H NMR (500 MHz, CDCl₃): δ 7.36 (d, J = 9.0 Hz, 2H), 7.28 (s, 1H), 7.24 (d, J = 9 Hz, 2H), 6.96 (bs, 1H), 6.22 (d, J = 14 Hz, 2H), 4.18 (s, 2H); ¹³C NMR (125 MHz, CDCl₃): δ 164.6, 150.5, 142.3, 136.5, 132.1, 131.9, 128.7, 121.2, 110.6, 108.1 and 27.0 ppm.

S-Benzyl N-phenylthiocarbamate (5b)³⁶. Brown solid, 0.48 g yield 44%; R_f = 0.5, (10% ethyl acetate/n-hexane); mp = 88–90 °C (lit. mp = 94.5 °C); MS: m/z 244 [M + H]⁺; ¹H NMR (500 MHz, CDCl₃): δ 7.34–7.16 (m, 9H), 7.06–7.03 (m, 1H), 6.97 (bs, 1H), 4.16 (s, 2H); ¹³C NMR (125 MHz, CDCl₃): δ 164.1, 137.9, 137.4, 129.1, 128.9, 128.8, 128.6, 127.3, 124.5, 119.7 and 34.4 ppm.

Typical experimental procedure for the synthesis of symmetric urea

To a stirring solution of compound 2 (1 equiv.) in mixture of THF/water (85 : 15) was added sodium azide (3.0 equiv.) in portions. The reaction mixture was stirred at 90–100 °C for 4 hours. After, completion of the reaction (monitored by TLC), the reaction mixture was concentrated in vacuo. The crude reaction mixture was purified directly using flash column chromatography to afford carbamates derivative (6a–g). Purified compounds were recrystallized using ethyl acetate/n-hexane.

1,3-Diphenylurea (6a)²⁵. White crystalline solid, 0.43 g yield 81%; R_f = 0.7, (25% ethyl acetate/n-hexane); mp = 240–242 °C (lit. mp = 238 °C); IR (KBr): ν_max 3328, 3034, 1646, 1594, 1543, 1497, 1440, 1314, 1231 cm⁻¹: ¹H NMR (300 MHz, DMSO-d⁶): δ 8.64 (bs, 2H), 7.46 (d, J = 7.8 Hz, 4H), 7.26 (t, J = 6.9 Hz, 4H), 6.95 (t, J = 6.9 Hz, 2H); ¹³C NMR (75 MHz, DMSO-d⁶): δ 152.5, 139.7, 128.7, 121.8 and 118.2 ppm; MS: m/z 213 [M + H]⁺.

1,3-Di-o-tolylurea (6b)²⁵. White crystalline solid, 0.33 g yield 65%; R_f = 0.6, (25% ethyl acetate/n-hexane); mp = 235–236 °C (lit. mp = 245–247 °C); ¹H NMR (300 MHz, DMSO-d⁶): δ 8.20 (bs, 2H), 7.78 (bs, 2H), 7.14 (bs, 4H), 6.94 (bs, 2H), 2.24 (m, 6H); ¹³C NMR (75 MHz, DMSO-d⁶): δ 154.3, 141.4, 130.1, 127.7, 126.0, 121.5, 119.4 and 17.9 ppm; MS: m/z 241 [M + H]⁺.

1,3-Bis(2-chlorophenyl)urea (6c)²⁵. White solid, 0.42 g yield 77%; R_f = 0.4, (10% ethyl acetate/n-hexane); mp = 242–244 °C (lit. mp = 240–241 °C); ¹H NMR (300 MHz, DMSO-d⁶): δ 8.97 (bs, 2H), 8.10 (d, J = 7.8 Hz, 2H) 7.38 (bs, 2H), 7.25 (bs, 2H), 7.01 (bs, 2H); ¹³C NMR (75 MHz, DMSO-d⁶): δ 148.4, 135.6, 128.9, 126.9, 122.6 and 122.1 ppm; MS: m/z 281 [M + H]⁺.

1,3-Bis(2-methoxyphenyl)urea (6d)²⁵. White solid, 0.47 g, yield 87%; R_f = 0.4, (15% ethyl acetate/n-hexane); mp = 180–184 °C (lit. mp = 185–186 °C); ¹H NMR (300 MHz, DMSO-d⁶): δ 8.13–8.10 (m, 2H), 7.19 (bs, 2H), 7.02–6.88 (m, 4H), 6.86 (d, J = 7.2 Hz, 2H), 3.85 (bs, 6H); ¹³C NMR (75 MHz, DMSO-d⁶): δ 152.4, 148.1, 128.1, 122.8, 121.1, 119.6, 110.0 and 55.6 ppm; MS: m/z 273 [M + H]⁺.

1,3-Bis(4-chlorophenyl)urea (6e)²⁵. White solid, 0.28 g, yield 52%; R_f = 0.5, (20% ethyl acetate/n-hexane); mp = 306–308 °C (lit. mp = 306–307 °C); IR (KBr): ν_max (cm⁻¹) 3296, 2923, 2852, 1633, 1590, 1560, 1491, 1395, 1298, 1237, 822, 639, 508; ¹H NMR (300 MHz, DMSO-d⁶): δ 8.34 (s, 2H), 7.35 (d, J = 9.0 Hz, 4H), 7.15 (d, J = 8.7 Hz, 4H); ¹³C NMR (75 MHz, DMSO-d⁶): δ 150.5, 137.7, 128.1, 126.2, 124.8 and 119.3 ppm; MS: m/z 281 [M + H]⁺.

1,3-Di-m-tolylurea (6f)²⁵. White solid, 0.36 g, yield 73%; R_f = 0.5, (25% ethyl acetate/n-hexane); mp = 224–228 °C (lit. mp = 217 °C); ¹H NMR (300 MHz, DMSO-d⁶): δ 8.55 (s, 2H), 7.30 (s, 2H), 7.23–7.12 (m, 4H), 6.78 (d, J = 6.9 Hz, 2H), 2.27 (s, 6H); ¹³C NMR (75 MHz, DMSO-d⁶): δ 152.4, 139.6, 137.9, 128.5, 122.4, 118.6, 115.2 and 21.2 ppm; MS: m/z 241 [M + H]⁺.

1,3-Bis(3-(trifluoromethyl)phenyl)urea (6g). White solid, 0.45 g yield 76%; R_f = 0.5, (30% ethyl acetate/n-hexane); mp = 198–200 °C (lit. mp = 194–196 °C); ¹H NMR (300 MHz, DMSO-d⁶): δ 8.93 (sbr, 2H), 7.97 (s, 2H), 7.56 (d, J = 7.8 Hz, 2H), 7.43 (t, J = 7.8 Hz, 2H), 7.24 (d, J = 7.5 Hz, 2H); ¹³C NMR (75 MHz, DMSO-d⁶): δ 152.2, 139.9, 130.1, 129.6, 129.0, 125.5, 121.9, 121.4, 118.0, 117.9 and 114.4 ppm; MS: m/z 349 [M + H]⁺; anal. calcd for C₁₅H₁₀F₆N₂O: C, 51.73; H, 2.89; N, 8.04. Found: C, 52.02; H, 2.95; N, 7.89.

Typical experimental procedure for the synthesis of triazolyl sugar derivatives

Method I. To a solution of compound 2 (1.0 equiv.) in mixture of propargyl alcohol/water (9 : 1) was added sodium azide (1.2 equiv.) and allow to stir for 10 hours at 90–100 °C. After completion of the reaction (monitored by TLC), then propargyl alcohol was removed and added ^tBuOH–water (8 : 2) and allow to stir. Sugar azide (1.1 equiv.) was added in portions followed by addition of CuSO₄·H₂O (0.02 mol%) and NaAsc (0.2 mol%) and stirred for 4 hours at rt. After completion of reaction the reaction mixture was concentrated in vacuum. The crude reaction mixture was purified directly using flash column chromatography to afford triazolyl derivatives of carbamate (8a–c).

Method II. A solution of compound 3 (1.0 equiv.) in a R.B. flask containing a mixture of ^tBuOH/water (8 : 2) was added sugar azide (1.1 equiv.), CuSO₄·H₂O (0.02 mol%) and NaAsc (0.2 mol%) and stirred for 4 hours at rt. After completion of reaction the reaction mixture was concentrated in vacuum. The crude reaction mixture was purified directly using flash column chromatography to afford triazolyl derivatives of carbamate (8a–c).

1′-(4-(((Phenylcarbamoyl)oxy)methyl)-1H-1,2,3-triazol-1-yl)-2,3,4,6-tetra-O-acetyl-α-D-glucopyranose (8a). Oil, yield = method I-302 mg (11%), method II-2.75 g (88%); R_f = 0.5, (60% ethyl acetate/n-hexane); ¹H NMR (300 MHz, CDCl₃): δ 7.99 (s, 1H), 7.40 (d, J = 7.8 Hz, 2H), 7.30 (t, J = 7.2 Hz, 2H), 7.27 (bs, 1H), 7.08–7.06 (m, 1H) 5.87 (d, J = 9.0 Hz, 1H), 5.55 (t, J = 9.9 Hz, 2H), 5.23–5.38 (m, 3H), 4.25–4.13 (m, 3H), 2.22 (s, 3H), 2.02–2.00 (m, 6H), 1.86 (s, 3H); ¹³C NMR (75 MHz, CDCl₃): δ 170.2, 169.9, 169.7, 168.9, 153.1, 143.6, 137.6, 128.9, 123.5, 122.5, 118.8, 86.1, 76.5, 73.9, 70.6, 67.8, 66.8, 61.1, 57.6, 20.6, 20.5, 20.3 and 20.1 ppm; MS: m/z 549 [M + H]⁺.

1′-(4-((((4-Bromophenyl)carbamoyl)oxy)methyl)-1H-1,2,3-triazol-1-yl)-2,3,4,6-tetra-O-acetyl-α-D-glucopyranose (8b). Oil, yield = method I-304 mg (13%), method II-2.34 g (94%); R_f = 0.5, (50% ethyl acetate/n-hexane); ¹H NMR (300 MHz, CDCl₃): δ 7.98 (s, 1H), 7.41 (d, J = 8.7 Hz, 2H), 7.32–7.29 (m, 2H), 7.18 (bs, 1H), 5.85 (d, J = 9.0 Hz, 1H), 5.57–5.50 (m, 2H), 5.38–5.24 (m, 3H), 4.24–4.14 (m, 3H), 2.22 (s, 3H), 2.06–2.01 (m, 6H), 1.86 (s, 3H); ¹³C NMR (75 MHz, CDCl₃): δ 170.3, 169.9, 169.7, 169.0, 152.9, 143.4, 136.8, 131.9, 122.5, 120.3, 86.2, 76.5, 74.0, 70.6, 67.8, 66.7, 61.1, 57.8, 20.6, 20.5, 20.3 and 20.1 ppm; MS: m/z 627 [M + H]⁺.

Ethyl-1′,2′-isopropylidine-3′-O-benzyl-5′-(4-((((4-bromophenyl)carbamoyl)oxy)methyl)-1H-1,2,3-triazol-1-yl)-5′-deoxy-α-D-xylo-heptofuranuronoate (8c). Oil, yield = method I-186 mg (8%), method II-2.33 g (92%); R_f = 0.5, (60% ethyl acetate/n-hexane); ¹H NMR (300 MHz, CDCl₃): δ 7.76 (s, 1H), 7.45 (s, 1H), 7.38–7.32 (m, 9H), 5.88 (d, J = 3.0 Hz, 1H), 5.28–5.19 (m, 2H), 5.14–5.08 (m, 1H), 4.59–4.56 (m, 1H), 4.76–4.67 (m, 2H), 4.44 (d, J = 11.7 Hz, 1H), 4.00–3.95 (m, 3H), 3.16 (dd, J = 10.5, 6.0 Hz, 1H), 2.43 (d, J = 15.0 Hz, 1H), 1.45 (s, 3H), 1.25 (s, 3H), 1.10 (t, J = 7.2 Hz, 3H); ¹³C NMR (75 MHz, CDCl₃): δ 169.2, 153.1, 141.6, 137.1, 136.2, 131.7, 128.6, 128.4, 128.1, 126.0, 120.2, 112.2, 104.8, 81.5, 80.6, 80.5, 71.6, 60.9, 60.3, 58.0, 57.3, 34.8, 29.6, 26.6, 26.1 and 13.8 ppm; MS: m/z 645 [M + H]⁺.

Acknowledgements

The authors thank Council of Scientific and Industrial Research (CSIR), New Delhi (Grant No. 02(0173)/13/EMR-II) for funding and CISC, Banaras Hindu University for providing basic infrastructure and spectroscopic studies.

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Footnote

† Electronic supplementary information (ESI) available: Copies of ¹H and ¹³C NMR for all the developed compounds has been provided. CCDC 1482293 and 1482294. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra14131e

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