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Synlett 2024; 35(20): 2477-2481
DOI: 10.1055/a-2316-5066
DOI: 10.1055/a-2316-5066
letter
Special Issue to Celebrate the 75th Birthday of Prof. B. C. Ranu
Metal-Free Synthesis of C-3-Alkoxycarbonylated 2H-Indazoles Using Alkyl Carbazates
A.H. acknowledges the financial support from the Council of Scientific and Industrial Research (CSIR), New Delhi (Grant no. 02(0455)/21/EMR-II). D.L. and S.B. thank the Council of Scientific and Industrial Research (CSIR), New Delhi (CSIR-SRF) for their fellowship.
This paper is dedicated to Professor Brindaban C. Ranu on the occasion of his 75th birthday.
Abstract
A simple, efficient, and environmentally benign method for the direct C-3-alkoxycarbonylation of 2H-indazoles using alkyl carbazates has been developed under metal-free conditions at room temperature. This current protocol represents a facile access to C-3-carboxylic ester derived 2H-indazoles with wide functional group tolerance in good to excellent yields. The mechanistic studies suggest that the reaction proceeds through a radical pathway.
Key words
metal-free - C-3-alkoxycarbonylation - 2H-indazole - synthetic transformations - alkyl carbazateSupporting Information
- Supporting information for this article is available online at https://doi.org/10.1055/a-2316-5066.
- Supporting Information
Publication History
Received: 23 March 2024
Accepted after revision: 29 April 2024
Accepted Manuscript online:
29 April 2024
Article published online:
16 May 2024
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- 15 General Experimental Procedure for the Synthesis of C-3-Alkoxycarbonylated 2H-Indazoles 3 and 5 A mixture of 2-arylindazoles 1/4 (0.2 mmol), alkyl hydrazinecarboxylate 2 (2.0 equiv), and TBHP (4.0 equiv, 0.14 mL) was taken in an oven-dried reaction tube under N2 atmosphere. Then 1,2-DCE (2 mL) was added to it using a syringe, and the reaction mixture was further stirred at room temperature for 8 h under N2 atmosphere. After completion of the reaction (TLC), the reaction mixture was extracted with DCM (10 mL). The organic phase was dried over anhydrous Na2SO4 and concentrated under reduced pressure to get the crude residue which was purified by column chromatography on silica gel (100–200 mesh) using a mixture of petroleum ether and ethyl acetate as an eluent to afford the corresponding products 3/5. Analytical Data for Compound 3ha White solid (67%, 42.9 mg); Rf = 0.50 (PE:EtOAc = 96:4); mp 155–156 °C. 1H NMR (400 MHz, CDCl3): δ = 8.10 (d, J = 8.4 Hz, 1 H), 7.85 (d, J = 8.4 Hz, 1 H), 7.80 (d, J = 8.4 Hz, 2 H), 7.68 (d, J = 8.0 Hz, 2 H), 7.46–7.42 (m, 1 H), 7.39–7.35 (m, 1 H), 3.94 (s, 3 H). 13C{1H} NMR (100 MHz, CDCl3): δ = 160.0, 148.9, 143.6, 131.6, 131.3, 127.7, 127.0, 126.2, 125.9 (q, J C–F = 4.0 Hz), 124.2, 123.8 (q, J C–F = 271.0 Hz), 121.6, 118.8, 52.3. FTIR: ν = 3055, 1708, 1616, 1462, 1296, 1103 cm–1. HRMS (ESI-TOF): m/z [M + H]+ calcd for [C16H12F3N2O2]+: 321.0845; found: 321.0841. Analytical Data for Compound 5aa White solid (77%, 45.6 mg); Rf = 0.5 (PE:EtOAc = 94 : 6); mp 134–135 °C. 1H NMR (400 MHz, CDCl3): δ = 7.72 (d, J = 9.2 Hz, 1 H), 7.39–7.37 (m, 2 H), 7.31–7.29 (m, 3 H), 7.10–7.07 (m, 1 H), 3.92 (s, 3 H), 3.89 (s, 3 H), 2.45 (s, 3 H). 13C{1H} NMR (100 MHz, CDCl3): δ = 160.3, 158.1, 145.2, 139.3, 138.7, 129.3, 126.0, 125.0, 123.9, 122.0, 120.1, 97.9, 55.6, 51.9, 21.4. FTIR: ν = 3032, 1712, 1631, 1442, 1211, 1126 cm–1. HRMS (ESI-TOF): m/z [M + H]+ calcd for [C17H17N2O3]+: 297.1234; found: 297.1225.