Nitro-facilitated ‘5-Exo-dig’ Intramolecular Cyclisation of 2-(2-Nitrophenyl-ethynyl)benzoic Acids: A New Total Synthesis of Indeno[1,2-b]indoles

Abstract

2-(2′-Nitrophenylethynyl)benzoic acid esters, obtained by Sonagashira coupling of o-bromobenzoates to 2-(2′-nitrophenyl)acetylenes, were converted into indeno[1,2-b]indoles by a two-step sequence, through the corresponding 2-nitrophenyl-1,3-indandiones.

alkynes - carboxylic acids - cyclizations - indoles - palladium - rearrangements

References

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All new compounds gave satisfactory analytical and spectroscopic data. Selected physical and spectroscopic data follow. Compound 3a. Mp 90-91 °C(hexane). ¹H NMR (ppm, CDCl₃): δ = 3.96 (s, 3 H), 7.38-7.63 (m, 4 H), 7.71 (dd, J = 7.4 Hz, J = 0.9 Hz, 1 H), 7.78 (dd, J = 7.5 Hz, J = 1.2 Hz, 1 H), 7.99 (dd, J = 7.8 Hz, J = 0.9 Hz, 1 H), 8.06 (dd, J = 8.2 Hz, J = 0.9 Hz, 1 H). MS: m/z (%) = 262(1) [M⁺], 163(100). Compound 3b. Mp 110-113 °C(methanol). ¹H NMR (ppm, CDCl₃): δ = 3.96 (s, 6 H), 3.99 (s, 3 H), 7.14 (s, 1 H), 7.40 (t, J = 6.3 Hz, 1 H), 7.50 (t, J = 6.3 Hz, 1 H), 7.64 (s, 1 H), 7.70 (d, J = 7.5 Hz, 1 H), 7.80 (d, J = 7.5 Hz, 1 H). MS: m/z (%, CI) = 342(37) [M + 1]⁺, 312(100). Compound 3c. Mp 130-133 °C (methanol/dichloro-methane). ¹H NMR (ppm, CDCl₃): δ = 3.96 (s, 3 H), 3.97 (s, 3 H), 3.99 (s, 3 H), 7.16 (s, 1 H), 7.47 (t, J = 6.9 Hz, 1 H), 7.53 (s, 1 H), 7.62 (t, J = 6.9 Hz, 1 H), 7.81 (d, J = 7.8 Hz, 1 H), 8.10 (d, J = 8.2 Hz, 1 H). MS m/z (%) = 341(4) [M⁺], 223(100). Compound 7a. Mp 178-180 °C(chloroform). ¹H NMR (ppm, CDCl₃): δ = 4.67 (s, 0.5 H), 7.46 (dd, J = 7.9 Hz, J = 1.4 Hz, 1 H), 7.58 (td, J = 7.4 Hz, J = 1.3 Hz, 1 H), 7.72 (td, J = 7.4 Hz, J = 1.4 Hz, 1 H), 7.90-8.10 (m, 4 H), 8.21 (dd, J = 6.4 Hz, J = 1.4 Hz, 1 H). MS: m/z (%) = 267 (0.5) [M⁺], 104(100). Compound 7b. Mp 196-198 °C(methanol). ¹H NMR (ppm, CDCl₃): δ = 3.98 (s, 3 H), 4.02 (s, 3 H), 4.51 (s, 1 H), 6.82 (s, 1 H), 7.80 (s, 1 H), 7.90 (dd, J = 5.6 Hz, J = 3.1 Hz, 2 H), 8.04 (dd, J = 5.6 Hz, J = 3.4 Hz, 2 H). MS (FAB): m/z (%) = 327(9) [M + 1]⁺, 104(100). Compound 7c. Mp 210-213 °C (decomp., methanol/dichloromethane). ¹H NMR (ppm, CDCl₃): δ = 4.03 (s, 6 H), 4.74 (s, 1 H), 7.39-7.44 (m, 3 H), 7.55 (t, J = 7.8 Hz, 1 H), 7.68 (t, J = 7.4 Hz, 1 H), 8.15 (d, J = 8.1 Hz, 1 H). MS: m/z (%) = 327(10) [M⁺], 164(100). Compound 10a. Mp 242-245 °C(methanol). ¹H NMR (ppm, CDCl₃/CD₃SOCD₃): δ= 3.61 (s, 2 H), 7.01-7.20 (m, 3 H), 7.32 (t, J = 7.1 Hz, 1 H), 7.62 (m, 4 H), 11.51 (s, 1 H). MS (CI): m/z (%) = 246(17) [M + 1]⁺, 205(100). Compound 10b. Mp 205-207 °C (ethyl acetate/hexane). ¹H NMR (ppm, CD₃SOCD₃): δ = 3.68 (s, 2 H), 3.94 (s, 3 H), 3.97 (s, 3 H), 6.96 (s, 1 H), 7.09 (s, 1 H), 7.16 (t, J = 7.5 Hz, 1 H), 7.30 (t, J = 7.5 Hz, 1 H), 7.40 (d, J = 7.5 Hz, 1 H), 7.51 (d, J = 7.2 Hz, 1 H), 8.17 (s, 1 H). MS (CI): m/z (%) = 265 [100) [M + 1]⁺. Compound 10c. Mp 245-250 °C (methanol/dichloro-methane). ¹H NMR (ppm, CDCl₃/CD₃SOCD₃): δ = 3.66 (s, 2 H), 3.96 (s, 3 H), 3.97 (s, 3 H), 7.06 (s, 1 H), 7.10-7.20 (m, 3 H), 7.40-7.44 (m, 1 H), 7.60-7.65 (m, 1 H), 8.30 (bs, 1 H). MS: m/z (%) = 265(100) [M⁺].

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