Synthesis of Racemosin B and Its Analogues by a Photochemical Cyclization

Abstract

A simple approach for the synthesis of the alkaloid racemosin B has been developed through a photochemical cyclization reaction with a 33% overall yield without any reagents. This method is sustainable and environmentally friendly because no reagents are required to effect the transformation.

Key words

photochemistry - alkaloids - indolocarbazoles - racemosin B - cyclization

• References and Notes

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• 15 Racemosin B (5a); Typical Procedure In a 10 mL Pyrex tube, compound 4a (100 mg) and a catalytic amount of I₂ were dissolved in anhyd MeCN (5 mL). The mixture was irradiated under N₂ in a photoreactor equipped with UV lamps (λ = 200–400 nm) for 12–24 h until the reaction was complete (TLC). The mixture was then concentrated in a rotary evaporator, and the crude product was purified by column chromatography [silica gel (100–200 mesh), 30% EtOAc–hexane] to give a white solid; yield: 60 mg (56%); mp 270 °C (Lit.¹² 265–267 °C); R_f = 0.59 (EtOAc–hexane, 3:7). IR (neat): 3452, 3385, 1661, 1571, 1426, 1233, 1093, 729 cm^–1. ¹H NMR (500 MHz, DMSO-d ₆): δ = 11.49 (s, 1 H), 10.63 (s, 1 H), 8.80 (d, J = 3.5 Hz, 1 H), 8.62 (d, J = 7.5 Hz, 1 H), 8.09 (d, J = 7.5 Hz, 1 H), 7.68–7.64 (m, 2 H), 7.46–7.39 (m, 2 H), 7.33 (t, J = 7.5 Hz, 1 H), 7.27 (t, J = 7.5 Hz, 1 H), 4.06 (s, 3 H). ¹³C {¹H} NMR (125 MHz, DMSO-d ₆): δ = 167.9, 140.3, 139.2, 138.4, 137.6, 124.6, 124.5, 123.8, 121.2, 121.1, 121.0, 119.8, 119.5, 119.0, 115.6, 111.3, 111.1, 106.8, 104.2, 51.5. HRMS (ESI); m/z [M + H]⁺ calcd for C₂₀H₁₅N₂O₂: 315.1134; found: 315.1133.