Metal-Free Visible-Light-Mediated Desulfurization and Aromatization of Dihydropyrimidine-2-thiones for Synthesis of 2-Unsubstituted Pyrimidines

Abstract

The visible-light-mediated aerobic desulfurization and aromatization of Biginelli 3,4-dihydropyrimidine-2(1H)-thiones for a one-step synthesis of 2-unsubstituted pyrimidines was established. The protocol uses molecular oxygen as an inexpensive oxidant, with visible-light irradiation, and eosin B as an organophotoredox catalyst.

Key words

photoredox reactions - organocatalysis - desulfurization - aromatization - dihydropyrimidinethiones

• References and Notes

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• 23 Ethyl 4-Methyl-6-phenylpyrimidine-5-carboxylate (3a); Typical Procedure An oven-dried tube was charged with ethyl 6-methyl-4-phenyl-2-thioxo-1,2,3,4-tetrahydropyrimidine-5-carboxylate (1a; 0.276 g, 1 mmol), eosin B (2; 0.012 g, 0.02 mmol), and DMF (3 mL). The stirred mixture was irradiated with a 15 W blue light for 12 h at r.t., while the reaction was monitored by TLC. The reaction was then quenched with sat. aq NH₄Cl (3 mL) and the mixture was extracted with EtOAc (3 × 15 mL). The organic layers were combined, washed with brine, and dried (MgSO₄). The crude product was purified by column chromatography [silica gel, PE–EtOAc (1:15)] to give a yellow oil; yield: 214 mg (0.88 mmol, 88%). ¹H NMR (600 MHz, CDCl₃): δ = 9.15 (s, 1 H), 7.65 (d, J = 7.6 Hz, 2 H), 7.46 (d, J = 7.4 Hz, 2 H), 4.20 (q, J = 7.2 Hz, 2 H), 2.63 (s, 3 H), 1.08 (t, J = 7.2 Hz, 3 H). ¹³C NMR (100 MHz, CDCl₃): δ = 167.67, 164.95, 163.20, 158.06, 137.53, 130.13, 128.58, 128.26, 125.95, 61.93, 22.56, 13.62. HRMS (ESI⁺): m/z [M + H]⁺ calcd for C₁₄H₁₅N₂O₂: 243.1128; found: 243.1123.