Synthesis of bis-benzoxazolyl phenol derivatives

The synthesis of bis-benzoxazolyl phenol derivatives is shown in Scheme S2. A mixture of 2-hydroxyisophthalaldehyde derivatives (3.0 mmol), 2-aminophenol (0.655 g, 6.0 mmol), and phenylboronic acid (0.109 g, 0.9 mmol) in 30 mL of MeOH was magnetically stirred for 5 min. Sodium cyanide (0.192 g, 4.8 mmol) was added. The mixture was thereafter stirred for 18 h at 25 °C. The flask was subsequently immersed in an ice bath. The precipitate was collected via vacuum filtration and washed with cold water. The solid was purified by recrystallization from MeOH/H₂O.
The compound 2,6-bis(2-benzoxazolyl)-4-methylphenol (C1) was obtained as an orange solid (0.569 g, 55.4%). m.p: 172.1–175.6 °C; ¹H-NMR (400 MHz, DMSO-d₆) δ 12.20 (s, 1H), 8.09 (s, 2H), 7.84–7.80 (m, 4H), 7.48–7.41 (m, 4H), 2.40 (s, 3H); ¹³C-NMR (101 MHz, DMSO-d₆) δ 161.8, 155.3, 149.9, 140.6, 133.5, 129.7, 126.5, 125.7, 120.1, 113.7, 111.6, 20.3; HRMS (ESI): m/z calcd for C₂₁H₁₃N₂O₃ [M–H]⁻ 341.0931, found 341.0923.