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Synthesis of symmetric dicarbonyl analogues of curcumin 1a–i, pyrazole derivative 2 and boron difluoride complexes 3f–i. Reagents and conditions: (i) B2O3, B[(OBu)3], 90 °C, 30 min; then, addition of nBuNH2, 70–100 °C, 24 h; (ii) NH2NH2·H2O, CH3COOH, EtOH, reflux, 24 h; (iii) BF3·OEt2, toluene, 65 °C, 5 h.
