Cleaner Citral Condensation - Synthesis of Pyrano [2,3-a] Carbazoles

Citral condensation¹ with phenols are characterized by the formation of multitudinous products, poor yields and unpredictable course as evidenced by conflicting reports.^2,3 They are sensitive to temperature and amount of catalyst -usually pyridine - used.⁴ In the attempts to synthesize pyrano 2,3-a] carbazole derivatives isomeric with Mahanimbine⁵ (I), a representative C-23 pyrano 3,2-a] carbazole alkaloid, the usual pyridine catalysed citral condensation was tried but the expected pyranocarbazole derivatives could not be isolated in pusstate. The reaction of l-hydroxy-6-methylcarbazole with citral in pyridine, for instance, gave a product on chromatographic purification followed by micro-distillation at 195–200° (bath)/0.01 mm. Though homogeneous on TLC, it indicated the presence of atleast two closely related pyranocarbazole derivatives when its PMR spectrum was examined. It showed two deuterium exchangeable protons at δ 7.86 and δ 8.00. The olefinic protons appeared as doublet of doublets at δ 5.52 and δ 6.52. Addition of benzoic acid (2% of thecatalyst) as recommended⁶ gave no better results.