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

Abstract

Abstract 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...