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Synthetic Communications
An International Journal for Rapid Communication of Synthetic Organic Chemistry
Volume 11, 1981 - Issue 10
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Original Articles

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

Pages 823-827 | Published online: 05 Dec 2006
 

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 δ 5.52 and δ 6.52. Addition of benzoic acid (2% of thecatalyst) as recommended6 gave no better results.

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