Syntheses of acridone and carbazole alkaloids involving pyridine-catalysed chromen formation: crystal and molecular structure of dibromocannibicyclol and its bearing on the structures of the ‘cyclol’ alkaloids
The pyrano[2,3-c]acridine alkaloid acronycine (2) and its [2,3-b] isomer (6) have been synthesised via pyridinecatalysed condensation of 1,3-dihydroxyacridone with 4,4-dimethoxy-2-methylbutan-2-ol: mono- and bis-prenylogues have been prepared similarly by aldehyde condensations. The 3-(4-methylpent-3-enyl)pyrano[2,3-c]acridone (9) cyclises further to give products with one (14) or two [(13) and (37)] further rings. The acridone anils (15) and (16) have also been encountered. The structure of cannabicyclol (the ‘type’ structure of the ‘cyclol’ system) has been elucidated by X-ray analysis of its dibromo-derivative. Dibromocannabicyclol formed triclinic crystals with petroleum of crysallisation, space group P, a= 9·14(3), b= 10·20(3), c= 15·51(4)Å, α= 112·9(4), β= 87·2(6), γ= 116·0(4)°, Z= 2. By using 2383 observed reflections the structure was solved by the heavy atom procedure and refined to R= 7·6%. In the light of the results, the structures of the carbazole alkaloids bicyclomahanimbine and bicyclomahanimbicine were revised to the 11,12-methanocyclopenta[5,6]pyrano[3,2-a]carbazole systems (35) and (36). Acid-catalysed cyclisation of (±)-mahanimbine (25) yields cyclomahanimbine (27) and its isomer (29): revision of structure (28) for curryanin to (27) is proposed.