Abstract
| - Several derivatives of the pyrido[1‘,2‘:1,2]pyrimido[4,5-b]indoles 4 and the pyrazino[1‘,2‘:1,2]pyrimido[4,5-b]indoles 14 were synthesized by treatment of the benzannulated enyne−isocyanates8 with the iminophosphoranes 9 and 13, respectively, for the aza-Wittig reaction followed bythermolysis. The reaction presumably proceeds through an initial formation of the correspondingbenzannulated enyne−carbodiimides, such as 10, followed by a formal intramolecular hetero Diels−Alder reaction. Surprisingly, when the iminophosphorane 17 was used for condensation with 8,the expected pyrimido[1‘,6‘:1,2]pyrimido[4,5-b]indoles 16 were not obtained. Instead, the isomericpyrimido[6‘,1‘:2,3]pyrimido[4,5-b]indoles 21 were isolated. Presumably, an alternative reactionpathway involving an initial [2 + 2] cycloaddition reaction to form 19 followed by ring openingcould lead to 20 and, after an intramolecular radical−radical coupling, 21. Treatment of the ureaderivatives 24 with dibromotriphenylphosphorane also produced in situ the benzannulated enyne−carbodiimides 25, which on thermolysis gave the isoquinolino[2‘,1‘:1,2]pyrimido[4,5-b]indoles 26.Methylation of 4a, 14a, and 26a with methyl iodide occurred exclusively at the site of the indolonitrogen. The planar geometry of those novel heteroaromatic compounds, resembling many DNA-binding agents, makes them potential candidates as DNA intercalators.
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