Iminophosphorane-mediated annelation of a pyridine or pyrimidine ring into an indole ring: synthesis of β-, γ-carbolines and pyrimido[4,5-b]indole derivatives

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Abstract
A number of pyrido[3,4-b] Indole, pyrido[4,3-b] Indole, and pyrimido[4,5-b] Indole derivatives have been prepared. Ethyl 3-(1-methylindol-3-yl)-2-triphenylphosphoranylideneaminoprop-2-enoate (2) reacts with aromatic isothiocyanates to yield the corresponding 1-arylamino-3-ethoxycarbonyl-9-methylpyrido[3,4-b]indoles [(3)–(6)]. Similarly, the ethyl 3-(1-methylindol-2-yl)-2-triphenylphosphoranylideneaminoprop-2-enoate (10) under similar reaction conditions leads to 1-arylamino-3-ethoxycarbonyl-5-methylpyrido[4,3-b]indoles [(11)–(14)]. Also, iminophosphoranes (2) and (10) react with carbon disulphide to give the 1-thioxopyrido[3,4-b]indole (8) and 1-thioxopyrido[4,3-b]indole (15) respectively. The reaction of the 3-formyl-1-phenyl-2-triphenylphosphoranylideneaminoindole (17) with isothiocyanates at room temperature leads directly to 3-aryl-2,3-dihydro-2-oxo-9-phenylpyrimido[4,5-b]indoles [(20)–(22)].