2-Amidinylindole-3-carbaldehydes: Versatile Synthons for the Preparation of α-Carboline Derivatives

The 2-amidinylindole-3-carbaldehydes 1 are the key starting materials for the preparation of three classes of carbolines 2, 6 and 7 in which the pyridine ring is characterised by a different substitution patterns. The carbolines 2 which are functionalized with an amino group in position 2, were obtained directly by heating 1 in presence of SiO2. The condensation of amidines 1 with arylmethylketones afforded unsaturated ketones 5 which on heating were transformed into 2,9-dialkyl-3-aroyl-9H-pyrido[2,3-b]indoles 7. Instead, prolonged reaction of amidines 1 with arylmethylketones in t-BuOH/t-BuOK gave 2-aryl-9H-pyrido[2,3-b]indoles 6.