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

Abstract 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 SiO 2 . The condensation of amidines 1 with arylmethylketones afforded unsaturated ketones 5 which on heating were transformed into 2,9-dialkyl-3-aroyl-9 H -pyrido[2,3- b ]indoles 7 . Instead, prolonged reaction of amidines 1 with arylmethylketones in t -BuOH/ t -BuOK gave 2-aryl-9 H -pyrido[2,3- b ]indoles 6 .