Carbocyclization versus Oxycyclization on the Metal-Catalyzed Reactions of Oxyallenyl C3-Linked Indoles

The preparation of previously unknown (indol-3-yl)-alpha-allenols and -allenones was accomplished from indole-3-carbaldehydes, through indium mediated Barbier allenylation reaction taking advantage of the N-(2-pyridyl)sulfonyl group. Metal catalyzed cyclizations of oxyallenyl C3-linked indoles proceeded in two ways depending on the presence or absence of the N- (2-pyridyl)sulfonyl group. For allenols, gold catalyzed oxycyclization occurred in the presence of the protecting group; in the absence of the protecting group, palladium- and gold-catalyzed benzannulations operated. On the contrary, under gold catalysis furyl-indoles were obtained as exclusive products from NH-allenones, while S-endo carbocyclization adducts were the major components starting from N-SO(2)py-protected allenones. These cyclization reactions have been developed experimentally, and their mechanisms have additionally been investigated by a computational study.