Sequential Knoevenagel condensation/cyclization leading to indene and benzofulvene derivatives have been developed. The reaction of 2-(1-phenylvinyl)benzaldehyde with malonates gave benzylidene malonates, cyclized indenes, and dehydrogenated benzofulvenes. The product selectivity depends on the reaction conditions. The reaction with piperidine, AcOH in benzene at 80 °C for 1.5 h gave a benzylidene malonate in 75% yield as a major product. The reactions with piperidine, AcOH in benzene at 80 °C for 17 h and with TiCl4-pyridine at room temperature gave an indene derivative in 56% and 79% yields, respectively as a major product. The reaction with TiCl4-Et3N gave a benzofulvene in 40% yield selectively. The indene was transformed to benzofulvene derivative using the reagents such as TiCl4-Et3N, FeCl3-Et3N, and DDQ. Reaction of variously substituted aryl derivatives with dimethyl malonate gave indene and benzofulvene derivatives as well. The reactions of 2-(1-phenylvinyl)benzaldehyde with Meldrum's acid or malononitrile also gave cyclized compounds in the suitable sequential or stepwise conditions. The reaction mechanism has been examined by the DFT calculations.

Sequential Knoevenagel condensation/cyclization for the synthesis of indene and benzofulvene derivatives

ACS Fall 2021

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