Preparation of stable fulvene and difulvene aldehydes from benzaldehydes and an indene-derived enamine: formation of novel indene-fused benzodiazepines and attempted syntheses of di- and tricarbaporphyrinoid systems

Although carbaporphyrins and related systems have been widely studied, far less work has been carried out on the synthesis of porphyrin analogues with more than one carbocyclic subunit. Fulvene aldehydes are potentially valuable intermediates for studies of this type. A versatile methodology has been developed where benzaldehydes are reacted with an indene-derived enamine in the presence of dibutylboron triflate to give fulvene monoaldehydes. This chemistry allows halo, alkyl, methoxy or cyanovinyl units to be introduced and the resulting fulvenes are stable compounds that are easily purified by column chromatography. Isophthalaldehydes afford difulvene dialdehydes that are equally stable and these can be reduced with CeCl3-NaBH4 to give the related dicarbinols. The difulvene dialdehydes failed to give macrocyclic products when reacted with o-phenylenediamine in the presence of CeCl3 but instead gave unprecedented bis-indene-fused benzodiazepines. Fulvene monoaldehydes also reacted under these conditions to give benzodiazepine products in good yields. These results highlight the potential utility of fulvene aldehydes for synthetic applications both inside and outside of the area of porphyrin analogue chemistry.