Annulation strategies for the synthesis of azulenes and polycyclic nitrogen heterocycles

Abstract

Highly convergent annulation strategies have been developed for the synthesis of azulenes and polycyclic nitrogen heterocycles. Specifically, substituted azulenes have been synthesized via a ring expansion-annulation strategy based on n-halo diazo ketones. The method employs the use of readily available benzenoid starting materials and achieves the synthesis of azulenes substituted on both the five- and seven-membered rings regioselectively. The synthetic utility of the ring expansion annulation products, in particular azulenyl halides and triflates, has been demonstrated by their successful application in transition metal-mediated carbon-carbon bond forming reactions. These reactions have been used to further functionalize the five- and seven-membered rings and also in the synthesis of oligoazulenes and azulenylamino acid derivatives. In addition, polycyclic nitrogen heterocycles have been synthesized via a tandem ring forming strategy. The strategy employs a novel benzannulation reaction of substituted cyclobutenones with ynamides, which are synthesized via the strategy for the N-alkynylation of amides strategy developed in the Danheiser laboratory. The products of these benzannulation reactions, highly-substituted anilines, then participate in ring closing metathesis reactions to form various nitrogen heterocycles. In particular dihydroquinolines, hydrobenzoazepines, and hydrobenzoazocines were synthesized via this tandem ynamide benzannulation ring closing metathesis strategy.