A modular catalytic approach towards pyridine and quinolone synthesis

N-Containing heterocycles such as substituted pyridines and 2-quinolones are commonly found in drugs and agrochemicals; however, classical methods towards their synthesis often lead to limited substitution patterns. We recently described the first examples of the organocatalytic
intramolecular aza-Wittig reaction in the synthesis of azoles and azines from isocyanates. The work described herein is divided into three chapters detailing attempts to expand the scope of the use of isocyanates in heterocycle synthesis.
The first chapter describes the development of multicomponent synthesis of substituted pyridines incorporating the intermolecular catalytic aza-Wittig and Diels–Alder reactions; in total 31 exemplar pyridines were prepared in up to 52% yield. The reaction works well for
electron-poor and heteroaromatic aldehydes, electron-rich and electron-poor cinnamic acids and push-pull enamines to give a range of substitution patterns. The use of commercially available starting materials and catalytic phosphine oxide means the process offers distinct advantages
over classical methods.
The development of a tandem catalytic aza Wittig/electrocyclisation process towards benzothienopyridines was also investigated. To this end benzothienoimines and azatrienes were prepared, however, the electrocyclic ring closure of the azatrienes was non-trivial with low yields of the desired pyridines even under harsh conditions.
The final chapter outlines the preparation of substituted 2-quinolones from readily available urea starting materials by two methods. The first is a two-pot three-step process incorporating urea-directed oxidative Heck reaction, isocyanate formation and electrocyclisation; in total 9
exemplar 2-quinolones were prepared in up to 61% overall yield. The second is a two-pot threestep process incorporating Heck reaction, isocyanate formation and electrocyclisation; 10 exemplar 2-quinolones were prepared in up to 59% overall yield. The use of the urea directinggroup in the subsequent isocyanate formation negates the necessity for acyl azides and means no further manipulation is required to remove the directing group after 2-quinolone synthesis