Abstract
The oxidative photocyclization of aromatic Schiff bases was investigated as a potential method for synthesis of phenanthridine derivatives, biologically active compounds with medical applications. Although it is possible to prepare the desired phenanthridines using such an approach, the reaction has to be performed in the presence of acid and TEMPO to increase reaction rate and yield. The reaction kinetics was studied on a series of substituted imines covering the range from electron-withdrawing to electron-donating substituents. It was found that imines with electron-withdrawing substituents react one order of magnitude faster than imines bearing electron-donating groups. The 1H NMR monitoring of the reaction course showed that a significant part of the Z isomer in the reaction is transformed into E isomer which is more prone to photocyclization. The portion of the Z isomer transformed showed a linear correlation to the Hammett substituent constants. The reaction scope was expanded towards synthesis of larger aromatic systems, namely to the synthesis of strained aromatic systems, e.g., helicenes. In this respect, it was found that the scope of oxidative photocyclization of aromatic imines is limited to the formation of no more than five ortho-fused aromatic rings.
Highlights
Phenanthridines consist of three nitrogen-containing, ortho-fused benzene rings
Phenanthridines can be prepared by oxidative photocyclization of stilbene-like molecules to other
The oxidative photocyclization of aromatic Schiff bases is a suitable method for synthesis of
Summary
Phenanthridines consist of three nitrogen-containing, ortho-fused benzene rings. This structural motif is abundant in many natural products and medicinally relevant biologically active compounds [1,2,3,4,5,6]. Since substitution of phenanthridines is an effective strategy to modify biological activity [7], numerous synthetic methods for preparation of substituted phenanthridines have been developed over the last decade. The known synthetic procedures are based on visible light photocatalysis [8,9], anionic ring closure [10], radical routes [11,12], oxidative cyclization of. Phenanthridines can be prepared by oxidative photocyclization of stilbene-like molecules to other.
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