The phenanthridine core is present in several natural products namely the phenanthridine alkaloids. Their synthesis is important in the pharmaceutical industry, so the development of new synthetic methods is welcomed by them.
Radical reactions are involved in the synthesis of several natural products, among them the phenanthridine alkaloids. The use of tributyltin hydride and azobisisobutyronitrile is a common approach for the thermal generation of radicals. Beyond being an effective method, they have associated the toxic effects of the tin.
For the past few years, N-heterocyclic carbene-boranes have being used in radical reactions, as radical mediators. Applications in radical reductions and, more recently, in intramolecular cyclizations have been reported.
Here we report studies with N-heterocyclic carbene-boranes for the intramolecular cyclization of appropriated substrates for the synthesis of the phenanthridines core. For that, the substrates used include secondary and tertiary amines, an amide, and an ether. Attempts has been made to synthesise a heteroatom free substrate in order to understand the effect of the heteroatoms in the reaction, but without success. Two different N-heterocyclic carbene-boranes were synthesized based on imidazolium salts, the (1,3-dimethylimidazolium-2-yl)trihydroborate and the (3-ethyl-1-methylimidazolium-2-yl)trihydroborate.
The reactions yields were below expectations, however the cyclization products were obtained. No reduction products were identified, showing the difficulty of the boranes to react with the substrate. It was identified a complex between a tertiary amide substrate and the borane, which may be the reason for the low yields. In the reaction with the ether it seems that the complex with the NHC-borane is also formed.
It was also identified an adduct between the borane and the thiophenol, when this last one was used as co-adjuvant. The formation of that adduct should be the reason why the reactions with thiophenol had a lower yield.Unfortunately, all the attempts to increase the yield were unsuccessful
