Bischler-Napieralski Synthesis of 6-Alkynyl Phenanthridines Based on Tf2O-Promoted Electrophilic Activation of N-Aryl-2-propynamides

An efficient method for the synthesis of 6-alkynyl phenanthridines was developed. The method offered the first example to use 2-propynamides as substrates in the Bischler-Napieralski reaction and to create alkynylnitrilium triflates as new active intermediates in organic synthesis.

1,5-Phosphonium betaines from N-triflylpropiolamides, triphenylphosphane, and active methylene compounds

N-Phenyl-N-(trifluoromethylsulfonyl)propiolamides react with triphenylphosphane in the presence of various active methylene compounds CH₂XY in a 1:1:1 molar ratio to furnish 1-phosphonium-5-oxabetaines, Ph₃P⁺-C(R)=CH-C(O^-)=CXY. These betaines are formed preferentially, but not exclusively, as E-diastereoisomers with respect to the vinylic double bond. In some cases, separation of the two diastereoisomers was achieved by fractionating crystallization. Structure determination by X-ray diffraction analysis revealed marked conformational differences around the CH-C(O^-) single bond of E and Z-isomers and extended charge delocalization in the anionic part.