Synthesis of porphyrinylamide and observation of N-methylation-induced trans–cis amide conformational alteration

We synthesized porphyrinylamide 4b and its N-methylated derivative 5b. Direct N-methylation of porphyrinylamides 4 proved unsuccessful, so 5b was obtained via N-methylation of 5-aminoporphyrin 10 with potassium hexamethyldisilazide. The secondary amide 4b exists in trans-amide form, while N-methylated amide 5b exists in the cis-amide form, both in solution and in the crystal. Thus, N-methylation of the amide bond of 4b resulted in trans to cis conformational alteration.