a little suggestion
Based on the reference, if you protect the imidazole, the yield will be improved.
In addition, tried the activated ester. first prepared the proline activated ester, purified, then coupling with histamine.
perhaps more powerful coupling agent work, like HATU, POP.
Tried the separated procedure. Fist, mixted DCC HObt and Pro for 30 min, then added the histamine.
I suspect that Pyr should be pro. If I was wrong, sorry about it.
