An object of the present invention is to provide methods of discovering drugs effective for tough targets, which have conventionally been discovered only with difficulty. The present invention relates to novel methods for cyclizing peptide compounds, and novel peptide compounds and libraries comprising the same, to achieve the above object.
Provided is a compound represented by the following formula (I) or a pharmaceutically acceptable salt thereof:
[in the formula, R
1
to R
5
, A, and *1 to *3 are as defined in Description].