Abstract: [Problem] To provide a guanidine derivative (in particular, an amino acid compound having a 4-guanidino group) which has a low production cost and is less likely to cause the problem of waste liquor, a method for producing the same and an intermediate to be used in the production of the same. [Solution] A compound represented by formula (I), a tautomer thereof, an enantiomer thereof, a pharmaceutically acceptable salt thereof or a pharmaceutically acceptable solvate thereof; a method for producing a compound represented by formula (I), a tautomer thereof, an enantiomer thereof, a pharmaceutically acceptable salt thereof or a pharmaceutically acceptable solvate thereof, said method comprising a mixing step for mixing a compound represented by formula (II) with a compound represented by formula (III); and a compound represented by formula (IV) or formula (V).

Abstract: A novel amino acid derivative is provided, wherein the amino acid derivative is expected to improve binding affinity of polypeptides comprising the derivative therein.

Abstract: A novel amino acid derivative is provided, wherein the amino acid derivative is expected to improve solubility of polypeptides comprising the derivative therein.

Abstract: The present invention is to provide a method for producing a peptide containing an N-alkylamino acid, which comprises the following Steps (1) to (3). Step (1): a step of mixing an N-terminal protected amino acid or an N-terminal protected peptide with a carboxylic acid halide or a halogenated alkyl formate; Step (2): a step of mixing an amino acid or a peptide in which the N-terminal and the C-terminal are not protected with a trialkylsilylating agent; and Step (3): a step of mixing the product obtained in Step (1) with the product obtained in Step (2).