AMINO ACID DERIVATIVE

PROBLEM TO BE SOLVED: To obtain a new amino acid derivative inhibiting proteinases originated from viruses and useful for treating the infectious diseases of viruses such as hepatitis C virus. SOLUTION: The compound of formula I [E is -CHO or -B(OH)2 ; R is a (halogenated) lower alkyl or the like; R is a (hydroxy) lower alkyl or the like; R is H or a lower alkyl; R and R together form dimethylene or trimethylene; R is a (hydroxy) lower alkyl or the like; R is an aryl-lower alkyl or the like; R is H or a lower alkyl; R is a (hydroxy) lower alkyl or the like; R is a carboxy-lower alkyl or the like; R is a lower alkylcarbonyl or the like]. For example, 2(RS)[[N-[N-[N-[N-[N-(3-carboxypropionyl)-L-α- aspartyl)-L-α-glutamyl]-2-methyl-L-phenylalanyl]-3-methyl-L-valyl]-O-benzyl-L- tyrosyl]amino-4,4,4-trifluorobutylaldehyde, is obtained by removing protecting groups from the acetal of formula II (R and R are each a lower alkyl).