| abstract |
An acid addition salt of a bis-dioxopiperazine derivative represented by general formula (I), wherein R¹ represents C₁ to C₆ alkyl; R² represents hydrogen or (a); R³ represents hydrogen, C₁ to C₆ alkyl or -(CH₂) n -A, wherein A represents carboxyl, benzyloxycarbonyl, mercapto, amino, benzyloxycarbonylamino or (b), wherein X represents hydrogen, trifluoromethyl, hydroxy, acetoxy or benzyloxy, and n represents an integer of 1 to 4; and R⁴ represents hydrogen or alternatively R⁴ and R³ are combined together to represent a 5-membered cyclic group optionally containing sulfur. |