in which

I) R represents H, simple alkyl chain (-SAC), simple cycloalkyl chain (-SCAC), aryl group (—Ar), or simple alkyl chain substituted by aryl (-SAC-Ar),

II) R¹ represents -SAC, -SCAC, —Ar, -SAC-Ar, or a side chain residue of all the natural amino acids; and the compound of formula (1) may exist in a specific diastereomeric form, or mixtures thereof when the carbon to which R¹ is attached becomes a stereocenter due to the R¹ group; or the compound of formula (1) may have a protecting group in an ester form (—CO₂R³ wherein R³ is -SAC) or a sulfonamide form (—CONHSO₂R⁴ wherein R⁴ is -SAC), or may exist in the form of pharmaceutically acceptable salt, when R¹ is a side chain residue of an amino acid containing carboxyl moiety; or the compound of formula (1) may also exist in the form of pharmaceutically acceptable salt when R¹ is a side chain residue of an amino acid containing a base moiety,

III) R² represents -SAC, -SCAC, —Ar, -SAC-Ar, or a side chain residue of the natural amino acids; and the compound of formula (1) may exist in a specific diastereomeric form, or mixtures thereof when the carbon to which R² is attached becomes a stereocenter due to the R² group; the compound of formula (1) may have a protecting group in an ester form (—CO₂R⁵ wherein R⁵ is -SAC) or a sulfonamide form (—CONHSO₂R⁶ wherein R⁶ is -SAC), or may exist in the form of pharmaceutically acceptable salt, when R² is a side chain residue of an amino acid containing carboxyl moiety; or the compound of formula (1) may also exist in the form of pharmaceutically acceptable salt when R² is a side chain residue of an amino acid containing a base moiety, or

R² further represents H; —(CH₂)_nOR⁷ wherein R⁷ is -SAC, -SCAC, —Ar, or -SAC-Ar, and n=1 or 2; or —(CH₂)_nOC(═O)R⁸ wherein R⁸ is -SAC, -SCAC, —Ar, or -SAC-Ar, and n=1 or 2,

IV) A represents —(CH₂)_n— (n=0-4), —O—(CH₂)_n— (n=0-4), or —NR⁹—(CH₂)_n— (n=0-4) wherein R⁹ is -SAC, -SCAC, —Ar, or -SAC-Ar,

V) B represents H, -SAC, -SCAC, —Ar, or -SAC-Ar, or

VI) R and R¹ may form a cycle together with the carbon atom to which they are attached, where —R—R¹— is —(CH₂)_n—, —(CH₂)_n—O—(CH₂)_m—, or —(CH₂)_n—NR¹⁰—(CH₂)_m— wherein n+m<9 and R¹⁰ is -SAC, -SCAC, —Ar, -SAC-Ar, —C(═O)— SAC, —C(═O)-SCAC, —C(═O)—Ar, or —C(═O)-SAC-Ar,

VII) X represents —C(═O)CH₂OR¹¹ wherein R¹¹ is -SAC, -SCAC, —Ar, or -SAC-Ar; —C(═O)CH₂OC(═O)R¹² wherein R¹² is -SAC, -SCAC, —Ar, or -SAC-Ar; —CH═CH—CO₂R¹³ wherein R¹³ is -SAC, -SCAC, —Ar, or -SAC-Ar; —CH═CH—SO₂R¹⁴ wherein R¹⁴ is -SAC, -SCAC, —Ar, or -SAC-Ar; —C(═O)CH═CH₂; or —COCH₂—W wherein W is —N₂, —F, —Cl, —Br, —I, —NR¹⁵R¹⁶ (R¹⁵ and R¹⁶ each are -SAC, -SCAC, —Ar, or -SAC-Ar, or together may form 3- to 6-membered saturated or unsaturated cyclic group), —SR¹⁷ (R¹⁷ is -SAC, -SCAC, —Ar, or —SAC-Ar), or is the following formula:

wherein

Y is H, —OH, —OR¹⁸ (R¹⁸=-SAC or -SCAC), —C(═O)R¹⁹ (R¹⁹=—H, -SAC, or -SCAC), —F, —Cl, —Br, —I, —CN, —NC, —N₃, —CO₂H, CF₃, —CO₂R²⁰(R²⁰=-SAC or —SCAC), —C(═O)NHR²¹ (R²¹=-SAC or -SCAC), or —C(═O)NR²²R²³ (R²² and R²³ each are -SAC, -SCAC, —Ar, or -SAC-Ar),

R²⁴ is H, -SAC, -SAC-Ar, or —Ar, salt, or stereoisomer thereof.