Actinomycins with altered threonine units in the beta-peptidolactone

Five new members of the actinomycin family, actinomycins G2-G6 (2-6), are produced by Streptomyces iakyrus strain DSM 41873. Their structures were established by spectroscopic methods. Unlike actinomycin D (1), the alpha-ring of the novel compounds contains the unusual amino acid 3-hydroxy-5-methylproline, while the beta-ring includes N-methylalanine and either a chlorinated or hydroxylated threonine moiety. The chlorine-containing actinomycin G2 (2) is the main product; it exhibits strong cytotoxic and antibacterial activities. Actinomycin G5 (5) is the first actinomycin with an additional ring closure between the beta-peptidolactone and the actinoyl chromophore. Actinomycin G6 (6) resulted from the 4-hydroxythreonine-containing actinomycin G3 (3) by a 2-fold acyl shift of the beta-unit, which has not been observed before for this class of chromopeptides. The structural modification of compounds 5 and 6 goes along with an evident reduction of the biological activity. The biosynthesis of aniso-actinomycins is discussed.