Tyr) (maculosin-1, (3S,8aS)-3-(4-hydroxybenzyl)hexahydropyrrolo[1,2-a]pyrazine-1,4-dione (1)), a host-specific phytotoxin, was isolated, together with some analogous cyclic dipeptides, from Alternaria alternata, the causal agent of black blight of spotted knapweed [1]. Compound 1 was tested against 19 plant species, and it showed phytotoxic activity only on knapweed [1]. Cyclo(L-Pro-L-Phe) (maculosin-2, 2), as well as cyclo(L-Pro-D-Phe), cyclo(Pro-Hle), cyclo(Pro-Val), cyclo(Pro-Leu), and cyclo(Pro-Ala), were isolated from the same fungus that was subsequently proposed as mycoherbicide for knapweed control. However, these cyclic dipeptides were less phytotoxic than compound 1 [1]. Cyclo(L-Pro-L-Tyr), cyclo(L-Pro-L-Phe), and several of their hemisynthetic derivatives were used in a structure-activity relationship (SAR) study aimed to obtain a safe and environmentally friendly specific bioherbicide [2-4]. 1 HN N O O H 2 HN N O O HO H 1 2 3 4 5 6 7 8 8a 1' 2' 3' 4' 5' 6' Cyclo(L-Pro-L-Tyr) (1) belongs to the family of 2,5-diketopiperazines, whose natural occurrence, biogenesis, synthesis, chemical reactivity, physicochemical properties, bioactivity, and pharmacological _______ *To whom correspondence should be addressed,