Maculosin (cyclo-Pro-Tyr), a host-specific phytotoxin produced by Alternaria alternata on spotted knapweed (Centaurea maculosa), is an ideal prototype for creating a safe and an environmentally friendly antiknapweed herbicide. To evaluate this possibility, a series of 18 maculosin analogs was synthesized and tested in the greenhouse on whole knapweed plants by spray or brush application. Interestingly, many of the maculosin analogs have significant potential as natural herbicides against spotted knapweed. Even the simplest analog, cyclo-Pro-Phe, destroys two thirds of the spotted knapweed foliage at a concentration of 6 × 10-2 mol/L within 15 days. Structure−activity relationships are discussed. Keywords: Cyclodipeptides; herbicides; maculosin; spotted knapweed

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