Probing Host-Selective Phytotoxicity: Synthesis and Biological Activity of Phomalide, Isophomalide, and Dihydrophomalide.

The cyclic depsipeptide phomalide [cyclo(Val-(E)-Aba-Hpp-Hmp-(R)-Leu); Aba = 2-amino-2-butenoic acid, Hpp = (2S)-2-hydroxy-3-phenylpropanoic acid, Hmp = (2S)-2-hydroxy-4-methylpentanoic acid] is the host-selective phytotoxin produced by the fungus [Leptosphaeria maculans (Desm.) Ces. et de Not., asexual stage Phoma lingam (Tode ex Fr.) Desm.] which causes blackleg disease (a devastating disease of several economically important brassica crops). Efficient total syntheses of phomalide, its (Z)-isomer isophomalide, and the two dihydro analogues [(R)-dihydrophomalide and (S)-dihydrophomalide] are described. A [2 + 3] fragment coupling of Cbz-Val-(Z)-Aba with Hpp-Hmp-D-Leu-OBn followed by deprotection and cyclization gave isophomalide which was diastereoselectively isomerized to phomalide by conjugate addition of PhSeH followed by elimination of the corresponding selenoxide. The dihydro analogues were prepared similarly using Cbz-Val-(R)-Abu or Cbz-Val-(S)-Abu (Abu = 2-aminobutanoic acid) in place of Cbz-Val-(Z)-Aba. Biological evaluations of phomalide, isophomalide, and the dihydrophomalides revealed that only phomalide (10(-)(5) M) caused necrotic, chlorotic, and reddish lesions on canola (Brassica napus and Brassica rapa; susceptible to blackleg) leaves whereas no damage was observed on brown mustard (Brassica juncea; resistant to blackleg) or white mustard (Sinapis alba; resistant to blackleg) leaves, even at significantly higher concentrations (10(-)(4) M). Thus, both the presence and configuration of the double bond is crucial for selective phytotoxicity. This is the first reported synthesis of an (E)-Aba-containing natural product, and importantly, the (Z) --> (E) isomerization approach should be applicable to other (depsi)peptide targets thereby allowing investigation of the effect of the double-bond configuration on various properties.