Gulypyrones A and B and Phomentrioloxins B and C Produced by <i>Diaporthe gulyae</i>, a Potential Mycoherbicide for Saffron Thistle (<i>Carthamus lanatus</i>)

Abstract

A virulent strain of <i>Diaporthe gulyae</i>, isolated from stem cankers of sunflower and known to be pathogenic to saffron thistle, has been shown to produce both known and previously undescribed metabolites when grown in either static liquid culture or a bioreactor. Together with phomentrioloxin, a phytotoxic geranylcyclohexenetriol recently isolated from a strain of <i>Phomopsis</i> sp., two new phytotoxic trisubstituted α-pyrones, named gulypyrones A and B (<b>1</b> and <b>2</b>), and two new 1,<i>O</i>- and 2,<i>O</i>-dehydro derivatives of phomentrioloxin, named phomentrioloxins B and C (<b>3</b> and <b>4</b>), were isolated from the liquid culture filtrates of <i>D. gulyae</i>. These four metabolites were characterized as 6-[(2<i>S</i>)­2-hydroxy-1-methylpropyl]-4-methoxy-5-methylpyran-2-one (<b>1</b>), 6-[(1<i>E</i>)-3-hydroxy-1-methylpropenyl]-4-methoxy-3-methylpyran-2-one (<b>2</b>), 4,6-dihydroxy-5-methoxy-2-(7-methyl-3-methylene­oct-6-en-1-ynyl)­cyclohex-2-enone (<b>3</b>), and 2,5-dihydroxy-6-methoxy-3-(7-methyl-3-methylene­oct-6-en-1-ynyl)­cyclohex-3-enone (<b>4</b>) using spectroscopic and chemical methods. The absolute configuration of the hydroxylated secondary carbon of the 2-hydroxy-1-methylpropyl side chain at C-6 of gulypyrone A was determined as <i>S</i> by applying a modified Mosher’s method. Other well-known metabolites were also isolated including 3-nitropropionic, succinic, and <i>p</i>-hydroxy- and <i>p</i>-methylbenzoic acids, <i>p</i>-hydroxybenzaldehyde, and nectriapyrone. When assayed using a 5 mM concentration on punctured leaf disks of weedy and crop plants, apart from 3-nitropropionic acid (the main metabolite responsible for the strong phytotoxicity of the culture filtrate), phomentrioloxin B caused small, but clear, necrotic spots on a number of plant species, whereas gulypyrone A caused leaf necrosis on <i>Helianthus annuus</i> plantlets. All other compounds were weakly active or inactive