O-Methylation of phenylphenalenone phytoalexins in Musa acuminata and Wachendorfia thyrsiflora

Biosynthetic O-methylation at various sites along the backbone of inducible phenylphenalenones in Musa acuminata var. “Williams” (Musaceae) and Wachendorfia thyrsiflora (Haemodoraceae) was investigated using ¹³C-labelled precursors. The inducibility of O-methylated metabolites was demonstrated in both species and the origin of methoxyl group from [methyl-¹³C]l-methionine was confirmed. In addition to known phenylphenalenones, a methoxylated metabolite, 4-(4-hydroxy-3-methoxy-phenyl)-benzo[de]isochromene-1,3-dione, was detected and its structure elucidated mainly by NMR spectroscopic techniques. The experiments were used to discriminate methionine-derived and artificial methoxy groups formed during methanolic extraction. Finally, demethylation of 4′-methoxycinnamic acid and subsequent conversion to 3′,4′-methylenedioxycinnamic acid was demonstrated in M. acuminata.