Three new dibenzofurans, porric acids A (1), B (2) and C (3), have been isolated from the bulbs of Allium porrum L. Their structures have been elucidated by a detailed spectroscopic
To date, a large number of antifungal metabolites have been characterized in plants . About half of these are constitutive agents, whereas the remainder are induced as phytoalexins, but there is no well-defined chemical division between constitutive and induced antifungal agents. Chemically, the most common antifungal compounds of plant origin are terpenoids or phenols. A subgroup of the latter metabolites are dibenzofurans, which are produced by living wood tissues of the plants as part of their dynamic defense system and are of relatively rare occurrence . They are reported as constitutive agents of lichens , as well as phytoalexins produced by sapwood of the woody members of Rosaceae in response to fungal inoculation .
As part of a systematic screening of the bioactive compounds in the Allium species from southern Italy [6] [7] [8] [9] , we have now found three novel dibenzofurans in the bulbs of the leek, Allium porrum L. We report herein on the isolation and characterization of these compounds, as well as on their activity towards the fungus Fusarium culmorum. This is the first report of the presence of dibenzofurans in plants belonging to the Allium genus.
Samples of A. porrum were collected and extracted first with n-hexane, and then with CHCl 3 , CHCl 3 /MeOH (9:1), and MeOH. Repeated column chromatography of the CHCl 3 /MeOH (9:1) extract yielded the pure dibenzofurans, porric acids A (1), B (2) and C (3).
Compound 1, isolated as pale-yellow microcrystals (m.p. 200Ϫ202°C), showed a molecular ion peak at m/z 302.0789 in the HREIMS, in accordance with the composition C 16 H 14 O 6 . This was also supported by 13 C-NMR data. The molecular formula indicated 10 degrees of unsaturation.
A sharp singlet at δ ϭ 11.89 in the 1 H-NMR spectrum, as well as a carbon signal at δ ϭ 164.2, interrelated in the HMBC spectrum, were diagnostic of a carboxylic group. The UV spectrum (bands at λ ϭ 338, 260, 238, 206 nm) was suggestive of a dibenzofuran skeleton , which was confirmed by the presence of 12 sp 2 carbon signals, other