Echinulin, a novel cyclic dipeptide carrying a triprenylated indole moiety from an anacardiaceae, a Cucurbitaceae and two Orchidaceae plants: detailed high resolution 2D-NMR and mass spectral studies

A levorotatory compound, C 2 9 H 3 9 N 3 O 2 was isolated from the edible fruits of Rhus parviflora (Anacardiaceae) along with citric acid 2-methyl ester. A detailed 1 H and 1 3 C NMR spectral studies including 1 H- 1 H correlation, long range 1 3 C- 1 H correlation (HMBC and HMQC) and also NOESY confirmed its structure as 3S,6S,3-[[2-(1,1-dimethylallyl)-5,7-bis(3,3-dimethylallyl-1H-indo-3-yl[methyl]-6-methyl-2,5-piperazinedione (1) and allowed the unambiguous assignments of each carbon and proton signals. This biogenetically interesting novel cyclic dipeptide was designated echinulin, being reported first in 1943 from the moulds of Aspergillus echinulatus; later on it was reported from the moulds of the A. glaucus group. Echinulin has since been isolated in this laboratory from three other plants, viz. two Orchidaceae plants, Dendrobium fimbriatum and Cymbidium aloifolium and the peels of an edible vegetable (fruits, Bengali name Potol) of Trichosanthes dioica (Cucurbitaceae). To our knowledge, this is the first report on the occurrence of this mould metabolite, echinulin in four higher plants belonging to three different families. The absolute configuration (S) at each of its two chiral centres and its mass fragmentation have also been discussed.