Isoflavonoid Phytoalexins from Leaflets of Dalbergia sericea

Simple 7,2',4'-oxygenated isoflavans accumulate in the fungus-inoculated tissues of many papilionate legumes where they appear to function as defensive compounds (phytoalexins) [1, 2], Vestitol (7,2'-dihydroxy-4'-methoxyisoflavan, 1) is the most common isoflavan phytoalexin [3] and occurs widely in the Papilionoideae subfamily of the Leguminosae; in contrast, the related compound, isovestitol (7,4'-dihydroxy-2'-methoxyisoflavan, 2), the three possible monomethyl ethers of 1 and 2 (sativan, 3; isosativan, 4; and arvensan, 5) and demethylvestitol (7,2',4'-trihydroxyisoflavan, 6) have a more restricted distribu­ tion being found principally in temperate legume tribes such as the Loteae and Trifolieae [3-6], It has recently been possible to investigate the phytoalexin response of several species belonging to Dalbergia (tribe Dalbergieae), a relatively large (approx. 100 species) tropical/subtropical genus of trees, shrubs and climbers. During the course of this work, it was found that the fungus (Helminthosporium carbonwm)-inoculated leaflets of the Himalayan species, D. sericea, produced substantial quantities of 1 to­ gether with a previously undescribed isoflavan (7) for which the common name, neovestitol, is proposed. This paper describes the isolation, purification and identification of neovestitol as 7-methoxy-2/,4'-dihydroxyisoflavan. Extraction and TLC purification of fungus-in­ duced leaf diffusates (see Experimental and [7, 8]) gave the known phytoalexins, vestitol (1), medicarpin (3-hydroxy-9-methoxypterocarpan, 8) and maackiain (3-hydroxy-8,9-methylenedioxypterocarpan, 9); all