A furocoumarin, Imperatorin isolated from Urena lobata L. (Malvaceae)

The plant, Urena lobata L. (Malvaceae) is reputed for its use in indigenous system of medicine.¹ Phytochemical study of the aerial part of this plant showed the presence of mangiferin and quercetin.² The roots have not been investigated so far for their active constituents. The isolation of imperatorin is being reported for the first time from this source.

Air-dried and pulverised roots (1.5 kg) of this plant were exhaustively extracted under reflux with benzene. The solvent was distilled off under the reduced pressure to yield 8 g of the extract which was then column chromatographed over silica gel. Elution of the column with petroleum-ether: benzene (4:3) eluate yielded an impure solid which on rechromatography over silica-gel column afforded a pure solid crystallized from petroleum-ether as crystals (I), yield: 34mg (0.0023 %).

M.p. 98-99°C(petroleum-ether); Optically inactive.

UV (l_max , MeOH, nm) : 301 (log e 3.86), 266 (log e 3.97).

IR (n_max , KBr, cm^-1): 3080 (C=C), 1720 (C=O of lactone ring), 1680, 1640, 1580 (aromatic),1440, 1220 (C-O-C stretching, asymmetric), 1150, 1085, 1030 (benzofuran), 990, 910 and 880.

¹H-NMR (300 MHz, CDCl₃): d: 7.69 (1H, d, J=2 Hz, H−2¢), 6.81(1H, d, J=2 Hz, H-3¢), 6.37(1H, d, J=9.6 Hz, H-3), 7.76 (1H, d, J=9.6 Hz, H-4), 7.36 (1H, s, H-5), 5.00 (2H, d, J=7 Hz, H-1²), 5.61 (1H, t, J= 7 Hz, H-2²), 1.72 (3H, s, Me-4² or 5²) , 1.74 (3H, s, Me-4² or 5²).

¹³C-NMR (75 MHz, CDCl₃): 146.5 (C-2¢), 106.6 (C-3¢), 160.2 (C-2), 114.6 (C-3), 144.1 (C-4), 116.5 (C-4a), 113.0 (C-5), 125.6 (C-6), 148.1 (C-7), 125.6 (C-8), 139.2 (C-8a), 70.1 (C-1²), 119.8 (C-2²), 131.2 (C-3²), 18.0 (C-4²), 25.6 (C-5²).

EIMS: m/z : 270 [M]⁺ (C₁₆H₁₄O₄), 202 (100%), 174, 146, 145, 136, 117, 90, 89.

The ¹H- & ¹³C-NMR analyses were in consonance with the molecular architecture of imperatorin.^3,4