Chemical constituents of Plumeria acutifolia leaves

A phytochemical investigation on the leaves of Plumeria acutifolia Poir., also known as Plumeria rubra f. acutifolia Poir. (Apocynaceae) has lead to the isolation and identification of three triterpenes (lupeol, uvaol and ursolic acid), one iridoid (15-demethylplumieride) [1], and a new alkaloid (1) with an iridoid-like skeleton for which the name plumerianine was adopted. Air-dried and powdered leaves were extracted with MeOH. Liquid/liquid partition and preparative TLC on silica gel using CHCl₃: MeOH (9: 1.5) yielded compound 1. Its structure was elucidated using ¹H and ¹³C NMR, DEPT-135 and DEPT-90, COSY, HSQC and HMBC experiments, mass spectrometry and measurement of the specific optical rotation [α]²⁵ _D=18 (c 0.18, CHCl₃). Based on this information, and because of the presence of iridoids in the genus Plumeria, an iridoid-like alkaloid (1) was proposed, with the same relative configuration as plumieride. This structural proposal was in complete agreement with the one- and two-dimensional NMR spectra. The ESI-MS spectrum recorded in the positive ion mode showed a [M+H]⁺ peak at m/z 232, and a [M+Na]⁺ peak at m/z 254, which was in agreement with the expected molecular weight of 231 (C₁₃H₁₃NO₃). It has been observed that alkaloids such as gentianine may be formed as artefacts by the incorporation of ammonia in iridoid structures during extraction. In principle this possibility cannot be excluded here. Nevertheless compound 1 cannot be formed from plumieride, the iridoid showing the best similarity, since the methoxycarbonyl group is not present, and since plumieride is unsaturated at position 6,7. Until now no 6,7-saturated iridoids have been reported in the genus Plumeria.

References: 1. Kardono, L.B. et al. (1990)J. Nat. Prod. 53: 1447–1455.