A New Pentacyclic Triterpenoid from Albizia lebbekoides (DC) Benth (Leguminosae)

Albizia lebbekoides (DC) Benth is one of the five species of the genus Albizia found in the Philippines. Not much is known about the chemistry and potential uses of this species. Solvent partition followed by sequential and repeated liquid chromatographic purification over silica led to the isolation of a compound from the hexane extract of A. lebbekoides leaves. A review of related literature and spectral analyses showed that the isolated compound is a new pentacyclic triterpenoid with an oleanane skeleton. Its structure was determined as 3β, 16α-dihydroxyolean6(7),12(13)-dien-28-oic acid.

There is a dearth of studies on A. lebbeckoides and its synonyms Mimosa carisquis Blanco and Pithecellobium myriophyllum Gagnep (http://www.theplantlist.org/tpl/record/ild-46234). Most of the studies were done by a research group from Chulalongkorn University. They documented the α-glucosidase inhibitory activity Tunsaringkarn et al., 2009), antioxidant activity (Ramli et al., 2008), inhibition of Heinz body induction (Tunsaringkarn et al., 2012), and stimulation of white blood cell proliferation (Tunsaringkarn et al., 2014) of A. lebbekoides extracts. Another study showed the antibacterial and antioxidant activities of the leaf 1 H and 13 C, DEPT, HSQC, HMBC, and DQF-COSY NMR spectra were recorded on a Bruker AVIII HD 600 NMR spectrometer (Scripps Research Institute USA). ESIMS was measured on an Agilent Q-TOF mass spectrometer in a positive-ion mode using an ESI ion source, with scan ranges (m/z) from 100 to 1000 (Scripps Research Institute USA). IR and UV spectra were recorded on a Nicolet Magna Infrared Spectrometer and UV-3101 PC double beam scanning spectrophotometer, respectively.
Leaves were collected from the University of the Philippines Diliman (UPD), Quezon City. A voucher specimen was submitted to the Jose Vera Santos Herbarium, Institute of Biology, UPD and was given accession number 14578.

Extraction and Isolation.
The dried leaves of A. lebbekoides (741.38 g) were homogenized and soaked in methanol (1kg:10 L). The methanol extract (5.8% yield) was filtered and concentrated in vacuo. The crude methanol extract was partitioned between water and hexane (1:6). The resultant dried hexane extract (3.7% yield) was subjected to VLC over a silica gel column (8 x 15.44 cm) and eluted with gradient mixtures of 500 mL hexane, hexane in 10% increments of ethyl acetate, ethyl acetate, ethyl acetate in 25% increments of ethanol, and ethanol under reduced pressure. Fractions were pooled by TLC analysis to yield 8 combined fractions.

RESULTS AND DISCUSSION
Sequential and repeated normal phase column chromatographic techniques led to the isolation of a TLC pure compound. The 13 C NMR spectrum (Table 1) showed 29 carbon resonances and comprising seven methyls, seven methylenes, eight methines and seven quaternary carbons. The IR spectrum showed the presence of C=O at 1698 cm -1 but signals were absent in the NMR spectra. The COOH proton at 10-12 ppm is usually very broad, often to the point of being hard to see above the baseline. The carboxyl carbon at 160-185 ppm is often weak because of nOe effects on other carbons in a proton-decoupled spectrum.
In conjunction with the DEPT, HSQC, and IR data, a molecular formula of C30H46O4 was deduced.

Figure 1. HMBC Cross Peaks 1 H → 13 C of the E Ring of Oleanane
The proposed structure of the isolated compound ( Fig. 2) was supported by the HMBC correlations of H23 with C4 and C5; H2 with C5; H9/H11 with C10; and H1/H12/ H25/H26/H27 with C9. A diagnostic HMBC cross peak between H27 and C7 established the second double bond at C6-C7.