3-Epiabruslactone A , a New Triterpene Lactone Isolated from Austroplenckia populnea

The investigation of chemical constituents of plants of the Celastraceae family has aroused considerable attention on account of the antitumoral and anti-leukemic activities found in maytansinoids and quinonemethide triterpenes isolated from plants of that family. In previous papers we described the chemical constitution of the root bark, bark wood and root wood of Austroplenckia populnea (Reiss) Lundell, specimen of the Celastraceae family. The plant is popularly known in the state of Minas Gerais as ‘‘mangabeira-brava’’, and it is a source of material for the industry of vimen furniture. The decoction of its branches is used as a traditional antidysenteric medicine. During our continued investigation of this species, one new oleanane derivative (1) has been isolated along with an extensive range of triterpenes including the known lactonic triterpene abruslactone A (2). The compound 1 showed spectral characteristics of an abruslactone A derivative. Thus, in this paper we present the identification of abruslactone A and structural elucidation of a new triterpene lactone, the epimer in C-3 (1) of abruslactone A, whose chemical structures are shown in Fig. 1. Both triterpenes were isolated from the ethanolic extract of the heartwood of Austroplenckia populnea, from which were also isolated the following triterpenes previously found in other parts of the same plant: 3-oxofriedelan-29α-oic acid (populnonic acid), 3-hydroxy-2-oxofriedelan-3-en-29αoic acid, 3β-hydroxyolean-12-en-29α-oic acid (epikatonic acid), and 3-oxoolean-12-en-29α-oic acid (katononic acid).


Introduction
The investigation of chemical constituents of plants of the Celastraceae family has aroused considerable attention on account of the antitumoral and anti-leukemic activities found in maytansinoids and quinonemethide triterpenes isolated from plants of that family 1 .
In previous papers we described the chemical constitution of the root bark 2 , bark wood 3 and root wood 4 of Austroplenckia populnea (Reiss) Lundell, specimen of the Celastraceae family.The plant is popularly known in the state of Minas Gerais as ''mangabeira-brava'', and it is a source of material for the industry of vimen furniture.The decoction of its branches is used as a traditional antidysenteric medicine 5 .

Experimental
General experimental procedure IR, NMR, mass spectroscopy, and melting point.IR spectra were obtained with a Galaxy FTIR 3000-Mattson in KBr pellets.A Bruker AC-200 ( 1 H: 200 MHz; 13 C: 50,3 Article MHz) was used for the measurement of the 13 C-NMR and 1 H-NMR, using CDCl3 as solvent and TMS as internal standard.The multiplicities of the 13 C signals were determined by means of the DEPT method.Mass spectra were obtained using an HP mass spectrometer.Melting points were determined using a Leitz thermic microscope without corrections.

X-ray crystallography
Data collection and processing.The X-ray diffraction data were obtained at ca. 293 K on a CAD-4 Enraf Nonius diffractometer using Mo Kα radiation monochromated by graphite.Intensity data for both crystals were collected in the 2θ range from 1 to 21°, using ω − 2θ scans with the scan width given by (0.8 + 0.35tnθ) and corrected for the Lp factors using the Enraf Nonius Structure Determination Package 6 , hkl range: 0 < h < 11, 0 < k < 14, and -2 < l < 16.Single crystal size 0.53 x 0.45 x 0.35 mm.Total number of reflections measured: 1849 of which 1798 independent and 1677 had I > 2σ, and two standard reflections were monitored every hour which however did not present any significant changes in intensities.
The MS spectrum of compound 1 presented a retro-Diels Alder standard fragmentation 10 [m/z 246 (base peak) and 207], indicating that the lactonic ring was localized at rings D and/or E.
The above data strongly suggested that 2 could be identical with abruslactone A isolated by Chang et al. 11 from Abrusprecatorius (Leguminosae).

Conclusion
All the experimental findings in this work are conclusive of the existence of the two proposed configurations of the herein studied compound in the plant, compound 1 is 3α-hydroxyolean-12-en-29,22α-olide and compound 2 is 3β-hydroxyolean-12-en-29,22α-olide.The isolation of the epimeric pair gives rise to doubts on abruslactone precursor.Hitherto the maytenfolic acid (3β-22α-dihydroxyolean-12-en-29α-oic acid) has been considered as abruslactone A precursor.But what could be the 3-epiabruslactone A precursor?In the course of our future chemical investigations we will we will attempt to resolve this question.It would be very interesting to further investigate if the epimers present different biological activities.

Figure 2 .
Figure 2. ORTEP representation of the superposition of the relative molecular structures of compounds 1 and 2 (primed = epiabruslactone) showing the different orientation of the hydroxyl group at C-3.

Table 2 .
Atomic coordinates (x 10 4 ) and equivalent isotropic displacement parameters (Å 2 x 10 3 ) for Epiabruslactone.U(eq) is defined as one third of the trace of the orthogonalized Uij tensor.