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

Abstracts

A new lactonic triterpene isolated from the heartwood of Austroplenckia populnea (Celastraceae) was characterized as 3alpha-hydroxyolean-12-en-29,22alpha-olide (the gamma-lactone of the 3alpha,22alpha-dihydroxyolean-12-en-29alpha-oic acid), the 3-epimer of the abruslactone A, on the basis of its spectral data, chemical transformations, and single crystal X-ray analysis.

Austroplenckia populnea; Celastraceae; 3-epiabruslactone A; structure elucidation

Um triterpeno lactônico inédito isolado do cerne de Austroplenckia populnea (Celastraceae) foi caracterizado como 3alfa-hidroxiolean-12-en-29,22alfa-olídeo (a gama-lactona do ácido 3alfa,22alfa-diidroxiolean-12-en-29alfa-óico), o epímero em C-3 da abruslactona A, com base em dados espectrais, oxidação e análise cristalográfica.

Article

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

Grácia Divina de Fátima Silva ^a* , Lucienir Pains Duarte ^a , Helena Clara da Silva Paes ^a , José Rêgo de Sousa ^a , Maria Cristina Nonato ^b , Paulo José Portezani ^b and Yvonne Primerano Mascarenhas ^b

^a Departamento de Química, ICEx, Univ. Fed. de Minas Gerais, C.P. 702, 30123-970 Belo Horizonte - MG, Brazil

^bDepartamento de Física e Informática, IFSC, USP, 13560-280, S. Carlos - SP, Brazil

Received: May 17, 1997

Um triterpeno lactônico inédito isolado do cerne de

Austroplenckia populnea

(Celastraceae) foi caracterizado como 3a-hidroxiolean-12-en-29,22a-olídeo (a g-lactona do ácido 3a,22a-diidroxiolean-12-en-29a-óico), o epímero em C-3 da abruslactona A, com base em dados espectrais, oxidação e análise cristalográfica.

A new lactonic triterpene isolated from the heartwood of

Austroplenckia populnea

(Celastraceae) was characterized as 3a-hydroxyolean-12-en-29,22a-olide (the g-lactone of the 3a,22a-dihydroxyolean-12-en-29a-oic acid), the 3-epimer of the abruslactone A, on the basis of its spectral data, chemical transformations, and single crystal X-ray analysis.

Keywords

: Austroplenckia populnea

, Celastraceae, 3-epiabruslactone A, structure elucidation

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 family¹.

In previous papers we described the chemical constitution of the root bark², bark wood³ and root wood⁴ 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 medicine⁵.

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-29a-oic acid (populnonic acid)^1,2,3, 3-hydroxy-2-oxofriedelan-3-en-29a-oic acid, 3b-hydroxyolean-12-en-29a-oic acid (epikatonic acid)², and 3-oxoolean-12-en-29a-oic acid (katononic acid)².

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 (¹H: 200 MHz; ¹³C: 50,3 MHz) was used for the measurement of the ¹³C-NMR and ¹H-NMR, using CDCl₃ as solvent and TMS as internal standard. The multiplicities of the ¹³C 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.

3-Epiabruslactone A ( 1 )

Mp. 295-296 °C (CHCl₃/MeOH), [a]²⁰ + 40.1° (c = 1.0, CHCl₃), MS: m/z 454 [M]⁺, 246 (100) and 207 [RDA C ring]⁺, IR max: 3550 and 1755 cm^-1, ¹H NMR (CDCl₃, 200 MHz): d (m) 5.30 (t, 1H, J = 4.4 Hz), 4.14 (d, 1H, J = 5.6 Hz), 3.40 (t, 1H, J = 2.9Hz), 1.20, 1.09, 0.95, 0.95, 0.92, 0.86, 0.84 (s, 3H each), ¹³C-NMR: Table 1.

Abruslactone A ( 2 )

Mp. 319.5-322.0 °C (CHCl₃/MeOH), [a]²⁰ -34.2° (c = 1.0, CHCl₃), ¹H NMR (CDCl₃, 200 MHz): d (m) 5.30 (t, 1H, J = 3.0 Hz), 4.17 (d, 1H, J = 5.0 Hz), 3.2 (1H, dd, J = 6.0, 9.0 Hz), 1.21, 1.07, 0.99, 0.94, 0.93, 0.87, 0.79 (s, 3H each) in good agreement with previously reported data by Budzikiewicz, Wilson and Djerassi¹⁰, Chang, Chiang and Mak¹¹ and Nozaki et at.¹²

3-oxoolean-12-en-29,22-olide (Oxidation of 1 or 2)

10.0 mg of 1 (or 2) and 200.0 mg of BaMnO₄ in 10.0 mL of CH₂Cl₂ were refluxed for 120 h. Filtration and evaporation of the solvent furnished a residue that was submitted to SiO₂ gel column chromatography. The product of the reaction was obtained in 40% yield by elution with CHCl₃/MeOH (19:1). Mp. 289-290 °C, IR max: 1770 and 1700 cm^-1.

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 Ka radiation monochromated by graphite. Intensity data for both crystals were collected in the 2q range from 1 to 21°, using w - 2q scans with the scan width given by (0.8 + 0.35tnq) and corrected for the Lp factors using the Enraf Nonius Structure Determination Package⁶, 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 > 2s, and two standard reflections were monitored every hour which however did not present any significant changes in intensities.

Results and Discussion

Compounds 1 and 2 analyzed as C₃₀H₄₆O₃ ([M]⁺, m/z 454), and presented a positive Liebermann-Burchard test for a pentacyclic triterpene. The presence of hydroxyl and lactonic carbonyl groups for 1 or 2 was shown by the bands at 3550 and 1755 cm^-1 in the IR spectra. The ¹H-NMR spectrum of 1 presented singlet signals corresponding to the seven tertiary methyl groups (d 1.20, 1.09, 0.95, 0.95, 0.92, 0.86 and 0.84), a signal attributed to a vinylic proton at d 5.30 (1H, t, J 4.4 Hz, H-12) and two signals assigned to protons of carbinol type: d 4.14 (1H, d, J 5.6 Hz, H-22) and d 3.4 (1H, t, J 2.9 Hz, H-3). The spectral analysis IR, ¹H-NMR and ¹³C-NMR, which revealed the presence of thirty signals, including d 182.4 (C-29); 140.2 (C-13); 124.6 (C-12); 76.0 (C-3) and 83.1 (C-22), was used to establish that compound 1 contains the backbone of b-amirine. Both compounds exhibited similar NMR spectra, but the doublet of doublets observed at d 3.22 (J 6.0 and 10.5 Hz) for H-3 in 2 was replaced by a triplet at d 3.4 (J 2.9 Hz) for H-3 in 1, indicating an axial position for the hydroxyl group at C-3 for 1. Both triterpenes on oxidation with barium manganate in dichloromethane¹³, gave 3-oxoolean-12-en-29,22a-olide, confirming that they are epimeric at C-3.

However, compounds 1 and 2 differ significantly in their melting points and optical rotations [295-296 °C, CHCl₃/MeOH; [a]²⁰ +40.1° (c. 1.0 CHCl₃)] and [329-330 °C; 317-318 °C; [a]²⁰ -33.4° (c. 0.2 CHCl₃)]^10,11.

The MS spectrum of compound 1 presented a retro-Diels Alder standard fragmentation¹⁰ [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).

As a further confirmation of the structure and stereochemistry of 1 and 2, an X-ray analysis was carried out for both. As the structure abruslactone was fully confirmed, the X-ray crystal data will be given only for 3-Epiabruslactone. Crystal data. C₃₀H₄₆O₃. M = 454.7 u., orthorhombic system, space group P2₁2₁2₁, Z = 4, D_x = 1.202 g cm^-3m = 0.7 cm^-1, F(000) = 1,000. Cell dimensions a = 11.205(1), b = 14.164(6), c = 15.894(3) Å, V = 2522(1) ų.

Structure analysis and refinement. The structures were solved by direct methods using the program SHELXS-86⁷ and refined by full matrix least-squares method using the program SHELX-93⁸ considering anisotropic temperature factors for all atoms except for the hydrogen atoms that had their positional parameters fixed as determined from Fourier difference maps and a refined single isotropic temperature factor common for all hydrogens. Atomic scattering factors from International Tables for X-ray Crystallography⁹. FinalR, (R = S (|F_obs - F_calc |)/S |F_obs |), for all observed reflections with I > 2s (I) and 300 refined parameters (atomic parameters, scale, extinction coeficint) was 5.3%. Final positional and equivalent isotropic temperature factors for all non-hydrogen atoms with estimated standard deviation in parentheses are given in Table 2. All calculated interatomic distances and angles were in agreement with the expected values for the respective bond types. Tables of hydrogen parameters, anisotropic temperature factors and observed and calculated structure factors are available from one of the authors (YPM). Similar conformation of the rings A, B, C, D, E and F were found for compounds 1 and 2 in the crystal and are exactly the same as for the structure found for abruslactone A³ with the only difference residing, as expected, in the relative position of O(1) of compound 1, the epimer of 2. In both crystal structures an intermolecular hydrogen bond is established in a head and tail fashion between O(1) and O(3) of molecules related, for epi-abruslactone by the symmetry operations (x, y, z) and (0.5+x, 0.5-y, -z) with O(1)-O(3), O1-H1, and O3-H1 distances of 2.919(8), 1.157(8) and 1.902(8) Å O(1)-H(1)-O(3) angle of 144.1(6)°. Figure 2 is an ORTEP¹⁴ representation of the superposition of the relative molecular structures of compounds 1 and 2 including only the non-hydrogen atoms.

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 3a-hydroxyolean-12-en-29,22a-olide and compound 2 is 3b-hydroxyolean-12-en-29,22a-olide. The isolation of the epimeric pair gives rise to doubts on abruslactone precursor. Hitherto the maytenfolic acid (3b-22a-dihydroxy-olean-12-en-29a-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.

Acknowledgments

The authors are grateful to Prof. Maria das Graças Lins Brandão from the Faculdade de Farmácia, UFMG for the determination of the optical rotation at the Institute of Pharmaceutical Biological, Karlstrasse, Germany, to Professor José Auri Pinheiro from the Universidade Federal do Ceará for the NMR and Mass spectra, to Prof. José Luis Pedersoli from ICB, UFMG for the identification and collection of the botanical material and to CNPq, CAPES, FINEP, FAPEMIG and FAPESP for financial support.

FAPESP helped in meeting the publication costs of this article

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Publication Dates

• Publication in this collection
  06 Nov 2002
• Date of issue
  Sept 1998

History

• Received
  17 May 1997