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Anais da Academia Brasileira de Ciências

versão impressa ISSN 0001-3765versão On-line ISSN 1678-2690

An. Acad. Bras. Ciênc. v.76 n.4 Rio de Janeiro dez. 2004

http://dx.doi.org/10.1590/S0001-37652004000400004 

CHEMICAL SCIENCES

 

Flavonoids from Lonchocarpus muehlbergianus

 

 

Aderbal F. MagalhãesI; Ana M.G.A. TozziII; Eva G. MagalhãesI; Ivani S. BlancoI; Maria-Del-Pilar C. SorianoI

IDepartamento de Química Orgânica, Instituto de Química, UNICAMP, Cx. Postal 6154, 13083-970 Campinas, SP, Brasil
IIDepartamento de Botânica, Instituto de Biologia, UNICAMP, Cx. Postal 6109, 13083-970 Campinas, SP, Brasil

Correspondence

 

 


ABSTRACT

The light petroleum extract from the roots of Lonchocarpus muehlbergianus Hassl contained nine flavonoids, including six new ones. These are 2,4-cis-2,4,5,8-tetramethoxy-(2´´,3´´:6,7)-furanoflavan; 2,4-cis-4-hydroxy-2,5,8-trimethoxy-(2´´,3´´:6,7)-furanoflavan; 2,4-cis-2-prenyloxy-4,5,8-trimethoxy-(2´´,3´´:6,7)-fu-ranoflavan; 2,4-cis-2-prenyloxy-4-hydroxy-5,8-dimethoxy-(2´´,3´´:6,7)-furanoflavan; 2',5',6'-trimethoxy-9-(1,1-dimethylallyoxy)-[2´´,3´´:3´,4´]-furanochalcone; 5,6-dimethoxy-(2´´,3´´:7,8)-furanoflavone, identi-fied by analysis of their spectral data (UV, IR, 1H and 13C NMR, 2D-NMR, NOE and MS). The natural occurrence of 2,4-dioxygenated flavan derivatives is being reported for the first time.
Quantitative analysis of the petrol extract, by using reversed-phase HPLC, showed that the most abundant flavonoid in the extract is 2,4-cis-2,4,5,8- tetramethoxy-(2´´,3´´:6,7)-furanoflavan.

Key words: Lonchocarpus muehlbergianus, Leguminosae, flavonoids, flavans.


RESUMO

O extrato éter de petróleo das raízes de Lonchocarpus muehlbergianus foi submetido a sucessivas análises cromatográficas (CC, CCD e CDC preparativa) levando ao isolamento de nove flavonóides (1-9) dos quais seis são inéditos na literatura (1-6) destacando-se as quatro flavanas 2,4-dioxigenadas (1-4) que representam uma nova classe de flavonóides.
As estruturas moleculares foram determinadas através da análise dos respectivos espectros de RMN 1H, RMN 13C e DEPT, RMN-2D (COSY, HETCOR e COLOC), NOE, IV, UV e EM.
A análise quantitativa por CLAE, mostrou que a nova flavana 1 é o flavonóide que ocorre em maior abundância no extrato.

Palavras-chave: Lonchocarpus muehlbergianus, Leguminosae, flavonóides, flavanas.


 

 

1 INTRODUCTION

In continuation of our studies on the flavonoids of the Lonchocarpus species (Leguminosae), occurring in Brazil, we have examined L. muehlbergianus Hassl, which was allocated in Lonchocarpus subgenus Punctati together with L. subglaucescens (Magalhães et al. 1996).

Phytochemical data, obtained with several Lonchocarpus species previously investigated, allowed the characterization of many secondary metabolites mainly consisting of flavonoid structural types. Up to the present nothing is found in the literature about the natural occurrence of 2,4-dioxygenated flavans, while in Lonchocarpus, the occurrence of 4-oxygenated flavans, which are rarely found in nature, has only been observed in L. subglaucescens (Magalhães et al. 1996) and L. orotinus (Mahmoud and Waterman 1987). On the other side only three b-hydroxychalcones have been isolated from Lonchocarpus species (Magalhães et al. 1996, Waterman and Mahmoud 1985).

 

2 MATERIALS AND METHODS

2.1 GENERAL

Melting points were determined on a Kofler block and are uncorrected. IR spectra were recorded in CH2Cl2 and as KBr discs, UV spectra in MeOH or CHCl3. 1H and 13C NMR: run in CDCl3 with TMS as internal standard; field strength given in text. EIMS: direct probe insert at 70 eV. HREIMS: measurements were made on a VG Auto Spec-Fisions Instrument. HPLC: UV detector, reversed-phase column Varian C18, MCH, 10 mm (300 mm ×4.0 mm) and isocratic elution with CH3CN:H2O (70:30) as the mobile phase, at a flow rate of 0.8 ml/min. CC and TLC: silica gel 35-70 mesh, flash chromatography: silica gel 230-400 mesh.

2.2 PLANT MATERIAL

Roots of L. muehlbergianus were collected in the Ecological Park - Unicamp, Campinas (SP) in February 1992. Voucher specimens have been deposited at the herbarium (A.M.G.A. Tozzi 95-30) of Campinas State University (UNICAMP), Campinas-SP, Brazil.

2.3 EXTRACTION AND ISOLATION

Dry roots (931.7 g) of L. muehlbergianus were successively extracted with petrol (30-60ºC), chloroform and methanol in a Soxhlet apparatus. After solvent evaporation, the petrol extract gave a viscous yellow oil (9.9 g), while the chloroform extract gave a brown oil (4.0 g) and the methanol extract gave a brown gum (3.5 g). The petrol extract (9.9 g) was applied to a silica gel column eluted first with petrol. The eluent polarity was gradually increased by addition of ethyl acetate to furnish 230 fractions of 200 mL each which were reduced to 23 groups after TLC analysis. Most of the compounds were found in six groups ranging from fractions 47 to 199. A sample of each was further fractionated by successive preparative TLC (silica gel) respectively run with hexane:diethyl ether (95:05), petrol ether:ethyl acetate (90:10), dichloromethane:methanol (98:02) and dichloromethane:methanol (95:05). Flavonoids were visualized under UV light (l = 254 and 366 nm) and recovered from TLC plates by extraction with mixtures of CH2Cl2 and MeOH to furnish compounds 1 (56.0 mg), 2 (12.0 mg), 3 (6.8 mg), 4 (16.0 mg), 5 (3.7 mg), 6 (5.9 mg), 7 (14.3 mg), 8 (11.0 mg) and 9 (7.0 mg). Analogously, a part of the chloroform extract (1 g) furnished compounds 1 (7.0 mg), 2 (1.7 mg), 3 (15.0 mg), 4 (5.0 mg), 5 (3.7 mg), 6 (3.0 mg), 7 (4.3 mg), 8 (11.0 mg) and 9 (3.4 mg).

2.4 2,4-Cis-2,4,5,8-TETRAMETHOXY-(2´´,3´´:6,7)-FURANOFLAVAN (1)

Viscous oil; [a]20D + 100.59 (CHCl3; c 21.95); UV (MeOH) lmax (log e): 257 (3.66) nm; IR (CH2C2l) nmax 2933, 2833, 1628, 1545, 1484, 1449, 1408, 1364, 1251, 1188, 1159, 1118, 1090, 1054, 1044, 990, 967, 921, 760, 701 cm-1; 1H NMR (300 MHz, CDCl3/TMS): see Table I; 13C NMR (75.4 MHz, CDCl3): see Table II; EIMS m/z 370 [M] (7), 369 (25), 338 (32), 337 (93), 307 (76), 236 (100), 134 (4), 133 (9), 105 (18), 77 (19), 57 (38). HREIMS m/z: found 370.1333 [M]+ (C21H22O6 requires 370.1416).

 

 

 

 

2.5 2,4-Cis-4-HYDROXY-2,5,8-TRIMETHOXY-(2´´,3´´:6,7)-FURANOFLAVAN (2)

Viscous oil; [a]20D + 103.25 (CHCl3; c 4.45); UV (MeOH) lmax (log e): 257 (3.51) nm; IR (CH2Cl2) nmax 3508, 3055, 2932, 2850, 1629, 1545, 1485, 1437, 1407, 1364, 1265, 1189, 1155, 1116, 1063, 991, 736, 702 cm-1; 1H NMR (300 MHz, CDCl3/ TMS): see Table I; 13C NMR (75.4 MHz, CDCl3): see Table II; EIMS m/z 356 [M+] (absent), 307 (5), 222 (100), 207 (49), 179 (2), 134 (5), 105 (16),77 (23). HR-EIMS m/z: found 356.1208 [M]+ (C20H20O6 requires 356.1260).

2.6 2,4-Cis-2-PRENYLOXY-4,5,8-TRIMETHOXY-(2´´,3´´:6,7)-FURANOFLAVAN (3)

Viscous oil; [a]20D + 60.11 (CHCl3; c 2.83); IR (CH2C2l) nmax 3044, 2922, 1635, 1484, 1364, 1265, 1117, 1065, 736, 703 cm-1; 1H NMR (300 MHz, CDCl3/TMS): see Table I; 13C NMR (75.4 MHz, CDCl3): see Table II; EIMS m/z 424 [M]+ (8), 338 (7), 324 (55), 292 (95), 236 (11), 221 (15), 105 (100), 77 (23), 69 (15).

2.7 2,4-Cis-2-PRENYLOXY-4-HYDROXY-5,8-DIMETHOXY (2´´,3´´:6,7) FURANOFLAVAN (4)

Viscous oil; [a]20D + 84.18 (CHCl3; c 5.66); UV (MeOH) nmax (log e): 256 (3.76), 217 (4.31) nm; IR (CH2Cl2) nmax 3526, 2924, 2852, 1627, 1485, 1353, 1247, 1150, 1113, 1061, 989, 761, 738, 700 cm-1; 1H NMR (300 MHz, CDCl3/TMS): see Table I; 13C NMR (75.4 MHz, CDCl3): see Table II; EIMS m/z (410 [M+] (2), 324 (20), 222 (57), 207 (17), 149 (17), 105 (100), 102 (4), 77 (16), 69 (25).

2.8 2',5',6'-TRIMETHOXY-9-(1,1-DIMETHYLALLYOXY)- [2´´,3´´:3´,4´]-FURANOCHALCONE (5)

Viscous oil; UV (MeOH) lmax (log e): 341 (3.50), 272 (4.35), 214 (4.36) nm; 1H NMR (300 MHz, CDCl3/TMS): see Table V; EIMS m/z 422 [M+] (absent), 322 (83), 307 (100), 279 (31), 221 (8), 107 (17), 77 (8).

 

 

 

 

 

 

2.9 5,6-DIMETHOXY-(2´´,3´´:7,8)-FURANOFLAVONE (6)

Viscous oil; UV (MeOH) lmax (log e): 350 (3.87), 270 (4.72) nm; IR(CH2Cl2) nmax 3450, 3044, 2930, 2855, 1641, 1479, 1450, 1370, 1265, 1195, 1132, 1067, 738, 704 cm-1; 1H NMR (300 MHz, CDCl3/ TMS): see Table V; 13C NMR (75.4 MHz, CDCl3): see Table II; EIMS m/z 322 [M+] (98), 307 (100), 220 (4), 105 (20), 102 (15), 77 (17).

2.10 2,4-Cis-4,5,6-TRIMETHOXY-(2´´,3´´:7,8)-FURANOFLAVAN (7)

White needles; mp 59-61ºC; [a]20D - 28,98º (CHCl3; c 22.0); UV (MeOH) lmax (log e): 292 (3.14), 284 (3.15), 254 (3.86) nm; IR (KBr) nmax 2943, 1627, 1479, 1408, 1365, 1354, 1263, 1195, 1161, 1130, 1108, 1068, 1002, 964, 907, 877, 842, 764, 746, 698 cm-1; COSY (Table VI); HETCOR (Table VI); COLOC (Table VI) and EIMS m/z 340 [M+] (14), 339 (61), 308 (13), 236 (100), 221 (98), 105 (6), 104 (14), 91 (15), 77 (11).

 

 

2.11 HYDROLYSIS of 7

To a solution of 1 (17.4 mg) in CHCl3 (2 ml) was added 1 molL-1 HCl (0.5 ml). Work-up gave 7a (8.4 mg) and 7b (4.5 mg).

2.12 2,4- Trans-4,5,6-TRIMETHOXY-(2´´,3´´:7,8)-FURANOFLAVAN (7a)

Viscous oil. 1H NMR spectral data (300 MHz, CDCl3/TMS): see Table V. 13C NMR spectral data (75.4 MHz, CDCl3): see Table II.

2.13 2,4-Trans-4-HYDROXY-5,6-DIMETHOXY-(2´´,3´´:7,8)-FURANOFLAVAN (7b)

Light yellow needles; mp 126-128ºC; UV (CHCl3) lmax (log e): 289 (3.39), 283 (3.40), 257 (4.05), 254 (4.06), 216 (4.53) nm; IR (KBr) nmax 3364, 2934, 1618, 1542, 1482, 1348, 1310, 1245, 1067, 954, 761, 699 cm-1; 1H NMR (300 MHz, CDCl3/TMS): see Table V; 13C NMR (75.4 MHz, CDCl3): see Table II; EIMS m/z 326 [M+] (12), 308 (8), 293 (6), 222 (95), 207 (100), 192 (14), 179 (5), 164 (8), 147 (12), 104 (13), 77 (25).

 

3 RESULTS AND DISCUSSION

The petrol extract from the roots of L. muehlbergianus was submitted to adsorption chromatographic separation analysis (column, chromatotron, TLC and preparative TLC) furnishing nine flavonoids (1-9, Fig. 1).

 

 

Flavans 1-4, showed similar 1H NMR spectra (Table I) as all of them have resonances for the hydrogens of a furan ring, two aromatic methoxyl groups and an unsubstituted B ring, where two of its hydrogens are deshielded. By comparison with flavan 7 1H NMR data (Magalhães et al. 1996), the lack of H-2 signal and a much simpler multiplet in the region expected for C-3 hydrogens, suggest that flavans 1-4 have an OR group (R = methoxyl or prenyl) at C-2 causing the paramagnetic shift of H-2´ and H-6´ hydrogens. The 13C NMR spectrum (Table II) and DEPT (90º and 135º) allowed the assignment of all carbons; a peak around d 100 (Cº) in place of a peak around d 77 (CH) was taken as evidence of an OR group at C-2. In the NOE difference spectra irradiation of the methoxyl group hydrogens at C-4 enhanced the signal of one aromatic methoxyl group while irradiation of H-3´´ hydrogens caused enhancement of H-2´´ and of the same aromatic methoxyl group signal, indicating a linear fusion of furan with A ring (Fig. 2).

 

 

Flavan 1 was isolated as a yellow oil. Its HREIMS showed a molecular ion [M]+ of m/z 370.1333, corresponding to C21H22O6 (required M+ 370.1416). The base peak at m/z 236 [C12H12O5]+corresponds to the fragment bearing A ring from C-ring retro Diels-Alder (RDA) cleavage, while those at m/z 337 (93%) and m/z 307 (76%) can be represented by I and II fragments (Fig. 3). The UV spectrum indicated that the A and B rings are unconjugated. The 1H NMR spectrum (Table I) also showed the signals of two aliphatic methoxyl groups which were located at C-2 and C-4, in accordance with the double doublets assigned to the H-4 and 2H-3 hydrogens. The respective chemical shifts were confirmed by COSY (Table III) and NOE data (Fig. 4).

 

 

 

 

Flavan 2 was also isolated as a yellow oiland showed a similar UV spectrum. Its HREIMS showed a molecular ion [M]+ of m/z 356.1208, corresponding to C20H20O6 (required M+ 356.1260), which is fourteen mass units lower than flavan 1, suggesting that one methoxyl was replaced by one hydroxyl. The base peak at m/z 222 [C11H10O5]+, originating from RDA cleavage of the C-ring, indicates that the hydroxyl group is part of the fragment including A-ring. The 1H NMR spectrum (Table I) showed all resonances of the A and B rings that were seen in flavan 1, except for a signal corresponding to an aliphatic methoxyl group and the presence of a much more complex multiplet for H-4 as well as a doublet integrating for one hydrogen which was attributed to a hydroxyl group at C-4. The chemical shifts of C-ring hydrogens and carbons were confirmed by COSY (Table III) and HETCOR (Table III).

The 1H NMR (Table I) and 13C NMR (Table II) spectra of flavans 3 and 4 show the resonances for an O-prenyl group. In the case of flavan 3 there is also a resonance for an aliphatic methoxyl group while in flavan 4 there are the multiplets for a hydroxyl group at C-4 as in flavan 2. The MS spectrum of flavan 3 showed [M]+ at m/z 424 while in that of flavan 4 [M+] at m/z 410 is again fourteen mass units lower, confirming the replacement of a methoxyl group by a hydroxyl group. Both spectra showed a peak at m/z 324, which can be respectively rationalized by the simultaneous loss of methyl-1,1-dimethyl-1-vinyl ether [CH3OC(CH3)2CH = CH2] and 1,1-dimethyl-1-vinyl alcohol [HOC(CH3)2CH= CH2] (Fig. 5). The molecular structure was also supported by COSY (Table IV) and NOE differential experiments (Fig. 2).

 

 

The buds of Populus nigra excudate (Wollenweber and Egger 1971) furnished 2,5-dihydroxy-7-methoxyflavanone, but the natural occurrence of 2,4-dioxygenated flavans is now being registered for the first time. A recent publication (Knaggs 2000) about the biosynthesis of shikimate metabolites mentions that a (2S)-flavanone 2-hydroxylase is involved in biosynthetic pathways to flavones and dibenzoylmethanes. Based on these findings itseems reasonable to suggest a biogenetic route where a 2-hydroxyflavanone derivative is also reduced to a 4-hydroxy derivative which, in turn, could be further alkylated.

The1H NMR spectrum of flavonoid 5 (Table V) is very similar to that of dibenzoylmethane 8 (Magalhães et al. 1997) because both have resonances for the hydrogens of an unsubstituted aromatic ring, three aromatic methoxyl groups and one dimethylallyl group. The singlet of a methinic hydrogen on 5, however, is deshielded suggesting a corresponding enol ether. Through a NOE experiment it was observed that irradiation of the H-8 enhanced (21%) H-2 and H-6 signals, while irradiation of H-2 and H-6 has not caused an enhancement of dimethylallyl resonances, suggesting that flavonoid 5 is the Z regioisomer of the corresponding 9-OR enol ether of 8. In the 13C NMR spectrum two signals of low intensity at d 191.6 and d 169.1 were attributed to C-7 and C-9, respectively while a signal at d 107,4 was attributed to C-8. Many weak signals in the 13C-NMR spectrum may be due to the presence of several tautomeric forms. In the UV spectrum, an additional band at lmax 341 nm was attributed to the cinamoyl system, now present in the enol ether. The main peaks in the MS spectrum can be explained by the simultaneous loss of methoxyl and 1,1-dimethyl-1-vinyl radicals to give a flavone type fragment m/z 322 (83%), followed by the loss of a methyl radical to give the fragment m/z 307 (100%) (Fig. 6).

 

 

The UV spectrum of compound 6 suggesteda flavone structure where the bands at lmax 270and 350 nm indicate the presence of benzoyl and cinamoyl systems, respectively. Its IR spectrum exhibited the carbonyl group at 1641 cm-1. The MS spectrum displayed [M]+ at m/z 322 [98%] and the peaks at m/z 307 [100%, M - 15]+, 105 [20%, C7H5O] and 77 [17%, C6H5]+. The base peak can be taken as evidence of a methoxyl group on C-6. The 1H NMR (Table V) and 13C NMR (Table II) spectral data are compatible with a flavone analogous of flavan 7.

Flavan 7 was isolated in much lower amounts from L. subglaucescens (Magalhães et al. 1996), when its structure was established by 1H NMR, 13C NMR and NOE data. We now report additional spectral data (UV, IR and MS) including the use of 2D-NMR techniques such as COSY, HETCOR and COLOC (Table VI) which support its molecular constitution. The UV spectrum is in accordance with the lack of conjugation between the aromatic rings while the MS spectrum displayed a molecular ion at m/z 340 [M]+, together with peaks at m/z 236 [C12H12O5]+ and 104 [C8H8]+ corresponding to fragments derived from C ring RDA cleavage. Flavan 7 was submitted to a hydrolysis reaction. After reaction work up two major products were obtained: 7a and 7b while some of the starting material was recovered.

The structure of 7a was deduced through the comparison of its 1H NMR spectrum with that of flavan 7 (Table V). The summation of the coupling constant values obtained for H-2, H-3 and H-4 signals [J 2,3ax+ J 2,3eq= 14.1 Hz and J 4,3ax(3.0) + J 4,3eq (2.0) = 5.0 Hz], now are nearly those attributed to trans relative configuration. The H-4 signal also appeared as a triplet with a coupling constant of J=3.0 Hz (Clark-Lewis 1968, Bolger et al. 1966). So in flavan 7, the ''triplet'' corresponding to H-4 signal is in fact, a superimposed double doublet. Finally, the downfield chemical shift value of H-2 is strong evidence that it is being deshielded by the 4-OMe group which is on the same side of the C-ring in 7a. These findings suggest that the relative configuration of 4,5,7-trimethoxy-8-prenyl flavan, previously isolated from Tephrosia quercetorum (Gómez-Garibay et al. 1988) is also 2:4-trans, since in its 1H NMR spectrum the signal at d 5.29 (1H, dd, J= 12 and 4 Hz) is very close to that of H-2 in 7a (Table V).

The 1H NMR spectrum of 7b (Table V) closely resembled that of 7a, except for the lack of a singlet corresponding to the methoxyl group at C-4 and the presence of an absorption corresponding to hydroxyl groups (d 2.63). Significant differences caused by the presence of the hydroxyl group, however, were observed in the C NMR spectrum (Table II); the chemical shifts that were assigned to C-3 and C-4 appeared deshielded (+3.5 ppm) and shielded (-8.3 ppm), respectively. The NOE spectrum exhibited the expected interactions (Fig. 7). The IR spectrum showed the presence of a hydroxyl group (n = 3364 cm-1). The MS spectrum displayed the molecular ion peak at m/z 326 [M]+, other significant peaks at m/z 308 [M - H2O]+ and those originating from RDA cleavage of the C-ring at m/z 222 [C11H10O5]+ and m/z 104 [C8H8]+.

 

 

The spectral data obtained for compound 9 agree with those previously reported (Nascimento et al. 1976, Nascimento and Mors 1981). Additionally a NOE experiment was carried out in order to confirm the angular closure of the dimethylchromene ring in the A-ring and the location of the methoxyl group on C-6.

HP-HPLC showed that 1 is the most abundant constituent in the roots (26.3 mg/g of dried roots).

Analogously, compounds 1, 2, 3, 4, 6, 7, 8 and 9 were isolated from the chloroform extract.

 

ACKNOWLEDGMENTS

The authors are grateful to Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) for a scholarship awarded to Ivani da Silva Blanco, to Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) for financial support and to Dr. Carol Collins for writing revision.

 

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Correspondence to
Aderbal F. Magalhães
E-mail: aderbal@iqm.unicamp.br

Manuscript received on November 11, 2003; accepted for publication on July 7, 2004; presented by FERNANDO GALEMBECK

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