Three new compounds from Piper montealegreanum Yuncker (Piperaceae)

Abstracts

Three new compounds: two flavonoids [(S)-8-formyl-3',5-dihydroxy-7-methoxy-6methylflavanone (1) and 3'-formyl-3,4',6'-trihydroxy-2'-methoxy-5'-methylchalcone (2)] and one phenylpropanoid [ethyl 3,4-methylenedioxy-5-methoxy-7,8-dihydrocinnamate (3)] were isolated from dried branches of Piper montealegreanum. Their structures were established by UV, IR, MS, 1D and 2D (¹H and 13C) NMR spectroscopic techniques, besides interpretation of spectral data (¹H and 13C NMR) of methylated derivatives of 1 and 2 compounds.

Piper montealegreanum; flavanone; chalcone; phenylalcanoid

Três novos compostos: dois flavonóides [(S)-8-formil-3',5-diidroxi-7-metoxi-6-metilflavanona(1) e 3'-formil-3,4',6'-triidroxi-2'-metoxi-5'-metilchalcona (2)] e um fenilpropanóide [3,4-metilenodioxi-5-metoxi-7,8-diidrocinamato de etila (3)] foram isolados dos ramos secos de Piper montealegreanum. As estruturas desses compostos foram estabelecidas através das técnicas espectroscópicas UV, IV, EM e RMN (¹H e 13C, 1D e 2D), além da interpretação dos dados de RMN de ¹H e 13C dos derivados metilados dos compostos 1 e 2.

SHORT REPORT

Three new compounds from Piper montealegreanum Yuncker (Piperaceae)

Harley da S. Alves; Maria de F. V. de Souza; Maria C. de O. Chaves^*

Laboratório de Tecnologia Farmacêutica, Universidade Federal da Paraíba, CP 5009, 58051-970 João Pessoa-PB, Brazil

ABSTRACT

Three new compounds: two flavonoids [(S)-8-formyl-3',5-dihydroxy-7-methoxy-6methylflavanone (1) and 3'-formyl-3,4',6'-trihydroxy-2'-methoxy-5'-methylchalcone (2)] and one phenylpropanoid [ethyl 3,4-methylenedioxy-5-methoxy-7,8-dihydrocinnamate (3)] were isolated from dried branches of Piper montealegreanum. Their structures were established by UV, IR, MS, 1D and 2D (¹H and ¹³C) NMR spectroscopic techniques, besides interpretation of spectral data (¹H and ¹³C NMR) of methylated derivatives of 1 and 2 compounds.

Keywords:Piper montealegreanum, flavanone, chalcone, phenylalcanoid

RESUMO

Três novos compostos: dois flavonóides [(S)-8-formil-3',5-diidroxi-7-metoxi-6-metilflavanona(1) e 3'-formil-3,4',6'-triidroxi-2'-metoxi-5'-metilchalcona (2)] e um fenilpropanóide [3,4-metilenodioxi-5-metoxi-7,8-diidrocinamato de etila (3)] foram isolados dos ramos secos de Piper montealegreanum. As estruturas desses compostos foram estabelecidas através das técnicas espectroscópicas UV, IV, EM e RMN (¹H e ¹³C, 1D e 2D), além da interpretação dos dados de RMN de ¹H e ¹³C dos derivados metilados dos compostos 1 e 2.

Introduction

Piper montealegreanum Yuncker (Piperaceae) is a shrub, native to the north Brazil¹ and has no previous chemical studies reported. A continuing search on the chemistry and bioactive agents from Brazilian north-northeast Piperaceae species have resulted in the isolation of amides,^2-5 aristolactams^6,7 and propenylphenols.^8-12 In this paper, we report the isolation and structure elucidation of (S)-8-formyl-3',5-dihydroxy-7-methoxy-6-methylflavanone (1), 3'-formyl-3,4',6'-trihydroxy-2'-methoxy-5'-methylchalcone (2), and ethyl 3,4-methylenedioxy-5-methoxy-7,8-dihydrocinnamate (3) from the branches of P. montealegreanum (Figure 1). The structures of the compounds were determined by interpretation of the spectral data analysis of UV, IR, MS, ¹H and ¹³C NMR, including 2D NMR HMQC (heteronuclear multiple quantum coherence), HMBC (heteronuclear multiple bond correlation), and by comparison with those reported in the literature.¹³

Results and Discussion

Compound 1 was obtained as orange-yellow crystals. The MS spectrum presented a molecular ion peak at m/z 327.0887 (M-1)^-, in LC-MS-IT-TOF apparatus (ion trap-time of flight liquid chromatography mass spectrometry). The ¹H NMR spectrum showed the presence of four singlets at δ_H 12.63 (1H), 10.15 (1H), 3.99 (3H) and 2.05 (3H) consistent with the presence of chelated hydroxyls, aldehyde, methoxyl and methyl groups, respectively. The presence of three signals at δ_H, 5.43 (dd,1H, J 10.8 and4.6 Hz), 2.97 (dd, 1H, J 17.0 and 10.8 Hz), 2.85 (dd,1H, J4.6 and 17.0 Hz), and also signals for four coupled aromatic protons at δ_H, 7.26 (t, 1H, J 8.0 Hz), 6.85 (brd, 1H, J 8.0 Hz) and 6.93 (m, 2H), suggested a flavanone nucleus¹⁴ with a 3'-monosubstituted B ring, deduced by analysis of the multiplicity and coupling constants of the aromatic protons.¹⁵ The above data and UV spectrum, with λ_max at 266 nm, reinforced a flavanone nature for compound 1.¹³ Since no further aromatic protons were evident, ring A should be fully substituted. The presence of the signal to methine carbon at δ_C 193.8 in the ¹³C NMR APT (attached proton test) spectrum was taken as a proof for the presence of an aldehyde group, and the low-field hydroxyl hydrogen [δ_H 12.63 (s, 1H)] evidenced the chelated hydroxyl at C-5 with the C=O of the α,β-unsaturated carbonyl group.¹³

In the HMBC spectrum, the presence of cross peaks at δ_H 12.63 with δ_C 166.3 and 109.3, besides the cross peak at δ_H 2.05 with δ_C 166.3, 166.1 and 109.3, evidenced the existence of a methyl group at C-6 and also suggested the attachment position of the methoxyl group at C-7. This was confirmed by correlation of the peak at δ_H 10.15 (aldehyde hydrogen) and δ_H 3.99 (methoxyl hydrogen) with δ_C 166.1, and consequently, the placement of the formyl group at C-8 (Figure 2).

The compound 1a (Figure 1), a methylated derivative of 1, showed correlations in HMQC spectrum between δ_H 3.95 /δ_C 64.3 and δ_H 3.82 / δ_C 55.3. The HMBC spectrum displayed correlations between δ_H 3.95 and 2.14 / δ_C 164.6 confirming the methoxyl group at C-5. The presence of cross peaks between signals in δ_H 3.82 / δ_C 159.9 supported the methoxyl group at the C-3'.

Compound 2 was obtained as yellow crystals. The MS spectrum gave a molecular ion peak at m/z 327.0870 (M-1)^-, in LC-MS-IT-TOF apparatus. The ¹H NMR spectrum indicated signals at δ_H 7.78 and 7.84 suggesting the presence of protons on a α,β-unsaturated ketone moiety.^13,16 The above data and UV absorption bands (λ_max) at 317 and 282 nm suggested a chalcone structure for compound 2.¹³ Signals for four coupled aromatic protons: a triplet at δ_H 7.33 (1H, J 7.8 Hz), a multiplet at δ_H 7.28-7.23 (m, 2H) and a double double doublet at δ_H 6.97 (1H, J 7.8, 2.0 and1.6 Hz) suggested a 3'-substituted B ring.¹⁵ Additionally, the ¹H NMR spectrum showed tree singlets at δ_H, 2.01 (3H),4.01 (3H) and 10.17 (1H), consistent with the presence of methyl, methoxyl and aldehyde groups, respectively, besides two hydroxyl group singlets at δ_H 12.83 and14.21. The two low-field hydroxyl protons evidenced a formyl group at C-3'. This conclusion is achieved since the down-field shift of OH-4' (δ_H 12.83) can be explained by hydrogen-bonding with the oxygen atom of the formyl substituent at a neighboring carbon atom and at the same time that the down-field shift of OH-6' (δ_H 14.21) is caused by chelation between the 6'-hydroxyl proton and the carbonyl oxygen of the α,β-unsaturated carbonyl group function.¹³ This intramolecular hydrogen bonding corroborated the assignments of the signals at δ 7.78 (d, 1H) and δ 7.84 (d, 1H) to α and β positions, respectively.¹⁶ Spectral analysis of 2a showed H_α and H_β signals with a marked difference in chemical shifts [δ 7.29 (d, 1H, J 16.0 Hz, H-7) and δ 6.98 (d, 1H, J 16.0 Hz, H-8)] related with the absence of the intramolecular hydrogen bonding with the C=O of the α,β-unsaturated carbonyl group. The coupling constant (16.0 Hz) observed in 2a indicated the E-isomer for the double bond.¹⁶ The placement of the other groups in the A-ring was made on the basis of the HMBC correlations (Figure 3): δ_H 14.21 (s, 1H, OH-6') / δ_C 169.8 and 109.0; δ_H 12.83 (s, 1H, OH-4') / δ_C 166.6, 109.2 and 109.5; δ_H 2.01 (s, 3H) / δ_C 109.2; δ_H 10.17 (s, 1H) / δ_C 166.6 and δ_C 109.5; δ_H 4.01 / δ_C 168.5 that evidenced the hydroxyl, formyl, methyl and methoxyl groups at C-6'-4', C-3', C-5' and C-2', respectively.

The hydroxyl group in the C-3 was confirmed for the methylated derivative of 2 (Figure 1) which showed correlation between δ_H 3.80 / δ_C 55.3 in the HMQC spectrum and δ_H 3.80 / δ_C 159.9 (C-3) in the HMBC spectrum. Other correlations observed in the HMQC spectrum of 2a were δ_H 3.86 / δ_C 62.7 and δ_H 3.75 / δ_C 61.9. In the HMBC spectrum, the signals at δ_H 3.86 and 3.75 showed correlations with δ_C 163.0 and 161.8, respectively, providing support for the presence of the methoxyl groups at the C-6' and C-4' position.

Formyl flavonoids have been reported from a few species in the plant kingdom. Early reports included a description of 2',4-dihydroxy-4'-methoxy-5'formylchalcone from Psoralea corylifolia (Fabaceae).¹⁷ Its isomeric compound, with methoxy group at the 2'-position, has been reported from the same species.¹⁸ 2',4',6'-trihydroxy-3'-formylchalcone has been reported from Psidium acutangulum (Myrtaceae)¹⁹ and its retrochalcone derivative was obtained from Anredera scandens (Basellaceae).²⁰ 3'-formyl-4',6'-dihydroxy2'-methoxy-5'-methylchalcone and (2S)-8-formyl-5-hydroxy-7-methoxy-6-methylflavanone were isolated from Cleistocalyx operculatus (Myrtaceae).¹³

The compound 3 was obtained as yellow powder. The ¹H spectrum showed two doublets in δ_H 6.37 (J 1.4 Hz, 1H) and δ_H 6.34 (J 1.4 Hz, 1H) typical of meta aromatics hydrogens in addition to two singlets in δ_H 5.91 (2H) and 3.86 (3H) typical of methylenedioxy and methoxyl groups, respectively. These data suggested the presence of tetrasubstituted aromatic ring. The signals in δ_H 4.11(q, 2H, J 7.0 Hz) and 1.23 (t, 3H, J 7.0 Hz) supported the presence of ethyl group attached to heteroatom and also two signals at δ_H 2.84 (t, 2H, J 7.5 Hz) and 2.55 (t, 2H, J 7.5 Hz) typical of methylene hydrogens.

The ¹³C NMR spectrum of 3 showed 13 signals. The signals at δ_H 2.84, 2.55 and 4.11 are, in the HMBC spectrum, correlated with a signal characteristic of carbonyl ester at δ_C 172.8, confirming the assignment of the chemical shifts for H-7, H-8 and H-10, respectively, and δ_C 172.8 for C-9 of a dihydrocinnamoyl group. The HMBC spectrum also showed correlations between δ_H 3.86 / δ_C 148.8, δ_H 5.91 / δ_C 143.5 and 133.5, confirming the methoxyl and methylenedioxy groups at C-5 and C-3-4, respectively.

Experimental

General procedures

Melting points (mp) were determined on a MQAPF-302 melting point digital apparatus. UV spectra were recorded on a Vankel-50 UV-Vis spectrophotometer. IR spectra were obtained on an FT-IR-1750 spectrophotometer, Perkin-Elmer apparatus. The spectra mass were obtained on a SHIMADZU LCMS-IT-TOF (225-07100-34) equipped with a Z-spray ESI (electrospray) source and operated in negative mode.

¹H and ¹³C NMR (1D and 2D) spectra were recorded on a Varian Mercury 200 spectrometer in CDCl₃ and (CD₃)₂CO, with TMS as internal standard. Sephadex LH-20 and silica gel 60 (PF₂₅₄ art. 7749 and art. 7731) were purchased from Merck.

The methylated flavonoids were obtained by treatment of the sample, dissolved in dry propanone, with 1.1 equiv. of dimethyl sulphate (Me₂SO₄) and 1.1 equiv. of potassium carbonate (K₂CO₃) to each free hydroxyl. The reactions were carried at room temperature during 12 h. After removal of the solvent in vacuum, the residue was suspended in H₂O (50 mL), treated with 5 mL of ammonia and extracted with CHCl₃ (3 × 15 mL). The CHCl₃ solution was dried with Na₂SO₄, filtrated and concentrated to dryness.¹³

Plant material

Branches of Piper montealegreanum Yuncker was collected in Belém (Pará State, Brazil), in December 2002 and identified in the Botanical Garden, Rio de Janeiro (Rio de Janeiro State, Brazil). A voucher specimen (MSP-010) was deposited at Emilio Goeldi Museum, Belém.

Extraction and isolation

The powered material of P. montealegreanum (1.3 kg) was exhaustively extracted with EtOH (4 × 2.0 L), the solvent removed under reducted pressure furnished a green residue (115.0 g). The crude extract amount of 13.5 g was chromatographed over Sephadex gel LH-20 and eluted with methanol (column 1) yielding 43 fractions. Fraction 17 was further fractionated over Sephadex gel LH-20 column providing 5 fractions. Fraction 3 after submitted to recristalization with a chloroform and methanol mixture yielding (S)-8-formyl-3',5-dihydroxy-7-methoxy-6-methyl-flavanone (1) (30 mg).

Fraction 18 (column 1) was chromatographed over Sephadex LH-20 and yielded five fractions. Fraction 4 gave 3'-formyl-3,4',6'-trihydroxy-2'-methoxy5'-methylchalcone (2) (40 mg), after submitted to recristalization with a chloroform and methanol mixture.

Fractions 8-11 (column 1) were also fractionated over Sephadex gel yielding 29 fractions. Fraction 8-14, after recrystallization with chloroform, gave ethyl 3,4-methylenedioxy-5-methoxy-7,8-dihydrocinnamate(3) (12 mg).

Compound 1. Orange yellow crystals (CDCl₃:MeOH); mp 162 ºC; [α]_D²⁵ -20 (MeOH, 0.025); UV λ_max/nm (MeOH): 266; UV λ_max/nm (AlCl₃): 286, 323 (sh); IR ν_max/cm^-1 (KBr): 3425, 1688, 1620, 1570, 1464; MS [M-1]^- 327.0887; ¹H and ¹³C NMR spectral data, see Table 1.

Compound 2. Yellow crystals (CHCl₃:MeOH); mp 164 ºC; UV λ_max/nm (MeOH): 317, 282; UV λ_max/nm (AlCl₃): 349, 306; IR (KBr) ν_max/cm^-1: 3445, 1616, 1581, 1422; MS [M-1]^- 327.0870; ¹H and ¹³C NMR spectral data, see Table 1.

Compound 1a and 2a.¹H and ¹³C NMR spectral data, see Table 2.

Compound 3. Amorphous green powder; IR (KBr) ν_max/cm^-1: 2924, 2853, 1734; MS [M⁺] 252; ¹H and ¹³C NMR spectral data, see Table 3.

Acknowledgements

The authors thank CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico) for financial support, Laboratório de Espectrometria de Massas do Nordeste (LEMANOR) for MS spectra and Maria das Graças Bichara Zoghbi for plant material.

Supplementary Information

Supplementary information (¹H and ¹³C NMR data for compounds 1, 2, 3, 1a and 2a, Figures S1-S15) is available free of charge at http://jbcs.org.br as a PDF file.

 

Submitted: December 8, 2009

Published online: April 19, 2011

Supplementary Information

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Publication Dates

• Publication in this collection
  04 Aug 2011
• Date of issue
  Aug 2011

History

• Accepted
  19 Apr 2011
• Received
  08 Dec 2009