STRUCTURAL MODIFICATION OF LIGNAN COMPOUNDS ISOLATED FROM NECTANDRA SPECIES (LAURACEAE)

 

VÍCTOR MACÍAS-VILLAMIZAR ^1,1*,§, LUÍS CUCA-SUÁREZ ¹

¹ Departamento de Química, Laboratorio de Productos Naturales, Universidad Nacional de Colombia. Av. Carrera 30 # 45-03. Cód. postal 111321; Edificio 476-Oficina 11; Bogotá D.C., Colombia.
^§ Profesor Universidad del Magdalena, Colombia.
* e-mail: vemaciasv@unal.edu.co

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1-INTRODUCTION

In the genus Nectandra, the presence of certain types of secondary metabolites has been determined, including sesquiterpenes, phytosterols, polyalcohols, arylpropionic acid derivatives, flavonols, arylpropanoids, furofuran lignans, dihydrobenzofuran neolignans [1], and certain norlignans [2], alkaloids [3], tannins [4], diterpenes [5], and components of essential oils [6]. However, the chemotaxonomic characteristics are determined by the presence of lignan-type compounds [7]. The ultimate goal of structural modification of natural products is to obtain new drugs [8]. In that sense, there is a growing interest in lignans and their synthetic derivatives due to applications in cancer chemotherapy and various other pharmacological effects [9]. This work corresponds to the first report of this type of structural modification of lignan compounds (7,7'-epoxylignans and diaryldimethylbutane lignans) isolated from Nectandra species. Therefore, this work can be used as a starting point for structure-activity relationship studies.

EXPERIMENTAL

Materials and reagents

Benzyl bromide (Merck), acetone and toluene were freshly distilled before use. 2,3-Dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) (Fluka purum ,97%), 1,4-dioxane (Aldrich, anhydrous 99.8%), pyridine (ACS reagent, ≥99.0%, Sigma-Aldrich) and acetic anhydride (ACS reagent, ≥98.0%, Sigma-Aldrich) were used. Purification of the products was carried out on a short silica gel column (100-200 mesh, Merck) using increasing percentage of ethyl acetate in hexane as elutant. NMR spectra: were recorded on a Bruker Avance 400 spectrometer (¹H 400 MHz, ¹³C 100 MHz) using TMS as internal standard, in deuterated chloroform (CDCl₃) as solvent. The product ethers were characterized by comparing spectral data of known compounds described in the literature and analysis of the spectral data.

 

Figure 1. ¹H NMR (400 MHz, CDCl₃) Spectrum of Compound 1
(mixture of veraguensin/galgravin)

 

Veraguensin: ¹H NMR (400 MHz, CDCl₃), δ: 0.67 (3H, d, J = 7.0, H-9'), 1.07 (3H, d, J = 6.6, H-9), 1.79 (1H, m, H-8), 2.25 (1H, m, H-8'), 3.86 (s, OCH₃), 3.88 (s, OCH₃), 3.89 (s, OCH₃), 3.91 (s, OCH₃), 4.42 (1H, d, J = 9.3, H-7), 5.14 (1H, d, J = 8.6, H-7'), 6.86-7.08 (6H, m, H-2/5/6, H-27576'). [v=veraguensin].

Galgravin: ¹H NMR (400 MHz, CDCl₃), δ: 1.05 (6H, d, J = 6.7, H-9/9'), 2.34 (2H, m, H-8/8'), 3.87 (s, OCH₃), 3.88 (s, OCH₃), 4.52 (2H, d, J = 6.4, H-7/7'), 6.85-6.99 (6H, m, H-2/5/6, H-27576'). [g=galgravin].

 

Figure 2. ¹³C NMR (100 MHz, CDCl₃) Spectrum of Compound 1
(mixture of veraguensin/galgravin)

 

Veraguensin: ¹³C NMR (100 MHz, CDCl₃) δ: 149.1 (C), 148.7 (C), 148.7 (C), 148.2 (C), 133.9 (C), 133.6 (C), 119.3 (CH), 118.8 (CH), 111.1 (CH), 110.8 (CH), 110.5 (CH), 110.1 (CH), 87.4 (CH), 83.1 (CH), 56.1 (CH₃), 56.0 (CH₃) (x2), 55.9 (CH₃), 48.0 (CH), 46.1 (CH), 15.2 (CH₃), 15.1 (CH₃).

Galgravin: ¹³C NMR (100 MHz) δ: 12.9 (CH₃), 44.3 (CHCH₃), 55.8 (OCH₃), 55.9 (OCH₃), 87.2 (OCH(Ar)), 109.7 (Ar-C2), 110.9 (Ar-C5), 118.5 (Ar-C6), 134.8 (Ar-C1), 148.4 (Ar-C4), 148.9 (Ar-C3).

Extraction of Secondary Metabolites

Secondary metabolites were previously isolated from species of Nectandra sp. in the Natural Products Laboratory of the National University of Colombia and correspond to 7,7'-epoxilignan and diaryldimethylbutane lignans.

General Procedure

Aromatization of veraguensina and galgravin (1).

A solution of veraguensin and galgravin (90/10) 1, (5,0 mmol) and DDQ (15,0 mmol) in toluene (20 mL) (the reaction mixture immediately turned deep green) was refluxed (100°C) for 6 h. The mixture was cooled, the precipitate collected, the solvent evaporated under reduced pressure, and the resulting residue purified by flash chromatography on silica gel (n-hexane/AcOEt= 7/3) to give 2,5-bis(3,4-dimethoxyphenyl)-3,4-dimethylfuran (45%) (TL-1) [10].

¹H NMR (400MHz, CDCl₃) spectral data of Veraguensin: δ: 0.67 (3H, d, J = 7.0, H-9'), 1.07 (3H, d, J = 6.6, H-9), 1.79 (1H, m, H-8), 2.25 (1H, m, H-8'), 3.86 (s, OCH₃), 3.88 (s, OCH₃), 3.89 (s, OCH₃), 3.91 (s, OCH₃), 4.42 (1H, d, J = 9.3, H-7), 5.14 (1H, d, J = 8.6, H-7'), 6.86-7.08 (6H, m, H-2/5/6, H-2'/5'/6').

¹H NMR (400MHz, CDCl₃) spectral data of Galgravin: δ: 1.05 (6H, d, J = 6.7, H-9/9'), 2.34 (2H, m, H-8/8'), 3.87 (s, OCH₃), 3.88 (s, OCH₃), 4.52 (2H, d, J = 6.4, H-7/7'), 6.85-6.99 (6H, m, H-2/5/6, H-2'/5'/6').

¹H NMR (400MHz, CDCl₃) spectral data of TL-1: δ 7.21 (4H, dd, J = 6.1, 1.9, H-2/2' and H-6/6'), 6.94 (2H, d, J = 8.9, H-5/5'), 3.95 (3H, s, OCH₃-3/3'), 3.92 (3H, s, OCH₃-4/4'), 2.22 (s, 6H, H-9/9') (see supporting information,, Figure 3).

 

Figure 3. ¹H NMR (400 MHz, CDCl₃) Spectrum of Compound 1 modified (TL-1)
(mixture of veraguensin/galgravin modified)

 

¹H NMR (400 MHz, CDCl₃) δ 7.21 (4H, dd, J = 6.1, 1.9, H-2/2' and H-6/6'), 6.94 (2H, d, J = 8.9, H-5/5'), 3.95 (6H, s, OCH₃-3/3'), 3.92 (6H, s, OCH₃-4/4'), 2.22 (6H, s, H-9/9').

 

Figure 4. ¹³C NMR (400 MHz, CDCl₃) Spectrum of Compound 1 modified (TL-1)
(mixture of veraguensin/galgravin modified)

 

¹³C NMR (100 MHz, CDCl₃) δ 149.2 (C) (x2), 148.3 (C) (x2), 147.1 (C) (x2), 125.3 (C) (x2), 118.6 (CH) (x2), 117.9 (C) (x2), 111.5 (CH) (x2), 109.5 (CH) (x2), 56.2 (CH₃), 56.1 (CH₃), 10.0 (CH₃) (x2).

 

Figure 5. COSY Spectrum of Compound 1 modified (TL-1)
(mixture of veraguensin/galgravin modified)

 

Figure 6. Expansion of the COSY Spectrum of Compound 1 modified (TL-1)
(From 4.80 to 3.40 _f1 and From 7.45 to 6.65 _f2)
(mixture of veraguensin/ galgravin modified)

 

Figure 7. Expansion of the COSY Spectrum of Compound 1 modified (TL-1)
(From 7.60 to 6.70 _f1 and From 7.36 to 6.88 _f2)
(mixture of veraguensin/ galgravin modified)

 

Figure 8. HMQC Spectrum of Compound 1 modified (TL-1)
(mixture of veraguensin/galgravin modified)

 

Figure 9. Expansion of the HMQC Spectrum of Compound 1 modified (TL-1)
(From 124.0 to 102.0 _f1 and From 7.34 to 6.90 _f2)
(mixture of veraguensin/galgravin modified)

 

Figure 10. Expansion of the HMQC Spectrum of Compound 1 modified (TL-1)
(From 70.0 to 0.0 _f1 and From 4.20 to 1.80 _f2)
(mixture of veraguensin/ galgravin modified)

 

Figure 11. DEPT-135° Spectrum of Compound 1 modified (TL-1)
(mixture of veraguensin/galgravin modified)

 

Figure 12. ¹H NMR (400 MHz, CDCl₃) Spectrum of Compound 2
(schi-neolignin B)

 

¹H NMR (400 MHz, CDCl₃) δ 6.76 (2H, d, J = 8.1, ArH), 6.65 (1H, d, J = 1.9, ArH), 6.63 (1H, dd, J = 8.1, 1.8, ArH), 6.58 (1H, d, J = 1.8, ArH), 2.56 (2H, dd, J = 13.5, 6.8, H-7/7'), 2.40 (2H, dd, J = 13.7, 7.8, 7/7'), 1.76 (2H, dd, J = 12.9, 6.5, H-8/8'), 0.83 (6H, d, J = 6.6, H-9/9').

Acetylation of schineolignin B (2).

Schineolignin B 2 (2,1 mmol), in a mixture of acetic anhydride and pyridine (5 mL/ 5 mL) was placed in a 50 mL pear-shaped flask. The mixture was stirred at 100 C for 15 h. Removal of the solvent under reduced pressure afforded a crude mixture, which was extracted with HCl solution followed by extracted with NaHCO₃ solution to give the products, which was purified by column chromatography on silica gel (n-hexane/AcOEt= 8/2) Sephadex LH-20 in open column chromatography to give 5-(4-(3,4-dimethoxyphenyl)-2,3-dimethylbutyl)-2,3-dimethoxyphenyl acetate, 85% (TL-2) [11].

¹H NMR (400 MHz, CDCl₃) spectral data of Schineolignin B: ¹H NMR (400 MHz, CDCl₃) δ: 6.76 (1H, d, J = 8.1, Ar-H), 6.65 (1H, d, J = 1.9, Ar-H), 6.63 (1H, dd, J = 8.1, 1.8, Ar-H), 6.58 (1H, d, J = 1.8, Ar-H), 2.56 (2H, dd, J = 13.5, 6.8, H-7/7'), 2.40 (2H, dd, J = 13.7, 7.8, H-7/7'), 1.76 (2H, dd, J = 12.9, 6.5, H-8/8'), 0.83 (6H, d, J = 6.6, H-9/9').

¹H NMR (400MHz, CDCl₃) spectral data of TL-2: δ: 6.76- 6.57 (5H, m, Ar-H), 3.86-3.81 (12H, s, 3 x OCH₃), 2.56 (2H, dd, J = 13.5, 6.7, H-7/7'), 2.40 (2H, dd, J = 13.5, 7.8, H-7/7'), 2.30 (3H, s, CH₃-CO₂-Ar), 1.79-1.73 (m, 2H, H-8/8'), 0.83 (d, J = 6.6, 6H, H-9/9') (see supporting information, Figure 13).

 

Figure 13. ¹H NMR (400 MHz, CDCl₃) Spectrum of Compound 2 modified (TL-2)
(schineolignin B modified)

 

Benzylation of meso-dihydroguaiaretic acid and threo-dihydroguaiaretic acid (3).

A mixture of 3(0.567 mmol) and sodium carbonate (11.4 mmol) in dry acetone (36 ml) was heated to reflux for 1 h under nitrogen. Then, benzyl bromide (0.63 ml, 5.67 mmol) was added and the mixture was heated under reflux for an additional 3 h. After cooling to room temperature, the reaction mixture was filtered. The filtrate was concentrated and distilled under reduced pressure in a rotary evaporator to remove the excess unreacted benzyl bromide. The residue was chromatographed on silica gel (hexane/AcOEt= 8/2) and Sephadex LH-20 in open column chromatography to give 1-(benzyloxy)-4-(4-(3,4-dimethoxyphenyl)-2,3-dimethylbutyl)-2-methoxybenzene, 80% (TL-3) [12, 13].

¹H NMR (400 MHz, CDCl₃) spectral data of meso-dihydroguaiaretic acid and threo-dihydroguaiaretic acid: δ: 6.82 (d, J = 8.0, 2H), 6.78 (dd, J = 8.2, 2.3, 2H), 6.67 (dd, J = 8.1, 1.9, 1H), 6.63 (d, J = 1.8, 1H), 6.60 (dd, J = 8.0, 1.8, 2H), 6.59 (d, J = 1.9, 1H), 6.54 (d, J = 1.8, 2H), 2.75 (dd, J = 13.5, 5.0, 2H), 2.54 (dd, J = 13.5, 7.1, 2H), 2.40 (dd, J = 13.6, 7.6, 2H), 2.30 (dd, J = 13.5, 9.2, 1H), 1.76 (dd, J = 13.3, 6.7, 2H), 1.75 (dd, J = 13.0, 6.6, 2H), 0.85 (dd, J = 6.6, 2.5, 6H).

¹H NMR (400MHz, CDCl₃) spectral data of TL-3: δ: 7.46 (2H, d, J = 7.3, H-2"/2"' and 6"/6"'), 7.38 (2H, t, J = 7.4, H-3"/3"' and H-5"/5"'), 7.32 (1H, t, J = 7.2, H-4''/4'''), 6.81 (2H, d, J = 9.3, Ar-H), 6.79 (2H, d, J = 8.3, Ar-H), 6.69 (1H, d, J = 1.6, Ar-H), 6.64 (1H, dd, J = 9.4, 1.5, Ar-H), 6.57 (1H, dd, J = 8.1, 1.6, Ar-H), 5.14 (4H, s, H-7"/7"'), 2.75 (2H, dd, J = 13.4, 4.9, H- 7/7'), 2.57 (2H, dd, J = 13.6, 6.7, H-7/7'), 2.40 (2H, dd, J = 13.6, 7.8, H-7/7'), 2.30 (2H, dd, J = 13.4, 9.3, H-7/7'), 0.86 (3H, d, J = 7.2, H-9/9'), 0.84 (3H, d, J = 6.8, H-9/9') (see supporting information, Figure 15).

 

Figure 14. ¹H NMR (400 MHz, CDCl₃) Spectrum of Compound 3
(mixture of meso-dihydroguaiaretic acid and threo-dihydroguaiaretic acid)

 

Figure 15. ¹H NMR (400 MHz, CDCl₃) Spectrum of Compound 3 modified (TL-3)
(mixture of meso-dihydroguaiaretic acid and threo-dihydroguaiaretic acid modified)

 

¹H NMR (400 MHz, CDCl₃) δ 6.76- 6.57 (5H, m, ArH), 3.86-3.81 (12H, s, 3 x OCH₃), 2.56 (2H, dd, J = 13.5, 6.7, H-7/7'), 2.40 (2H, dd, J = 13.5, 7.8, H-7/7'), 2.30 (3H, s, CH₃-CO), 1.79 - 1.73 (2H, m, H-8/8'), 0.83 (6H, d, J = 6.6, H-9/9').

¹H NMR (400 MHz, CDCl₃) δ 6.82 (2H, d, J = 8.0), 6.78 (2H, dd, J = 8.2, 2.3), 6.67 (1H, dd, J = 8.1, 1.9), 6.63 (1H, d, J = 1.8), 6.60 (2H, dd, J = 8.0, 1.8), 6.59 (1H, d, J = 1.9), 6.54 (2H, d, J = 1.8), 5.44 (1H, OH), 2.75 (2H, dd, J = 13.5, 5.0), 2.54 (2H, dd, J = 13.5, 7.1), 2.40 (2H, dd, J = 13.6, 7.6), 2.30 (2H, dd, J = 13.5, 9.2), 1.76 (2H, dd, J = 13.3, 6.7), 1.75 (2H, dd, J = 13.0, 6.6), 0.85 (6H, dd, J = 6.6, 2.5).

¹H NMR (400 MHz, CDCl₃) ¹H NMR (400 MHz, CDCl₃) δ 7.46 (2H, d, J = 7.3, H-2"/2"' and 6"/6"'), 7.38 (2H, t, J = 7.4, H-3"/3"' and H-5"/5"'), 7.32 (1H, t, J = 7.2, H-4"/4"'), 6.81 (2H, d, J = 9.3, Ar-H), 6.79 (2H, d, J = 8.3, Ar-H), 6.69 (1H, d, J = 1.6, Ar-H), 6.64 (1H, dd, J = 9.4, 1.5, Ar-H), 6.57 (1H, dd, J = 8.1, 1.6, Ar-H), 5.14 (4H, s, H-7"/7"'), 2.75 (2H, dd, J = 13.4, 4.9, H-7/7'), 2.57 (2H, dd, J = 13.6, 6.7, H-7/7'), 2.40 (2H, dd, J = 13.6, 7.8, H-7/7'), 2.30 (2H, dd, J = 13.4, 9.3, H-7/7'), 0.86 (3H, d, J = 7.2, H-9/9'), 0.84 (3H, d, J = 6.8, H-9/9').

RESULT S AND DISCUSSION

Three structural transformation process are presented in this article; and corresponds to the first report of this type of structural modification of lignans isolated from Nectandra species. A direct method was developed for the conversion of compound 1 to furan-type lignan. Additionally, the structural transformation of compounds 2 [14] (benzylation); and compound 3 [15] (acetylation). The spectroscopic data comparison between the initial and the transformed compound showed formation of derivatives compounds (see supporting information for details). Interestingly, few reports describe dehydrogenation, benzylation or acetylation of natural products isolates; to our knowledge the direct structural transformation of lignan compounds isolated from Nectandra species has yet to be documented.

Comparison of spectroscopic data between the starting material (veraguensin and galgravin) and the product (TL- 1) show the absence some characteristics signals [such as: 4.42 (1H, d, J = 9.3, H-7), 5.14 (1H, d, J = 8.6, H-7'), and 4.52 (2H, d, J = 6.4, H-7/7')], allow suggest the formation of TL-1.

The compound TL-2 has a signal 2.30 (3H, s, CH₃-CO), among others; which it is characteristic of the formation of the product.

The compound TL-3 has a signal 5.14 (4H, s, H-7''/7'''), among others; which it is characteristic of the formation of the product. Additionally, the compound formed is absent the signal generated by the hydroxyl group [5.44 (1H, s, OH)].

 

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