Other chemical constituents isolated from Solanum crinitum Lam. (Solanaceae)

Abstracts

The phytochemical investigation of Solanum crinitum Lam led to the isolation from the fruit trichomes of four flavonoids, tiliroside (1), astragalin (2), kaempferol (3), biochanin A-7-O-β-D-apiofuranosyl-(1→5)-β-D-apiofuranosyl-(1→6)-β-D-glucopyranoside (7), along with 4-hydroxybenzoic acid (12), and four cinnamic acid derivatives, cis- and trans- coumaric acids (10 and 11) and cis- and trans- ethyl coumarate (8 and 9). Three tri-glycosyl-steroidal alkaloids, solamargine (13), 20-epi-solamargine (14) and solasonine (16) were isolated from the methanolic extract of the green fruits. The derivatives 3,5,7,4'-tretra-O-methyl-kaempferol (4), 3,7,4'-tri-O-methyl-kaempferol (5), 3,7,4'-tri-O-methyl-5-O-acetyl-kaempferol (6), the peracetyl-epi-solamargine (15) and peracetyl-solasonine (17) were prepared. The structures were established through the analysis of their spectral data. The complete ¹H and 13C NMR data assignments of the new peracetyl derivatives of the alkaloids were made.

Solanum crinitum; Solanaceae; steroidal glycoalkaloids; flavonoids; cinnamic acids

ARTICLE

Other chemical constituents isolated from Solanum crinitum Lam. (Solanaceae)

Marli T. F. Cornelius^I; Mário G. de Carvalho^I,*; Tania M. S. da Silva^I; Cassia C. F. Alves^I; Ana P. N. Siston^I; Kelly Z. Alves^I; Carlos M. R. Sant'Anna^I; Mario B. Neto^II; Marcos N. Eberlin^II; Raimundo Braz-Filho^III

^IDepartamento de Química, ICE, Universidade Federal Rural do Janeiro, BR 465 km 07, 23890-000 Seropédica-RJ, Brazil

^IIInstituto de Química, Universidade Estadual de Campinas, 13084-862 Campinas-SP, Brazil

^IIISetor de Química de Produtos Naturais, LCQUI, CCT, Universidade Estadual do Norte Fluminense Darcy Ribeiro, 28013-602 Campos dos Goytacazes-RJ, Brazil

ABSTRACT

The phytochemical investigation of Solanum crinitum Lam led to the isolation from the fruit trichomes of four flavonoids, tiliroside (1), astragalin (2), kaempferol (3), biochanin A-7-O-β-D-apiofuranosyl-(1→5)-β-D-apiofuranosyl-(1→6)-β-D-glucopyranoside (7), along with 4-hydroxybenzoic acid (12), and four cinnamic acid derivatives, cis- and trans- coumaric acids (10 and 11) and cis- and trans- ethyl coumarate (8 and 9). Three tri-glycosyl-steroidal alkaloids, solamargine (13), 20-epi-solamargine (14) and solasonine (16) were isolated from the methanolic extract of the green fruits. The derivatives 3,5,7,4'-tretra-O-methyl-kaempferol (4), 3,7,4'-tri-O-methyl-kaempferol (5), 3,7,4'-tri-O-methyl-5-O-acetyl-kaempferol (6), the peracetyl-epi-solamargine (15) and peracetyl-solasonine (17) were prepared. The structures were established through the analysis of their spectral data. The complete ¹H and ¹³C NMR data assignments of the new peracetyl derivatives of the alkaloids were made.

Keywords:Solanum crinitum, Solanaceae, steroidal glycoalkaloids, flavonoids, cinnamic acids.

RESUMO

O estudo fitoquímico de Solanum crinitum Lam forneceu quatro flavonóides: tilirosídeo (1), astragalina (2), kaempferol (3) e biochanina A-7-O-β-D-apiofuranosil-(1→5)-β-D-apiofuranosil-(1→6)-β-D-glucopiranosideo (7), ácido 4-hidroxibenzoico (12), e quatro derivados do ácido cinâmico: cis- e trans- cumárico (10 e 11), cis- e trans- cumarato de etila (8 e 9), isolados de tricomas do fruto. Do extrato metanólico de frutos verdes foram isolados três alcalóides esteroidais glicosilados: solamargina (13), 20-epi-solamargina (14) e solasonina (16). Os derivados 3,5,7,4'-tetra-O-metil-kaempferol (4), 3,7,4'-tri-O-metil-kaempferol (5), 3,7,4'-tri-O-metil-5-O-acetil-kaempferol (6), peracetil-epi-solamargina (15) e peracetil-solasonina (17) foram sintetizados e estão sendo registrados pela primeira vez na literatura. As estruturas foram definidas através de análise de dados espectrométricos.

Introduction

Solanum (L) is the most representative genus of Solanaceae, containing about 1,500 species, and 5,000 epithets. It is widespread in tropical and subtropical regions of all the world, but its highest diversity occurs in South America.¹Solanum species are a rich source of steroidal alkaloids, flavonoids and their glycosides which are known to possess a variety of biological activities. The glycoalkaloids are natural toxins with ecological and human health importance, such as the allelopathic effect against herbivores, against pathogenic microorganism^2,3 and molluscicidal activity,⁴ and are also of interest as starting material for anabolic, anti-fertility, anti-inflammatory, anti-allergic drugs.⁵ As the alkaloids, the flavonoids are a frequent group of compounds in Solanum species, and can be used as systematic markers for the family taxons.^6-8

Solanum crinitum Lam (Solanaceae), popularly known as "jurubeba" and "fruto-de-lobo", is a shrub or small tree in South America, including southern Brazil and Colombia.^9,10 In the course of our pharmacological and phytochemical investigations of Solanum species,^3,4,8,11-13 we have reported the cytotoxic and antitumoral activities of flavonoids isolated from trichomes of young branches and also of a rich glycoalkaloids total fraction from the green fruits of Solanum crinitum Lam¹⁴ along with its allelopathic activity.³

This work describes the isolation and structural characterization of the flavonoids tiliroside (1), astragalin (2), kaempferol (3) and biochanin A-7-O-β-D-apiofuranosyl-(1→5)-β-D-apiofuranosyl-(1→6)-β-D-glucopyranoside (7), cis- (10) and trans- (11) cumaric acids and ethyl cis- (8) and trans- (9) cumarate and 4-hydroxybenzoic acid (12), which were isolated from the trichomes, and the glycoalkaloids solamargine (13), 20-epi-solamargine (14) and solasonine (16) isolated from the green fruits. The derivatives 3,5,7,4'-tetra-O-methyl-kaempferol (4), 3,7,4'-tri-O-methyl-kaempferol (5), 3,7,4'-tri-O-methyl-5-O-acetyl-kaempferol (6), peracetyl-epi-solamargine (15) and peracetyl-solasonine (17) were prepared and used to confirm the structures of the natural compounds. The structures were established through analysis of their spectral data, mainly NMR (1D and 2D) and mass spectra. NMR data for the new acetyl derivatives of the alkaloids are described herein for the first time.

Results and Discussion

From the trichomes isolated from green fruits of Solanum crinitum Lam were isolated the flavonoids: tiliroside (1), astragalin (2), kaempferol (3), and biochanin A-7-O-β-D-apiofuranosyl-(1→5)-β-D-apiofuranosyl-(1→6)-β-D-glucopyranoside (7), cis- (10) and trans- (11) cumaric acids and ethyl cis- (8) and trans- (9) cumarate esters and 4-hydroxybenzoic acid (12); from green fruits methanolic extracts were isolated three tri-glycosyl-steroidal alkaloids, solamargine (13), 20-epi-solamargine (14) and solasonine (16). The derivatives 3,5,7,4'-tetramethylkaempferol (4), 3,7,4'-trimethylkaempferol (5), 3,7,4'-trimethyl-5-acetylkaempferol (6), the peracetyl-20-epi-solamargine (15) and peracetyl-solasonine (17) were prepared. The structures were established on the basis of IR, NMR and MS data analysis of the natural compounds and of the derivatives 4-6, 15 and 17.

Compounds 1-3 were identified by the analysis of ¹H and ¹³C NMR spectra, including HMQC and HMBC experiments, and comparison with literature data for tiliroside, kaempferol and astragalin.^15-18 The first report of 1 in Solanum species was made by Souza et al.¹⁴ Kaempferol and astragalin have been isolated from some Solanum species.⁸ The treatment of 3 with diazomethane yielded the methyl derivatives 4 and 5, described in the literature,¹⁹ and the derivative 6 was obtained by treating 5 with Ac₂O/Pyridine. The 2D NMR spectra, including NOESY experiments, of these derivatives were used to confirm the proposed structure of 3 and to carry out the complete assignment of the ¹H and ¹³C chemical shifts of 6 (see Experimental).

The ¹H and ¹³C NMR spectra of compound 7 revealed characteristic resonances of the isoflavonoid biochanin A besides additional signals for three sugars unities, one glucopyranoside and two apiofuranosides. Comparison of these data with those of glycosides isolated from Dalbergia nigra²⁰ and Andira anthelmia,²¹ besides the analysis of mass spectrum obtained by FAB-MS in positive mode {m/z 733.20820 ([M+Na]⁺, C₃₂H₃₈O₁₈+Na⁺, calc.: m/z 733.19558), m/z 601.12300 [M+Na-132]⁺, m/z 451.0600 (M+Na-282)⁺, m/z 449.27710 [M+Na-284]⁺, m/z 431.16210 [M+Na-302]⁺, m/z 317.1338 [M+Na-416]⁺, m/z 287.0865 [M+Na-446]⁺}, led us to identify the compound as biochanin A, 7-O-β-D-apiofuranosyl-(1→5)-β-D-apiofuranosyl-(1→6)-β-D-glucopyranoside (7). This is the first report of this apiofuranosyl derivative in Solanum species.

Compounds 8-12 were identified by the ¹H and ¹³C NMR spectral data analysis of the mixtures of 8+9, 10+11 and of 12 along with comparison with literature data.^21-25 The integration of the ¹H NMR signals allowed us to calculate the approximate relative percentage of both compounds: 36.16% of 8 and 63.84% of 9.

The fractions containing compounds 13, 14 and 16 showed a positive test for alkaloids. The detailed analysis of the ¹H and ¹³C NMR spectra of the isolated compounds allowed to identify characteristic signals corresponding to the same aglycone as that of the steroidal spirazolane-type alkaloid in the three glycoalkaloids: four quaternary carbon atoms, including one linked to oxygen and nitrogen atoms: (δ_C 98.7 to 98.5 and one sp² at d_c 141.2 to 140.4), nine methine groups (including two oxygenated at δ_C 78.5 to 78.0 and 81.1 to 78.6), ten methylene groups and four methyl groups = (C)₃(O-C-NH)(CH)₇ (HC-O)₂(CH₂)₁₀(CH₃)₄ = C₂₇H₄₂NO₃ = C₂₇H₄₂NO₂ considering the presence of an ether function (Table 1) having a trisaccharide moiety (three anomeric carbon atoms: δ_C 100.6 to 100.3) attached to the oxygen atom of carbon CH-3, with δ_C 78.5 to 78.0 (Table 1 and 2), which is significantly higher when compared with the ¹³C chemical shift of the methyl carbon CH-3 sustaining free hydroxyl group (about δ_C 71). The ¹H and ¹³C chemical shifts of the trisaccharide moieties of 13 and 14, had practically the same values (Table 2), indicating identical the partial structure O-[a-L-rhamnopyranosyl-(1→2)-O-[α-L-rhamnopyranosyl-(1→4)]-β-D-glucopyranoside = C₁₈H₃₂O₁₃ (three degrees of unsaturation, C₁₈H₃₈O₁₃ – C₁₈H₃₂O₁₃ = H₆). On the other hand, the ¹H and ¹³C signals observed in NMR spectra of 16 allowed to characterize the trisaccharide as O-[a-L-rhamnopyranosyl-(1→2)-O-[b-glucopyranosyl-(1→3)]-β-D-galactopyranoside = C₁₈H₃₂O₁₄ (three degrees of unsaturation, C₁₈H₃₈O₁₄ – C₁₈H₃₂O₁₄ = H₆). All these NMR spectral data allowed to deduce the same molecular formulas C₄₅H₇₃NO₁₅ to 13 and 14 and C₄₅H₇₃NO₁₆ to 16, all with ten degrees of unsaturation corresponding to one double bond and six rings). In fact, the values of pseudomolecular peaks in the positive HRMS spectra at m/z 868.5239 (M + H·⁺, 100%) of 13 (C₄₅H₇₄NO₁₅ = m/z 868.5058) and at m/z 868.5305 (M + H·⁺, 100%) of 14 (C₄₅H₇₄NO₁₅ = m/z 868.5058), besides additional peaks compatible with loss of sugar moieties, were used to confirm the molecular formula of these isomeric compounds (13 and 14). The hydrogen and carbon atoms signals observed in the ¹H and ¹³C NMR spectra of 13-17 (Tables 1 and ³) were also assigned with aid of the homonuclear 2D ¹H-¹H-COSY and heteronuclear 2D HMQC (¹H-¹³C-COSY-¹J_CH) and HMBC (¹H-¹³C-COSY-ⁿJ_CH, n = 2 and 3), allowing to identify the ¹H chemical shifts of the methyl group signals of the aglycone and of the trisaccharide moieties (^{Table 3} and 4): δ_H 0.83 (d, 3H-27), 0.88 (s, 3H-18), 1.07 (s, 3H-19) and 1.09 (d, 3H-21) of 13, 0.79 (d, 3H-27), 0.85 (s, 3H-18), 1.06 (s, 3H-19) and 1.28 (d, 3H-21) of 14, and 0.76 (d, 3H-27), 0.82 (s, 3H-18), 1.03 (s, 3H-19) and 1.17 (d, 3H-21) of 16; signals of the hydrogen H-6 linked to sp² carbons of double bounds at δ_H: 5.32 (13), 5.33 (14), 5.30 (16), besides the signals of H-16 located in the spyrazolane ring (d, J 6 Hz) at δ_H 4.42 (13); 4.82 (14) and 4.21 (16). The additional doublets (J 6 Hz) corresponding to methyl groups were revealed at d_3H 1.64 and d_3H 1.78 in the spectra of 13 and 14 suggested the presence of two rhamnose moieties in both compounds and only one rhamnose for 16 as indicated by signal at δ_H 1.78. The differences observed in the ¹H NMR spectra of 13 and 14, recorded in same apparatus (¹H: 500 MHz; ¹³C: 125 MHz) and solvent pyridine-d₅, involving mainly the chemical shifts of the hydrogen atoms H-16 (δ_H 4.42 and 4.82), H-17 (δ_H 1.78 and 2.02), H-20 (δ_H 1.98 and 2.14), 2H-26 (δ_H 2.78 and 3.04/2.89) and 3H-21 (δ_H 1.09 and 1.28), which revealed correlation in the HMQC with the ¹³C signals of the corresponding carbon atoms: CH-16 (δ_C 79.2 and 81.1), CH-17 (δ_C 63.9 and 63.0), CH-20 (δ_C 42.0 and 42.0), CH₂-26 (δ_C 48.2 and 47.1) and 3H-21 (δ_C 16.1 and 15.7), as summarized in Figure 1. Comparative analysis of these data was used to suggest the two stereoisomers H-20β (13) and H-20α (14), since the 22αN and 22βN possibilities were eliminated considering the absence of ¹³C signal corresponding to a methylene carbon CH₂-23 at about δ_C 27 which is consistent with the ¹³C NMR chemical shift of the 22βN stereoisomer (Figure 1). The β-position of the methyl Me-21 located at carbon atom 20 of 14 may be used to justify the ¹³C chemical shifts of the CH-16 (δ_C 81.1, absence of the γ-effect from Me-21) and CH₂-23 (δ_C 34.0, presence of the γ-effect from Me-21).^26-30 In order to compare the relative stabilities of the epimeric structures 13/14, a molecular modeling study was implemented using the Spartan 06 for Linux program (Wavefunction, Inc.), see Supplementary Information.³¹

The complete ¹H and ¹³C chemical shift assignments of the signals of CH₃, CH₂, CH and C observed in the ¹H (including ¹H-¹H-COSY) and ¹³C ({¹H} and DEPT) NMR spectra (including 2D experiments HMQC and HMBC) (Tables 1, 2, ³ and 4) and comparison with values described in the literature for solamargine (13)^26-29 and solasonine (16)^28,31 led to the proposition of the structures (25R)(20S)(16S)-3β-{O-α-L-rhamnopyranosyl-(1→2)-O-[a-L-rhamnopyranosyl-(1→4)]-β-D-glucopyranosyl}-22aN-spirosol-5-en (solamargine, 13), the new glycoalkaloid (25R)(20R)(16S)-3β-{O-α-L-rhamnopyranosyl-(1→2)-O-[α-L-rhamnopyranosyl-(1→4)]-β-D-glucopyranosyl}-22αN-spirosol-5-en (16-epi-solamargine, 14), and (25R)(20S)(16S)-3β-{O-α-L-rhamnopyranosyl-(1→2)-O-[β-glucopyranosyl-(1→3)]-β-D-galactopyranosyl} -22αN-spirosol-5-en (solasonine, 16) for the three steroidal glycoalkaloids isolated from this plant (Figure 1).

The natural alkaloids 14 and 16 were treated with Ac₂O/pyridine to yield the peracetyl derivatives 15 and 17. The ¹H and ¹³C NMR spectral data of these peracetyl derivatives (15 and 17), obtained through the analysis of extensive 1D and 2D NMR experiments (Tables 1-4), were also used to confirm the postulated structures to 14 and 16. The analysis of 1D and 2D NMR spectra was also used to make the complete hydrogen and carbon-13 chemical shift assignments for the alkaloid, the new 16-epi-solamargine (14) and for the two peracetyl derivatives 15 and 17.

Experimental

General procedure

Melting points have not been corrected. IR, NMR and mass spectra were recorded on the same equipments used in previous papers.^21,32 Column chromatography was carried out with silica gel (Vetec and Aldrich 0.05-0.20 mm) and Sephadex LH-20 (Sigma, USA); silica gel F254 G (Vetec) was used for preparative TLC; aluminum backed (Sorbent) silica gel plates W/UV254 were used for analytical TLC, with visualization under UV (254 and 366 nm), with AlCl₃-ETOH (1%), Liebermann-Burchard and/or Godin reagents, or exposure to iodine vapor.

Plant material

The green fruits of Solanum crinitum Lam. were collected in September, 2001, in the campus of Universidade Federal Rural do Rio de Janeiro (UFRRJ), Seropédica-RJ, Brazil. They were collected by M.Sc. José Milton Alves (Agronomy Institute, UFRRJ). The identification was made by Dr. Maria de Fátima Agra, Laboratório de Tecnologia Farmacêutica, Universidade Federal da Paraíba, João Pessoa-PB. A voucher specimen (No. JP-28000) was deposited at the Herbarium Prof. Lauro Pires Xavier, Universidade Federal da Paraíba, João Pessoa-PB, Brazil.

Extraction and isolation

The trichomes (9.7 g) were isolated by scraping the green fruits with a glass slide and were subsequently extracted with CHCl₃ and MeOH in an ultrasound bath to furnish the CHCl₃ extract (310.0 mg) and MeOH (3.0 g) residues. The CHCl₃ residue was chromatographed on a sephadex CC, using MeOH as eluent, and 13 fractions were collected and analyzed by TLC and ¹H NMR spectroscopy. Fractions 7-8 yielded astragaline (1, 10.0 mg) and fractions 9-10 yielded the tilirozide (2, 25.0 mg). The methanolic extract was chromatographed on a silica gel column (col A) and 20 fractions were collected and analyzed by TLC plate. Fractions 5-8 (2.0 g) were dissolved in methanol and addition of CH₂Cl₂ yielded a precipitate that was separated by filtration to afford a solid (1.6 g) that was identified as kaempferol (3, mp 282-284 ºC) and the mother liquor (AM-5-8). The reaction of kaempferol (65.0 mg) with diazomethane yielded 16.1 mg of 4 (mp 128-130 ºC) and 36.6 mg of 5 (mp 140-142 ºC) that were separated by preparative TLC. 20.3 mg of 5 were treated with Ac₂O/pyridine (1:1) to afford 6 (14.8 mg, mp 166-168 ºC). The mother liquor (AM 5-8, 360.0 mg) was submitted to silica gel CC and fractions 8-12 yielded a mixture of 8 and 9 (1.2 mg, gum). Fraction 10 (850 mg) from col A was chromatographed on a silica gel column and 130 fractions of 50 mL were collected. Fractions 32-40 yielded 35.5 mg of a material corresponding to the mixture of 10, 11 and 12. Fractions 94-114 (102.3 mg) were chromatographed on a silica gel column and fractions 3-5 yielded 7 (9.5 mg) after filtration on Sephadex LH20.

The green fruits (2.6 g) of Solanum crinitum Lam were powdered and extracted with ethanol + acetic acid (2%) and 900 mL of solution was obtained. 900 mL of acetic acid (10%) were added, and the solution was left to stand and chill overnight. The solution was filtered under vacuum using Hirsch funnel. NH₄OH (pH 9-10) was added to the filtrate which was allowed to stand overnight in the fridge affording a precipitate (96.4 g) corresponding to the glycoalkaloids fraction was obtained. 90 g of the glycoalkaloids was adsorbed on silica gel and applied on a silica gel column, eluted with hexane, dichloromethane, ethyl acetate, acetone and methanol. 14 fractions of 500 mL were collected. Fraction 8 (1.0 g), collected with ethyl acetate, was chromatographed in a silica gel CC and fractions analyzed by silica gel TLC plate. Fraction 84 (52.6 mg) from this column was filtered on a Sephadex LH20 column, eluted with methanol, and the alkaloid solamargine 13 (268-270 ºC) was obtained. Fraction 10 (8.67 g) was extracted with methanol and the alkaloids were detected with Dragendorff and Libermann Burchard. This fraction was chromatographed on a silica gel column using CH₂Cl₂/MeOH (3:1) as initial eluent. 79 fractions were collected and were analyzed by TLC plate and reunited in group. Fractions 24-29 (1.2 g) were filtered on a Sephadex LH-20 column and the epi-solamargine (14, 225.8 mg, 238-250 ºC) was obtained. Acetic anhydride:pyridine (1:1) treatment of 14 (44.5 mg) yielded 44.5 mg of the peracetyl derivative 15 (mp 128-130 ºC). Fractions 39-44 (1.3 g) were filtered on Sephadex LH20 to isolate solasonine (16, 93.5 mg, mp 244-246 ºC). Treatment of 16 (79.1 mg) with acetic anhydride:pyridine (1:1) gave the peracetyl-solasonine (17, 27.6 mg, mp 148-150 ºC).

3,7,4'-trimethoxy-5-acethoxyflavone (6)

Yellow powder, mp 166-168 ºC; IR (KBr) ν_max/cm^-1: 1762, 1632, 1606, 1511; ¹H NMR (CDCl₃, 500 MHz) δ 6.60 (d, J 2.5 Hz), 6.82 (d, J 2.5 Hz), 8.02 (dd, J 9.0 Hz), 7.02 (d, J 9.0 Hz), 3.78, 3.90 (s), 3.89 (s), 2.47 (s), ¹³C NMR (CDCl₃, 125 MHz) δ 154.8 (C-2), 140.8 (C-3), 173.2 (C-4), 150.5 (C-5), 108.0 (CH-6), 163.1 (C-7), 98.6 (CH-8), 157.8 (C-9), 111.4 (C-10), 122.9 (C-1'), 129.9 (CH-2',6'), 114.0 (CH-3',5'), 161.4 (C-4'), 59.9, 55.9, 55.3 (MeO-3,7,4', respectively), 169.7/21.2 (COCH₃-5).

Epi-solamargine (14)

Colorless crystals, mp 238-250 ºC; [α]²⁵_D – 98 (c 0.1, MeOH); IR (KBr) ν_max/cm^-1: 3417, 2927, 1453, 1384, 1044 cm^-1. HR-FABMS positive ions at m/z 868.5239 [M+H]⁺ (calc. for C₄₅H₇₄NO₁₅: m/z 868.505846), 722.4643 (calc. for C₃₉H₆₄NO₁₁: 722.447937) [M+H-rhamnose]⁺, m/z 576.4037 (calc. for C₃₃H₅₄NO₇: 576.390028) [M+H-rhamnose-rhamnose]⁺, m/z 445.7553 (calc. for C₂₁H₃₃O₁₀: 445.207372) [M-423]⁺, m/z 414.3467 (calc. for C₂₇H₄₄NO₂: 414.337204) [M+H-rhamnose-rhamnose-glucose]⁺, m/z 413.2782 (calc. for C₂₇H₄₃NO₂: 413.329379) [M-rhamnose-rhamnose-glucose]⁺. ¹³C RMN: Tables 1 and 2. ¹H RMN: ^{Tables 3} and 4.

Peracetyl-epi-solamargine (15)

Crystal, mp 128-130 ºC, [α]²⁵_D – 165 (c 0.1, MeOH); IR (KBr) ν_max/cm^-1: 2943. 2728, 1751, 1438, 1374, 1045; Positive ion FABMS m/z 1246.5522 [M+H]⁺ (calc. for C₆₃H₉₂NO₂₄: 1246.600928), m/z 1204.5278 (calc. for C₆₁H₉₀NO₂₃: 1204.590364) [M+H-42]⁺, m/z 974.5256 (calc. for C₅₁H₇₆NO₁₇: 974.511325) [M+H-rhamnose]⁺, m/z 932.5163 (calc. for C₄₉H₇₄NO₁₆: 932.500760) [M-314]⁺, m/z 634.7963 (calc. for C₃₉H₅₆NO₆: 634.410763) [M+H-612]⁺, m/z 413.2715 (calc. for C₂₇H₄₃NO₂: 413.329379) [M-acetate-rhamnose-rhamnose-glucose]⁺. ¹³C RMN: Tables 1 and 2. ¹H RMN: ^{Tables 3} and 4.

Peracetyl-solasonine (17)

Crystal, mp 148-150 ºC; ¹³C RMN: Tables 1 and 2; ¹H RMN: ^{Tables 3} and 4.

Supplementary Information

Supplementary data associated with this paper are available free of charge at http://jbcs.sbq.org.br, as a PDF file including a molecular modeling study in order to compare the relative stabilities of the epimeric structures 13 and 14.

Acknowledgments

The authors are grateful to CNPq, FAPERJ and CAPES for research fellowships and financial support. They also thank CENAUREMN for the 500 MHz NMR spectra, Laboratório Plataforma de Métodos Analíticos de Farmanguinhos/FIOCRUZ for the 400 MHz NMR spectra. They thank MSc J.M. Alves and Dr. M. de F. Agra for the collection and classification of plant material.

Submitted: March 23, 2009

Published online: August 12, 2010

Supplementary Information

Molecular Modeling

In order to compare the relative stabilities of the epimeric structures 13/14, a molecular modeling study was implemented using the Spartan 06 for Linux program (Wavefunction, Inc.). Because the long chain attached to C-3 should only have a small influence on the relative stabilities, it was replaced by a methoxy group to reduce the computational cost for the calculations. The conformer distribution of the resulting alpha and beta epimer models (13a and 14a) was determined with the Monte Carlo approach using the MMFF molecular mechanics force field. The most stable conformers of 13a and 14a were submitted to a previous energy minimization with the PM3 semi empirical method.³¹ The PM3 optimized structures were then submitted to a complete energy minimization with the B3LYP/6-31G* DFT method. The B3LYP method was chosen because it usually yields results for many properties in close agreement with those obtained from MP calculations, and is more efficient than conventional ab initio correlated methods for larger-scale calculations. The possibility of existence of this epimer as a stable species was verified by a molecular modeling study at the DFT B3LYP/6-31G* level with models of both epimers. The b-epimer model at C-20 (14a) is less stable than the a-epimer one (13a), but the energy difference between both structures is only 4.85 kcal mol^-1 (20.30 kJ mol^-1), as calculated with the B3LYP/6-31G* DFT method. Because of this small energy difference, the b-epimer is expected to exist in appreciable amount in an equilibrium mixture with the more stable a-epimer. The main reason for the lower stability of the b-epimer should be the proximity between the C-21 methyl group and carbons C-23 and C-18. The corresponding C-C distances, which are equal to 4.22 Å and 3.47 Å, respectively, in 13a, are considerably shorter in 14a, 3.43 Å and 2.98 Å, respectively. This closer proximity would raise more unfavorable steric interactions in 14a than in 13a (Figure S15).

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Publication Dates

• Publication in this collection
  16 Dec 2010
• Date of issue
  Dec 2010

History

• Received
  23 Mar 2009
• Accepted
  12 Aug 2010