The complete assignment of ¹H- and 13C-NMR of prenylated xanthones from Tovomita spp. (Guttiferae)

Abstracts

The ¹H-and 13C-NMR spectral data of 1,6-dihydroxy-5-(3-methylbut-2-enyl)-6’,6’-dimethyl-pyrano(2’,3’;7,8)-6",6"-dimethyl-pyrano(2",3";3,2) xanthone and 1,3,6-trihydroxy-6’,6’-dimethyl-pirano(2’,3’;7,8)-2,5-di-(3-methylbut-2-enyl)xanthone have been unambiguously assigned by the analysis of homonuclear (¹H-¹H-COSY) and heteronuclear [¹H-13C-COSY: modulated with ¹J CH and nJ CH (n = 2 and 3, COLOC)] shift-correlated and NOE difference spectra.

A 1,6-dihidroxy-5-(3-metilbut-2-enil)-6’,6’-dimetil-pirano(2’,3’;7,8)-6",6"-dimetil-pirano (2",3";3,2)xantona e a 1,3,6-trihidroxi-6’,6’-dimetil-pirano(2’,3’;7,8)-2,5-di-(3-metilbut-2-enil)xantona tiveram seus deslocamentos químicos de hidrogênio e carbono-13 atribuídos de forma inequívoca, com base nas informações obtidas através dos espectros de RMN de ¹H[1D e 2D(¹H-¹H-COSY) e de correlação heteronuclear ¹H-13C-COSY[nJ CH(n = 1, 2 e 3)]. Estes dados ainda foram confirmados utilizando-se a técnica de NOE diferencial.

Tovomita; Guttiferae; prenylated xanthone; NMR data

Note

The complete assignment of ¹H- and ¹³C-NMR of prenylated xanthones from Tovomita spp. (Guttiferae)

Tanus Jorge Nagem^a, Alceni Augusta Werle^a, Mário Geraldo de Carvalho^b, and Antônio Augusto Lins Mesquita ^{* c}

^aDepartamento de Química, Universidade Federal de Ouro Preto, Campus do Morro do Cruzeiro, 354000-000 Ouro Preto - MG, Brazil

^b Departamento de Química, ICE, Universidade Federal Rural do Rio de Janeiro, 23851-000 Seropédica-Itaguaí, Rio de Janeiro - RJ, Brazil

^cDepartamento de Química, Universidade Federal de Minas Gerais, Av. Antônio Carlos, 6627, 31270-010 Belo Horizonte - MG, Brazil

Received: April 5, 1995; November 29, 1996

A 1,6-dihidroxy-5-(3-metilbut-2-enil)-6’,6’-dimetil-pirano(2’,3’;7,8)-6",6"-dimetil-pirano (2",3";3,2)xantona e a 1,3,6-trihidroxi-6’,6’-dimetil-pirano(2’,3’;7,8)-2,5-di-(3-metilbut-2-enil)xantona tiveram seus deslocamentos químicos de hidrogênio e carbono-13 atribuídos de forma inequívoca, com base nas informações obtidas através dos espectros de RMN de ¹H[1D e 2D(¹H-¹H-COSY) e de correlação heteronuclear ¹H-¹³C-COSY[ⁿJ_CH(n = 1, 2 e 3)]. Estes dados ainda foram confirmados utilizando-se a técnica de NOE diferencial.

The ¹H-and ¹³C-NMR spectral data of 1,6-dihydroxy-5-(3-methylbut-2-enyl)-6’,6’-dimethyl-pyrano(2’,3’;7,8)-6",6"-dimethyl-pyrano(2",3";3,2) xanthone and 1,3,6-trihydroxy-6’,6’-dimethyl-pirano(2’,3’;7,8)-2,5-di-(3-methylbut-2-enyl)xanthone have been unambiguously assigned by the analysis of homonuclear (¹H-¹H-COSY) and heteronuclear [¹H-¹³C-COSY: modulated with ¹J_CH and ⁿJ_CH (n = 2 and 3, COLOC)] shift-correlated and NOE difference spectra.

Keywords: Tovomita, Guttiferae, prenylated xanthone, NMR data

Introduction

Several compounds have been isolated from two species of Tovomita (T. macrophylla and T. pyrifolium)^1,2. Among them, two prenylated xanthones, the structures of which were defined as 1,6-dihydroxy-5-(3-methylbut-2-enyl)-6’,6’-dimethyl-pyrano(2’,3’;7,8)-6",6"-dimethyl-pyrano (2",3" ;3,2)xanthone (1) and 1,3,6-trihydroxy-6’,6’-dimethyl-pyrano(2’,3’;7,8)-2,5-di-(3-methylbut-2-enyl)xanthone (2). Despite the fact that xanthones have been studied by ¹H-and ¹³C-NMR spectroscopy, these structures have not been previously studied using these methods. This investigation led us to confirm the proposed structures and unambiguously establish the chemical shifts for the protons and carbons.

Experimental

1D and 2D NMR spectra were recorded at 200 MHz for ¹H, and at 50.3 MHz for ¹³C, using a Bruker AC-200 spectrometer. The pulse sequences used are contained in the XHCORR > AU Bruker program for heteronuclear correlation (¹H-¹³C-COSY), with D₃ = 0.5/J_CH and D₄ = 0.25/J_CH, modulated for ¹J_CH = 135 Hz and ⁿJ_CH = 8.0 Hz (long range, n = 2 and 3). The COSY.AU program was applied for homonuclear (¹H-¹H-COSY), NOEDIFF.AU to observe NOE, and signal multiplicities were established by DEPTVAR.AU (Q = 90° and 135°).

Results and Discussion

The observed signals in the NMR ¹H-spectra [1D and 2D (¹H-¹H-COSY)], in conjunction with the observed interaction in the heteronuclear correlation spectra ¹H-¹³C-COSY[ⁿJ_CH (n = 1,2 and 3)] (Tables 1 and 2, Fig. 1), were compatible with the presence of two pyrano and one prenyl unit in Structure 1, and one pyrano and two prenyl units in Structure 2.

The carbon chemical shift data for the xanthone skeleton obtained by ¹³C-NMR (PND) from 1 and 2 (Tables 1 and 2) when compared to models 3³ and 4⁴, showed the same substitution pattern on ring A of 1 and 2 respectively. The location of the pyrano units in 1 and the prenyl units in 2 was confirmed by long-range heteronuclear chemical shift correlation (Fig. 1). The long-range coupling between H-1" (d 3.18), 1-OH(d 13.58), H-4(d 6.15), and C-2(d 109.6) in Structure 2 are in accordance with ring A non-substituted at C-4 position. Consequently, the long-range coupling between H-5"( d 5.55) and C-2(d 103.7) in structure 1 (Fig. 1) confirms the same hydrogenation pattern of ring A.

The other ¹³C signals of the xanthone skeleton were non-hydrogenated. Two of them are oxygenated carbons (d 150.3 and 137.6) of 1 and (d 149.9 and 137.3) of 2, and two did not have any heteroatom attached to them (d 115.1 and 116.9) for 1 and (d 115.4 and 116.9) for 2, which are in accordance with a fully substituted ring B.

The long-range coupling between H-1"’ (d 3.40) and C-10a (d 150.6), and H-1"’ (d 3.40) and C-6 (d 150.3) of 1, and H-1’’’ (d 3.42) and C-6 (d 149.9) of 2 are in accordance with the prenyl unit attached to C-5. The same kind of observation was made in relation to H-4’ (d 7.80) and C-7 (d 137.6) of 1 and H-4’ (d 7.84) and C-7 (d 137.3) of 2 which was in accordance with the presence of the pyrano unit at C-7/C-8. In addition the hydroxyl group should be attached at C-6. This experimental information suggests that the attribution of the chemical shift for C-4’ and C-5’ reported in the literature for 3^3,5 an 5⁵ must be exchanged.

The observed NOE for 1, with double irradiation, defined the attribution of methyl groups from pyrano and prenyl units of 1 and 2 (Tables 1 and 2). In the prenyl units, NOE experiments were very important for establishing the stereochemistry of the methyl groups. In Structure 1, the irradiation in the H-1"’ frequency generated NOE of 10.0 and 6.0 % in H-2"’ and H-4"’, respectively. On the other hand, irradiation in the frequency of H-2’’’ gives a NOE of 7.0% in H-5"’, while the irradiation of the H-4"’ signal gives a NOE of 3.0 % in H-1"’ and H-5"’. Again, irradiation at the frequency of H-5"’ produces NOE of 7.0 and 4.0 % in H-2"’ and H-4"’, respectively (Table 1).

Acknowledgments

The authors thank CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico), CAPES (Coordenação de Aperfeiçoamento de Ensino Superior), and FINEP/PADCT (Financiadora de Estudos e Projetos) for financial support.

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• 3. Westerman, P.W.; Gunasekera, S.P.; Uvais, M.; Sultambawa, S.; Kazlauskas, R. Org. Magn. Reson. 1977, 9, 631.
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• 5. Dharmaratne, H.R.W.; Sotheeswaram, S.; Bala-subramanam, S.; Reisch, J. Phytochemistry 1986, 25, 1957.

(*

) The authors dedicate this article to the memory of Professor Mesquita, for his contributions to the field of xanthones.

Publication Dates

• Publication in this collection
  31 Jan 2011
• Date of issue
  June 1997

History

• Accepted
  29 Nov 1996
• Received
  05 Apr 1995