Anthraquinones from the bark of Senna macranthera

2-acetyl physcion (2-acetyl-1,8-dihydroxy-6-methoxy-3-methyl-9,10-anthraquinone, 2), a rare anthraquinone, was isolated from Senna macranthera var. nervosa (Vogel) H.S. Irwin & Barneby (Fabaceae). The chemical structure was elucidated and all 1H and 13C NMR chemical shifts were assigned by NMR one(1HNMR, {1H}-13CNMR, and APT-13CNMR) and two (COSY, NOESY, HMQC and HMBC) dimensional of this natural compound. Furthermore, the minor anthraquinones chrysophanol (3), chrysophanol-8-methyl ether (4) and physcion (5) were characterized by GC-MS analysis. The occurrence of the anthraquinones 3-5 confirms that S. macranthera is a typical representative of the genus Senna.


INTRODUCTION
The genus Senna is known to produce various classes of aromatic compounds, e.g.quinones, normal (Barba et al. 1992) and dimeric anthraquinones (Koyama et al. 2001), naphthopyrones (Barbosa et al. 2004) and flavonoids (Baez et al. 1999).S. macranthera var.nervosa (Vogel) H.S. Irwin & Barneby (syn.Cassia macranthera), commonly known as "alleluia" (subfamily Papilionoidae, family Fabaceae), is a 6-8 m high tree with a trunk of up to 30 cm of diameter.The leaves are composed of two pairs of opposing leaflets.In Brazil, the tree is found from Ceará in the North down to São Paulo in the South.It is used mainly as an ornamental plant in cities because of lush yellow flowers and rapid growth (Lorenzi 2000).Previous phytochemical studies on the bark of S. macranthera collected in Belo Horizonte, Brazil, revealed the presence of rubrofusarin (1), 6-O-galactosylrubrofusarin, 3,5-dihydroxy-8-isobutenyl-2-methyl-7-methoxychromone and β-sitosterol (Oliveira et al. 1977).A galactomannan with anticoagulant activity has been isolated from the endosperm of seeds (Pires et al. 2001).
The anthraquinones are traditionally used as pigments and medicines, and are widely distributed in nature, occurring in both free and glycosidic form (Bruneton 1991).Several analytical methods have been used to analyse these compounds, including gas chromatography coupled to mass spectrometry (GC-MS) (Waterman and Mole 1994).This method permits the analysis of non-glycosylated anthraquinones with (Zuo et al. 2008) or without derivatisation (Mueller et al. 1999, Liu et al. 2007).
In this paper we report the identification of 2-acetyl physcion (2) by spectrometric methods, together with the characterisation of others minor anthraquinones (3-5) (Fig. 1) using GC-MS analysis from S. macranthera.

GENERAL EXPERIMENTAL PROCEDURES
NMR spectra were recorded in CDCl 3 solution at 400 MHz for 1 H and 100 MHz for 13 C on a JEOL Eclipse+ 400 spectrometer.Chemical shift values are reported relative to TMS, which was either used as internal standard or by reference to solvent signals: CHCl 3 at δ H 7.26 and at δ C 77.00.GC analyses were recorded on a Hewlett Packard model 5790 A gas chromatograph using glass capillary column (11 m × 0.25 μm) coated with SE-54 (df = 0.25 μm).GC-MS spectra were run at 70 eV on a Shimadzu QP-2000 spectrometer.The data were collected on an HP 3396-II integrator.TLC: silica gel (Merck, Kieselgel 60), spots visualised by UV (254 and 360 nm) and exposure to I 2 vapour.TLC was used to analyse fractions collected from CC.

PLANT MATERIAL
The bark of S. macranthera was collected in Campinas, São Paulo State, Brazil, in September 2000.The identification was performed by Marcia Dias Campos, and a voucher specimen (n.89281) is deposited at the Herbarium of the State University of Feira de Santana, Brazil.
Thus, the structure of 2 was established as 1,8dihydroxy-2-acetyl-3-methyl-6-methoxy-9, 10-anthraquinone (2-acetyl physcion).This is the first report of the antraquinone 2 in a Senna species.All 1 H and 13 C chemical shifts were unambiguously assigned.This compound has been reported only once in the literature (Wei et al. 2007), and the attributions to the carbons C-4a (δ C 135.03) and C-10a (δ C 133.01) were corrected.
The fractions of the dichloromethane extract obtained by SiO 2 fractionation were analysed by GC-MS to detect other polyketid.Only the fraction 8-9 showed the presence of peaks the typical fragmentation pattern of anthraquinones: intense molecular ion, loss of CO (28 Da) and other fragmentations with weak intensity (Song et al. 2009).Figure 2 and Table I show the chromatogram and mass spectra of this fraction, respectively.The peaks corresponding to Tr at 14.6, 16.1 and 17.3 min showed molecular ion base peaks ([M] •+ , 100%) at m/z 254, 268 and 284, and loss of CO to furnish peaks at m/z 226 (13%), 240 (4%) and 256 (5%), respectively.The molecular mass of 4 ([M] •+ , m/z 268) is 14 Da higher than 3 ([M] •+ , m/z 254), indicating the presence of an additional methylene unit.This -CH 2 -unit was attributed to formation of one methoxyl group at C-8 (C-OH to C-OCH 3 ).