A New Cytotoxic Naphthoquinone from Paepalanthus latipes.

Abstract

Quinones are widely distributed in nature and constitute an important class of naturally occurring compounds. They are found in plants, fungi and bacteria. 1) These compounds are aromatic rings with two ketone substitutions. 2) Large number of quinones has been associated with antitumor, antibacterial, antimalarial and antifungal activities.3) The antitumor activity is exhibited predominantly by three main groups of naturally occurring quinones such as benzoquinone, naphthoquinone and anthraquinone. Mitomycin and streptonigrin possess p-benzoquinone moiety with heterocyclic groups whereas anthracyclines, doxorubicin and daunorubicin consist of anthraquinone moiety. Some naphthaquinone antibiotics such as lapachol and lapinone are also found to be cytotoxic to tumor cells. 4) Plants of the Eriocaulaceae family are widespread in the region of the Serra do Cipó, State Minas Gerais, Brazil. Several plants from this family are known as 'everlasting plants' because they appear to be alive even for years after being haversted. Paepalanthus is the largest genus of this family with approximately 500 species. Among there more than 400 species are endemic in Brazil. 5) In previous works we have described some biological activity (cytotoxicity, antimicrobial and antioxidant) from naphthopyrones isolated from the capitula of P. bromelioides. [6][7][8] In this work we describe the isolation, structure determination and the cytotoxic effect in vitro of a new 1,4-naphthoquinone isolated from the capitula of Paepalanthus latipes. Results and DiscussionCompound 1 was obtained as a red powder. The ES-MS spectrum of 1 (positive mode) exhibited protonated molecular ion [MϩH] ] ϩ were also observed. The IR spectrum showed absorptions for hydroxyl (3300 cm Ϫ1 ) and carbonyl groups (1680, 1620 cm Ϫ1 ) indicating a para-quinone system. 9,10) The complete structure of 1 was elucidated by 1D and 2D NMR experiments at 500 MHz. The 13 C-NMR spectrum showed 16 signals. The 1 H-NMR spectrum displayed singlets of aromatic protons at d 6.60 (1H) and 6.05 (1H). Also evident were signals at d 3.86 and 3.81 typical of aromatic methoxyl groups. Another signal at d 2.28 (3H) corresponds to a methyl group. Assignments of 1 H-and 13 C-NMR data were based on HMBC and HSQC experiments (Table 1). In the HMBC spectrum we observed diagnostic long-range correlations between the proton signal at d 6.60 (H-4) and the carbon resonances at d 167.0 (C-1), 161.4 (C-10), 113.0 (C-10a), 148.0 (C-5) and 20.3 (C-11); the proton at d 6.08 (H-8) and the carbonyl carbon resonances at d 177.9 (C-6) and at d 189.2 (C-9) and the carbon resonances at 160.9 (C-7) and 111.2 (C-9a); the proton at d 3.86 (OCH 3 -7) and the carbon resonance at d 160.9 (C-7); the proton signal at d 3.81 (OCH 3 -5) and the carbon resonance at d 147.9 (C-5); the methyl group at d 2.28 (CH 3 -11) and the carbon signals at d 160.7 (C-3) and d 98.2 (C-4). NOESY experiment displayed correlations between the signal at d 6.60 (H-4) and the signals at d 2.28 (CH 3 -11) and at d 3.81 (OCH 3 -5), and betw...