Two New 2'-Oxygenated Flavones from Andrographis elongata.

Abstract

Andrographis elongata T. And (Acanthaceae) is an herbaceous plant distributed throughout the Western Ghats of South India.1) As part of our continuing investigation on the flavonoid constituents of Andrographis species, [2][3][4][5][6] we have examined the whole plant of A. elongata, a plant hitherto not investigated for its chemical constituents and report here the isolation and structure elucidation of two new 2Ј-oxygenated flavones (3, 5), in addition to three known flavones (1, 2, 4). Results and DiscussionCompound 3, obtained as yellow crystals, mp 210-211°C, showed the pseudomolecular ion at m/z 301.0714 in its positive high resolution chemical ionization mass spectrometry (HR-CI-MS) corresponding to the molecular formula C 16 H 12 O 6 . This was corroborated by decoupled 13 C-NMR spectrum which showed signals for all the sixteen carbons of the molecule. The UV absorption maxima of 3 in MeOH (258, 303 nm) and a bathochromic shift of 62 nm with AlCl 3 / HCl suggested that it is a 5-hydroxyflavone.The 1 H-NMR spectrum of 3 showed the presence of a methoxyl (d 3.83), a chelated hydroxyl (d 12.88), two nonchelated hydroxyls (d 9.88, 2H) and a singlet at d 6.26 assigned to H-3. A set of meta coupled doublets (Jϭ2.2 Hz) resonating at d 6.38 and 6.61 were ascribed to H-6 and H-8 protons, respectively. From heteronuclear multiple bond connectivity (HMBC) and rotating frame Overhauser enhancement spectroscopy (ROESY) studies the methoxyl group at d 3.83 was placed at C-7 as the methoxyl protons showed correlation with C-7 at 165.1 ppm in its HMBC spectrum ( Fig. 1) and two strong nuclear Overhauser effects (NOEs) were observed between methoxyl protons at C-7 and H-6 (d 6.38) and H-8 (d 6.61) protons (Fig. 1). The presence of AB 2 type aromatic proton signals of ring B at d 7.10 (1H, t, Jϭ8.2 Hz) and 6.40 (2H, d, Jϭ8.2 Hz) were typical of 4Ј, and 3Ј and 5Ј protons of 2Ј,6Ј-dihydroxy (or methoxy) flavones.7-9) The presence of 2Ј,6Ј-dioxygenation in 3 was also supported by the chemical shift values of the B-ring carbons which were very similar to those observed for the B-ring carbons of 2Ј,6Ј-dihydroxyflavones. 9,10) The electron impact-mass spectra (EI-MS) of compound 3 exhibited the molecular ion as the base peak at m/z 300, with the diagnostic peaks of retro-Diels-Alder cleavage of ring C at m/z 167 and 134 supporting the presence of a hydroxyl and a methoxyl in ring A, and two hydroxyls in ring B. From HMBC correlations the two non-chelated hydroxyls were located at C-2Ј and C-6Ј positions as both these hydroxyl bearing carbons at 156.6 ppm showed cross peaks with H-3Ј and H-5Ј (d 6.40), and H-4Ј (d 7.10) protons (Fig. 1). Thus the structure of compound 3 was elucidated as 5,2Ј,6Ј-trihydroxy-7-methoxyflavone.Compound 5, obtained as yellow amorphous powder showed [MϩH] ϩ peak at m/z 607.1807 in its HR-FAB-MS corresponding to the molecular formula C 32 H 30 O 12 , and was corroborated by 13 C-NMR spectrum which showed signals for all the 32 carbons of the molecule. The UV spectrum exhibited absorption maxima at 2...

Anthocyanins and other flavonoids

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