Chemical study of Petasites albus

Abstract

A new compound was isolated pure by column chromatography of the acetone extract of Petasites albus roots. Its structure was established as 5-isopropenyl-3-methyl-7-carboxybenzofuran (1) based on physicochemical and spectral (IR, PMR, 13 C NMR, 13 C DEPT 135) data.In continuation of the chemical study of plants from the family Asteraceae growing in Azerbaijan, we investigated roots of Petasites albus (L.) Gaertn. (white butterbur) collected at the end of vegetation (November 2009) near the village Sebetkechmez, Kedabek Region, Azerbaijan Republic.Chromatography of the total extracted substances obtained by acetone extraction (3u) of P. albus roots isolated pure crystalline 1, R f 0.49 (hexane), C 13 H 12 O 3 , mp 128-130°C.The IR spectrum contained characteristic frequencies bands for a carboxylic carbonyl and benzene C=C (broad overall band with a maximum at 1640 cm -1 ) and strong absorption bands in the fingerprint region (800, 840, 905, 1160, 1280, 1300) indicating the presence of a carboxyl and benzene ring [1, 2]. The strong band at 905 was consistent with a methylene double bond in the structure [3].The NMR spectrum taken with full proton decoupling exhibited 13 singlets characteristic of 13 C atoms in the basic structure of the compound. Of these, those with chemical shifts 17.5 and 31.5 ppm belonged to two methyls on a double bond; 98.0, 103.0, 113.0, and 124.0, to protonated olefinic C atoms; 116.5, 123.0, 133.0, 151.0, 158.5, and 159.0, to unprotonated olefinic C atoms; and 162.0, to a carboxyl C atom. The correctness of the assignments in the 13 C NMR spectrum was confirmed by the 13 C DEPT 135 spectrum, where resonances of two methyl C atoms at 17.5 and 31.5 ppm, three (98.0, 103.0, 124.0) protonated olefinic C atoms, and one (113.0) methylene double bond were found.A comparison of the data given above with the literature, especially with physicochemical and spectral data of compounds obtained from representatives of the genus Petasites Milli [4, 5], led to the conclusion that the studied compound was new. We called it petalbusin.The PMR spectrum of petalbusin exhibited eight singlets: two resonances with an area of 3H and six singlets with an area of 1H. Thus, resonances at 3.10 and 2.70 ppm provided evidence that petalbusin contained two vinyl methyls. The chemical shifts of these resonances were slightly lower than that of ordinary vinyl methyls. This was apparently due to the presence in the molecule of a more conjugated system such as a benzene ring condensed with a furan ring.Highly oxygenated sesquiterpenes, mainly containing a furan ring, and sesquiterpene lactones of the eremophilane type, for example, furanoeremophilane, petasalbin, 9D-hydroxyfuroeremophilane, furanopetasol, furanopetasin, albopetasol, etc., were isolated from roots of species from the genus Petasites [5]. The furan ring in all these compounds contains a methyl group. It is possible from a biogenesis viewpoint that one of the methyls (2.7 ppm) in petalbusin is also located on the furan ring (in the E-position to the fura...