Phenolics from Rhagadiolus stellatus (Asteraceae, Cichorieae).

Rhagadiolus stellatus Gaertn., a Mediterranean member of the Cichorieae tribe of the Asteraceae family used as a food plant, was analyzed for its spectrum of phenolic compounds. Kaempferol 3-O-β-glucoside 1, kaempferol 3-O-β-rutinoside (nicotiflorin) 2, quercetin 3-O-β-glucoside 3, and luteolin 4 were isolated from the n-butanol layer of a methanolic extract of whole plants of Rh. stellatus of Spanish origin by repeated Sephadex LH-20 column chromatography. Structures were determined based on NMR and MS data as well as by comparison with literature data. Additionally, chlorogenic acid 5 and 3,5-dicaffeoylquinic acid 6 were detected by HPLC/DAD and HPLC/MS. Chemosystematic implications of the presented findings are discussed in comparison with other members of the Cichorieae tribe.

Introduction

Rhagadiolus stellatus (L.) Gaertn. is one of two species of the genus Rhagadiolus. According to recent molecular studies Rhagadiolus is closest related to the genus Lapsana and both genera cluster within the genus Crepis s.l. [1]. Rhagadiolus stellatus is a herb of up to 50 cm height, with small flowering heads composed of yellow ligulate flowers. The achenes are narrowly cylindrical and bear no pappus. The outer achenes are long-persistent and form a characteristic radiating infructescence, as also indicated in the specific epithet. The natural distribution area of Rh. stellatus encompasses Southern Europe, Northern Africa, the Macaronesian Archipelago, and the West and Southwest of Asia [2, 3]. In the only phytochemical investigation of Rh. stellatus so far, quercetin was found to be the major aglycon in leaves after hydrolysis of the genuine flavonoids [4].

Results and Discussion

Flavonoids (Fig. 1) kaempferol 3-O-β-glucoside 1, kaempferol 3-O-β-rutinoside (nicotiflorin) 2, quercetin 3-O-β-glucoside 3, and luteolin 4 were identified based on NMR and MS data as well as by comparison with literature data of the above and related compounds [5-12]. Phenolic acids chlorogenic acid 5 and 3,5-dicaffeoylquinic acid 6 (Fig. 1) were detected by HPLC/DAD and HPLC/MS in comparison with authentic reference compounds using an established system [13].

Fig. 1.

Structures of phenolics isolated from and detected in Rhagadiolus stellatus.

Chlorogenic acid 5 and 3,5-dicaffeoylquinic acid 6 occur ubiquitously in the Asteraceae. Luteolin 4 is a very common flavonoid in the Cichorieae and has also been reported from both Crepis and Lapsana, which are closely related to Rhagadiolus [1, 14]. Quercetin 3-O-glucoside 3 was also reported from aerial parts of Lapsana communis L. but has not yet been found in Crepis [14]. In contrast, kaempferol 3-O-derivatives 1 and 2 are rather rare flavonoids in the Cichorieae tribe. Kaempferol 3-O-β-glucoside 1 has only been reported from the genus Cichorium (detected in leaves of four species), aerial parts of Lactuca tatarica C.A.Mey., and in whole plants of five species of Stephanomeria. Kaempferol 3-O-β-rutinoside 2 has only been reported from aerial parts of Pinaropappus roseus Less. and Scolymus hispanicus L. [14].

A deeper chemosystematic interpretation of the above findings is difficult because of the poor or missing phytochemical data for most of the related genera. Phenolics of many species of Crepis were reported recently but these data are of limited value in the present context because this study was focused on flowering heads, only [15]. Nonetheless, the data presented here suggest that Rhagadiolus is not only morphologically but also chemically (prevalence of flavonols) differentiated from the genus Crepis (prevalence of flavones) and thus the decision of Enke and Gemeinholzer [1] to keep Rhagadiolus as a genus separate from Crepis is compatible with the available phytochemical data.

Experimental

Plant material

Rh. stellatus was collected in April 2009 between Vélez Rubio and Santa Maria de Nieva/Almeria/Andalucia/Spain; N 37°37′26″; W 02°00′53″; alt.: 890 m. Voucher specimens are deposited in the herbarium of the Institut für Botanik, Universität Innsbruck, Austria (voucher codes: IB-33270) and in the private herbarium of CZ (CZ-20090417A-2).

Natural product isolation and identification

Air-dried, ground whole plants (719 g) of Rh. stellatus were exhaustively macerated with MeOH to yield 101 g of crude extract after evaporation of the solvent in vacuo. The crude extract was re-dissolved in a mixture of MeOH and H2O (1/2, v/v) and successively partitioned with petrol ether, EtOAc, and n-BuOH. The BuOH layer was brought to dryness in vacuo to yield 11.8 g of residue.

Kaempferol 3-O-β-glucoside 1 (22.4 mg), kaempferol 3-O-β-rutinoside (nicotiflorin) 2 (9.0 mg), quercetin 3-O-β-glucoside 3 (62.6 mg), and luteolin 4 (3.8 mg) were isolated from the BuOH layer of a MeOH extract of whole plants of Rh. stellatus by repeated Sephadex LH-20 column chromatography using a mixture of MeOH, (CH3)2CO, and H2O (3/1/1, v/v/v) as mobile phase. Chlorogenic acid 5 and 3,5-dicaffeoylquinic acid 6 were detected by HPLC/DAD and HPLC/MS in comparison with authentic reference compounds using the methodology described by Fusani and Zidorn in 2010 [13].

NMR spectra were measured at 300 MHz (1H NMR) and 75 MHz (13C NMR), respectively. Spectra of compounds 1, 2, and 4 were recorded in CD3OD and referenced to solvent residual and solvent signals at 3.31 ppm (1H NMR) and 49.0 ppm (13C NMR), respectively. Spectra of compound 3 were recorded in DMSO-d6 and referenced to solvent residual and solvent signals at 2.50 ppm (1H NMR) and 39.5 ppm (13C NMR), respectively.