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Within a systematic phytochemical investigation of the leaves of RHODODENDRON FERRUGINEUM L. (Ericaceae), the volatile oil was isolated (0.7 %) and its constituents were characterized. Eleven flavonoids were isolated and characterized, with quercetin 3- O-[6''- O-(2-methyl-1-oxobutyl)]-β- D-glucopyranoside and 2 R,3 R-dihydromyricetin 3- O- β- L-arabinopyranoside as new natural products. Beside monomeric flavan-3-ols (catechin, epicatechin, gallocatechin, epigallocatechin) from the tannin fraction (about 3.5 % calculated as pyrogallol), the dimeric procyanidins B1 to B7 were identified, as well as the trimeric compounds procyanidin C1, epicatechin-(4 β → 8)-epicatechin-(4 β → 8)-catechin and the trimeric A type-linked cinnamtannin B1. Additionally, phloroacetophenon 4- O- β- D-glucopyranoside and chlorogenic acid were isolated. Water-soluble carbohydrates comprised about 13.5 % of the dried leaves, including fructans (3 %), polysaccharides (1 %) (mainly type II arabinogalactans), glucose, fructose, sucrose, stachyose and raffinose. The IN VITRO effects on cellular vitality (MTT test), proliferation (BrdU incorporation) and necrosis (LDH release) of an aqueous extract were investigated. The extract did not exert any toxic effects, while the vitality and the proliferation rates of epithelial HaCaT keratinocytes were significantly increased at 100 µg/mL, indicating that the aqueous extract does not have negative effects against cellular activity.

For quantitative determination of grayanotoxin I (1) in plant material, a GC/MS method was developed after trimethylsilyl derivatisation of the analytes. Forskolin (5) was used as an internal standard for quantification. ICH-compliant method validation indicated sufficient specificity, precision, quantitation (15 µg/mL) and detection (5 µg/mL) limits. Regression analysis showed that a non-linear (polynomial) model was preferable to a linear one. For isolation of grayanotoxin I reference material from Rhododendron ponticum leaves, an efficient two-step fast centrifugal partition chromatography isolation protocol is described. A survey of 17 different plant species from the genus Rhododendron revealed high grayanotoxin I content for R. catawbiense, R. ponticum, R. degronianum subsp. yakushimanum, R. × sochadzeae, R. moupinense, R. galactinum, and R. mucronatum var. ripense. The content of this compound in leaf material from R. ponticum decreased rapidly during drying process. Grayanotoxin I was not detected in different batches of fresh leaves and fruits from R. ferrugineum. In contrast to the claims of German health authorities, this traditionally used herb therefore cannot be evaluated as toxic due to the presence of grayanotoxin I.

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