Diterpenes from Arnica angustifolia.

From flowerheads of Arnica angustifolia ssp. attenuata the labdane diterpenes labd-13(Z)-ene-8α,15-diol (main component) and (13R,14R)8,13-epoxylabdane-14,15-diol were isolated and their structures established by extensive NMR and MS measurements. Both compounds are reported as natural products for the first time. Oxidative degradation of the main component was observed and the oxidation products 8α-hydroxylabd-13(Z)-ene-15-al, 8α-hydroxylabd-13(E)-ene-15-al and 8,13-epoxylabdane-15-al were identified. Additionally, the tetranorlabdanolide norambreinolide, most probably also derived from the labdanes, was detected in minor amounts. A search for diterpenes in flowers of A. montana revealed the presence of small amounts of labd-13(Z)-ene-8α,15-diol.