New Cell Growth‐Inhibitory Cyclohexadienone Derivatives from Hypericum calycinum L.

The crude petroleum‐ether extract of the aerial parts of Hypericum calycinum L. (Guttiferae) exhibited in vitro growth‐inhibitory activity against the Co‐115 human colon carcinoma cell line. Bioassay‐guided fractionation of this extract allowed the isolation of the cyclohexadienone derivatives 1–5, four of which are previously undescribed compounds. The structures of the known chinesin II (1) and of 2 (hypercalin A) were established by ¹H‐ and ¹³C‐NMR spectroscopy and were confirmed by X‐ray analysis of their crystalline mixture which revealed the complete relative configuration of both compounds. The structure of 3 (hypercalin B) was elucidated by means of extensive 1D‐ and 2D‐NMR experiments, including DQ‐COSY, HETCOR and LR‐HETCOR. The structure of compound 4 (hypercalin C) was established by ¹H‐ and ¹³C‐NMR spectroscopy and confirmed by X‐ray analysis to be the 3,5‐dihydroxy‐4‐{[(1R*,2S*, 5S*)‐2‐hydroxy‐2‐methyl‐5‐(1‐methylethenyl)cyclopentyl]methyl}‐6,6‐bis‐(3‐methylbut‐2‐enyl)‐2‐(2‐methylpropanoyl)cyclohexa‐2,4‐dien‐1‐one. The structures of the higher isomeric homologues 5a/5b were deduced by comparison of their UV, ¹H‐, and ¹³C‐NMR spectra with those of 4. The isolated compounds appeared to be related to chinesin I and II previously isolated from Hypericum chinense L. and were responsible for the growth‐inhibitory activity of the extract against the Co‐115 human carcinoma cell line. Moreover, 1/2 and 3 showed molluscicidal activity against the schistosomiasis‐transmitting snail Biomphalaria glabrata.