1. E. Weiss, O. Schindler and T. Reichstein, Helv. Chim. Acta, 40, 980 (1957).
2. B. Fechtig, O. Schindler and T. Reichstein, Helv. Chim. Acta, 43, 727 (1960).
3. R.-F. Chen, F. Abe, T. Yamauchi and M. Taki, Phytochemistry, 26, 2351 (1987).
4. W. Bleier, W. Kubelka, E. Weiss and M. Wichtl, Pharm. Acta Helv., 40, 554 (1965).
5. R. Ferth and B. Kopp, Pharmazie, 47, 626 (1992).