1. Maneva, L.S., Slavcheva, N.N., Videnov, G.I., Mancheva, I.N., Petkov, D.D., Stoev, S.B. and Aleksiev, B.V., C.R. Acad. Bulg. Sci., 41(1988)83.
2. Petkov, D.D. and Stoineva, I., Tetrahedron Lett., 25(1984)317.
3. Koenig, W. and Geiger, R., Chem. Ber., 105(1972)2872.
4. Mancheva, I., Nikolov, R. and Pfletschinger, J., J. Chromatogr., 213(1981)99.
5. Leslie, F.M., Pharmacol. Rev., 39(1987)197.