1. F. Effenberger, Angew. Chem. Int. Ed. Engl. 33 (1994) 1555.
2. F. Effenberger, Chimia 53 (1999) 3.
3. R.G.H. Gregory, Chem. Revs. 99 (1999) 3649.
4. U. Burkard, F. Effenberger, Chem. Ber. 119 (1986) 1594.
5. H. Ohta, Y. Kimura, Y. Sugano, T. Sugai, Tetrahedron 45 (1989) 5469.