1. Institut für Organische Chemie, Johannes Gutenberg-Universität Mainz, J. J. Becher Weg 34 SB1, D-55099 Mainz, Germany, Institute of Organic Chemistry, Polish Academy of Science, 01-224 Warsaw, ul. Kasprzaka 44, Poland, and Institute of Organic Chemistry National Academy of Sciences of Ukraine, Murmans'ka Street 5, 253-660 Kyiv 94, Ukraine