1. Institute of Organic Chemistry, University of Bremen, Postfach 33 04 40, 28334 Bremen, Germany, Institute of Inorganic and Physical Chemistry, University of Bremen, Postfach 33 04 40, 28334 Bremen, Germany, Institute of Organic Chemistry, Russian Academy of Sciences, Siberian Division, 630090 Novosibirsk, Russia, and Facultad de Bioquimica, Quimica y Farmacia, Instituto de Quimica Fisica, Universidad Nacional de Tucuman, Ayacucho 491, 4000 Tucuman, Argentina