1. Department of Chemistry, University of Malaya, 50603 Kuala Lumpur, Malaysia, The Kitasato Institute, 5-9-1 Shirokane, Minato-ku, Tokyo 108-8642, Japan, Faculty of Pharmacy, Iwaki Meisei University, 5-5-1, Chuodai Iino, Iwaki, Fukushima 970-8551, Japan, and Temko Corporation, 4-27-4 Honcho, Nakano-ku, Tokyo 164, Japan