Since the first limonoid, limonin, was elucidated in 1960, over 300 limonoids have been characterized (Fig. 1). Most of these compounds show various biological activities, e. g., insect antifeeding, cytotoxic, antitumor, and other activities. Their structure-activity relationship is very interested but it is little clarified due to their complex structures. On the other hand, a series of compounds known as degraded limonoids are biogenetically related to limonoids and have rather simple structures (Fig. 2). But their biological activities are almost unknown and only a few syntheses have been reported. Now we would like to report here the synthesis of fraxinellonone (3a) and related compounds including their stereoisomers (1a,b, 2a,b, and 3b), and their biological activity. The synthesis of these compounds were accomplished from common key intermediates (10) as shown in Scheme 2 and 3. Their ichtyotoxicity was examined and the resulting data was listed in Table 2. Compounds 1, 2, and 13 showed particularly strong toxicity. Their antifeedant test is now in progress.