2: PDE-I 3: PDE-I1 resulted in three clever, and conceptually different, syntheses. 2 The dioxygenated and essentially identical B and C units are like the natural products PDE-I (2) and PDE-I1 (3), which were isolated by Umezawa's group and shown to exhibit inhibitory activity against cyclic adenosine-3', 5'-monophosphate phosphodie~ terase.~ Furthermore, both PDE-I and PDE-I1 were found to have remarkably low toxicity, no signs of toxicity ...