“…[2] During the course of our development of an FXa inhibitor as an antithrombotic agent, we have reported the unique FXa inhibitor M55529, containing a cyclic N,O-acetal structure, [3] and an asymmetric synthesis of M55529 based on the enantioselective cyclic N,O-acetal formation using a chiral salen-manganese complex (Scheme 1). [4] Quite recently, we also reported other oral FXa inhibitors M58163 and M58169, [5] with characteristic features of a spiro unit and an imidazopyrazinone as a cyclic N,N-acetal structure instead of the cyclic N,Oacetal of M55529. These compounds exhibit higher FXa inhibitory activity [M58163 (IC 50 = 0.61 nM), M58169 (IC 50 = 0.58 nM)] than M55529 (IC 50 = 2.0 nM).…”