Ir-Catalyzed Double Asymmetric Hydrogenation of 3,6-Dialkylidene-2,5-diketopiperazines for Enantioselective Synthesis of Cyclic Dipeptides

An Ir/spiro­[4,4]-1,6-nonadiene-based phosphine-oxazoline ligand (SpinPHOX) complex-catalyzed double asymmetric hydrogenation of 3,6-dialkylidene-1,4-dimethylpiperazine-2,5-diones has been developed, providing efficient and practical access to a wide variety of chiral 3,6-disubstituted-2,5-diketopiperazines in high yields with exclusive cis-diastereo- and excellent enantioselectivities (>99% de, up to 98% ee). The synthetic utilities of the protocol have been demonstrated in a gram scale synthesis of 6a and efficient construction of chiral products 8, 14, and 17 as well as a 2-butenyl-bridged bicyclic diketopiperazine 10 and hydroxydiketopiperazine 11. With an analogous achiral Ir catalyst, the hydrogenation of enantiopure monohydrogenated intermediate 7a gave cis-6a as the only product, indicating that the second-step hydrogenation of the titled transformation is a chiral substrate controlled process. The reaction profile study for asymmetric hydrogenation (AH) of 5a revealed that the concentration of the monohydrogenation intermediate 7a remained at a low level (