Stereoselective synthesis of protected l-allo-Enduracididine and l-Enduracididine via asymmetric nitroaldol reaction

The diastereoselecetive and scalable synthesis of cyclic guanidine-containing nonproteinoginic amino acids, enduracididines, has been achieved. Both diastereomers, l-allo-enduracididine and l-enduracididine, were prepared via catalyst-controlled asymmetric nitroaldol reaction with the aldehyde precursor derived from l-aspartic acid. The cyclic guanidine of di-Cbz-protected l-allo-enduracididine was fully protected with an allyl group to suppress nucleophilic side reactions. Introduced allyl group was efficiently removed via π-allylpalladium chemistry without attaching the Cbz group on the cyclic guanidine moiety.