Development of a Stereoselective Synthesis of (1R,4R)- and (1S,4S)‑2-Oxa-5-azabicyclo[2.2.2]octane

Despite the prevalence of morpholine derivatives and bridged heterocycles in medicinally relevant compounds, bridged bicyclic morpholines remain scarce because of the challenges associated with their synthesis. MRK A, an IDH1mut inhibitor for the treatment of glioma, derives its potency in part from substitution of a zigzag 2,5-bicyclic morpholine, 2-oxa-5-azabicyclo[2.2.2]­octane, at C8. While existing entries suffered from low yields and lack of stereochemical control, we developed concise stereospecific routes toward both enantiomers of the zigzag morpholine antipode. The key common intermediate in the two routes was a chiral bicyclic lactone, which was readily synthesized following our previous synthesis of relebactam from optically pure (2S,5S)-5-hydroxypiperidine-2-carboxylic acid (HPA). The desired (R,R) enantiomer for incorporation into MRK A required inversion of both stereocenters of the bicyclic lactone intermediate, which was accomplished by epimerization via a crystallization-induced diastereomer transformation process followed by a key Ti­(OiPr)4-mediated intramolecular SN2 ring closure. By this method, the (R,R)-zigzag morpholine was synthesized in six steps from HPA in 25% overall yield.