Trifluoroethanol-Mediated Cyclization of Two-Carbon-Tethered Epoxide- N -Boc Pairs: Completely Regioselective Synthesis of 3,6-Disubstituted 1,3-Oxazinan-2-ones

The regio-and diastereoselective construction of biologically relevant cyclic carbamates under operationally simple and mild transition-metal-free conditions is challenging and has led to a demand for efficient methods for their synthesis. The intramolecular ring -opening cyclization of N-Boc-tethered epoxides leading to the formation of cyclic carbamates is equipped with many favorable synthetic features, including easy accessibility of starting materials in a stereodefined form, high diversification points in the substrates, and a favorable entropy factor. However, its use in the construction of 1,3-oxazinan-2-ones remains largely neglected. Specifically, prior to 2020, only few 1,3-oxazinan-2-ones were successfully synthesized using this strategy. Moreover, our very own recent attempt to access these heterocycles using one carbon-tethered N-Boc-epoxide pairs was met with little success as the process furnished either only 1,3-oxazolidin-2-ones or nearly equal amounts of 1,3-oxazolidin-2-ones and 1,3-oxazinan-2-ones. Herein, we demonstrate that when the epoxide and N-Boc moieties are connected by a two-carbon tether, the cyclization could deliver 1,3-oxazinan-2ones containing vicinal stereocenters, as sole regioisomers and diastereomers in high yields (up to 95% yield), irrespective of whether the distal epoxide substituent is alkyl or aryl, or the epoxide is cis- or trans -configured.