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

Abstract

The regio- and diastereoselective construction of biologically relevant cyclic carbamates under operationally simple and mild transition metal-free 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 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 a 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-2-ones containing vicinal stereocenters, as sole regio- 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.

Base-Mediated, Chemo- and Regioselective (4+2) Annulation of Indole-2-carboxamides with 2,3-Epoxy Tosylates toward 1,2-Fused Indoles

“…Surprisingly, however, the observed 6-exo selectivity in the epoxide-opening process remained unaltered when epoxy tosylate 2k was treated with indole-2-carboxamide 1b , enabling the formation of product 3r via S N 2-like inversion of the configuration at the reacting tertiary epoxide carbon atom (Scheme ). It should be noted that the S N 2 reaction in the tertiary epoxide carbon atom, while not especially common, is not unheard of …”