Ring-Opening Cyclization (ROC) of Aziridines with Propargyl Alcohols: Synthesis of 3,4-Dihydro-2 H -1,4-oxazines.

Activated aziridines react with propargyl alcohols in the presence of Zn(OTf) 2 as the Lewis acid catalyst following an S N 2-type ring-opening mechanism to furnish the corresponding amino ether derivatives. Those amino ethers further undergo intramolecular hydroamination via 6-exo-dig cyclization in the presence of Zn(OTf) 2 as the catalyst and tetrabutylammonium triflate salt as an additive under one-pot two-step reaction conditions. However, for nonracemic examples, ring-opening and cyclization steps were conducted under two-pot conditions. The reaction works well without any additional solvents. The final 3,4-dihydro-2 H -1,4-oxazine products were obtained with 13 to 84% yield and 78 to 98% enantiomeric excess (for nonracemic examples).