A comparative mechanistic analysis of the stereoselectivity trends observed in the oxidation of chiral oxazolidinone-functionalized enecarbamates by singlet oxygen, ozone, and triazolinedione

The stereoselectivity in the reactions of the E/Z enecarbamates 1, equipped with the oxazolidinone chiral auxiliary, has been examined for singlet oxygen (¹O₂), ozone (O₃), and 4-phenyl-1,2,4-triazoline-3,5-dione (PTAD) in a variety of solvents as a function of temperature. The oxidative cleavage of the alkenyl functionality by ¹O₂ and O₃ releases the enantiomerically enriched methyldesoxybenzoin (MDB) product. The extent (% ee) as well as the sense (R vs S) of the stereoselectivity in the MDB formation depends on the electronic nature of the oxidant. A high stereoselectivity, substantially dependent on solvent and temperature, is displayed for the reactions with ¹O₂, whereas the ground-state reactants O₃ and PTAD are rather unaffected by solvent and temperature variations. The present comparative analysis clearly substantiates our hypothesis of stereoselective vibrational quenching of the attacking ¹O₂, whereas O₃ and PTAD are only subject to steric impositions. The electronically excited ¹O₂ is sensitive to all three stereochemically relevant structural characteristics embodied in the chiral enecarbamates, namely the R/S configuration at the C₄ position of the oxazolidinone chiral auxiliary, the Z/E geometry of the ‘alkene’ functionality, and R/S configuration at the C_3′ position of the enecarbamate side chain.