Abstract
The stereoselectivity in the reactions of the E/ Z enecarbamates 1, equipped with the oxazolidinone chiral auxiliary, has been examined for singlet oxygen ( 1O 2), ozone (O 3), 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 1O 2 and O 3 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 1O 2, whereas the ground-state reactants O 3 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 1O 2, whereas O 3 and PTAD are only subject to steric impositions. The electronically excited 1O 2 is sensitive to all three stereochemically relevant structural characteristics embodied in the chiral enecarbamates, namely the R/ S configuration at the C 4 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.
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