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

• Published in: Tetrahedron Vol. 62; no. 46; pp. 10647 - 10659
• Main Authors: Sivaguru, J., Poon, Thomas, Hooper, Catherine, Saito, Hideaki, Solomon, Marissa R., Jockusch, Steffen, Adam, Waldemar, Inoue, Yoshihisa, Turro, Nicholas J.
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 13.11.2006
• Subjects: Kinetic resolution; Ozonolysis; Photooxygenation; Solvent and temperature effects; Substituent; Vibrational quenching; Ozonolysis; Vibrational quenching; Solvent and temperature effects; Photooxygenation; Substituent; Kinetic resolution
• ISSN: 0040-4020; 1464-5416
• DOI: 10.1016/j.tet.2006.07.105

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. [Display omitted]