Steric and conformational control of the regioselectivities in the ene reaction with trisubstituted cycloalkenes: comparison of the enophiles singlet oxygen, triazolinedione, and nitrosoarene
The nitrosoarene ene reaction with the cycloalkenes 1-3 and E-4 proceeds in high twix regioselectivity to afford the hydroxylamine ene products 1a-4a (twix) and 1b-4b (twin, except far E-4 twix). Steric interactions in the enophile attack are responsible for the skew trajectory of the nitrosoarene enophile. For Z-1-methylcyclooctene (Z-4), twin abstraction dominates, caused by conformational constraints (transannular interactions) in the hydrgogen-atom abstraction. The balance between these steric and conformational factors dictates the regioselectivity in the ene reaction
A comparative mechanistic analysis of the stereoselectivity trends observed in the oxidation of chiral oxazolidinone-functionalized enecarbamates by singlet oxygen, ozone, and triazolinedione
Abstract 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 C3′ position of the enecarbamate side chain.
Regioselective reaction of singlet oxygen with cis-alkenes
Abstract The reaction of singlet oxygen with cis olefins is regioselective and shows a general preference for hydrogen abstraction on the larger alkyl group of the double bond.