Abstract
1H NMR chemical shifts were used to assign the structures of the regioisomeric products obtained from the reactions of
C-ethoxycarbonyl-
N-arylnitrilimines
2A–E to α,β-unsaturated ketones
3a–j. The assignments were based on the large observed difference between chemical shifts of the H-4 and H-5 of the 2-pyrazoline ring residue. Values of 1.29 ± 0.06 and 0.34 ± 0.03 ppm were found for Δδ
4.5 for the 5-aroyl- and 4-aroyl-2-pyrazoline regioisomers
4 and
5, respectively. The regioselectivity in the studied cycloaddition reactions is interpreted in terms of FMO method.