Catalytic Enantioselective Synthesis of 2-Pyrazolines

Significance

Reported here is a highly enantio-selective 1,3-dipolar cycloaddition between alkyl diazoacetates 1 and acroleins 2 to give 2-pyrazolines 3. The catalysts 4-6 were prepared in situ by activation of the corresponding Corey-Bakshi-Shibata oxazaborolidines with triflimide. High yields and excellent enantioselectivities were obtained in almost all cases, with both α-substituted and α,β-disubstituted acroleins tolerated in the reaction. The observed stereochemistry may be rationalized via a standard transition state model for this class of catalyst, whereby a hydrogen-bonded complex is formed leading to endo attack of the 1,3-dipole.