Facile synthesis of novel tetrasubstituted 1-pyrazolines from Baylis–Hillman adducts and acyl diazomethanes

An efficient method for the regioselective synthesis of potentially biologically active tetrasubstituted 1-pyrazolines has been achieved via a 1,3-dipolar cycloaddition reaction. A range of tetrasubstituted 1-pyrazolines bearing one Boc group and two ester groups were obtained in high yields (up to 99%). The structure and relative stereochemistry of cycloadducts were confirmed by NMR spectra and single crystal X-ray diffraction. The possible mechanism was proposed and the major Z-cycloadducts as a single diastereomer could be separated from each other by chromatography.