Regioselective Synthesis of 3,4,5-Trisubstituted Pyrazoles from 2,3-Allenoates and a Sterically Hindered Diazo Ketone: Expanding the Scope of 1,3-Dipolar Cycloaddition

Novel 3,4,5-trisubstituted pyrazoles were synthesized from keto-stabilized 2,3-allenoates and a sterically hindered diazoketone (derived from α-amino-β-phenylpropionic acid) in acetonitrile under thermal conditions. The reaction occurred in a regio- and stereoselective fashion, positioning the bulky substituents in the diazoketone moiety of the resulting pyrazoles at the maximal distance from the ester group. The physical properties of the synthesized compounds were investigated.