Cycloadditions. Part II. A steric effect in the 1,3-dipolar addition of 2-diazopropane to methyl but-2-ynoate

Abstract

The predominant mode of 1,3-dipolar addition of 2-diazopropane to methyl but-2-ynoate is the opposite of that predicted on electronic grounds, 4-methoxycarbonyl-3,3,5-trimethyl-3H-pyrazole and 5-methoxycarbonyl-3,3,4-trimethyl-3H-pyrazole being obtained in the ratio 6 : 1. In contrast, methyl propiolate behaves normally with 2-diazopropane, giving solely 3,3-dimethyl-5-methoxycarbonyl-3H-pyrazole. The preference for reverse addition to methyl but-2-ynoate is attributed to steric factors in the transition states for cycloaddition.

The structures of the 3H-pyrazoles were established by photolysis, the diazoalkenes initially generated being trapped with acetic acid as acetoxy-esters. In the absence of acetic acid, the diazo-intermediates underwent further photolysis to cyclopropenes.