Cycloadditions. Part I. Steric effects in the addition of diazoalkanes to αβ-unsaturated esters

Abstract

The addition of 2-diazopropane to methyl acrylate and its methyl-substituted analogues normally (i.e., when electronic effects are dominant) gives methyl 1-pyrazoline-3-carboxylates, of which those bearing a hydrogen atom at C(3) tautomerise readily to the Δ²-isomers. The orientation of addition is reversed in the reactions of methyl 3-methylbut-2-enoate with 2-diazopropane and diazoethane. Methyl 4,4-dimethylpent-2-enoate undergoes reverse addition with 2-diazopropane, but not with diazoethane or diazomethane. The reversals are attributed to the presence of unfavourable eclipsing interactions in the transition state for normal addition.