Selective Non-catalytic Cyclopropanation of Methyl (E)-Pent-3-en-1-yn-3-ylcarboxylates with Diazomethane

1,3-Dipolar cycloaddition of diazomethane to methyl (E)-pent-3-en-1-yn-3-ylcarboxylates proceeds selectively with participation of only the double bond, resulting in unstable methyl 3-alkynylpyrazolinylcarboxylates, thermolysis of which leads to methyl 1-alkynylcyclopropanecarboxylates and 4-methylpent-3-en-1-yn-3-ylcarboxylates.