Cycloaddition

In the Diels-Alder cycloaddition, a 1,3-diene compound and a conjugated alkene (dienophile) come together to form a cyclic compound.

The azide-alkyne Huisgen cycloaddition, also known as 'click reaction', involves the reaction of an azide and an alkyne to create a 1,2,3-triazole. This reaction has found extensive use in bioconjugation, drug discovery, and materials science.

In this reaction, two ethene molecules (each with a double bond) form a cyclobutane ring. The process can be illustrated as: \( \text{{Ethene}} + \text{{Ethene}} \rightarrow \text{{Cyclobutane}} \)

This reaction proceeds as: \( \text{{Ethylene}} + \text{{Ketene}} \rightarrow \text{{Oxetane}} \)

The 3+2 cycloaddition reaction between cyclopentadiene and ethylene produces a 7-membered ring with one double bond: \( \text{{Cyclopentadiene}} + \text{{Ethylene}} \rightarrow \text{{Product}} \)

An azomethine ylide and an alkene can come together to form a pyrrolidine in the following manner: \( \text{{Azomethine Ylide}} + \text{{Alkene}} \rightarrow \text{{Pyrrolidine}} \)

Fulvene and a carbonyl compound react to form a fused cyclohexane ring. It's done in the following manner: \( \text{{Fulvene}} + \text{{Carbonyl Compound}} \rightarrow \text{{Cyclohexane Ring}} \)

Here, the silver carbene and alkene react to yield a 6-membered ring: \( \text{{Silver Carbene}} + \text{{Alkene}} \rightarrow \text{{Cyclohexene Ring}} \)

The reaction can be represented as follows: \( \text{{Alkyne}}(2 \pi \text{{ electrons}}) + \text{{Azide}}(2 \pi \text{{ electrons}}) \rightarrow \text{{1,2,3-Triazole}} \) The generation of 1,2,3-triazole demonstrates how the 1,3 dipolar cycloaddition mechanism helps synthesizing heterocyclic compounds.

This can be represented as: \( \text{{Carbonyl Oxide}}(2 \pi \text{{ electrons}}) + \text{{Alkene}}(2 \pi \text{{ electrons}}) \rightarrow \text{{Five-membered cyclic product}} \) Here, the 1,3-dipole (carbonyl oxide) and dipolarophile (alkene) interact to form a five-membered cyclic product called an isoxazoline.

One such case is the formation of natural products called gibberellins, plant hormones responsible for growth and development. The final step in the biosynthesis of the backbone of these hormones is a [4+2] cycloaddition. \( \text{{Conjugated diene}} + \text{{Dienophile}} \rightarrow \text{{Gibberellin Backbones}} \)

\( \text{{Conjugated diene}} + \text{{Intramolecular Dienophile}} \rightarrow \text{{Strychnine}} \) This illustrates the powerful application of cycloaddition in the synthesis of highly complex organic compounds.