cycloadduct

A cycloadduct is the product formed when two unsaturated molecular fragments undergo a cycloaddition reaction to create a new ring system. The process typically involves the formation of two or more new sigma bonds from pi bonds and proceeds through a pericyclic, often concerted, mechanism. Cycloadducts are common in organic synthesis and can be bicyclic or polycyclic, depending on the geometry and substituents of the starting fragments.

The best-known example is the Diels–Alder reaction, a [4+2] cycloaddition between a conjugated diene and a dienophile. The reaction yields a cyclohexene framework and is frequently stereospecific, producing distinct endo and exo products with the endo preference common for many dienes and dienophiles. The Diels–Alder cycloadducts are widely used as building blocks in natural product synthesis and material science.

Other cycloadditions include [2+2] cycloadditions, which are thermally forbidden in many cases but can proceed under photochemical activation to give cyclobutane derivatives. [3+2] cycloadditions, such as nitrones or azomethine ylides reacting with dipolarophiles, form five-membered rings and are prominent in 1,3-dipolar cycloadditions. The governing principles of orbital symmetry explain which cycloadditions are allowed under thermal conditions versus those requiring light.

Cycloadducts can sometimes be isolated as stable compounds, while others are reactive intermediates that undergo subsequent transformations. The term is used broadly to describe any ring-containing product resulting from a cycloaddition.