Highly strained unsaturated carbocycles

Highly strained unsaturated carbocyclic rings are high-energy compounds which are attracting increasing interest – not just for their peculiar structural and reactivity features – but also as modern chemical tools for bioorthogonal and in vivo chemistry, especially via inverse-electron demand Diels–Alder (iEDDA) and strain-promoted azide-alkyne cycloaddition (SPAAC) reactions. This mini-review covers two main classes of compounds: 3 to 10 membered-ring cycloalkynes and trans-cycloalkenes, including some examples of cyclic enynes, dienes and bridgehead alkenes. Their molecular properties, synthesis and reactivity are presented and discussed, with an emphasis on their functionalization and reactivity.