Ring contraction and ring expansion reactions in terpenoid biosynthesis and their application to total synthesis

Terpenoids exhibit remarkable structural diversity, including highly complex ring-expanded or contracted carbocyclic skeletons. This review aims to explore intriguing examples of such ring-size alterations in all aspects of terpenoid synthesis. The current state-of-the-art regarding proposed biosynthetic pathways for terpenoids with unusual carbon skeleta, occurring either during initial cyclisation or subsequent oxidative tailoring, will be examined and discussed. Where possible, biogenetic relationships of closely related families of natural products will be contextualised by showing the mechanistic rationale for their interconversion. In the second part of this article, the application of bioinspired ring contraction and ring-expansion strategies in relevant natural product syntheses will be presented, demonstrating how synthetic chemistry can help to elucidate plausible biogenetic routes for structurally complex natural products.