Naturally Occurring [4+2] Type Terpenoid Dimers Assembled through Unmatched-electron-demand Cycloaddition

Terpenoid dimers of the [4+2] type, which are naturally occurring compounds biosynthetically derived from the [4+2] cycloaddition of two precursors, have garnered considerable attention due to their complex molecular structures, diverse biological activities, and intriguing biosynthetic pathways. We have previously summarized the advancements in three types of [4+2] terpenoid dimers. In this review, we will focus on the lesser-explored class of [4+2] terpenoid dimers which assembled from two electron-deficient precursors via the unmatched-electron-demand Diels-Alder reaction (UMEDDA). We will summarize their sources, biological activities, proposed biosynthesis, and chemical syntheses. Finally, a summary and outlook for this fascinating class of compounds will be presented.