Biogenesis of Triterpene Dimers from Orthoquinones Related to Quinonemethides: Theoretical Study on the Reaction Mechanism

Naturally occurring dimeric triterpenoids: Occurrence, chemistry and bioactivities

The triterpenes represent one of the most reported subclasses of specialized metabolites from the plant kingdom. They play a key role in the protection of plants and their metabolism in addition to displaying a high structural diversity and large scale of biological activities. The scaffold can undergo several reactions like oxidation or substitution at different positions of the skeleton leading to the formation of several types of compounds. More specifically, triterpene dimer is a small group of compounds found in nature (from plants precisely). Until 2021, the chemical and pharmacological works reported in the literature indicated the identification of 90 natural dimeric triterpenes and 11 synthetic derivatives from 19 plants species and very few of them have been biologically evaluated for their antibacterial, antioxidant, antiproliferative or molluscicide activities. This review aims to compile the literature on the occurrence, chemistry and biological activities of the triterpenoid dimers. To attend this goal, a literature survey has been done in a number of online libraries including Scifinder, PubMed, Web of Science and Google Scholar using keywords terpene, triterpene, dimer, celastroloid without language restriction. This paper provides the easiest access to the information on triterpene dimers for readers and researchers in view to enhancing the continuity of research works on this topic.

Theoretical insight into the on-water catalytic effect in the biogenesis of triterpene dimers: from one-step to two-step hetero Diels–Alder reactions

An alternative pathway to the hetero Diels–Alder reaction for the biogenic origin of triterpene dimers is presented here. In this new pathway, the explicit water molecules take a fundamental role.

Biosynthesis of Grandione: An Example of Tandem Hetero Diels-Alder/Retro-Claisen Rearrangement Reaction?

Mechanistic theoretical studies about the feasibility of the traditional proposed mechanism of formation for icetexane diterpene dimer grandione were assessed using density functional method at the M06-2X/6-31G(d,p) level of theory. Bulk water solvent effects were taken into account implicitly using the polarizable continuum model (SCI-PCM). The results were compared with the selectivity found in the biomimetic synthesis performed by experimental research groups. The relative free energy calculation shows that the one-step H-DA formation mechanism nominated in the literature is not a viable mechanism. We found that an alternative competing Tandem pathway is consistent with the experimental trends. Thus, our results suggested that the compound grandione is formed via a H-DA/retro-Claisen rearrangement and not by the traditional H-DA mechanism proposed early in the experimental studies. The H-DA initial step produce a biecyclic adduct followed by a domino retro-Claisen rearrangement that releases the energy strain of the bicyclic intermediary. Steric issues and hyperconjugation interactions are the mainly factors driving the reaction nature and the selectivity in the formation reaction. Finally, the enzymatic assistance for dimer formation was analyzed in terms of the calculated transition state energy barrier.