Biogenesis of sesquiterpene lactones pseudoguaianolides from germacranolides: theoretical study on the reaction mechanism of terminal biogenesis of 8-epiconfertin

Biomimetic Domino Knoevenagel/Cycloisomerization Strategy for the Synthesis of Citridone A and Derivatives

Studies on Knoevenagel condensations between conjugated dienals and 4-hydroxy-2-pyridone/quinolone-type 1,3-dicarbonyl equivalents led to the development of a simple one-pot strategy to access citridone A and related synthetic cyclopenta[b]furopyridones/quinolones. The present work highlights the power of domino cascades in the synthesis of natural product frameworks and may help promote future studies on this promising new class of pyridone alkaloids.

Germacrone Derivatives as new Insecticidal and Acaricidal Compounds: A Structure-Activity Relationship

Currently, the use of synthetic pesticides is the main method of plant protection applied in agri- and horticulture. However, its excessive use leads to the development of pesticide resistance, a contamination of the environment, toxicity to non-target organisms, and risks for human health. With the ultimate aim of contributing to the develop of a more sustainable pest management, we used the natural product germacrone (compound 1), reported to possess significant insecticidal activity, as starting material for the generation of molecular diversity (2-24). Some of the generated derivatives are natural compounds, such as 1,10-epoxygermacrone (2), 4,5-epoxygermacrone (3), gajutsulactone A (7), germacrol (11), isogermacrone (14), 9-hydroxyeudesma-3,7(11)dien-6-one (19), eudesma-4,7(11),dien-8-one (20), eudesma-3,7(11)-dien-8-one (21) and eudesma-4(15),7(11)-dien-8-one (22). Compounds, 7,11-9,10-diepoxigermacr-4,5-en-8-ol (17), 7,11-epoxieudesma-4,7(11)-dien-8-one (23) and 7,11-epoxieudesma-3,7(11)-dien-8-one (24) are described for the first time. The biocidal activity of most of these compounds was assayed against the tick Hyalomma lusitanicum. The acaricidal effects of compound 24 were four times higher than that of germacrone (1). Compound 2 is an insect antifeedant a thousand times more potent than germacrone against Rhopalosiphum padi, which makes this substance a promising selective antifeedant against this cereal pest.

15-Hydroxygermacranolides as Sources of Structural Diversity: Synthesis of Sesquiterpene Lactones by Cyclization and Rearrangement Reactions. Experimental and DFT Study

A study on the electrophile-induced rearrangement of two 15-hydroxygermacranolides, salonitenolide and artemisiifolin, was carried out. These compounds underwent electrophilic intramolecular cyclizations or acid-mediated rearrangements to give sesquiterpene lactones with different skeletons such as eudesmanolides, guaianolides, amorphanolides, or other germacranolides. The cyclization that gives guaianolides can be considered a biomimetic route to this type of sesquiterpene lactones. The use of acetone as a solvent changes the reactivity of the two starting germacranolides to the acid catalysts, with a 4,15-diol acetonide being the main product obtained. The δ-amorphenolide obtained by intramolecular cyclization of this acetonide is a valuable intermediate for accessing the antimalarials artemisinin and its derivatives. Mechanistic proposals for the transformations are raised, and to provide support them, quantum chemical calculations [DFT B3LYP/6-31+G(d,p) level] were undertaken.

Are boat transition states likely to occur in Cope rearrangements? A DFT study of the biogenesis of germacranes

It has been proposed that elemanes are biogenetically formed from germacranes by Cope sigmatropic rearrangements. Normally, this reaction proceeds through a transition state with a chair conformation. However, the transformation of schkuhriolide (germacrane) into elemanschkuhriolide (elemane) may occur through a boat transition state due to the final configuration of the elemanschkuhriolide, but this transition state is questionable due to its high energy. The possible mechanisms of this transformation were studied in the density functional theory frame. The mechanistic differences between the transformation of (Z,E)-germacranes and (E,E)-germacranes were also studied. We found that (Z,E)-germacranolides are significantly more stable than (E,E)-germacranolides and elemanolides. In the specific case of schkuhriolide, even when the boat transition state is not energetically favored, a previous hemiacetalization lowers enough the energetic barrier to allow the formation of a very stable elemanolide that is even more stable than its (Z,E)-germacrane.

A theoretical biogenesis overview of diterpenes isolated from Salvia microphylla

Establishing the reaction mechanisms involved in the biosynthesis of natural products plays an important role in phytochemistry and pharmacology. Mechanistic studies of the biogenesis of natural products have been mainly explored by means of theoretical calculations, and taking into account experimental structures of reagents and products. Using a hybrid meta density functional theory method (mPW1B95), we studied the mechanisms associated with the biogenesis of five neo-clerodanes isolated from Salvia mycrophylla. The reaction mechanisms presented here explain the formation of the five neo-clerodanes and coincides with the formulated biogenetic hypothesis. Graphical Abstract Proposed biogenesis of diterpenes isolated from Salvia microphylla.

Memory effects in carbocation rearrangements: structural and dynamic study of the norborn-2-en-7-ylmethyl-X solvolysis case

The solvolysis of two diastereomers may give the same two products, but in different ratios, notwithstanding the fact that the two reaction pathways share an apparently identical intermediate carbocation. This has been dubbed the "memory effect", since the initial carbocation seems to "remember" its origin when undergoing further evolutions through multistep rearrangements. This puzzling result was studied theoretically for the case of the solvolysis of norborn-2-en-7-ylmethyl-X systems by defining the reaction potential energy surface (PES) and then carrying out a dynamical study. The PES shows that upon X(-) loss, multiphase rearrangements concertedly yield the two stablest carbocations, G and L. These carbocations are connected by a transition structure. The carbocation intermediates proposed in the literature do not correspond to any stationary point. The preference for the rearrangement to G or L (the memory effect) is determined by structural and stereoelectronic effects: the competitive interaction between an empty p orbital with a σ orbital or a p/π orbital is guided by geometrical aspects present in the starting carbocations. The dynamical study shows that (1) G and L do not interconvert and (2) the evolving system can switch from one pathway to the other to different extents, thus determining a more or less pronounced memory loss (the leakage).

Biosynthesis via carbocations: theoretical studies on terpene formation

This review describes applications of quantum chemical calculations in the field of terpene biosynthesis, with a focus on insights into the mechanisms of terpene-forming carbocation rearrangements arising from theoretical studies.

The renaissance of alpha-methylene-gamma-butyrolactones: new synthetic approaches

The amount of research activity concerning alpha-methylene-gamma-butyrolactones and alpha-alkylidene-gamma-butyrolactones has increased dramatically in recent years. This Review summarizes the structural types, biological activities, and biosynthesis of these compounds, concentrating on publications from the past 10 years. Traditional approaches to alpha-methylene-gamma-butyrolactones and alpha-alkylidene-gamma-butyrolactones are then reviewed together with novel approaches, including those from our own research group, reported more recently.