Synthesis of aromatic spiroacetals related to γ-rubromycin based on a 3H-spiro[1-benzofuran-2,2′-chromane] skeleton

Computational insight into the mechanism and origin of high regioselectivity in the ring-opening cyclization of spirocyclopropanes with stabilized sulfonium ylides by the DFT

Chromanes with high bioactivity play an important role in nature, and cyclization reactions of cyclopropanes with sulfonium ylides to form chromane skeletons have attracted great attention of scientists. The mechanism as well as origins of regioselectivity and stereoselectivity for the ring-opening/cyclization reactions between cyclohexane-1,3-dione-2-spirocyclopropanes and stabilized sulfonium ylides in CH₂Cl₂ were investigated by using the density functional theory (DFT) M06-2X/6-311+G(d,p)//M06-2X/6-31G(d,p) method combined with the SMD model. The calculated results revealed that the reaction process involved two key steps: the ring-opening step and the cyclization step, with the former being the rate-determining and stereoselectivity-determining step. The regioselectivity of the ring-opening step of spirocyclopropane indicated that the tertiary carbon was more preferential than the secondary one when sulfonium ylide attacked spirocyclopropane. The theoretical results confirmed that the stereoselectivity of the reaction to form the trans-isomer product is more favorable than the cis-isomer, and the calculated trans/cis ratio is in accordance with the experiment. Moreover, the conceptual density functional theory reactivity indices suggest that the electronic effect controls the regioselectivity. What is more, the stereoselectivity analyzed by weak non-covalent interaction also shows the importance of electronic effect. Graphical Abstract.

Retrosynthetic strategies and their impact on synthesis of arcutane natural products

Decisions, decisions, decisions: the interplay between different retrosynthetic strategies in the synthesis of the highly bridged, polycyclic arcutane natural products.

Retrosynthetic analysis is a cornerstone of modern natural product synthesis, providing an array of tools for disconnecting structures. However, discussion of retrosynthesis is often limited to the reactions used to form selected bonds in the forward synthesis. This review details three strategies for retrosynthesis, focusing on how they can be combined to plan the synthesis of polycyclic natural products, such as atropurpuran and the related arcutane alkaloids. Recent syntheses of natural products containing the arcutane framework showcase how these strategies for retrosynthesis can be combined to plan the total synthesis of highly caged scaffolds. Comparison of multiple syntheses of the same target provides a unique opportunity for detailed analysis of the impact of retrosynthetic disconnections on synthesis outcomes.

Antimicrobial Activity of Naturally Occurring Phenols and Derivatives Against Biofilm and Planktonic Bacteria

Biofilm-forming bacteria present formidable challenges across diverse settings, and there is a need for new antimicrobial agents that are both environmentally acceptable and relatively potent against microorganisms in the biofilm state. The antimicrobial activity of three naturally occurring, low molecular weight, phenols, and their derivatives were evaluated against planktonic and biofilm Staphylococcus epidermidis and Pseudomonas aeruginosa. The structure activity relationships of eugenol, thymol, carvacrol, and their corresponding 2- and 4-allyl, 2-methallyl, and 2- and 4-n-propyl derivatives were evaluated. Allyl derivatives showed a consistent increased potency with both killing and inhibiting planktonic cells but they exhibited a decrease in potency against biofilms. This result underscores the importance of using biofilm assays to develop structure-activity relationships when the end target is biofilm.

An Approach to the Synthesis of 1-Propenylnaphthols and 3-Arylnaphtho[2,1-b]furans

A simple and efficient strategy for the synthesis of 1-propenylnaphthols from readily accessible 3-arylallylnaphthyl ethers has been developed. By using K₂CO₃ as base and 2-methoxyethanol as solvent, direct access to a wide range of 1-propenylnaphthols can be achieved in generally good yield (up to 99%) with high stereoselectivity toward the Z isomer. The control experiments indicate that the reaction proceeds through a sequential Claisen rearrangement/isomerization process. Furthermore, starting from the same material, the highly valuable 3-arylnaphtho[2,1-b]furans can be obtained in N,N-dimethylformamide and in the presence of Ag₂O as the oxidant via a one-pot sequential Claisen rearrangement/isomerization/cyclization reaction. Mechanistic studies confirm that 1-propenylnaphthols are the key intermediates to form the 3-arylnaphtho[2,1-b]furans. In addition, these two operationally simple and practical protocols could be scaled up to a gram level.

Total synthesis of (±)-δ-rubromycin

Transition-metal-catalyzed spiroketalization cyclization has been performed successfully and has led to the first total synthesis of (±)-δ-rubromycin with a longest linear sequence of 18 steps from commercially available guaiacol in a 2.7% overall yield.

Stereoselective synthesis of the rocaglamide skeleton via a silyl vinylketene formation/[4 + 1] annulation sequence

The tricyclic core of the cyclopentabenzofurans has been prepared in an efficient and stereoselective manner utilizing an intramolecular silyl vinylketene formation/[4 + 1] annulation sequence. This novel approach affords the ABC ring system where the adjacent phenyl and aryl substituents of the C ring have the required cis relationship.

Facile synthesis of naphthoquinone spiroketals by diastereoselective oxidative [3 + 2] cycloaddition

A highly selective oxidative [3 + 2] cycloaddition of chiral enol ethers and hydroxynaphthoquinone is described. This convergent strategy is amenable to an enantioselective synthesis of beta-rubromycin and related naphthoquinone spiroketals. Several compounds were found to inhibit DNA-polymerase and telomerase in a manner resembling alpha-rubromycin and beta-rubromycin.