Convergent Synthesis of 2H-Chromenes - a Formal [3+3] Cycloaddition by a One-pot, Three-Step Cascade

Synthesis of novel pyrano[2,3-f]chromene-dione derivatives using phosphoric acid-functionalized silica-coated Fe3O4 nanoparticles as a new reusable solid acid nanocatalyst

In this research, the synthesis of novel indeno[1,2-b]pyrano[2,3-f]chromene-2,12(13H)-dione derivatives in the presence of a newly introduced magnetically recoverable nanosolid acid catalyst is reported. At the first, phosphoric acid-functionalized silica-coated Fe₃O₄ nanoparticles (Fe₃O₄@SiO₂-(CH₂)₃OPO₃H₂) were prepared and well characterized using infrared spectroscopy (FT-IR), X-ray diffraction (XRD), scanning electron microscopy (SEM), vibrating sample magnetometer (VSM), and energy-dispersive X-ray spectroscopy (EDS) techniques. Then, the catalytic activity of the prepared Fe₃O₄@ SiO₂-(CH₂)₃OPO₃H₂ nanocatalyst was investigated for the synthesis of novel indeno[1,2-b]pyrano[2,3-f]chromene-2,12(13H)-dione derivatives via a one-pot and three-component condensation between 5,7-dihydroxy-4-methylcoumarin, indane-1, 3-dione, and various aromatic aldehydes under solvent-free condition. All the products are unknown, and their characterization was performed with the spectral data information obtained from their FT-IR, ¹H and ¹³CNMR, elemental analysis, and their melting points. The reusability study of the introduced nanosolid acid catalyst showed that the catalytic stability is almost completely remained up to five consecutive runs.

Recent Advances in Oxa-6π Electrocyclization Reactivity for the Synthesis of Privileged Natural Product Scaffolds

The stunning advances in understanding the reactivity and selectivity principles of asymmetric pericyclic reactions have had a profound impact on the synthetic planning of complex natural products. Indeed, electrocyclizations, cycloadditions, and sigmatropic rearrangements enable synthetic chemists to craft highly functionalized scaffolds that would not otherwise be possible with a similar atom-, step-, and redox-economy. In this review, selected examples from the last two decades of research (2003–2020) on tandem processes combining oxa-6π electrocyclic reactions are discussed in terms of reactivity challenges, inherent reversibility, and key structural bond formation in the assembly of natural products. A particular emphasis is given to the electrocyclic ring-closures in the tandem processes featuring Knoevenagel-type condensations, Diels–Alder cycloadditions, Stille couplings, and oxidative dearomatizations. The synthetic manifolds reviewed here illustrate how oxa-6π electrocyclizations are intimately linked to the construction of complex natural product scaffolds and have inspired a number of biomimetic syntheses in the laboratory.

Nanofibre Sepiolite Catalyzed Green and Rapid Synthesis of 2-Amino-4H-chromene Derivatives

Nanofibre sepiolite catalyzed the rapid, clean, and highly efficient synthesis of 2-amino-4H-chromene derivatives by a one-pot, three-component condensation of a series of aldehydes, various enolizable C–H bonds (such as dimedone, α-naphthol, resorcinol, and 4-hydroxy-2H-chromen-2-one), and malononitrile in a mixture of water/ethanol. The present method offers several advantages such as high to excellent yields, short reaction times, mild reaction conditions, simple procedure, use of inexpensive, non-toxic, and naturally available catalyst, easy isolation of the products, and no need for column chromatography. The catalyst could be easily separated from the reaction mixture and can be reused for many consecutive trials without a significant decline in its reactivity.

A cascade Claisen rearrangement/o-quinone methide formation/electrocyclization approach to 2H-chromenes

A new approach to 8-substituted 2H-chromenes is developed, featuring a novel cascade aromatic Claisen rearrangement/o-quinone methide formation/6π-electrocyclization.

Total Syntheses of Proposed (±)-Trichodermatides B and C

Total syntheses of putative (±)-trichodermatides B and C are described. These efficient syntheses feature the oxa-[3 + 3] annulation strategy, leading to B and C along with their respective C2-epimers. However, these synthetic samples are spectroscopically very different from the natural products. DFT calculations of C13 chemical shifts are conducted and the predicted values are in good agreement with those of synthetic samples, thereby questioning in the accuracy of structural assignments of trichodermatides B and C.