Brønsted Acid-Catalyzed Intramolecular Tandem Double Cyclization of γ-Hydroxy Acetylenic Ketones with Alkynes into Naphtho[1,2-b]furan-3-ones

A metal-free and atom-economic route for the synthesis of naphtho[1,2-b]furan-3-ones has been realized via p-TsOH·H2O-catalyzed intramolecular tandem double cyclization of γ-hydroxy acetylenic ketones with alkynes in formic acid. The benzene-linked furanonyl-ynes are the key intermediates obtained by the scission/recombination of C-O double bonds. Further, the structural modifications of the representative product were implemented by reduction, demethylation, substitution, and [5 + 2]-cycloaddition.

Rh2(II)-Catalyzed Enantioselective Intramolecular Büchner Reaction and Aromatic Substitution of Donor-Donor Carbenes

The chiral dirhodium(ii) tetracarboxylate-catalyzed enantioselective intramolecular Büchner reaction of donor/donor-carbenes was reported and a series of valuable chiral polycyclic products were synthesized. Both aryloxy enynones and diazo compounds were efficient carbene precursors for this reaction. Excellent yields (up to 99%) and outstanding enantioselectivities (up to >99% ee) were achieved under standard conditions. For furyl substituted chiral cyclohepta[b]benzofurans bearing a substituent at the C4 position on cycloheptatrienes, control reactions showed that the chiral Büchner products could slowly racemize either under dark or natural light conditions. A diradical-involved mechanism rather than a zwitterionic intermediate was proposed to explain the racemization. Furthermore, furyl substituted chiral fluorene derivatives were obtained via asymmetric aromatic substitution when biaryl enynones were employed as carbene precursors.

The chiral dirhodium(ii) tetracarboxylate-catalyzed enantioselective intramolecular Büchner reaction and aromatic substitution of donor/donor-carbenes were reported and a series of valuable chiral polycyclic products were synthesized.

Enantioselective Rh(II)-Catalyzed Desymmetric Cycloisomerization of Diynes: Constructing Furan-Fused Dihydropiperidines with an Alkyne-Substituted Aza-Quaternary Stereocenter

Described herein is an enantioselective dirhodium(II)-catalyzed cycloisomerization of diynes achieved by the strategy of desymmetrization, which not only represents a new cycloisomerization reaction of diynes but also constitutes the first Rh(II)-catalyzed asymmetric intramolecular cycloisomerization of 1,6-diynes. This protocol provides a range of valuable furan-fused dihydropiperidine derivatives with an enantiomerically enriched alkynyl-substituted aza-quaternary stereocenter in high efficiency, complete atom economy, and excellent enantioselectivity (up to 98% ee). Besides, the highly functionalized products could be easily transformed into various synthetically useful building blocks and conjugated with a series of pharmaceutical molecules. The mechanism involving a concerted [3+2] cycloaddition/[1,2]-H shift of the Rh(II) carbenoid intermediate was elucidated by DFT calculations and mechanistic studies. More importantly, the first single crystal of alkyne-dirhodium(II) was obtained to show that a η²-coordinating activation of alkynal by dirhodium(II) was involved. Weak hydrogen bondings between the carboxylate ligands and alkynal were found, which probably made the well-defined paddlewheel-like dirhodium(II) distinctive from other metal complexes in catalyzing this transformation. Furthermore, the origin of the enantioselectivity was elucidated by a Rh₂(R-PTAD)₄-alkyne complex and additional calculational studies.

Gold-Catalyzed Synthesis of Small Rings

Three- and four-membered rings, widespread motifs in nature and medicinal chemistry, have fascinated chemists ever since their discovery. However, due to energetic considerations, small rings are often difficult to assemble. In this regard, homogeneous gold catalysis has emerged as a powerful tool to construct these highly strained carbocycles. This review aims to provide a comprehensive summary of all the major advances and discoveries made in the gold-catalyzed synthesis of cyclopropanes, cyclopropenes, cyclobutanes, cyclobutenes, and their corresponding heterocyclic or heterosubstituted analogs.