Gosteli−Claisen Rearrangement of Propargyl Vinyl Ethers: Cascading Molecular Rearrangements

One-pot synthesis of tetrasubstituted 2-aminofurans via Au(I)-catalyzed cascade reaction of ynamides with propargylic alcohols

One-pot synthesis of fully substituted 2-aminofurans via a Au(I)-catalyzed cascade reaction of ynamides and propargylic alcohol was realized. Hydroalkoxylation of ynamide with propargylic alcohol, Saucy-Marbet rearrangement, and cyclization of the resultant 3,4-dienamide sequentially proceeds in a one-pot reaction under highly mild conditions to give fully substituted 2-aminofurans.

Cascade Radical Cyclization on Alkynyl Vinylogous Carbonates for the Divergent Synthesis of Tetrasubstituted Furans and Dihydrofurans

A single alkynyl vinylogous carbonate was elaborated to tetrasubstituted furan or dihydrofuran via a cascade inter-intramolecular radical reaction by changing the radical being added. The strategy could be used in the synthesis of polycyclic heterocycles as well as bis-furan exhibiting atropisomerism. Installation of a new furan motif on the existing one was feasible by iteration. Stannyl dihydrofuran derivative was used in Stille coupling, whereas intramolecular Friedel-Crafts acylation on the furan gave furanonaphthol.

Silver & gold-catalyzed routes to furans and benzofurans

Silver and gold have become incredibly versatile and mild catalysts for numerous transformations, especially in heterocycle synthesis. For the most prominent of them, i.e. furans, silver and gold, with their unique reactivity and mildness, allow numerous possible routes to highly substituted and/or functionalized furans from a large variety of starting materials. Silver and gold catalysis provide thus the most flexible way to this important family of compounds. The present review describes these silver and gold-catalyzed routes, with some emphasis on mechanistic aspects, and proposes a comparison of both the silver and the gold-catalyzed syntheses of furans.

Synthesis of Allenes through Triazole Gold(III) Catalysed Rearrangement of Propargyl Vinyl Ethers

An efficient and simple method to the synthesis of allene derivatives was developed through pyridyltriazole gold(III) catalysed rearrangement of propargyl vinyl ethers with moderate to good yields. It was found that the pyridyltriazole gold(III) complex is a good chemoselective catalyst in selective activation of alkynes and allenes. Compared to gold(I) catalysts, the pyridyltriazole gold(III) shows much better substrate tolerance and much higher air and moisture stability.

Direct access to 2-amino-5-azidomethylfurans through palladium-catalyzed azidative cycloisomerization of homoallenyl amides

A room temperature Pd-catalyzed azidative cycloisomerization of homoallenyl amides using TMSN₃ as the azidation reagent and PhI(O₂CCF₃)₂ as the oxidant is described, producing various 2-amino-5-azidomethylfurans in moderate to excellent yields.

Crystal structure of N'-[(E)-(1S,3R)-(3-isopropyl-1-methyl-2-oxo-cyclo-pent-yl)methyl-idene]-4-methyl-benzene-sulfono-hydrazide

The title compound, C17H24N2O3S, was synthesized in order to determine the relative configuration of the corresponding β-keto aldehyde. In the U-shaped mol-ecule, the five-membered ring approximates an envelope, with the methyl-ene C atom adjacent to the quaternary C atom being the flap, and the methyl and isopropyl substituents lying to the same side of the ring. The dihedral angles between the four nearly coplanar atoms of the five-membered ring and the flap and the aromatic ring are 35.74 (15) and 55.72 (9)°, respectively. The bond angles around the S atom are in the range from 103.26 (12) to 120.65 (14)°. In the crystal, mol-ecules are linked via N-H⋯O hydrogen bonds, forming a chain along the a axis.

Divergent Synthesis of 2-Aminofurans via Palladium-Catalyzed Acetoxylative, Alkoxylative, and Hydroxylative Cycloisomerization of Homoallenyl Amides

A fine-tunable transformation, including Pd-catalyzed acetoxylative, alkoxylative, and hydroxylative cycloisomerization of homoallenyl amides, has been realized with hypervalent iodine organic compounds as the oxidants, giving polysubstituted 2-aminofurans in promising yields at room temperature. The selective formation of three different types of products from the same starting materials makes this reaction particularly attractive and useful for organic synthesis.

Crystal structure of (E)-N′-{[(1R,3R)-3-isopropyl-1-methyl-2-oxocyclopentyl]methylidene}-4-methylbenzenesulfonohydrazide

The title compound, C₁₇H₂₄N₂O₃S, was synthesized in order to determine the relative configuration of the corresponding β-keto aldehyde. In the U-shaped mol­ecule, the five-membered ring approximates an envelope with the methyl­ene atom adjacent to the quaternary C atom being the flap. The dihedral angles between the four nearly coplanar atoms of the five-membered ring and the flap and the aromatic ring are 38.8 (4) and 22.9 (2)°, respectively. The bond angles around the S atom are in the range 104.11 (16)–119.95 (16)°. In the crystal, mol­ecules are linked via N—H⋯O by hydrogen bonds, forming a chain along the a-axis direction.

DABCO-catalyzed synthesis of trifluoromethylated furans from propargyl alcohols and methyl 2-perfluoroalkynoate

The DABCO-catalyzed reaction of propargyl alcohols with methyl 2-perfluoroalkynoate to give trifluoromethylated furans in up to 98% yield under mild conditions has been developed. The established allene-enol and control experiments indicate that the reaction should proceed through a Michael addition and Claisen rearrangement/cyclization process.

Chiral N-Heterocyclic Carbene Catalyzed Annulations of Enals and Ynals with Stable Enols: A Highly Enantioselective Coates-Claisen Rearrangement

A combination of a chiral N-heterocyclic carbene catalyst and α,β-unsaturated aldehyde leads to a catalytically generated α,β-unsaturated acyl azolium, which participates in a highly enantioselective annulation to give dihydropyranone products. This full account of our investigations into the scope and mechanism of this reaction reveals the critical role of both the type and substitution pattern of the chiral triazolium precatalyst in inducing and controlling the stereochemistry. In an effort to explain why stable enols such as naphthol, kojic acid, and dicarbonyl are uniquely efficient, we have postulated that this annulation occurs via a Coates-Claisen rearrangement that invokes the formation of a hemiacetal prior to a sigmatropic rearrangement. Detailed kinetic investigations of the catalytic annulation are consistent with this mechanistic postulate.