Synthesis of [n,5]-spiroketals by ring enlargement of donor-acceptor-substituted cyclopropane derivatives

Organocatalytic Cloke-Wilson Rearrangement: Carbocation-Initiated Tandem Ring Opening/Cyclization of Cyclopropanes under Neutral Conditions

We report a metal-, acid-, and base-free 2-(bromomethyl)naphthalene (2-BMN)-promoted organocatalytic Cloke-Wilson rearrangement of chain doubly activated cyclopropanes for the construction of 2,3-dihydrofurans via a carbocation-initiated tandem intramolecular ring-opening/recyclization process. The strategy is especially suitable for the construction of furan units in complex molecules, providing a solution to the problem of heavy-metal residues in dihydrofuran-containing drugs synthesized by traditional metal-based protocols. Thus, it is of potential interest in synthetic and medicinal chemistry.

Synthesis of Spirocyclic Cyclopropyl Glycosyl-1-phosphate Analogues

A general methodology allowing the preparation of phosphonylated 1-spirocyclopropyl analogues of glycosyl-1-phosphates is reported. The scope of this reaction has been assessed using various exo-glycals easily obtained from the corresponding pyranoses and furanoses. The cyclopropanation was found to be stereospecific, and the cis/trans selectivity only depends on the E/Z configuration of the starting exo-glycal. The four possible isomers of spirocyclopropyl ribose-1-phosphonate could thus be prepared in a controlled manner, protected and deprotected.

Stereodivergent Synthesis of Biologically Active Spironucleoside Scaffolds via Catalytic Cyclopropanation of 4-exo-Methylene Furanosides

Cyclopropane fusion of the only rotatable carbon-carbon bond in furanosyl nucleosides (i.e., exocyclic 4'-5') is a powerful design strategy to arrive at conformationally constrained analogues. Herein, we report a direct stereodivergent route toward the synthesis of the four possible configurations of 4-spirocyclopropane furanoses, which have been transformed into the corresponding 4'-spirocyclic adenosine analogues. The latter showed differential inhibition of the protein methyltransferase PRMT5-MEP50 complex, with one analogue inhibiting more effectively than adenosine itself, demonstrating the utility of rationally probing 4'-5' side chain orientations.

Cu-catalyzed hydroxycyclopropanol ring-opening cyclization to tetrahydrofurans and tetrahydropyrans: short total syntheses of hyperiones

Tetrahydrofurans (THFs) and tetrahydropyrans (THPs) are important core scaffolds frequently found in many molecules of medicinal importance. Herein, we report a novel copper-catalyzed hydroxycyclopropanol ring-opening cyclization methodology to synthesize di- or tri-substituted THFs and THPs. In this reaction, a strained C–C bond was cleaved and a new Csp³–O bond was formed to produce the aforementioned O-heterocycles. The new THF synthesis features a broad substrate scope, scalability, and good functional-group tolerability. It enabled us to complete the shortest enantioselective syntheses of hyperiones A and B (3 and 4 steps, respectively), which is significantly shorter than the previously reported two total syntheses (≥10 steps).

A novel Cu-catalyzed hydroxycyclopropanol ring-opening cyclization was developed to synthesize substituted tetrahydrofuran/tetrahydropyran molecules including two norlignan natural products hyperiones A and B.

Recent Advances in Stereoselective Ring Expansion of Spirocyclopropanes: Access to the Spirocyclic Compounds

Spirocyclopropane represents a privileged structural scaffold for accessing synthetic libraries of densely functionalized spirocarbo- and heterocyclic compounds. Due to the ubiquity of spirocyclic motifs as a potent pharmacophore in natural products and pharmaceuticals, recent years have witnessed significant advances in developing synthetic strategies that exploits carbon-carbon bond scission in spirocyclopropanes. This paper summarizes the recent developments in stereoselective ring expansion of spirocyclopropanes in diversity-oriented synthesis and highlights the synthetic as well as mechanistic rationale of those methodologies. This review also encompasses the applicability of the protocols in bioactive natural product syntheses.

Ring-Opening 1,3-Aminochalcogenation of Donor-Acceptor Cyclopropanes: A Three-Component Approach

A 1,3-aminothiolation was realized by reacting 2-substituted cyclopropane 1,1-dicarboxylates with sulfonamides and N-(arylthio)succinimides. Under Sn(OTf)2 catalysis the transformation proceeded smoothly to the corresponding ring-opened products bearing the sulfonamide in the 1-position next to the donor and the arylthio residue in the 3-position next to the acceptor. The procedure was extended to the corresponding selenium analogues by employing N-(phenylseleno)succinimides as an electrophilic selenium source.

New Dyes Based on Extended Fulvene Motifs: Synthesis through Redox Reactions of Naphthoquinones with Donor-Acceptor Cyclopropanes and Their Spectroelectrochemical Behavior

Novel dyes based on extended fulvene motifs are reported. The carbon skeleton was generated by a catalyzed addition of donor-acceptor cyclopropanes to naphthoquinone. The hydroxy group at the central ring of the tricyclic fulvene motif was converted into the triflate, which reacted efficiently with a wide range of nucleophiles, resulting in substitution and thereby providing new derivatives. The synthetic versatility allowed us to investigate the absorption, electrochemical, and UV/Vis-NIR spectroelectrochemical properties of these dyes as a function of the substituents. The dyes were shown to participate in reductive electrochemistry, the reversibility of which can be improved by appropriate selection of the substituents. Additionally, first signs of NIR electrochromism are presented, opening new avenues for the future investigations of such dyes.

Asymmetric Sulfur-Ylide-Mediated Formal [4+1]-Annulation Reaction: Scope and Mechanism

A formal [4+1]-annulation strategy between sulfur ylides and 1,3-dienes was developed to afford functionalized cyclopentanoids. The process consists of a stereoselective cyclopropanation reaction followed, in situ, by a stereospecific MgI₂ -catalyzed vinylcyclopropane-cyclopentene rearrangement. The use of chiral sulfur ylides provided cyclopentanoids with excellent enantiocontrol. A combined experimental and computational mechanistic study showed that the stereospecificity of the rearrangement could be accounted for by a double S_N 2 reaction mechanism involving iodide.

Synthesis of 5-C-Methylated d-Mannose, d-Galactose, l-Gulose, and l-Altrose and Their Structural Elucidation by NMR Spectroscopy

C5/C6-Spirocyclopropanation of exocyclic enol esters followed by alkali ring-opening of the three-membered ring was used for the diastereoselective preparation of 5-C-methylated d-mannose, d-galactose, l-gulose, and l-altrose. Extensive NMR studies demonstrated an increase of furanose form by 5-C-methylation in almost all cases.

The synthesis of pyrroles and oxazoles based on gold α-imino carbene complexes

Cationic gold complexes of α-oximimino carbenes have been identified to react with weak nucleophiles including enol ethers and nitriles. These findings allowed us to develop the highly efficient synthesis of pyrroles and oxazoles.

Ring-Opening 1,3-Halochalcogenation of Cyclopropane Dicarboxylates

Donor-acceptor cyclopropanes with two geminal carboxylic esters are reacted with chalcogenyl chlorides and bromides to afford ring-opened products bearing the halogen atoms in the 1-position, adjacent to the donor, and the chalcogenyl residue in the 3-position next to the two acceptor groups. A variety of different donors (e.g., aryl, N, and O) are used. The stereospecificity of the reaction is demonstrated by using a chiral starting material.

Copper/P(tBu)3 -Mediated Regiospecific Synthesis of Fused Furans and Naphthofurans

A novel [3+2] cycloaddition between a variety of cyclic ketones and diverse olefins or alkynes can be effectively promoted by copper in combination with the tri-tert-butylphosphine [P(tBu)₃ ] ligand. This protocol exhibits excellent selectivity and provides an exemplary set of fused heterocycles in good to excellent yields. Present strategy also represents an extremely simple and atom-economic way to construct substituted fused furans and naphthofurans from readily available starting materials under mild reaction conditions. The utility of the method is further demonstrated by the synthesis of chiral furans from (R)-(-)-carvone and (S)-(+)-carvone. A plausible mechanism involving the oxidative radical cyclization has been suggested based on experimental observations.

[3 + 3]-Cycloaddition of Donor-Acceptor Cyclopropanes with Nitrile Imines Generated in Situ: Access to Tetrahydropyridazines

Donor-acceptor cyclopropanes are reacted under the influence of a Lewis acid with hydrazonyl chlorides to afford tetrahydropyridazines. Formally, this transformation can be regarded as a [3 + 3]-cycloaddition of three-membered rings and nitrile imines generated in situ. This efficient method provides fast access to a variety of structurally diverse pyridazine derivatives. The structure of a typical product was confirmed by X-ray crystallography.

Stereoselective synthesis of 1,6-dioxaspirolactones from spiro-cyclopropanecarboxylated sugars: total synthesis of dihydro-pyrenolide D

A one-pot protocol for the stereoselective construction of γ-spiroketal γ-lactone frameworks from sugar derived spiro-cyclopropanecarboxylic acids involving a ring enlargement and cyclization reaction is revealed.