Recent advances in the semi-pinacol rearrangement of α-hydroxy epoxides and related compounds

9-Membered Carbocycles: Strategies and Tactics for their Synthesis

Many natural products comprising a nine-membered carbocyclic core structure exhibit interesting biological effects. However, only a minority have succumbed to their synthesis in the past. The synthesis of functionalized nine-membered carbocycles still remains a challenging goal for synthetic chemists, mainly due to their high ring strain. Different strategies to overcome the unfavorable enthalpic and entropic factors associated with their formation are highlighted in this Concept article. The presented methods are classified into two different categories: (1) the ring-expansion of smaller rings or the ring-contraction of larger rings and (2) the direct cyclization of acyclic precursors.

Asymmetric Synthesis of Cyclobutanone via Lewis Acid Catalyzed Tandem Cyclopropanation/Semipinacol Rearrangement

Chiral Lewis acid catalyzed asymmetric formation of cyclobutanones from α-silyloxyacroleins and α-alkyl or α-aryl diazoesters has been developed. In the presence of a chiral oxazaborolidinium ion catalyst, various α-silyloxycyclobutanones possessing a chiral β-quaternary center were synthesized in high yield (up to 91%) with excellent enantio- and diastereoselectivity (up to 98% ee and up to >20:1 dr) through tandem cyclopropanation/semipinacol rearrangement. The synthetic potential of this method was illustrated by conversion of the product to various cyclic compounds such as γ-lactone, cyclobutanol, and cyclopentanone.

Synthesis of α-oxygenated β,γ-unsaturated ketones by a catalytic rearrangement strategy

A straightforward two-step entry to α-oxgenated β,γ-unsaturated ketones from readily available α,β-unsaturated ketones is disclosed. It was found that bis(allylic) alcohols undergo a skeletal rearrangement in the presence of 1 mol% of cheap and non-corrosive p-toluenesulfonic acid. Computational studies were conducted to support the mechanism and to rationalise the influence of the catalyst acidity on the product selectivity.

Photoredox-promoted alkyl radical addition/semipinacol rearrangement sequences of alkenylcyclobutanols: rapid access to cyclic ketones

Two photo-catalytic tandem alkyl radical addition/semipinacol rearrangement reactions of cycloalkanol-substituted styrenes with N-acyloxyphthalimides and O-acyl oximes have been documented. These protocols provide efficient access to functionalized cyclic ketones, and feature mild conditions (i.e., visible light irradiation, redox neutral and room temperature), broad substrate scope and excellent functional group tolerance.

A catalytic allylic cation-induced intermolecular allylation-semipinacol rearrangement

A catalytic intermolecular semipinacol rearrangement induced by allylic carbocations has been realized. This tandem reaction is highly efficient under the catalysis of ZnBr₂, generating a wide range of α-homoallyl substituted ketones which contain all-carbon quaternary centres in good to excellent yields (up to 98%) with moderate to high diastereoselectivities (up to >20 : 1). Synthetic application of this novel methodology in the construction of core structures of natural products is also reported.

Total Synthesis of ent-Ascospiroketal B

Ascospiroketal B was isolated from a marine-derived fungus as a structurally unique polyketide possessing a rare tricyclic core including 5,5-spiroketal-γ-lactone. An asymmetric total synthesis of ent-ascospiroketal B was achieved using an original synthetic route. The synthesis included the stereoselective construction of 5,5-spiroketal for ascospiroketal B and stereocontrolled construction of a quaternary asymmetric carbon by rearrangement of a trisubstituted epoxide.

Gold-catalyzed stereoselective dearomatization/metal-free aerobic oxidation: access to 3-substituted indolines/oxindoles

An unprecedented dearomatization of indoles with diazoesters has been developed via cationic gold(i) catalysis. The functionalization selectively occurs at the C3-position to deliver methylene indole derivatives in good yields with excellent Z-selectivity, demonstrating unusual reactivity and selectivity compared with other noble metal catalysis. Importantly, simply followed by silica gel adsorption, an unprecedented metal-free aerobic oxidation occurs for indoles bearing N-electron donating substituents, providing a novel and efficient approach towards 3-substituted indolin-2-ones with a newly formed quaternary stereocenter in excellent stereoselectivity. Notably, these processes afford direct and selective access to a variety of valuable intermediates from abundant feedstock chemicals.

Copper-catalyzed three-component phosphorylation–peroxidation of alkenes

A copper-catalyzed three-component phosphorylation–peroxidation of alkenes with P(O)–H compounds and TBHP has been developed.

Visible light-induced aryltrifluoromethylation of hydroxy alkenes via radical trifluoromethylation-triggered aryl and heteroaryl migration

An efficient protocol was developed to achieve the aryltrifluoroalkylation of hydroxy alkenes via radical trifluoromethylation-triggered aryl migration.

SN 2 Reactions at Tertiary Carbon Centers in Epoxides

Described herein is a novel concept for S_N 2 reactions at tertiary carbon centers in epoxides without activation of the leaving group. Quantum chemical calculations show why S_N 2 reactions at tertiary carbon centers are proceeding in these systems. The reaction allows flexible synthesis of 1,3-diol building blocks for natural product synthesis with excellent control of the relative and absolute configurations.

Cyclohelminthol X, a Hexa-Substituted Spirocyclopropane from Helminthosporium velutinum yone96: Structural Elucidation, Electronic Circular Dichroism Analysis, and Biological Properties

Helminthosporium velutinum yone96 produces cyclohelminthol X (1), a unique hexa-substituted spirocyclopropane. Although its molecular formula and NMR spectral data resemble those of AD0157, being isolated from marine fungus Paraconiothyrium sp. HL-78-gCHSP3-B005, our detailed analyses disclosed a totally different structure. Chemical shift calculations and electronic circular dichroism spectral calculations were quite helpful to establish the structure, when those were performed based on density functional theory. The carbon framework of cyclohelminthols I-IV is found at the C1-C8 propenylcyclopentene substructure of 1. Thus, 1 is assumed to be biosynthesized by cyclopropanation between an oxidized form of cyclohelminthol IV and a succinic anhydride derivative 4. Cytotoxicity for two cancer cell lines and proteasome inhibition efficiency are measured.

Organocatalytic Enantioselective Acyloin Rearrangement of α-Hydroxy Acetals to α-Alkoxy Ketones

We report an unprecedented organocatalytic enantioselective acyloin rearrangement of α,α-disubstituted α-hydroxy acetals. In the presence of a catalytic amount of chiral binol-derived N-triflyl phosphoramide, α-hydroxy acetals rearranged to α-alkoxy ketones in good to high yields with high enantioselectivities. Formation of an ion pair between the in situ generated oxocarbenium ion and the chiral phosphoramide anion was proposed to be responsible for the highly efficient transfer of chirality. Conditions for removal of cyclohexyl and cyclopentyl groups from the corresponding α-alkoxy ketones were uncovered underpinning their potential general utility as hydroxy protecting groups. Conversion of the rearranged products to the enantioenriched α-hydroxy ketone, 1,2-diol, β-amino alcohol and 1,4-dioxane was also documented.

1,2-Difunctionalization-type (hetero)arylation of unactivated alkenes triggered by radical addition/remote (hetero)aryl migration

A novel (hetero)arylation of unactivated alkenes has been developed via 1,4(5)-(hetero)aryl migration triggered by radical alkene azidation, trifluoromethylation, or phosphonylation.

Ring Expansion and Skeletal Rearrangement of Propargyl Alcohol Substituted Aziridines Induced by Ruthenium Complexes

The ring expansion and skeletal rearrangement of two types of propargyl alcohol substituted aziridines with or without cycloalkane moieties was induced by a ruthenium cyclopentadienyl phosphine complex. In the simple aziridine system with no cycloalkane, the unique cycloisomerization process altered the absolute connectivity of the two-carbon unit in the three-membered ring to give organometallic products with substituted pyridine or dihydropyridine ligands. For the aziridine on a cyclohexyl ring, the cycloisomerization process was controlled by an interchange process between vinylidene and allenylidene species, thus creating a better relative configuration of the aziridinyl and the alkynyl units. This determines the stereochemistry of the metal carbene products of the octahydroindole derivatives. The structures of five products were determined by X-ray diffraction analysis.

Sodium-Ketyl Radical Anions by Reverse Pinacol Reaction and Their Coupling with Iodoarenes

Transition-metal-free C-C bond formation between aryl iodides and pinacols is presented. Pinacols are readily transformed by deprotonation with NaH to their Na-pinacolates. C-C-bond homolysis at room temperature generates in situ the corresponding Na-ketyl radical anions which react with various aryl iodides in a homolytic aromatic substitution reaction to form tertiary Na-alcoholates. Protonation eventually provides tertiary alcohols in an unprecedented route.

Enantioselective total synthesis of (+)-Lingzhiol via tandem semipinacol rearrangement/Friedel–Crafts type cyclization

Enantioselective total synthesis of (+)-Lingzhiol has been achieved. Tandem semipinacol rearrangement/lactone formation/Friedel–Crafts cyclization/elimination reaction was developed to construct the four-ring core skeleton.

Cycloaddition of cyclobutenone and azomethine imine enabled by chiral isothiourea organic catalysts

The addition of an organic catalyst to the ketone moiety of a γ-mono-chloride substituted cyclobutenone destroys its stable, conjugated and nearly planar structure. The C-C bond in the resulting less stable anionic oxy-substituted non-planar intermediate is then activated. The breaking of one C-C single bond leads to a catalyst-bound intermediate that undergoes α-carbon selective reactions with azomethine imines to afford nitrogen-containing heterocyclic compounds with excellent diastereo- and enantio-selectivities. Our organocatalytic approach provides a new reaction pattern for C-C bond activation of cyclobutenones that is unavailable with transition metal catalysis. In addition, the present study with isothioureas as the organocatalysts expands the potential in using organocatalysts for C-C bond breaking and selective reactions.