Catalytic and enantioselective oxa-Piancatelli reaction using a chiral vanadium complex

Asymmetric Construction of Functionalized Cyclopenta[b]pyrrolines via Cascade Aza-Piancatelli/Hydroamination Reactions

A chiral Brønsted acid/Pd that cooperatively catalyzed the asymmetric cascade aza-Piancatelli rearrangement/hydroamination of readily accessible alkynyl-functionalized tertiary furylcarbinols with anilines has been developed. This protocol provides expedient access to a variety of densely functionalized cyclopenta[b]pyrroline derivatives in high yields with excellent enantioselectivities.

Organocatalytic enantioselective oxa-Piancatelli rearrangement

The first highly enantioselective oxa-Piancatelli rearrangement has been developed. This process which is catalyzed by a chiral BINOL-derived phosphoric acid rearranges a wide range of furylcarbinols into densely substituted γ-hydroxy cyclopentenones in high yield with excellent diastereo- and enantioselectivities (up to 99 : 1 er). This reaction exhibits a high functional group tolerance and was applied to complex bioactive molecules as well. The products were further manipulated into value-added molecular scaffolds further highlighting their versatility and synthetic utility.

AlCl3-catalyzed chemoselective cascade reactions of 4-anilinocoumarins with 2-furylcarbinols: access to densely functionalized chromeno[4,3-b]pyrrol-4(1H)-one derivatives

An unprecedented protocol has been developed for the preparation of highly functionalized chromeno[4,3-b]pyrrol-4(1H)-ones, which are not only valuable architectures of many biologically active molecules but also key building blocks for rich photophysical properties. The transformation proceeded through chemoselective intermolecular α-carbon nucleophilic attacking/ring-opening/Michael addition/deprotonation aromatization processes from 4-aminocoumarins and 2-furylcarbinols.

Atroposelective Synthesis of C-C Axially Chiral Compounds via Mono- and Dinuclear Vanadium Catalysis

Axially chiral compounds with rotationally constrained σ-bonds that exhibit atropisomerism are lucrative synthetic targets because of their ubiquity in functional materials and natural products. The metal complex-catalyzed enantioselective fabrication of axially chiral scaffolds has been widely investigated, and thus far, considerable progress has been made. Over the past two decades, we have developed a highly efficient strategy for constructing axially chiral biarenol derivatives using chiral mono- and dinuclear vanadium complexes. These complexes are readily prepared from vanadium(IV) salts and Schiff base ligands (generated from the condensation of (S)-tert-leucine and di- or monoformyl-(R)-1,1'-bi-2-naphthol (BINOL) derivatives) under O₂ and act as highly active catalysts for highly stereoselective C-C bond formation. In particular, the vanadium complex-catalyzed enantioselective oxidative coupling of 2-naphthols 1 under oxygen or in air, which is a green oxidant, affords the desired axially chiral molecules in high yields and high stereoselectivity (up to quantitative yield and 97% ee), along with water as the sole coproduct. This coupling reaction tolerated various functional groups (such as halogens, alkoxys, and boryls) and avoided overoxidation of coupling products.The key feature of dinuclear vanadium(V) catalysts such as (R_a,S,S)-5a is an outstanding mode of the homocoupling reaction, in which a single molecule of the catalyst activates two molecules of the starting material (e.g., 2-naphthols) simultaneously. With this "dual activation" mechanism, the oxidative coupling promoted by the dinuclear catalyst proceeds in an intramolecular manner. The homocoupling rate using 5 mol % of the dinuclear vanadium(V) complex (R_a,S,S)-5a was measured to be 111 times faster than that of the mononuclear vanadium(IV) complex (S)-4a bearing a half motif of the dinuclear vanadium complex.In the case of the heterocoupling reaction utilizing two different kinds of arenol derivatives, only a starting arenol having lower oxidation potential seems to be activated by the mononuclear vanadium complex. The reaction rate of the heterocoupling using either mono- or dinuclear vanadium complexes showed no difference to give the coupling product in high yields but with a different enantioselective manner; chiral mononuclear vanadium(V) complexes showed better enantioselectivites than that of the dinuclear vanadium(V) complexes. A competing heterocoupling study and a linear correlation between the ee of the mononucaler vanadium catalyst and ee of the heterocoupling suggested that the heterocoupling involves an intermolecular radical-anion coupling pathway.In this Account, we summarize the recent advances in vanadium-catalyzed coupling reactions that produced important chiral molecules, such as biresorcinols, polycyclic biphenols, oxa[9]helicenes, bihydroxycarbazoles, and C₁-symmetrical biarenols, and their coupling reaction mechanisms. By pursuing vanadium catalysis, we believe numerous additional transformations as well as a renewed interest in catalytic and chemo-, regio-, and enantioselective aryl-aryl bond constructions will be manifested.

Organocatalytic Asymmetric Synthesis of Bridged Tetrahydrobenzo[b]azepines/oxepines

The asymmetric synthesis of bridged tetrahydrobenzo[b]azepine and oxepine derivatives through chiral Brønsted acid catalyzed asymmetric aza-Piancatelli rearrangement/Michael addition sequence has been developed. The reaction proceeds under mild reaction conditions to afford the final bridged cyclic products in good yields with excellent enantio- and diastereoselectivities.

Enantiopure chiroptical probes for circular dichroism and absorbance based detection of nerve gas simulants

A series of oxovanadium(V) compounds 1-4 were prepared and explored as stereodynamic chiroptical probes to detect a simulant of sarin known as diethyl chlorophosphate (DCP) without any interference from the competing analytes. Simultaneous CD cum UV/vis based bimodal instant recognition of DCP using optically active probes is unprecedented. Upon fabricating the vanadium compound with a polymer has yielded a chiroptical membrane, which showed a change in its dichroic as well as colorimetric signals on interaction with DCP vapour at 1 ppm. EPR and UV/vis studies revealed an irreversible change of the CD-active V(V) to the CD-silent ternary V(V) species in presence of DCP via a transient V(IV) species. Nucleophilic attack of the alkoxo oxygen of 1-4 to the electrophilic P atom of DCP resulted in the formation of ternary V(V) compounds as confirmed by ⁵¹V/³¹P NMR.

Mono Versus Dinuclear Vanadium(V) Complexes: Solvent Dependent Structural Versatility and Electro Syntheses of Mixed-Valence Oxovanadium(IV/V) Entities in Solution

Two mononuclear oxidovanadium(V) complexes type of [V^VO(L¹)(OMe)(MeOH)] (1), [V^VO(L²)(OMe)(MeOH)] (2) and two [V₂O₃]⁴⁺ core of μ-oxidodioxidodivanadium(V) complexes (L¹)(O)V^V-O-V^V(O)(L¹) (3) and (L²)(O)V^V-O-V^V(O)(L²) (4) and two complexes [V^VO(L¹)(8-Hq)] (5) and [V^VO(L²)(8-Hq)] (6) incorporating 8-hydroxyquinoline (8-hq) as co-ligand have been reported where L¹ [(E)-N'-(2-hydroxybenzylidene)cinnamohydrazide] and L² [(2E,N'E)-N'-(2-hydroxybenzylidene)-3-(naphthalen-1-yl)acrylohydrazide] are the dianionic forms of the conjugated keto-imine functionalized substituted hydrazone ligands. The μ-oxidodioxidodivanadium complexes are generated upon switching the solvent from methanol to acetonitrile. The X-ray analysis showed octahedral geometry for the mononuclear complexes 1, 2 and 5 but oxido-bridged dinuclear complexes 3 and 4 formed penta-coordinated square-pyramidal geometry about metal atoms. Two mixed-valence species of type II, 3a and 4a, of general formulae (L)(O)V^IV-O-V^V(O)(L), are being generated upon constant potential electrolysis (CPE) of 3 and 4 respectively. Frozen solution EPR spectra have 13 hyperfine lines, revealing the unpaired electron is majorly localized on one of the two vanadium centres. All these complexes have been well characterized by physio-chemical techniques and the density functional theory (DFT) calculations were applied to obtain further insight into the electronic structure of this type of molecule. The oxidomethoxido complexes 1 and 2 were taken to investigate the catechol oxidase mimicking activity following the oxidation of 3,5-di-tert-butyl catechol (3,5-DTBC) to 3,5-di-tert-butyl benzoquinone (3,5-DTBQ).

Recent Advances on Piancatelli Reactions and Related Cascade Processes

The Piancatelli reaction, which is the rearrangement of 2-furylcarbinol to cyclopentenone, involves a key furanoxonium ion intermediate and a furan ring opening-4π electrocyclization process. In recent years, the original oxa-Piancatelli reaction has been extended to a large family of aza- and carbo-Piancatelli reactions and related cascade processes, providing a powerful platform for the construction of diverse functionalized cyclopentenones and polycyclic cyclopentanones. Meanwhile, chiral Brønsted/Lewis acid based catalytic asymmetric approaches to Piancatelli reactions have also been achieved for the assembly of highly valued chiral cyclopentenone scaffolds. In this short review, we present an overview of the recent developments in these areas and focus primarily on reports published in the last five years.

1 Introduction

2 Diastereoselective Oxa-, Aza- and Carbo-Piancatelli Reactions

3 Diastereoselective Cascade Piancatelli Reactions

4 Asymmetric Piancatelli Reactions and Related Cascade Processes

5 Miscellaneous Furanoxonium Ion-Based Rearrangements

6 Conclusion

Retro-Aza-Piancatelli Rearrangement Triggered Cascade Reaction of Methyl Furylacrylates with Anilines to Access Cyclopenta[b]pyrrolidinones

A novel aza-Piancatelli rearrangement triggered cascade reaction has been developed by utilizing methyl furylacrylates as a new type of functionalized furanoxonium ion precursor, permitting rapid and flexible construction of diverse cyclopenta[b]pyrrolidinone derivatives. The unprecedented and highly efficient bicyclic γ-lactam product formation is originated from an unusual retro-aza-Piancatelli rearrangement of the major cis-fused multifunctionalized cyclopentenone to the minor trans-fused one followed by a lactamization reaction.

Asymmetric Hydrogenation of Racemic α-Aryl-β-ethoxycarbonyl Cyclopentanones via Dynamic Kinetic Resolution and Its Application to the Synthesis of (+)-Burmaniol A

An efficient asymmetric hydrogenation of racemic α-aryl-β-ethoxycarbonyl cyclopentanones via dynamic kinetic resolution is reported. Via catalysis by a chiral iridium Ir-SpiroPAP catalyst, a range of racemic α-aryl-β-ethoxycarbonyl cyclopentanones were hydrogenated to the corresponding ester-functionalized chiral 2-arylcyclopentanols with three contiguous stereocenters in high yields with excellent enantio- and diastereoselectivities. This method was successfully applied in the enantioselective synthesis of cyclopentane-based γ-amino ester/alcohol derivatives and phenylpropanoid (+)-burmaniol A.

Chemo- and enantioselective hetero-coupling of hydroxycarbazoles catalyzed by a chiral vanadium(v) complex

The catalytic enantioselective oxidative hetero-coupling of arenols using a chiral vanadium(v) complex has been developed.