Direct and Highly Diastereoselective Synthesis of Azaspirocycles by a Dysprosium(III) Triflate Catalyzed Aza-Piancatelli Rearrangement

Stereoselective Access to Spiro-isoindolinone Scaffolds via Catalytic Asymmetric aza-Piancatelli Rearrangement

A chiral Brønsted acid catalyzed asymmetric aza-Piancatelli rearrangement of 3-furyl-3-hydroxyisoindolinones with anilines has been developed, enabling the divergent construction of chiral spiro-cyclopentenone-isoindolinones in high yields with excellent diastereo- and enantioselectivities even under a low catalyst loading of 0.5 mol%.

From 5-HMF to Novel Cyclopentenone-Based Aza Spirocycles: An Intramolecular Aza-Piancatelli Reaction in Action

With a double objective to upgrade biobased 5-HMF and to access to original spirocycles via an intramolecular aza-Piancatelli reaction, a multistep sequence was designed toward appropriate furylcarbinols. The impacts of both the nucleophiles, arylamines compared to alkoxyamines, and the length of the intramolecular tether were studied. After an in-depth evaluation of the different parameters, an extension of the scope provided a library of original azaspiro[4.5]non-8-en-7-ones and azaspiro[4.6]dec-3-en-2-ones whose skeletons have so far never been listed. The application of the aza-Piancatelli reaction associated with the use of biobased HMF in fine chemistry gives credit to the development of novative structures, as raised by the green chemistry community. Combining efforts in synthetic methodology with integration of biosourced platforms could open the way to new molecules exhibiting different properties from the ones raised from petrochemical sources.

Development and characterization of amino donor-acceptor Stenhouse adducts

Donor-acceptor Stenhouse adducts (DASAs) are molecular photoswitches spurring wide interest because of their dynamic photophysical properties, complex photoswitching mechanism, and diverse applications. Despite breakthroughs in modularity for the donor, acceptor, and triene compartments, the backbone heteroatom remains static due to synthetic challenges. We provide a predictive tool and sought-after strategy to vary the heteroatom, introduce amino DASA photoswitches, and analyze backbone heteroatom effects on photophysical properties. Amino DASA synthesis is enabled by aza-Piancatelli rearrangements on pyrrole substrates, imparting an aromaticity-breaking rearrangement that capitalizes on nitrogen's additional bonding orbital and the inductive properties of sulfonyl groups. Amino DASA structure is confirmed by single crystal X-ray diffraction, the photochromic properties are characterized, and the photoswitch isomerization is investigated. Overall, the discovered pyrrole rearrangement enables the study of the DASA backbone heteroatom compartment and furthers our insight into the structure-property relationship of this complex photoswitch.

Robust, Ultrafast and Reversible Photoswitching in Bulk Polymers Enabled by Octupolar Molecule Design

Improving the photoswitching rate and robustness of photochromic molecules in bulk solids is paramount for practical applications but remains an on-going challenge. Here, we introduce an octupolar design paradigm to develop a new family of visible light organic photoswitches, namely multi-branched octupolar Stenhouse Adducts (MOPSAs) featuring a C3-symmetrical A3-(D-core) architecture with a dipolar donor-acceptor (D-A) photochrome in each branch. Our design couples multi-dimensional geometric and electronic effects of MOPSAs to enable robust ultrafast reversible photoswitching in bulk polymers. Specifically, the optimal MOPSA (4 wt %) in commercial polyurethane films accomplishes nearly 100 % discoloration in 6 s under visible light with ∼ 100 % thermal-recovery in 17.4 s at 60 °C, while the acquired kinetics constants are 3∼7 times that of dipolar DASA counterpart and 1∼2 orders of magnitude higher than those of reported DASAs in polymers. Importantly, the MOPSA-doped polymer films sustain 500 discoloration/recovery cycles with slow degradation, superior to the existing DASAs in polymers (≤30 cycles). We discover that multi-dipolar coupling in MOPSA enables enhanced polarization and electron delocalization, promoting the rate-determining thermal cyclization, while the branched and non-planar geometry of MOPSA induces large free volume to facilitate the isomerization. This design can be extended to develop spiropyran or azobenzene-based ultrafast photochromic films. The superior photoswitching performance of MOPSAs together with their high-yield and scalable synthesis and facile film processing inspires us to explore their versatile uses as smart inks or labels for time-temperature indicators, optical logic encryption and multi-levelled data encryption.

Synthesis of cis-fused cyclopentenone-pyrrolidine scaffolds via sequential aza-Piancatelli and Conia-ene type reactions in one pot

A novel one-pot protocol that enables sequential execution of an aza-Piancatelli rearrangement and a Conia-ene type reaction has been developed under Lewis acid catalysis. Here, a combination of B(C6F5)3 and Cu(OTf)2, triethylamine, and triphenylphosphine yielded a wide range of cis-fused cyclopentenone-pyrrolidine scaffolds in one pot with good yields and diastereoselectivity.

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.

Tethered together: DASA design towards aqueous compatibility

Donor-acceptor Stenhouse adducts (DASAs) are an exciting class of photoswitches due to their facile tunability, visible light absorbance, and negative photochromism. While they have shown use in a variety of applications, to date all reported DASA derivatives have low equilibrium and/or poor photoswitching in polar protic solvents, which is vital for moving towards applications in biological systems. We demonstrate a strategy to introduce a substitution on the DASA triene that results in derivatives that are stable and have high dark equilibrium of the open form in polar protic solvents. Decreasing the charge separation of these new derivatives also allows for reversible switching in polar and protic solvents including THF : water mixtures.

Recent Advances in Photochemical Asymmetric Three-Component Reactions

Over the past decades, asymmetric photochemical synthesis has garnered significant attention for its sustainability and unique ability to generate enantio-enriched molecules through distinct reaction pathways. Photochemical asymmetric three-component reactions have demonstrated significant potential for the rapid construction of chiral compounds with molecular diversity and complexity. However, noteworthy challenges persist, including the participation of high-energy intermediates such as radical species, difficulties in precise control of stereoselectivity, and the presence of competing background and side reactions. Recent breakthroughs have led to the development of sophisticated strategies in this field. This review explores the intricate mechanisms, synthetic applications, and limitations of these methods. We anticipate that it will contribute towards advancing asymmetric catalysis, photochemical synthesis, and green chemistry.

Construction of Octahydro-4H-cyclopenta[b]pyridin-6-one Skeletons using Pot, Atom, and Step Economy (PASE) Synthesis

Cascade aza-Piancatelli reaction and [3+3]/[4+2] cycloaddition reactions are carried out using the ideality principles of pot, atom, and step economy (PASE) synthesis. The reaction resulted in generation of octahydro-4H-cyclopenta[b]pyridin-6-one scaffolds. Moreover, octahydro-5,7a-epoxycyclopenta[cd]isoindol-4-one frameworks of gracilamine alkaloid and a novel decahydro-1H-dicyclopenta[cd,hi]isoindol-6-one were also realized in good yields with excellent regio- and diastereo-selectivities.

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.

Asymmetric Ketoalkylation/Rearrangement of Alkyenlfurans via Synergistic Photoredox/Brønsted Acid Catalysis

An enantioselective three-component rearrangement of alkenylfurans with various cycloalkyl silyl peroxides and anilines has been developed by merging photoredox catalysis with chiral Brønsted acid catalysis. This protocol provides expedient access to a broad spectrum of ketoalkyl-functionalized 4-aminocyclopentenones in high yields with excellent enantio- and diastereoselectivities. Diverse functional groups can be introduced via facile product derivations.

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.

Access to densely functionalized spirocyclopentenonyl oxindole frameworks via aza- and carbo-Piancatelli rearrangement

A new strategy for access to spirocyclopentenonyl oxindole frameworks is disclosed. Suitably anchored furfuryl alcohol at C3 of an oxindole was used for the aza-Piancatelli rearrangement, which furnished spirocyclic aminocyclopentenone frameworks with catalytic phosphomolybdic acid. The scope of the transformation was extended to the carbo-Piancatelli rearrangement with various indole derivatives.

Quaternary carbon construction through Piancatelli rearrangement: easy access to spirocyclopentenones

A new and efficient approach to a series of novel multifunctionalized spirocyclopentenone scaffolds through Piancatelli rearrangement was developed under metal-free conditions. This method has been successfully applied to O-, N- and C-nucleophiles with excellent yields.

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

An Aza-Piancatelli Templated Reaction Manifold of 4-Aminocyclopentenones Enables Access to Complex Carbocyclic Assemblies

Capitalizing on the propensity of 1, 2-amino group migration in γ-aminocyclopentenone with a suitable promoter, gem-diaryl equipped systems unfolded an unprecedented avenue for the Lewis acid promoted displacement of γ-aniline group with nucleophiles such as indole. Such transformation, besides providing a means for direct γ-functionalization of cyclopentenones, presented innumerable scope for β, γ-annulation. Various tailored indolo bis-nucleophiles were explored in the current study that rendered an array of indole alkaloid-like compounds in excellent yields and selectivity through one-pot operation. Analysis of collective experimental observation along with designed control experiments strongly suggested the possibility of a retro aza-Piancatelli rearrangement, which is hitherto unknown in the context. Such repertoire could find potential applications in the synthesis of complex assemblies from the Piancatelli rearrangement and related processes. 1. Introduction 2. Aza-Piancatelli rearrangement and related domino processes 3. An unprecedented aza-Piancatelli-templated strategy for polycyclic assemblies 4. Summary and Outlook

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.

One-Pot Transformation of Furans into 1-Azaspirocyclic Alkaloid Frameworks Induced by Visible Light

High-value 1-azaspirocyclic scaffolds have been made from simple and readily accessible furan precursors in a single operation. The protocol is a one-pot sequence using highly sustainable conditions (oxygen, visible light, and a favored green solvent) that leads to a dramatic increase in molecular complexity. The initial substrates can include functionalities that are suitable for further elaboration; in this way, the pruned polycyclic skeletons of the stemonamine, cylindricine, and lepadiformine natural products were rapidly accessed.

Unprecedented Reactivity of γ-Amino Cyclopentenone Enables Diversity-Oriented Access to Functionalized Indoles and Indole-Annulated Ring Structures

Observation of an unexpected, Lewis acid promoted displacement of latent reactive γ-amino group on cyclopentenone presented unparalleled opportunity for enone functionalization and annulations with indole derivatives, which is developed in the current study. Herein, a vast range of C3/N-indolyl enones and indole alkaloid-like compound were accessed in excellent yields (up to 99 %) and selectivity through a one-pot operation. The mechanism most likely involves an unprecedented trait of Piancatelli-type rearrangement where influence of the gem-diaryl group appeared crucial.

Total Synthesis of (+)-Hyacinthacine A1 Using a Chemoselective Cross-Benzoin Reaction and a Furan Photooxygenation-Amine Cyclization Strategy

We report the shortest synthesis of glycosidase inhibitor (+)-hyacinthacine A₁ using a highly chemoselective N-heterocyclic carbene-catalyzed cross-benzoin reaction as well as a furan photooxygenation-amine cyclization strategy. This is the first such cyclization on a furylic alcohol, an unprecedented reaction due to the notorious instability of the formed intermediates. The photooxygenation strategy was eventually incorporated into a three-step one-pot process that formed the requisite pyrrolizidine framework of (+)-hyacinthacine A₁.

The Piancatelli rearrangement of non-symmetrical furan-2,5-dicarbinols for the synthesis of highly functionalized cyclopentenones

Piancatelli rearrangement on substituted non-symmetrical furan-2,5-dicarbinols has been developed using an effective combination of microwave activation and Lewis acid catalyst.

Cation Triggered Domino Aza-Piancatelli Rearrangement/Friedel-Crafts Alkylation of Indole-Tethered Furfuyl Alcohols to Access Cycloocta[b]indole Core of Alkaloids

A domino approach to bridged cycloocta[b]indolone through a cascade of aza-Piancatelli rearrangement/Friedel-Crafts alkylation is developed. This transformation has been realized by reaction of an indole-tethered 2-furylcarbinol and substituted aniline in the presence of a Lewis acid to initiate aza-Piancatelli rearrangement followed by an in situ intramolecular Friedel-Crafts alkylation to access bridged tetracyclic frameworks in one pot.

A chiral cobalt(ii) complex catalyzed enantioselective aza-Piancatelli rearrangement/Diels-Alder cascade reaction

A chiral N,N′-dioxide/cobalt(ii) complex catalyzed highly diastereoselective and enantioselective tandem aza-Piancatelli rearrangement/intramolecular Diels–Alder reaction has been disclosed. Various valuable hexahydro-2a,5-epoxycyclopenta[cd]isoindoles bearing six contiguous stereocenters have been obtained in good yields with excellent diastereo- and enantio-selectivities from a wide range of both readily available 2-furylcarbinols and N-(furan-2-ylmethyl)anilines.

An asymmetric aza-Piancatelli rearrangement/Diels–Alder cascade reaction between 2-furylcarbinols and N-(furan-2-ylmethyl)anilines was realized by using a chiral N,N′-dioxide/cobalt(ii) complex catalyst.

A designed and potentially decadentate ligand for use in lanthanide(iii) catalysed biomass transformations: targeting diastereoselective trans-4,5-diaminocyclopentenone derivatives

The catalysis of furfural to diastereoselective trans-4,5-diaminocyclopentenone building blocks for natural product synthesis for a wide range of amines was achieved under mild aerobic conditions.

Enantioselective PCCP Brønsted acid-catalyzed aza-Piancatelli rearrangement

An enantioselective aza-Piancatelli rearrangement has been developed using a chiral Brønsted acid based on pentacarboxycyclopentadiene (PCCP). This reaction provides rapid access to valuable chiral 4-amino-2-cyclopentenone building blocks from readily available starting material and is operationally simple.

Direct Conversion of Activated 5-Hydroxymethylfurfural into δ-Lactone-Fused Cyclopentenones

Valorization of biomass derived feedstock (e.g., 5-hydroxymethylfurfural platform) is a very active field of chemical research. In this study, 5-hydroxymethylfurfural is converted into cyclopenten-2-ones by virtue of furfural's activation and Meldrum's acid's tendency to undergo decomposition/esterification. Experimental and computational studies suggest a domino rearrangement-lactonization reaction involving BINOL-catalyzed lactonization as the rate-determining step. The novel lactone-fused cyclopenten-2-ones, which bear a quaternary carbon and resemble a didemnenone natural product structure, are converted into several derivatives with potential interest for the fields of synthetic and medicinal chemistry.

Multi-stimuli responsive trigger for temporally controlled depolymerization of self-immolative polymers

The design and development of a multi-stimuli trigger enables temporal control over trigger cleavage and subsequent depolymerization of self-immolative polymer.

The Piancatelli reaction and its variants: recent applications to high added-value chemicals and biomass valorization

The conversion of furfuryl alcohols by the Piancatelli reaction (and its C- and N-variants) provided highly functionalized cyclopentenones (intermolecular) and spirocycles (intramolecular).

Lewis Acid Mediated Cyclizations: Diastereoselective Synthesis of Six- to Eight-Membered Substituted Cyclic Ethers

Cyclic ethers are widely abundant in natural products. Cyclic ether templates are also utilized in drug design and medicinal chemistry. Although the synthetic processes for this class of compounds have been studied extensively with respect to five- and six-membered rings, medium-sized cyclic ethers are synthetically more challenging due to a variety of factors. Herein, we report our results on the Lewis acid catalyzed synthesis of medium-sized cyclic ethers in a diastereoselective manner.

Catalyst-Controlled Chemodivergent Modification of Indoles with 2-Furylcarbinols: Piancatelli Reaction vs Cross-Dehydrative Coupling Reaction

A divergent synthetic strategy to functionalize the indole nucleus with readily available 2-furylcarbinols was developed. It was found that the 3-(4-oxo-2-cyclopentenyl)indoles were obtained in moderate to good yields (up to 89%) through Piancatelli reaction catalyzed by ZnCl₂, whereas employment of Brønsted acid TFA afforded directly coupled product 3-(2-furyl)indoles in moderate to good yields (up to 87%) via the deprotonation-rearomatization route.