A Domino Aza-Piancatelli Rearrangement/Intramolecular Diels–Alder Reaction: Stereoselective Synthesis of Octahydro-1H-cyclopenta[cd]isoindole

Rapid Access to Divergent Fused Polycycles Via One-Pot A3 Coupling and Intramolecular Diels-Alder Reaction

Divergent nitrogen-containing fused polycyclic ring systems are constructed from simple starting materials via a one-pot aldehyde-alkyne-amine (A3) coupling and intramolecular Diels-Alder reaction. This domino reaction directly furnishes linear 5/5/5 and 5/5/6, or nonlinear 5/5/6/5, polycyclic rings containing an oxa-bridged fused 5/5 bicycle and a 1,6-enyne substructure. One-step derivation of the oxa-bridged 5/5 bicycle leads to a polyfunctionalized 5/5 bicycle with tetrahydrofuran fused back-to-back to pyrroline or a 6/5 bicycle with the hexahydro-1H-isoindole structure, while cycloisomerizing the enyne substructure adds an extra fused 5-membered ring to afford functionalized linear 5/5/5/5 or 5/5/5/5/5 fused ring systems from selected substrates. In addition, the one-pot product can be designed so that the alkyne moiety is hydroalkoxylated to form an additional heterocyle in a linear 5/5/5/6 or nonlinear 5/5/6/5/5 ring system. This diversity-oriented synthetic approach thus allows rapid access to an under-explored structural space for discovery of new biological or non-biological activities or functions.

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.

Rapid Access to Densely Functionalized Cyclopentenyl Sulfoximines through a Sc-Catalyzed Aza-Piancatelli Reaction

Sulfoximines make up a class of compounds of growing interest for crop science and medicinal chemistry, but methods for directly incorporating them into complex molecular scaffolds are lacking. Here we report a scandium-catalyzed variant of the aza-Piancatelli cyclization that can directly incorporate sulfoximines as nucleophiles rather than the classical aniline substrates. Starting from 2-furylcarbinols and sulfoximines, the reaction provides direct access to 4-sulfoximinocyclopentenones, a new scaffold bearing cyclopentenone and sulfoximine motifs, both of interest for bioactive compounds.

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.

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 acid-promoted pseudocine substitution manifold of γ-aminocyclopentenone enables divergent access to polycyclic indole derivatives

We demonstrated γ-aminocyclopentenones to be a suitable surrogate for reactive cyclopentadienone via a pseudocine-substitution manifold. This approach enabled its orchestrated annulation with "tailored" bis-nucleophiles and to furnish complex β,γ-annulated cyclopentanoids or indole-based polycyclic architectures. This strategy represents a generalized means for direct, regioselective and stereoselective β,γ-functionalization of monosubstituted or unsubstituted aminocyclopentenones.

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.

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.

Remote asymmetric conjugate addition catalyzed by a bifunctional spiro-pyrrolidine-derived thiourea catalyst

A novel spiro-pyrrolidine (SPD)-derived bifunctional thiourea catalyst has been developed and used in a stereoselective conjugate addition of furfurals to β,γ-unsaturated α-ketoesters.

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.