[3 + 3] Formal Cycloadditions of Nitrones from Isatins and Azaoxyallyl Cations for Construction of Spirooxindoles

Synthesis of Spirooxindole-1,2-oxazinan-5-ones through 2,2,2-Trifluoroethanol Promoted [3 + 3] Cycloaddition of N-Vinyl Oxindole Nitrones and Oxyallyl Cations

A variety of spirooxindole-1,2-oxazinan-5-one derivatives were prepared in moderate to excellent yields through 2,2,2-trifluoroethanol (TFE)-promoted [3 + 3] cycloaddition of N-vinyl oxindole nitrones with oxyallyl cations generated from α-tosyloxy ketones under mild reaction conditions. Mechanistic studies revealed that [3 + 3] cycloaddition might involve two possible reaction pathways, including direct [3 + 3] cycloaddition of N-vinyl oxindole ntirones with oxyallyl cations, or the addition of TFE to N-vinyl oxindole nitrones, sequential addition to oxyallyl cations, elimination, and cyclization. The present method features mild reaction conditions, broad substrate scope, good functional group tolerance, easy gram scalable preparation, and new applications of TFE.

HFIP in Organic Synthesis

1,1,1,3,3,3-Hexafluoroisopropanol (HFIP) is a polar, strongly hydrogen bond-donating solvent that has found numerous uses in organic synthesis due to its ability to stabilize ionic species, transfer protons, and engage in a range of other intermolecular interactions. The use of this solvent has exponentially increased in the past decade and has become a solvent of choice in some areas, such as C-H functionalization chemistry. In this review, following a brief history of HFIP in organic synthesis and an overview of its physical properties, literature examples of organic reactions using HFIP as a solvent or an additive are presented, emphasizing the effect of solvent of each reaction.

(3 + 2) cycloaddition of 2-alkoxynaphthalenes with azaoxyallyl cations: access to benzo[e]indolones

A route to functionalized benzo[e]indolones has been reported via (3 + 2) cycloaddition of in situ generated azaoxyallyl cations with alkoxynaphthalene followed by aryl C–O bond cleavage. The intermediate adduct has been isolated and characterized.

Synthesis, Characterization of Spirocyclic λ3 -Iodanes and Their Application to Prepare 4,1-Benzoxazepine-2,5-diones and 1,3-Diynes

Herein, a [3+2] cycloaddition of aza-oxyallylic cations with ethynylbenziodoxolones for synthesis of new λ³ -iodanes containing spirocyclic 4-oxazolidinone has been developed. This cyclic λ³ -iodanes display stability in air and excellent solubility in organic solvent. Using them as substrate, both the 4,1-benzoxazepine-2,5-diones and symmetrical 1,3-diynes derivatives were afforded in high yield under copper(I)-catalyzed conditions.

Diastereoselective [3 + 1] Cyclization Reaction of Oxindolyl Azaoxyallyl Cations with Sulfur Ylides: Assembly of 3,3'-Spiro[β-lactam]-oxindoles

Oxindoles and β-lactams are attractive structural motifs because of their unique biological importance. However, the fusion of the two moieties featuring 3,3'-spirocyclic scaffolds is a challenging task in organic synthesis. Herein we designed a novel type of oxindole-based azaoxyallyl cation synthons, which could readily participate in the [3 + 1] cyclization with sulfur ylides. With this protocol, a collection of 3,3-spiro[β-lactam]-oxindoles were facilely produced in up to 94% yield with perfect diastereoselectivity.

1,3-Dipolar cycloaddition of nitrones: synthesis of multisubstituted, diverse range of heterocyclic compounds

The 1,3-dipolar cycloaddition reaction of nitrone is one of the most important methods for the synthesis of different sizes of heterocycles which have enormous applications in natural products, biologically active molecules and pharmaceuticals.

A Role for Isatin Azomethine Imines as a Dipolarophile in Cycloaddition Reactions

An unusual [2 + 3] cycloaddition of isatin azomethine imines (AIs) and in situ generated azaoxyallyl cations has been developed. It is the first example where AIs serve as the [C,O] 2-atom synthon in organic synthesis. This work not only reveals a new role of isatin AIs in cycloaddition reaction but also provides an efficient access to unprecedented spiroheterocycle compounds.

Enantioselective construction of 3-substituted 3-amino-2-oxindoles containing an N,N-ketal skeleton via organocatalyzed aza-addition of isatin imines

The first example of organocatalytic chemo-, regio- and enantioselective aza-Mannich reactions of triazoles and arylamines, respectively, with isatin-derived imines has been achieved in the presence of double hydrogen bonding organocatalysts, affording the valuable optically active 3-substituted 3-amino-2-oxindoles featuring N,N-ketal structural motifs in high yields. This strategy was featured by low catalyst loading, mild conditions, broad substrate scope, and high efficiency and selectivity.

[3 + 2]-Cycloaddition of Azaoxyallyl Cations with 1,2-Benzisoxazoles: A Rapid Entry to Oxazolines

A novel and efficient [3 + 2] cycloaddition reaction of azaoxyallyl cations and 1,2-benzisoxazoles to give oxazoline derivatives has been developed. The transformation provides a rapid entry to functionalized oxazoline scaffolds under mild and transition-metal-free conditions, which will greatly expand the reaction types of heterocycle chemistry and pave the way for syntheses of bioactive compounds.

α-Halogenoacetamides: versatile and efficient tools for the synthesis of complex aza-heterocycles

This review presents the use of α-alkyl- and α-alkoxy-halogenoacetamides as powerful partners for domino and 1,3-dipolar cycloaddition reactions resulting in a ring closure.

Synthesis of 1,2-Oxazinanes via Hydrogen Bond Mediated [3 + 3] Cycloaddition Reactions of Oxyallyl Cations with Nitrones

Reported herein is the development of [3 + 3] cycloaddition reactions between oxyallyl cations and nitrones to yield 1,2-oxazinane heterocycles. Oxyallyl cation intermediates, generated in situ from α-tosyloxy ketones in the presence of hexafluoro-2-propanol (HFIP), a cosolvent, and a base, are found to react with a range of nitrones to afford 1,2-oxazinanes in good to high yields. The reactions are catalyzed by hydrogen-bond donors such as phenols and squaramides, and dramatically higher diastereoselectivities are observed with 4-nitrophenol.

Brønsted Acid-Promoted Friedel-Crafts Alkylation/Cyclization of (7-Hydroxynaphthalenyl)pyrrole or (2-Hydroxyphenyl)pyrroles with Isatins for the Construction of Pyrrolospirooxindole Derivatives

An efficient trifluoroacetic acid-catalyzed cascade Friedel-Crafts alkylation/cyclization of 1-(7-hydroxynaphthalenyl)pyrrole or 1-(2-hydroxyphenyl)pyrroles with isatins has been developed, providing practical access to a variety of biologically important pyrrole-containing spirooxindoles.

Synthesis of 2-Aminoimidazolones and Imidazolones by (3 + 2) Annulation of Azaoxyallyl Cations

The first examples of (3 + 2) annulations between azaoxyallyl cations and cyanamides and nitriles to give the corresponding 2-aminoimidazolones and imidazolones are reported. On the basis of the isolation of unexpected imidate products with certain substrates, it is proposed that the reaction proceeds via fast kinetic O-alkylation followed by rearrangement to the thermodynamically favored 2-aminoimidazolones and imidazolones. The method was applied to the formal synthesis of the antihypertensive drug irbesartan.

Advances in heterocycle synthesis via [3+m]-cycloaddition reactions involving an azaoxyallyl cation as the key intermediate

This feature article highlights recent advances in heterocycle synthesis via [3+m]-cycloaddition reactions involving an azaoxyallyl cation as the key intermediate.

The synthesis of multi-substituted pyrrolidinones via a direct [3 + 2] cycloaddition of azaoxyallyl cations with aromatic ethylenes

A novel [3 + 2] cycloaddition reaction of azaoxyallyl cations and aromatic ethylenes has been developed to afford multi-substituted pyrrolidinones in moderate to good yields.

Base-mediated [2 + 4] cycloadditions of in situ formed azaoxyallyl cations with N-(2-chloromethyl)aryl amides

A base-mediated [2 + 4] annulation of in situ formed azaoxyallyl cations with in situ generated aza-oQMs has been realized.