Phosphine-Catalyzed [3 + 2] Cycloaddition of Sulfamate-Derived Cyclic Imines with Allenoate: Synthesis of Sulfamate-Fused Dihydropyrroles

An Overview of the Synthesis of 3,4-Fused Pyrrolocoumarins of Biological Interest

3,4-Fused pyrrolocoumarins, synthetically prepared or naturally occurring, possess interesting biological properties. In this review, the synthetic strategies for the synthesis of the title compounds are presented along with their biological activities. Two routes are followed for that synthesis. In one, the pyrrole ring is formed from coumarin derivatives, such as aminocoumarins or other coumarins. In the other approach, the pyranone moiety is built from an existing pyrrole derivative or through the simultaneous formation of coumarin and pyrrole frameworks. The above syntheses are achieved via 1,3-dipolar cycloaddition reactions, Michael reaction, aza-Claisen rearrangement reactions, multi-component reactions (MCR), as well as metal-catalyzed reactions. Pyrrolocoumarins present cytotoxic, antifungal, antibacterial, α-glucosidase inhibition, antioxidant, lipoxygenase (LOX) inhibition, and fluorescent activities, as well as benzodiazepine receptor ability.

Photoinduced Metal- and Photosensitizer-Free Decarbonylative C-H Alkylation of Cyclic Sulfamidate Imines

Photoinduced decarbonylative C-C bond formation with readily accessible aldehydes as alkyl sources is described. This protocol provides a sustainable alternative for the effective construction of diverse valuable 4-alkylated sulfonyl ketimines under metal- and photosensitizer-free conditions. Significantly, in this reaction, air serves as the green oxidant, and cyclic sulfamidate imines play a dual role of substrate and photocatalyst, thus affording a concise reaction system for C-H alkylation of cyclic sulfamidate imines.

[4+2]-Cycloaddition of 2-Aminophenyl Enones with Cyclic N-Sulfimines to Access Enantioenriched Ring-Fused Aminals

Ring-fused aminal is an interesting structural skeleton in biologically active and pharmaceutically relevant compounds. A novel and efficient method for synthesizing benzosulfamidate-fused tetrahydroquinazolines is described. By employing the [4+2]-cycloaddition of 2-aminophenyl enones with cyclic N-sulfimines in the presence of DMAP as a base, the desired benzosulfamidate-fused tetrahydroquinazolines were obtained in good yields with high diastereoselectivities. Furthermore, an organocatalytic asymmetric [4+2]-cycloaddition was successfully achieved using a squaramide-based catalyst, enabling the enantioselective synthesis of chiral ring-fused tetrahydroquinazolines with high yields and enantio- as well as diastereoselectivities (up to 89 % yield, 94 % ee, and >30 : 1 dr).

Synthesis of Polycyclic Imidazolidinones via Cascade [3 + 2]-Annulation of β-Oxo-acrylamides with Cyclic N-Sulfonyl Imines

An Et3N-catalyzed cascade [3 + 2]-annulation of β-oxo-acrylamides with cyclic N-sulfonyl ketimines or sulfamate-derived imines is developed under mild reaction conditions, which provides a concise and efficient route to access valuable sultam- or sulfamidate-fused imidazolidinone derivatives in good to excellent yields (80-95% yields) with excellent diastereoselectivities (>20:1 drs). The current protocol features atom economy, a transition-metal-free process, and broad functional group tolerance. Moreover, the asymmetric variant of the [3 + 2]-cycloaddition reaction was achieved in the presence of diphenylethanediamine or quinine-based bifunctional squaramide organocatalysts C-1 and C-11, giving the corresponding chiral polycyclic imidazolidinones in 68-90% yields with 25-94% ees and >20:1 drs in all cases.

Organocatalytic Asymmetric [3 + 2]-Annulations of γ-Sulfonamido/γ-Hydroxy-α,β-Unsaturated Ketones with Cyclic N-Sulfimines: Synthesis of Chiral Polyheterotricyclic Imidazolidines and Oxazolidines

The first organocatalytic asymmetric [3 + 2]-annulation of γ-sulfonamido-α,β-unsaturated ketones with cyclic N-sulfimines has been developed, and enantioenriched functionalized polyheterotricyclic imidazolidines were obtained in good yields and with excellent enantioselectivities. This approach was also extended to the asymmetric [3 + 2]-annulation of γ-hydroxy-α,β-unsaturated ketones, affording enantioenriched polyheterotricyclic oxazolidines. In addition, base-catalyzed [3 + 2]-annulations of γ-sulfonamido/γ-hydroxy-α,β-unsaturated ketones with cyclic N-sulfimines were re-investigated under mild reaction conditions for the synthesis of racemic polyheterotricyclic imidazolidines and oxazolidines with excellent diastereoselectivities.

Concerted Enantioselective [2+2] Cycloaddition Reaction of Imines Mediated by a Magnesium Catalyst

Enantioselective [2 + 2] cyclization between an imine and a carbon-carbon double bond is a versatile strategy to build chiral azetidines. However, α-branched allenoates have never been successfully applied in [2 + 2] cyclization reactions with imines, as they always undergo Kwon's [4 + 2] annulation in previous catalytic methods. Herein, a simple in situ generated magnesium catalyst was employed to successfully achieve the enantioselective [2 + 2] cyclization reaction of DPP-imines and α-branched allenoates for the first time. Insightful experiments including KIE experiments, controlled experiments, Hammett plot analysis, and ³¹P NMR studies of initial intermediates indicate that the current [2 + 2] cyclization of imine most likely involves an asynchronous concerted transition state. Further mechanistic investigations by combining kinetic studies, ESI experiments, ³¹P NMR studies of coordination complexes, and controlled experiments on reaction rates under different catalyst loading amounts provided the coordination details for this [2 + 2] cyclization reaction between DPP-imines and α-branched allenoates. This new approach was applied to the synthesis of various chiral aza-heterocycles, including the enantioselective synthesis of the key intermediate of a lipid-lowering agent Ezetimibe.

Pd-Catalyzed N–H or C–H Functionalization/Oxidative Cyclization for the Efficient Synthesis of N-Aryl-Substituted [3,4]-Fused Pyrrolo­coumarins

1-Aryl-2-methyl- or 3-methylchromeno[4,3-b]pyrrol-4(1H)-ones have been synthesized in excellent yields by the Pd-catalyzed intramolecular aza-Wacker-type cyclization of 3-allyl-4-arylaminocoumarins or C–H insertion/oxidative cyclization of N-allyl-N-aryl-4-aminocoumarins, respectively, in the presence of Cu(OAc)2 in acetic acid under heating. The starting allylcoumarins have been prepared by the allylation of 4-arylaminocoumarins with allyl bromide in CH3CN in the presence of Cs2CO3 at room temperature. Preliminary biological tests indicated interesting antioxidant activity and significant levels of inhibition of soybean lipoxygenase.

Phosphine-Catalyzed Asymmetric Tandem Isomerization/Annulation of Allyl Amines with Allenoates: Enantioselective Annulation of a Saturated C-N Bond

Under catalysis by chiral phosphine, an asymmetric isomerization/annulation cascade reaction of allylamines with allenoates was realized. A wide range of γ-substituted allenoates were tolerated to afford chiral pyrroline derivatives in high yields with excellent enantioselectivities. In the reaction, isomerization of readily available N-allylamines to reactive aliphatic imines through a 1,4-proton shift is a key step, which circumvents the isolation of highly unstable alkyl N-sulfonylimines.

Synthesis of heterocyclic compounds through nucleophilic phosphine catalysis

Nucleophilic phosphine catalysis is a practical and powerful tool for the synthesis of various heterocyclic compounds with the advantages of environmentally friendly, metal-free, and mild reaction conditions. The present report summarizes the construction of four to eight-membered heterocyclic compounds containing nitrogen, oxygen and sulphur atoms through phosphine-catalyzed intramolecular annulations and intermolecular [2+2], [3+2], [4+1], [3+1+1], [5+1], [4+2], [2+2+2], [3+3], [4+3] and [3+2+3] annulations of electron-deficient alkenes, allenes, alkynes and Morita-Baylis-Hillman carbonates.

Phosphine-Catalyzed (4+1) Annulation: Rearrangement of Allenylic Carbamates to 3-Pyrrolines through Phosphonium Diene Intermediates

We have developed a phosphine-catalyzed (4+1) annulative rearrangement for the preparation of 3-pyrrolines from allenylic carbamates via phosphonium diene intermediates. We employed this methodology to synthesize an array of 1,3-disubstituted- and 1,2,3-trisubstituted-3-pyrrolines, including the often difficult to prepare 2-alkyl variants. A mechanistic investigation employing allenylic acetates and mononucleophiles unexpectedly unveiled that a phosphine-catalyzed (4+1) reaction for the construction of cyclopentene products, previously reported by Tong, might not occur through a phosphonium diene, as had been proposed, but rather through multiple mechanisms working in concert. Consequently, our phosphine-catalyzed rearrangement is most likely the first transformation to involve the unequivocal formation of a phosphonium diene intermediate along the reaction pathway. To demonstrate the synthetic utility of this newly developed reaction, we have completed concise formal syntheses of the pyrrolizidine alkaloids (±)-trachelanthamidine and (±)-supinidine.

Highly Regio- and Enantioselective Dienylation of p-Quinone Methides Enabled by an Organocatalyzed Isomerization/Addition Cascade of Allenoates

A novel catalytic asymmetric dienylation of para-quinone methides with allenoates has been developed. Under mild conditions catalyzed by ( R)-SITCP, various dienylated bisarylmethides were obtained in moderate to good yields (up to 82% yield) and excellent enantioselectivities (90-98% ees). The efficacy and robustness were demonstrated by 27 examples of chiral dienylation products. A plausible mechanism, which involved 1,2 H-shift and umpolung of allenoates, was proposed based on deuterium labeling experiments and previous reports.

Intramolecular hydrogen-bonding-assisted phosphine-catalysed [3 + 2] cyclisation of ynones with o-hydroxy/amino benzaldehydes

A new strategy for the synthesis of functionalized tetrahydrofuran derivatives was developed via a phosphine-catalysed [3 + 2] annulation reaction of aromatic aldehydes with 4-phenylbut-3-yn-2-one.

Chemoselective cyclization of N-sulfonyl ketimines with ethenesulfonyl fluorides: access to trans-cyclopropanes and fused-dihydropyrroles

A stereo- and chemoselective ring closing reaction of N-sulfonyl ketimines with ethene sulfonyl fluorides promoted by DBU is reported. This selective C–C vs. C–N bond cyclization process delivers to trans-cyclopropanes (dr up to ≤99 : 1) and fused-dihydropyrroles.

Phosphine Organocatalysis

The hallmark of nucleophilic phosphine catalysis is the initial nucleophilic addition of a phosphine to an electrophilic starting material, producing a reactive zwitterionic intermediate, generally under mild conditions. In this Review, we classify nucleophilic phosphine catalysis reactions in terms of their electrophilic components. In the majority of cases, these electrophiles possess carbon-carbon multiple bonds: alkenes (section 2), allenes (section 3), alkynes (section 4), and Morita-Baylis-Hillman (MBH) alcohol derivatives (MBHADs; section 5). Within each of these sections, the reactions are compiled based on the nature of the second starting material-nucleophiles, dinucleophiles, electrophiles, and electrophile-nucleophiles. Nucleophilic phosphine catalysis reactions that occur via the initial addition to starting materials that do not possess carbon-carbon multiple bonds are collated in section 6. Although not catalytic in the phosphine, the formation of ylides through the nucleophilic addition of phosphines to carbon-carbon multiple bond-containing compounds is intimately related to the catalysis and is discussed in section 7. Finally, section 8 compiles miscellaneous topics, including annulations of the Hüisgen zwitterion, phosphine-mediated reductions, iminophosphorane organocatalysis, and catalytic variants of classical phosphine oxide-generating reactions.

Multifunctional chiral phosphine-catalyzed [3+2] annulation of Morita-Baylis-Hillman carbonates with cyclopentenones: asymmetric synthesis of 4-oxo-hexahydropentalenes

A multifunctional chiral phosphine-catalyzed enantioselective [3+2] annulation of Morita-Baylis-Hillman carbonates with cyclopentenones provides a direct approach to access functional 4-oxo-1,3a,4,5,6,6a-hexahydropentalene compounds. Moderate to good yields and excellent enantioselectivities (>90% ee) were obtained for a series of substrates under mild reaction conditions.

Pd-Catalyzed diastereoselective [3 + 2] cycloaddition of vinylcyclopropanes with sulfamate-derived cyclic imines

The palladium-catalyzed diastereoselective [3 + 2] cycloaddition reaction of vinylcyclopropanes with sulfamate-derived cyclic imines worked efficiently under mild conditions, providing sulfamate-fused pyrrolidines in good to excellent yields.

PPh3-Catalyzed [3 + 2] Spiroannulation of 1C,3N-Bisnucleophiles Derived from Secondary β-Ketoamides with δ-Acetoxy Allenoate: A Route to Functionalized Spiro N-Heterocyclic Derivatives

A [3 + 2] annulation of α-substituted secondary β-ketoamides with δ-acetoxy-modified allenoate has been developed in the presence of phosphine catalyst. In this spiroannulation reaction, β-ketoamides were used as the bis-nucleophilic partner while the γ,δ-carbon of 5-acetoxypenta-2,3-dienoate participated as a C2 synthon, affording the desired functionalized five-membered N-heterocyclic derivatives in moderate to excellent yields and diastereoselectivities under mild conditions. Preliminary attempts on the asymmetric variant of this reaction have been also examined, giving the corresponding products with moderate ee values.

PPh3-Mediated [4 + 2]- and [4 + 1]-Annulations of Maleimides with Azoalkenes: Access to Fused Tetrahydropyridazine/Pyrrolidinedione and Spiro-dihydropyrazole/Pyrrolidinedione Derivatives

Unprecedented PPh₃-mediated [4 + 2]- and [4 + 1]-annulation of maleimides with in situ formed azoalkenes have been successfully developed, affording fused tetrahydropyridazine/pyrrolidinedione and spiro-dihydropyrazole/pyrrolidinedione derivatives in good yields under mild reaction conditions. Maleimides serve as C2 synthons in the [4 + 2]-annulation using 1,2-dichloroethane as the solvent in the presence of 20 mol % of PPh₃. With a stoichiometric amount of PPh₃ in acetone, maleimides serve as C1 synthons, and the in situ formed phosphorus ylide is the key intermediate to realize this [4 + 1]-annulation.

Lewis base-catalyzed diastereoselective [3 + 2] cycloaddition reaction of nitrones with electron-deficient alkenes: an access to isoxazolidine derivatives

A Lewis base-catalyzed [3 + 2] cycloaddition reaction of nitrones with electron-deficient alkenes has been achieved under mild reaction conditions, affording functionalized isoxazolidines as single diastereomers in moderate to excellent yields.

Divergent synthesis of hydropyridine derivatives via nitrogen-containing Lewis base mediated regioselective [4 + 2] cyclizations

A nitrogen-containing Lewis base mediated regioselective [4 + 2] cyclization strategy between allenic esters and α,β-unsaturated ketimines was discovered.

An asymmetric Brønsted acid-catalyzed Friedel–Crafts reaction of indoles with cyclic N-sulfimines

A highly enantioselective organocatalytic Friedel–Crafts reaction of indoles with cyclic N-sulfimines using a chiral phosphoric acid as an organocatalyst has been developed.

Lu's [3 + 2] cycloaddition of allenes with electrophiles: discovery, development and synthetic application

The discovery of Lu's [3 + 2] cycloaddition reaction is briefly introduced, and the recent important developments and synthetic applications of Lu's [3 + 2] cycloaddition reaction are highlighted and overviewed.