Synthesis of Pyrimidopyrrolopyridazines via a Tandem Reaction of Heterocyclic Ketene Aminals with 1,2-Diaza-1,3-dienes

Synthesis of 3-indolyl all-carbon quaternary centers via Rh/Brønsted acid co-catalyzed three-component reactions of azoalkenes with indoles and diazoacetates

Highly diastereoselective three-component reactions of azoalkenes with indoles and diazoacetates have been developed via a Rh/Brønsted acid co-catalyzed strategy. These transformations provide efficient construction of a range of 3-indolyl all-carbon quaternary centers in good yields and diastereoselectivities. This reaction is proposed to proceed through Michael-type trapping of zwitterionic intermediates generated from metal carbenes and indoles.

Highly Stereoselective Synthesis of α-Oxoketene N,X-Acetals (X = O, S, or N) under Mild Conditions

A straightforward synthesis of α-oxoketene N,X-acetals (X = O, S, or N) from β-oxoamides has been described. The oxyphosphonium intermediates are quickly generated in situ from β-oxoamides in the presence of Hendrickson reagent and triflic anhydride (Tf2O) in a chemoselective manner and undergo a chemoselective nucleophilic substitution with phenols, thiophenols, or aromatic amines to deliver the desired products with high stereoselectivity. The one-pot synthetic protocol features readily available starting materials, mild conditions, simple execution, and a wide range of synthetic potentials of the products.

Cascade [3 + 2]/[4 + 2] Cycloaddition of Enaminones with Vinylene Carbonate for the Synthesis of Functionalized Pyrrolo[2,1-a]isoquinoline Derivatives

We developed a protocol for the synthesis of functionalized pyrrolo[2,1-a]isoquinoline derivatives (PIQDs) 3 from enaminones 1 using vinylene carbonate 2. This strategy involved [3 + 2] and [4 + 2] cycloadditions via heating a mixture of substrates 1 with vinylene carbonate 2 and DCE at 60 °C, catalyzed by [Cp*RhCl2]2 and oxidized with Cu(OAc)2 and AgSbF6 promoted by NaOAc. As we increased the reaction temperature to 110 °C under the same conditions, we synthesized PIQDs 4 through sequential C-H activation, alkene insertion, migratory insertion, C-N reductive elimination, β-O elimination, and finally dehydration. As a result, a series of PIQDs 3-4 were generated by forming four bonds (2 C-C and 2 C-N bonds) in a single step. This strategy realizes the synthesis of linear molecules with potential biological activity, specifically natural-like heterocycles (3-4). It expands the application of vinylene carbonate as a C2 synthon in the construction of pyrrole and isoquinoline skeletons for the synthesis of functionalized PIQDs in combinatorial and parallel syntheses via one-pot reactions.

Radical Cascade Reaction of Heterocyclic Ketene Aminals with Thiocyanates Promoted by Visible Light: Synthesis of Functionalized Fused 2-Iminothiazolines

Herein, a visible-light-promoted radical cascade cyclization of heterocyclic ketene aminals (HKAs) and thiocyanates was developed to access functionalized fused 2-iminothiazolines. This novel cascade reaction can be realized under only visible-light irradiation without the help of external photocatalysts, oxidants, and additives. These multicomponent cascade reactions demonstrate excellent selectivity for the Z-isomers and ensure the formation of the products only in their isomeric form. Preliminary mechanism investigations demonstrated that HKAs and thiocyanates can form electron donor-acceptor complexes for harvesting the energy of visible light to activate substrates and generate reactive radicals. This protocol can be used for synthesizing various natural-like products such as fused 2-iminothiazolines. This approach demonstrates multiple advantages such as commercially available substrates, convenient operation, environmentally friendly, mild conditions, and an efficient multicomponent reaction (2A + B).

Hydroxyl-Directed Rh(III)-Catalyzed C-H Functionalization: Access to Benzo[de]chromenes

A cascade oxidative annulation reaction of heterocyclic ketene aminals (HKAs) with internal alkynes catalyzed by [Cp*RhCl₂]₂ and oxidized by Cu(OAc)₂·H₂O was developed to efficiently synthesize highly functionalized benzo[de]chromene derivatives in good to excellent yields. The reaction proceeded by the sequential cleavage of C(sp²)-H/O-H and C(sp²)-H/C(sp²)-H bonds. These multicomponent cascade reactions were highly regioselective. In addition, all of the benzo[de]chromene products exhibited intense fluorescence emission in the solid state, and they demonstrated concentration-dependent quenching in the presence of Fe³⁺, indicating that these compounds could be used in the recognition of Fe³⁺.

Catalytic [2,3]-sigmatropic rearrangement of sulfonium ylides derived from azoalkenes: non-carbenoid Doyle-Kirmse reaction

The Sc(III)-catalyzed [2,3]-sigmatropic rearrangement of sulfonium ylides derived from azoalkenes has been established. Owing to the absence of a carbenoid intermediate, this protocol represents the first non-carbenoid variant of the Doyle-Kirmse reaction. Under mild conditions, a variety of tertiary thioethers have been readily prepared in good to excellent yields.

Multicomponent Cascade Reaction of 3-Formylchromones: Highly Selective Synthesis of Functionalized 9-Azabicyclo[3.3.1]nonane Derivatives

A novel protocol for the preparation of functionalized 9-azabicyclo[3.3.1]nonane (ABCN) derivatives from 3-formylchromones, enaminones, and heterocyclic ketene aminals (HKAs) through an unprecedented cascade reaction has been developed by simply refluxing the mixture of the substrates 1-3. As a result, a series of ABCNs were produced through a very complex cascade reaction. This protocol can be used in the synthesis of ABCNs that are suitable for combinatorial and parallel syntheses of ABCN natural-like products in a one-pot reaction.

Substrate-Tuned Domino Annulation for Selective Synthesis of Poly-substituted Benzo[f]imidazo[2,1-a][2,7]naphthyridines and 3-Azaheterocyclic Substituted 2-Arylquinolines

A domino annulation/oxidation of heterocyclic ketene aminals (HKAs) and 2-aminochalcones has been developed for the selective synthesis of poly-substituted benzo[f]imidazo[2,1-a][2,7]naphthyridines and 3-azaheterocyclic substituted 2-arylquinolines. These reactions proceed well under mild conditions without any additives. Plausible mechanisms for such a polycyclic ring system assembly were also proposed. Moreover, benzo[f]imidazo[2,1-a][2,7]naphthyridine 3g displayed a fluorescence effect, demonstrating the potential applications in organic optical materials.

Cascade reaction of isatins with nitro-substituted enamines: highly selective synthesis of functionalized (Z)-3-(1-(arylamino)-2-oxoarylidene)indolin-2-ones

A novel protocol for the construction of functionalized (Z)-3-(1(-arylamino)-2-oxoarylidene)indolin-2-ones (AOIDOs) from isatins 1 with nitro-substituted enamines 2via an unprecedented cascade reaction catalyzed by sulfamic acid is developed.