Development of Methods for the Synthesis of Libraries of Substituted Maleimides and α,β-Unsaturated-γ-butyrolactams

Pd(II)-catalyzed regioselective ring opening/[3+2] annulation reaction of enaminones with cyclopropenones: divergent synthesis of γ-butenolides and γ-lactams

A practical, effective, and regioselective palladium-catalyzed ring opening/[4+2] annulation of enaminones with cyclopropenones for the controllable synthesis of highly substituted γ-butenolides and γ-lactams has been described. This method for the first time reports the regio-selective annulation reaction on the carbon and amine groups of the enaminone structure. This reaction is characterized by its wide substrate scope, good functional group compatibility, moderate to good yields, scale-up synthesis, and versatile transformations, providing a versatile and general protocol to construct γ-butenolides and γ-lactams.

Developing a straightforward route toward the synthesis of arylaminomaleimides by palladium-catalyzed arylation of one-pot synthesized aminomaleimides

3-Aryl-4-aminomaleimides have well-demonstrated applications, such as being used as fluorophores and inhibitors. However, their previous synthesis methods have involved tedious multi-step procedures or methods that need pre-functionalized maleimides and toxic or unstable reagents. Here, a feasible method is developed to synthesize these useful compounds. This includes the one-pot preparation of 3-aminomaleimides, followed by their direct arylation through a palladium-catalyzed Heck reaction with various aryl iodides regioselectively at the β-position of their amine substituents. The results show that this method efficiently exhibits a broad scope.

Access to thiionized-, selenolized-, and alkylated 5-alkylidene 3-pyrrolin-2-one derivatives via a regioselective oxidative annulation reaction

A metal-free regioselective oxidative annulation reaction of readily available 2,4-pentanediones with primary amines has been described. This protocol provides a divergent strategy for the incorporation of various radical donors into 5-alkylidene 3-pyrrolin-2-one skeletons, producing a variety of thiionized-, selenolized-, and alkylated 5-alkylidene 3-pyrrolin-2-one derivatives. Moreover, the diverse synthetic transformations of the 5-alkylidene 3-pyrrolin-2-one products were also investigated.

Synthesis of 2-carboxyaniline-substituted maleimides from 2'-nitrochalcones

A practical, one-pot approach to 3-anilino-4-(het)arylmaleimides by simple heating of aqueous DMSO solution of 2'-nitrochalcones with potassium cyanide in the presence of formic acid has been developed. This new reaction provides effective access to a variety of β-substituted α-aminomaleimides which have recently become a subject of growing interest as small, easily modified and environmentally responsive fluorescent probes.

Total Synthesis of Aqabamycin G, a Nitrophenyl Indolylmaleimide Marine Alkaloid from Vibrio sp. WMBA

The first total synthesis of the marine alkaloid aqabamycin G is disclosed. The synthetic sequence involved the stepwise addition to maleimide of an indole motif and a substituted diazo-benzenoid unit derived from acetaminophen. An alternative strategy using a protected phenol is also reported.

Facile approach to multifunctionalized 5-alkylidene-3-pyrrolin-2-ones via regioselective oxidative cyclization of 2,4-pentanediones with primary amines and sodium sulfinates

Highly functionalized 5-alkylidene-3-pyrrolin-2-ones were efficiently synthesized via a four-component cascade cyclization/sulfonylation reaction between readily available 2,4-pentanediones, primary amines and sodium sulfinates under mild conditions.

Rhodium(iii)-catalyzed [5+1] annulation of 2-alkenylphenols with maleimides: access to highly functionalized spirocyclic skeletons

A new edition of [5+1] annulation reaction of maleimides with 2-alkenylphenols has been discovered under a Rh(iii)-catalytic system. The process leads to an efficient synthesis of valued spirocyclic scaffolds bearing an oxygen-containing spiro carbon in a single step and shows a broad substrate scope with good functional group tolerance. The synthetic utility has been exemplified by synthesizing highly functionalized 2,2-disubstituted-2H-chromene skeletons and a gram-scale synthesis with a low catalyst loading.

Alkynoates as Versatile and Powerful Chemical Tools for the Rapid Assembly of Diverse Heterocycles under Transition-Metal Catalysis: Recent Developments and Challenges

Heterocycles, heteroaromatics and spirocyclic entities are ubiquitous components of a wide plethora of synthetic drugs, biologically active natural products, marketed pharmaceuticals and agrochemical targets. Recognizing their high proportion in drugs and rich pharmacological potential, these invaluable structural motifs have garnered significant interest, thus enabling the development of efficient catalytic methodologies providing access to architecturally complex and diverse molecules with high atom-economy and low cost. These chemical processes not only allow the formation of diverse heterocycles but also utilize a range of flexible and easily accessible building units in a single operation to discover diversity-oriented synthetic approaches. Alkynoates are significantly important, diverse and powerful building blocks in organic chemistry due to their unique and inherent properties such as the electronic bias on carbon-carbon triple bonds posed by electron-withdrawing groups or the metallic coordination site provided by carbonyl groups. The present review highlights the comprehensive picture of the utility of alkynoates (2007-2019) for the synthesis of various heterocycles (> 50 types) using transition-metal catalysts (Ru, Rh, Pd, Ir, Ag, Au, Pt, Cu, Mn, Fe) in various forms. The valuable function of versatile alkynoates (bearing multifunctional groups) as simple and useful starting materials is explored, thus cyclizing with an array of coupling partners to deliver a broad range of oxygen-, nitrogen-, sulfur-containing heterocycles alongside fused-, and spiro-heterocyclic compounds. In addition, these examples will also focus the scope and reaction limitations, as well as mechanistic investigations into the synthesis of these heterocycles. The biological significance will also be discussed, citing relevant examples of drug molecules highlighting each class of heterocycles. This review summarizes the recent developments in the synthetic methods for the synthesis of various heterocycles using alkynoates as readily available starting materials under transition-metal catalysis.

Pd(II)-Catalyzed oxidative alkenylation of 4-hydroxycoumarin with maleimide via a C-H bond activation strategy

A Pd(ii)-catalyzed oxidative alkenylation of 4-hydroxycoumarins with maleimides for the synthesis of 4-hydroxy-3-maleimidecoumarins has been described. This methodology proceeds via C-H activation and C(sp2)-C(sp2) bond formation providing a series of alkenylated Heck-type products.

Scalable Synthesis of 3-Ethyl-4-methyl-1,5-dihydro-2H-pyrrol-2-one: An Important Building Block of the Antidiabetic Drug Glimepiride

A four-step, practical, and easily scalable synthesis of 3-ethyl-4-methyl-1,5-dihydro-2H-pyrrol-2-one, an important building block of the antidiabetic drug glimepiride, has been accomplished. Key features are the synthesis of 3-methyl-4-hydroxy-2-butenolide in water and triflic acid mediated N-benzyl lactam N-deprotection. The main advantages of this process are the scalable synthetic route and decreased number of reaction steps, which paves the way for the industrial-scale synthesis of 3-ethyl-4-methyl-1,5-dihydro-2H-pyrrol-2-one.

Zn(OAc)2-catalyzed tandem cyclization of isocyanides, α-diazoketones, and anhydrides: a general route to polysubstituted maleimides

A zinc-catalyzed three-component reaction of isocyanides, α-diazoketones, and anhydrides has been realized as a novel and efficient method for the synthesis of polysubstituted maleimides.

Copper-Catalysed Bromoamination of Maleimides using NBS as the Bromine Source

CuBr2-catalysed bromoamination of maleimides has been achieved in THF with alkylamines and N-bromosuccinimide as nitrogen and bromine sources respectively. The reaction conditions were optimised. A series of bromoamination products such as 3-amino-4-bromomaleimides was synthesised with satisfactory yields. Furthermore, a plausible mechanism of the bromoamination of maleimides was discussed.

Rational design of substituted maleimide dyes with tunable fluorescence and solvafluorochromism

A series of maleimide derivatives were systematically designed and synthesized with tunable fluorescent properties. The facile modifications herein provide a simple methodology to expand the scope of maleimide-based dyes and also provide insight into the relationship between substitution pattern and optical properties.

Rhodium(iii)-catalyzed C–H activation at the C4-position of indole: switchable hydroarylation and oxidative Heck-type reactions of maleimides

A Rh(iii)-catalyzed C–H activation of indole at the C4-position leading to novel and switchable functionalization has been reported by employing a weakly co-ordinating COCF₃ group as a directing group.

Quantitatively monitoring polymer chain growth and topology formation based on monodisperse polymers

In this work, a novel technique for monitoring polymer chain growth and topology formation was demonstrated by precision synthesis of monodisperse polymers, which opened a novel avenue for obtaining real-time polymer structure information.

Assembly of Polysubstituted Maleimides via Palladium-Catalyzed Cyclization Reaction of Alkynes with Isocyanides

An efficient and convenient palladium-catalyzed cyclization reaction of alkynes with isocyanides is described herein. This protocol allows the practical synthesis of many valuable polysubstituted maleimide derivatives after hydrolysis with a broad scope of substrates and mild reaction conditions. C-C, C═O, and C-N bonds were constructed in this transformation with isocyanide serving as both C and N sources.

Aminomaleimide fluorophores: a simple functional group with bright, solvent dependent emission

Amino-substituted maleimides form a new class of highly emissive compounds, with large Stokes shifts (>100 nm) and high quantum yields (up to ∼60%).

Amino-substituted maleimides form a new class of highly emissive compounds, with large Stokes shifts (>100 nm) and high quantum yields (up to ∼60%). Emission is responsive to the maleimide's environment with both a red-shift, and quenching, observed in protic polar solvents. Aminomaleimides are easily functionalised, providing a versatile fluorescent probe.

Palladium-catalyzed cross-dehydrogenative coupling of maleimides with simple arenes: a fast track to highly substituted maleimides

A combination of PivOH/TFA, accelerates an unprecedented palladium-catalyzed CDC diarylation of unfunctionalized maleimides with simple arenes.

A challenging Heck reaction of maleimides

A difficult Heck reaction of base-sensitive maleimides is realized by using a weak base in a carbonate solvent.

Preparation of dibenzo[e,g]isoindol-1-ones via Scholl-type oxidative cyclization reactions

A flexible synthesis of dibenzo[e,g]isoindol-1-ones has been developed. Dibenzo[e,g]isoindol-1-ones represent simplified benzenoid analogues of biological indolo[2,3-a]pyrrolo[3,4-c]carbazol-5-ones (indolocarbazoles), compounds that have demonstrated a wide range of biological activity. The synthesis of the title compounds involved tetramic acid sulfonates. Different aryl groups were introduced at C4 of the heterocyclic ring via Suzuki-Miyaura cross-coupling reactions. Finally, mild Scholl-type oxidative cyclizations mediated by phenyliodine(III) bis(trifluoroacetate) (PIFA) converted some of the latter compounds into the corresponding dibenzo[e,g]isoindol-1-ones. A systematic study of the oxidative cyclization revealed the following reactivity trend: 3,4-dimethoxyphenyl ≫ 3-methoxyphenyl > 3,4,5-trimethoxyphenyl > 4-methoxyphenyl ≈ phenyl. Overall, the oxidative cyclization required at least two methoxy groups distributed in the aromatic rings, at least one of which had to be located para to the site of the cyclization.

Structural complexity through multicomponent cycloaddition cascades enabled by dual-purpose, reactivity regenerating 1,2,3-triene equivalents

Multicomponent reactions allow for more bond-forming events per synthetic operation, enabling more step- and time-economical conversion of simple starting materials to complex and thus value-added targets. These processes invariably require that reactivity be relayed from intermediate to intermediate over several mechanistic steps until a termination event produces the final product. Here, we report a multicomponent process in which a novel 1,2,3-butatriene equivalent (TMSBO: TMSCH2C≡CCH2OH) engages chemospecifically as a two-carbon alkyne component in a metal-catalysed [5 + 2] cycloaddition with a vinylcyclopropane to produce an intermediate cycloadduct. Under the reaction conditions, this intermediate undergoes a remarkably rapid 1,4-Peterson elimination, producing a reactive four-carbon diene intermediate that is readily intercepted in either a metal-catalysed or thermal [4 + 2] cycloaddition. TMSBO thus serves as an yne-to-diene transmissive reagent coupling two powerful and convergent cycloadditions--the homologous Diels-Alder and Diels-Alder cycloadditions--through a vinylogous Peterson elimination, and enabling flexible access to diverse polycycles.

A mild synthesis of N-functionalised bromomaleimides, thiomaleimides and bromopyridazinediones

Bromomaleimides are useful building blocks in synthesis and powerful reagents for the selective chemical modification of proteins. A mild new synthesis of these reagents is described, along with the convenient transferability of the approach to dithiomaleimides and bromopyridazinediones.