Carbamoyl radical-mediated synthesis and semipinacol rearrangement of β-lactam diols

Facile synthesis of tetrahydroquinoline containing dithiocarbamate derivatives via one-pot sequential multicomponent reaction

An efficient sequential multi-component method for the synthesis of tetrahydroquinoline containing dithiocarbamates has been developed. This reaction involved a boronic acid-catalysed reduction of quinolines to tertrahydroquinolines, followed by nucleophilic addition reaction with carbon disulphide to form dithiocarbamic acids and subsequent S-arylation via external base-free Chan-Evans-Lam coupling in a one-pot operation. The methodology is compatible with a wide variety of functional groups and also useful in the late-stage functionalization of pharmaceuticals. The dual role of the boronic acid as a catalyst (in the reduction of quinolines) and as a reagent (in the S-arylation) has been demonstrated for the first time herein.

Visible light mediated iron-catalyzed addition of oxamic acids to imines

Oxamic acids where shown to add to imines, providing a broad range of α-aminoacid amides in generally good yields. The process is efficient on pre-formed imines but may also be conducted using a 3-component strategy by simply mixing aldehydes, amines and oxamic acids in the presence of ferrocene, acting both as a photocatalyst under visible light and as a Lewis acid. The reaction proceeds through the addition onto the imine of a carbamoyl radical intermediate generated through a charge transfer from the carboxylate ligand to a Fe(iii) species (LMCT).

Regioselective Synthesis of the Tetrahydrocarbazole Core of Akuammiline Alkaloids via Palladium-Catalyzed Intramolecular Arylation Reaction

Tetrahydrocarbazole is the central core for several biologically active alkaloids, and regioselective synthesis of this core is a challenging task. Herein, we report an efficient strategy for the synthesis of this core involving palladium-catalyzed intramolecular arylation reaction with excellent regioselectivity (>99%) starting from N-phenyl-bromoalkene without having any relocation of double bonds via competitive palladium-catalyzed isomerization reaction. Broad functional group tolerance and exclusive regioselectivity have been observed for meta-substituted halide substrates. Furthermore, this reaction can be scalable on the gram scale.

Multicomponent reactions to access S-aryl dithiocarbamates via an electron donor-acceptor complex under open-to-air conditions

A simple and efficient transition-metal/photocatalyst-free visible-light-driven one-pot three-component reaction between thianthrenium salts, carbon disulfide and amines under an air atmosphere for the preparation of biologically relevant S-aryl dithiocarbamates is developed. This methodology is robust and scalable, and exhibits a broad substrate scope and excellent functional group tolerance. Of note, a wide range of primary aliphatic amines bearing different groups are suitable for this strategy. The synthetic utility was further demonstrated by a two-step one-pot multi-component reaction and photo-flow decagram-scale synthesis. Preliminary mechanistic studies suggest that the association of the dithiocarbamate anion with thianthrenium salts formed an electron donor-acceptor complex, which upon excitation with visible light produced an aryl radical via single-electron transfer.

S-alkyl Dithiocarbamates Synthesis through Electrochemical Multicomponent Reaction of Thiols, Hydrogen Sulfide, and Isocyanides

Dithiocarbamates synthesis is extremely important in plenty of biomedical and agrochemical applications, especially fungicide development, but remains a great challenge. In this work, we have successfully developed a multicomponent reaction protocol to convert H2S into S-alkyl dithiocarbamates under constant current conditions. No additional oxidants nor additional catalysts are required, and due to mild conditions, the reactions display a broad substrate scope, including varieties of thiols or disulfides.

Multicomponent Reaction of CS2, Amines, and Sulfoxonium Ylides in Water: Straightforward Access to β-Keto Dithiocarbamates, Thiazolidine-2-thiones, and Thiazole-2-thiones

Simple, versatile, and catalyst-free synthetic methods for β-keto dithiocarbamates, thiazolidine-2-thiones, and thiazole-2-thiones via the multicomponent reaction of CS₂, amines, and sulfoxonium ylides have been described. The β-keto sulfoxonium ylides furnished β-keto dithiocarbamates in the presence of CS₂ and secondary amines, whereas primary amines afforded thiazolidine-2-thiones or thiazole-2-thiones after dehydration in an acidic environment. With simple procedures, the reaction has a wide substrate scope and excellent functional group tolerance.

An Enantiospecific Synthesis of Isoneoamphilectane Confirms Its Strained Tricyclic Structure

We describe a total synthesis of the rare isocyanoterpene natural product isoneoamphilectane and two of its unnatural diastereomers. The significantly strained ring system of the reported natural product─along with a hypothesis about a biosynthetic relationship to related family members─inspired us to consider a potential misassignment in the structure's relative configuration. As a result, we initially targeted two less strained, more accessible, stereoisomers of the reported natural product. When these compounds failed to exhibit spectroscopic data that matched those of isoneoamphilectane, we embarked on a synthesis of the originally proposed strained structure via an approach that hinged on a challenging cis-to-trans decalone epimerization. Ultimately, we implemented a novel cyclic sulfite pinacol-type rearrangement to generate the strained ring system. Additional features of this work include the application of a stereocontrolled Mukaiyama-Michael addition of an acyclic silylketene acetal, an unusual intramolecular alkoxide-mediated regioselective elimination, and an HAT-mediated alkene hydroazidation to forge the C-N bond of the tertiary isonitrile. Throughout this work, our synthetic planning was heavily guided by computational analyses to inform on key issues of stereochemical control.

Studies towards the Synthesis of (+)‐Dictyoxetane

The dolabellane-type diterpene dictyoxetane represents a significant challenge to synthetic organic chemistry. Methodology directed towards the total synthesis of naturally occurring (+)-dictyoxetane is reported. Catalytic asymmetric synthesis of the trans-hydrindane ring system is achieved through chemoselective deoxygenation of the Hajos-Parrish ketone. An alternative to the Garst-Spencer furan annulation is developed for the synthesis of a 2,5-dimethyl, tetrasubstituted furan, employing a tandem 5-exo-dig alcohol to alkyne cyclisation/aromatisation reaction as a key step. The (4+3) cycloaddition reaction of an oxyallyl cation with a tetrasubstituted furan is established on a cyclohexanone-derived model system, and a range of related (4+3) cycloadditions investigated on a homochiral, trans-hydrindane-fused furan, where regio- and diastereoselectivity is required for the natural product synthesis. In an alternative (4+2) Diels-Alder approach, a C₂ -symmetric vinyl sulfoxide-based chiral ketene equivalent is used to prepare oxanorbornenes with the same oxygen bridge stereochemistry found in the 2,7-dioxatricyclo[4.2.1.0^3,8 ]nonane ring system of the natural product.

Synthesis of functionalized S-benzyl dithiocarbamates from diazo-compounds via multi-component reactions with carbon disulfide and secondary amines

Triflic acid promoted multi-component synthesis of functionalized S-benzyl dithiocarbamates from diazo compounds, carbon disulfide and secondary amines is reported. The reactions proceeded at room temperature and gave the desired dithiocarbamates in good yields. Wide-substrate scope and easy operation are the important features of this methodology.

Decarboxylative 1,2-rearrangement of cyclic carbonates promoted by Lewis acid

A Lewis acid-mediated decarboxylative 1,2-rearrangement reaction of cyclic carbonates was developed. The selectivity of the migration in the decarboxylative 1,2-rearrangement of cyclic carbonates was opposite to that of the corresponding 1,2-diols under the same reaction conditions. This contrasting selectivity of the migration was confirmed in a variety of substrates.

Domino assembly of dithiocarbamates via Cu-catalyzed denitrogenative thiolation of iodotriazole-based diazo precursors

An efficient domino approach to assemble benzoxazoles and anthranilamides bearing dithiocarbamate moieties has been developed. The proposed route represents a Cu-catalyzed three-component reaction between readily available 5-iodo-1,2,3-triazoles, amines, and CS₂. The cascade transformation is based on a denitrogenative coupling of in situ formed dithiocarbamic acids with diazo intermediates, generated via annulation-triggered triazole ring-opening. This method is applicable to nucleophilic secondary amines and features good functional group compatibility.

Photoinduced carbamoylation reactions: unlocking new reactivities towards amide synthesis

The preparation of amide-containing compounds is among the most interesting and challenging topics for the synthetic community. Such relevance is given by their reactive aspects explored in the context of organic synthesis and by the direct application of these compounds as pharmaceuticals and useful materials, and their key roles in biological structures. A simple and straightforward strategy for the amide moiety installation is the use of carbamoyl radicals - this nucleophilic one-electron intermediate is prone to undergo a series of transformations, providing a range of structurally relevant derivatives. In this review, we summarize the latest advances in the field from the perspective of photoinduced protocols. To this end, their synthetic applications are organized accordingly to the nature of the radical precursor (formamides through HAT, 4-substituted-1,4-dihydropyridines, oxamic acids, and N-hydroxyphthalimido esters), the mechanistic aspects also being highlighted. The discussion also includes a recent approach proceeding via photolytic C-S cleavage of dithiocarbamate-carbamoyl intermediates. By exploring fundamental concepts, this material aims to offer an understanding of the topic, which will encourage and facilitate the design of new synthetic strategies applying the carbamoyl radical.

Oxamic acids: useful precursors of carbamoyl radicals

This review article describes the recent development in the chemistry of carbamoyl radicals generated from oxamic acids. This mild and efficient method compares well with previous methods of generation of these nucleophilic radicals. The oxidative decarboxylation of oxamic acids can be mediated through thermal, photochemical, electrochemical or photoelectrochemical means, generating carbamoyl radicals, which may further add to unsaturated systems to provide a broad range of important amides. Oxidative decarboxylation of oxamic acids also offers a straightforward entry for the preparation of urethanes, ureas, and thioureas.

Multicomponent Synthesis of S-Benzyl Dithiocarbamates from para-Quinone Methides and Their Biological Evaluation for the Treatment of Alzheimer's Disease

Multicomponent synthesis of biologically relevant S-benzyl dithiocarbamates from para-quinone methides, amines, and carbon disulfide are described under catalyst and additive-free conditions. The reactions proceeded at room temperature in a short span of time with excellent yields. One of the synthesized compounds, 3e showed considerable acetylcholinesterase (AChE) inhibitory (51.70 + 5.63% at 20 μm) and antioxidant (63.52 ± 1.15 at 20 μm) activities.

One-pot two-step dithiocarbamylation of styrenes: metal-free stereoselective synthesis of styrenyl dithiocarbamates

Styrenes have been functionalized to produce styrenyl dithiocarbamates by a one-pot two-step procedure without using any metal catalysts. Styrene was transformed into a bromo-derivative, which undergoes a domino nucleophilic substitution followed by elimination in the presence of a dithiocarbamate anion and triethylamine to produce trans-styrenyl dithiocarbamates exclusively. The reaction shows a wide substrate scope and good yields of products.

Aryne Three-Component Coupling Involving CS2 for the Synthesis of S-Aryl Dithiocarbamates

A facile synthesis of biologically important S-aryl dithiocarbamates has been demonstrated by the aryne three-component coupling involving CS₂ and aliphatic amines. This transition-metal-free and mild reaction is scalable and operates with good functional group compatibility. Preliminary mechanistic experiments, including density functional theory studies, are also provided. Moreover, with 3-triflyloxybenzynes, a unique four-component coupling incorporating tetrahydrofuran was observed.

Cascade Vinyl Radical Ipso-Cyclization Reactions and the Formation of α,β-Unsaturated-β-aryl-γ-lactams from N-Propargyl Benzamides

Various N-(2-bromo-allyl) benzamides were used as the starting materials to study vinyl radical cyclization reactions. The vinyl radicals underwent ipso-cyclization, fragmentation, and cyclization reactions to produce β-aryl-γ-lactams with the carbonyl group remaining intact. To further study this cascade radical reaction, vinyl radicals were generated by the addition of a tributyltin radical to alkyne moieties, followed by radical ipso-cyclization, fragmentation, cyclization, and β-scission reactions with the production of a series of α,β-unsaturated-β-aryl-γ-lactam derivatives. This new type of radical reaction was examined from the substituent effects on both the amino groups and the aryl groups. A bulky tert-butyl substituent on the amino group enhanced the formation of a Z-conformation of the benzamides and facilitated vinyl radical ipso-cyclization reactions. A synthetic method for preparing α,β-unsaturated-β-aryl-γ-lactams from N-propargyl benzamides was developed.

Multicomponent Synthesis of Biologically Relevant S-Aryl Dithiocarbamates Using Diaryliodonium Salts

A transition-metal-free one-pot three-component annulation between diaryliodonium triflates, cyclic and acyclic aliphatic amines, and carbon disulfide providing a convenient and efficient access to biologically relevant S-aryl dithiocarbamates is developed. The reaction does not require metal, base, or any other additive and operates under mild and ambient conditions. This methodology is robust, scalable, and exhibits a broad substrate scope. The in silico analysis revealed that the majority of the compounds have a drug-likeness and good ADMET characteristics.

Recent advances in transition metal-catalyzed reactions of carbamoyl chlorides

This review highlights the recent advancements of carbamoyl chlorides in transition metal-catalyzed reactions to access various amide-containing molecules and heterocycles.

Organocatalytic Enantioselective Conia-Ene-Type Carbocyclization of Ynamide Cyclohexanones: Regiodivergent Synthesis of Morphans and Normorphans

Described herein is an organocatalytic enantioselective desymmetrizing cycloisomerization of arylsulfonyl-protected ynamide cyclohexanones, representing the first metal-free asymmetric Conia-ene-type carbocyclization. This method allows the highly efficient and atom-economical construction of a range of valuable morphans with wide substrate scope and excellent enantioselectivity (up to 97 % ee). In addition, such a cycloisomerization of alkylsulfonyl-protected ynamide cyclohexanones can lead to the divergent synthesis of normorphans as the main products with high enantioselectivity (up to 90 % ee). Moreover, theoretical calculations are employed to elucidate the origins of regioselectivity and enantioselectivity.

Chemo- and Regioselective Functionalization of Isotactic Polypropylene: A Mechanistic and Structure-Property Study

Polyolefins represent a high-volume class of polymers prized for their attractive thermomechanical properties, but the lack of chemical functionality on polyolefins makes them inadequate for many high-performance engineering applications. We report a metal-free postpolymerization modification approach to impart functionality onto branched polyolefins without the deleterious chain-coupling or chain-scission side reactions inherent to previous methods. The identification of conditions for thermally initiated polyolefin C-H functionalization combined with the development of new reagents enabled the addition of xanthates, trithiocarbonates, and dithiocarbamates to a variety of commercially available branched polyolefins. Systematic experimental and kinetic studies led to a mechanistic hypothesis that facilitated the rational design of reagents and reaction conditions for the thermally initiated C-H xanthylation of isotactic polypropylene (iPP) within a twin-screw extruder. A structure-property study showed that the functionalized iPP adheres to polar surfaces twice as strongly as commercial iPP while demonstrating similar tensile properties. The fundamental understanding of the elementary steps in amidyl radical-mediated polyolefin functionalization provided herein reveals key structure-reactivity relationships for the design of improved reagents, while the demonstration of chemoselective and scalable iPP functionalization to realize a material with improved adhesion properties indicates the translational potential of this method.

Deciphering the exceptional selectivity of semipinacol rearrangements in cis-fused β-lactam diols using high-level quantum chemical methods

The semipinacol rearrangement mechanism in cis-fused β-lactam diols has been found to kinetically and thermodynamically favor acyl migration. The semipinacol rearrangement was also investigated for trans-fused β-lactam diol, in which the reversed selectivity that favors the alkyl migration was observed.

Visible-light mediated carbamoyl radical addition to heteroarenes

Decarboxylative carbamoylation of heteroarenes using oxamic acids under visible-light irradiation is described.

A General Approach to Site-Specific, Intramolecular C-H Functionalization Using Dithiocarbamates

Intramolecular hydrogen atom transfer is an established approach for the site-specific functionalization of unactivated, aliphatic C-H bonds. Transformations using this strategy typically require unstable intermediates formed using strong oxidants and have mainly targeted C-H halogenations or intramolecular aminations. Herein, we report a site-specific C-H functionalization that significantly increases the synthetic scope and convergency of reactions proceeding via intramolecular hydrogen atom transfer. Stable, isolable N-dithiocarbamates are used as precursors to amidyl radicals formed via either light or radical initiation to efficiently deliver highly versatile alkyl dithiocarbamates across a wide range of complex structures.

Recent advances in β-lactam synthesis

During the past century, β-lactams have been identified as the core of penicillin and since then several strategies have been developed for their synthesis.

Synthesis of a bicyclic oxo-γ-lactam from a simple caprolactam derivative

Reaction of caprolactams with an axial ester substituent with base gives the 6-azabicyclo-[3.2.1]octane ring system via Dieckmann cyclization.

Diastereoselective synthesis of 3-acetoxy-4-(3-aryloxiran-2-yl)azetidin-2-ones and their transformation into 3,4-oxolane-fused bicyclic β-lactams

cis-3-Acetoxy-4-(3-aryloxiran-2-yl)azetidin-2-ones were prepared through a Staudinger [2+2]-cyclocondensation between acetoxyketene and the appropriate epoxyimines in a highly diastereoselective way. Subsequent potassium carbonate-mediated acetate hydrolysis, followed by intramolecular ring closure through epoxide ring opening, afforded stereodefined 3-aryl-4-hydroxy-2-oxa-6-azabicyclo[3.2.0]heptan-7-ones as a novel class of C-fused bicyclic β-lactams. Selective benzylic oxidation of bicyclic N-(4-methoxybenzyl)-β-lactams with potassium persulfate and potassium dihydrogen phosphate provided the corresponding N-aroyl derivatives as interesting leads for further β-lactamase inhibitor development.

Diverse N-Heterocyclic Ring Systems via Aza-Heck Cyclizations of N-(Pentafluorobenzoyloxy)sulfonamides

Aza‐Heck cyclizations initiated by oxidative addition of Pd⁰‐catalysts into the N−O bond of N‐(pentafluoro‐benzoyloxy)sulfonamides are described. These studies, which encompass only the second class of aza‐Heck reaction developed to date, provide direct access to diverse N‐heterocyclic ring systems.

Synthesis of functionalized 5-substituted thiazolidine-2-thiones via adscititious xanthate-promoted radical cyclization of allyl(alkyl/aryl)dithiocarbamates

An adscititious radical precursor-promoted cyclization is designed and realized in efficient synthesis of functionalized 5-substituted thiazolidine-2-thiones from alkyl allyl(alkyl/aryl)dithiocarbamates.

An unprecedented benzannulation of oxindoles with enalcarbenoids: a regioselective approach to functionalized carbazoles

An unusual rhodium-catalyzed tandem reaction of oxindoles with diazoenals involving the cleavage of sp² C–O and sp² C–C bonds resulted in the formation of 1-hydroxy-2-acylcarbazoles.