α-Lactams(Aziridinones)

Asymmetric Synthesis of α-Chloroamides via Photoenzymatic Hydroalkylation of Olefins

Photoenzymatic intermolecular hydroalkylations of olefins are highly enantioselective for chiral centers formed during radical termination but poorly selective for centers set in the C-C bond-forming event. Here, we report the evolution of a flavin-dependent "ene"-reductase to catalyze the coupling of α,α-dichloroamides with alkenes to afford α-chloroamides in good yield with excellent chemo- and stereoselectivity. These products can serve as linchpins in the synthesis of pharmaceutically valuable motifs. Mechanistic studies indicate that radical formation occurs by exciting a charge-transfer complex templated by the protein. Precise control over the orientation of molecules within the charge-transfer complex potentially accounts for the observed stereoselectivity. The work expands the types of motifs that can be prepared using photoenzymatic catalysis.

Zinc-Mediated Radiosynthesis of Unprotected Fluorine-18 Labelled α-Tertiary Amides

This report describes the development of a Zn(OTf)2 -mediated method for converting α-tertiary haloamides to the corresponding fluorine-18 labelled α-tertiary fluoroamides with no-carrier-added [18 F]tetramethylammonium fluoride. 1,5,7-Triazabicyclo[4.4.0]dec-5-ene is an essential additive for achieving high radiochemical conversion. Under the optimised conditions, radiofluorination proceeds at sterically hindered tertiary sites in high radiochemical conversions, yields, and purities. This method has been successfully automated and applied to access >200 mCi (>7.4 GBq) of several model radiofluorides. Mechanistic studies led to the development of a new, nucleophilic C-H radiofluorination process using N-sulphonyloxyamide substrates.

α-Lactam Electrophiles for Covalent Chemical Biology

Electrophilic groups are one of the key pillars of contemporary chemical biology and medicinal chemistry. For instance, 3-membered N-heterocyclic compounds-such as aziridines, azirines, and oxaziridines-possess unique electronic and structural properties which underlie their potential and applicability as covalent tools. The α-lactams are also members of this group of compounds, however, their utility within the field remains unexplored. Here, we demonstrate an α-lactam reagent (AM2) that is tolerant to aqueous buffers while being reactive towards biologically relevant nucleophiles. Interestingly, carboxylesterases 1 and 2 (CES1/2), both serine hydrolases with key roles in endo- and xenobiotic metabolism, were found as primary covalent targets for AM2 in HepG2 liver cancer cells. All in all, this study constitutes the starting point for the further development and exploration of α-lactam-based electrophilic probes in covalent chemical biology.

Copper-catalysed enantioconvergent alkylation of oxygen nucleophiles

Carbon-oxygen bonds are commonplace in organic molecules, including chiral bioactive compounds; therefore, the development of methods for their construction with simultaneous control of stereoselectivity is an important objective in synthesis. The Williamson ether synthesis, first reported in 18501, is the most widely used approach to the alkylation of an oxygen nucleophile, but it has significant limitations (scope and stereochemistry) owing to its reaction mechanism (SN2 pathway). Transition-metal catalysis of the coupling of an oxygen nucleophile with an alkyl electrophile has the potential to address these limitations, but progress so far has been limited2-7, especially with regard to controlling enantioselectivity. Here we establish that a readily available copper catalyst can achieve an array of enantioconvergent substitution reactions of α-haloamides, a useful family of electrophiles, by oxygen nucleophiles; the reaction proceeds under mild conditions in the presence of a wide variety of functional groups. The catalyst is uniquely effective in being able to achieve enantioconvergent alkylations of not only oxygen nucleophiles but also nitrogen nucleophiles, giving support for the potential of transition-metal catalysts to provide a solution to the pivotal challenge of achieving enantioselective alkylations of heteroatom nucleophiles.

Chemo- and Enantioselective Reduction of α-Keto Amides to α-Hydroxy Amides using Reusable CuO-Nanoparticles as Catalyst

An efficient, commercially available, and reusable copper-oxide nanoparticle (CuO-NPs) and (R)-(-)-DTBM SEGPHOS catalyzed chemo- and enantioselective reduction of α-keto amides to α-hydroxy amides has been developed. The scope of the reaction has been studied with various α-keto amides containing electron-donating and electron-withdrawing groups affording the enantiomerically enriched α-hydroxy amides in good yields with excellent enantioselectivity. The CuO-NPs catalyst has been recovered and reused up to four catalytic cycles without any significant change in particle size, reactivity, and enantioselectivity.

Total Synthesis of (±)-N-Methyldibromoisophakellin and N-Methylugibohlin via Net [3+2] Cycloguanidinylations Employing 2-Amido-1,3-Diamino-Allyl Cations

A concise total synthesis of (±)-N-methyldibromoisophakellin, a member of the monomeric pyrrole-aminoimidazole alkaloid family isolated from the marine sponge Stylissa carbica, was achieved via a net [3+2] cycloaddition to install the cyclic guanidine. This ring annulation employs a 2-amido-1,3-aminoallyl cation obtained under oxidative conditions from variously N-substituted guanidines which in one instance led to isolation of a tetracycle bearing a carbinolamine center through subsequent benzylic oxidation. Finally, the serendipitous formation of a unique, related alkenyl guanidine, N-methylugibohlin, achieved via ring opening of cyclic guanidine under acidic conditions suggests that ugibohlin may be an artifact of isolation.

Access to Sterically Hindered Thioethers (α-Thioamides) Under Mild Conditions Using α-Halohydroxamates: Application toward 1,4-Benzothiazinones and 4,1-Benzothiazepinones

Herein, we report a new and highly efficient approach for synthesizing congested α-thioamides under mild reaction conditions (mild base, room temperature, and short duration) using α-halo hydroxamates as direct alkylating agents. The reaction works well with both (hetero)aryl and alkyl thiols, tolerating a broad functional group and diverse substrate scope, including benzeneselenol for selenoether construction. The strategy enables efficient synthesis of biologically relevant 1,4 benzothiazinone and 4,1-benzothiazepinone cores, along with various other functionalized sulfur-based scaffolds of biological importance.

Transition metal-free ether coupling and hydroamidation enabling the efficient synthesis of congested heterocycles

In this study, we discovered that α-bromocarboxamides react with alkynols containing tertiary alcohol moieties to produce congested ethers or heterocycles. Here, the etherification and hydroamidation reactions can be controlled by a suitable base. Both C-O and C-N bond formations occurred without a transition-metal catalyst. The stereospecific etherification and cyclization of diastereo-enriched α-bromocarboxamide afforded the corresponding diastereo-enriched ether and heterocyclic compound.

Aza-Oxyallyl Cation Driven 3-Amido Oxetane Rearrangement to 2-Oxazolines: Access to Oxazoline Amide Ethers

Herein, we report a highly facile and unprecedented activation of 3-amido oxetanes to synthesize 2-oxazoline amide ethers using a transient electrophilic aza-oxyallyl cation as an activating as well as an alkylating agent under mild reaction conditions. The aza-oxyallyl cation driven intramolecular rearrangement of 3-amido oxetanes to 2-oxazolines is the hallmark of this transformation and is a new addition to the reactivity profile of aza-oxyallyl cations.

(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.

Trifluoromethylthiolation of Hindered α-Bromoamides with Nucleophilic Trifluoromethylthiolating Reagents

General methods have not been previously developed for the synthesis of sterically hindered α-SCF₃-substituted carbonyl compounds using nucleophilic trifluoromethylthiolating reagents. Thus, we herein report sp³C-SCF₃ bond formation in hindered α-bromoamides containing 3-bromo-oxindoles and linear α-bromoamides using CuSCF₃ or AgSCF₃ under mild conditions to access sterically hindered α-SCF₃-substituted amides. This transformation is applicable to not only 3-SCF₃-substituted oxindoles but also primary and secondary amides and reveals a broad functional group tolerance. This method will benefit the fields of medicinal and agricultural chemistry.

Fungal Dioxygenase AsqJ Is Promiscuous and Bimodal: Substrate-Directed Formation of Quinolones versus Quinazolinones

Previous studies showed that the Fe^II/α‐ketoglutarate dependent dioxygenase AsqJ induces a skeletal rearrangement in viridicatin biosynthesis in Aspergillus nidulans, generating a quinolone scaffold from benzo[1,4]diazepine‐2,5‐dione substrates. We report that AsqJ catalyzes an additional, entirely different reaction, simply by a change in substituent in the benzodiazepinedione substrate. This new mechanism is established by substrate screening, application of functional probes, and computational analysis. AsqJ excises H₂CO from the heterocyclic ring structure of suitable benzo[1,4]diazepine‐2,5‐dione substrates to generate quinazolinones. This novel AsqJ catalysis pathway is governed by a single substituent within the complex substrate. This unique substrate‐directed reactivity of AsqJ enables the targeted biocatalytic generation of either quinolones or quinazolinones, two alkaloid frameworks of exceptional biomedical relevance.

Silver-Promoted Fluorination Reactions of α-Bromoamides

Silver-promoted C-F bond formation in α-bromoamides by using AgF under mild conditions is reported. This simple method enables access to tertiary, secondary, and primary alkyl fluorides involving biomolecular scaffolds. This transformation is applicable to primary and secondary amides and shows broad functional-group tolerance. Kinetics experiments revealed that the reaction rate increased in the order of 3°>2°>1° α-carbon atom. In addition, it was found that the acidic amide proton plays an important role in accelerating the reaction. Mechanistic studies suggested generation of an aziridinone intermediate that undergoes subsequent nucleophilic addition to form the C-F bond with stereospecificity (i.e., retention of configuration). The synthesis of sterically hindered alcohols and ethers by using Ag^I is also demonstrated. Examples of reactions of α-bromoamides with O nucleophiles are presented.

Chemistry of Tertiary Carbon Center in the Formation of Congested C-O Ether Bonds

Nucleophilic substitutions, including S_N 1 and S_N 2, are classical and reliable reactions, but a serious drawback is their intolerance for both bulky nucleophiles and chiral tertiary alkyl electrophiles for the synthesis of a chiral quaternary carbon center. An S_RN 1 reaction via a radical species is another conventional method used to carry out substitution reactions of bulky nucleophiles and alkyl halides, but chiral tertiary alkyl electrophiles cannot be used. Therefore, a stereospecific nucleophilic substitution reaction using chiral tertiary alkyl electrophiles and bulky nucleophiles has not yet been well studied. In this paper, we describe the reaction of tertiary alkyl alcohols and non-chiral or chiral α-bromocarboxamides as a tertiary alkyl source for the formation of congested ether compounds possessing two different tertiary alkyl groups on the oxygen atom with stereoretention.

Metal free amination of congested and functionalized alkyl bromides at room temperature

Herein, we report a highly facile and unprecedented approach to synthesize congested N-(hetero)aryl amines en route to α-amino acid amides using α-bromoamides as alkylating agents under mild reaction conditions (room temperature). The involvement of aza-oxyallyl cations as alkylating agents is the hallmark of this reaction. The method was readily adapted for the rapid synthesis of coveted 1,4-benzodiazepine-3,5-diones.

Generation and Reactivity of 2-Amido-1,3-diaminoallyl Cations: Cyclic Guanidine Annulations via Net (3 + 2) and (4 + 3) Cycloadditions

Toward a method for direct conversion of alkenes to cyclic guanidines, we report that 1,3-dipolar cycloadditions of 2-amido-1,3-diamino allylic cations with alkenes provide a new method for direct cyclic guanidine annulation. Generated under oxidative conditions, the 2-amido-1,3-diaminoallyl cations react as 1,3-dipoles providing rapid access to 2-amino imidazolines through net (3 + 2) cycloadditions. The utility is demonstrated through a concise synthesis of the oroidin alkaloid, phakellin. The described 1,3-dipole also participates in net (4 + 3) cycloadditions with dienes.

The Fascinating Chemistry of α-Haloamides

The aim of this review is to highlight the rich chemistry of α-haloamides originally mainly used to discover new C-N, C-O and C-S bond forming reactions, and later widely employed in C-C cross-coupling reactions with C(sp3), C(sp2) and C(sp) coupling partners. Radical-mediated transformations of α-haloamides bearing a suitable located unsaturated bond has proven to be a straightforward alternative to access diverse cyclic compounds by means of either radical initiators, transition metal redox catalysis or visible light photoredox catalysis. On the other hand, cycloadditions with α-halohydroxamate-based azaoxyallyl cations have garnered significant attention. Moreover, in view of the important role in life and materials science of difluoroalkylated compounds, a wide range of catalysts has been developed for the efficient incorporation of difluoroacetamido moieties into activated as well as unactivated substrates.

[3 + 2]-Annulation of Azaoxyallyl Cations and Thiocarbonyls for the Assembly of Thiazolidin-4-ones

A base-promoted, efficient [3 + 2] annulation between azaoxyallyl cations and thiocarbonyls is reported for flexible access to highly functionalized thiazolidin-4-one derivatives in good to excellent yields. An intriguing feature of this method is the metal or Lewis acid free late-stage entry of distinct set of functional groups at C2 of thiazolidin-4-ones via substitution of a latent amino functional group. Overall, this approach constitutes a general platform for convenient access to this medicinally important scaffold.

Sodium carbonate promoted [3 + 2] annulation of α-halohydroxamates and isocyanates

A new [3 + 2] annulation of readily available α-halohydroxamates and isocyanates has been developed. This protocol allows a rapid assembly of various hydantoins in good to high yields as well as a broad substrate scope and good functional group tolerance.

Transition-Metal-Free Synthesis of N-Hydroxy Oxindoles by an Aza-Nazarov-Type Reaction Involving Azaoxyallyl Cations

A novel transition-metal-free method to construct N-hydroxy oxindoles by an aza-Nazarov-type reaction involving azaoxyallyl cation intermediates is described. A variety of functional groups were tolerated under the weak basic reaction conditions and at room temperature. A one-pot process was also developed to make the reaction even more practical. This method provides alternative access to oxindoles and their biologically active derivatives.

Metal- and Oxidant-Free Modular Approach To Access N-Alkoxy Oxindoles via Aryne Annulation

An unprecedented metal- and oxidant-free (intermolecular) approach to access N-alkoxy oxindoles via [3 + 2] cycloadition of in situ generated electrophilic species viz. aryne and (putative) aza-oxyallyl cation is reported. This approach is amenable to both C3-unsubstituted as well as C3-substituted oxindoles. A one-pot manipulation further makes this reaction highly practical. The versatility of this approach was demonstrated through valuable synthetic transformations.

Catalyst-Free Ring Opening of Spiroaziridine Oxindoles by Heteronucleophiles: An Approach to the Synthesis of Enantiopure 3-Substituted Oxindoles

A simple catalyst-free method was developed for the ring opening of spiroaziridine oxindoles by three different nucleophiles, namely, amines, thiols, and methanol, to produce enantiopure (up to 99%) vicinal diaminooxindoles, β-aminosulfides, and β-amino-3-methoxyoxindole, respectively, in good to excellent yields. In contrast to the spiroepoxides, spiroaziridines are opened regio- and stereospecifically through the pseudobenzylic spirocenter under catalyst-free conditions. Moreover, unlike simple 2-substituted aziridines, these spiroaziridines are opened up with retention in configuration at the C3-spirocenter.

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.

Silver(I)- and Base-Mediated [3 + 3]-Cycloaddition of C,N-Cyclic Azomethine Imines with Aza-oxyallyl Cations

A silver(I) and base-mediated [3 + 3]-cycloaddition reaction of in situ generated C,N-cyclic azomethine imines with in situ formed aza-oxyallyl cations is reported. This one-pot cycloaddition process shows broad substrate scope an excellent functional group tolerance and provides the corresponding biologically important isoquinoline-fused triazines in good to excellent yields.

[3 + 2] Cycloaddition of aza-oxyallyl cations with aldehydes

We report here the [3 + 2] cycloaddition reactions of in situ-generated aza-oxyallyl cations with commercially available aldehydes.

One-Step Transformation of Trichloroacetamide into Isonitrile

A one-step transformation of trichloroacetamide, a protective group for the amine function, into isonitrile was successfully developed. The substrate scope and functional group tolerance of this procedure are also described.

Access to the Pyrroloindoline Core via [3 + 2] Annulation as well as the Application in the Synthetic Approach to (±)-Minfiensine

A [3 + 2] formal cycloaddition reaction using aza-oxyallyl cation as a synthetic synthon was developed to construct the pyrroloindololine core. With this novel method, a variety of C3-substituted indoles were readily converted into the corresponding pyrroloindoline analogues at room temperature in the mixed solvents. To further demonstrate the utility of this method, a synthetic approach to the total synthesis of (±)-minfiensine was developed in quite concise fashion.