Zinc Chloride Promoted Formal Oxidative Coupling of Aromatic Aldehydes and Isocyanides to α-Ketoamides

Practice of green chemistry strategies in synthetic organic chemistry: a glimpse of our sincere efforts in green chemistry research

This feature article summarises our recent contributions (2019-2023) in designing and developing a handful of promising organic transformations for accessing several diversely functionalised biologically relevant organic scaffolds, following the green chemistry principles, particularly focusing on the application of low-energy visible light, electrochemistry, ball-milling, ultrasound, and catalyst- and additive-free synthetic strategies.

2-Methylimidazole-1-(N-tert-octyl)sulfonimidoyl Fluoride: A Bench-Stable Alternative to SOF4 as Precursor to N,O-Substituted S(VI) Compounds

S(VI) compounds with multiple N or O substituents are often difficult to make and several crucial routes, such as multimodal SuFEx (Sulfur (VI) Fluoride Exchange) chemistry, rely on the highly useful but hazardous SOF4 gas. Safety issues and inaccessibility of SOF4 strongly hamper the developments of these organic compounds. Here we describe the synthesis and applications of 2-methylimidazole-1-(N-tert-octyl)sulfonimidoyl fluoride (ImSF), a novel bench-stable analogue of SOF4. ImSF is synthesized on a gram scale via a double fluorination of t-OctNSO. We show ImSF can undergo substitution reactions with phenols and amines, which lead to sulfurimidates and sulfuramidimidates, respectively, the intrinsically chiral analogous of medicinally relevant sulfates and sulfamates in which an S=O moiety is replaced by S=NR unit. Finally we demonstrate that such substitutions can occur enantiospecifically, providing the first entry to chiral sulfurimidates and sulfuramidimidates.

Recent developments in the total synthesis of natural products using the Ugi multicomponent reactions as the key strategy

The total syntheses of selected natural products using different versions of the Ugi multicomponent reaction is reviewed on a case-by-case basis. The revision covers the period 2008-2023 and includes detailed descriptions of the synthetic sequences, the use of state-of-the-art chemical reagents and strategies, as well as the advantages and limitations of the transformation and some remedial solutions. Relevant data on the isolation and bioactivity of the different natural targets are also briefly provided. The examples clearly evidence the strategic importance of this transformation and its key role in the modern natural products synthetic chemistry toolbox. This methodology proved to be a valuable means for easily building molecular complexity and efficiently delivering step-economic syntheses even of intricate structures, with a promising future.

Transition Metal-Free N-S Bond Cleavage and C-N Bond Activation of Ugi-Adducts for Rapid Preparation of Primary Amides and α-Ketoamides

A novel method of transition metal-free N-S bond cleavage and subsequent C-N bond activation of Ugi-adducts was developed. Diverse primary amides and α-ketoamides were prepared in a rapid, step-economical and highly efficient manner in two steps. This strategy features excellent chemoselectivity, high yield and functional-group tolerance. Primary amides derived from the pharmaceuticals probenecid and febuxostat were prepared. This method opens a new pathway for the simultaneous synthesis of primary amides and α-ketoamides in an environmentally friendly manner.

Catalytic Enantioselective Pictet-Spengler Reaction of α-Ketoamides Catalyzed by a Single H-Bond Donor Organocatalyst

The asymmetric Pictet–Spengler reaction (PSR) with aldehydes is well known. However, PSR involving ketones as electrophilic partners is far‐less developed. We report herein the first examples of catalytic enantioselective PSR of tryptamines with α‐ketoamides. A new class of easily accessible prolyl‐urea organocatalysts bearing a single H‐bond donor function catalyzes the title reaction to afford 1,1‐disubstituted tetrahydro‐β‐carbolines in excellent yields and enantioselectivities. The kinetic isotope effect using C2‐deuterium‐labelled tryptamine indicates that the rearomatization of the pentahydro‐β‐carbolinium ion intermediate might be the rate‐ and the enantioselectivity‐determining step.

One-Pot Synthesis of α-Ketoamides from α-Keto Acids and Amines Using Ynamides as Coupling Reagents

A one-pot strategy for α-keto amide bond formation have been developed by using ynamides as coupling reagents under extremely mild reaction conditions. Diversely structural α-ketoamides were afforded in up to 98% yield for 36 examples. This reaction features advantages such as practical coupling procedure, wide functional group tolerance, and extremely mild conditions and has potential applications in synthetic and medicinal chemistry.

Controllable methylenation with ethylene glycol as the methylene source: bridging enaminones and synthesis of tetrahydropyrimidines

Controllable methylenation using renewable ethylene glycol as the methylene source has been developed for the introduction of one or two methylene building blocks.

I2-DMSO mediated oxidative amidation of methyl ketones with anthranils for the synthesis of α-ketoamides

An I₂-DMSO mediated oxidative amidation of methyl ketones using anthranils as masked N-nucleophiles has been developed for the direct synthesis of α-ketoamides with high atom-economy. This metal-free process involves reductive N-O bond cleavage of anthranils and oxidative C-N bond formation of methyl ketones under mild conditions. The iodo group and electrophilic formyl group provide multiple possibilities for further functionalization of α-ketoamides.

Metal-free electrochemical synthesis of α-ketoamides via decarboxylative coupling of α-keto acids with isocyanides and water

A mild and convenient electrochemical protocol has been developed for the preparation of α-ketoamides via a decarboxylative coupling reaction of α-keto acids with isocyanides and water.

Copper-catalyzed synthesis of α-ketoamides using water and dioxygen as the oxygen source

The reaction employing H₂O and O₂ as the co-oxygen source in the catalytic synthesis of α-ketoamides is described. This copper-catalyzed reaction is carried out in a tandem manner constituted by the hydroamination of alkyne, hydration of vinyl–Cu complex and subsequent oxidation. Isotope labeling and radical capture experiments reveal that the oxygen atom of α-ketone at α-ketoamides derives from O₂ and the oxygen atom of amide group originates from H₂O.

The reaction employing H₂O and O₂ as the co-oxygen source in the catalytic synthesis of α-ketoamides is described. This Cu-catalyzed reaction is carried out in a tandem manner constituted by hydroamination of alkyne, hydration of vinyl–Cu complex and subsequent oxidation.

Copper-catalyzed oxidative cleavage of Passerini and Ugi adducts in basic medium yielding α-ketoamides

The present study provides an insight into the reactivity of Passerini and Ugi adducts in basic medium leading to α-ketoamides.

Employing thiocyanate salts as a nitrogen source via CN bond cleavage: divergent synthesis of α-ketoamides and 2-acyloxazoles

A switchable procedure for the synthesis of α-ketoamides and 2-acyloxazoles from aryl methyl ketones and thiocyanate salts is reported.

Copper-catalyzed coupling of anthranils and α-keto acids: direct synthesis of α-ketoamides

Copper-catalyzed coupling of α-keto acids with anthranils is reported for the synthesis of α-ketoamides bearing an aldehyde group via N–O/C–O bond cleavages and C–N bond formation.

nBu4NI-Mediated oxidation of methyl ketones to α-ketoamides: using ammonium, primary and secondary amine-salt as an amine moiety

Presented is the first example of synthesizing an array of primary-, secondary-, and tertiary-α-ketoamides in moderate to excellent yields with a catalyst nBu₄NI from methyl ketones and amine/ammonium salts under mild conditions using oxidant TBHP.

Ketenimines from Isocyanides and Allyl Carbonates: Palladium-Catalyzed Synthesis of β,γ-Unsaturated Amides and Tetrazoles

The reaction of allyl ethyl carbonates with isocyanides in the presence of a catalytic amount of Pd(OAc)₂ provided ketenimines through β-hydride elimination of the allyl imidoylpalladium intermediates. The insertion of the isocyanide into the π-allyl Pd complex proceeded via an unusual η¹ -allyl Pd species. The resulting ketenimines were hydrolyzed to β,γ-unsaturated carboxamides during purification by flash column chromatography on silica gel or converted in situ into 1,5-disubstituted tetrazoles by [3+2] cycloaddition with hydrazoic acid or trimethylsilyl azide.

Enantioselective Synthesis of (+)-Peganumine A

A gram-scale enantioselective total synthesis of (+)-peganumine A was accomplished in 7 steps from commercially available 6-methoxytryptamine. Key steps included (a) a Liebeskind-Srogl cross-coupling; (b) a one-pot construction of the tetracyclic skeleton from an ω-isocyano-γ-oxo-aldehyde via a sequence of an unprecedented C-C bond forming lactamization and a transannular condensation; (c) a one-pot organocatalytic process merging two achiral building blocks into an octacyclic structure via a sequence of enantioselective Pictet-Spengler reaction followed by a transannular cyclization. This last reaction created two spirocycles and a 2,7-diazabicyclo[2.2.1]heptan-3-one unit with excellent control of both the absolute and relative stereochemistry of the two newly created quaternary stereocenters.

Spontaneous Oxygenation of Siloxy-N-silylketenimines to α-Ketoamides

Siloxy-N-silylketenimines generated in situ from O-silyl cyanohydrins were converted to α-ketoamides by brief exposure to air or oxygen. Oxidation under extremely mild conditions can be explained by assuming the intermediacy of a 3-imino-1,2-dioxetane derivative generated via triplet-singlet intersystem crossing after the reaction of siloxy-N-silylketenimines with triplet oxygen.

A two-step continuous synthesis of α-ketoamides and α-amino ketones from 2° benzylic alcohols using hydrogen peroxide as an economic and benign oxidant

A practical two-step flow synthesis of α-ketoamides and α-amino ketones via direct oxidative coupling between 2° benzylic alcohols and amines was developed.

Copper-catalyzed, hypervalent iodine mediated CC bond activation of enaminones for the synthesis of α-keto amides

An unprecedented approach toward the general synthesis of α-keto amides has been established by tailoring the CC double bond of enaminones in the presence of CuI and hypervalent iodine.

I2-promoted aerobic oxidative coupling of acetophenes with amines under metal-free conditions: facile access to α-ketoamides

A novel and efficient I₂-promoted oxidative coupling of acetophenes with amines to α-ketoamides is presented, which employs molecular oxygen as an environmentally friendly oxidant under metal-free conditions.

Phosphinic acid-promoted addition reaction of isocyanides to (Z)-hydroximoyl chlorides: efficient synthesis of α-(hydroxyimino)amides

The reaction of (Z)-hydroximoyl chlorides with isocyanides promoted by phosphinic acid in the presence of triethylamine proceeds smoothly to afford α-(hydroxyimino)amides in good to high yields.

Room temperature copper-catalyzed oxidative amidation of terminal alkynes for the synthesis of α-ketoamides using O-benzoyl hydroxylamines as aminating reagent and oxidant

A novel and convenient copper-catalyzed oxidative amidation for the synthesis of α-ketoamides has been successfully developed, which uses easily available O-benzoyl hydroxylamines as aminating reagent and oxidant.

Silver-Catalyzed Cross-Coupling of Isocyanides and Active Methylene Compounds by a Radical Process

Isocyanides are versatile building blocks, and have been extensively exploited in C-H functionalization reactions. However, transition-metal-catalyzed direct C-H functionalization reactions with isocyanides suffer from over-insertion of isocyanides. Reported herein is a radical coupling/isomerization strategy for the cross-coupling of isocyanides with active methylene compounds through silver-catalysis. The method solves the over-insertion issue and affords a variety of otherwise difficult to synthesize β-aminoenones and tricarbonylmethanes under base- and ligand-free conditions. This report presents a new fundamental C-C bond-forming reaction of two basic chemicals.

Formamidine hydrochloride as an amino surrogate: I2-catalyzed oxidative amidation of aryl methyl ketones leading to free (N–H) α-ketoamides

A highly efficient molecular iodine catalyzed oxidative amidation of aryl methyl ketones has been developed. This reaction represents a novel strategy for the synthesis of free (N–H) α-ketoamides.