Terminal Alkyne Addition to Diazodicarboxylates: Synthesis of Hydrazide Linked Alkynes (Ynehydrazides)

Synthesis of Pentafluorosulfanylated Ynamides and Further Functionalizations

Herein is described the first synthesis of SF5-ynamides, versatile building blocks featuring the pentafluorosulfanyl motif. This synthesis proceeds through a two-step sequence of radical SF5-addition onto the π-system of a wide range of terminal ynamides and derivatives substituted with various nitrogen fragments followed by a dehydrochlorination reaction. A selection of downstream functionalization reactions, including nucleophilic additions, cycloadditions, and sulfur ylide synthesis, highlights the wide-ranging applications of this novel type of functionalized SF5-building block.

Recent progress in alkynylation with hypervalent iodine reagents

Although alkynes are one of the smallest functional groups, they are among the most versatile building blocks for organic chemistry, with applications ranging from biochemistry to material sciences. Alkynylation reactions have traditionally relied on the use of acetylenes as nucleophiles. The discovery and development of ethynyl hypervalent iodine reagents have allowed to greatly expand the transfer of alkynes as electrophilic synthons. In this feature article the progress in the field since 2018 will be presented. After a short introduction on alkynylation reactions and hypervalent iodine reagents, the developments in the synthesis of alkynyl hypervalent iodine reagents will be discussed. Their recent use in base-mediated and transition-metal catalyzed alkynylations will be described. Progress in radical-based alkynylations and atom-economical transformations will then be presented.

Copper-Catalyzed C(sp)-H Bond Hydrazidation

A catalytic, direct synthetic strategy for preparing ynehydrazides with terminal alkynes and dialkyl azodicarboxylates is described. The protocol utilizes a cheap copper catalyst in combination with a catalytic amount of a weak base. The high sustainability, good practicality, broad substrate scope, and wide functional group tolerance comprised the advantages of this reaction. Synthetic applications and preliminary mechanistic studies have been conducted.

Copper-Catalyzed Alkynylation of Hydrazides: An Easy Access to Functionalized Azadipeptides

We report a copper-catalyzed alkynylation of azadipeptides using ethynylbenziodoxolone (EBX) reagents. Nonsymmetrical ynehydrazides could be obtained in 25-97% yield using azaglycine derivatives as nucleophiles. The transformation is compatible with most functional groups naturally occurring on amino acid side chains and allows the transfer of silyl-, alkyl-, and aryl-substituted alkynes. The obtained α-alkynyl azaglycine products could be further functionalized by nucleophilic attack or cycloaddition on the triple bond.

A Modular Approach to Arylazo-1,2,3-triazole Photoswitches

Azoheteroarenes make up an emerging class of photoswitchable compounds with unique photophysical properties and advantages over traditional azobenzenes. Therefore, methods for synthesizing azoheteroarenes are highly desirable. Here, we utilize azide-alkyne click chemistry to access arylazo-1,2,3-triazoles, a previously unexplored class of azoheteroarenes that exhibit high thermal stabilities and near-quantitative bidirectional photoconversion. Controlling the catalyst or 1,3-dipole grants access to both regioisomeric arylazotriazoles and arylazoisoxazoles, highlighting the versatility of our approach.

Short and Modular Synthesis of Substituted 2-Aminopyrroles

We herein describe a simple and metal-free domino methodology to synthesize 2-aminopyrroles from alkynyl vinyl hydrazides. The domino reaction involves a novel propargylic 3,4-diaza-Cope rearrangement and a tandem isomerization/5-exo-dig N-cyclization reaction. By using this approach, a number of 2-aminopyrroles with diverse substituents have been prepared.

Ligand-Enabled Copper-Catalyzed N-Alkynylation of Sulfonamide with Alkynyl Benziodoxolone: Synthesis of Amino Acid-Derived Ynamide

Ynamides are versatile building blocks in organic synthesis. However, the synthesis of amino acid-derived ynamides is difficult but in high demand. Herein, we disclose the copper-catalyzed Csp-N coupling of sulfonamide, including amino acid and peptide derivatives, to give ynamides by using alkynyl benziodoxolones with broad functional group tolerance under mild reaction conditions. The electron-rich bipyridine as a ligand and ethanol as solvent were used for the success of this reaction. The usefulness of the obtained amino acid-derived ynamide as building block was showcased by further derivatization to unique amino acid derivatives. A control experiment to elucidate the mechanistic insight was also described.

Hydrazine derivative synthesis by trifluoroacetyl hydrazide alkylation

N′-Alkyl hydrazides were effectively synthesized by routes featuring installation, alkylation, and removal of a trifluoroacetyl group. A set of amino acid derived hydrazides were acylated using trifluoroacetic anhydride, and the resulting trifluoroacetyl hydrazides were alkylated with alcohols in Mitsunobu reactions and with alkyl halides under alkaline conditions. Removal of the trifluoroacetyl group was affected under reductive and hydrolytic conditions to provide the respective N′-alkyl hydrazides. This three-step process may be performed without isolation of intermediates to yield N′-alkyl hydrazide after a single chromatographic purification.

Di-tert-butyl Ethynylimidodicarbonate as a General Synthon for the β-Aminoethylation of Organic Electrophiles: Application to the Formal Synthesis of Pyrrolidinoindoline Alkaloids (±)-CPC-1 and (±)-Alline

The reagent di-tert-butyl ethynylimidodicarbonate is demonstrated as a β-aminoethyl anion synthetic equivalent. It can be used to install ethyleneamine groups by exploiting its terminal alkyne reactivity with common organic electrophiles. Reactions exemplified with this terminal ynimide reagent include additions to imines, aldehydes, ketones, pyridinium salts, Michael acceptors, epoxides, and Pd-catalyzed Sonogashira couplings. Subsequent regioselective [3 + 2] cycloadditions of the alkynyl-imides (ynimides) generate N,N-di-Boc imide-functionalized triazole and isoxazole heterocycles. Reduction of the ynimides with Pd-catalyzed hydrogenation generates ethyleneimides with easily removable N,N-di-Boc-carbamate protecting groups, allowing for a flexible ynimide-based approach to ethyleneamine installation. The utility of this two-step aminoethylation strategy was demonstrated in the short formal syntheses of pyrrolidinoindoline alkaloids (±)-CPC-1 and (±)-alline. Analogously, the reagent (N,N,N')-tri-Boc 2-ethynylhydrazine serves as a β-hydrazinoethyl anion synthetic equivalent.

Synthesis of Hydrazines via Radical Generation and Addition of Azocarboxylic tert-Butyl Esters

A new chemistry of azo compounds that is a radical generation and addition in situ of azocarboxylic tert-butyl esters to synthesize hydrazines has been described. The protocol provides a novel strategy for the synthesis of various hydrazines. The advantages of the transformation include broad substrate scope, benign conditions, and convenient operation.

Ynamide carbopalladation: a flexible route to mono-, bi- and tricyclic azacycles

Bromoenynamides represent precursors to a diversity of azacycles by a cascade sequence of carbopalladation followed by cross-coupling/electrocyclization, or reduction processes. Full details of our investigations into intramolecular ynamide carbopalladation are disclosed, which include the first examples of carbopalladation/cross-coupling reactions using potassium organotrifluoroborate salts; and an understanding of factors influencing the success of these processes, including ring size, and the nature of the coupling partner. Additional mechanistic observations are reported, such as the isolation of triene intermediates for electrocyclization. A variety of hetero-Diels-Alder reactions using the product heterocycles are also described, which provide insight into Diels-Alder regioselectivity.

S3S63 Terminal Ynamides: Synthesis, Coupling Reactions and Additions to Common Electrophiles

Ynamides consist of a polarized triple bond that is directly attached to a nitrogen atom carrying a sulfonyl, an alkoxycarbonyl, an acyl or another electron withdrawing group. The triple bond polarization renders ynamides broadly useful building blocks with synthetic opportunities that go far beyond traditional alkyne chemistry. The versatile reactivity of ynamides in cycloadditions, cycloisomerizations, regioselective additions with various nucleophiles or electrophiles, ring-closing metathesis, and many other reactions has been investigated in detail during the past decades. A common feature of these methods is that the triple bond is consumed and either cleaved or transformed to a new functionality. The wealth of reports on these ynamide reactions is in stark contrast to the dearth of carbon-carbon bond formations that leave the triple bond of terminal ynamides intact. The recent introduction of effective synthetic methods for the preparation of terminal ynamides has set the stage to fully explore the synthetic potential of this intriguing class of compounds. This digest letter summarizes the most effective routes to terminal ynamides and the current state of selective nucleophilic addition, substitution and coupling reactions, including the first examples of asymmetric synthesis.

Ynamides in ring forming transformations

The ynamide functional group activates carbon-carbontriple bonds through an attached nitrogen atom that bears an electron-withdrawing group. As a result, the alkyne has both electrophilic and nucleophilic properties. Through the selection of the electron-withdrawing group attached to nitrogen, chemists can modulate the electronic properties and reactivity of ynamides, making these groups versatile synthetic building blocks. The reactions of ynamides also lead directly to nitrogen-containing products, which provides access to important structural motifs found in natural products and molecules of medicinal interest. Therefore, researchers have invested increasing time and research in the chemistry of ynamides in recent years. This Account surveys and assesses new organic transforma-tions involving ynamides developed in our laboratory and in others around the world. We showcase the synthetic power of ynamides for rapid assembly of complex molecular structures. Among the recent reports of ynamide transformations, ring-forming reactions provide a powerful tool for generating molecular complexity quickly. In addition to their synthetic utility, such reactions are mechanistically interesting. Therefore, we focus primarily on the cyclization chemistry of ynamides. This Account highlights ynamide reactions that are useful in the rapid synthesis of cyclic and polycyclic structural manifolds. We discuss the mechanisms active in the ring formations and describe representative examples that demonstrate the scope of these reactions and provide mechanistic insights. In this discussion, we feature examples of ynamide reactions involving radical cyclizations, ring-closing metathesis, transition metal and non-transition metal mediated cyclizations, cycloaddition reactions, and rearrangements. The transformations presented rapidly introduce structural complexity and include nitrogen within or in close proximity to a newly formed ring (or rings). Thus, ynamides have emerged as powerful synthons for nitrogen-containing heterocycles and nitrogen-substituted rings, and we hope this Account will promote continued interest in the chemistry of ynamides.

SYNTHESIS OF DE NOVO CHIRAL γ-AMINO-YNAMIDES USING LITHIATED YNAMIDES. OBSERVATION OF A UNIQUE 5-ENDO-DIG CYCLIZATION WITH AN INVERSION OF S-CENTER

We describe herein details of our efforts in developing a highly stereoselective synthesis of de novo chiral γ-amino-ynamides through additions of lithiated ynamides to Ellman-Davis chiral N-tert-butanesulfinyl imines. While additions of ynamides could be highly stereoselective even without Lewis acids, the use of BF3-OEt2 completely reversed the stereoselectivity. On the other hand, additions of oxazolidinone-substituted, oxazinanone-substituted and tetrahydropyrimidinone-substituted ynamides behaved quite differently and functioned better with BF3-OEt2. The chirality of the oxazolidinone ring exerts no impact on the selectivity. This work also features a unique 5-endo-dig cyclization of oxazolidinone-substituted γ-amino-ynamides that could be promoted with acid, leading to isothiazoles and 2,3-dihydro-isothiazole S-oxides.

A Convenient Synthesis of γ-Amino-Ynamides via Additions of Lithiated Ynamides to Aryl Imines. Observation of an Aza-Meyer-Schuster Rearrangement

Efforts in developing an expeditious and convenient method for synthesizing γ-amino-ynamides via nucleophilic addition of lithiated ynamides to aryl imines are described. This work also features an aza-variant of a Meyer-Schuster rearrangement of γ-amino-ynamides and the synthetic utility of γ-amino-ynamides in an intramolecular ketenimine-[2 + 2] cycloaddition.

Synthesis of cyclopentenimines from N-allyl ynamides via a tandem aza-Claisen rearrangement-carbocyclization sequence

We describe here details of our investigations into Pd-catalyzed and thermal aza-Claisen-carbocyclizations of N-allyl ynamides to prepare a variety of α,β-unsaturated cyclopentenimines. The nature of the ynamide electron-withdrawing group and β-substituent plays critical roles in the success of this tandem cascade. With N-sulfonyl ynamides, the use of palladium catalysis is required, as facile 1,3-sulfonyl shifts dominate under thermal conditions. However, since no analogous 1,3-phosphoryl shift is operational, N-phosphoryl ynamides could be used to prepare similar cyclopentenimines under thermal conditions through zwitter ionic intermediates that undergo N-promoted H-shifts. Alternatively, by employing ynamides bearing tethered carbon nucleophiles, the zwitter ionic intermediates could be intercepted, giving rise rapidly to more complex fused bi- and tricyclic scaffolds.

Total synthesis of the cytotoxic enehydrazide natural products hydrazidomycins A and B by a carbazate addition/Peterson olefination approach

The first total syntheses of two natural antitumor enehydrazide compounds (hydrazidomycins A and B) and a related positional isomer of hydrazidomycin B (elaiomycin B) have been accomplished in a rapid and stereocontrolled fashion using a Peterson elimination approach. A regioselective silyl epoxide ring opening reaction with Boc-carbazate followed by base-mediated Peterson siloxide elimination stereospecifically installed the key Z-enehydrazide functionality. The use of Boc-carbazate allowed for the differential functionalization of the hydrazide nitrogens.

A highly stereoselective addition of lithiated ynamides to Ellman-Davis chiral N-tert-butanesulfinyl imines

A highly diastereoselective addition of lithiated ynamides to Ellman-Davis chiral imines is described. While additions of N-sulfonyl ynamides are highly stereoselective even without Lewis acids, the use of BF3-OEt2 completely reversed the stereoselectivity. In addition, oxazolidinone-substituted ynamides behaved differently and functioned better with BF3-OEt2, and the chirality of the oxazolidinone ring exerts no impact on the selectivity.

Synthesis of diversely 1,3,5-trisubstituted pyrazoles via 5-exo-dig cyclization

5-Exo-dig cyclocondensation of alk-3-yn-1-ones with hydrazines, in the presence of montmorillonite K-10, provides an effective method with a high atom economy for the synthesis of diversely 1,3,5-trisubstituted pyrazoles. The microwave-accelerated reaction proceeds in the absence of solvent and leads to 5-benzyl substituted pyrazoles with good yields (72-91%). The regiochemistry of the process was confirmed by the X-ray crystallographic structure determination of 1-(2-fluorophenyl)-5-(4-methylbenzyl)-3-phenyl-1H-pyrazole.

Carbocyclization cascades of allyl ketenimines via aza-Claisen rearrangements of N-phosphoryl-N-allyl-ynamides

A series of carbocyclization cascades of allyl ketenimines initiated through a thermal aza-Claisen rearrangement of N-phosphoryl-N-allyl ynamides is described. Interceptions of the cationic intermediate via Meerwein-Wagner rearrangements and polyene-type cyclizations en route to fused bi- and tricyclic frameworks are featured.