Direct Synthesis of α-Amino Acid Derivatives by Hydrative Amination of Alkynes

α-Amino acid derivatives are key components of the molecules of life. The synthesis of α-amino carbonyl/carboxyl compounds is a contemporary challenge in organic synthesis. Herein, we report a practical method for the preparation of α-amino acid derivatives via direct hydrative amination of activated alkynes under mild conditions, relying on sulfinamides as the nitrogen source. Computational studies suggest that the reaction is enabled by a new type of sulfonium [2,3]-sigmatropic rearrangement.

Synthese von α-Aminosäurederivaten durch hydrative Aminierung von Alkinen

α‐Aminosäurederivate sind Kernbestandteile jeglichen Lebens. Die Synthese von α‐Aminocarbonyl/carboxyl Verbindungen ist jedoch weiterhin eine Herausforderung für die organische Synthese. In dieser Arbeit berichten wir von einer praktischen Herstellungsmethode für α‐Aminosäurederivate durch direkte hydrative Aminierung von aktivierten Alkinen mit Sulfinamiden unter milden Bedingungen. Computergestützte Untersuchungen legen nahe, dass eine [2,3]‐sigmatrope Sulfoniumumlagerung der zentrale Schritt der Reaktion ist.

Regioselective access to di- and trisubstituted pyridines via a metal-oxidant-solvent-free domino reaction involving 3-chloropropiophenones

A remarkable metal-oxidant-solvent- and base-free domino route for regioselective access to a wide range of 2,4-di- and 2,3,4/6-trisubstituted pyridines including carbo- and heterocyclic fused pyridines is reported. This [3C + 2C + 1N] cyclization reaction occurs between 3-chloropropiophenones (3C units), enolizable acyclic/cyclic ketones (2C sources) and NH₄OAc as a robust N source under neat conditions under an open atmosphere, producing new C=C and C=N-C bonds in highly chemo- and regioselective manners. Interestingly, this eco-friendly method has many positive features: excellent functional group tolerance, broad substrate scope, good to excellent regioselectivities, promising yields, no-unwanted products, neutral reaction conditions and appropriateness for large-scale synthesis. Mechanism studies reveal that the in situ generated β-amino ketone from 3-chloropropiophenone and an ammonium salt undergoes C=N bond formation with a ketone followed by an intramolecular cyclization process (C=C bond), which are the decisive steps for pyridine synthesis.

Gold-Catalyzed Annulations with Nucleophilic Nitrenoids Enabled by Heteroatom-Substituted Alkynes

The combination of a nucleophilic nitrene equivalent, a triple bond and a π-acid catalyst has underpinned numerous efficient transformations for the preparation of azacycles. This personal account details our efforts in developing an annulation strategy. Adding a nucleophilic nitrenoid to an activated alkyne can generate carbenoid character that is then quenched by a cyclisation onto the nitrenoid substituent. The use and development of N-acyl and N-heterocyclic pyridinium-N-aminides as 1,3-N,O and 1,3-N,N-dipole equivalents is discussed in the context of oxazole and heterocycle-fused imidazole formation, respectively. The resulting processes are highly efficient, practically straightforward, and tolerate considerable structural and functional group variation. Our use of heteroatom-substituted alkynes as enabling tools for reaction discovery is discussed. The reactivity accessed from the strong donor-like properties of ynamides is complemented by that obtained from alkynyl thioethers, which are emerging as interesting substrates for π-acid catalysis.

Regioselective difunctionalization of pyridines via 3,4-pyridynes

A new regioselective 3,4-difunctionalization of 3-chloropyridines via 3,4-pyridyne intermediates is reported. Regioselective lithiation of 3-chloro-2-ethoxypyridine and a related 2-thio-derivative followed by treatment with aryl- and alkylmagnesium halides as well as magnesium thiolates at -78 °C produced 3,4-pyridynes during heating to 75 °C. Regioselective addition of the Grignard moiety in position 4 followed by an electrophilic quench in position 3 led to various 2,3,4-trisubstituted pyridines. This method was adapted into a continuous flow set-up. As an application, we have prepared a key intermediate for (±)-paroxetine.

NH4OAc-Promoted Domino Route to Hydroxyarylated Unsymmetrical Pyridines under Neat Conditions

A new metal-, oxidant-, and solvent-free ecofriendly domino method has been established for modular synthesis of a diverse range of medicinally promising hydroxyarylated unsymmetrical pyridines in good to high chemical yields with an excellent regioselectivity. This domino process involves a range of N-sulfonyl ketimines as C,N-binucleophiles, enolizable ketones, and aromatic/heteroaromatic aldehydes using ammonium acetate as an ideal promoter under neat conditions, which creates two new C-C bonds and one C-N bond. Notably, the neutral reaction conditions are mild enough to tolerate a range of functionalities and cover a variety of substrates, thus bestowing a powerful avenue to access tri- and tetrasubstituted pyridines including carbo- and heterocyclic fused ones. Interestingly, a practical, scalable, and high-yielding synthesis of pyridylphenol derivatives was successfully accomplished by our unique method.

Facile Synthesis of Pyridines from Propargyl Amines: Concise Total Synthesis of Suaveoline Alkaloids

A general and efficient protocol was developed for the synthesis of polysubstituted pyridines from propargyl amines and unsaturated carbonyl compounds through a tandem condensation/alkyne isomerization/6π 3-azatriene electrocyclization sequence. This process was found to be applicable to a wide range of readily available substrates (30 examples, up to 95 % yield) and could be readily performed on a preparative (20 g) scale. By taking advantage of this method for late-stage pyridine incorporation, we successfully completed the collective total synthesis of suveoline, norsuveoline, and macrophylline.

Catalytic [1,3] O-to-C Rearrangement: Rapid Access to Bridged Bicyclic Systems

A catalytic [1,3] O-to-C rearrangement from enyne-ethers was developed for the rapid synthesis of diverse bridged bicyclic systems. In this reaction, a vinyl oxonium intermediate, generated in situ from enyne-ether, was the precursor for the [1,3] O-to-C rearrangement. This versatile protocol represents the first example of catalytic [1,3] O-to-C rearrangement based on ring-expansion strategy, enabling efficient access to bridged bicyclic scaffolds.

Alkynyl Thioethers in Gold-Catalyzed Annulations To Form Oxazoles

Non-oxidative, regioselective, and convergent access to densely functionalized oxazoles is realized in a functional-group tolerant manner using alkynyl thioethers. Sulfur-terminated alkynes provide access to reactivity previously requiring strong donor-substituted alkynes such as ynamides. Sulfur does not act in an analogous donor fashion in this gold-catalyzed reaction, thus leading to complementary regioselective outcomes and addressing the limitations of using ynamides.

Copper-Catalyzed [2+2+2] Modular Synthesis of Multisubstituted Pyridines: Alkenylation of Nitriles with Vinyliodonium Salts

A [2+2+2] modular synthesis of multisubstituted pyridines, with excellent regioselectivity, has been realized by copper catalysisand involves three distinct components: vinyliodonium salts, nitriles, and alkynes. The reactions proceeded with the facile formation of an aza-butadienylium intermediate by alkenylation of the nitrile with a vinyliodonium salt. Moreover, the alkynes in the reaction were extended to alkenes, which are an advantage of expense and relative scarceness of alkynes.