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.

Alkyl/Glycosyl Sulfoxides as Radical Precursors and Their Use in the Synthesis of Pyridine Derivatives

We report here the use of simple and readily available alkyl sulfoxides as precursors to radicals and their application in the preparation of pyridine derivatives. We show that alkyl sulfoxides, N-methoxy pyridinium salts and fluoride anions form electron donor-acceptor (EDA) complexes in solution, which, upon visible light irradiation, undergo a radical chain process to afford various pyridine derivatives smoothly. This reaction displays broad scope with respect to both sulfoxides and N-methoxy pyridiniums. The synthetic versatility of sulfoxides as a handle in chemistry adds to their power as radical precursors. Glycosyl sulfoxides are converted to the corresponding pyridyl C-glycosides with high stereoselectivities. Computational and experimental studies provide insights into the reaction mechanism.

Recent Progress in Enolonium Chemistry under Metal-Free Conditions

Umpolung approach through inversion of the polarity of conventional enolates, has opened up an unprecedented opportunity in the cross-coupling via alkylation. The enolonium equivalents can be accessed either by hypervalent iodine reagents, activation/oxidation of amides, or the oxidation of alkynes. Under umpolung conditions, highly basic conditions required for classical enolate chemistry can be avoided, and they can couple with unmodified nucleophiles such as heteroatom donors and electron-rich arenes.

Computational and Experimental Study of Turbo-Organomagnesium Amide Reagents: Cubane Aggregates as Reactive Intermediates in Pummerer Coupling

The dynamic equilibria of organomagnesium reagents are known to be very complex, and the relative reactivity of their components is poorly understood. Herein, a combination of DFT calculations and kinetic experiments is employed to investigate the detailed reaction mechanism of the Pummerer coupling between sulfoxides and turbo-organomagnesium amides. Among the various aggregates studied, unprecedented heterometallic open cubane structures are demonstrated to yield favorable barriers through a concerted anion-anion coupling/ S-O cleavage step. Beyond a structural curiosity, these results introduce open cubane organometallics as key reactive intermediates in turbo-organomagnesium amide mixtures.

Synthesis and Diversification of Macrocyclic Alkynediyl Sulfide Peptides

The synthesis of rare macrocyclic alkynediyl sulfides by a Cu-catalyzed C_sp -S cross-coupling is presented. The catalytic protocol (Cu(MeCN)₄ PF₆ /dtbbpy) promotes macrocyclization of peptides, dipeptides and tripeptides at ambient temperature (14 examples, 23→73 % yields) via thiols and bromoalkynes, and is chemoselective with regards to terminal alkynes. Importantly, the underexplored alkynediyl sulfide functionality incorporates a rigidifying structural element and opens new opportunities for diversification of macrocyclic frameworks through S oxidation, halide addition and azide-alkyne cycloaddition chemistries to integrate sulfones, halides or valuable fluorophores (7 examples, 37→92 % yields).

α-Functionalisation of Ketones Through Metal-Free Electrophilic Activation

Triflic anhydride mediated activation of acetophenones leads to highly electrophilic intermediates that can undergo a variety of transformations when treated with nucleophiles. This electrophilic ketone activation gives access to α‐arylated and α‐oxyaminated acetophenones under metal‐free conditions in moderate to excellent yields and enables extension to the synthesis of arylated morpholines via generation of vinylsulfonium salts. Computational investigations confirmed the transient existence of intermediates derived from vinyl triflates and the role of the oxygen atoms at the para position of aromatic ring in facilitating their stabilisation.

Efficient and Divergent Synthesis of α-Halogenated Amides and Esters by Double Electrophilic Activation of Ynamides

An efficient and modular entry to α-halogenated amides and esters is reported. This reaction is based on an underestimated double electrophilic activation of ynamides sequentially involving highly reactive activated keteniminium and iminium ions. Upon simple reaction with HCl and an electrophilic halogenation reagent in the presence of water or an alcohol, a broad range of ynamides can be transformed, in a highly divergent manner, to α-halo amides and esters with high efficiency and under mild conditions.

Vinyl Cation Stabilization by Silicon Enables a Formal Metal-Free α-Arylation of Alkyl Ketones

The ability of silicon to stabilize vinyl cationic species leads to a redox arylation of alkynes whereby the stringent limitations of reactivity and regioselectivity of alkyl-substituted alkynes are lifted. This allows the synthesis of a range of α-silyl-α'-arylketones under mild conditions in good to excellent yields and with high functional group tolerance, whereby the silicon moiety in the final products can either be removed for a formal acetone monoarylation transform, or capitalized upon for subsequent electrophilic substitutions at either side of the carbonyl group.

Highlights from the 53rd EUCHEM conference on stereochemistry, Bürgenstock, Switzerland, May 2018

When we first heard of the Bürgenstock conference it was described as a guarded meeting in a remote location, undisturbed by modern diversions, with mysterious customs and a secret handshake. All of these rumors turned out to be completely true. We arrived at an undisclosed location and three young men, who knew our names upon sight, greeted us and gave us an agenda in which we discovered the identity of the speakers, moderators and other participants. We then had a few moments to bask in their glory before being treated to a delicious dose of chemistry. During the banquet, before the first lecture, this year's president, Prof. Ilan Marek, gave the opening address from a balcony reserved only for these meetings, rumors hold that it stands vacant all year in anticipation. He mentioned several of the traditions of the meeting, which we are not allowed to share of course, and described the joy of putting together this year's exciting program with the help of an exceptional organizing committee that included Prof. Cristina Nevado, Prof. Christian Bochet, Dr Fabrice Gallou and Dr Alain De Mesmaeker. He also introduced this year's guest of honor, Prof. Yitzhak Apeloig, and wished the best luck to the current Vice President and future President, Prof. Véronique Gouverneur, not only for preparing the 54th Bürgenstock Conference, but also with her duty this year, maintaining the good weather for the whole week. Although the official title of the conference is the EUCHEM conference on stereochemistry, the topics covered span a broad range of cutting edge chemical transformations and insights, which can appeal to anyone working in chemistry and its interfaces with other disciplines. We will briefly describe each of the talks following one of our favorite citable quotes from these inspiring speakers.

Merging Art and Science-The 53rd Bürgenstock Conference

For the 53rd time, the Bürgenstock Conference gathered some of the most gifted scientists and rising stars in organic, physical, and bioorganic chemistry. Orchestrated by Ilan Marek (President) and his successor, Véronique Gouverneur, the synergy between art and science took place in Brunnen, Switzerland, with a beatiful view over Lake Lucerne.

An Interrupted Pummerer/Nickel-Catalysed Cross-Coupling Sequence

An interrupted Pummerer/nickel-catalysed cross-coupling strategy has been developed and used in the elaboration of styrenes. The operationally simple method can be carried out as a one-pot process, involves the direct formation of stable alkenyl sulfonium salt intermediates, utilises a commercially available sulfoxide, catalyst, and ligand, operates at ambient temperature, accommodates sp-, sp² -, and sp³ -hybridised organozinc coupling partners, and delivers functionalised styrene products in high yields over two steps. An interrupted Pummerer/cyclisation approach has also been used to access carbo- and heterocyclic alkenyl sulfonium salts for cross-coupling.

Ynamide Preactivation Allows a Regio- and Stereoselective Synthesis of α,β-Disubstituted Enamides

A novel ynamide preactivation strategy enables the use of otherwise incompatible reagents and allows preparation of α,β-disubstituted enamides with high regio- and stereoselectivity. Mechanistic analysis reveals the intermediacy of a triflate-bound intermediate as a solution-stable, effective keteniminium reservoir, whilst still allowing subsequent addition of organometallic reagents.

Synthesis of Terminal Ethynyl Aryl Selenides and Sulfides Based on the Retro-Favorskii Reaction of Hydroxypropargyl Precursors

The retro-Favorskii reaction is an excellent way to achieve terminal alkynes. Methodologies that connect the synthesis of terminal alkynes and organochalcogen motifs are important for the construction of novel compounds. Fourteen new terminal alkynes containing either C_sp -S or C_sp -Se bonds were selectively prepared through the retro-Favorskii reaction from the respective carbinol precursors. It was discovered that terminal chalcogen alkynes were stable for weeks if stored as a solution in hexanes.

Photochemical Dual-Catalytic Synthesis of Alkynyl Sulfides

A photochemical dual-catalytic cross-coupling to form alkynyl sulfides via C(sp)-S bond formation is described. The cross-coupling of thiols and bromoalkynes is promoted by a soluble organic carbazole-based photocatalyst using continuous flow techniques. Synthesis of alkynyl sulfides bearing a wide range of electronically and sterically diverse aromatic alkynes and thiols can be achieved in good to excellent yields (50-96 %). The simple continuous flow setup also allows for short reaction times (30 min) and high reproducibility on gram scale. In addition, we report the first application of photoredox/nickel dual catalysis towards macrocyclization, as well as the first example of the incorporation of an alkynyl sulfide functional group into a macrocyclic scaffold.

Investigations on Gold-Catalyzed Thioalkyne Activation Toward Facile Synthesis of Ketene Dithioacetals

Nucleophilic addition to thioalkynes was investigated under various catalytic conditions with gold(I) complexes being identified as the optimal catalysts. Structural evaluation of the product revealed an unexpected cis-addition, arising from a gold-associated thioketene intermediate. Based on this interesting mechanistic insight, a gold(I)-catalyzed thioether addition to thioalkynes was developed as a novel approach to prepare ketene dithioacetals with good yields and high efficiency.