Disulfonimide-Catalyzed Asymmetric Synthesis of β3-Amino Esters Directly from N-Boc-Amino Sulfones

Pyrazole based palladacycles for the synthesis of β-aminoketones by redox coupling of allyl alcohols and anilines

Herein, we report the synthesis of β-aminoketones by oxidative coupling of allyl alcohols at room temperature using a pyrazole based palladacycle and BINOL-phosphoric acid system. This method avoids the use of any base, external oxidant, and additive. The reaction of p-anisidine and 1-penten-3-ol under the optimized conditions using isolated palladacycle-BINOL-phosphoric acid produced the desired product in 24 h, which suggests that the active species involved in this reaction is the palladacycle-BINOL-phosphoric acid.

Elucidating Mechanism and Selectivity in Pyridine Functionalization Through Silylium Catalysis

The functionalization of aromatic N-heterocycles through silylium activation demonstrates exceptional selectivity and efficiency. Density functional theory (DFT) calculations unveil the detailed silylium catalysis mechanism and elucidate the origins of selectivity in this reaction. The phosphoramidimidate sulfonamide (PADI) precatalyst orchestrates of the catalytic cycle via three elementary steps. The Brønsted acidity of precatalyst significantly influences both the formation of silylium-based Lewis acid active species and the silylium activation of pyridine. Unlike disulfonimide (DSI)-type precatalysts, both Tf2NH and PADI precatalysts with strong acidities can easily promote the generation of activated silylium pyridine species. A semi-enclosed 'rigid' electronegative cavity in PADI-type anions constructs a well-defined recognition site, facilitating engagement with the positively charged silylium pyridine species. Due to the high electrophilicity and less steric demand at the C4-position of the pyridine substrate, the product with C4-regioselectivity was predominantly generated.

Harnessing Dpp-Imine as a Powerful Achiral Cocatalyst to Dramatically Increase the Efficiency and Stereoselectivity in a Magnesium-Mediated Oxa-Michael Reaction

Dpp-imines are classic model substrates for synthetic method studies. Here, we disclose their powerful use as achiral coligands in metal-catalyzed reactions. It is highly interesting to find that the Dpp-imine can not only act as powerful ligand to create excellent chiral pockets with magnesium complexes but also, more importantly, this coligand can dramatically enhance the catalytic ability of the metal catalyst. The underlying reaction mechanism was extensively explored by conducting a series of experiments, including 31P NMR studies of the coordination complex between the Dpp-imine coligand and magnesium complexes, ESI capture results, multiple control experiments, studies and comparison of different coligands, 1H NMR studies on the relationship between the substrate and Dpp-imine coligand, as well as the relationship between the substrate and the full complexes. Furthermore, DFT calculation provided valuable insights in the role of the imine additive and demonstrated that adding the Dpp-imine coligand in the magnesium catalyst can switch the deprotonation/nucleophilic addition steps from a stepwise mechanism to a concerted process during the oxa-cyclization reaction. The crucial factors responsible for the excellent enantioselectivity and enhanced reaction efficiency brought by Dpp-imine have been extracted from the calculation model. These mechanistic experiments and DFT calculation data clearly disclose and prove the powerful and interesting functions of the Dpp-imine coligand, which also direct a novel application of this type of active imine as useful ligands in metal-catalyzed asymmetric reactions.

Organocatalytic DYKAT of Si-Stereogenic Silanes

Chiral organosilanes do not exist in nature and are therefore absent from the "chiral pool". As a consequence, synthetic approaches toward enantiopure silanes, stereogenic at silicon, are rather limited. While catalytic asymmetric desymmetrization reactions of symmetric organosilicon compounds have been developed, the utilization of racemic silanes in a dynamic kinetic asymmetric transformation (DYKAT) or dynamic kinetic resolution (DKR) would significantly expand the breadth of accessible Si-stereogenic compounds. We now report a DYKAT of racemic allyl silanes enabled by strong and confined imidodiphosphorimidate (IDPi) catalysts, providing access to Si-stereogenic silyl ethers. The products of this reaction are easily converted into useful enantiopure monohydrosilanes. We propose a spectroscopically and experimentally supported mechanism involving the epimerization of a catalyst-bound intermediate.

Design of an Organocatalytic Asymmetric (4 + 3) Cycloaddition of 2-Indolylalcohols with Dienolsilanes

Here we present the design of a highly enantioselective, catalytic (4 + 3) cycloaddition of gem-dialkyl 2-indolyl alcohols and dienolsilanes, enabled by strong and confined IDPi Lewis acids. The method furnishes novel bicyclo[3.2.2]cyclohepta[b]indoles with up to three stereogenic centers, one of which is quaternary. A broad substrate scope is accompanied by versatile downstream chemical modifications. Density functional theory-supported mechanistic studies shed light on the importance of the in situ generated silylium species in an overall concerted yet asynchronous cycloaddition.

Catalytic Asymmetric Additions of Enol Silanes to In Situ Generated Cyclic, Aliphatic N-Acyliminium Ions

Strong and confined imidodiphosphorimidate (IDPi) catalysts enable highly enantioselective substitutions of cyclic, aliphatic hemiaminal ethers with enol silanes. 2-Substituted pyrrolidines, piperidines, and azepanes are obtained with high enantioselectivities, and the method displays a broad tolerance of various enol silane nucleophiles. Several natural products can be accessed using this methodology. Mechanistic studies support the intermediacy of non-stabilized, cyclic N-(exo-acyl)iminium ions, paired with the confined chiral counteranion. Computational studies suggest transition states that explain the observed enantioselectivity.

A Powerful Chiral Super Brønsted C-H Acid for Asymmetric Catalysis

A new type of chiral super Brønsted C-H acids, BINOL-derived phosphoryl bis((trifluoromethyl)sulfonyl) methanes (BPTMs), were developed. As compared to widely utilized BINOL-derived chiral phosphoric acids (BPAs) and N-triflyl phosphoramides (NTPAs), BPTMs displayed much higher Brønsted acidity, resulting in dramatically improved activity and excellent enantioselectivity as demonstrated in catalytic asymmetric Mukaiyama-Mannich reaction, allylic amination, three-component coupling of allyltrimethylsilane with 9-fluorenylmethyl carbamate and aldehydes, and protonation of silyl enol ether. These new strong Brønsted C-H acids have provided a platform for expanding the chemistry of asymmetric Brønsted acid catalysis.

Unified Synthesis of Polycyclic Alkaloids by Complementary Carbonyl Activation*

A complementary dual carbonyl activation strategy for the synthesis of polycyclic alkaloids has been developed. Successful applications include the synthesis of tetracyclic alkaloids harmalanine and harmalacinine, pentacyclic indoloquinolizidine alkaloid nortetoyobyrine, and octacyclic β-carboline alkaloid peganumine A. The latter synthesis features a protecting-group-free assembly and an asymmetric disulfonimide-catalyzed cyclization. Furthermore, formal syntheses of hirsutine, deplancheine, 10-desbromoarborescidine A, and oxindole alkaloids rhynchophylline and isorhynchophylline have been achieved. Finally, a concise synthesis of berberine alkaloid ilicifoline B was completed.

Silylium Ions: From Elusive Reactive Intermediates to Potent Catalysts

The history of silyl cations has all the makings of a drama but with a happy ending. Being considered reactive intermediates impossible to isolate in the condensed phase for decades, their actual characterization in solution and later in solid state did only fuel the discussion about their existence and initially created a lot of controversy. This perception has completely changed today, and silyl cations and their donor-stabilized congeners are now widely accepted compounds with promising use in synthetic chemistry. This review provides a comprehensive summary of the fundamental facts and principles of the chemistry of silyl cations, including reliable ways of their preparation as well as their physical and chemical properties. The striking features of silyl cations are their enormous electrophilicity and as such reactivity as super Lewis acids as well as fluorophilicity. Known applications rely on silyl cations as reactants, stoichiometric reagents, and promoters where the reaction success is based on their steady regeneration over the course of the reaction. Silyl cations can even be discrete catalysts, thereby opening the next chapter of their way into the toolbox of synthetic methodology.

Catalytic Asymmetric Synthesis of Unprotected β2-Amino Acids

We report here a scalable, catalytic one-pot approach to enantiopure and unmodified β²-amino acids. A newly developed confined imidodiphosphorimidate (IDPi) catalyzes a broadly applicable reaction of diverse bis-silyl ketene acetals with a silylated aminomethyl ether, followed by hydrolytic workup, to give free β²-amino acids in high yields, purity, and enantioselectivity. Importantly, both aromatic and aliphatic β²-amino acids can be obtained using this method. Mechanistic studies are consistent with the aminomethylation to proceed via silylium-based asymmetric counteranion-directed catalysis (Si-ACDC) and a transition state to explain the enantioselectivity is suggested on the basis of density functional theory calculation.

Functionalisation of esters via 1,3-chelation using NaOtBu: mechanistic investigations and synthetic applications

Unusual nucleophilic behavior of a metal t-butoxide in a transesterification reaction was demonstrated by NMR and real-time IR spectroscopies and deuterium-labeling experiments.

Chiral disulfonimides: a versatile template for asymmetric catalysis

Since the emergence of pseudo-C2-symmetric chiral phosphoric acids (CPA), much work has been done to utilize these systems in stereoselective, organocatalytic processes. Despite the success in this field, reasonably basic substrates such as imines are often required to achieve appreciable activation. In order to access a wider variety of potential reaction partners, many related organocatalysts with enhanced Brønsted acidity have since been developed. Chiral disulfonimides (DSIs) have materialized as one such powerful class of organocatalysts and have been shown to expand the list of potential substrates to include aldehydes and ketones via Brønsted, Lewis, or bifunctional acid activation. This versatility renders DSIs amenable to an impressive scope of reaction types, typically with remarkable stereoselectivity induced by asymmetric counteranion-directed catalysis (ACDC). This review serves to provide a complete analysis of the successful applications, mechanistic insights, and unmet challenges exhibited to date in DSI-catalyzed and -assisted processes.

Organocatalytic Enantioselective Mukaiyama-Mannich Reaction of Isatin-Derived Ketimines for the Synthesis of Oxindolyl-β3, 3 -Amino Acid Esters

Mukaiyama-Mannich reactions of ester enolate equivalents with aldimines have been elegantly used for the asymmetric synthesis of β-amino acids; nevertheless, the corresponding asymmetric reaction employing ketimines are unexplored. Herein, the first organocatalytic enantioselective Mukaiyama-Mannich reaction employing isatin-derived ketimines with unsubstituted silyl ketene acetals is disclosed towards the scalable synthesis of 2-oxoindolinyl-β^3, 3 -amino acid esters at room temperature with excellent enantioselectivities (ee >99.5 %). Ultra-low catalyst loadings (as low as 250 ppm) could be used for the quantitative product formation with high enantiopurity. The synthetic utility of this protocol has been showcased in the short formal synthesis of pharmaceutically demanded (+)-AG-041R, a potent gastrin/CCK-B receptor antagonist.

Total synthesis of (±)-fumimycin and analogues for biological evaluation as peptide deformylase inhibitors

A concise 7-step total synthesis of (±)-fumimycin in 11.6 % overall yield is reported. An acid-catalyzed intramolecular aza-Friedel-Crafts cyclization was developed to construct the benzofuranone skeleton of the natural product bearing an α,α-disubstituted amino acid moiety in a single step. Regioselective chlorination followed by a Suzuki-Miyaura cross-coupling rapidly enabled the preparation of a library of analogues which were evaluated against peptide deformylase for antibacterial activity.

Organocatalytic Enantioselective Mannich Reaction: Direct Access to Chiral β-Amino Esters

An asymmetric Mannich reaction has been developed to generate chiral β-amino esters in good yields with excellent enantiomeric excesses (ee, up to 99%) using a chiral bifunctional thiourea catalyst derived from (R,R)-cyclohexyldiamine. This is the first report on the addition of 3-indolinone-2-carboxylates to N-Boc-benzaldimines generated in situ from α-amidosulfones, which proceeds under mild conditions.

Recent synthetic applications of α-amido sulfones as precursors of N-acylimino derivatives

α-Amido sulfones can be directly used as N-acylimine or N-acyliminium ion precursors in several synthetic processes aimed at the preparation of nitrogen containing compounds. This review collects the most relevant and practical utilizations of α-amido sulfones appeared in the literature after 2005.

A highly efficient Hg(OTf)2-mediated Sakurai–Hosomi allylation of N-tert-butyloxycarbonylamino sulfones, aldehydes, fluoroalkyl ketones and α,β-unsaturated enones using allyltrimethylsilane

The cheap and easily available Hg(OTf)₂ can efficiently mediate the Sakurai–Hosomi reaction of N-Boc amino sulfones, aldehydes, α-fluoroalkyl ketones and α,β-unsaturated enones using allyltrimethylsilane with the catalyst loading down to 0.5–5.0 mol%.

Enantioselective Reaction between 2-(Cyanomethyl)azaarenes and N-Boc-amino Sulfones

A series of 2-(cyanomethyl)azaarenes containing benzothiazole or benzoxazole were designed and synthesized for asymmetric α-functionalization with N-Boc-amino sulfones. The Mannich adducts were obtained in high yields with good diastereo- and enantioselectivities. Aryl-substituted amino sulfones were tolerated under the current conditions, and the reaction can be performed on gram scale in good results.

Can a Ketone Be More Reactive than an Aldehyde? Catalytic Asymmetric Synthesis of Substituted Tetrahydrofurans

O-heterocycles bearing tetrasubstituted stereogenic centers are prepared via catalytic chemo- and enantioselective nucleophilic additions to ketoaldehydes, in which the ketone reacts preferentially over the aldehyde. Five- and six-membered rings with both aromatic and aliphatic substituents, as well as an alkynyl substituent, are obtained. Moreover, 2,2,5-trisubstituted and 2,2,5,5-tetrasubstituted tetrahydrofurans are synthesized with excellent stereoselectivities. Additionally, the synthetic utility of the described method is demonstrated with a three-step synthesis of the side chain of anhydroharringtonine.

Formal [4 + 2] cycloaddition of imines with alkoxyisocoumarins

A new preparation of δ-lactams is reported. In the presence of a Lewis acid promoter, alkoxyisocoumarins engage a range of N-aryl and N-alkyl imines to form δ-lactams with a pendent carboalkoxy substituent. A sulfonamide-thiourea catalyst enables the synthesis of these products in moderate to good enantioselectivities.

An asymmetric acetate-Mannich reaction of chiral isatin derived ketimines and its applications

A highly efficient TMSOTf-mediated asymmetric acetate-Mannich reaction of isatin derived tert-butylsulfinyl ketimines and S-phenyl thioacetate was developed to afford the direct synthesis of indole-based β^3,3-amino acid thioester with excellent selectivity (dr > 98 : 2). Syntheses of (+)-AG-041R and 3-aminopyrroloindoline have been accomplished utilizing the developed method.

Palladium-catalyzed oxidative amination of homoallylic alcohols: sequentially installing carbonyl and amino groups along an alkyl chain

A novel Pd-catalyzed intermolecular oxidative amination of homoallylic alcohols affording valuable β-amino acetones under mild conditions has been developed.

Tunable regiodivergent phosphine-catalyzed [3 + 2] cycloaddition of alkynones and trifluoroacetyl phenylamides

The regiodivergent phosphine-catalyzed [3 + 2] cycloaddition of 2-(2,2,2-trifluoroacetyl)phenylamides and alkynones have been achieved through adjusting the acidities of substrates, changing reaction temperature and addition of protic additive H₂O. The detailed mechanism was systematically studied.

A Disulfonimide Catalyst for Highly Enantioselective Mukaiyama-Mannich Reaction

A new BINOL-derived chiral disulfonimide has been developed by introducing 4-methyl-3,5-dinitrophenyl substituents at its 3- and 3'-positions. This chiral disulfonimide catalyst displays high catalytic efficacy toward the asymmetric Mukaiyama-Mannich reaction of imines with ketene silyl acetals leading to β-amino acid esters in good yields (up to 99%) with high diastereoselectivities (syn/anti up to 97:3) and enantioselectivities (up to 98% ee). The long-standing problem of the chiral phosphoric acid-catalyzed asymmetric Mukaiyama-Mannich reaction that requires a 2-hydroxyphenyl moiety was solved by this disulfonimide catalyst.

Asymmetric counteranion-directed Lewis acid organocatalysis for the scalable cyanosilylation of aldehydes

Due to the high versatility of chiral cyanohydrins, the catalytic asymmetric cyanation reaction of carbonyl compounds has attracted widespread interest. However, efficient protocols that function at a preparative scale with low catalyst loading are still rare. Here, asymmetric counteranion-directed Lewis acid organocatalysis proves to be remarkably successful in addressing this problem and enabled a molar-scale cyanosilylation in quantitative yield and with excellent enantioselectivity. Also, the catalyst loading could be lowered to a part-per-million level (50 ppm: 0.005 mol%). A readily accessible chiral disulfonimide was used, which in combination with trimethylsilyl cyanide, turned into the active silylium Lewis acid organocatalyst. The nature of a peculiar phenomenon referred to as a "dormant period", which is mainly induced by water, was systematically investigated by means of in situ Fourier transform infrared analysis.