Stereoselective Synthesis of Pyrido- and Pyrrolo[1,2-c][1,3]oxazin-1-ones via a Nucleophilic Addition–Cyclization Process of N,O-Acetal with Ynamides

AgSbF6-catalyzed C3 aza-Friedel-Crafts alkylation of N,O-acetals with indoles for the synthesis of N-α indole substituted pyrrolidine and piperidine derivatives

An efficient approach to access chiral N-α indole substituted pyrrolidine and piperidine skeletons has been developed through a AgSbF₆-catalyzed N-α aza-Friedel-Crafts alkylation of N,O-acetals 6a, 6b, 9, and 11a-11d with indoles. As a result, a series of 2,3-trans N-α indole substituted pyrrolidines 8a-8x and piperidines 10a-10j were prepared in moderate to excellent yields and with excellent diastereoselectivities (dr up to 99 : 1). Moreover, several 2,5-cis-N-α indole substituted pyrrolidine derivatives 12a-12k were synthesized according to this strategy with moderate to good yields and diastereoselectivities (dr up to 99 : 1).

AgNTf2 catalyzed cycloaddition of N-acyliminium ions with alkynes for the synthesis of the 3,4-dihydro-1,3-oxazin-2-one skeleton

A catalyzed process for the synthesis of the 4,6-substituted 3,4-dihydro-1,3-oxazin-2-one skeleton has been developed through cycloaddition of in situ generated acyliminium intermediates with alkynes. A range of chain N,O-acetals and terminal alkynes were amenable for this mild transformation. As a result, a series of desired cycloaddition products were obtained in moderate to good yields.

Cu(OTf)2-catalyzed C3 aza-Friedel-Crafts alkylation of indoles with N,O-acetals

An efficient approach to access functionalized indole derivatives has been developed through Cu(OTf)₂-catalyzed C3 aza-Friedel-Crafts alkylation of substituted indoles 5a-5m, N-methyl-pyrrole with linear N,O-acetals 4a-4l. As a result, a series of C3 amide aza-alkylated indole derivatives 6a-6ag and 7 were synthesized in moderate to excellent yields.

A Ag2CO3/TFA-catalyzed intramolecular annulation approach to imidazo[1,2-c][1,3]oxazin-5-one derivatives

A series of 2,7-disubstituted 3-methylimidazo[1,2-c][1,3]oxazin-5-ones were synthesized in good yields via Ag₂CO₃/TFA-mediated intramolecular annulation of N-Boc-2-alkynyl-4-bromo(alkynyl)-5-methylimidazoles. This methodology was carried out in the presence of a catalytic amount of silver carbonate and trifluoroacetic acid in dichloroethane at 60 °C. In all experiments, only the six-membered ring product was obtained since the possible five-membered compound was not observed, proving the high regioselectivity of this approach. A complementary computational study was performed in order to rationalize the mechanism of 6-endo-dig heterocycle formation. In addition, 2-bromo-3-methyl-7-phenylimidazo[1,2-c][1,3]oxazin-5-one was used as a building block to synthesize a small library of new 2-substituted imidazo[1,2-c][1,3]oxazin-5-one derivatives through the Suzuki, Sonogashira and Heck cross coupling reactions.

AgNTf2-Catalyzed Regioselective C-H Alkenylation of N,N-Dialkylanilines with Ynamides

Regioselective C-H alkenylation of N,N-dialkylanilines with ynamides was developed using AgNTf₂ as a catalyst. This approach represents a facile hydroarylation of ynamides, allowing for the introduction of an alkenyl group exclusively at the para position of aniline derivatives. As a result, a series of 4-alkenyl N,N-dialkylanilines were synthesized with excellent regioselectivities.

SmI2‑Mediated Intermolecular Addition-Elimination of Piperidine and Pyrrolidine N-α-Radicals with Arylacetylene Sulfones

An efficient approach to access α-arylacetylene substituted pyrrolidine and piperidine derivatives has been developed through a samarium diiodide mediated addition-elimination process of pyrrolidine and piperidine N-α-radicals with arylacetylene sulfones.

C-H bond cleavage-enabled aerobic ring-opening reaction of in situ formed 2-aminobenzofuran-3(2H)-ones

A C-H bond cleavage-enabled aerobic ring-opening reaction of 2-aminobenzofuran-3(2H)-ones formed in situ by hemiacetals with a variety of amines is reported. This simple one-pot reaction provides an alternative approach to obtain o-hydroxyaryl glyoxylamides in excellent yields of up to 97%. Alkylamines react with hemiacetals via a catalyst-free dehydration condensation to generate 2-aminobenzofuran-3(2H)-ones. The in situ formed semicyclic N,O-acetals undergo the same amine-initiated C-H bond hydroxylation in air under mild conditions to afford ring-opening products. Similarly, arylamines were investigated as substrates for a two-step tandem process involving a DPP-catalyzed condensation followed by a Et₂NH-mediated C-H hydroxylation. Unlike the previously reported functionalization of N,O-acetals via a C-O or C-N cleavage, the aerobic oxidative C-H hydroxylation in this reaction, which is promoted by using stoichiometric amounts of alkylamines as both a Lewis base and a reductant at room temperature under atmospheric air, proceeds via α-carbonyl-stabilized carbanion intermediates from the C-H cleavage of N,O-acetals.

PhI(OAc)2-mediated trifluoromethylthiolation/oxidative cyclization of ynamides

A PhI(OAc)₂-mediated trifluoromethylthiolation/oxidative cyclization of ynamides with the Shen reagent has been established herein, providing a facile access to CF₃S-substituted oxazolidine-2,4-diones bearing a quaternary carbon center in 38-85% yields with chemoselectivities of up to 99/1.

TMSOTf-mediated synthesis of skipped dienes through the addition of olefins to imines and semicyclic N,O-acetals

A novel approach to skipped dienes has been developed through the TMSOTf-mediated one-pot addition-substitution of olefins 2a, 2f and 2g with imines 1a-1g, and a series of aryl substituted skipped dienes 3aa-3gf were accordingly obtained in 62%-94% yields. Moreover, semicyclic N,O-acetals 5 and 7 could also undergo this transformation to produce the corresponding skipped dienes 6aa and 6af-6al and 8ba and 8bf-8bk in moderate to good yields.

Cu(OTf)2 catalyzed Ugi-type reaction of N,O-acetals with isocyanides for the synthesis of pyrrolidinyl and piperidinyl 2-carboxamides

The Cu(OTf)₂ catalyzed Ugi-type reactions of N,O-acetals with isocyanides have been described for the synthesis of pyrrolidinyl and piperidinyl 2-carboxamides. 4-Hydroxy-5-substituted-prolinamides can be obtained in high diastereoselectivities (2,4-cis/trans > 19 : 1) and a stereoselective model was proposed for 2,4-cis selectivity. Moreover, 4-F-VH 032, a novel analog of the VHL ligand, was conveniently obtained by utilizing the present method.

A One-Pot Approach to 2-Substituted-2-(Dimethoxyphosphoryl)-Pyrrolidines from Substituted tert-Butyl 4-Oxobutylcarbamates and Trimethyl Phosphite

A novel approach to 2-substituted-2-(dimethoxyphosphoryl)-pyrrolidines 7a-7o and 9a-9r has been developed, which features a TMSOTf-mediated one-pot intramolecular cyclization and phosphonylation of substituted tert-butyl 4-oxobutylcarbamates. The major advantages of this method include simple operation under mild reaction conditions, the use of cheap Lewis acid, and good to excellent yields with high diastereoselectivities (dr up to 99:1).

Palladium-catalysed cyclisation of ynamides and propargyl tethered iodosulfonamides with boronic acids leading to benzosultams

An efficient and straightforward Pd-catalysed synthesis of diversely substituted sultams utilising ynamides and boronic acids is disclosed; toluene was found to be the most suitable solvent for this transformation. This strategy has been successfully applied to generate dihydrobenzo[d]isothiazole 1,1-dioxides and dihydro-2H-benzo[e][1,2]thiazine 1,1-dioxides. The advantages of this protocol are good functional group tolerance, broad substrate scope, high-yielding reaction and low catalyst loading to access benzofused sultams with five-/six-membered rings. The synthetic utility has been demonstrated by a gram-scale synthesis. A plausible catalytic cycle involving carbopalladation has been proposed for this transformation.

Zn(OTf)2-catalyzed hydroamination of ynamides with aromatic amines

The Zn(OTf)₂-catalyzed hydroamination of ynamides 2a-2l with aromatic amines 1a-1r was developed. This protocol features broad substrate scope of aromatic amines, good functional group tolerance for ynamides, and excellent regioselectivities. As a result, a variety of substituted amidine compounds 3aa-3oa, 3ab-3al and 3pa-3rk were prepared in moderate to excellent yields and with high regioselectivities.

Reactivity of ynamides in catalytic intermolecular annulations

Ynamides are unique alkynes with a carbon-carbon triple bond directly attached to the nitrogen atom bearing an electron-withdrawing group. The alkyne is strongly polarized by the electron-donating nitrogen atom, but its high reactivity can be finely tempered by the electron-withdrawing group. Accordingly, ynamides are endowed with both nucleophilic and electrophilic properties and their chemistry has been an active research field. The catalytic intermolecular annulations of ynamides, featuring divergent assembly of structurally important amino-heterocycles in a regioselective manner, have gained much attention over the past decade. This review aims to provide a comprehensive summary of the advances achieved in this area involving transition metal and acid catalysis. Moreover, the intermolecular annulations of ynamide analogs including ynol ethers and thioalkynes are also discussed, which can provide insights into the reactivity difference caused by the heteroatoms.

Copper-catalyzed alkynylation/annulation cascades of N-allyl ynamides: regioselective access to medium-sized N-heterocycles

A synthetic strategy based on sequential application of aza-Claisen rearrangement, C–H functionalization, C–N coupling and cyclization as key steps has been developed for the synthesis of 6-, 7-, 8-, and 9-membered rings N-heterocycles.

Asymmetric synthesis with ynamides: unique reaction control, chemical diversity and applications

Ynamides are among the most powerful building blocks in organic synthesis and have become invaluable starting materials for the construction of multifunctional compounds and challenging architectures that would be difficult to prepare otherwise. The rapidly growing popularity originates from the unique reactivity and ease of manipulation of the polarized ynamide triple bond, the advance of practical methods for making them, and the simplicity of storage and handling. These attractive features and the demonstration of numerous synthetic applications have spurred the development of intriguing asymmetric reaction strategies during the last decade. An impressive variety of chemo-, regio- and stereoselective carbon-carbon and carbon-heteroatom bond forming reactions with ynamides have been developed and now significantly enrich the toolbox of synthetic chemists. This review provides a comprehensive overview of asymmetric ynamide chemistry since 2010 with a focus on the general scope, current limitations, stereochemical reaction control and mechanistic aspects.

Influence of solvent mixture on nucleophilicity parameters: the case of pyrrolidine in methanol-acetonitrile

The course of organic chemical reactions is efficiently modelled through the concepts of “electrophiles” and “nucleophiles” (meaning electron-seeking and nucleus-seeking reactive species). On the one hand, an advanced approach of the correlation of the nucleophilicity parameters N and electrophilicity E has been delivered from the linear free energy relationship log k (20 °C) = s(N + E). On the other hand, the general influence of the solvent mixtures, which are very often employed in preparative synthetic chemistry, has been poorly explored theoretically and experimentally, to date. Herein, we combined experimental and theoretical studies of the solvent influence on pyrrolidine nucleophilicity. We determined the nucleophilicity parameters N and s of pyrrolidine at 20 °C in CH₃OH/CH₃CN mixtures containing 0, 20, 40, 60, 80 and 100% CH₃CN by kinetic investigations of their nucleophilic substitution reactions to a series of 2-methoxy-3-X-5-nitrothiophenes 1a–e (X = NO₂, CN, COCH₃, CO₂CH₃, CONH₂). Depending on the resulting solvation medium, the N parameters range from 15.72 to 18.32 on the empirical nucleophilicity scale of Mayr. The nucleophilicity parameters N first evolve linearly with the content of acetonitrile up to 60% CH₃CN by volume, but is non linear for higher amounts. We designed a general computation protocol to investigate the solvent effect at the atomistic scale. The nucleophilicity in solvent mixtures was evaluated by combining classical molecular dynamic (MD) simulations of solvated pyrrolidine and a few density functional theory (DFT) calculations of Parr nucleophilicity. The pyrrolidine theoretical nucleophilicity 1/ω obtained in various CH₃OH/CH₃CN mixtures are in excellent agreement with Mayr's nucleophilicity (N) parameters measured. Analyses of the molecular dynamic trajectories reveal that the decrease of the nucleophilicity in methanol rich mixtures arises predominantly from the solvation of the pyrrolidine by methanol molecules through strong hydrogen bonds. Last, we proposed a simple model to predict and accurately reproduce the experimentally obtained nucleophilicity values.

Combined experiments and modelling rationalize the large influence of solvent composition on pyrrolidine nucleophilicity.

Divergent synthesis of N-heterocyclic 1,6-enynes through a zinc-catalyzed decarboxylative A3 reaction

A zinc-catalyzed decarboxylative A³ reaction of cyclic amino acids, α,β-unsaturated aldehydes and terminal alkynes has been developed. A series of functionalized N-heterocyclic 1,6-enynes have been successfully obtained with excellent regioselectivities through this novel approach. In addition, the utility of this straightforward process is demonstrated by the preparation of a polycyclic nitrogen-containing heterocyclic compound.

Synthesis of 1,4- and 1,5-Amino Alcohols via Nucleophilic Addition of Semicyclic N,O-Acetal with Organozinc Reagents

An efficient approach to access functionalized 1,4- and 1,5-amino alcohols has been developed through the nucleophilic addition of semicyclic N,O-acetal with organozinc reagents. A number of substituted benzyl zinc reagents (including nitrile and ester substituted) could react with semicyclic N,O-acetals 1 and 2, affording the desired products 3a-3p and 4a-4o in good to excellent yields.

Chemoselective Strategy for the Direct Formation of Tetrahydro-2,5-methanobenzo[ c]azepines or Azetotetrahydroisoquinolines via Regio- and Stereoselective Reactions

The present study reports regio- and highly diastereoselective preparative methods for the synthesis of versatile alkaloid-type compounds from oxiranylmethyl tetrahydroisoquinolines. 2,5-Methanobenzo[ c]azepines or azetidine-fused heterocycles were synthesized in tandem reactions depending on the absence or presence of a BF₃ co-reagent. A high functional group tolerance has also been demonstrated. DFT calculations with an explicit solvent model confirmed the proposed reaction mechanisms and the role of kinetic controls on the stereochemical outcome of the reported new methods.

Annulation Cascades of N-Allyl- N-((2-bromoaryl)ethynyl)amides Involving C-H Functionalization

An annulation cascades of N-allyl- N-((2-bromoaryl)ethynyl)amides with terminal alkynes or 1,3-dicarbonyls involving C-H functionalization for producing 2,3-functionalized indoles has been first developed by means of Cu catalysis. The method is enabled by the formation of the ketenimine intermediates to deliver 2,3-disubstituted indoles through a sequence of aza-Claisen rearrangement, C-H functionalization, Ullmann C-N coupling, and cyclization.