Isothiourea-mediated one-pot synthesis of functionalized pyridines

Boron-Catalyzed Michael Reaction of Donor-Acceptor Carboxylic Acid Pairs Enabling Direct Synthesis of 1,5-Dicarboxylic Acids

A boron-catalyzed Michael reaction using pairs of carboxylic acids was developed. The reaction occurs through dual activation of the two substrates by a boron catalyst, which facilitates boron enolate formation from the donor carboxylic acid with simultaneous activation of the α,β-unsaturated carboxylic acid as the acceptor. α-Aryl and α-alkenyl carboxylic acids were applicable as donors. The versatility and utility of this reaction were demonstrated by the direct use of pharmaceuticals as donor carboxylic acids.

Organocatalytic access to 3-pyridylphosphonates from vinyl phosphonates and aldehydes

The preparation of 3-phosphorylated pyridines has remained the most challenging compared to the corresponding 2- or 4-functionalized pyridines. Herein, an unprecedented amino-organocatalyzed direct synthesis of 3-pyridylphosphonates from vinylphosphonates and aldehydes has been achieved. This allows access to a wide range of multi-substituted phosphorylated pyridines in excellent yields under transition metal-free conditions.

Isothiourea-Catalyzed Enantioselective Functionalisation of Glycine Schiff Base Aryl Esters via 1,6- and 1,4-Additions

The enantioselective α-functionalisation of glycine Schiff base aryl esters through isothiourea catalysis is successfully demonstrated for 1,6-additions to para-quinone methides (21 examples, up to 95:5 dr and 96:4 er) and 1,4-additions to methylene substituted dicarbonyl or disulfonyl Michael acceptors (17 examples, up to 98:2 er). This nucleophilic organocatalysis approach gives access to a range of α-functionalised α-amino acid derivatives and further transformations of the activated aryl ester group provide a straightforward entry to advanced amino acid-based esters, amides or thioesters.

A Desilylative Approach to Alkyl Substituted C(1)‐Ammonium Enolates: Application in Enantioselective [2+2] Cycloadditions

The catalytic generation of C(1)‐ammonium enolates from the corresponding α‐silyl‐α‐alkyl substituted carboxylic acids using the isothiourea HyperBTM is reported. This desilylative approach grants access to α‐unsubstituted and α‐alkyl substituted C(1)‐ammonium enolates, which are typically difficult to access through traditional methods reliant upon deprotonation. The scope and limitations of this process is established in enantioselective [2+2]‐cycloaddition processes with perfluoroalkylketones (31 examples, up to 96 % yield and >99 : 1 er), as well as selective [2+2]‐cycloaddition with trifluoromethyl enones (4 examples, up to 75 % yield and >99 : 1 er). Preliminary mechanistic studies indicate this process proceeds through an initial kinetic resolution of an in situ prepared (±)‐α‐silyl‐α‐alkyl substituted anhydride, while the reaction process exhibits overall pseudo zero‐order kinetics.

Synthesis of Unsymmetrically Tetrasubstituted Pyrroles and Studies of AIEE in Pyrrolo[1,2-a]pyrimidine Derivatives

Pyrroles are among the most important heterocycles in pharmaceuticals and agrochemicals. Construction of pyrrole scaffolds with different substituents and free NH group however, is challenging. Herein, a metal-free method for...

Bifunctional Urea/Hg(OAc)2-Mediated Synthesis of 4-Aryl-6-oxycarbonyl-2-pyrones and 2-Pyridones from Dithiomalonate and β,γ-Unsaturated α-Keto Esters

Disubstituted 2-pyrones and 2-pyridones were obtained by bifunctional urea-catalyzed Michael addition/lactonization or lactamization followed by a Hg(OAc)₂- or Hg(OAc)₂/DBU-mediated hydrolysis/decarboxylation/dehydrogenation process. This one-pot two-stage protocol enabled the rapid synthesis of 4,6-disubstituted 2-pyrones and 2-pyridones from dithiomalonate and β,γ-unsaturated α-keto esters in practical yields under mild reaction conditions. Additionally, the obtained 2-pyridones were facilely transformed to 2,4,6-trisubstituted pyridines in excellent yields.

Mechanism and regio- and stereoselectivity in an NHC-catalyzed Mannich/lactamization domino reaction

Density functional theory (DFT) calculations (M06-2X) have been employed to disclose the mechanisms and regio- and stereo-selectivities of the N-heterocyclic carbene (NHC)-catalyzed reaction of 2-benzothiazolimines and α-chloroaldehydes. The preferred mechanism is initiated by the nucleophilic attack of NHC on α-chloroaldehyde (first step), followed by 1,2-proton transfer which was assisted by the Brønsted acid DABCO·H+ to generate the Breslow intermediate (second step). The cleavage of the C-Cl bond (third step) and deprotonation (fourth step) form the enolate intermediate. This further reacts with 2-benzothiazolimine which leads to the formation of a new C-C bond (fifth step). Subsequent cyclization takes place via the formation of a new C-N bond (sixth step). Catalyst regeneration completes the whole catalytic cycle and affords the final product (seventh step). The DFT results indicate that the fifth step determines the stereochemistry of the reaction and leads to benzothiazolopyrimidinone with the SS configuration, which agrees well with experimental observations. Intramolecular cyclization is found to be the regioselectivity-determining step, for which the [4+2] annulation pathway is more preferred than that via [2+2] annulation, which again agrees well with experimental observations. Based on the mechanism proposed, the origins of regio- and stereoselectivities have also been investigated by performing distortion/interaction, natural bond orbital (NBO) and non-covalent interaction (NCI) analyses. The mechanistic insights gained in this work should be helpful in the rational design of potential catalysts for analogous reactions.

Sustainable Four-Component Annulation for the Synthesis of 2,3,4,6-Tetraarylpyridines

A one-pot, four-component annulation of 2,3,4,6-tetraarylpyridines from aromatic aldehydes, methyl ketones, diaryl ethanones, and ammonium acetate is described. The reaction features high functional group compatibility in air under solvent-free conditions without any additive and only water as the nontoxic byproduct, providing a strategy for the facile, economical, and eco-friendly construction of multiaryl-substituted pyridines from simple and readily available reactants.

Generation and Reactivity of C(1)-Ammonium Enolates by Using Isothiourea Catalysis

C(1)-Ammonium enolates are powerful, catalytically generated synthetic intermediates applied in the enantioselective α-functionalisation of carboxylic acid derivatives. This minireview describes the recent developments in the generation and application of C(1)-ammonium enolates from various precursors (carboxylic acids, anhydrides, acyl imidazoles, aryl esters, α-diazoketones, alkyl halides) using isothiourea Lewis base organocatalysts. Their synthetic utility in intra- and intermolecular enantioselective C-C and C-X bond forming processes on reaction with various electrophiles will be showcased utilising two distinct catalyst turnover approaches.

Rapid, two-pot procedure for the synthesis of dihydropyridinones; total synthesis of aza-goniothalamin

A fast, protecting-group-free synthesis of dihydropyridinones has been developed. Starting from commercially available aldehydes, a novel one-pot amidoallylation gave access to diene compounds in good yields. Ring-closing metathesis conditions were then employed to produce the target dihydropyridinones efficiently and in high yields.

Synthetic and mechanistic aspects of sulfonyl migrations

Sulfonyl migrations, frequently described as ‘unusual’ or ‘unexpected’, from the last 20 years, including 1,2-, 1,3-, 1,4-, 1,5-, 1,6- and 1,7-sulfonyl shifts, through either radical or polar processes, either inter- or intramolecularly are reviewed.

Insights into isothiourea-catalyzed asymmetric [3 + 3] annulation of α,β-unsaturated aryl esters with 2-acylbenzazoles: mechanism, origin of stereoselectivity and switchable chemoselectivity

The switchable chemoselectivity of isothiourea-catalyzed asymmetric [3 + 3] annulation of α,β-unsaturated aryl esters with 2-acylbenzazoles has been predicted successfully.

Organocatalytic asymmetric [4+2] cyclization of 2-benzothiazolimines with azlactones: access to chiral benzothiazolopyrimidine derivatives

A squaramide catalyzed regiospecific and stereoselective [4+2] cyclization of 2-benzothiazolimines with azlactones has been established.

Facile One-pot Protocol of Derivatization Nitropyridines: Access to 3-Acetamidopyridin-2-yl 4-methylbenzenesulfonate Derivatives

This paper discloses an efficient one-pot protocol to convert easily accessible 3-nitropyridines to 3-acetamidopyridin-2-yl 4-methylbenzenesulfonate derivatives which are core structures of many pharmaceutical molecules. The strategy successfully combined a three-step reaction in one pot via progressively adding different reactants at rt. The reaction displays good functional group tolerance and regioselectivity. Structurally diversified 3-nitropyridine could be time-efficiently (3.5 h) derivatized to various functional 2-O,3-N-pyridines which are apt for further elaborations. The transformation was amenable to gram-scale synthesis.

Perfluorocarboxylic Anhydrides Triggered Cyclization: Access to 4-Perfluoroalkylpyridines

A double nucleophilic addition-cyclization-elimination cascade is developed, that allows various 2,6-diaryl-4-perfluoroalkylpyridines to be synthesized in one step from easily available enamides and perfluorocarboxylic anhydrides. The procedure is also operationally simple and scalable and provides access to the facial construction of 4-fluoroalkylpyridines, which are of great interest in medicinal chemistry.

Chiral phosphoric acid-catalyzed enantioselective construction of structurally diverse benzothiazolopyrimidines

A highly efficient catalytic enantioselective [4+2] cycloaddition was developed between 2-benzothiazolimines and enecarbamates. A wide range of benzothiazolopyrimidines bearing three contiguous stereogenic centers was obtained in high to excellent yields and with excellent diastereo- and enantioselectivities (d.r. > 98 : 2 and up to >99% ee). Furthermore, this chiral phosphoric acid-catalyzed strategy was scalable and enabled access to a new class of optically pure Lewis base isothiourea derivatives.

Insights into the isothiourea-catalyzed asymmetric [4 + 2] annulation of phenylacetic acid with alkylidene pyrazolone

A computational study on the isothiourea-catalyzed asymmetric [4 + 2] annulation of phenylacetic acid with alkylidene pyrazolone was performed using the DFT method.

Copper/amine-catalyzed formal regioselective [3 + 2] cycloaddition of an α,β-unsaturated O-acetyl oxime with enals

A novel copper/amine co-catalyzed formal regioselective [3 + 2] cycloaddition reaction of an O-acyl oxime with α,β-unsaturated aldehydes is developed.

Cu-Catalyzed switchable synthesis of functionalized pyridines and pyrroles

A switchable synthesis of functionalized pyridines and pyrroles via Cu-catalyzed cascade reactions with N-sulfonyl azadienes and alkynes.

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.

Carbene-catalyzed LUMO activation of alkyne esters for access to functional pyridines

A carbene-catalyzed activation of alkyne ester for quick access to functional pyridines is reported. The reaction involves a previously unexplored unsaturated acyl azolium intermediate bearing a carbon–carbon triple bond that shall exhibit rich chemical reactivities.

Acid-catalyzed tandem reaction for the synthesis of pyridine derivatives via CC/C(sp3)–N bond cleavage of enones and primary amines

2,4,6-Trisubstituted pyridines can be generated from an acid-catalyzed tandem reaction via CC/C(sp³)–N bond cleavage of enones and primary amines.

Isothiourea-catalysed enantioselective pyrrolizine synthesis: synthetic and computational studies

The catalytic enantioselective synthesis of a range of cis-pyrrolizine carboxylate derivatives with outstanding stereocontrol (14 examples, >95 : 5 dr, >98 : 2 er) through an isothiourea-catalyzed intramolecular Michael addition-lactonisation and ring-opening approach from the corresponding enone acid is reported. An optimised and straightforward three-step synthetic route to the enone acid starting materials from readily available pyrrole-2-carboxaldehydes is delineated, with benzotetramisole (5 mol%) proving the optimal catalyst for the enantioselective process. Ring-opening of the pyrrolizine dihydropyranone products with either MeOH or a range of amines leads to the desired products in excellent yield and enantioselectivity. Computation has been used to probe the factors leading to high stereocontrol, with the formation of the observed cis-steroisomer predicted to be kinetically and thermodynamically favoured.