Chemo- and Diastereoselective Construction of Indenopyrazolines via a Cascade aza-Michael/Aldol Annulation of Huisgen Zwitterions with 2-Arylideneindane-1,3-diones

Syntheses of Phosphonylated Pyrido-Dihydrofurans by Regioselective 5-Exo-dig Addition to Carbon-Carbon Triple Bond Catalyzed by AgNO3

The tandem nucleophilic addition-cyclization reaction of o-alkynylpyridinecarbaldehyde 1 with dialkyl phosphites 2 took place very smoothly in the presence of AgNO3 under base conditions in DCE at 50 °C. In all cases, the reaction proceeded in a regioselective manner leading to the 5-exo-dig phosphonylated pyrido-dihydrofurans 3 in moderate to good yields.

Organocatalytic Dearomatization of 5-Aminopyrazoles: Synthesis of 4-Hydroxypyrazolines

Dearomatization is a fundamental chemical reaction that affords complex three-dimensional heterocyclic frameworks. We disclose the first organocatalytic dearomatization of 5-aminopyrazoles, which yields a range of structurally diversified C4-hydroxylated pyrazolines with yields of ≤95% in <1.5 h at room temperature. This catalytic process is achieved using in situ-generated hypervalent iodine. The method also yields a spirolactone via an intramolecular dearomatization process. Furthermore, we demonstrate that substrate-directed reduction of the resulting iminopyrazoline leads to 4,5-difunctionalized pyrazoline as a single diastereomer. Mechanistic studies suggest that the reaction proceeds through a dearomatized cationic intermediate.

Sequential annulation and isomerisation reaction of 3-acylmethylidene oxindoles with Huisgen zwitterions and synthesis of 5-(3-oxindolyl)oxazoles

Herein, we report a facile synthesis of 5-(3-oxindolyl)oxazole derivatives via a sequential annulation and isomerisation reaction of 3-acylmethylidene oxindoles with in situ generated Huisgen zwitterions (HZs) from PPh3 and azodicarboxylates. This reaction exhibits good functional group tolerance with 30 examples of structurally diverse products prepared with moderate to good efficiencies (up to 88% yield), thus providing a generally applicable route to the biologically important 5-(3-indolyl)oxazole structural motifs. Key to the success of this sequential one-pot strategy is the utilization of DBU as a base to promote the isomerisation process of the corresponding intermediate annulation products.

ASYMMETRIC SYNTHESIS: A GLANCE AT VARIOUS METHODOLOGIES FOR DIFFERENT FRAMEWORKS

Abstract:

Asymmetric reactions have made a significant advancement over the past few decades and involved the production of enantiomerically pure molecules using enantioselective organocatalysis, chiral auxiliaries/substrates, and reagents via controlling the absolute stereochemistry. The laboratory synthesis from an enantiomerically impure starting material gives a combination of enantiomers which are difficult to separate for chemists in the fields of medicine, chromatography, pharmacology, asymmetric synthesis, studies of structure-function relationships of proteins, life sciences and mechanistic studies. This challenging step of separation can be avoided by the use of asymmetric synthesis. Using pharmacologically relevant scaffolds/pharmacophores, the drug designing can also be achieved using asymmetric synthesis to synthesize receptor specific pharmacologically active chiral molecules. This approach can be used to synthesize asymmetric molecules from wide variety of reactants using specific asymmetric conditions which is also beneficial for environment due to less usage and discharge of chemicals into the environment. So, in this review, we have focused on the inclusive collation of diverse mechanisms in this area, to encourage auxiliary studies of asymmetric reactions to develop selective, efficient, environment-friendly and high yielding advanced processes in asymmetric reactions.

Phosphine-Mediated [4 + 3] Annulation of Diynoates and 2-Arylidene Indane-1,3-diones: Access of Indeno[1,2-b]oxepin-4-ylidenes and Beyond

A novel unprecedented triphenylphosphine-mediated [4 + 3] annulation reaction of 2-benzylidene indane-1,3-diones and -diynoates through initial phosphine α-addition was discovered and found to result in biologically interesting indeno[1,2-b]oxepin-4-ylidenes in up to 75% yield. The seven-membered separable Z and E isomeric oxepins were confirmed using single-crystal X-ray diffraction.

Diastereoselective Synthesis of Tetrabenzohydrofuran Spirooxindoles via Diethyl Phosphite-Mediated Coupling of Isatins with o-Quinone Methides

Diethyl phosphite-initiated coupling of isatins with o-quinone methides (o-QMs) is reported. This reaction involves a cascade transformation initiated by base-promoted addition of phosphite to isatins, followed by [1,2]-phospha-Brook rearrangement. This generates α-phosphonyloxy enolates that are subsequently intercepted by o-QMs and finally intramolecular ring closure. This protocol was used to diastereoselectively synthesize a range of trans-tetrabenzohydrofuran spirooxindoles in moderate to good yields with moderate to excellent diastereoselectivities.

A PPh3-catalyzed sequential annulation reaction to construct cyclopentane-fused dihydropyrazolone-pyrrolidinediones

A PPh3-catalyzed sequential cycloaddition of maleimides with unsaturated pyrazolones has been developed. This protocol provides a simple and practical strategy for the construction of dispirocyclopentyl-[dihydropyrazolone-pyrrolidinedione] pyrrolidinedione skeletons containing five contiguous stereogenic centers, including two spiro-quaternary centers, with moderate yields (34-73%) and excellent diastereoselectivities (>20 : 1).

Catalytic Asymmetric Construction of a 1,2,4-Benzotriazepine Skeleton via Diastereo- and Enantioselective Decarboxylative [4 + 3] Cyclization

We have developed a protocol for palladium-catalyzed decarboxylative [4 + 3] cycloaddition reactions between 4-vinyl benzoxazinanones and azomethine imines to generate moderate to good yields of structurally diverse 1,2,4-benzotriazepines bearing two stereogenic centers with good to excellent stereoselectivities. This protocol not only addresses the challenge of asymmetrically constructing compounds with a 1,2,4-benzotriazepine skeleton but also demonstrates the utility of decarboxylative cycloadditions for the synthesis of enantioenriched polycyclic compounds.

Highly diastereoselective spiro-cyclopropanation of 2-arylidene-1,3-indanediones and dimethylsulfonium ylides

High diastereoselectivity, mild conditions and excellent functional group compatibility.

Condensation of 1,2-dicarbonyl compounds with modified Huisgen zwitterions: synthesis of N-aryl-N-acyl hydrazones

A phosphine-mediated deoxygenative condensation of 1,2-dicarbonyl compounds such as aroylformates, α-diketones, and isatins with arylazocarboxylates has been developed for a facile synthesis of N-aryl-N-acyl hydrazones in moderate to excellent yields under very mild conditions. Mechanistic investigation based on 31P NMR tracking experiments unveils that the reaction is initiated with the in situ formation of the modified Huisgen zwitterions from arylazocarboxylates and PPh3 and proceeds via a nitrogen to nitrogen ester group migration process. This study also represents the first exploration of the reactivity patterns of the modified Huisgen zwitterions derived from arylazocarboxylates toward electrophiles such as 1,2-dicarbonyl compounds.

Synthesis of methylene cyclopropane-fused chromenes and dihydroquinolines by sequential [4 + 2]- and [1 + 2]-annulation

A base promoted sequential [4 + 2]- and [1 + 2]-annulation of 2-hydroxychalcones or 2-tosylaminochalcones with prop-2-ynylsulfonium salts was developed.

Recent advances in the asymmetric synthesis of pharmacology-relevant nitrogen heterocycles via stereoselective aza-Michael reactions

In this review, recent applications of a stereoselective aza-Michael reaction for asymmetric synthesis of naturally occurring N-containing heterocyclic scaffolds and their usefulness to pharmacology are summarized.

Lewis base catalyzed regioselective cyclization of allene ketones or α-methyl allene ketones with unsaturated pyrazolones

A nitrogen-containing Lewis base catalyzed highly regioselective [4 + 2] cycloaddition of allene ketones or α-methyl allene ketones with unsaturated pyrazolones has been disclosed to give the corresponding tetrahydropyrano [2,3-c] pyrazoles in moderate to good yields under mild conditions. High regioselectivity, 100% atom-economy, broad substrate scope and good functional group tolerance are attractive features of this process and make it a practical and versatile transformation.

Bisphosphine catalyzed sequential [3 + 2] cycloaddition and Michael addition of ynones with benzylidenepyrazolones via dual α′,α′-C(sp3)–H bifunctionalization to construct cyclopentanone-fused spiro-pyrazolones

A sequential [3 + 2] cycloaddition and Michael addition reaction of ynones with benzylidenepyrazolones afforded cyclopentanone-fused spiro-pyrazolones catalyzed by DPPB via dual α′,α′-C(sp³)–H bifunctionalization.