Direct Organocatalytic Reductive Alkylation of Syncarpic Acid: Scope and Applications

Two-step, high-yielding total synthesis of antibiotic pyrones

A simple two-step dialkylation protocol was developed to synthesize biologically active antibiotics photopyrones, pseudopyronines, and violapyrones from bio-renewable triacetate lactone in excellent yields. These pyrones are functionally modified into another set of pyrone natural products by a single O-methylation reaction. The high-yielding gram scale synthesis of four natural products [pseudopyronine A, photopyrone A, pseudopyronine B and photopyrone C] demonstrated the viability for industrial applications.

Organocatalytic Reductive Amination of the Chiral Formylcyclopropanes: Scope and Applications

We developed a sustainable three-component reductive amination protocol for the chemoselective coupling of optically active functionally rich donor-acceptor carbonyl-cyclopropanes with various amines under 10 mol % of diphenyl phosphate in the presence of Hantzsch ester as a hydride source. The catalytic selective reductive C-N coupling has wide advantages like no epimerization, no ring opening, large substrate scope, generating only mono N-alkylation products and simultaneously resulting in chiral cyclopropane-containing amines possessing many applications in the medicinal chemistry. In this article, we have shown the synthetic applications of reductive C-N coupling reaction to make chiral α-carbonyl-cyclopropane containing amines 8, double C-N coupled cyclopropane-amines 10, unusual C-N/C-C coupled cyclopropane-amines 12, chiral tert-butylsulfinamide containing cyclopropanes 14/15, and functionally rich chiral cyclopropane-fused N-heterocycles 16/18/19. Many of these chiral cyclopropane-amines 5-19 can serve as building blocks for the synthesis of drug-like small molecules, natural products, pharmaceuticals, and their analogues.

Direct organocatalytic transfer hydrogenation and C-H oxidation: high-yielding synthesis of 3-hydroxy-3-alkyloxindoles

Readily available 3-alkylideneoxindoles were effectively reduced to 3-alkyloxindoles through transfer hydrogenation using Hantzsch ester as a reducing agent at ambient temperature and the greenness/sustainability of this protocol was assessed by correlation with Pd/C-mediated hydrogenation with hydrogen gas. Furthermore, an organocatalytic method was developed to access drug-like 3-alkyl-3-hydroxyoxindoles by C-H oxidation of 3-alkyl-indolin-2-one, using a catalytic amount of 1,1,3,3-tetramethylguanidine (TMG) as an organic base and dissolved oxygen in THF as an oxidant at room temperature. Key reaction intermediates were observed by controlled on-line ESI-HRMS experiments and identified by their corresponding mass (m/z) analysis. This two-step high-yielding transfer hydrogenation/C-H oxidation protocol was used for the total synthesis of medicinally important 3-cyanomethyl-3-hydroxyoxindole and formal total synthesis of (±)-alline and (±)-CPC-I in very good overall yields compared to previous methods.