Dynamic Kinetic Resolution of Carboxylic Esters Catalyzed by Chiral PPY N-Oxides: Synthesis of Nonsteroidal Anti-Inflammatory Drugs and Mechanistic Insights

Reversal of Enantioselectivity for the Desymmetrization of meso-1,2-Diols Catalyzed by Pyridine-N-oxides

The desymmetrization of meso-vic-diols with a reversal of enantioselectivity catalyzed by chiral pyridine-N-oxides with l-proline as a single source of chirality is reported. With chiral 3-substituted ArPNO C2c and 2-substituted 4-(dimethylamino)pyridine-N-oxide C3b as catalysts, a wide range of monoesters were obtained with satisfactory results with a complete and controlled switch in stereoselectivity (up to 97:3 and 1:99 er). Chiral six-membered carbocyclic uracil nucleosides were generated with excellent enantioselectivities after derivatization. A series of control experiments and density functional theory (DFT) calculations supported that the reaction proceeded in a bifunctional activated manner, where the N-oxide groups and N-H proton of the amides were vital for catalytic reactivity and stereocontrol. The DFT calculation also supported the distance-directed switching of enantioselectivity, in which the l-prolinamide moiety moved from the C3 to C2 position on the pyridine ring, resulting in the H-bond interaction between the amide N-H and OH group of meso-vic-diol also shifted from one hydroxyl group to another.

Kinetics and Mechanism of Azole n-π*-Catalyzed Amine Acylation

Azole anions are highly competent in the activation of weak acyl donors, but, unlike neutral (aprotic) Lewis bases, are not yet widely applied as acylation catalysts. Using a combination of in situ and stopped-flow 1H/19F NMR spectroscopy, kinetics, isotopic labeling, 1H DOSY, and electronic structure calculations, we have investigated azole-catalyzed aminolysis of p-fluorophenyl acetate. The global kinetics have been elucidated under four sets of conditions, and the key elementary steps underpinning catalysis deconvoluted using a range of intermediates and transition state probes. While all evidence points to an overarching mechanism involving n-π* catalysis via N-acylated azole intermediates, a diverse array of kinetic regimes emerges from this framework. Even seemingly minor changes to the solvent, auxiliary base, or azole catalyst can elicit profound changes in the temporal evolution, thermal sensitivity, and progressive inhibition of catalysis. These observations can only be rationalized by taking a holistic view of the mechanism and a set of limiting regimes for the kinetics. Overall, the analysis of 18 azole catalysts spanning nearly 10 orders of magnitude in acidity highlights the pitfall of pursuing ever more nucleophilic catalysts without regard for catalyst speciation.

Silver/chiral pyrrolidinopyridine relay catalytic cycloisomerization/(2 + 3) cycloadditions of enynamides to asymmetrically synthesize bispirocyclopentenes as PDE1B inhibitors

Significant progress has been made in asymmetric synthesis through the use of transition metal catalysts combined with Lewis bases. However, the use of a dual catalytic system involving 4-aminopyridine and transition metal has received little attention. Here we show a metal/Lewis base relay catalytic system featuring silver acetate and a modified chiral pyrrolidinopyridine (PPY). It was successfully applied in the cycloisomerization/(2 + 3) cycloaddition reaction of enynamides. Bispirocyclopentene pyrazolone products could be efficiently synthesized in a stereoselective and economical manner (up to >19:1 dr, 99.5:0.5 er). Transformations of the product could access stereodivergent diastereoisomers and densely functionalized polycyclic derivatives. Mechanistic studies illustrated the relay catalytic model and the origin of the uncommon chemoselectivity. In subsequent bioassays, the products containing a privileged drug-like scaffold exhibited isoform-selective phosphodiesterase 1 (PDE1) inhibitory activity in vitro. The optimal lead compound displayed a good therapeutic effect for ameliorating pulmonary fibrosis via inhibiting PDE1 in vivo.

ArPNO-catalyzed acylative kinetic resolution of tertiary alcohols: access to 3-hydroxy-3-substituted oxindoles

Bifunctional chiral 4-aryl-pyridine-N-oxides (ArPNO) were reported for the acylative kinetic resolution of 3-hydroxy-3-substituted oxindoles, where the oxygen acts as the nucleophilic site. Using less sterically hindered acetic anhydride, both the recovered tertiary heterocyclic alcohols and the ester products exhibited good to excellent results with s-factors up to 167. Control experiments supported the dual activation manner, where the N-oxide group and N-H proton in ArPNO were crucial for high selectivity and enhanced catalytic reactivity. Compared with the extensively used chiral NHC, isochalcogenourea, and DMAP catalysts, we found that chiral ArPNO were also efficient organocatalysts in the kinetic resolution of tertiary alcohols.

New tertiary amine-derived C2-symmetric chiral pyridine-N,N′-dioxide ligands and their applications in asymmetric catalysis

Developing efficient methods for preparing privileged chiral ligands is an important endeavor in synthetic chemistry.