Kinetic Resolution of Tertiary Alcohols by Chiral DMAP Derivatives: Enantioselective Access to 3-Hydroxy-3-substituted 2-Oxindoles

One-Pot Access to Functionalised Malamides via Organocatalytic Enantioselective Formation of Spirocyclic β-Lactone-Oxindoles and Double Ring-Opening

Malamides (diamide derivatives of malic acid) are prevalent in nature and of significant biological interest, yet only limited synthetic methods to access functionalised enantiopure derivatives have been established to date. Herein, an effective synthetic method to generate this molecular class is developed through in situ formation of spirocyclic β-lactone-oxindoles (employing a known enantioselective isothiourea-catalysed formal [2+2] cycloaddition of C(1)-ammonium enolates and isatin derivatives) followed by a subsequent dual ring-opening protocol (of the β-lactone and oxindole) with amine nucleophiles. The application of this protocol is demonstrated across twelve examples to give densely functionalised malamide derivatives with high enantio- and diastereo-selectivity (up to >95:5 dr and >99:1 er).

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.

Recent Advances in Catalytic Nonenzymatic Kinetic Resolution of Tertiary Alcohols

The kinetic resolution (KR) of racemates is one of the most widely used approaches to access enantiomerically pure compounds. Over the past two decades, catalytic nonenzymatic KR has gained popularity in the field of asymmetric synthesis due to the rapid development of chiral catalysts and ligands in asymmetric catalysis. Chiral tertiary alcohols are prevalent in a variety of natural products, pharmaceuticals, and biologically active chiral compounds. The catalytic nonenzymatic KR of racemic tertiary alcohols is a straightforward strategy to access enantioenriched tertiary alcohols. This short review describes recent advances in catalytic nonenzymatic KR of tertiary alcohols, including organocatalysis and metal catalysis.

1 Introduction

2 Organocatalysis

2.1 Peptide Catalyst

2.2 Chiral Phosphoric Acid Catalyst

2.3 Chiral Lewis Base Catalyst

2.4 Chiral Quaternary Ammonium Salt Catalyst

3 Metal Catalysis

3.1 Mixed La-Li Heterobimetallic Catalyst

3.2 Rh Catalyst

3.3 Hf Catalyst

3.4 Pd Catalyst

3.5 Cu Catalyst

3.6 Ag Catalyst

4 Conclusion and Outlook

Zn-Catalyzed enantioselective allylation and allenylation of isatins by virtue of a proline-derived chiral ligand

An asymmetric allylation and allenylation of isatins with facile organoboron reagents was developed under the catalysis of a Lewis acid. A series of optically pure 3-allyl-3-hydroxyoxindoles and 3-allenyl-3-hydroxyoxindoles can be obtained in excellent yields (up to 99% yield) and high enantioselectivities (up to 97% ee). The possible transition state was supported by DFT calculation and the corresponding mechanism was proposed. A gram scale experiment and further functionalization of these chiral 3-hydroxyoxindoles are established.

Asymmetric Enamide-Imine Tautomerism in the Kinetic Resolution of Tertiary Alcohols

An efficient protocol for kinetic resolution of tertiary alcohols has been developed through an unprecedented asymmetric enamide-imine tautomerism process enabled by chiral phosphoric acid catalysis. A broad range of racemic 2-arylsulfonamido tertiary allyl alcohols could be kinetically resolved with excellent kinetic resolution performances (with s-factor up to >200). This method is particularly effective for a series of 1,1-dialkyl substituted allyl alcohols, which produced chiral tertiary alcohols that would be difficult to access via other asymmetric methods. Facile and versatile transformations of the chiral α-hydroxy imine and enamide products, especially the efficient stereodivergent synthesis of all four stereoisomers of β-amino tertiary alcohols using one enantiomer of the catalyst, demonstrated the value of this kinetic resolution method.

Synthesis of Carbamoyl Fluorides via a Selective Fluorinative Beckmann Fragmentation

A fluorinative Beckmann fragmentation of α-oximinoamides was devised to provide synthetically useful carbamoyl fluorides. High selectivity for fragmentation over a potentially competing Beckmann rearrangement was observed. This protocol has a distinct mechanism and thus a different substrate scope compared with other synthetic methods. α-Oximinoamides derived from the readily available secondary amines, lactams, or isatins were converted into structurally diverse carbamoyl fluorides.