Asymmetric carbonyl reduction with borane catalyzed by chiral phosphinamides derived from l-amino acid

Microwave-Assisted Ruthenium- and Rhodium-Catalyzed Couplings of α-Amino Acid Ester-Derived Phosphinamides with Alkynes

Two different types of new phosphinamide α-amino ester derivatives have been prepared in moderate to high yields via ruthenium(II) and rhodium(III)-catalyzed ortho-C-H functionalization under microwave irradiation. Specifically, the ortho-alkenylated phosphinamides were produced through coupling of phosphinamides containing an α-substituted or α,α-disubstituted α-amino ester with internal alkynes under ruthenium catalysis. In contrast, Ru and the more effective Rh-catalyzed coupling of the α-unsubstituted glycine ester phosphinamide with alkynes resulted in formation of oxidative annulation products, phosphaisoquinolin-1-ones. The developed methods feature the use of easily accessible starting materials, short reaction time, exclusive E-stereoselectivity (for ortho-alkenylation) and good functional group tolerance. The alkenylation reaction was readily scaled up to gram scale. Furthermore, the obtained alkenylated phosphinamide could be transformed into P-containing dipeptides through hydrolysis of the ester group in the catalysis product and subsequent condensation with an α-amino ester.

Desymmetric Enantioselective Reduction of Cyclic 1,3-Diketones Catalyzed by a Recyclable P-Chiral Phosphinamide Organocatalyst

The P-stereogenic phosphinamides are a structurally novel skeletal class which has not been investigated as chiral organocatalysts. However, chiral cyclic 3-hydroxy ketones are widely used as building blocks in the synthesis of natural products and bioactive compounds. However, general and practical methods for the synthesis of such chiral compounds remain underdeveloped. Herein, we demonstrate that the P-stereogenic phosphinamides are powerful organocatalysts for the desymmetric enantioselective reduction of cyclic 1,3-diketones, providing a useful method for the synthesis of chiral cyclic 3-hydroxy ketones. The protocol displays a broad substrate scope that is amenable to a series of cyclic 2,2-disubstituted five- and six-membered 1,3-diketones. The chiral cyclic 3-hydroxy ketone products bearing an all-carbon chiral quaternary center could be obtained with high enantioselectivities (up to 98% ee) and diastereoselectivities (up to 99:1 dr). Most importantly, the reactions could be practically performed on the gram scale and the catalysts could be reused without compromising the catalytic efficiency. Mechanistic studies revealed that an intermediate formed from P-stereogenic phosphinamide and catecholborane is the real catalytically active species. The results disclosed herein bode well for designing and developing other reactions using P-stereogenic phosphinamides as new organocatalysts.

Absolute Configuration and Conformation of (-)-R-t-Butylphenylphosphinoamidate: Chiroptical Spectroscopy and X-ray Analysis

The absolute configuration and conformations of (−)-tert-butylphenylphosphinoamidate were determined using three different chiroptical spectroscopic methods, namely vibrational circular dichroism (VCD), electronic circular dichroism (ECD), and optical rotatory dispersion (ORD). In each of the spectroscopic methods used, experimental data for the (−)-enantiomer of tert-butylphenylphosphinoamidate were measured in the solution phase. Using the concentration-dependent experimental infrared spectra, the existence of dimers in the solution was investigated, and the monomer–dimer equilibrium constant was determined. Concomitant quantum mechanical predictions of the VCD, ECD, and ORD for monomeric tert-butylphenylphosphinoamidate were carried out using density functional theory (DFT) calculations using the B3LYP functional and the 6-31G(d), 6-311G(2d,2p) and aug-cc-pVDZ basis sets. Similar predictions for dimeric tert-butylphenylphosphinoamidate were also obtained using the B3LYP/6-31G(d) method. A comparison of theoretically predicted data with the corresponding experimental data led to the elucidation of the absolute configuration as (−)-(R)-tert-butylphenylphosphinoamidate with one predominant conformation in the solution. This conclusion was independently supported by X-ray analysis of the complex with (+)-R-2,2′-dihydroxy-1,1′-binaphthol ((+)-R- BINOL).

Recent trends in organocatalyzed asymmetric reduction of prochiral ketones

This review depicts the recent practices followed in organocatalyzed asymmetric reduction of prochiral ketones, highlighting the main organocatalysts used for the past seven years.

Highly diastereo- and enantioselective organocatalytic addition of acetone to beta-substituted alpha-ketoesters via dynamic kinetic resolution

L-proline catalyzes the aldol addition reaction of acetone to beta-substituted alpha-ketoesters with dynamic kinetic resolution, providing the desired adduct in good yield with excellent diastereoselectivity (up to >99:1 dr) and enantioselectivity (up to 98% ee). The absolute configuration of the chiral adduct was assigned by single-crystal X-ray diffraction analysis. A tentative explanation of the stereochemical outcome is proposed.