A new chiral catalytic source with an N–PO structural framework containing a proximal hydroxyl group for the borane-mediated asymmetric reduction of prochiral ketones

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.

Highly enantioselective production of (R)-halohydrins with whole cells of Rhodotorula rubra KCh 82 culture

Biotransformation of ten α-haloacetophenones in the growing culture of the strain Rhodotorula rubra KCh 82 has been carried out. Nine of the substrates underwent an effective enantioselective reduction to the respective (R)-alcohols according to Prelog's rule, with the exception of 2-chloro-1,2-diphenylethan-1-one that was not transformed by this strain. The expected reduction proceeded without dehalogenation, leading to the respective (R)-halohydrins in high yields. The use of this biocatalyst yielded (R)-2-bromo-1-phenyl-ethan-1-ol (enantiomeric excess (ee) = 97%) and its derivatives: 4'-Bromo- (ee = 99%); 4'-Chloro- (ee > 99%); 4'-Methoxy- (ee = 96%); 3'-Methoxy- (ee = 93%); 2'-Methoxy- (ee = 98%). There were also obtained and characterized 2,4'-dichloro-, 2,2',4'-trichloro- and 2-chloro-4'-fluoro-phenyetan-1-ol with >99% of enantiomeric excesses.

Biotransformation of acetophenone and its halogen derivatives by Yarrowia lipolytica strains

The ability of 16 strains of Yarrowia lipolytica to biotransform acetophenone and its derivatives has been studied. Thirteen of these strains were derived from a wild-type strain Y. lipolytica A-101; six had the invertase gene (SUC2) from Saccharomyces cerevisiae integrated into their genome, as well as the damaged or undamaged gene encoding orotidine-5'-phosphate decarboxylase (URA3), three had integrated the damaged URA3 gene into their genome and three were UV acetate-negative mutants, not able to growth on acetate as the sole carbon source. The other tested strains included two wild strains, A-101 and PMR-1, and an adenine auxotroph ATCC 32-338A. All strains were capable of reducing acetophenone to the R-alcohol in high enantiomeric excess (80-89 %). In all of the cultures tested, reversibility of the reduction was observed, which led to an increase in the enantiomeric excess. nantioselective reduction of the acetophenone halogen derivatives revealed that the nature and location of the halogen atom had a significant influence on the enantioselectivity of the reduction. In the culture of ATCC 32-338A, after a 3-day biotransformation of 2,4'-dibromoacetophenone the enantiopure R-alcohol was obtained at a rate of 100 % of substrate conversion. In conclusion, using these invertase-containing strains or uracyl auxotrophs provided no additional benefit in terms of biotransformation capacity over the parental strain.

Synthesis of a new C(2)-symmetric chiral catalyst and its application in the catalytic asymmetric borane reduction of prochiral ketones

A new C(2)-symmetric chiral catalyst 3,5-bis[(2S)-(hydroxy-diphenylmethyl)- pyrrolidin-1-ylmethyl]-1,3,4-oxadiazole was successfully synthesized by the reaction of 2,5-dichloromethyl-1,3,4-oxadiazole with (S)-alpha,alpha-diphenyl-2-pyrrolidinemethanol, and applied to the catalytic asymmetric reduction of prochiral ketones with borane. When the catalyst loading was 1 mol %, enantiomeric excesses of up to 86.8% and 94.5% were observed in reduction of aromatic and alpha-halo ketones, respectively.