AbstractCyclic phosphoric acids have attracted much attention as chiral organocatalysts. While binaphthol based ligands have been extensively used in various transformations, the analogous TADDOL-type ligands are less explored. A library of seventeen cyclic phosphoric acids with structural variation of the TADDOL backbone were synthesized and an optimization study of the Mannich reaction of aromatic N-(2-hydroxyphenyl)imines with 2,2-dimethyl-1-methoxy-silylenolate was performed. Enantioselectivities between 96% (S) and 70% (R) were observed and a rationalisation, based on crystal structure analysis and DFT calculations, is provided.
AbstractHydroamination of unsaturated carbon-carbon bonds is a useful C-N bond forming reaction as it leads to products (amines, imines, enamines) of high synthetic value. While the hydroamination of terminal alkenes and alkynes under metal catalysis has been widely developed, the corresponding reactions with internal substrates remain a challenge. Two recent studies by Buchwald and by Dong which focus on enantioselective hydroamination of internal alkenes and alkynes respectively are of significant importance. [43,44] The results reported broaden the scope of hydroamination as a useful C-N bond forming process and allow acess to chiral amines which are difficult to synthesize by other methods.
Abstract Solvent effects in the L-tryptophan-catalyzed Mannich reaction between hydroxyacetone and glyoxylate imines have been examined. The use of a DMSO/1-butanol (4:1 v/v) mixture as solvent at rt provided the expected Mannich adducts in good yields, high anti-diastereoselectivity (up to 10.3:1 anti/syn ratio) and excellent enantioselectivities (up to >99.9% ee for the anti isomer).
AbstractThe [3+2]-cycloaddition reaction between acrolein and in situ generated acyclic azomethine ylides promoted by a chiral secondary amine to generate C-3 unsubstituted pyrrolidines has been studied in detail. Optimum conditions involve the use of the cheap and readily available l-Proline as catalyst to afford the pyrrolidine cycloadduct with complete diastereomeric control and up to 80% enantiomeric control.
The application of a convenient one-pot synthetic strategy, utilizing an in situ formed organocatalyst, to the enantioselective synthesis of anti-leukaemia agent (R)-convolutamydine A has been demonstrated.
Abstract Novel enantiomerically pure tetramethylbithiophene diphosphine oxides (tetraMe-BITIOPO) featuring differently substituted aromatic rings at the phosphorous atoms were synthesized, fully characterized and isolated in enantiomerically pure form. The new Lewis bases were tested as organocatalysts in two different reactions involving trichlorosilyl compounds. The introduction of electron donating substituents on the aromatic rings connected to the phosphine oxide groups positively affected the chemical and stereochemical catalytic efficiency of these ligands. The new catalysts were able to promote the allylation of aldehydes with allyltrichlorosilane in up to 76% yield and up to 86% enantiomeric excess (ee), and the direct aldol reaction to afford β-hydroxy ketones in high diastereoselectivity (up to 88:12 anti:syn ratio) and up to 72% ee.
Application of a chiral counter-ion strategy in the Kharasch-Sosnovsky reaction is reported. Copper(I) phosphates obtained from axially chiral, BINOL-derived phosphoric acids were investigated as catalysts in theenantioselective allylic oxidation of acyclic alkenes. Good yields and high regioselectivity towards branched allylic ester have been achieved. The chiral phosphates investigated exhibited modest enantioselectivities.
AbstractThis article describes a study on the Michael addition reaction of cyclohexanone to nitroolefins catalyzed by the chiral secondary amine (1S,4S)-2-tosyl- 2,5-diazabicyclo[2.2.1]heptane. Reactions were carried out under solvent-free conditions to make them more environmentally friendly. Initially, the observed diastereoand enantioselectivities were moderate to good, but were significantly improved by lowering the reaction temperature. Furthermore, a variety of chiral acids were also tested as co-catalysts in both of their enantiomeric forms, which revealed that (R)-mandelic acid affords excellent results in terms of yield and stereoselectivity. Monitoring the reaction by MS-TOF allowed for the detection of key reaction intermediates, and a reasonable reaction mechanism in which both catalysts are involved is proposed.
