Ai-Bao Xia

In situ enamine activation in aqueous salt solutions: highly efficient asymmetric organocatalytic Diels-Alder reaction of cyclohexenones with nitroolefins.

Deep-sea Diels-Alder: The asymmetric organocatalytic Diels-Alder reaction of cyclohexenones with aromatic nitroolefins can be carried out in seawater and brine. The reaction proceeds by an in situ enamine activation involving a one-step concerted addition pathway (see scheme).

One‐Pot Organocatalytic Asymmetric Synthesis of 3‐Nitro‐1,2‐dihydroquinolines by a Dual‐Activation Protocol

Generally, amine-catalyzed enantioselective transformations rely on chiral enamine or unsaturated iminium intermediates. Herein, we report a protocol involving dual activation by an aromatic iminium and hydrogen-bonding. An enantioselective aza-Michael-Henry domino reaction of 2-aminobenzaldehydes with nitroolefins has been developed through this protocol using primary amine thiourea catalysts to provide a variety of 3-nitro-1,2-dihydroquinolines in moderate yields and with up to 90% ee. The mechanism for the catalytic enantioselective reaction was confirmed by ESI mass spectrometric detection of the reaction intermediates. The products formed are substructures found in skeletons of important biological and pharmaceutical molecules.

Enantioselective synthesis of functionalized 3,4-disubstituted dihydro-2(1H)-quinolinones via Michael–hemiaminalization/oxidation reaction

A novel method is developed for the enantioselective synthesis of highly functionalized 3,4-disubstituted dihydro-2(1H)-quinolinones bearing two trans contiguous stereogenic centers with excellent diastereoselectivities (up to >99 : 1 dr) and high to excellent enantioselectivities (up to >99% ee). The process combines an enantioselective organocatalytic Michael–hemiaminalization reaction and a highly efficient oxidation reaction sequence with good yields and stereoselectivity.

Highly enantioselective Michael reaction employing cycloheptanone and cyclooctanone as nucleophiles

An organocatalytic Michael reaction of cycloheptanone and cyclooctanone with nitrodienes and nitroolefins catalyzed by a hydroquinine-based primary amine catalyst has been accomplished. The corresponding Michael adducts were obtained in good yields (up to 88%) with good to excellent diastereoselectivities (up to >100 : 1) and enantioselectivities (up to >99% ee). The absolute configuration of the Michael product was assigned by TDDFT simulation of the ECD spectrum. And the Michael products can be readily converted into analogues of cycloalkano[b] fused pyrrolidines.

(S)-2-[(S,Z)-3-Bromo-1-nitro-4-phenyl-but-3-en-2-yl]cyclo-hexa-none

In the crystal structure of the title compound, C16H18BrNO3, the two stereogenic centres both have an S configuration. The cyclohexyl ring adopts a chair conformation. In the crystal, molecules are linked by weak N—O⋯Br contacts [O⋯Br = 3.289 (4) Å].

(2S,4R)-2-[(1R)-1-(4-Bromo-phen-yl)-2-nitro-eth-yl]-4-ethyl-cyclo-hexa-none.

The crystal structure of the title compound, C16H20BrNO3, contains three chiral centers in the configuration 1R,2S,6R. The cyclohexane ring is in a chair conformation. In the crystal, molecules are linked by weak C—H⋯O interactions, forming chains along the a-axis direction.

(R)-7-Bromo-2,3,4,4a-tetra­hydro-1H-xanthen-1-one

The title compound, C13H11BrO2, contains a tricyclic ring system with one chiral center which exhibits an R configuration. The crystal structure is devoid of any classical hydrogen bonding.

1-[3,5-Bis(trifluoro­meth­yl)phen­yl]-3-(2-pyrid­yl)thio­urea

The title compound, C14H9F6N3S, exhibits a nearly planar conformation in the solid state, with a dihedral angle between the planes of the benzene and pyridine rings of 14.86 (3)°. The pyridine N atom allows for the formation of a six-membered N—H⋯Npy hydrogen-bonded ring, thus forcing the two amide H atoms of the thiourea group to point in opposite directions. The second N—H group forms an intermolecular N—H⋯S hydrogen bond with the S atom of an adjacent molecule. The F atoms of the two trifluoromethyl groups display rotational disorder around the C—CF3 axis, with an occupancy ratio of 0.54 (1):0.46 (1).

Asymmetric synthesis of polysubstituted chiral chromans via an organocatalytic oxa-Michael-nitro-Michael domino reaction

A catalytic asymmetric method for the synthesis of polysubstituted chromans via an oxa-Michael-nitro-Michael reaction has been developed. The squaramide-catalyzed domino reaction of 2-hydroxynitrostyrenes with trans-β-nitroolefins produced chiral chromans with excellent enantioselectivities (up to 99% ee), diastereoselectivities (up to >20 : 1 dr), and moderate to good yields (up to 82%).

Successive Waste as Reagent: Two More Steps Forward in a Pinnick Oxidation

The successful development of the classical Pinnick oxidation into a new and promising oxidative lactonization reaction is reported. Chiral 3-oxindolepropionic aldehydes, Michael adducts of 3-olefinic oxindoles with aliphatic aldehydes, are directly converted to spirocyclic oxindole-γ-lactones solely by sodium chlorite via a tandem Pinnick oxidation/chlorination/substitution sequence. This reaction uses waste ClO- generated in the initial Pinnick oxidation as an ecofriendly halogenating agent for the subsequent chlorination, and then it utilizes the byproduct OH- formed in the chlorination to facilitate the final internal substitution.