Kinetic Resolution of 2-Aryldihydroquinolines Using Lithiation - Synthesis of Chiral 1,2- and 1,4-Dihydroquinolines

Highly enantiomerically enriched dihydrohydroquinolines were prepared in two steps from quinoline. Addition of aryllithiums to quinoline with tert-butoxycarbonyl (Boc) protection gave N-Boc-2-aryl-1,2-dihydroquinolines. These were treated with n-butyllithium and electrophilic trapping occurred exclusively at C-4 of the dihydroquinoline, a result supported by DFT studies. Variable temperature NMR spectroscopy gave kinetic data for the barrier to rotation of the carbonyl group (ΔG≠ ≈49 kJ mol-1 , 195 K). Lithiation using the diamine sparteine allowed kinetic resolutions with high enantioselectivities (enantiomer ratio up to 99 : 1). The enantioenriched 1,2-dihydroquinolines could be converted to 1,4-dihydroquinolines with retention of stereochemistry. Further functionalisation led to trisubstituted products. Reduction provided enantioenriched tetrahydroquinolines, whereas acid-promoted removal of Boc led to quinolines, and this was applied to a synthesis of the antimalarial compound M5717.

Dihydroquinolines, Dihydronaphthyridines and Quinolones by Domino Reactions of Morita-Baylis-Hillman Acetates

An efficient synthetic route to highly substituted dihydroquinolines and dihydronaphthyridines has been developed using a domino reaction of Morita-Baylis-Hillman (MBH) acetates with primary aliphatic and aromatic amines in DMF at 50–90 °C. The MBH substrates incorporate a side chain acetate positioned adjacent to an acrylate or acrylonitrile aza-Michael acceptor as well as an aromatic ring activated toward S_NAr ring closure. A control experiment established that the initial reaction was an S_N2′-type displacement of the side chain acetate by the amine to generate the alkene product with the added nitrogen nucleophile positioned trans to the S_NAr aromatic ring acceptor. Thus, equilibration of the initial alkene geometry is required prior to cyclization. A further double bond migration was observed for several reactions targeting dihydronaphthyridines from substrates with a side chain acrylonitrile moiety. MBH acetates incorporating a 2,5-difluorophenyl moiety were found to have dual reactivity in these annulations. In the absence of O₂, the expected dihydroquinolines were formed, while in the presence of O₂, quinolones were produced. All of the products were isolated in good to excellent yields (72–93%). Numerous cases (42) are reported, and mechanisms are discussed.

NHC-Catalyzed Chemoselective Reactions of Enals and Aminobenzaldehydes for Access to Chiral Dihydroquinolines

An N-heterocyclic carbene (NHC)-catalyzed reaction between α-bromoenals and 2-aminoaldehydes has been developed. Key steps include chemoselective reaction of the NHC catalyst with one of the aldehyde substrates (the bromoenal) to eventually generate an α,β-unsaturated acylazolium intermediate. Addition of the nitrogen atom of aminoaldehyde to the unsaturated azolium ester intermediate followed by intramolecular aldol reaction, β-lactone formation, and decarboxylation leads to chiral dihydroquinolines with high optical purity. The dihydroquinoline products, which are quickly prepared by using this method, can be readily transformed into a diverse set of functional molecules such as pyridines and chiral piperidines.

Rhodium-catalyzed intramolecular hydroarylation of 1-halo-1-alkynes: regioselective synthesis of semihydrogenated aromatic heterocycles

The regioselective intramolecular hydroarylation of (3-halo-2-propynyl)anilines, (3-halo-2-propynyl) aryl ethers, or (4-halo-3-butynyl) aryl ethers was efficiently catalyzed by Rh2(OCOCF3)4 to give semihydrogenated aromatic heterocycles, such as 4-halo-1,2-dihydroquinolines, 4-halo-3-chromenes, or 4-(halomethylene)chromans, in good to excellent yields. Some synthetic applications taking advantage of the halo-substituents of the products are also illustrated.