Versatile and Enantioselective Preparation of Planar-Chiral Metallocene-Fused 4-Dialkylaminopyridines and Their Application in Asymmetric Organocatalysis

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A series of ferrocene-fused planar-chiral N -tosyl-4-pyridones ( S )- 2b – d were prepared in enantiomerically pure forms. Starting with the chiral ferrocenyl acetals, 1-[(2 S,4 S )-4-methoxymethyl-1,3-dioxan-2-yl]-1′,2′,3′,4′,5′-R 5 -ferrocenes ((−)- 3b, R = Me; (−)- 3c, R = Ph; (−)- 3d, R = Bn), N -tosylamino and formyl groups were introduced at the 1- and 2-positions of the ferrocene cores in ( S )- 11b – d with control of the planar chirality. After the reaction with ethynylmagnesium bromide, generated propargyl alcohol derivatives ( S )- 17 were treated with MnO 2 and catalytic tetrabutylammonium iodide to give the planar-chiral pyridones by the iodide-catalyzed cyclization. This method is highly practical with a shorter and higher-yield sequence without using noble metal catalysts. Planar-chiral ferroco-pyridones ( S )- 2b–d were reacted with various Me 3 SiNR 2 compounds to give a library of ferrocene-fused 4-dialkylaminopyridines (( S )- 1, DAAPs) in high yields as single enantiomers by the detosylative amination. The cymantrene-fused DAAPs were also prepared in the same way. The library of chiral DAAPs was examined in the two asymmetric reactions as organocatalysts, and some newly prepared Fc-DAAPs showed better enantioselectivity than the known species.