A couple of planar-chiral ferrocene-fused 4-pyridone derivatives 2a and 2b were synthesized in enantiomerically pure form by scalable asymmetric transformations. Pyridones 2 are versatile precursors to various ferrocene-fused pyridine derivatives, which are useful nucleophilic asymmetric organocatalysts.
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-[(2S,4S)-4-methoxymethyl-1,3-dioxan-2-yl]-1′,2′,3′,4′,5′-R5-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 MnO2 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 Me3SiNR2 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.
The synthesis of substituted styrenes 3 and 3-vinylphenols 9 was achieved by ring-closing enyne metathesis (RCEM)/dehydration of 7 and RCEM/tautomerization of 8, respectively. Those methods provide selective access to unique aromatic compounds and solve the problem of regioisomer formation.
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