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Xiao L, Kitzler R, Weissensteiner W: O-Methylephedrine: a versatile and highly efficient ortho-directing group. J Org Chem. 2001 Dec 28;66(26):8912-9.
Synthesis of enantiopure 1,2-disubstituted ferrocene derivatives.. O-Methylephedrine was identified as a very efficient chiral auxiliary for ortho-lithiation reactions of ferrocenes. (1R,2S)-O-Methylephedrine [CH (3) NHCH (CH (3)) CH (Ph) OCH (3)] was reacted with N-ferrocenylmethyl-N,N,N-trimethylammonium iodide [FcCH (2) N (CH (3))(3) I; Fc = ferrocenyl] to give (1R,2S)-N-ferrocenylmethyl-O-methylephedrine. Treatment of this compound with t-BuLi in pentane followed by quenching with the electrophiles iodine, dibromotetrafluoroethane, chlorodiphenylphosphine or benzophenone gave 2-substituted ferrocenes in 98% de and with the (R (p))-ferrocene configuration. Subsequently, the chiral auxiliary could be replaced by systems including dimethylamine, acetate, diaryl- or dialkylphosphines to give a number of enantiopure bifunctional 1,2-disubstituted ferrocene derivatives such as (R (p))-N-2-iodo- or (R (p))-N-2-bromoferrocenylmethyldimethylamine or (R (p))-2-acetoxymethyl-1-diphenylphosphinoferrocene. As an application, ferrocenyl diphosphines possessing a planar (R (p))-ferrocene configuration only [1,2-(PPh (2)) FcCH (2) PR (2), R = Cy, Ph, [3,5-(CF (3))(2) Ph]] were synthesized in three steps from O-methylephedrine and N-ferrocenylmethyl-N,N,N-trimethylammonium iodide in up to 77% overall yield. |
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