engleski

Unsymmetrical 1, n '-disubstituted ferrocenoyl peptides: convenient one pot synthesis and solution structures by CD and NMR spectroscopy

We present a systematic study on unsymmetrical amino acid derivatives of 1, 1-ferrocene dicarboxylic acid. A convenient and general one pot synthetic procedure is presented in which the unsymmetrical amino acid derivatives are readily separated by column chromatography. The following ferrocene derivatives Fe[C5H4-CO-Aaa1-OMe][C5H4-CO-Aaa2-OMe] with different amino acids were prepared and their solution structures studied by NMR and CD spectrosocopy (Aaa1= Aaa2= Phe, 1 ; Aaa1= Aaa2=DPhe, 1a ; Aaa1= Aaa2= Ala, 2 ; Aaa1= Phe, Aaa2= Ala, 3 ; Aaa1=DPhe, Aaa2= Ala, 4 ; Aaa1= Aaa2= Pro, 5 ; Aaa1= Aaa2= Gly, 6 ; all amino acids are pure L enantiomers except where stated otherwise). NMR spectroscopy in CDCl3 confirms intramolecular hydrogen bonds for the disubstituted derivatives 1– 4 and 6. CD spectra were recorded for all derivatives in CH2Cl2. They clearly show a P helical isomer of the ferrocene moiety for the L amino acid derivatives 1– 3 and M helicity for the DPhe derivative 1a. The racemic derivative 6 shows only intermolecular interactions, while the Pro derivative 5, which is devoid of amide protons, most likely exists in an open conformation. Most importantly, the unsymmetrical mixed D, L derivative 4 has only a weak CD signal in the ferrocene region which is interpreted as an equilibrium between the P and M helical isomers with a slight excess of the latter.

Ferrocene; Hydrogen bond; CD spectroscopy

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