Synthesis and Solid State Conformation of Tetrapeptide Amides Containing two Aib and two (αMe)Phe Residues - Use of Enantiomerically Pure 2-Benzyl-2-methyl-2H-azirin-3-amines as (αMe)Phe-Synthons

Synthesis and crystal structures of new chiral 3-amino-2H-azirines and the Pd complex of one of them

3-Amino-2H-azirines are potentially versatile building blocks in heterocyclic and peptide synthesis. Three new 3-amino-2H-azirines have been synthesized as racemates or mixtures of diastereoisomers in cases where another chiral residue is incorporated as the exocyclic amine. The crystal structures of two of them, an approximately 1:1 diastereoisomeric mixture of (2R)- and (2S)-2-ethyl-3-[(2S)-2-(1-methoxy-1,1-diphenylmethyl)pyrrolidin-1-yl]-2-methyl-2H-azirine, C₂₃H₂₈N₂O, 11, and 2-benzyl-3-(N-methyl-N-phenylamino)-2-phenyl-2H-azirine, C₂₂H₂₀N₂, 12, and the third as its diastereoisomeric trans-PdCl₂ complex, trans-dichlorido[(2R)-2-ethyl-2-methyl-3-(X)-2H-azirine][(2S)-2-ethyl-2-methyl-3-(X)-2H-azirine]palladium(II), where X = N-{[(1S,2S,5S)-6,6-dimethylbicyclo[3.1.1]heptan-2-yl]methyl}-N-phenylamino, [PdCl₂(C₂₁H₃₀N₂)₂], 14, have been determined and the geometries of the azirine rings compared with those of 11 other 3-amino-2H-azirine structures reported in the literature. Most notable is the very long formal N-C single bond, which is, with one exception, around 1.57 Å. Each compound has crystallized in a chiral space group. The Pd atom in the trans-PdCl₂ complex is coordinated by one of each of the pair of diastereoisomers, while both of the diastereoisomers share the same crystallographic site in the structure of 11; this property thereby manifesting itself as disorder. The chosen crystal of 12 is either an inversion twin or composed of a pure enantiomorph, but this could not be established specifically.