Chiral Selectivity in the Achiral Amino Acid Glycine

A Cambridge Structural Database (CSD) search for the
zwitterion
of glycine, the only natural amino acid which does not have a chiral
center at Cα, and its derivatives H3N–CαH2–C′(Ocis)­Otrans(H/R/M) gave 425 hits, 420 of which are chiral because
of the ψ conformation with regard to the central C′–Cα bond (ψ = OcisC′CαN ≠ 0 or 180°). Chiral ψ conformations distort
the planar carboxylic group CαC′(Ocis)­Otrans group stereoselectively to an asymmetric flat
tetrahedron with four different corners, specified by (Rθ)/(Sθ). Negative
rotation angles ψ predominantly induce pyramidalization angles
θ = OtransC′CαOcis below −180°, resulting in a high diastereoselectivity
for (Mψ,Sθ)-glycine. Positive rotation angles ψ preferentially give (Pψ,Rθ)-glycine. Density functional theory calculations confirm the results
of the data analysis, indicating an increase of pyramidalization θ
for increasing rotation angles ψ. This is corroborated by the
analysis of section averages of the structural data, showing an increase
of the ψ/θ selectivity in the series 1.5, 2.8, 3.5, and
6.0 for increasing rotation angles ψ. Without any chiral bias,
419 glycine derivatives crystallize in the ratio racemate/conglomerate
359:65.