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Ring conformations of 4-substituted prolines. The Cγ -endo conformation is favored strongly by stereoelectronic effects when R1 = H, R2 = F (flp) or Cl (clp) and by steric effects when R1 = Me (mep), R2 = H. The Cγ -exo conformation is favored strongly by stereoelectronic effects when R1 = OH (Hyp), F (Flp) or Cl (Clp), R2 = H and by steric effects when R1 = H, R2 = Me (Mep).
