Predominant (<i>S</i>)-Enantioselective Inclusion of Aryl Methyl Sulfoxides by (<i>S</i>)-Isoleucyl-(<i>S</i>)-phenylglycines

In terms of chiral recognition for racemic aryl methyl sulfoxides in the solid state, three kinds of crystalline (S)-alkylglycyl-(S)-phenylglycines were examined as potential dipeptides host molecules. When (S)-alanyl-(S)-phenylglycines [(S,S)-Ala-Phg] crystallized with aryl methyl sulfoxides, the stereochemistry of preferentially included sulfoxides depended on the individual shapes of the sulfoxides and the enantiomeric excess was relatively low. Although (S)-leucyl-(S)-phenylglycines [(S,S)-Leu-Phg] and (S)-isoleucyl-(S)-phenylglycines [(S,S)-Ile-Phg] mainly included the S-form of aryl methyl sulfoxides, the enantiomeric recognition of (S,S)-Ile-Phg was superior to that of (S,S)-Leu-Phg. Single-crystal X-ray analysis of these inclusion compounds revealed that the dipeptide molecules self-assembled to form layer structures and included sulfoxides between these layers through hydrogen bonding between the proton of +NH3 and the oxygen of the sulfoxide. Besides these host−guest interactions, the phenyl groups of the sulfoxides interacted with each other through the phenyl−phenyl interaction. Two adjacent homochiral sulfoxides make a pair having a 2-fold screw axis along the channel cavity. Thus, the self-recognition of sulfoxides made 21 helical column structures and had high enantioselectivity.