A series of N-benzylpyroglutamyl-l-phenylalanine derivatives bearing carbamoyl substituents in the 3-or 4-positions was prepared and assayed for inhibition of the interaction between VCAM and VLA-4. Potent inhibition was observed in a number of analogues with substitution in the 4-position favored over the 3-position. A crystal structure of the key intermediate 25 indicates that it accesses a low energy conformation which closely ...