Topographically designed analogues of [D-Pen,D-Pen5]enkephalin.

The conformationally restricted, cyclic disulfide-containing δ opioid receptor selective enkephalin analogue [D-Pen 2 ,D-Pen 5 ] enkephalin (1, DPDPE) was systematically modified topographically by addition of a methyl group at either the pro-S or pro-R position of the β carbon of an L-Phe 4 or D-Phe 4 residue to give [(2S,3S)-β-MePhe 4 ] DPDPE (2), [(2R,3R)-β-MePhe 4 ] DPDPE (3), [(2S,3R)-β-MePhe 4 ] DPDPE (4), and [(2R,3S)-β-MePhe 4 ] DPDPE (5)