A shape-persistent D,L-dipeptide building block for the assembly of rigidified oligopeptides.

Mannuronic acid (5) was transformed into the densely functionalised D,L-dipeptide mimic 1 and subsequently inserted into the cyclic hexapeptide 3. The gulo-configurated seven-membered lactam 1 exhibits an inverted ring conformation compared to the previously described L,L-dipeptide mimic 2. In spite of this considerable difference, both prefer the same i to i + 1 positions of a beta-turn within a cyclic hexapeptide.