Design, Synthesis, and X-ray Crystallographic Analysis of Two Novel Spirolactam Systems as β-Turn Mimics

Two novel 5.4-spirolactam systems have been developed as β-turn mimics. The (R)-5.4-spirolactam system of 4 was designed to mimic the type-II β-turn, and the (S)-5.4-spirolactam system of 5 was designed to mimic the type-II′ β-turn. The 5.4-spirolactam dipeptide mimic 11 was synthesized in racemic form from pipecolic acid. This intermediate was then converted to the diastereoisomeric mixture of peptidomimetics 4 and 5, which were separated by fractional recrystallization. X-ray crystallographic analysis of 4 and 5 indicated that both of these compounds adopted hydrogen-bonded β-turn conformations. As predicted, the (R)-5.4-spirolactam system of 4 induced a type-II β-turn while the (S)-5.4-spirolactam system of 5 induced a type-II′ β-turn. The backbone torsion angles of 4 and 5 were close to those of the classical type-II and -II′ β-turns, respectively. A computer-generated fit between nine atoms of the backbone of 4 and their counterparts in an ideal type-II β-turn yielded an RMS fit of 0.218 Å. A similar comparison between 5 and an ideal type-II′ β-turn produced an RMS fit of 0.392 Å.