Synthesis and Structural Studies of/-Peptides Derived from Fused Furano-pyran -Amino Acid and L-Ala

New fused cis- and trans-furano-pyran -amino acids were synthesized. The cis-monomer was utilized for the synthesis of /-peptides (tetra-, penta-, hexa- and hepta-mers), to understand its impact on helix formation. Conformational analysis of the peptides with a -- sequence at the C-terminus showed the presence of a left-handed 9/11-helix in which the nine-membered H-bond motif is found to be weak and contributes only a small amount of electrostatic interaction to helix stabilization. Peptides with an -- sequence at the C-terminus revealed the presence of a left-handed 9/11-helix involving the central residues and display a clear 12-membered turn at the C-terminus. In both the cases, the 9/11-helix was stabilized by a seven-membered H-bond at the N-terminus.