Folding propensity of cyclohexylether-δ-peptides

Linear (n = 2-18) and cyclic oligomers (n = 3-8) of a cyclohexylether-delta-amino acid (COA) were prepared in high yield and stereopurity. CID spectra of the linear oligomers were indicative of secondary structure formation. X-ray crystal structures of cyclic COA oligomers revealed hydrophobic packing and internal 5- and 10-membered-ring hydrogen bonds. Ether and amide oxygens reside preferably in an ap orientation. This conformational locking is apparently broken by a C-2 substituent in an asymmetric cyclotrimer, for which a zeolithe-like tubular structure was found.