Conformational studies of oligomeric oxetane‐based dipeptide isosteres derived from L ‐rhamnose or D ‐xylose

Conformational investigations have been undertaken on oligomers (dimers, tetramers, hexamers) of five closely related oxetane‐based dipeptide isosteres. All the oligomers were subjected to a range of studies by NMR, FT‐IR and CD spectroscopy. The oligomers derived from methyl 2,4‐anhydro‐5‐azido‐3‐ O ‐ tert ‐butyldimethylsilyl‐5‐deoxy‐ L ‐rhamnonate ‘monomer’ all exhibited evidence of ordered conformations in chloroform and 2,2,2‐trifluoroethanol (TFE) solution. 5‐Acetamido and N ‐methylamide derivatives of the L ‐rhamnonate ‘monomer’, along with a ‘dimer’ lacking silyl protection at C‐3, were synthesized to ascertain the role of intramolecular interactions. This led to the conclusion that, for the L ‐rhamnonate oligomers, steric interactions govern the conformational preference observed. The equivalent silyl‐protected D ‐lyxonate oligomers gave ordered CD spectra in TFE solution, but NMR and FT‐IR spectroscopy in chloroform solution suggested an irregular, non‐hydrogen bonded system. The remaining silyl‐protected 6‐deoxy‐ L ‐altronate, 6‐deoxy‐ D ‐gulonate and D ‐fuconate oligomers appear to be characterized by their lack of ordered conformation in TFE and chloroform solution. Copyright © 2005 European Peptide Society and John Wiley & Sons, Ltd.