The synthesis of oligomers of oxetane-based dipeptide isosteres derived from L-rhamnose or D-xylose.

Routes to oligomers (dimers, tetramers, hexamers) of five oxetane-based dipeptide isosteres have been established. Methyl 2,4-anhydro-5-azido-5-deoxy-L-rhamnonate 'monomer' led, by coupling the corresponding carboxylic acid and amine, to a 'dimer'. Reverse-aldol ring-opening occurred on attempted saponification of the dimer, so all further oligomerization was performed using TBDMS C-3 hydroxyl protection. The silyl protected L-rhamnonate monomer led in turn to the dimer (via the monomer acid and amine), the tetramer (via the dimer acid and amine) and finally the hexamer (via the tetramer acid and dimer amine). In each case the acids were obtained through saponification of the respective methyl esters and the amines were obtained by hydrogenation of the azides; coupling was TBTU-mediated. Essentially the same strategy was employed on equivalent D-lyxonate, 6-deoxy-L-altronate, 6-deoxy-D-gulonate and D-fuconate dipeptide isosteres to give the respective dimers, tetramers and hexamers.