Cyclic oligomers of oxetane-based dipeptide isosteres derived from L-rhamnose.

Two new cyclic oligomers, cyclo-tetra-[2,4-anhydro-3-O-tert-butyldimethylsilyl-5-deoxy-L-rhamnonamido-(N-->5)] and the corresponding 6-deoxy-D-gulonate cyclic "tetramer", have been synthesised from linear tetrameric oligomers, using TBTU- and pentafluorophenyl ester-based methodologies, respectively. These two compounds constitute a novel class of cyclic oligomers derived from oxetane-based sugar amino acids.

Syntheses and bio-activities of the L-enantiomers of two potent transition state analogue inhibitors of purine nucleoside phosphorylases.

(1R)-1-(9-Deazahypoxanthin-9-yl)-1,4-dideoxy-1,4-imino-L-ribitol [(+)-5] and (3S,4S)-1-[(9-deazahypoxanthin-9-yl)methyl]-4-(hydroxymethyl)pyrrolidin-3-ol [(-)-6] are the L-enantiomers of immucillin-H (D-ImmH) and DADMe-immucillin-H (D-DADMe-ImmH), respectively, these D-isomers being high affinity transition state analogue inhibitors of purine nucleoside phosphorylases (PNPases) developed as potential pharmaceuticals against diseases involving irregular activation of T-cells. The C-nucleoside hydrochloride D-ImmH [(-)-5) x HCl], now "Fodosine" is in phase II clinical trials as an anti-T-cell leukaemia agent, while D-DADMe-ImmH is a second generation inhibitor with extreme binding to the target enzyme and has entered the clinic for phase I testing as an anti-psoriasis drug. Since the enantiomers of some pharmaceuticals have revealed surprising biological activities, the L-nucleoside analogues (+)-5 x HCl and (-)-6, respectively, of D-ImmH and D-DADMe-ImmH, were prepared and their PNPase binding properties were studied. For the synthesis of compound (-)-6 suitable enzyme-based routes to the enantiomerically pure starting material (3S,4S)-4-(hydroxymethyl)pyrrolidin-3-ol [(-)-6] and its enantiomer were developed. The L-enantiomers (+)-5 x HCl and (-)-6 bind to the PNPases approximately 5- to 600-times less well than do the D-compounds, but nevertheless remain powerful inhibitors with nanomolar dissociation constants.

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.

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.

An unsaturated peptidomimetic assembly derived from a carbohydrate.

A strategy has been established for the synthesis of peptidomimetics derived from unsaturated carbohydrates, and exemplified by the use of methyl 2,6-anhydro-7-azido-3,7-deoxy-4,5-O-isopropylidene-D-lyxo-hept-2-enonate 9 as a dipeptide 'monomer' which can be elaborated from either end. Selective reduction of 9 gives a protected pseudodipeptide ester suitable for use as an amino component, and saponification gives an azido acid suitable for use as a carboxyl component. The 'dimer' product of coupling these two components with TBTU can be similarly elaborated at either end to give a 'trimer' and a further cycle of selective reduction and coupling gave a 'tetramer', 17, a pseudo-octapeptide.