| Résumé | A qualitative summary of our canonical vector representation of the conformations of six membered rings and its quantitative application to sugar pyranose rings is presented first. This representation is illustrated for the results of Density Functional Theory (DFT) optimized structures of per O-methylated D-lyxo, D-manno and D-gluco-configured oxacarbenium ions where two conformers are consistently found. Nucleophilic attack modelled with methanol is found to lead to intramolecular hydrogen bonding from the hydroxylic proton to electronegative oxygens of the oxacarbenium ion. As well, the ring conformation is found to change with the C-5-O-5--C-1-C-2 torsion angle moving from nearly planar in the isolated cations to more negative values in the case of α-approach and to more positive values for β-attack. This can be favorable or unfavorable depending on the protecting groups present with for example 4,6-benzylidene protection favoring β-attack with D-manno-configured ions whereas for D-gluco-configured ions α-approach is favorable. Extending the DFT studies to 2-O-acetyl analogues finds similar pairs of cations and raises the important issue of the order of ring inversion or O-2 bond rotation during the formation of ring inverted dioxolenium ions which in some cases are calculated to be very stable. Finally, an example using complex constraints is described which led to the design and development of 2,6-dimethylbenzoyl and 2,6-dimethoxybenzoyl as O-2 protecting groups which greatly minimize the acyl transfer side reaction. |
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