Differential reactivity of carbohydrate hydroxyls in glycosylations. III. Structure, stability, and reactivity of 2′-deoxycytidine model compound-BF3 complexes

The possibility of complexation between an acidic promoter like BF3 and the alcoholic aglycone has been studied as a key mechanistic intermediate in glycosylation reactions operating via an electron-deficient anomeric carbon. The BF3 complexes at O3′,O4′,O5′, and N3 of the model compound of 3′-,4-N-diacetyl-2′-deoxyribocytosine (1) have been studied. The last regioisomer of the four BF3 complexes was found to have the weakest hydrogen bond and yet was the most stable isomer energetically. Furthermore, the O5′ nucleophilicity of this isomer was the most pronounced, as evaluated by Bader-type charge density topological analysis at the HF/6-31G**//HF/3-21G level of theory. This indicates that such a complex will make glycosylation at the O5′ oxygen possible.