THE ELECTRONIC NATURE OF THE AGLYCONE DICTATES THE DRIVE OF THE PSEUDOROTATIONAL EQUILIBRIUM OF THE PENTOFURANOSE MOIETY IN C-NUCLEOSIDES

We herein show for the first time that the specific electronic charact er of a C-aglycone dictates the thermodynamic preference of the two-st ate N reversible arrow S pseudorotational equilibrium to either N- or S-type sugar. As the electron-deficiency of the C-aglycone increases, a more favourable O4'(n) --> sigma(C1'-C(sp2)) interaction results in to anomeric stabilization as indicated by more positive Delta H degree s for the drive of the N reversible arrow S equilibrium to more N-type sugar conformation with pseudoaxial aglycone. Alternatively, as the C -aglycone becomes more electron-rich, an unfavourable O4'(n) --> sigma (C1'-C(sp2)) interaction results into anomeric destabilization as ind icated by more negative Delta H degrees for the drive of the N reversi ble arrow S equilibrium to more s-type sugar conformation with pseudoe quatorial aglycone.