Stereochemical Features of Lewis Acid-Promoted Glycosidations Involving 4‘-Spiroannulated DNA Building Blocks

Tin tetrachloride-catalyzed glycosidation of persilylated nucleobases with acetate donor 6 in CH2Cl2 solution followed by deprotection gave rise very predominantly to α-spironucleosides. These
stereochemical assignments stem from the determination of NOE interactions and an X-ray
crystallographic analysis of the latter product. Computational studies revealed that these results
are consistent with the fact that the C5‘ substituent shields the β-face of the oxonium ion involved
in the coupling reaction while the C3‘ substituent is projected away from the α-underside. Attack
from the more open direction is therefore kinetically favored. Entirely comparable calculations
suggested that donor 19 should behave comparably. Experimentation involving this donor gave
results consistent with this model although more equitable α/β spironucleoside product ratios were
seen when acetonitrile was employed as the reaction medium.