Stereoselective synthesis of 2 prime ,3 prime -dideoxy-3 prime -fluoro-2 prime -phenylselenenyl-β @-nucleoside s from phenyl 1-seleno-α-arabino-furanosides through consecutive 1,2-migration and glycosylation under Mitsunobu conditions. A new entry to 2 prime ,3 prime -dideoxy-3 prime -fluoronucleosides