A compound of formula: ##STR36##wherein: R.sub.62 is H or a hydroxyl protecting group; R.sub.1 is H, hydroxyl, a protected hydroxyl, a 2'-substituent group or a protected 2'-substituent group; B is a heterocyclic base moiety; and R.sub.61 is a chiral auxiliary selected fromformulas I-VI: ##STR37##50.