Upon borane-pyridine reduction, a series of nucleoside dialdehyde dioximes 2 underwent cyclization to the corresponding 4'-hydroxymorpholin-2'-ylpurines or pyrimidines 3 from which the peracetyl derivatives 4 were prepared. At room temperature, compounds 3 and 4 exist as a mixture of invertomers in which the 4'S (equatorial 4'-OH or 4'-OAc) predominates. A 14 kcal/mol, nitrogen inversion barrier was estimated from variable temperature experiments. N.O.E. and 3JCH measurements established the anti conformation of the base-"sugar" bond. Compounds 3 spontaneously oxidized to the corresponding aminoxyl free radicals, EPR spectra of which showed that they existed in a chair conformation.