SPIN-LABELED NUCLEOSIDE ANALOGS - 4'-HYDROXYMORPHOLIN-2'-YLPURINES AND PYRIMIDINES

Upon borane-pyridine reduction, a series of nucleoside dialdehyde diox imes 2 underwent cyclization to the corresponding 4'-hydroxymorpholin- 2'-ylpurines or pyrimidines 3 from which the peracetyl derivatives 4 w ere prepared. At room temperature, compounds 3 and 4 exist as a mixtur e of invertomers in which the 4'S (equatorial 4'-OH or 4'-OAc) predomi nates. A 14 kcal/mol, nitrogen inversion barrier was estimated from va riable temperature experiments. N.O.E. and 3J(CH) measurements establi shed the anti conformation of the base-''sugar'' bond. Compounds 3 spo ntaneously oxidized to the corresponding aminoxyl free radicals, EPR s pectra of which showed that they existed in a chair conformation.