The complete set of the 4'-aza analogues of 2',3'-dideoxynucleosides was sy
nthesized by cycloaddition of N-tetrahydropiranyl or N-trityl methylene nit
rones on suitably protected vinyl nucleobases. The convertible nucleoside a
pproach was used in the preparation of cytosine and 5-methyl cytosine analo
gues.