N6-Monoalkylated, N6-dialkylated and N6-acylated (RP)-adenosine 3′,5′-cyclic phosphorothioic acids have been prepared by stereoselective syntheses from cAMP for a study of protein kinase A antagonist activity. The antagonist activity of the parent primary 6-amino cAMP derivative was reduced after N-monoalkylation. No significant activity was detected in the N,N-dialkylated derivative. Mono N-acylation