Adenine derivatives bearing substituents in the 2-, N 6-, 7-, 8-, and/or 9-position and a series of deazapurines were synthesized and investigated in [3H] adenine binding studies at the adenine receptor in rat brain cortical membrane preparations (rAde1R). Steep structure− activity relationships were observed. Substitution in the 8-position (amino, dimethylamino, piperidinyl, piperazinyl) or in the 9-position (2-morpholinoethyl) with basic residues or ...