Purines. LXV. Preparatory Study for the Syntheses of the Marine Sponge Purines Agelasimines-A and -B : synthesis and Acetylation of Their N(7)-Benzyl Analogues

Abstract

Four-step synthetic routes from 3-methyladenine (10) to 7-benzyl-N⁶, 3-dimethyladenine (1b) and 7-benzyl-1, 2-dihydro-1, 3-dimethyladenine (2b), selected as models for the marine sponge alkaloids agelasimine-A (1a) and agelasimine-B (2a), respectively, have been established. The key steps involved are regioselective methylations of 7-benzyl-3-methyladenine (8) and 7-benzyl-1, 2-dihydro-3-methyladenine (11). The reaction of 1b with acetic anhydride in pyridine was found to give the monocyclic imidazole derivative 29b. A similar acetylation of 2b yielded the N⁶-acetyl derivative 20b. When treated with boiling H₂O, 20b afforded 7-benzyl-2, 3-dimethylhypoxanthine (21b) and a compound inferred to be the dihydrohypoxanthine derivative 30. Probable pathways to 29b from 1b and to 21b and 30 from 20b are proposed.