A CASE OF UNUSUAL STERICALLY DRIVEN C-TRITYLATION REACTION OF TRICYCLIC ANALOGS OF ACYCLOVIR

3,9-Dihydro-3-[(2-hydroxyethoxy)methyl] 6-methyl-9-oxo-5H-imidazo[1,2- a]purine (1) and its silylated derivative (2) when tritylated under co nditions suitable for regioselective N-5 alkylation underwent instead C-substitution to give 7-trityl (12, 3) and 7-(4-benzhydrylphenyl) (13 , 4) derivatives as major and minor products, respectively. Substrates lacking 6-Me group (5, 6) yielded 5-tritylated (10, 7) and 5,7-ditrit ylated (11, 8) major products and 7-tritylated (9) minor product. The regioselectivity of the reaction seems to be driven mainly by steric h indrance of the 6-Me substituent. (C) 1998 Elsevier Science Ltd. All r ights reserved.