Modified Tricyclic Analogues of Acyclovir. A Direct Alkynylation in the Fused Ring

Two types of new 7-alkynylated tricyclic analogues (3,9-dihydro-5 H -imidazo[1,2- a ]purin-9-ones) of acyclovir differing by the presence of N-5 substituent, a temporary 2-(4-nitrophenyl)ethyl or a permanent 3-hydroxypropyl were obtained by a Sonogashira coupling. 7-Alk-1-ynyl-5-(3-acetoxypropyl) compounds ( 19a - 19d , 21a - 21c ) were efficiently prepared from 7-iodo, 7-iodo-6-methyl precursors 12 and 11 , respectively, and deprotected while the products with unsubstituted N-5 were unstable (e.g. 17 ). Iodide 12 was generally less reactive than 11 and underwent a preferable reduction (48%) to deiodinated 8 when coupled with ethynyltrimethylsilane.