Fluorescent nucleoside analogues : Synthesis of fluorescent pyrido[2,1-i]purines, and their corresponding ribosides

Abstract

Fluorescent pyrido[2,1-i]purines can in principle be obtained via Michael-addition of a suitable anion of a purine derivative to an acetylenic ester, followed by based-catalyzed cyclization, as depicted in Scheme II. 6-Substituted purine-derivatives are obtained via nucleophilic substitution of 6-chloro- and 6-methylsulfonylpurine ( 8a and 8b ). In the presence of methyl propiolate and sodium methoxide, before cyclization, two consecutive Michael-additions take place, leading to 13 and 14 . With substituted acetylenic esters, cyclization occurs after one Michael-addition. Michael-additions with ethylenic esters did not lead to expected cyclization products, except in cases where oxidation took place. -For the conversion of the pyrido[2,1-i]purines into the corresponding ribosides protection against nucleophilic attack was necessary.