NUCLEIC-ACID RELATED-COMPOUNDS .93. A SOLUTION FOR THE HISTORIC PROBLEM OF REGIOSELECTIVE SUGAR-BASE COUPLING TO PRODUCE 9-GLYCOSYLGUANINESOR 7-GLYCOSYLGUANINES

Per(trimethylsilyl)-2-N-acylguanine derivatives and tetra-O-acylpentof uranoses were coupled [tin-(IV) chloride or titanium(IV) chloride cata lysis] to give predominant formation of 7-glycosylguanines. With TiCl4 , a fortuitous organic/aqueous partitioning allowed isolation of 7-gly cosylguanines from the 7/9 isomer mixtures. thylsilyl)-2-N-acyl-6-O-(d iphenylcarbamoyl)guanine derivatives and tetra-O-acylpentofuranoses un derwent regioselective coupling (trimethylsilyl trifluoromethane-sulfo nate catalysis) to give 9-glycosylguanines. The -(diphenylcarbamoyl)pe racyl-9-beta-D-ribofuranosyl isomer was shown to be both the kinetic a nd thermodynamic coupling product. Deprotection of all of the peracyl coupling products was effected under mild conditions to give good to h igh yields of guanine nucleoside analogues. These methodologies provid e solutions for the regioselective synthesis of 7- and 9-glycosylguani ne nucleosides.