One-pot montmorillonite K-10 clay-supported reactions of substituted/unsubstituted salicylaldehyde and ribosyl/deoxyribosyl thioureas expeditiously yielded novel N-nucleosides, 4-hydroxy-3,4-dihydro-3-(beta-D-ribofuranosyl or beta-D-2'-deoxyribofuranosyl)-2''-benz[e]-1,3-oxazin-2-thione via cycloisomerization of aldehyde intermediate under solvent-free microwave irradiation conditions.