Abstract: 4-Amino-7-(5′-deoxy-β-D-xylofuranosyl)-5-iodo-pyrrolo [2, 3-d] pyrimidine 1, an unusual naturally occurring marine nucleoside isolated from an ascidan, Diplosoma sp., was synthesized from D-xylose in seven steps with 28% overall yield on 10 g scale. The key step was Vorbrüggen glycosylation of 5-iodo-pyrrolo [2, 3-d] pyrimidine with 5-deoxy-1, 2-O- diacetyl-3-O-benzoyl-D-xylofuranose. Its absolute configuration was confirmed.