Preparation of 5-Selenopentopyranose Sugars from Pentose Starting Materials by Samarium(II) Iodide or (Phenylseleno)formate Mediated Ring Closures

2,3,4-Tri-O-benzyl-1,5-dideoxy-5-seleno-d-pentopyranose sugars (16, 23, 24) are readily prepared by thermolysis of 2,3,4-tri-O-benzyl-5-benzylseleno-d-ribit-1-yl formate, 2,3,4-tri-O-benzyl-5-benzylseleno-d-xylit-1-yl formate and 2,3,4-tri-O-benzyl-5-benzylseleno-d-arabit-1-yl formate (13, 21, 22) in transformations which involve intramolecular nucleophilic attack of the benzylseleno moiety with concomitant loss of carbon dioxide and phenylselenoate. In a complementary procedure, treatment of 2,3,4-tri-O-benzyl-5-benzylseleno-5-deoxyribose (19) with samarium(II) iodide in THF affords 2,3,4-tri-O-benzyl-5-deoxy-5-seleno-d-ribopyranose (26) in 50% isolated yield in a process most likely involving intramolecular homolytic substitution at the selenium atom in the selenosugar.