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 format e 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 phenylselen oate. In a complementary procedure, treatment of 2,3,4-tri-O-benzyl-5-benzy lseleno-5-deoxyribose (19) with samarium(II) iodide in THF affords 2,3,4-tr i-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. (C) 2000 Elsevier Science Ltd. All rights reserved.