When subjected to a Ruff degradation, a d-aldopentose, A, is converted to an aldotetrose, B. When reduced with sodium borohydride, the aldotetrose B forms an optically active alditol. The 13C NMR spectrum of this alditol displays only two signals. The alditol obtained by direct reduction of A with sodium borohydride is not optically active. When A is used as the starting material for a Kiliani-Fischer synthesis, two diastereomeric aldohexoses, C and D, are produced. On treatment with sodium borohydride, C leads to an alditol E, and D leads to F. The 13C NMR spectrum of E consists of three signals; that of F consists of six. Propose structures for A-F.