Upon treatment of (Z)-3-deoxy-1,2-O-isopropylidene-3-C-methoxycarbonylmethylidene-5-O-TBDMS-α-D-erythro-pentofuranose [(Z)-3b] with lithium methylsulfonylmethanide in THF /HMPA at-78 °C, both the product of the carbonyl and conjugate attacks were obtained. Optimizing the reaction conditions favored the tandem reaction leading to the corresponding title spiro compound 4 obtained highly chemoselectively in 75% isolated yield. The (E) isomer in any condition, yielded only the product from the carbonyl attack. Another significant difference in the behavior of the two geometrical isomers consists in the fact the only (Z)-3b oxidized in the air, to give a novel example of a stable crystalline sugar hydroperoxide derivative (7). The configuration of 4 was assessed by X-ray diffraction.