Treatment of methyl 4,6-O-benzylidene-α-D-erythro-hex-2-enopyranoside (5) with the Simmons–Smith reagents gives 85% of the expected cyclopropane adduct 6 and about 10% of a second product 4. When the cyclopropane derivative 6 is allowed to react further with the Simmons–Smith reagents, it is 40% converted to 4. The structure of compound 4 is established to be 4,6-O-benzylidene-3-deoxy-3-C-(iodomethyl)-D-allal (4,6-O-benzylidene-1,2,3-trideoxy-3-C-iodomethyl-D-ribo-hex-1-enopyranose), and the unusually high solvolytic reactivity observed for it, as well as its formation from 6, are rationalized as proceeding through the cyclopropylcarbinyl-oxo-carbonium ion3.