Synthesis of <i>C</i>‑Septanosides from Pyranoses via Vinyl Addition and Electrophilic Cyclization

A two-step synthesis of <i>C</i>-septanosides from pyranoses is reported. Vinyl addition to tetra-<i>O</i>-benzyl d-glucose, d-galactose, and d-mannose gave the corresponding allylic alcohols. Electrophilic cyclization followed by treatment with iodine gave iodomethyl <i>C</i>-septanosides suitable for substitution reactions. The cyclizations were diastereoselective, giving <i>cis</i>-1,2 configured <i>C</i>-septanosides. Selectivity is rationalized through a model for electrophilic additions that invokes the conformation of the allylic system. This new approach should be generally applicable to the synthesis of a variety of <i>C</i>-septanosides