Cascade Reaction of 6-Deoxy-6-iodohexopyranosides Promoted by Samarium Diiodide: A New Ring Contraction of Carbohydrate Derivatives.

Sequential transformations1 allow the assembly of complex molecules in a simple manner, with a minimum of purification steps, and are accordingly ideal components of elegant and efficient synthetic strategies. In recent years, samarium diiodide2 has evolved as a unique single electron reducing agent that is especially well suited to promote sequential processes that combine radical and anionic steps with a high degree of chemoand stereoselectivity.3 On the basis of the rich chemistry uncovered for this reagent, we hypothesized that its interaction with a 6-deoxy-6-iodohexopyranoside derivative A (Scheme 1) could trigger a reaction cascade that would eventually lead to a novel ring contraction4,5 of the pyranose moiety. This sequence requires four SET steps (i.e., 4 mol equiv of SmI2) and consists of (1) a reductive dealkoxyhalogenation to give the ring-opened hex-5-enal B;6 (2) an intramolecular ketyl-olefin reductive coupling affording the ring-contracted organosamarium intermediate C,3a-c,7 and (3) the intermolecular trapping of this organosamarium with appropriate electrophiles to produce finally the branched cyclopentitol derivative D.3a-c We report here the successful implementation of the postulated process for a series of 6-deoxy-6-iodohexopyranosides (1-7)8,9 (Table 1) of different configuration and substitution pattern and initial studies of the influence