Reaction of an aldehyde or ketone with samarium(II) iodide produces a samarium ketyl. 26–39 These ketyls add intramolecularly to appro­priately positioned carbon–carbon 25 –33 (Scheme 10) 26 and carbon–nitrogen 34 –37 (eq 10) 34 double bonds. Such reactions are reminiscent of the addition of typical carbon-centered radicals to multiple bonds. When samarium(II) iodide reacts with compounds containing two aldehydo groups, the first is converted into a samarium ketyl that then adds to the second. This addition depends upon pro­per separation between the reacting groups; 40–53 accordingly, pinacols with five-mem­bered 40 ,49‑53 (eq 11) 40 and six-membered 41 –48 (eq 12) 41 rings form easily. It is not necessary for both interacting groups in a molecule to be aldehydo groups; pinacols also arise when one 49 –52 (eq 13, R = H) 49 or both (eq 13, R = CH 2 SiMe 2 C 6 H 5 ) 53 are keto groups. Complex­ation of the ketyl and carbonyl oxygen atoms with SmI 2 forces a cis relation between the hydroxyl groups in the products (Scheme 11). 42 Pinacol formation and other reactions of aldehydes and ketones with samarium(II) iodide is revisited in Chapter 20 , where a broader discussion of the interaction of SmI 2 with carbohydrate derivatives takes place.