Samarium diiodide pinacol coupling reactions

Samarium diiodide has also been utilized as a reductant to promote pinacolic coupling reactions mediated by low-valent titanium species (equation 71). Utilizing this protocol, high diastereoselectivity can be achieved, although the yields for Ais particular process were not reported. 

This method was later applied to samarium diiodide initiated reactions [26] and to tita-nocene-catalyzed pinacol couplings [27]. The first examples of enantioselective reactions using Me3SiCl as a mediator for catalysis have very recently been reported by Cozzi et al., as shown in Scheme 12.14 [28]. 

Several examples of the stereoselective formation of cyclopentane rings promoted by samarium diiodide have been reported. Thus a pinacol-like coupling of a 1,5-dialdehyde derivative produced 75 in which the newly formed hydroxyl groups R, R were in a y/z-arrangement. (See Vol. 29, p.238, ref. 95 for a related reaction). In contrast, reaction of a 5-hexenal, produced by zinc-assisted Grob fragmentation of a methyl 6-deoxy-6-iodopyranoside, produced 76 and 77 in which the exocyclic groups R, R (or R, R ) were anti to each other. ... 

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