Fram-Diols

When the two oxygen atoms involved in the stannylene acetal are diaste-reotopic, three dimers, two with C2 symmetry, can be formed, as shown in Fig. 7 for those obtained from 1,2-propanediol. Dimers are named by means of the numbers of the tricoordinate oxygen atoms.44 Steric effects appear to be the most important factor in determining the relative populations of the three dimers. In particular, stannylene acetals derived from fram-diols with one adjacent axial substituent exist in solution, to the level of detection of 119Sn NMR spectroscopy, as the symmetric dimer in which the tricoordinate oxygen atom is not adjacent to the axial substituent.44 Similarly, dialkylstannylene acetals from carbohydrate-derived terminal 1,2-diols exist predominantly as symmetric dimers with the primary oxygen atoms tricoordinate (Fig. 8).19,37... 

When the fram-1,2-diols are flanked by two axial substituents, mixtures are always obtained, except in oxidation reactions. 

Vic-diols can thus be easily converted to alkenes through their reaction with /V,A"-thiocarbonyldiinidazole. The reported synthesis of irans-cyclooctene is illustrative of the method [219]. It should be noted that continuous elimination of /ram-cyclooctene by a stream of argon was necessary to avoid isomerization to the cis isomer. The conversion of cis-cyclooctene to fram-cyclooctene through a trithiocarbonate is described in the same paper. 

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