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We have designed and synthesized a highly lipophilic boronic acid (11) with a molecular shape that makes it much more effective at carrying sugars through organic membranes than a previously used steroidal boronic acid. The corresponding diboronic acid (12) was also found to transport fructose ahead of glucose with a very high selectivity (7.6:1.0). Modeling suggests that 12 is able to carry two fructose molecules at once in a complex stabilized through hydrogen bonding and ion pairing.

The 4-(alkyldithio) butyl radicals 8a and c, and the 4-(pheny1dithio)butyl radical 8b, generated from the corresponding esters of Nhydroxypyridine-2( 1 H ) -thione, undergo fast exo-cyclisation by SHi attack at sulfur. Similarly, the 5-(a1kyldithio)pentyl radical 8d undergoes 1.6-ring formation. The rate constants for cyclisation were determined by photolysis of the radical precursors in the presence of appropriate thiols. Butyl and pentyl radicals bearing o-acetylthio or a-benzoylthio substituents also undergo ring closure but much more slowly. The kinetics of these intramolecular SH2 reactions are discussed and compared with those for the intermolecular attack of hexyl radicals on diphenyl disulfide and on dibutyl disulfide.Although ring formation by intramolecular homolytic substitution at a hetero atom is of synthetic utility14 and may have biological significance, 5 , 6 this type of process has received much less attention than have cyclisations involving intramolecular radical addition to multiple bonds. Recent reports include investigations of the attack of carbon-centred radicals on sulfur in sulfides, 1 , 2 , 8 -1 di~ulfides,~.' ' 9 ' sulfoxides,' 3 3 1 thioesters,' 3*1 8-20 sulfinyl esters 2 1 and S-sulfonates,22 on selenium in ~e l e n i d e s , ~. ~. ~ 5 3 2 3 and on oxygen in peroxides 24 and sulfonates.22 Intramolecular displacements of carbon-centred radicals by attack of iminyl radicals on sulfides and selenides have also been described.25In the first reported study of intramolecular radical substitution at sulfur, Kampmeier found that the radical 1, derived from the corresponding iodide, gave only dibenzothiophene 2 by substitution in the exo mode,26 and the same behaviour was observed later for radicals of the general type 3.17,27,28 No products derived from the radical 5 generated by endo substitution could be detected. Exclusive exo substitution was 2 SMe 1 3 endo f 5 6a R = But, n = 1 b R = P h , n = 1 c R = B u , / I = 1 d a = B u ' , n = 2 4 S 7a R = But, n = 1 b R=Ph, n = l c R=Bu, n = 1 d R = B d , n = 2 11 12 a R=Bu',n = 1 c R=Bu, n = 1 b R = P h , n = 1 d R=Bu',n = 2 13 Paper 4/00839A

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