Symmetric bridgehead substitution

However, in the case of 116 when R = Br, the presence of the A-2 cycloreversion product in equilibrium with A-l has been established by chemical means [63], Symmetrical bridgehead substitution of the lepidopterene skeleton may drastically increase the equilibrium ratio (see Table 26). For dimethyllepidopterene 118 (L Y = methyl), the [L]/[A] ratio is > 107, i.e., the cycloreversion product 119 (A Y = methyl) is not detectable by absorption spectroscopy. 

Contrary to the rotationally symmetric bridgehead substitution (azoalkanes 5a,b,c), which leads to the unselective triplet reaction channel and selective formation of anti-configured (retained) housanes (Scheme 7), the direct photolysis of the rotationally unsymmetric bridgehead-substituted azoalkanes 5d,e is moderately diastereoselective for both the singlet and triplet modes of photolysis (Scheme 9), as evidenced by the synjanti (inversion/retention) ratio. Thus, the photolysis of 5d,e affords under singlet conditions (high-temperature direct photolysis) predominantly the retained housanes anti-14d,e syn anti 21 79 for housane 14d and 33 67 for 14e) while under triplet conditions (low-temperature direct or benzophenone-sensitized photolysis), the inverted diastereomer syn-14d,e is favored syn anti 61 39 for 14d and 70 30 14e). 

The 1,6-, or bridgehead, carbon atoms in the beautifully symmetric molecule (I) can theoretically be substituted with any elements capable of (roughly) tetrahedral coordination, the most likely candidates being in Groups III, IV, V, and VI of the periodic table (see Fig. 1). The problem of their study is almost entirely one of synthesis, not stability. Although the first member of the series, 1,6-diazatriptycene (II), was apparently synthesized more than 100 years ago, (J) progress has been slow because only the 1,-6-diarsatriptycene skeleton can be built up in a stepwise fashion from stable intermediates (2). Special techniques must be devised for the other triptycenes and thus far, with one exception, only Group V derivatives have been synthesized. 

Besides the popular bridgehead TREN for cryptands, substituted benzene rings provide an alternative for bicyclic and tricyclic receptors with a 3D cavity for anion recognition. Anslyn and co-workers prepared amide-linked Cs-symmetric cryptand 15 with a cavity of 78.3 A [47]. X-ray crystal structure of its chloride complex showed a 1 2 stoichiometry, and each chloride formed two N C1 H-bonds with... 

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