A high-yield synthesis of o,o' crown ether-bridged stilbenes 3b-e by reductive McMuny condensation of (2-formylphen-y1)oligoethylene glycols 2b-e with facile (E/Z) diastereomer separation by selective cation complexation is described. Derivatization of the stilbene double bonds of ( E )or (Z)-3b-e affords dihydroxy crown ethers 4c, d and 512, d diastereo-and enantioselectively. Likewise, trans-and cis-epoxides 1 lb-d and 12b-d are stereospecifically obtained. A crown ether-

The conformations of (±)‐trans‐2,3‐Bis(1‐naphthyl)‐15‐crown‐5 (1) and ‐18‐crown‐6 (2) are investigated by MM2 and dynamic 1H‐NMR spectroscopy. The most stable conformation is the diequatorial one in both the free ligands and the alkali metal complexes 1 · NaClO4 and 2 · KClO4. Three rotamers, pseudo‐aa, pseudo‐ee, and pseudo‐ae, with respect to the spatial arrangement of the naphthyl groups, were found as calculated minima by MM2 and experimentally by low‐temperature NMR, with fair agreement of the rotational barriers. In the cation‐reinforced crowns the rotation rates are significantly diminished. The distribution of the rotamers depends on the size of the crown and its state, free or complexed.

The synthesis of 2,3-didehydro [ 15]crown-5 and -[ 18]crown-6 derivatives 4a, b with vicinal a-naphthyl groups at the double bonds starting from a-naphthoin (3) is described. The barriers to rotation of the naphthyl groups were probed by MM2 and by dynamic 'H-NMR spectroscopy, An achiral syn and a chiral anti conformation were experimentally observed at -90°C for the free ligands and at -4OOC for the comple-xes. The experimental AG' and the calculated AH' values are in fair agreement. The barriers are substantially higher in the alkali metal complexes than in the free ligands. In the presence of racemic and potassium S-(+)-mandelate nonequilibrating ion pairs with 4b were observed by low-temperature 'H-NMR spectroscopy for the anti conformation.

The conformations of (±)-trans-2,3-Bis(1-naphthyl)-15-were found as calculated minima by MM2 and experimentally by low-temperature NMR, with fair agreement of the crown-5 (1) and -18-crown-6 (2) are investigated by MM2 and dynamic 1 H-NMR spectroscopy. The most stable confor-rotational barriers. In the cation-reinforced crowns the rotation rates are significantly diminished. The distribution of the mation is the diequatorial one in both the free ligands and the alkali metal complexes 1 · NaClO 4 and 2 · KClO 4 . Three rotamers depends on the size of the crown and its state, free or complexed. rotamers, pseudo-aa, pseudo-ee, and pseudo-ae, with respect to the spatial arrangement of the naphthyl groups, Scheme 1The conformations of crown ethers are quite different in their free states and complexes. Well known examples are the oblong shape of 18-crown-6 and the typical crown arrangement in the potassium complex. [1] In 2,3-disubstituted crown ethers the substituents can be located at gauche or anti sites of the oblong macrocycle, and the resulting subconformations for the trans arrangement of the substituents are the diequatorial (A), diaxial (B), and axial-equatorial (C) ones (Scheme 1). The trans substitution is particularly interesting due to the chiral C 2 symmetry, [2] [3] and analog conformations apply for the cis substitution patterns. The relative importance of the conformations A and B depends on the nature of the substituents. If, for instance, transannular interactions or synergistic effects with guest molecules are possible, the diaxial arrangement will be preferred. [4] Although, in the crystal, [7] 1 has an axial-equatorial ar-The X-ray crystal-structures of the trans-syn-trans and rangement of the naphthyl groups at a close-to-anti confortrans-syn-cis isomers [5] of the 2,3,11,12-tetraphenyl-18mation (ψ (OϪCϪCϪO) ഠ 21°) of the crown site (Figure 1), crown-6 series of stereoisomers [3] provide an example of the we can show that the only significant conformation in solubehavior of noninteracting substituents in both free ligands tion is the diequatorial one in both the free ligands and the and sodium complexes.complexes of 1 and 2.Here we report on the conformations of (±)-trans-2,3-Syntheses bis(2-naphthyl)-15-crown-5 (1) and the homologous 18crown-6 derivative 2. In these two crown ethers there is an 1-Naphthaldehyde was converted into (E)-2-naphthostilbene by the McMurry reaction [8] and subsequently oxidized additional conformational bias, namely the mutual rotation of the vicinal 1-naphthyl groups, a phenomenon which we by the catalytic Sharpless oxidation with OsO 4 /NMO [9] to give the racemic diol 3 whose (R,R) enantiomer was re-have previously investigated for the corresponding 2,3-dihydro crown ethers. [6] The question was to which extent cently obtained by asymmetric Sharpless dihydroxylation. [10] The (R,R)-acetonide 4 was investigated by CD complexation would influence the rotation rate of a single naphthyl group and the equilibrium distribution of pos-spectroscopy, and its most stable co...

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