Synthesis and structure of mono-bridged resorcinarene host: a ditopic receptor for ammonium guests

Abstract

The synthesis and structural properties of tetramethoxy resorcinarene mono-crown-5 (1) are described. The binding characteristics of 1 toward acetylcholine and tetramethylammonium salts were investigated by 1H NMR titration. It was observed that the cavity of 1 provides a better fit to acetylcholine compared to the smaller tetramethylammonium cation, as acetylcholine is able to interact with both the crown ether moiety and the free hydroxyl groups of receptor 1 simultaneously.

The Structural Diversity of Benzofuran Resorcinarene Leads to Enhanced Fluorescence

“…[13] The ratio of the three resorcinarene crown derivatives 1-3 was determined to be 2:3:5, respectively, and the isolated yield of the by-product 1 was 5.2% (Table 1, entry 1). The unusual structure of compound 1 with benzofuran ring as a part of the resorcinarene skeleton was characterized by means of NMR spectroscopy ( 1 H, 13 C, HMBC and HMQC), mass spectrometry (accurate mass, ESI + ) and X-ray crystallography, which revealed the unexpected formation of the benzofuran unit inducing an elongated conjugated system with the alternating σ-and π-bonds into the resorcinarene macrocycle (highlighted in Scheme 1).…”

“…Even though it lacks the additional rigidity brought by a benzofuran moiety, the bridging and lack of intramolecular hydrogen bonds favour the boat conformation in the solid state and in solution. [13] A linear relation between the emission and the excitation of the oval shape signal is 0.55 for 1 and 0.19 for 2. [28] This ratio illustrates that the longer wavelength absorbing moieties of compound 1 and 2 have inhomogeneous broadening.…”

“…The center of the cavity is outlined by two upright aromatic rings (A and C), the other being the benzofuran ring (A) fused into the resorcinarene framework ( Figure 1). In comparison to the boat conformations of the biscrown [12] 3 and mono-crown [13] 2 derivatives, the resorcinarene skeleton of 1 is severely distorted due to the 180 rotation of the lower rim ethyl groups about the methine bridge (C14) as it takes part in forming the benzofuran ring. This causes the methine bridge (C14) to shift 2.40-2.79 Å up from the methine plane (C21-C28-C7, Figure 1a).…”

“…The crown bridge flexibility is also seen in the crystal structures ( Figure 2) and in the 1 H NMR spectrum as the multiplicity of the crown ether proton resonances. [13] …”

“…guanosine 5′-triphosphate. [11] Recently, by serendipity we succeeded to isolate a previously unknown resorcinarene by-product (1) from a reaction mixture of tetramethoxy resorcinarene bis-crown (3) [12] and mono-crown (2) [13] derivatives. To our surprise, the previously untouched resorcinarene core had one of the aromatic rings been replaced by a benzofuran moiety (Scheme 1) and, as at first observed with a naked eye under UV-light (366 nm, Figure S3), the benzofuran mono-crown derivative 1 showed intrinsic enhanced photophysical properties without having a separate fluorescent functionalization.…”