Photoinduced formation of a cryptand from a coronand: an unexpected switch in cation binding affinity

Aza-crown ethers 2 and 3 with anthracene-containing pendant arms have been synthesised and characterised. Both compounds bind Group 1 metal cations in solution, forming complexes of 1:1 stoichiometry. The properties of compound 2 and its complexes have been studied by a range of techniques, including NMR, UV and fluorescence spectroscopy and X-ray crystallography. The pendant arms can adopt either a cis or a trans geometry, the cis geometry favoured with larger cations. The geometry of the complex affects the fluorescence properties of the system, with larger cations giving higher excimer/monomer ratios. Upon irradiation at lambda>300 nm, coronand 2 forms the cryptand 5 through a reversible intramolecular [4pi+4pi] cycloaddition reaction. The rates of the forward and reverse reactions of this photochromic process are cation dependent; in particular the rate of the thermal reverse reaction is decreased by smaller cations and increased by larger cations, especially Rb(+). The metal binding constants in methanol for 2 and 5 have been determined, revealing that the cryptand 5 binds Na(+) and Rb(+) more weakly than crown ether 2 by over two orders of magnitude.     

A short,highly efficient synthesis of coenzyme Q(10)

The most efficient synthesis reported to date of ubiquinone (CoQ10) is described. A sequence consisting of six operations is involved which leads to crystalline material in an overall yield of >64%.     

Novel acid-catalyzed rearrangement of tetrahydro-1,2,3,4-tetrazines: unexpected formation of glycosazones

The present contribution discloses a simple and unexpected acid-catalyzed cleavage of tetrahydrotetrazines leading to 1,2-bis(hydrazones). Incorporation of a chiral fragment derived from carbohydrates enables the rapid preparation of glycosazones, a family of compounds employed by Emil Fischer to elucidate the configuration of sugars. In addition, a mechanistic proposal accounts for experimental observations.     

Pendant arm pyrrolic amide cleft anion receptors

The propensity of amine, ammonium and amide pendant arm 2,5-diamidopyrrole derivatives to act as anion receptors has been investigated; the anion-coordination ability of these species has been determined by 1H NMR titration techniques revealing a marked selectivity of the amine functionalised receptor for hydrogen sulfate anions.     

Liquid-phase EPR, ENDOR, and TRIPLE resonance studies on corrole and isocorrole cation radicals

The “Contracted Porphyrins” corrole and the recently synthesized isocorrole (as octaethyl derivatives) represent novel porphyrinoid macrocycles of biochemical interest. The radical cations of the free-bases of both corrole and its isomer are studied by EPR, ENDOR, and TRIPLE resonance in liquid solution to measure isotropic hyperfine couplings including signs. They yield detailed information of the electronic structure of the radical cations. The experimental findings are compared with results of all-valence-electrons self-consistent field molecular orbital calculations (RHF-INDO/SP). The comparison shows that the free-bases of corrole and isocorrole undergo protonation when oxidized to cation radicals in CH₂Cl₂ solution. Excellent agreement of the measured and calculated spin density distributions is achieved for the protonated dication radicals, which exhibit the C₂ symmetry observed for the hyperfine structure. The analysis of the hyperfine data discriminates against NH tautomerism as the sole source for an effective C₂ symmetry of the macrocycle.     

Shape change and fast M1 transitions in Kr

A shape transition from a probably asymmetric shape at low excitation to a more axial-symmetric shape above 21/2⁺ has been found in ⁸¹Kr. This shape change and the drastic increase of the M1 transition probabilities above spin 21/2 are attributed to the alignment of two g_9/2 protons.     

A stereoselective cyclization to carbafuranose derivatives starting from 1,4-bis-epoxides

[reactions: see text] A concise synthesis of highly functionalized cyclopentane derivatives via a Brook rearrangement mediated stereoselective linchpin cyclization reaction involving tert-butyldimethylsilyl-1,3-dithianyllithium and homochiral 1,4-bis-epoxides is described.