Efficient synthesis of tricyclic benzobisoxazines by silica gel catalysis

A new method for the synthesis of tricyclic benzobisoxazines, based upon silica gel-catalyzed formation of two 3,1-oxazine rings, is reported. The reversibility of the condensation reaction, forming an oxazine ring, allows for implementation of silica gel catalyzed ketone exchange in benzobisoxazine, thus enabling access to nonsymmetric derivatives of benzobisoxazines.     

Total synthesis of malyngamide X and its 7′S-epi isomer

Stereoselective syntheses of malyngamide X (1) and its 7′(S)-epimer are described. A Lewis acid (Et₂AlCl) mediated anti-aldol reaction was employed to generate the stereocenters C-7 and C-8. The route is convergent and provides a convenient access to the synthesis of structural variants of malyngamide X. Stereochemistry at C-7′ in the molecules of natural and synthetic 1, and 7′(S)-epi1 was confirmed by NMR chiral solvation experiments.     

Synthesis, structure, and conformation of aza[1n]metacyclophanes

N-Benzyl substituted aza[1n]metacyclophanes (n = 4, 6, 8, and 10) were prepared in overall 40% isolated yields via Pd-catalyzed aminations. Analyses of the reaction mixtures showed that aza[14]metacyclophane and the related polymer were the primary products ( approximately 60% overall yield); aza[1n]metacyclophanes up to n = 14 and linear oligomers with up to 20 nitrogen atoms (with at least three types of end groups) were detected. Macrocyclic structures for n = 4, 6, and 10 were confirmed by X-ray crystallography. 1,3-Alternate (D(2d)) and 1,3,5-alternate (S(6)) conformations in solution on NMR time scale at low temperatures were found for macrocycles with n = 4 and n = 6, respectively; the barrier for ring inversion was considerably lower for the larger macrocycle.     

Recognition of a dicarboxylic acid with dipicolyl urea in solution and in solid phases: intramolecular hydrogen bond inhibiting both pyridine nitrogens from binding carboxyl groups

Recognition of a dicarboxylic acid in solution as well as in the solid phase by the pyridyl urea based pseudoditopic receptor 1 has been studied. The X-ray structures of both the receptor and its complex with 1,4-phenylenediacetic acid are also presented. Intramolecular hydrogen bonding inhibits both the pyridine ring nitrogens from forming hydrogen bonds with the carboxyl group and force the receptor to behave in a monotopic manner, using the syn urea amide moiety to bind carboxyl group of a dicarboxylic acid to form a 2:1 complex. Binding of receptor 1 with a monocarboxylic acid is also compared.     

Impact of electron-electron spin interaction on electron spin relaxation of nitroxide diradicals and tetraradical in glassy solvents between 10 and 300 k

To determine the impact of electron-electron spin-spin interactions on electron spin relaxation rates, 1/T1 and 1/Tm were measured for nitroxide monoradical, diradical, and tetraradical derivatives of 1,3-alternate calix[4]arenes, for two pegylated high-spin nitroxide diradicals, and for an azine-linked nitroxide diradical. The synthesis and characterization by SQUID (superconducting quantum interference device) magnetometry of one of the high-spin diradicals, in which nitroxides are conformationally constrained to be coplanar with the m-phenylene unit, is reported. The interspin distances ranged from about 5-9 A, and the magnitude of the exchange interaction ranged from >150 to >0.1 K. 1/T1 and 1/Tm were measured by long-pulse saturation recovery, three-pulse inversion recovery, and two-pulse echo decay at X-band (9.5 GHz) and Q-band (35 GHz). For a diradical with interspin distance about 9 A, relaxation rates were only slightly faster than for a monoradical with analogous structure. For interspin distances of about 5-6 A, relaxation rates in glassy solvents up to 300 K increased in the order monoradical < diradical < tetraradical. Modulation of electron-electron interaction enhanced relaxation via the direct, Raman, and local mode processes. The largest differences in 1/T1 were observed below 10 K, where the direct process dominates. For the three diradicals with comparable magnitude of dipolar interaction, 1/Tm and 1/T1 were faster for the molecules with more flexible structures. Relaxation rates were faster in the less rigid low-polarity sucrose octaacetate glass than in the more rigid 4:1 toluene/chloroform or in hydrogen-bonded glycerol glasses, which highlights the impact of motion on relaxation.     

Band gap of carbon-sulfur [N]helicenes

Asymmetric synthesis of (-)-[9]helicene, as well as preparation of its lower homologues, completes the series of carbon-sulfur [5]-, [7]-, [9]-, and [11]helicenes. Spectroscopic and electrochemical studies of this series provide an absorption onset-based band gap, E(g) = 3.40 eV, for a cross-conjugated (C(2)S)(n) helix; this value may be compared to E(g) = 3.59 eV obtained from TD-DFT computed excitation energies for a series of dimethyl-substituted [n]helicenes (n ≤ 31).     

Exchange coupling mediated through-bonds and through-space in conformationally constrained polyradical scaffolds: calix[4]arene nitroxide tetraradicals and diradical

Calix[4]arenes constrained to the 1,3-alternate conformation and functionalized at the upper rim with four and two tert-butylnitroxides have been synthesized and characterized by X-ray crystallography, magnetic resonance (EPR and (1)H NMR) spectroscopy, and magnetic studies. The 1,3-alternate nitroxide tetraradical and diradical provide unique polyradical scaffolds for dissection of the through-bond and through-space intramolecular exchange couplings. In addition, detailed magnetic studies of the previously reported calix[4]arene nitroxide tetraradical, which possesses cone conformation in solution, reveal conformational dependence of exchange coupling. Through-bond coupling between the adjacent nitroxide radicals is mediated by the nitroxide-m-phenylene-CH(2)-m-phenylene-nitroxide coupling pathway, and through-space coupling is found between the diagonal nitroxide radicals at the conformationally constrained N...N distance of 5-6 A. Magnetic studies of the calix[4]arene polyradical scaffolds in frozen solutions show that the through-bond exchange coupling in the 1,3-alternate calix[4]arene tetraradical is antiferromagnetic, while that in cone calix[4]arene tetraradical is ferromagnetic. The through-space exchange couplings are antiferromagnetic in both cone and 1,3-alternate calix[4]arene tetraradical, as well as in the 1,3-alternate calix[4]arene diradical. The exchange coupling constants (|J/k|) are of the order of 1 K.