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The concept that resorcinarenes can be used as templates for the synthesis of large macrocycles is introduced. By way of example, previously inaccessible, aromatic crown ethers compounds are synthesized.
Synthesis and characterization of organic spin clusters, high-spin poly(arylmethyl) polyradicals with 24 and 8 triarylmethyls, are described. Polyether precursors to the polyradicals are prepared via modular, multistep syntheses, culminating in Negishi cross-couplings between four monofunctional branch (dendritic) modules and the tetrafunctional calix[4]arene-based macrocyclic core. The corresponding carbopolyanions are prepared and oxidized to polyradicals in tetrahydrofuran-d(8). The measured values of S, from numerical fits of magnetization vs magnetic field data to Brillouin functions at low temperatures (T = 1.8-5 K), are S = 10 and S = 3.6-3.8 for polyradicals with 24 and 8 triarylmethyls, respectively. Magnetizations at saturation (M(sat)) indicate that 60-80% of unpaired electrons are present at T = 1.8-5 K. Low-resolution shape reconstructions from the small-angle neutron scattering (SANS) data indicate that both the polyradical with 24 triarylmethyls and its derivatives have dumbbell-like shapes with overall dimensions 2 x 3 x 4 nm, in agreement with the molecular shapes of the lowest energy conformations obtained from Monte Carlo conformational searches. On the basis of these shapes, the size of the magnetic anisotropy barrier in the polyradical, originating in magnetic shape anisotropy, is estimated to be in the milliKelvin range, consistent with the observed paramagnetic behavior at T >or= 1.8 K. For macromolecular polyradicals, with the elongated shape and the spin density similar to the polyradical with 24 triarylmethyls, it is predicted that the values of S on the order of 1000 or higher may be required for "single-molecule-magnet" behavior, i.e., superparamagnetic blocking (via coherent rotation of magnetization) at the readily accessible temperatures T > 2 K.
The synthesis of a variety of shape-persistent macrocycles with either one (1a-d, 2) or two (opposing) terpyridine units (3, 4, 5a-c) and inner diameters of up to 2 nm is described. The sequences are mainly based on transition metal cross-coupling reactions and, whenever appropriate, compared with one another regarding their respective efficiency. Typical overall yields and amounts prepared range from 8% (4) to 27% (3) and 25 mg (1a) to 290 mg (1b), respectively. For solubility and processing of the targeted cycles, all precursors have already been decorated with flexible side chains (hexyloxy or hexyloxymethyl). The cycles' characterization is based on MALDI-TOF mass spectrometry, 2D NMR spectroscopy, and/or low-temperature single-crystal X-ray diffraction. Their packing in the crystal is discussed in terms of both number and length of side chains. Cycle 1d was physisorbed into an ordered structure at the solution-HOPG interface and investigated by scanning tunneling microscopy (STM).
Synthesis, crystallography, and magnetic characterization of a stable macrocyclic tetranitroxide 1, a calix[4]arene, which is functionalized with four tert-butylnitroxides at the upper rim, is described. In solution, 1 has a 4-fold symmetric fixed cone conformation on the NMR time scale and small, but nonnegligible, exchange interactions between the radicals (30 K > /J/k/ >>1.8 mK). In the solid state, dimerization of one diagonal pair of nitroxides leads to a pinched cone conformation for 1 with strong intradimer antiferromagnetic coupling with /J/k/ = 200-300 K (singlet-triplet energy gap, DeltaE(ST) approximately 1 kcal/mol).
Macrocyclization of a bioactive peptide with a bifunctional near-infrared fluorescent optical probe gives a compound that retained the receptor binding affinity of the peptide and the photophysical properties of the optical probe. The robust nature of the new compound provides a structural framework for optimizing the activity of bioactive molecules and for monitoring chemical or biological processes in vivo and in vitro by near-infrared optical methods.
Self-assembly of rigid oxygen donor dicarboxylates such as terephthalate and fumarate with a platinum-containing clip-like acceptor unit afforded neutral macrocyclic rectangles which have been characterized by X-ray single-crystal structure analysis as well as NMR spectroscopy.
Two nickel-catalyzed reductive coupling reactions of alkynes were instrumental in a modular, stereoselective synthesis of amphidinolide T1 (1). The C13-C21 fragment was prepared from two simple starting materials that were joined in a catalytic alkyne-epoxide fragment coupling operation, whereas an intramolecular aldehyde-alkyne reductive coupling simultaneously formed the final carbon-carbon bond of the macrocycle and established the C13 carbinol configuration with complete selectivity in the desired fashion.
The coordination-driven self-assembly of discrete 2D macrocyclic species from ambidentate pyridyl-carboxylate-based donor ligands and platinum-containing acceptors is presented. All these species are characterized by electrospray ionization mass spectrometry (ESIMS), multinuclear NMR, and in one example, X-ray crystallography. In each case only one isomeric assembly is selectively formed in high yield, despite the potential for more than one product as a consequence of differences in connectivity.
Coordination-assembled porphyrin macrocycles reinforced with covalent bondings were deposited on a metal surface by a pulse injection method, and their scanning tunneling microscopy (STM) images were recorded under ultrahigh vacuum conditions at liquid nitrogen temperature. The decamer ring consisting of 30 porphyrins gave clear circular STM images with hollow structure, whereas that without covalent linking did not give clear circular images, showing that covalent linking of the coordination pairs by ring-closing metathesis reaction was effective to reinforce the supramolecular structure on a metal surface. This strategy will be applicable to a variety of supramolecular assemblies.
For a number of phosphoryltransfer enzymes, including the exonuclease subunit of DNA polymerase I, a mechanism involving two-metal ions and double Lewis-acid activation of the substrate, combined with leaving group stabilization, has been proposed. Inspired by the active site structure of this enzyme, we have designed as a synthetic phosphoryl transfer catalyst the dicopper(II) macrocyclic complex LCu(2). Crystal structures of complexes [(L)Cu(2)(mu-NO(3))(NO(3))](NO(3))(2) (1), [(L)Cu(2)(mu-CO(3))(CH(3)OH)](BF(4))(2) (2), and [(L)Cu(2)(mu-O(2)P(OCH(3))(2))(NO(3))](NO(3))(2) (3) illustrate various possibilities for the interaction of oxoanions with the dicopper(II) site. 1 efficiently promotes the transesterification of dimethyl phosphate (DMP) in CD(3)OD, k(cat) = 2 x 10(-)(4) s(-)(1) at 55 degrees C. 1 is the only available catalyst for the smooth transesterification of highly inert simple dialkyl phosphates. From photometric titrations and the pH dependence of reactivity, we conclude that a complex [(L)Cu(2)(DMP)(OCH(3))](2+) is the reactive species. Steric bulk at the -OR substituents of phosphodiester substrates O(2)P(OR)(2)(-) drastically reduces the reactivity of 1. This is explained with -OR leaving group stabilization by Cu coordination, an interaction which is sensitive to steric crowding at the alpha-C-atom of substituent R. A proposed reaction mechanism related to that of the exonuclease unit of DNA polymerase I is supported by DFT calculations on reaction intermediates. The complex [(L)Cu(3)(mu(3)-OH)(mu-CH(3)O)(2)(CH(3)CN)(2)](ClO(4))(3) (4) incorporates a [Cu(OH)(OCH(3))(2)(CH(3)CN)(2)](-) complex anion, which might be considered as an analogue of the [PO(2)(OCH(3))(2)(OCD(3))](2)(-) transition state (or intermediate) of DMP transesterification catalyzed by LCu(2).
Diastereomeric proton-bound complexes between some phenylalanine derivatives (A) and chiral tetramide macrocycles (M) exhibit an uncommon enantioselectivity when reacting with the enantiomers of 2-aminobutanes in the gas phase (B). The measured enantioselectivity depends mainly on two distinct factors: (i) the configuration of the A guest; and (ii) the structure and the relative stability of isomeric [MHA]+ complexes. No significant effects of the B configuration are observed. The diastereomeric [MHA]+ complexes with A = 1-naphthylalanine ethyl ester exhibit the largest enantioselectivity factor ever measured in the gas phase (khomo/khetero = 0.046). The origin of such an exceptional enantioselectivity is mainly attributed to the relative stability of the diastereomeric [MHA]+ complexes, as demonstrated by the comparison of the kinetic results with those from collision-induced dissociation of the trimeric [M2HA]+ adducts and with computational evidence.
Ni-catalyzed reductive macrocyclizations of ynals are reported. Disubstituted alkynes afford either endocyclic or exocyclic allylic alcohols depending on the ligand. Phosphine ligands favor the formation of endocyclic olefins, whereas N-heterocyclic carbene ligands favor the formation of exocyclic olefins. Terminal alkynes provide 1,2-disubstituted olefins with N-heterocyclic carbene ligands.
Taking into consideration the model geometry of the macrocyclic hexaporphyrin 1 as a host molecule, the structure of a benzene-centered porphyrin trimer bearing pyridine rings at the apical positions has been designed with the aim to use the latter as a template for the synthesis of its own host. Indeed, in the presence of the porphyrin trimer 5, the yield of the cyclization of a linear porphyrin hexamer, as a precursor of 1, could be improved from 8 to 30% (variable yield) to 50% (reproducible yield). Even the condensation of equimolecular amounts of porphyrin monomers 20b and 21b in the presence of 5 led--probably through a loose preorganized complex between the latter and the Zn(II) chelate 20b--to the formation of 1 in only five steps from 19, as compared with 13 steps of the synthesis via linear porphyrin hexamer in the absence of template. As evidenced by 1H NMR spectroscopic analysis of the supramolecular complex between 5 and an analogue of 1b in which all H-atoms at the pyrrole rings have been replaced by deuterium, in the presence of unlabeled 1b, a rapid dissociation and recombination of the host and guest molecules forming the supramolecular complex takes place even at low temperature (-40 degrees C). As at 55 degrees C all six Zn(II) porphyrinate rings of the complex 1b + 5 become magnetically equivalent in the 500 MHz 1H NMR time scale, approximate kinetic data for the ligand exchange process could be obtained.
Paramagnetic lanthanide(III) complexes that contain hyperfine-shifted exchangeable protons offer considerable advantages over diamagnetic molecules as chemical exchange saturation transfer (CEST) agents for MRI. As part of a program to investigate avenues to improve the sensitivity of such agents, the CEST characteristics of europium(III) macrocyclic complexes having appended hydroxyethyl groups were investigated. The CEST spectrum of the asymmetrical complex, EuCNPHC3+, shows five distinct peaks for each magnetically nonequivalent exchangeable proton in the molecule. The CEST spectra of this complex were fitted to NMR Bloch theory to yield exchange rates between each of six exchanging proton pools (five on the agent plus bulk water). Exchange between the Eu3+-bound hydroxyl protons and bulk water protons was slow in dry acetonitrile but accelerated incrementally upon stepwise addition of water. In pure water, exchange was too fast to observe a CEST effect. The utility of this class of europium(III) complex for CEST imaging applications is ultimately limited by the small chemical shifts induced by the hydroxyl-appended ligands of this type and the resulting small Deltaomega values for the exchangeable hydroxyl protons.
Two-dimensional supramolecular honeycomb networks with cavities of an internal diameter of 2.95 nm were formed by the self-assembly of 4,4',4' '-benzene-1,3,5-triyl-tribenzoic acid (BTA) on a Ag(111) surface at room temperature. Annealing to higher temperatures resulted in two sequential phase transformations into closer-packed supramolecular arrangements. The phase transformations are associated with stepwise deprotonation of the carboxylic acid groups. The voids of the honeycomb network of BTA have a suitable size for the construction of hierarchical structures with guest molecules. Single molecules of the macrocyclic compound mt-33 were successfully confined inside 2D nanocavities of the honeycomb networks and released when the phase was transformed to the close-packed structure.
The "cyclamen" red (or pink) colors in carnation flowers-cultivars Red Rox and eight others-are based on the presence of a new macrocyclic anthocyanin, pelargonidin 3,5-di-O-beta-glucoside(6' ', 6' "-malyl diester) identified by spectroscopic methods. The instability of the bridging malyl group with sugars in acidic medium readily causes the formation of the opened ring form, 3-O-(6' '-O-malylglucoside)-5-O-glucoside. The issue of cyclamen colors based in carnations on this original acylated pelargonidin derivative simulating those based on simpler cyanidin glycosides in Rosa cultivars is discussed using CIELAB colorimetric coordinates.
Design modifications to the lead HIV-PR inhibitor 1 (MDL 73,669, Ki = 5 nM) have been postulated based on a computational model of the 1/HIV-PR complex. A novel macrocyclic inhibitor 8 (MDL 104,168) wherein the P1 and P3 side chains of the original acyclic inhibitor have been joined retains good biological activity (Ki = 20 nM). NMR analysis of the precursor alcohol (S)-7 shows the conformation of the cyclic region to be very similar to that observed in the enzyme-bound 8 as determined by the computational model. Consistency of the computational model with NMR data and in vacuo molecular dynamics simulations provide the basis for postulating further modifications of the cyclic inhibitor expected to optimize its interactions with HIV-PR.
Replacing one amide bond in macrocyclic renin inhibitors of the general structure 1 and 2 with an ester linkage gave glutamate-derived inhibitors 3 and serine-derived inhibitors 4. While this oxygen-for-nitrogen exchange had little effect on potency in the glutamate series, potency was dramatically increased in the serine series. In this series, the 14-membered ring compounds proved to be more potent than the corresponding 13-membered ring derivatives. Substitution of the ring at the position corresponding to P2' generally increased potency. The absolute configuration at this center was shown to be R for the 4-morpholinomethyl derivative (4o), both by asymmetric synthesis and X-ray crystallography. Replacing the "Boc-Phe" moiety of inhibitor 4o with a variety of substituents led to subnanomolar inhibitors, one of which (the "3(S)-quinuclidinyl-Phe" derivative 33) lowered blood pressure 20 mmHg and completely inhibited plasma renin activity for 6 h in sodium-depleted rhesus monkeys. This compound proved to have limited bioavailability (1% in rats) due to cleavage of the serine ester bond and rapid hepatic extraction.