Synthesis of C-Unsubstituted 1,2-Diazetidines and Their Ring-Opening Reactions via Selective N-N Bond Cleavage.

C-Unsubstituted 1,2-diazetidines, a rarely studied type of four-membered heterocyclic compounds, were synthesized through an operationally simple intermolecular vicinal disubstitution reaction. 1,2-Diazetidine derivatives bearing various N-arylsulfonyl groups were readily accessed and studied by experimental and computed Raman spectra. The ring-opening reaction of the diazetidine was explored and resulted in the identification of a selective N-N bond cleavage with thiols as nucleophiles, which stereoselectively produced a new class of N-sulfenylimine derivatives with C-aminomethyl groups.

Bis-corannulenoanthracene: An Angularly Fused Pentacene as a Precursor for Barrelene-Tethered Receptors for Fullerenes.

Bis-corannulenoanthracene (C50H22, 5) was prepared by the Diels-Alder double cycloaddition of isocorannulenofuran with "bis-benzyne", followed by deoxygenation of the adducts. Despite the presence of a pentacene core, 5 is stable enough to be isolated and stored. A cycloaddition reaction of 5 with maleic anhydride produces 10 which exhibits strong affinity toward C60, as evidenced by (1)H NMR titration experiment. Synthesis of 10 demonstrates the synthetic utility of hydrocarbon 5 in the preparation of the barrelene-based molecular clips with two benzocorannulene pincers adorned with polar substituents on their tethers, which will allow for immobilization of the receptors on solid supports.

Bis-corannulene Receptors for Fullerenes Based on Klärner's Tethers: Reaching the Affinity Limits.

Bis-corannulene receptors 4 and 5 with Klärner's tethers prepared by the Diels-Alder cycloaddition form inclusion complexes with C60 and C70, as evidenced by (1)H NMR titration. While 4 exhibits affinity toward fullerenes comparable to the previously reported corannulene-based receptors, 5 exceeds the performance of the former systems by ca. 2 orders of magnitude and, in addition, shows an enhanced preference for C70 over C60. The X-ray crystal structure of C60@5 and DFT calculations indicate that the tether in 5 not only preorganizes the pincers into a proper topology of the host but also contributes to the dispersion-based binding with the fullerene guests.

A 2:1 receptor/C60 complex as a nanosized universal joint.

Buckycatcher II, a C51 H24 hydrocarbon with two corannulene pincers on a dibenzonorbornadiene tether, exhibits an affinity toward C60 in organic solvents that is dramatically higher than the original buckycatcher C60 H28 and other corannulene-based molecular receptors for fullerenes. In addition to the formation of an usual 1:1 C60 @catcher inclusion complex, a trimeric C60 @(catcher)2 assembly is detected in solutions and in the solid state. X-ray structure determination reveals a remarkable "universal joint" solvent-free crystal arrangement of the trimer, with a single fullerene cage wrapped by four corannulene subunits of two cooperating catcher receptors.

Thermodynamics of host-guest interactions between fullerenes and a buckycatcher.

(1)H NMR and isothermal titration calorimetry (ITC) experiments were employed to obtain reliable thermodynamic data for the formation of the 1:1 inclusion complexes of fullerenes C(60) and C(70) with the buckycatcher (C(60)H(28)). NMR measurements were done in toluene-d8 and chlorobenzene-d5 at 288, 298, and 308 K, while the ITC titrations were performed in toluene, chlorobenzene, o-dichlorobenzene, anisole, and 1,1,2,2-tetrachloroethane at temperatures from 278 to 323 K. The association constants, K(a), obtained with both techniques are in very good agreement. The thermodynamic data obtained by ITC indicate that generally the host-guest association is enthalpy-driven. Interestingly, the entropy contributions are, with rare exceptions, slightly stabilizing or close to zero. Neither ΔH nor ΔS is constant over the temperature range studied, and these thermodynamic functions exhibit classical enthalpy/entropy compensation. The ΔCp values calculated from the temperature dependence of the calorimetric ΔH values are negative for the association of both fullerenes with the buckycatcher in toluene. The negative ΔCp values are consistent with some desolvation of the host-cavity and the guest in the inclusion complexes, C(60)@C(60)H(28) and C(70)@C(60)H(28).

An unsolvated buckycatcher and its first dianion.

The X-ray crystallographic study of C60H28 consisting of two tethered corannulene bowls revealed a unique solid-state packing based on tight convex-concave π-π interactions. The controlled reduction of C60H28 resulted in the isolation and structural characterization of its dianion in the form of the rubidium salt that shows an entrapment of counterions by an anionic pincer.

Corannulene subunit acts as a diene in a cycloaddition reaction: synthesis of C80H32 corannulyne tetramer.

Distortion of six-membered rings in corannulene subunits of corannulyne (1,2-didehydrocorannulene) cyclotrimer activates the system for a cycloaddition reaction with another corannulyne unit. This unprecedented cycloaddition in which a corannulene fragment acts as a diene produces the largest oligomer of corannulyne reported to date. X-ray crystallography reveals the highly nonplanar structure of the tetramer which exhibits conformational and optical absorption properties very different from those of the cyclotrimer.

(E)-3,3',4,4',7,7',8,8'-Octa-methyl-2H,2'H-1,1'-bi(cyclo-penta-[fg]acenaphthyl-enyl-idene)-2,2',5,5',6,6'-hexa-one dichloro-methane monosolvate.

The title compound, C(36)H(24)O(6)·CH(2)Cl(2), is a dimer of two essentially planar (r.m.s., deviations of fitted plane of 14 pyracene C atoms = 0.0539 and 0.0543 Å) tetra-cyclic pyracene frameworks (each with four methyl groups and three carbonyl groups on the peripheral carbon atoms) twisted along a central C=C bond with an angle of 50.78 (3)° at 90 K. There are notably long Csp(2)-Csp(2) bonds associated with the carbonyl groups, the longest being 1.601 (3) Šbetween two carbonyl C atoms. There are also intermolecular carbonyl⋯carbonyl interactions of both parallel and antiparallel types, with C⋯O distances in the range 3.041 (3) to 3.431 (2) Å. This compound is of inter-est with respect to the synthesis of fullerene fragments, such as corannulene and semibuckminsterfullerene derivatives (or 'buckybowls'), and is a side product of the previously reported oxidation reaction. Structural details, such as planarity analysis of fused rings, out-of-plane deviation of substituents, inter-molecular inter-actions, and longer than typical bond lengths, will be discussed as well as comparisons to structurally related compounds.

Epirubicin-[Anti-HER2/neu] Synthesized with an Epirubicin-(C13-imino)-EMCS Analog: Anti-Neoplastic Activity against Chemotherapeutic-Resistant SKBr-3 Mammary Carcinoma in Combination with Organic Selenium.

PURPOSE: Discover the anti-neoplastic efficacy of epirubicin-(C13-imino)-[anti-HER2/neu] against chemotherapeutic-resistant SKBr-3 mammary carcinoma and delineate the capacity of selenium to enhance it's cytotoxic anti-neoplastic potency. METHODS: In molar excess, EMCH was combined with epirubicin to create a covalent epirubicin-(C13-imino)-EMCH-maleimide intermediate with sulfhydryl-reactive properties. Monoclonal immunoglobulin selective for HER2/neu was then thiolated with 2-iminothiolane at the terminal ε-amine group of lysine residues. The sulfhydryl-reactive epirubicin-(C13-imino)-EMCH intermediate was then combined with thiolated anti-HER2/neu monoclonal immunoglobulin. Western-blot analysis was utilized to characterize the molecular weight profiles while binding of epirubicin-(C13-imino)-[anti-HER2/neu] to membrane receptors was determined by cell-ELISA utilizing populations of SKBr-3 mammary carcinoma that highly over-expresses HER2/neu complexes. Anti-neoplastic potency of epirubicin-(C13-imino)-[anti-HER2/neu] between the epirubicin-equivalent concentrations of 10-12 M and 10-7 M was determined by vitality staining analysis with and without the presence of selenium (5 µM). RESULTS: Epiribucin-(C13-imino)-[anti-HER2/neu] between epirubicin-equivalent concentrations of 10-8 M to 10-7 M consistently evoked higher anti-neoplastic potency than "free" non-conjugated epirubicin which corresponded with previous investigations utilizing epirubicin-(C3-amide)-[anti-HER2/neu] and epirubicin-(C3-amide)-[anti-EGFR]. Selenium at 5 mM consistently enhanced the cytotoxic anti-neoplastic potency of epirubicin-(C13-imino)-[anti-HER2/neu] at epirubicin equivalent concentrations (10-12 to 10-7 M). CONCLUSIONS: Epirubicin-(C13-imino)-[anti-HER2/neu] is more potent than epirubicin against chemotherapeutic-resistant SKBr-3 mammary carcinoma and selenium enhances epirubicin-(C13-imino)-[anti-HER2/neu] potency. The methodology applied for synthesizing epirubicin-(C13-imino)-[anti-HER2/neu] is relatively time convenient and has low instrumentation requirements.

Acid cleavable surface enhanced raman tagging for protein detection.

Dye conjugation is a common strategy improving the surface enhanced Raman detection sensitivity of biomolecules. Reported is a proof-of-concept study of a novel surface enhanced Raman spectroscopic tagging strategy termed as acid-cleavable SERS tag (ACST) method. Using Rhodamine B as the starting material, we prepared the first ACST prototype that consisted of, from the distal end, a SERS tag moiety (STM), an acid-cleavable linker, and a protein reactive moiety. Complete acid cleavage of the ACST tags was achieved at a very mild condition that is 1.5% trifluoroacetic acid (TFA) aqueous solution at room temperature. SERS detection of this ACST tagged protein was demonstrated using bovine serum albumin (BSA) as the model protein. While the SERS spectrum of intact ACST-BSA was entirely dominated by the fluorescent signal of STM, quality SERS spectra can be readily obtained with the acid cleaved ACST-BSA conjugates. Separation of the acid cleaved STM from protein further enhances the SERS sensitivity. Current SERS detection sensitivity, achieved with the acid cleaved ACST-BSA conjugate is ∼5 nM in terms of the BSA concentration and ∼1.5 nM in ACST content. The dynamic range of the cleaved ACST-BSA conjugate spans four orders of magnitudes from ∼10 nM to ∼100 µM in protein concentrations. Further improvement in the SERS sensitivity can be achieved with resonance Raman acquisition. This cleavable tagging strategy may also be used for elimination of protein interference in fluorescence based biomolecule detection.

Sensitive carbohydrate detection using surface enhanced Raman tagging.

Glycomic analysis is an increasingly important field in biological and biomedical research as glycosylation is one of the most important protein post-translational modifications. We have developed a new technique to detect carbohydrates using surface enhanced Raman spectroscopy (SERS) by designing and applying a Rhodamine B derivative as the SERS tag. Using a reductive amination reaction, the Rhodamine-based tag (RT) was successfully conjugated to three model carbohydrates (glucose, lactose, and glucuronic acid). SERS detection limits obtained with a 633 nm HeNe laser were ∼1 nM in concentration for all the RT-carbohydrate conjugates and ∼10 fmol in total sample consumption. The dynamic range of the SERS method is about 4 orders of magnitude, spanning from 1 nM to 5 µM. Ratiometric SERS quantification using isotope-substituted SERS internal references allows comparative quantifications of carbohydrates labeled with RT and deuterium/hydrogen substituted RT tags, respectively. In addition to enhancing the SERS detection of the tagged carbohydrates, the Rhodamine tagging facilitates fluorescence and mass spectrometric detection of carbohydrates. Current fluorescence sensitivity of RT-carbohydrates is ∼3 nM in concentration while the mass spectrometry (MS) sensitivity is about 1 fmol, achieved with a linear ion trap electrospray ionization (ESI)-MS instrument. Potential applications that take advantage of the high SERS, fluorescence, and MS sensitivity of this SERS tagging strategy are discussed for practical glycomic analysis where carbohydrates may be quantified with a fluorescence and SERS technique and then identified with ESI-MS techniques.

Ultrasensitive detection of malondialdehyde with surface-enhanced Raman spectroscopy.

Malondialdehyde (MDA) is a biomarker of lipid peroxidation that has been widely associated with food rancidity as well as many human diseases. Most current MDA detection methods involve MDA reaction with thiobarbituric acid (TBA), followed by UV-visible and/or fluorescence detection of high-performance liquid chromatography (HPLC)-separated TBA-MDA. Herein, we report the first proof-of-concept study of surface-enhanced Raman detection of a TBA-MDA adduct using silver nanoparticles as the SERS substrate and the 632.8 nm HeNe laser as a Raman excitation source. Current SERS detection limit of TBA-MDA is 0.45 nM, ~100 times higher than the 36 nM fluorescence sensitivity recently reported with the HPLC-purified TBA-MDA. Molecular specificity of the SERS technique was studied by comparing the SERS spectrum of TBA-MDA with those acquired with TBA adducts of other TBA-reactive compounds (TBARCs) that includes formaldehyde, acetaldehyde, butyraldehyde, trans-2-hexenal, and pyrimidine. Compared to TBA and TBA adducts with those TBARCs, the SERS activity of TBA-MDA adduct is significantly higher. The possibility of direct SERS detection of TBA-MDA in a reaction mixture (without HPLC separation) has also been investigated.

Inclusion complexes of buckycatcher with C(60) and C(70).

The dispersion corrected B97-D functional studies find a previously overlooked conformer of the buckycatcher C(60)H(28) (2) exhibiting intramolecular pi-pi stacking of its corannulene pincers to represent a global potential energy minimum conformation. B97-D/TZVP calculated geometry of C(60)@2 supramolecular assembly is in excellent agreement with the X-ray structure, slightly better than the previously reported M06-L results. In contrast, our calculated binding energy of C(60)@2 complex is dramatically higher than both M05-2X and M06-2X results and we conclude that the latter numbers are grossly underestimated. The lack of specificity of the buckycatcher for molecular recognition of C(60)vs. C(70) fullerenes is predicted by our calculations and confirmed by the NMR titration experiment which provides the equilibrium constant of 6800 +/- 400 M(-1) for association of C(70) with C(60)H(28) which is only slightly lower than the previously reported association constant of C(60)@2 assembly.

Evidence for pi-stacking as a source of stereocontrol in the synthesis of the core pyranochromene ring system common to calyxin I, calyxin J, and epicalyxin J.

A diastereoselective synthesis of the tetrahydropyranochromene ring system common to several natural product isolates of Alpinia blepharocalyx is reported. We have shown that a stereochemical preference exists for a syn configuration between the anomeric aryl substituents, representative of the C-7 and C-7' substituents in the natural products. Further, our results show that stereocontrol is under kinetic control, and calculations suggest that a favorable pi-stacking interaction may be the source of this stereocontrol.

Ratiometric Raman spectroscopy for quantification of protein oxidative damage.

A novel ratiometric Raman spectroscopic (RMRS) method has been developed for quantitative determination of protein carbonyl levels. Oxidized bovine serum albumin (BSA) and oxidized lysozyme were used as model proteins to demonstrate this method. The technique involves conjugation of protein carbonyls with dinitrophenyl hydrazine (DNPH), followed by drop coating deposition Raman spectral acquisition (DCDR). The RMRS method is easy to implement because it requires only one conjugation reaction, uses a single spectral acquisition, and does not require sample calibration. Characteristic peaks from both protein and DNPH moieties are obtained in a single spectral acquisition, allowing the protein carbonyl level to be calculated from the peak intensity ratio. Detection sensitivity for the RMRS method is approximately 0.33 pmol carbonyl per measurement. Fluorescence and/or immunoassay-based techniques only detect a signal from the labeling molecule and, thus, yield no structural or quantitative information for the modified protein, whereas the RMRS technique allows protein identification and protein carbonyl quantification in a single experiment.

2-Trimethylsilylcorannulenyl trifluoromethanesulfonate: an efficient precursor for 1,2-didehydrocorannulene.

2-Trimethylsilylcorannulenyl trifluoromethanesulfonate, prepared from bromocorannulene, provides a means for generating of 1,2-didehydrocorannulene (corannulyne) under very mild conditions as demonstrated by the high-yield formation of its adducts with dienes.

Isocorannulenofuran: a versatile building block for the synthesis of large buckybowls.

Isocorannulenofuran, synthesized in two steps from accessible bromocorannulene, gives Diels-Alder adducts with benzynes which can be deoxygenated to produce large polycyclic aromatic hydrocarbons (PAHs) combining the bowl-shaped corannulene subunit with planar fragments. [reaction: see text]

The first buckybowl aryne. Corannulyne: a nonplanar benzyne.

[reaction: see text] Treatment of bromocorannulene with NaNH(2)/tBuOK in THF in the presence of furans or amines leads to formation of addition products in good yields, presumably through formation of 1,2-didehydrocorannulene (corannulyne). HDFT calculations predict that corannulyne exists as a bowl-shaped entity and that the didehydrogenation energy of corannulene is slightly lower than that of benzene.