No abstract

The chemical properties of a 1,8-didehydronaphthalene derivative, the 4,5-didehydroisoquinolinium cation, were examined in the gas phase in a dual-cell Fourier-transform ion cyclotron resonance (FT-ICR) mass spectrometer. This is an interesting biradical because it has two radical sites in close proximity, yet their coupling is very weak. In fact, the biradical is calculated to have approximately degenerate singlet and triplet states. This biradical was found to exclusively undergo radical reactions, as opposed to other related biradicals with nearby radical sites. The first bond formation occurs at the radical site in the 4-position, followed by that in the 5-position. The proximity of the radical sites leads to reactions that have not been observed for related mono- or biradicals. Interestingly, some ortho-benzynes have been found to yield similar products. Since ortho-benzynes do not react via radical mechanisms, these products must be especially favorable thermodynamically.

The gas-phase reactivities of several protonated quinoline-based σ-type (carbon-centered) mono-, bi-, and triradicals toward dimethyl disulfide (DMDS) were studied by using a linear quadrupole ion trap mass spectrometer. The mono- and biradicals produce abundant thiomethyl abstraction products and small amounts of DMDS radical cation, as expected. Surprisingly, all triradicals produce very abundant DMDS radical cations. A single-step mechanism involving electron transfer from DMDS to the triradicals is highly unlikely because the (experimental) adiabatic ionization energy of DMDS is almost 3 eV greater than the (calculated) adiabatic electron affinities of the triradicals. The unexpected reactivity can be explained based on an unprecedented two-step mechanism wherein the protonated triradical first transfers a proton to DMDS, which is then followed by hydrogen atom abstraction from the protonated sulfur atom in DMDS by the radical site in the benzene ring of the deprotonated triradical to generate the conventional DMDS radical cation and a neutral biradical. Quantum chemical calculations as well as examination of deuterated and methylated triradicals provide support for this mechanism. The proton affinities of the neutral triradicals (and DMDS) influence the first step of the reaction while the vertical electron affinities and spin–spin coupling of the neutral triradicals influence the second step. The calculated total reaction exothermicities for the triradicals studied range from 27.6 up to 29.9 kcal mol–1.

Synthesis of a fluorescently labelled (dansylated) linear α(1→6)-linked octamannan, using glycosyl fluoride donors and thioglycosyl acceptors is described. A selective and convergent two-stage activation progression was executed to construct di-, tetra and octa-mannosyl thioglycosides in three glycosylation steps with excellent yield. Further a 5-N,N-Dimethylaminonaphthalene-1-sulfonamidoethyl (dansyl) group was coupled to 1-azidoethyl octamannosyl thioglycoside. Global deprotection of the coupled product afforded the desired dansylated homo-linear α(1→6)-linked octamannan.Mannans are constituents of many bacterial and fungal cell walls. 1 They are functional components of proteins and play important roles in defining the structures of proteins, their stabilization under physiological conditions, tuning of enzymatic activities, cell-cell recognition and in the adhesion of the microorganism to host cells. 2 The α(1→6)-linked oligomannans are found in the cell wall polysaccharides of yeast. 3 Polysaccharides, especially oligomannan, can be used to deliver drugs, genes and antigens through polysaccharide receptors present in cells and macrophages. 4 Macrophages are known to express high levels of specific polysaccharide receptors, e.g. mannan, glucan, and galactin receptors, on their membranes that generally bind neutral or charged polysaccharides and internalize these ligands. 5 Receptor mediated delivery of drug-polysaccharide conjugates is an approach that can deliver small and effective amounts of drugs specifically to target organisms, minimizing patient exposure and potential toxic side effects. We have recently demonstrated the selective, receptor-mediated delivery of an antibacterial drug to macrophages infected with Mycobacterium tuberculosis via conjugation of moxifloxacin with 1,3-β-glucan. 6 Drug loading on commercially available high molecular weight polysaccharides is very low and it is attributed to their poor solubility in solvents during the drug conjugation reactions. Therefore, we describe here a simple and efficient method for the synthesis of a low molecular weight homo-linear α(1→6)-linked octamannan fluorescent probe 1 (figure 1) that may be utilized to study uptake by macrophages through the mannan receptor present on the plasma membrane. Further, in the future, such conjugation can be exploited for selective receptormediated delivery of drugs. We have previously synthesized and reported dansylated disaccharide fluorescent probes to study glycosyltransferases in the biosynthesis of M. Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.Supplementar...

No abstract

No abstract