James L. Little

Artifacts in trimethylsilyl derivatization reactions and ways to avoid them

Trimethylsilyl derivatives are routinely employed in gas chromatography to increase the volatility and stability of organic compounds containing active hydrogens. Normally only the desired derivative is formed when organic compounds are derivatized with common silylation reagents. However, some compounds form additional unexpected derivatives or by-products (artifacts). Artifact formation leads to multiple peaks for the same compound or unexpected components in the gas chromatographic analysis of mixtures. This review includes silylation artifacts identified in our laboratory by mass spectrometry during the last 20 years and references to those found in the literature. Also, means of avoiding artifact formation are discussed in detail.

Inhibiting efflux with novel non-ionic surfactants: Rational design based on vitamin E TPGS.

Tocopheryl Polyethylene Glycol Succinate 1000 (TPGS 1000) can inhibit P-glycoprotein (P-gp); TPGS 1000 was not originally designed to inhibit an efflux pump. Recent work from our laboratories demonstrated that TPGS activity has a rational PEG chain length dependency. In other recent work, inhibition mechanism was investigated and appears to be specific to the ATPase providing P-gp energy. Based on these observations, we commenced rational surface-active design. The current work summarizes new materials tested in a validated Caco-2 cell monolayer model; rhodamine 123 (10microM) was used as the P-gp substrate. These results demonstrate that one may logically construct non-ionic surfactants with enhanced propensity to inhibit in vitro efflux. One new surfactant based inhibitor, Tocopheryl Polypropylene Glycol Succinate 1000 (TPPG 1000), approached cyclosporine (CsA) in its in vitro efflux inhibitory potency. Subsequently, TPPG 1000 was tested for its ability to enhance the bioavailability of raloxifene - an established P-gp substrate -in fasted male rats. Animals dosed with raloxifene and TPPG 1000 experienced an increase in raloxifene oral bioavailability versus a control group which received no inhibitor. These preliminary results demonstrate that one may prepare TPGS analogs that possess enhanced inhibitory potency in vitro and in vivo.

Identification of “Known Unknowns” Utilizing Accurate Mass Data and Chemical Abstracts Service Databases

In many cases, an unknown to an investigator is actually known in the chemical literature. We refer to these types of compounds as “known unknowns.” Chemical Abstracts Service (CAS) Registry is a particularly good source of these substances as it contains over 54 million entries. Accurate mass measurements can be used to query the CAS Registry by either molecular formulae or average molecular weights. Searching the database by the web-based version of SciFinder is the preferred approach when molecular formulae are available. However, if a definitive molecular formula cannot be ascertained, searching the database with STN Express by average molecular weights is a viable alternative. The results from either approach are refined by employing the number of associated references or minimal sample history as orthogonal filters. These approaches were shown to be successful in identifying “known unknowns” noted in LC-MS and even GC-MS analyses in our laboratory. In addition, they were demonstrated in the identification of a variety of compounds of interest to others.

High-performance liquid chromatographic analysis of sulfonated aromatics using a β-cyclodextrin-bonded phase

Abstract A cyclodextrin-bonded phase and methanol—aqueous ammonium acetate mobile phase were used in the liquid chromatographic analysis of a number of sulfon

Isolation and structural identification of the trihydroxamate siderophore vicibactin and its degradative products from Rhizobium leguminosarum ATCC 14479 bv. trifolii

The Rhizobia are a group of free-living soil bacteria known for their ability to symbiotically infect the roots of specific host plants as well as to produce siderophores in order to compete with other microorganisms for the limited availability of iron in the rhizosphere. In this study, Rhizobium leguminosarum ATCC 14479, which preferentially infects the red clover Trifolium pratense, was found to produce the trihydroxamate siderophore vicibactin (C33H55N6O15) under iron restricted conditions. In addition, two other iron-binding, siderophore-like compounds: C20H36N4O10, C31H55N6O15, were isolated and purified from the culture media. Due to the structural similarity of the latter compounds to vicibactin based on electrospray-mass spectrometry and nuclear magnetic resonance data, these heretofore unreported molecules are thought to be either modified or degraded products of vicibactin. Although vicibactin has previously been found to be commonly produced by other rhizobial strains, this is the first time it has been chemically characterized from a clover infecting strain of R. leguminosarum.

Qualitative Gas Chromatography–Mass Spectrometry Analyses Using Amines as Chemical Ionization Reagent Gases

AbstractAmmonia is a very useful chemical ionization (CI) reagent gas for the qualitative analyses of compounds by positive ion gas chromatography–mass spectrometry (GCMS). The gas is readily available, inexpensive, and leaves no carbon contamination in the MS source. Compounds of interest to our laboratory typically yield abundant protonated or ammoniated species, which are indicative of a compound’s molecular weight. Nevertheless, some labile compounds fragment extensively by substitution and elimination reactions and yield no molecular weight information. In these cases, a CI reagent gas mixture of methylamine in methane prepared dynamically was found to be very useful in obtaining molecular weight data. Likewise, deuterated ammonia and deuterated methylamine are useful CI reagent gases for determining the exchangeable protons in organic compounds. Deuterated methylamine CI reagent gas is conveniently prepared by dynamically mixing small amounts of methylamine with excess deuterated ammonia. Figureᅟ

Thermal Decomposition of Pyridine-Substituted Cobaltic Acetate in Acetic Acid

The thermal decomposition of [py(3)Co(3)O(OAc)(5)OH][PF(6)] in acetic acid solution in the absence of oxygen produced carbon dioxide, methane, carbon monoxide, picoline, and formic acid as the major products. The ratio of the products was affected by the water concentration and acidity of the mixture. Increased water concentration caused a decrease in methane and an increase in carbon monoxide. Decreased acidity resulted in an increase in methane and a decrease in carbon monoxide. Isotopic labeling experiments showed that some of the carbon monoxide originated as the carboxyl group of the acetic acid. Labeling experiments also showed that formaldehyde and formic acid could be converted to carbon monoxide under the reaction conditions. Two pathways leading to the formation of carbon monoxide were proposed; one involving the decomposition of glyoxylic acid and another involving the oxidation of the methyl radical by cobalt(III).

Thiophenol Analogs of Tocopherols and Tocotrienols

The tocol family of natural products includes tocopherols and tocotrienols. Although RRRα-tocopherol 1a (vitamin E) is the best known of the eight natural tocols, other members of the family such as RRR-γ -tocopherol (1b), R-γ -tocotrienol (1c), and R-δ-tocotrienol (1d) have shown a variety of important biological activities and are currently being investigated in such diverse areas as cancer cell apoptosis,1,2 protection from acute radiation damage,3 antiangiogenesis,4 and hypocholesterolemic activity5 in addition to their well-known antioxidant activities.6 As part of investigations on the use of natural antioxidants and their derivatives as therapeutic agents, we desired samples of the thio analogs of several of the natural tocols, in particular compounds such as 4a–4d.

On the preparation of carbohydrate sulfates by displacement of trifluoromethanesulfonates

A recent paper by Latif et.al.’ described the reaction of certain carbohydrate trifluoromethanesulfonates (triflates) with tetrabutylammonium hydrogensulfate in acetonitrile solution to produce hydrogensulfates. In a particular example, benzyl 2,3-anhydro-CO-(trifluoromethanesulfonyl)-cr-D-lyxopyranoside (1) was reported to undergo displacement of the triflate group by the hydrogensulfate ion to produce in 59% yield the product benzyl 2,3-anhydro-@=ribopyranoside 4-hydrogensulfate (2, Scheme 1). Compound 2 was reported to be a stable, crystalline solid that could be chromatographed on silica gel with ethyl acetate-hexane elution. ‘Hand ‘3C-n.m.r. spectra in support of structure 2 were provided; mass and infrared spectra were not reported’. In connection with some studies underway in these laboratories, there was a need for a secondary carbohydrate sulfate, and we repeated the process for preparing 2 from 1. From this reaction a compound was obtained whose physical properties (RF, m.p., and ‘Hand “C-n.m.r. spectra) were in excellent accord with those reported’ for 2.