Conversion techniques in flow injection analysis: Determination of Sulphide by Precipitation with Cadmium Ions and Detection by Atomic Absorption Spectrometry

A flow-injection system is described for on-line conversion of a soluble species to an insoluble compound by means of a tag-material which subsequently can be determined. This approach is used for the determination of sulphide by flame atomic absorption spectrometry, cadmium(II) ions being used as the precipitating tag-reagent. Excess of cadmium(II) is collected on a chelating ion-exchanger and later eluted. The detection limit for sulphide was 10 μg l^?1 and the sampling rate was 100 h^?1. Typical relative standard deviation was 1.2%. Of the potential interferences tested, only phosphate had any effect.     

A practical oxidative method for the cleavage of hydrazide N[bond]N bonds

The selective N-oxidation of the most nucleophilic amino nitrogen atom in hydrazides is central to the development of an unprecedented methodology for the cleavage of their N[bond]N bonds under oxidative conditions. Treatment of a series of hydrazides 1-9 with peracids such as magnesium monoperoxyphtalate hexahydrate (MMPP.6 H(2)O) or meta-chloroperbenzoic acid (m-CPBA) afforded the corresponding amides 10-16 in good-to-excellent yields (80-92 %). The extension of the methodology to carbamate-like substrates such as 17 and 18 was also investigated, but in this case the process is synthetically useless in view of the low yields observed of carbamates 19 and 20 (approximately 15 %). Experiments carried out with equivalent amounts of oxidant produced nitrones, such as 26, proceeding from the dialkylamino moiety, and (1)H NMR experiments indicated that this product is formed by fast conversion of the parent hydrazide, without detection of the expected hydrazide N-oxides. In addition, the over oxidation of 26 into nitronate 25 proceeds through an unknown intermediate. This oxidative N[bond]N bond cleavage by peracids is an alternative method for the deamination of hydrazides, and constitutes the only solution compatible with substrates carrying functionalities sensitive to reducing conditions.     

Determination of creatinine in undiluted blood serum enzymatic flow injection analysis with optosensing

A specific enzymatic assay for creatinine in undiluted serum samples is described, exploiting the generation of ammonia from creatinine by immobilized creatinine iminohydrolase. The ammonia produced is separated from the sample matrix by gas diffusion into an acceptor stream containing a pH-sensitive indicator. The creatinine content is quantified by monitoring the resulting colour change of the indicator by means of reflectance measurement via optical fibers, the hydrophobic gas-permeable membrane serving as a diffuse reflector. Two approaches are used to overcome the interference caused by endogenous ammonia. The first is based on enzymatic abatement of endogenous ammonia by immobilized glutamate dehydrogenase. In the second, preferable, approach, endogenous ammonia, itself a parameter of clinical interest, is measured separately prior to the enzymatic degradation by creatinine iminohydrolase. Each assay requires only 30 μl of sample solution, and the sampling frequency is 60 h^?1. The relative standard deviation is approximately 3%.     

Gold(I)-catalyzed cyclizations of 1,6-enynes: alkoxycyclizations and exo/endo skeletal rearrangements

Gold(I) complexes are the most active catalysts for alkoxy- or hydroxycyclization and for skeletal rearrangement reactions of 1,6-enynes. Intramolecular alkoxycyclizations also proceed efficiently in the presence of gold(I) catalysts. The first examples of the skeletal rearrangement of enynes by the endocyclic cyclization pathway are also documented. Iron(III) is also able to catalyze exo and endo skeletal rearrangements of 1,6-enynes, although the scope of this transformation is more limited. The gold(I)-catalyzed endocyclic cyclization proceeds by a mechanism different from those followed in the presence of PdII, HgII, or RhI catalysts.     

Mass Spectroscopy of the Antileukemic Drug MGBG and Related Bis(Amidinohydrazones) [‘Bis(Guanylhydrazones)’]

The first study on the mass spectroscopy of various bis(amidinohydrazones) is reported. The compounds studied included the investigational antileukemic drugs methylglyoxal bis(amidino-hydrazone) [‘methylglyoxal bis(guanylhydrazone)’, MGBG] and glyoxal bis(amidinohydrazone), as well as seven mono- and dialkylglyoxal analogs thereof. The results indicate that the free bases of these high-melting compounds are volatilized well enough to allow a facile detection of the molecular ions and to make mass spectroscopy of the underivatized compounds a suitable method for the verification of the identity of the substances. This result is of importance considering the development of novel analogs and derivatives. A compilation of electron-impact mass spectra is reported and possible fragmentation routes are outlined. The fragmentation of the various congeners appears to occur essentially similarly, the main paths involving breakage of the carbon-carbon single bond in the glyoxal moiety or breakage of either one of the nitrogen-nitrogen single bonds.     

Multinuclear PGSE diffusion and overhauser NMR studies on a variety of salts in THF solution

(1)H, (19)F, and (7)Li pulsed gradient spin-echo (PGSE) NMR measurements for a series of salts are reported. The (7)Li is shown to complement the (1)H and (19)F measurements; however, the use of higher concentrations (for the less-sensitive (7)Li) can lead to aggregation. For all of the salts discussed {Li(BF(4)); (n-Bu(4)N)(BF(4)), a trinuclear Ru cluster; [Ir(1,5-COD)(4)](BF(4)), where 4 is a chiral P,N ligand; and the crown ether stabilized potassium salt, [K(18-crown-6)(NPh(2))], 6}, the use of THF seems to promote strong ion pairing. In several cases, the degree of ion pairing approaches 100%. In THF solution, the potassium salt, 6, prefers to exist as a more classical ion pair rather than as the pi complex found in the solid state. In some cases, (1)H, (1)H NOESY and (1)H, (19)F HOESY spectra help to pinpoint the cation/anion spatial relationship.     

7Li, 31P, and 1H pulsed gradient spin-echo (PGSE) diffusion NMR spectroscopy and ion pairing: on the temperature dependence of the ion pairing in Li(CPh3), fluorenyllithium, and Li[N(SiMe3)2] amongst other salts

7Li, 31P, and 1H variable-temperature pulsed gradient spin-echo (PGSE) diffusion methods have been used to study ion pairing and aggregation states for a range of lithium salts such as lithium halides, lithium carbanions, and a lithium amide in THF solutions. For trityllithium (2) and fluorenyllithium (9), it is shown that ion pairing is favored at 299 K but the ions are well separated at 155 K. For 2-lithio-1,3-dithiane (13) and lithium hexamethyldisilazane (LiHMDS 16), low-temperature data show that the ions remain together. For the dithio anion 13, a mononuclear species has been established, whereas for the lithium amide 16, the PGSE results allow two different aggregation states to be readily recognized. For the lithium halides LiX (X = Br, Cl, I) in THF, the 7Li PGSE data show that all three salts can be described as well-separated ions at ambient temperature. The solid state structure of trityllithium (2) is described and reveals a solvent-separated ion pair formed by a [Li(thf)4]+ ion and a bare triphenylmethide anion.     

Studies on stereoselective [2+2] cycloadditions between N,N-dialkylhydrazones and ketenes

Staudinger-like cycloadditions between chiral, non-racemic N,N-dialkylhydrazones 1 and functionalized ketenes constitute an efficient methodology for the stereoselective construction of the beta-lactam ring. The potential for fine tuning of the dialkylamino auxiliary structure, the availability of a high-yielding deprotection method for the release of the free azetidinones, and the high thermal and chemical stability of hydrazones as N-dialkylamino imines are highlighted as the key elements for the success of the strategy. This last aspect is of particular importance concerning generality: even hydrazones from easily enolizable aldehydes or from formaldehyde reacted to afford the corresponding cycloadducts with high chemical and stereochemical yields. The syntheses of the beta-amino-alpha-hydroxyacids (2R,3S)-phenylisoserine (42) and (2R,3S)-norstatin (45) were accomplished as illustrative examples of the synthetic utility of this procedure. A model system for the cycloaddition of g series auxiliaries was studied by ab initio computational methods. The collected results support a two-step mechanism through zwitterionic intermediates, and explain the observed absolute and relative stereochemistry in terms of the preferred outward cycloaddition to the Re face of the hydrazone.