An effective enantioselective approach to the securinega alkaloids: total synthesis of (-)-norsecurinine

[reaction: see text] A highly versatile approach to the enantioselective synthesis of securinega alkaloids is presented. Crucial steps are a palladium-catalyzed enantioselective imide alkylation, a vinylogous Mannich reaction, and a ring-closing metathesis process. Through this strategy, the synthesis of (-)-norsecurinine has been accomplished in nine steps and 11% overall yield.     

A simple monitored thermometric determination of copper(II) by reaction-rate measurement based on the catalysis of the aerial oxidation of 2,2′-dipyridylketone hydrazone

The initial reaction rate is measured by monitoring the rate of increase of temperature of the solution with a thermistor; 0.2–2 mg l^?1 copper in the final solution is determined, with very few interferences.     

Observation of apigenin anionic clusters in the gas phase

The ubiquity and favorable medicinal properties of flavonoids make essential the determination of flavonoid levels in various matrices. While developing a liquid chromatography/tandem mass spectrometry method for the analysis of the flavonoid, apigenin, anionic oligomers and nitrate- and chloride-bound clusters of this compound were observed. Tandem mass spectrometry of these oligomers and cluster ions showed the cleavage of apigenin molecules from the precursor. The observation of these cluster ions shows the possibility of post-column derivatization techniques to enhance specificity in analysis. Copyright 2000 John Wiley & Sons, Ltd.     

Electrospray ionization tandem mass spectrometric study of salt cluster ions. Part 1--investigations of alkali metal chloride and sodium salt cluster ions

Salt cluster ions of alkali metal chlorides ACl (A = Li(+), Na(+), K(+), Rb(+) and Cs(+)) and sodium salts NaB (B = I(-), HCOO(-), CH(3)COO(-), NO(2)(-), and NO(3)(-)), formed by electrospray ionization, were studied systematically by mass spectrometry. The influences on the total positive ion and negative ion currents of variation of solvent, solution concentration, desolvation temperature, solution flow-rate, capillary voltage and cone voltage were investigated. Only cone voltage was found to influence dramatically the distribution of salt cluster ions in the mass spectra observed. Under conditions of normal cone voltage of approximately 70 V, cluster ions having magic numbers of molecules are detected with high relative signal intensity. Under conditions of low cone voltage of approximately 10 V, the distribution of cluster ions detected is characterized by a relatively low average mass/charge ratio due to the presence of multiply charged cluster ions; in addition, there is a marked reduction in cluster ions having a magic number of molecules. Product ion mass spectra obtained by tandem mass spectrometry of cluster ions are characterized by a base peak having a magic number of molecules that is less than and closest to the number of molecules in the precursor ion. Structures have been proposed for some dications and some quadruply charged ions. At pH 3 and 11, the mass spectra of NaCl clusters show the presence of mixed clusters of NaCl with HCl and NaOH, respectively. The effects of ionic radius on 20 distributions of cluster ions for 10 salts were investigated; however, the fine structure of these effects is not readily discerned.     

Building blocks for electron density in free molecules and in condensed matter phases

With the density functional theory as the underlying framework, the modeling of the ground-state density p( r ), using fragments or bonds as appropriate, is considered in both free molecules and in some condensed phases. Emphasis is placed on bringing the models into contact with both first-principles quantum mechanics and available diffraction experiments. Clusters of alkali metals are specifically referred to, since cluster science affords a bridge between the quantum chemistry of small molecules and condensed phases. Finally, amorphous silicon and both solid and liquid alkali metals are considered. © 1994 John Wiley & Sons, Inc.     

Effects of exchange on equilibrium bond lengths of heavy,almost spherical,tetrahedral molecules XH4

The simplest nonrelativistic density functional theory, namely, that of Thomas, Fermi, and Dirac, is used to study the effect of exchange on the equilibrium bond lengths of heavy tetrahedral molecules XH₄. In particular, the limiting bond length as the central atom X becomes infinitely heavy is shown to be reduced by exchange by some 0.34 Å. Comparison with experiment shows that the main features of the bond-length variation through the series CH₄? PbH₄ are reflected by the Thomas–Fermi–Dirac predictions, though the bond lengths remain too large for finite atomic number Z of the central atom. Therefore, a semiempirical correction is proposed, which, however, tends to zero as Z tends to infinity.     

Observation of Na(+)-bound oligomers of quercetin in the gas phase

While developing a liquid chromatography/tandem mass spectrometry method for the analysis of the flavonoid quercitin, it was observed that quercetin (3,3',4',5,7-pentahydroxyflavone) exhibited clustering in both the positive and negative ion mode. Two series of positive ion clusters were observed; the first series corresponds to singly charged [2M + Na](+) at m/z 627.2 to [13M + Na](+) at m/z 3947.5, while the second series corresponds to doubly charged [7M + 2Na](2+) at m/z 1080.4 to [25M + 2Na](2+) at m/z 3798.5. In the negative ion mode, the behavior of quercetin parallels that of apigenin (4',5,7-trihydroxyflavone) in that [M + NO(3)](-), [2M + NO(3)](-), and [3M + NO(3)](-) were observed at m/z 364.1, 666.0, and 968.9, respectively; in addition, quercitin clusters with chloride ions ([2M + Cl](-) at m/z 638.9 and [3M + Cl](-) at m/z 940. 9) were observed. The results of tandem mass spectrometric examination of several cluster ions are reported.     

Relativistic total energy of heavy atomic ions: Dimensionality dependent scaling

The scaling property of the relativistic total energy E (Z, N) established by Marconi and the writer for heavy positive ions with N electrons and atomic number Z is demonstrated, by setting up the self-consistent relativistic Thomas–Fermi equation in d dimensions, to be a special case of the scaling property α being the fine structure constant.     

Reactions of triacetic acid lactone with carbonyl compounds. X-Ray structure determination of 3-acetoacetyl-2-chromenone and 3,6,9,12-tetramethyl-1H,6H,7H,12H-6,12-methanodipyrano[4,3-b:4,3-f]-dioxocin-1,7-dione

The structure of 3-acetoacetyl-2-chromenone has been definitely assigned to the product resulting from the reaction of 4-hydroxy-6-methyl-2-pyrone, 1 , with 2-hydroxybenzaldehyde. Also, the reaction of the pyrone 1 with pentane-2,4-dione in a 2:1 molar ratio gives 3,6,9,12-tetramethyl-1H,6H,7H,12H-6,12-methanodypirano[4,3-b:4,3-f]dioxocin-1,7-dione. X-Ray analyses of both products are presented.     

Thermolytic carbonates for potential 5'-hydroxyl protection of deoxyribonucleosides

Thermolytic groups structurally related to well-studied heat-sensitive phosphate/thiophosphate protecting groups have been evaluated for 5'-hydroxyl protection of deoxyribonucleosides as carbonates and for potential use in solid-phase oligonucleotide synthesis. The spatial arrangement of selected functional groups forming an asymmetric nucleosidic 5'-O-carbonic acid ester has been designed to enable heat-induced cyclodecarbonation reactions, which would result in the release of carbon dioxide and the generation of a nucleosidic 5'-hydroxyl group. The nucleosidic 5'-O-carbonates 3-8, 10-15, and 19-21 were prepared and were isolated in yields ranging from 45 to 83%. Thermolytic deprotection of these carbonates is preferably performed in aqueous organic solvent at 90 degrees C under near neutral conditions. The rates of carbonate deprotection are dependent on the nucleophilicity of the functional group involved in the postulated cyclodecarbonation reaction and on solvent polarity. Deprotection kinetics increase according to the following order: 4 < 5 < 10 < 6 < 12 < 7 < 13 < 8 < 14 congruent with 19-21 and CCl4 < dioxane < MeCN < t-BuOH < MeCN:phosphate buffer (3:1 v/v, pH 7.0) < EtOH:phosphate buffer (1:1 v/v, pH 7.0). Complete thermolytic deprotection of carbonates 7, 8, 13, and 14 is achieved within 20 min to 2 h under optimal conditions in phosphate buffer-MeCN. The 2-(2-pyridyl)amino-1-phenylethyl and 2-[N-methyl-N-(2-pyridyl)]aminoethyl groups are particularly promising for 5'-hydroxyl protection of deoxyribonucleosides as thermolytic carbonates.