Carbodiimide-mediated preparation of the tricyclic pyrido[3',2':4,5]pyrrolo[1,2-c]pyrimidine ring system and its application to the synthesis of the potent antitumoral marine alkaloid variolin B and analog

A total synthesis of the marine alkaloid variolin B has been completed in 13 steps in an overall yield of 6.5% from 3-formyl-4-methoxypyridine. Our approach is based on the sequential formation of the 7-azaindole ring, the tricyclic pyrido[3',2':4,5]pyrrolo[1,2-c]pyrimidine ring system, and finally installation of the 2-aminopyrimidine ring at C5. The required 7-azaindole ring appropriately substituted is formed by a modified indole synthesis involving a nitrene insertion process (two steps). Formation of the annelated pyrimidine ring is achieved by two routes both involving a carbodiimide-mediated cyclization process, which allow incorporation of the amine functionality at C9 of the core tricyclic (six steps). Installation of the northeast 2-aminopyrimidine ring at C5 is performed using the Bredereck protocol (three steps). Ultimate, thermal decarboxylation with concomitant O-methyl deprotection and further N-benzyl deprotection by the action of triflic acid completed the synthesis of the target natural product variolin B.     

Electrochemical study of taxol (paclitaxel) by cathodic stripping voltammetry: determination in human urine

An analytical method for the determination of taxol (paclitaxel) via cathodic stripping square wave voltammetry has been developed. The method allows to achieve a detection limit of 5.2 ng ml⁻¹ and a determination limit of 11.0 ng ml⁻¹ working with a mercury drop electrode in 0.05 M boric acid/borate buffer pH 9.0, previously accumulated at −0.1 V (v.s. Ag/AgCl/KCl 3 M) for 360 s. Moreover, information about the mechanisms governing the reduction of taxol has been obtained from the studies of the instrumental parameters of square wave voltammetry technique. After addition of an internal standard, taxol was extracted from spiked human urine and the proposed method was successfully applied to the quantification of the analyte.     

Host-guest chemistry. 2. Amine inclusion compounds of 2-[o-(triphenylphosphoranylidenamino)benzyliden]amino-1H-2,3-dihydroindazol-3-one. X-ray structure of its 1∶1∶1 inclusion complex with isopropylamine and water

The crystal and molecular structure is reported for the inclusion compound 2-[o-(triphenylphosphoranylidenamino)benzyliden]amino-1H-2,3-dihydroindazol-3-one/isopropylamine/water3b. The crystal structure consists of discrete dimeric salt-like aggregates joined together by strong N⁺–H...^–O–C hydrogen bonds between pairs of centrosymmetrically-related indazolonate anions and isopropylammonium cations. Six other inclusion compounds have been prepared and characterized by NMR [with propylamine (3a), withtert-butylamine (3c), withsec-butylamine (3d), withtert-pentylamine (3e), with 1-methylbutylamine (3f) and withiso-pentylamine (3g)]. Two different arrangements are found, both with the host being in the anionic form. The guests are either: (i) one protonated amine and one water molecule (3b and3f); or (ii) one protonated amine and the corresponding neutral amine (3a, 3c, 3d, 3e and3g). Supplementary Data relating to this article (structure factors, thermal components, hydrogen parameters and bond distances and angles, and¹³C-NMR shifts) are deposited with the British Library at Boston Spa, Wetherby, West Yorkshire, U.K., as Supplementary Publication No. SUP 82155 (23 pages).For Part 1, see Reference [1].     

Micellar electrokinetic chromatography: current developments and future

This review highlights recent methodological and instrumental advances in micellar electrokinetic chromatography (MEKC). Enhancements in sensitivity and selectivity of the technique through the use of on-line preconcentration approaches (stacking and sweeping) and nonconventional pseudostationary phases, namely nonionic and zwitterionic surfactants, mixed micelles and polymers, are discussed in detail. Laser-induced fluorescence and mass spectrometry, as alternatives to UV-absorption detection, have been covered to evaluate their advantages and limitations when applied to analysis in an MEKC format. Some thoughts on future directions in this area such as in-capillary reactions, coated capillaries and MEKC on microchips are also presented.     

Application of the superposition principle to the study of multistep electrode processes and systems with several components in chronopotentiometry with programmed current. Part I

Summary In this paper we present the solution to the differential equations system corresponding to the problem of multistep processes and systems with several components when a programmed current-time function is applied to planar and spherical electrodes. The analytical solutions have been obtained in both cases, by a recurrent procedure, which facilitates the numerical determination of results. This solution has been found for an expanding spherical electrode, since by using simple substitutions it is transformed into that corresponding to a stationary spherical electrode, to an expanding plane electrode or to a stationary plane electrode.     

Synthesis,tautomerism and diels-alder reactions of 1, 4-dihydroxy-9, 10-anthraquinon-9-imines

The ammonolysis of quinizarin and simple derivatives thereof affords 9-imino-derivatives, the tautomeric forms of which act as dienophiles in Diels-Alder reactions to yield tetracyclic compounds of potential value as anthracyclinone precursors.     

Solid state features of electrocrystallized tungstate films

X-ray diffraction, scanning electron microscopy, and infra-red spectroscopy are applied to study the evolution of films electrodeposited from acidic tungstate solutions. Structural inhomogeneity is found to be responsible for the difference in rechargeability of films of different thickness. Voltammetric responses demonstrate pronounced sensitivity to the nature of crystalline phases, thus throwing light on the defects of the lattice features of nonstoichiometric W(V)–W(VI) oxides. One crystalline phase observed in the films under study and attributed to the layered nonstoichiometric oxohydroxide was never reported for oxotungstate films fabricated by other techniques. This phase is believed to be special to electrocrystallized films and to keep some structural features of dissolved isopolytungstate molecular precursors.Dedicated to Prof. G.Horanyi with the highest respect to his experimental talents, remarkable papers and important contributions in various fields of modern electrochemistry.     

New strained ferrocenophane-based receptors for the selective electrochemical recognition of Mg in the presence of Ca cations

Ferrocenophane derivatives 3 act as electrochemical sensors of Mg²⁺ ions: a new redox peak appears in the CVs anodically shifted compared to the E_1/2 of the free receptors by 268-382 mV. Receptors 3a, 3c and 3d are selective for Mg²⁺, whilst 3b is also responsive to Ca²⁺ ion (302 mV shift). Receptor 3c also gave a highly visual output response in the presence of Mg²⁺ consisting of a deep purple colour.     

Synthesis and properties of a new class of nitrogen-rich multinuclear[m.n] ferrocenophanes

The synthesis of the first tetraaza[3,3]ferrocenophane, from the previously unreported 1,1'-bis(triphenylphosphoranylidenamino)ferrocene and its conversion into a nitrogen-rich [2,2]bis[3,3]ferrocenophane is described. The electronic behaviour of these multinuclear ferrocenophanes indicates a strong electronic coupling between the iron centers.     

Computer simulation of cluster assembling

Density functional theory is used to investigate the assembling of metallic clusters to yield stable or metastable cluster solids. Motivated by the observed high stability of the Al₁₃H cluster, which has a substantial highest occupied and lowest unoccupied molecular orbitals (HOMO–LUMO) gap, we have modeled the assembling of those clusters. For a favorable relative orientation of each cluster with respect to all its neighbors, a cluster solid is predicted and its structure appears to be stable at least up to 150 K, which is the highest temperature in our simulations. We have also studied the chemical bonding in the stoichiometric solid alloys PbA, where A is one of the alkali elements Na, K, Rb, or Cs. Those crystals exist in an ordered phase formed by tetrahedral Pb₄ clusters surrounded by the alkali atoms. The study of this family of natural cluster compounds reveals the coating role played by the cations, providing further insight into the favorable conditions required for the formation of cluster solids. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001