Novel triterpenoids from the aerial roots of Ficus microcarpa

Three novel triterpenoids, 3beta-acetoxy-11alpha-hydroxy-11(12-->13)abeooleanan-12-al (1), 3beta-hydroxy-20-oxo-29(20-->19)abeolupane (2), and 29,30-dinor-3beta-acetoxy-18,19-dioxo-18,19-secolupane (3), and the known 4, 5a, and 5b were isolated from the aerial roots of Ficus microcarpa. Their structures were elucidated on the basis of 2D NMR and X-ray diffraction experiments. Compound 1, derived from the oleanane skeleton, has an unusual five-membered C ring. Compounds 2 and 3, derived from the lupane skeleton, have unique skeletons that may arise from the same biogenetic pathway.     

Flash photolytic generation of o-quinone alpha-phenylmethide and o-quinone alpha-(p-anisyl)methide in aqueous solution and investigation of their reactions in that medium. Saturation of acid-catalyzed hydration.

o-quinone alpha-phenylmethide was generated as a short-lived transient species in aqueous solution by flash photolysis of o-hydroxy-alpha-phenylbenzyl alcohol, and its rate of decay was measured in HClO4 and NaOH solutions as well as in CH3CO2H, H2PO4-, and HCO3- buffers. These data show that hydration of this quinone methide back to its benzyl alcohol precursor occurs by acid-, base-, and uncatalyzed routes. The acid-catalyzed reaction gives the solvent isotope effect kH+/kD+ = 0.34, whose inverse nature indicates that this reaction occurs via rapid preequilibrium protonation of the quinone methide on its carbonyl oxygen atom followed by rate-determining capture of the ensuing carbocationic intermediate by water, a conclusion supported by the saturation of acid catalysis in concentrated HClO4 solution. o-quinone alpha-(p-anisyl)methide was also generated by flash photolysis of the corresponding benzyl alcohol and of the p-cyanophenol ether of this alcohol as well, and its rate of decay was measured in HClO4 and NaOH solutions and in HCO2H, CH3CO2H, HN3, CF3CH2NH3+, imidazolium ion, H2PO4-, (CH2OH)3CNH3+, (CH3)3CPO3H-, and HCO3- buffers. Acid-, base-, and uncatalyzed hydration reaction routes were again found, and solvent isotope effects as well as saturation of acid catalysis, this time in dilute HClO4, confirmed a preequilibrium mechanism for the acid-catalyzed reaction. Analysis of the buffer data gave buffer-base rate constants that did not conform to the Br?nsted relation, consistent with the expected nucleophilic nature of the buffer reactions.     

Flash Photolysis of 5-Methyl-1,4-naphthoquinone in Aqueous Solution: Kinetics and mechanism of photoenolization and of enol trapping

5-Methyl-1,4-naphthoquinone ( 1 ) is a remarkable probe to study hydrogen and proton transfer reactions. The photoenol 4-hydroxy-5-methylidene naphthalen-1 (5H)-one ( 2 ) is formed in the ground state within 2 ps of excitation and with a quantum yield of unity, presumably through a conical intersection of the S₀ and S₁ hypersurfaces. In aqueous acid, enol 2 is hydrated to 5-(hydroxymethyl)naphthalene-1,4-diol 3 (X ? OH, Scheme 1). The rate of hydration of 2 increases linearly with acid concentration from ca. 1.5 × 10⁴ s^?1 at pH 6 to reach a maximum value of 9 ×10⁷ s^?1 when the remaining carbonyl function is protonated, pK_a(2⁺) = 1.1. Contrary to an earlier suggestion, the rate-determining step in the acid-catalyzed hydration of 2 is addition of water to the conjugate acid 2 ⁺. Pronounced acceleration of the decay rate of 2 by hydrazoic-acid buffers indicates competitive trapping of 2 ⁺ by the azide ion. In neutral-to-weakly-basic solutions, enol 2 reacts by ionization, pK^a( 2 ) = 6.5, and nearly diffusion-controlled condensation of the carbanionic species 2 ^? with quinone 1 . Proto-nation at the methylidene C-atom does not compete measurably with protonation on carbonyl O-atom, despite a Substsial thermodynamic basic for carbon Portoation of ca. 50 kJ mol^?1 for 2 and 100 mol^?1 for 2^?.     

Structure and topography of molecular assemblies on solid substrates by infrared spectroscopy and scanning tunneling microscopy

Recently we have combined infrared spectroscopy and atomic resolution scanning tunneling microscopy (STM) to probe the local structure and intermolecular arrangement of molecules within thin films. IR spectroscopy provides spatially averaged information about orientation of the molecules with respect to the surface and about intermolecular arrangement within the crystallographic unit cell. STM data yields a local picture of molecular packing within the film. The requirements of an atomically flat (over distances of hundreds of angstroms) conducting substrate for the STM are fulfilled by an epitaxially grown film of gold on a cleaved mica substrate which also provides a good infrared reflective surface, enabling IR and STM measurements on identical samples. Systems investigated include Langmuir-Blodgett films of cadmium arachidate and self-assembled films of octadecyltrichlorosilane.     

The first A-nor-hippuristanol and two novel 4,5-secosuberosanoids from the Gorgonian Isis hippuris

The first A-nor-hippuristanol, A-nor-22-epi-hippurin-2α-carboxylic acid (1), and two 4,5-secosuberosane sesquiterpenoids, isishippuric acids A and B (2 and 3), have been isolated from the gorgonian coral Isis hippuris. Those structures were deduced by extensive 1D and 2D NMR studies. The structure of 1 was further supported by a single crystal X-ray diffraction analysis. Isishippuric acid B has been shown to exhibit potent cytotoxicity toward a limited panel of cancer cells.     

Peri-bridged naphthalenes. 4. chalcogen-bridged acenaphthylenes

Three new electron donors, acenaphtho[5,6-$\text{cd}$]-1,2-dithiole ($\text{1}$), acenaphtho[5,6-$\text{cd}$]-1,2-diselenole ($\text{2}$), and acenaphtho[5,6-$\text{cd}$]-1,2-ditellurole ($\text{3}$), can be prepared in 28%, 22%, and 14% yields respectively by reaction of the elemental chalcogens (S, Se, Te) with 5,6-dilithioacenaphthylene ($\text{4}$). Compound $\text{4}$ is generated by treatment of 5,6-dibromoacenaphthylen ($\text{5}$), for which a convenient preparation is described, with $\text{n}$-butyllithium (2 equiv.) in THF at ?78°C.     

Intense near-infrared optical absorbing emerald green [60]fullerenes

Synthesis of emerald green fullerenes (EF) C60[C(CH3)(CO2Et)2]6 and C60[C(CH3)(CO2-t-Bu)2]6 was performed by using hexaanionic C60 intermediate (C60-6) as a reagent in one-pot reaction for attaching six alkyl ester addends on one C60 cage. These EF compounds exhibit intense optical absorption over 600-940 nm, the longest optical absorption of the C60 cage among many [60]fullerene derivatives synthesized.     

Efficient epoxidation of alkenes with aqueous hydrogen peroxide catalyzed by methyltrioxorhenium and 3-cyanopyridine

The epoxidation of alkenes with 30% aqueous hydrogen peroxide is catalyzed efficiently by methyltrioxorhenium (MTO) in the presence of pyridine additives. The addition of 1-10 mol % of 3-cyanopyridine increases the system's efficiency for terminal and trans-disubstituted alkenes resulting in high isolated yields of the corresponding epoxides. The system allows for epoxidation of alkenes with various functional groups. Alkenes leading to acid-sensitive products are efficiently epoxidized using a mixture of pyridine and 3-cyanopyridine as additives. This method is operationally very simple and uses an environmentally benign oxidant. The effects of different pyridine additives on the alkene conversion and the catalyst lifetime are discussed.     

Preparations and structures of titanium(IV) complexes containing 8-hydroxyquinolinate and a Schiff-base N-(2-oxyphenyl)salicylideneiminate

In methanol, the reaction of Ti(OⁱPr)₄, N-(2-hydroxyphenyl)salicylideneimine (H₂Sap) and 8-hydroxyquinoline (HQ) in stoichiometric ratio 1:1:x yielded Ti(Sap)₂ precipitate as initial product even when x was as high as 10. However, when the reaction mixture with x = 2 was left standing for 12 h or more, a small amount of red crystalline Ti(Sap)Q(OMe) was isolated. Addition of wet acetonitrile to the reaction mixture with x = 10, small amount of another red crystalline [Ti(Sap)Q]₂(μ-O) was obtained after standing for 2 days. The reaction between TiQ₂(OⁱPr)₂ and H₂Sap in methanol with stoichiometric ratio of y:1 also yielded Ti(Sap)₂ as initial product even for y as large as 10. ¹H NMR investigation of the reaction of TiQ₂(OMe)₂ with H₂Sap revealed that Ti(Sap)Q(OMe) was not detected initially. These experimental results can be explained based on a mechanism that includes: (i) rapid reaction of H₂Sap with Ti(IV) centers to form Ti(Sap)₂; (ii) equilibrium between TiQ₂(OMe)₂ and Ti(Sap)Q(OMe); (iii) equilibrium between Ti(Sap)Q(OR) and Ti(Sap)₂; and (iv) limited solubilities of Ti(Sap)Q(OR) and Ti(Sap)₂. The equilibrium constants and solubilities in the mechanism were determined by the ¹H NMR spectral method. The structures of Ti(Sap)Q(OMe) and [Ti(Sap)Q]₂(μ-O), consisting octahedrally coordinated Ti(IV), were determined by X-ray diffraction method.     

A Sequential Extraction Method Measures the Toxic Metal Content in Fly Ash from a Municipal Solid Waste Incinerator

The purpose of this research was to develop an optimized pretreatment procedure for toxic metals (Pb, Cd, Zn and Cu) content in fly ash from a municipal waste incinerator. In addition, modified sequential extraction procedures were used to characterize the chemical composition of the fly ash samples. The sequential extraction resolved the fly ash elements into the following chemical forms: soluble, exchangeable, carbonate, oxide, organic, and silicate compounds. Certified reference city waste incineration ash (BCR.176) was used as target ash samples. A H₂O₂+HNO₃+HF mixed acid digestion solution with a low temperature evaporation procedure was selected as optimal for the fly ash digestion. The digested solution was analyzed by inductively coupled plasma mass spectrometry (ICP‐MS), which effectively determined the concentrations of the toxic metal elements in BCR.176. Except for Cd, the recovery of Pb, Zn, and Cu under H₂O₂+HNO₃+HF digestion and their sequential extraction procedures were higher than 95%. The relative standard deviations (RSD) for recoveries of the four elements were within 10%. Furthermore, the sequential extraction procedure's results provided information on the potential mobility of the studied elements. Most of the Cd was bound to water‐soluble and carbonate material in the fly ash samples. Most of the Pb, Zn, and Cu was released to carbonates and bound to organic matter in the fly ash samples.