bluuebunny/crossref_metadata_2025_split huggingface dataset:
Ground magnetic field fluctuations associated with pulsating aurora
Authors: Michell R. G. , Samara M. 
Published: October 2015

AbstractA case study of an intense pulsating auroral event is presented where the large‐scale (100–200 km) optical intensity variations are anticorrelated with fluctuations in the ground magnetometer data at a frequency of 0.1 Hz. The auroral event occurred over Poker Flat, Alaska, on 1 March 2012 and was imaged optically with several different fields of view and filters. The fluctuations in the magnetometer data were most prominent in the D component and had magnitudes of 1 to 5 nT. The auroral intensity variations had amplitudes of 200 to 400 R, comprising 25% to 50% of the total auroral luminosity at 427.8 nm. The direction of the magnetometer deflections is consistent with a south‐to‐north ionospheric current present when each pulsation is on, thus providing closure for the field‐aligned currents associated with each of the pulsating patches.
Spatial variability of absorption coefficients over a biogeochemical gradient in a large and optically complex shallow lake
Authors: Riddick Caitlin A. L. , Hunter Peter D. , Tyler Andrew N. , Martinez‐Vicente Victor , Horváth Hajnalka , Kovács Attila W. , Vörös Lajos , Preston Tom , Présing Mátyás 
Published: October 2015

AbstractIn order to improve robustness of remote sensing algorithms for lakes, it is vital to understand the variability of inherent optical properties (IOPs) and their mass‐specific representations (SIOPs). In this study, absorption coefficients for particulate and dissolved constituents were measured at 38 stations distributed over a biogeochemical gradient in Lake Balaton, Hungary. There was a large range of phytoplankton absorption (aph(λ)) over blue and red wavelengths (aph(440) = 0.11–4.39 m−1, aph(675) = 0.048–2.52 m−1), while there was less variability in chlorophyll‐specific phytoplankton absorption (a*ph(λ)) in the lake (a*ph(440) = 0.022 ± 0.0046 m2 mg−1, a*ph(675) = 0.010 ± 0.0020 m2 mg−1) and adjoining wetland system, Kis‐Balaton (a*ph(440) = 0.017 ± 0.0015 m2 mg−1, a*ph(675) = 0.0088 ± 0.0017 m2 mg−1). However, in the UV, a*ph(350) significantly increased with increasing distance from the main inflow (Zala River). This was likely due to variable production of photoprotective pigments (e.g., MAAs) in response to the decreasing gradient of colored dissolved organic matter (CDOM). The slope of CDOM absorption (SCDOM) also increased from west to east due to larger terrestrial CDOM input in the western basins. Absorption by nonalgal particles (aNAP(λ)) was highly influenced by inorganic particulates, as a result of the largely mineral sediments in Balaton. The relative contributions to the absorption budget varied more widely than oceans with a greater contribution from NAP (up to 30%), and wind speed affected the proportion attributed to NAP, phytoplankton, or CDOM. Ultimately, these data provide knowledge of the heterogeneity of (S)IOPs in Lake Balaton, suggesting the full range of variability must be considered for future improvement of analytical algorithms for constituent retrieval in inland waters.
Novel Synthesis and Characterization of N‐Heterocyclic Azo Dyes with Acrylic Polymer for Thin Film Coatings
Authors: Walekar Balaji J. , Mane Sambhaji R. , Bhosale Popatrao N. 
Published: October 2015

ABSTRACTNitrogen‐containing heterocyclic compounds are an important class of compounds in dyeing and textile industries. New nitrogen containing heterocyclic azo compounds have been synthesized by using gold nanoparticles (AuNPs) as a catalyst having a particle size of 15 nm. AuNPs play an important role in the coupling reaction to produce new heterocyclic azo pigments. These azo pigments were tested to solubility, light fastness, and durability. These synthesized nitrogen‐containing heterocyclic pigments were characterized by melting points, UV–vis, FT‐IR, and 1H NMR spectroscopy for their characteristics and structural features. After analysis, these pigments at lower concentrations were blended with the n‐butyl acrylate–co‐methyl methacrylate (n‐BA‐co‐MMA) polymer. These blended pigments were coated on a simple silica glass surface by following the simple deep and dry method. The coupled azo pigment shows excellent binding strength with poly(n‐BA‐co‐MMA) acrylic polymers.
Investigation of Microstructure, Morphology, Mechanical, and Dielectric Properties of PVA/PbO Nanocomposites
Authors: Pasha S. K. Khadheer , Deshmukh Kalim , Ahamed M. Basheer , Chidambaram K. , Mohanapriya M. K. , Raj N. Arunai Nambi 
Published: October 2015

ABSTRACTThe present article deals with the preparation and characterization of pure and lead oxide (PbO) nanoparticles embedded polyvinyl alcohol (PVA) films by using a colloidal processing technique. PbO nanoparticles were successfully synthesized using the simple precipitation method. Polymer/ceramic‐based flexible and self‐standing films were obtained and further characterized using various analytical techniques. The mechanical and dielectric properties were also investigated. The Fourier Transform Infrared Spectroscopy (FTIR) results indicate that the structural characterization of PVA is strongly affected by the incorporation of PbO. Thermal analysis results indicate that the thermal stability of the PbO‐doped PVA film has improved as compared with the neat PVA film. The mechanical property of nanocomposites has improved significantly due to an increase in filler loadings, indicating that a good interaction exists between PbO nanoparticles and PVA matrix. The dielectric constant of PVA/PbO nanocomposites has significantly improved with comparatively low dielectric loss values, indicating that the nanocomposites can be considered as an attractive material for embedded capacitor applications.
Heterophasen‐Photokatalysatoren aus wasserlöslichen Polyelektrolyten: ein Beispiel für die Selbstinitiierung unter sichtbarem Licht
Authors: Ghasimi Saman , Prescher Simon , Wang Zi Jun , Landfester Katharina , Yuan Jiayin , Zhang Kai A. I. 
Published: October 2015

AbstractEin neues Konzept zur Herstellung stabiler Heterophasen‐Photokatalysatoren, die sowohl als hoch dispergierbare konjugierte Polymernanopartikel wie auch als poröse Monolithe im wässrigen Medium durch Bestrahlung von sichtbarem Licht funktionieren, wird vorgestellt. Beim Aufbau dieser Materialien wird 1‐Alkyl‐3‐vinylimidazoliumbromid, eine typische Einheit der ionischen Flüssigkeiten, in die Seitenketten eines photoaktiven Polymers eingebracht. Diese Strukturkonfiguration ermöglicht neben der Photokatalyse in wässriger Umgebung auch die Herstellung von Nanopartikeln oder porösen Monolithen. Durch die einzigartige Selbstinitiierung und anschließende radikalische Vernetzung wird das wasserlösliche photoaktive Polymer in einen Heterophasen‐System umgewandelt. Die hohe photokatalytische Aktivität und die Wiederverwendbarkeit des Heterophasen‐Systems wurden anhand der Zersetzung organischer Farbstoffe und der Reduktion von CrVI zu CrIII in Wasser unter Bestrahlung mit sichtbarem Licht demonstriert.
Transport in nanoporösen Materialien, einschließlich MOFs: über die Anwendbarkeit der Fickschen Gesetze
Authors: Titze Tobias , Lauerer Alexander , Heinke Lars , Chmelik Christian , Zimmermann Nils E. R. , Keil Frerich J. , Ruthven Douglas M. , Kärger Jörg 
Published: October 2015

AbstractDie Diffusion in nanoporösen Wirt‐Gast‐Systemen wird oft als viel zu kompliziert angesehen, als dass sie mit so “einfachen” Beziehungen wie dem ersten und zweiten Fickschen Gesetz vereinbar sein könnte. Wir zeigen nun, dass die mikroskopischen Diffusionsmesstechniken, vor allem die NMR‐Spektroskopie mit gepulsten Feldgradienten (PFG) und die Mikrobildgebung mittels Interferenzmikroskopie (IFM) und IR‐Mikroskopie (IRM), den direkten experimentellen Nachweis für die Anwendbarkeit der Fickschen Gesetze auf solche Systeme liefern. Dies gilt oft auch dann noch, wenn die zugrundeliegenden Transportmechanismen komplex sind. Die Grenzen des Diffusionsmodells werden unter Bezugnahme auf die umfangreiche Literatur diskutiert.
Dipolar Quinoidal Acene Analogues as Stable Isoelectronic Structures of Pentacene and Nonacene
Authors: Shi Xueliang , Kueh Weixiang , Zheng Bin , Huang Kuo‐Wei , Chi Chunyan 
Published: October 2015

AbstractQuinoidal thia‐acene analogues, as the respective isoelectronic structures of pentacene and nonacene, were synthesized and an unusual 1,2‐sulfur migration was observed during the Friedel–Crafts alkylation reaction. The analogues display a closed‐shell quinoidal structure in the ground state with a distinctive dipolar character. In contrast to their acene isoelectronic structures, both compounds are stable because of the existence of more aromatic sextet rings, a dipolar character, and kinetic blocking. They exhibit unique packing in single crystals resulting from balanced dipole–dipole and [CH⋅⋅⋅π]/[CH⋅⋅⋅S] interactions.
Ultrasensitive Direct Quantification of Nucleobase Modifications in DNA by Surface‐Enhanced Raman Scattering: The Case of Cytosine
Authors: Morla‐Folch Judit , Xie Hai‐nan , Gisbert‐Quilis Patricia , Pedro Sara Gómez‐de , Pazos‐Perez Nicolas , Alvarez‐Puebla Ramon A. , Guerrini Luca 
Published: October 2015

AbstractRecognition of chemical modifications in canonical nucleobases of nucleic acids is of key importance since such modified variants act as different genetic encoders, introducing variability in the biological information contained in DNA. Herein, we demonstrate the feasibility of direct SERS in combination with chemometrics and microfluidics for the identification and relative quantification of 4 different cytosine modifications in both single‐ and double‐stranded DNA. The minute amount of DNA required per measurement, in the sub‐nanogram regime, removes the necessity of pre‐amplification or enrichment steps (which are also potential sources of artificial DNA damages). These findings show great potentials for the development of fast, low‐cost and high‐throughput screening analytical devices capable of detecting known and unknown modifications in nucleic acids (DNA and RNA) opening new windows of activity in several fields such as biology, medicine and forensic sciences.
Effect of hydrochloric acid on the properties of biodegradable packaging materials of carboxymethylcellulose/poly(vinyl alcohol) blends
Authors: Harun‐or‐Rashid M. D. , Saifur Rahaman M. D. , Enamul Kabir Shariff , Khan Mubarak A 
Published: October 2015

ABSTRACTThe main focus of this research was to prepare ecofriendly biodegradable packaging materials with carboxymethylcellulose (CMC) and poly(vinyl alcohol) (PVA). Different blend films were prepared through the variation of the ratios of CMC to PVA (00:100, 30:70, 50:50, 70:30, 100:00 w/w) with and without hydrochloric acid. The mechanical properties, including the tensile strength (TS), percentage elongation at break (Eb), water uptake, and solubility in water, of the films were investigated. A soil burial test was also performed. The prepared films were characterized by attenuated total reflectance (ATR)–Fourier transform infrared (FTIR) spectroscopy, differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The highest TS and Eb values of the blend films were recorded as 19.52 MPa and 34.52%, respectively. ATR–FTIR spectroscopy, DSC, and TGA results provided that in the presence of HCl, a chemical reaction occurred between CMC and PVA. In addition, the water uptake, solubility in water, and biodegradability of the films were found to be reduced significantly. With all of these results, in presence of HCl, a blend made up of CMC and PVA with a ratio 30:70 was optimized as a biodegradable packaging material. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 42870.
A comparison of four different conformations adopted by human telomeric G‐quadruplex using computer simulations
Authors: Ray Angana , Panigrahi Swati , Bhattacharyya Dhananjay 
Published: October 2015

ABSTRACTThe telomeric G‐quadruplexes for their unique structural features are considered as potential anticancer drug targets. These, however, exhibit structural polymorphism as different topology types for the intra‐molecular G‐quadruplexes from human telomeric G‐rich sequences have been reported based on NMR spectroscopy and X‐ray crystallography. These techniques provide detailed atomic‐level information about the molecule but relative conformational stability of the different topologies remains unsolved. Therefore, to understand the conformational preference, we have carried out quantum chemical calculations on G‐quartets; used all‐atom molecular dynamics (MD) simulations and steered molecular dynamics (SMD) simulations to characterize the four human telomeric G‐quadruplex topologies based on its G‐tetrad core‐types, viz., parallel, anti‐parallel, mixed‐(3 + 1)‐form1 and mixed‐(3 + 1)‐form2. We have also studied a non‐telomeric sequence along with these telomeric forms giving a comparison between the two G‐rich forms. The structural properties such as base pairing, stacking geometry and backbone conformations have been analyzed. The quantum calculations indicate that presence of a sodium ion inside the G‐tetrad plane or two potassium ions on both sides of the plane give it an overall planarity which is much needed for good stacking to form a helix. MD simulations indicate that capping of the G‐tetrad core by the TTA loops keep the terminal guanine bases away from water. The SMD simulations along with equilibrium MD studies indicate that the parallel and non‐telomeric forms are comparatively less stable. We could come to the conclusion that the anti‐parallel form and also the mixed‐(3 + 1)‐form1 topology are most likely to represent the major conformation., 2016. © 2015 Wiley Periodicals, Inc. Biopolymers 105: 83–99, 2016
Adaptive Correction from Virtually Complex Dynamic Libraries: The Role of Noncovalent Interactions in Structural Selection and Folding
Authors: Lafuente Maria , Atcher Joan , Solà Jordi , Alfonso Ignacio 
Published: October 2015

AbstractThe hierarchical self‐assembling of complex molecular systems is dictated by the chemical and structural information stored in their components. This information can be expressed through an adaptive process that determines the structurally fittest assembly under given environmental conditions. We have set up complex disulfide‐based dynamic covalent libraries of chemically and topologically diverse pseudopeptidic compounds. We show how the reaction evolves from very complex mixtures at short reaction times to the almost exclusive formation of a major compound, through the establishment of intramolecular noncovalent interactions. Our experiments demonstrate that the systems evolve through error‐check and error‐correction processes. The nature of these interactions, the importance of the folding and the effects of the environment are also discussed.
[{Ni4(OH)3AsO4}4(B‐α‐PW9O34)4]28−: A New Polyoxometalate Structural Family with Catalytic Hydrogen Evolution Activity
Authors: Lv Hongjin , Chi Yingnan , van Leusen Jan , Kögerler Paul , Chen Zheyuan , Bacsa John , Geletii Yurii V. , Guo Weiwei , Lian Tianquan , Hill Craig L. 
Published: October 2015

AbstractA new structural polyoxometalate motif, [{Ni4(OH)3AsO4}4(B‐α‐PW9O34)4]28−, which contains the highest nuclearity structurally characterized multi‐nickel‐containing polyanion to date, has been synthesized and characterized by single‐crystal X‐ray diffraction, temperature‐dependent magnetism and several other techniques. The unique central {Ni16(OH)12O4(AsO4)4} core shows dominant ferromagnetic exchange interactions, with maximum χmT of 69.21 cm3 K mol−1 at 3.4 K. Significantly, this structurally unprecedented complex is an efficient, water‐compatible, noble‐metal‐free catalyst for H2 production upon visible light irradiation (photosensitizer=[Ir(ppy)2(dtbbpy)][PF6]; sacrificial electron donor=triethylamine or triethanolamine). The highest turnover number of approximately 580, corresponding to a best quantum yield of approximately 4.07 %, is achieved when using triethylamine as electron donor in the presence of water. The mechanism of this photodriven process has been probed by time‐solved luminescence and by static emission quenching.
ChemInform Abstract: Ti3GaC2 and Ti3InC2: First Bulk Synthesis, DFT Stability Calculations and Structural Systematics.
Authors: Cuskelly Dylan T. , Richards Erin R. , Kisi Erich H. , Keast Vicki J. 
Published: October 2015

AbstractAnalysis of systematic features of MAX phase systems reveal a method for predicting new 312 MAX phases.
ChemInform Abstract: Piezoelectrically‐Induced Stress‐Luminescence Phenomenon in CaAl2O4:Eu2+.
Authors: Wei Yongbin , Wu Zheng , Jia Yanmin , Liu Yongsheng 
Published: October 2015

AbstractCaAl2O4:Eu2+ (1 mol%) is prepared by firing stoichiometric amounts of ball milled CaCO3, α‐Al2O3, and Eu2O3 in 5% H2/Ar (1.
ChemInform Abstract: Na3V2(PO4)3/C Nanocomposite Synthesized via Pre‐Reduction Process as High‐Performance Cathode Material for Sodium‐Ion Batteries.
Authors: Zhu Xiaoming , Fang Yongjin , Ai Xinping , Yang Hanxi , Cao Yuliang 
Published: October 2015

AbstractThe title nanocomposite is prepared by ball milling of a stoichiometric mixture of V2O5, NH4H2PO4, and Na2CO3 together with oxalic acid as reducing agent (V5+→V3+) and sucrose as C precursor (10 h) and subsequent calcination under argon (700 °C, 8 h).
ChemInform Abstract: Unexpected Reactivity of Red Phosphorus in Ionic Liquids.
Authors: Groh Matthias F. , Paasch Silvia , Weiz Alexander , Ruck Michael , Brunner Eike 
Published: October 2015

AbstractReactions of red phosphorus and elemental iodine in [bmim]Cl·2AlCl3 are monitored by 31P liquid‐ and solid state NMR spectroscopy between room temperature and 80 °C.
ChemInform Abstract: Synthesis of MacMillan Catalyst Modified with Ionic Liquid as a Recoverable Catalyst for Asymmetric Diels—Alder Reaction.
Authors: Chauhan ManMohan Singh , Kumar Pramod , Singh Surendra 
Published: October 2015

AbstractThe catalyst can be recovered and reused up to five cycles with only a slight decrease in enantiomeric excess and product conversion.
ChemInform Abstract: Palladium/Zinc Co‐Catalyzed syn‐Stereoselectively Asymmetric Ring‐Opening Reaction of Oxabenzonorbornadienes with Phenols.
Authors: Li Sifeng , Xu Jianbin , Fan Baomin , Lu Zhiwu , Zeng Chaoyuan , Bian Zhaoxiang , Zhou Yongyun , Wang Jun 
Published: October 2015

Abstractleading to cis‐2‐aryloxy‐1,2‐dihydronaphthalen‐1‐ols (III) and (V) with high enantioselectivities
ChemInform Abstract: Use of Elemental Sulfur or Selenium in a Novel One‐Pot Copper‐Catalyzed Tandem Cyclization of Functionalized Ynamides Leading to Benzosultams.
Authors: Siva Reddy Alla , Kumara Swamy K. C. 
Published: October 2015

AbstractA novel copper‐catalyzed regio‐ and stereospecific tandem cyclization of ynamides to benzo[1,4,2]dithiazines and benzo[f][1,5,2]dithiazepines as their 1,1‐dioxides is reported.
ChemInform Abstract: Preparation of Palladium Nanoparticles Using Euphorbia thymifolia L. Leaf Extract and Evaluation of Catalytic Activity in the Ligand‐Free Stille and Hiyama Cross‐Coupling Reactions in Water.
Authors: Nasrollahzadeh Mahmoud , Sajadi S. Mohammad , Honarmand Ebrahim , Maham Mehdi 
Published: October 2015

AbstractA novel, environmentally friendly, and simple one‐step method to synthesize stable Pd nanoparticles is reported.
ChemInform Abstract: One‐Pot Cascade Hydration‐Asymmetric Transfer Hydrogenation as a Practical Strategy to Construct Chiral β‐Adrenergic Receptor Blockers.
Authors: Ye Qunqun , Cheng Tanyu , Zhao Yuxi , Zhao Junwei , Jin Ronghua , Liu Guohua 
Published: October 2015

AbstractThe title process allows for the conversion of aryl‐substituted haloalkynes into chiral halohydrins, useful precursors for the preparation of β‐andrenergic receptor blockers.
ChemInform Abstract: Diaryliodoniums by Rhodium(III)‐Catalyzed C—H Activation: Mild Synthesis and Diversified Functionalizations.
Authors: Xie Fang , Zhang Zhipeng , Yu Xinzhang , Tang Guodong , Li Xingwei 
Published: October 2015

AbstractThe rhodium(III)‐catalyzed C—H hyperiodination of electron‐poor arenes proceeds under chelation assistance at room temperature with high regioselectivity and functional group compatibility.
ChemInform Abstract: Palladium‐Catalyzed Reductive Coupling of Phenols with Anilines and Amines: Efficient Conversion of Phenolic Lignin Model Monomers and Analogues to Cyclohexylamines.
Authors: Chen Zhengwang , Zeng Huiying , Gong Hang , Wang Haining , Li Chao‐Jun 
Published: October 2015

AbstractA wide range of secondary and tertiary substituted cyclohexylamines are conveniently obtained using cheap and safe sodium formate as hydrogen source.
ChemInform Abstract: First Application of Heterogeneous Cobalt Catalysis in Csp2‐N Cross‐Coupling of Activated Chloroarenes under Ligand‐Free Conditions.
Authors: Mukherjee Nirmalya , Chatterjee Tanmay , Ranu Brindaban C. 
Published: October 2015

AbstractFunctionalized tertiary amines are obtained by N‐arylation of N‐heterocycles and aliphatic secondary amines with chloroarenes and chloroheteroarenes.
ChemInform Abstract: Catalytic Asymmetric α‐Amination of α‐Branched Ketones via Enol Catalysis.
Authors: Shevchenko Grigory A. , Pupo Gabriele , List Benjamin 
Published: October 2015

AbstractA chiral anthracenylbinaphthyl‐derived phosphoric acid catalyzes the title reaction in a regio‐ and enantioselective manner.
ChemInform Abstract: Highly Enantioselective Sulfa‐Michael Addition Reactions Using N‐Heterocyclic Carbene as a Non‐covalent Organocatalyst.
Authors: Chen Jiean , Meng Sixuan , Wang Leming , Tang Hongmei , Huang Yong 
Published: October 2015

AbstractA triazolium salt derived NHC serves as an excellent Broensted base to promote the enantioselective carbon‐sulfur bond formation in the asymmetric sulfa‐Michael addition.
ChemInform Abstract: Highly Active Bimetallic Nickel‐Palladium Alloy Nanoparticle Catalyzed Suzuki‐Miyaura Reactions.
Authors: Rai Rohit Kumar , Gupta Kavita , Behrens Silke , Li Jun , Xu Qiang , Singh Sanjay Kumar 
Published: October 2015

AbstractThe Ni‐Pd nanoparticle catalyst with low palladium content efficiently catalyzes the coupling of aryl bromides and iodides with arylboronic acids in water/ethanol solvent at moderate reaction temperatures.
ChemInform Abstract: Facile Synthesis of 2,2′‐Dinitrosubstituted Biaryls Through Cu‐Catalyzed Ligand‐Free Decarboxylative Homocoupling of ortho‐Nitrobenzoic Acids.
Authors: Fu Zhengjiang , Li Zhaojie , Xiong Qiheng , Cai Hu 
Published: October 2015

AbstractThe reaction tolerates various substituents in the meta‐ and the para‐position of the ortho‐nitrobenzoic acids, and the products are formed in moderate to high yields.
ChemInform Abstract: Enantiopure trans‐4,5‐Disubstituted 2‐Imidazolidinones via Copper(I)‐Catalyzed Ring Opening of 1,1′‐DiBoc‐2,2′‐Biaziridine with Grignard Reagents.
Authors: Kennedy Matthew D. , Bailey Stephen J. , Wales Steven M. , Keller Paul A. 
Published: October 2015

AbstractRing opening of 1,1′‐di‐Boc‐2,2′‐biaziridines with alkyl, cyclic, and aryl Grignard reagents affords imidazolidinones via a cascade process involving one of the Boc protecting groups.
ChemInform Abstract: A General and Practical Route to 4,5‐Disubstituted Oxazoles Using Acid Chlorides and Isocyanides.
Authors: Tiwari Dipak Kumar , Pogula Jaya , Tiwari Dharmendra Kumar 
Published: October 2015

AbstractAn efficient and mild method for the synthesis of 4,5‐disubstituted oxazoles via [3 + 2] cycloaddition of isocyanides with acid chlorides (I) is developed.
ChemInform Abstract: New 3,4,5‐Trisubstituted Isoxazole Derivatives with Potential Biological Properties.
Authors: Bustos Carlos , Molins Elies , Carcamo Juan‐Guillermo , Aguilar Marcelo N. , Sanchez Christian , Moreno‐Villoslada Ignacio , Nishide Hiroyuki , Mesias‐Salazar Angela , Zarate Ximena , Schott Eduardo 
Published: October 2015

AbstractTwo of the entitled compounds (IIa) and (IIb) exhibit significant cytotoxic activity.