Microwave assisted synthesis of 2-aminooxazolo [4,5-b] pyridine derivatives via intramolecular C–O bond formation in aqueous medium

A highly, efficient synthetic protocol for the synthesis of 2-aminooxazolo[4,5-b]pyridine derivatives is established via intramolecular C–O bond coupling using copper iodide as a catalyst and water as solvent. A variety of functionalized substrates were found to react under this reaction conditions to provide products in good to excellent yields.     

A-posteriori residual bounds for Arnoldi’s methods for nonsymmetric eigenvalue problems

Convergence of the implicitly restarted Arnoldi (IRA) method for nonsymmetric eigenvalue problems has often been studied by deriving bounds for the angle between a desired eigenvector and the Krylov projection subspace. Bounds for residual norms of approximate eigenvectors have been less studied and this paper derives a new a-posteriori residual bound for nonsymmetric matrices with simple eigenvalues. The residual vector is shown to be a linear combination of exact eigenvectors and a residual bound is obtained as the sum of the magnitudes of the coefficients of the eigenvectors. We numerically illustrate that the convergence of the residual norm to zero is governed by a scalar term, namely the last element of the wanted eigenvector of the projected matrix. Both cases of convergence and non-convergence are illustrated and this validates our theoretical results. We derive an analogous result for implicitly restarted refined Arnoldi (IRRA) and for this algorithm, we numerically illustrate that convergence is governed by two scalar terms appearing in the linear combination which drives the residual norm to zero. We provide a set of numerical results that validate the residual bounds for both variants of Arnoldi methods.     

Unprecedented Selectivity of Ruthenium Iodide Benzylidenes in Olefin Metathesis Reactions

The development of selective olefin metathesis catalysts is crucial to achieving new synthetic pathways. Herein, we show that cis‐diiodo/sulfur‐chelated ruthenium benzylidenes do not react with strained cycloalkenes and internal olefins, but can effectively catalyze metathesis reactions of terminal dienes. Surprisingly, internal olefins may partake in olefin metathesis reactions once the ruthenium methylidene intermediate has been generated. This unexpected behavior allows the facile formation of strained cis‐cyclooctene by the RCM reaction of 1,9‐undecadiene. Moreover, cis‐1,4‐polybutadiene may be transformed into small cyclic molecules, including its smallest precursor, 1,5‐cyclooctadiene, by the use of this novel sequence. Norbornenes, including the reactive dicyclopentadiene (DCPD), remain unscathed even in the presence of terminal olefin substrates as they are too bulky to approach the diiodo ruthenium methylidene. The experimental results are accompanied by thorough DFT calculations.     

Finding minimum-cost flows by double scaling

Several researchers have recently developed new techniques that give fast algorithms for the minimum-cost flow problem. In this paper we combine several of these techniques to yield an algorithm running in O(nm(log logU) log(nC)) time on networks withn vertices,m edges, maximum arc capacityU, and maximum arc cost magnitudeC. The major techniques used are the capacity-scaling approach of Edmonds and Karp, the excess-scaling approach of Ahuja and Orlin, the cost-scaling approach of Goldberg and Tarjan, and the dynamic tree data structure of Sleator and Tarjan. For nonsparse graphs with large maximum arc capacity, we obtain a similar but slightly better bound. We also obtain a slightly better bound for the (noncapacitated) transportation problem. In addition, we discuss a capacity-bounding approach to the minimum-cost flow problem.Research partially supported by an NSF Presidential Young Investigator Fellowship, Contract 8451517ECS, and grants from Analog Devices, Apple Computer Inc., and Prime Computer.On leave from Indian Institute of Technology, Kanpur, India.Research partially supported by an NSF Presidential Young Investigator Award.Research at Princeton University partially supported by National Science Foundation Grant DCR-8605962 and Office of Naval Research Contract N00014-87-K-0467.     

Structure and stability of BN microclusters: Ab initio calculations for (BN)n (n = 2–4)

Unrestricted Hartree–Fock calcuations coupled with second-order Møller–Plesset correlation correction were performed to study the structures and energetics of microlusters. For (BN)₂, linear and rhombus forms are almost isoenergetic, whereas cyclic forms are preferable for (BN)₃ and (BN)₄ clusters. As a general trend, linear isomers prefer the triplet spin state, whereas cyclic isomers prefer the singlet spin state. Total charge density plots show a strong dominace of the B? N bond, indicating that the extent of its polar character becomes stronger with the increase in the cluster size. The loss of a BN monomer is shown to be the most likely fragmentation channel for both neutral and single-ionized clusters. We find that neutral (BN)_n clusters have the same structural configurations as those of their corresponding C_2n counterparts. This similarity follows the isoelectronic principle and is of importance due to recent interest in the investigations of BN fullerene analogs. © 1994 John Wiley & Sons, Inc.     

Protein crystallography beamline (PX‐BL21) at Indus‐2 synchrotron

The protein crystallography beamline (PX‐BL21), installed at the 1.5 T bending‐magnet port at the Indian synchrotron (Indus‐2), is now available to users. The beamline can be used for X‐ray diffraction measurements on a single crystal of macromolecules such as proteins, nucleic acids and their complexes. PX‐BL21 has a working energy range of 5–20 keV for accessing the absorption edges of heavy elements commonly used for phasing. A double‐crystal monochromator [Si(111) and Si(220)] and a pair of rhodium‐coated X‐ray mirrors are used for beam monochromatization and manipulation, respectively. This beamline is equipped with a single‐axis goniometer, Rayonix MX225 CCD detector, fluorescence detector, cryogenic sample cooler and automated sample changer. Additional user facilities include a workstation for on‐site data processing and a biochemistry laboratory for sample preparation. In this article the beamline, other facilities and some recent scientific results are briefly described.     

Highly selective synthesis of the ring-B reduced chlorins by ferric chloride-mediated oxidation of bacteriochlorins: effects of the fused imide vs isocyclic ring on photophysical and electrochemical properties

The oxidation of bacteriopyropheophorbide with ferric chloride hexahydrate or its anhydrous form produced the ring-D oxidized (ring-B reduced) chlorin in >95% yield. Replacing the five-member isocyclic ring in bacteriopyropheophorbide- a with a fused six-member N-butylimide ring system made no difference in regioselective oxidation, and the corresponding ring-B reduced chlorin was isolated in almost quantitative yield. When the oxidant was replaced by 2,3-dichloro-5,6-dicyano-p-benzoquinone, which is frequently used at the oxidizing stage of the porphyrin synthesis, the ring-B oxidized (ring-D reduced) chlorins were obtained. With both ring-B reduced and ring-D reduced chlorins in hand, their photophysical and electrochemical properties were examined and compared for the first time. The ring-B reduced chlorine 20, with a fused six-member N-butylimide ring, exhibits the most red-shifted absorption band (at lambda(max) = 746 nm), the lowest fluorescence quantum yield (4.5%), and the largest quantum yield of singlet oxygen formation (67%) among the reduced ring-B and ring-D chlorins investigated in this study. Measurements of the one-electron oxidation and reduction potentials show that compound 20 is also the easiest to oxidize among the examined compounds and the third easiest to reduce. In addition, the 1.62 eV HOMO-LUMO gap of 20 is the smallest of the examined compounds, and this agrees with values calculated using the DFT method. Spectroelectrochemical measurements afforded UV-visible absorption spectra for both the radical cations and radical anions of the examined chlorins. The ring-B reduced compound 20, with a fused six-member N-butylimide ring, is regarded as the most promising candidate in this study for photodynamic therapy because it has the longest wavelength absorption and the largest quantum yield of singlet oxygen formation among the compounds investigated.     

Mercury-selective membrane electrode based on methyl substituted dibenzo tetraphenyl tetraaza macrocycle

A new polystyrene based membrane electrode of methyl substituted 6,7:13,14-dibenzo-2,4,9,11-tetraphenyl-1,5,8,12-tetraazacyclotetradeca-1,4,6,8,11,13-hexaene (I) with sodium tetraphenylborate (NaTPB) and dibutyl phthalate (DBP) as anion excluder and plasticizing agent was prepared and investigated as Hg (II)-selective electrode. The electrode exhibits a Nernstian response for Hg (II) ions over a wide concentration range of 1.0 × 10⁻¹–8.9 × 10⁻⁶ M with a slope of 30 ± 1 mV per decade concentration. It has a response time of 10 s and can be used for at least 4 months without any divergence in potentials. The membrane works satisfactorily in a partially non-aqueous medium up to a maximum 30% (v/v) content of methanol and ethanol. The proposed sensor revealed good selectivity over a wide variety of other cations including alkali, alkaline earth, heavy and transition metal ions and could be used in a pH range of 2.5–5.0. Normal interferents like Ag⁺, Cd²⁺ and Pb²⁺ low interfere in the working of the electrode. The electrode was successfully used in the direct determination of Hg²⁺ in aqueous solution.     

A direct and stereospecific approach to the synthesis of alpha-glycosyl thiols

A new simple method for the synthesis of alpha-glycosyl thiols is described. Ring-opening of 1,6-anhydrosugars with commercially available bis(trimethylsilyl)sulfide under the action of catalytic amounts of TMSOTf smoothly afforded alpha-glycosyl thiols in very high yields and in a stereospecific way.