Two-photon ionization of an inner shell electron of the Cl atom

We have calculated, using second order perturbation theory, the two photon ionization cross section of a K-shell electron of chlorine forE=1.6 keV incident photons. Two classes of intermediate states must be considered, those in which a 1s electron moves to an emptyp-orbital, and those in which an electron from an occupiedp-orbital moves into the continuum. The first class of intermediate states is followed by the ejection of ap-electron into the continuum. The second class of intermediate states is followed by the transfer of a 1s electron into an emptyp-state. The largest contribution comes from 3p →d-continuum followed by 1s → 3p transition. Our result is σ⁽²⁾/I=2.06×10⁴¹ cm⁴/W where σ⁽²⁾ is the two photon ionization cross-section andI is the light intensity.     

The formation of medium-sized rings by the intramolecular cyclization of arylnitrenium ions

The synthesis of an aminodibenzo[a,c]octadiene and an aminodibenzo[a,c]octatriene, guided by molecular mechanics calculations, was effected by the intramolecular cyclization of the corresponding open chain nitrenium ions. Also reported is the predicted failure to synthesize another aminodibenzo[a,c]octadiene using this method.     

N-Dealkylation of Amines

N-dealkylation, the removal of an N-alkyl group from an amine, is an important chemical transformation which provides routes for the synthesis of a wide range of pharmaceuticals, agrochemicals, bulk and fine chemicals. N-dealkylation of amines is also an important in vivo metabolic pathway in the metabolism of xenobiotics. Identification and synthesis of drug metabolites such as N-dealkylated metabolites are necessary throughout all phases of drug development studies. In this review, different approaches for the N-dealkylation of amines including chemical, catalytic, electrochemical, photochemical and enzymatic methods will be discussed.     

Synthesis and absolute configuration of an exomethylene portion of zooxanthellatoxin-A

The absolute configuration of the exomethylene portion of zooxanthellatoxin-A (ZT-A) was established as 71R,73R,74S,75R by comparing an MTPA ester of an acyclic degradation product of ZT-A with those of the compound synthesized from methyl-α-D-glucopyranoside.     

Asymmetric synthesis of (4S,5S)-2-oxo-4-phenyloxazolidine-5-carboxylic acid using a 1,2-addition of PhMgBr to an N-sulfinimine derived from (R)-glyceraldehyde acetonide and (S)-t-BSA

We report an asymmetric synthesis of (4S,5S)-2-oxo-4-phenyloxazolidine-5-carboxylic acid via stereoselective addition of phenylmagnesium bromide (PhMgBr) to an N-sulfinimine derived from (R)-glyceraldehyde acetonide. (S)- and (R)-Glyceraldehyde acetonides, important chiral synthons in synthetic organic chemistry, are prepared from the corresponding epichlorohydrin using an identical synthetic methodology.     

Synthesis of the all-cis-trimethyldecalin fragment of unusual terpenes by radical-mediated protonolysis of an alkylboron derivative

The synthesis of an enantiopure building-block with a contiguous all-cis-trimethyldecalin motif embedded in unusual terpenes is described. Starting from the Wieland–Miescher ketone, the key step is a one-pot hydroboration of an exocyclic methylene double bond promoted by disiamylborane and radical-mediated protonolysis of the corresponding alkylborane using 4-tert-butylcatechol.     

Aminolysis of glycal-derived allyl epoxides and activated aziridines. Effects of the absence of coordination processes on the regio- and stereoselectivity

The addition of primary and secondary aliphatic amines to glycal-derived allyl epoxides is completely 1,2-regio- and anti-stereoselective, whereas mixtures of the corresponding anti-1,2- [3-N-(substituted-amino) glycals] and anti-1,4-addition products (N-glycosyl amines) are obtained with N-(mesyl)-aziridines. In this way, structural moieties, otherwise difficult to synthesize, are obtained by means of a very simple protocol. The regio- and stereoselectivity observed with epoxides is the consequence of an isomerization process, whereas the result obtained with aziridines is explained by the absence of an effective substrate-nucleophile (amine) coordination.     

Synthesis of B-organo-substituted 1,2-, 1,7-, and 1,12-dicarbaclosododecarboranes(12)

A convenient new method is proposed for the synthesis of 9-organo-substituted o- and m-carboranes and 2-organo-substituted p-carborane by the substitution of iodine in 9-iodine-o-, 9-iodine-m-, and 2-iodine-p-carboranes by an organic group from an organomagnesium compound in the presence of catalytic amounts of phosphine complexes of palladium. For the first time the halogen in boron halogen carboranes has been substituted by an organic group.     

Synthesis of an O-acyl isopeptide by using native chemical ligation to efficiently construct a hydrophobic polypeptide

By using dimethylformamide to suppress the O-to-N acyl migration, we efficiently synthesized an O-acyl isopeptide by native chemical ligation of a peptide-thioester and a Cys-O-acyl isopeptide. The reaction mixture was then loaded onto an octadecylsilane reverse-phase HPLC column, and the isopeptide was purified by using a linear gradient of CH₃CN in 0.1% aqueous trifluoroacetic acid. The recovery rate of the O-acyl isopeptide was considerably higher than that of the corresponding native polypeptide. Synthesis of O-acyl isopeptides via native chemical ligation, with O-to-N acyl migration as the final step to give the native form, has potential as an efficient method of constructing hydrophobic polypeptides.     

Total synthesis of (−)-centrolobine

Stereoselective synthesis of (−)-centrolobine, an anti-Leishmania agent, was accomplished via an intramolecular Barbier-type reaction of iodo-ester with n- or t-butyllithium followed by Lewis acid-promoted Et₃SiH reduction of the resulting hemiacetal.     

An efficient synthesis of furyl sulfonamides from the reaction of furan with in situ generated N-tosyl imines

Treatment of an aldehyde and furan with N-sulfinyl-p-toluenesulfonamide/zinc chloride leads to the formation of furyl sulfonamides via an in situ generated N-tosyl imine intermediate. In one case, a novel 4-tosylamino-5,6-dihydro-4H-3-oxa-benz[e]azulene was obtained by intramolecular aromatic substitution of the activated imine at the 3-position of the furan ring.     

Stereoselective synthesis of 3a,7a-dihydro-3H,4H-furo[3,4-c]pyran-1-ones via intramolecular hetero-Diels-Alder reaction

The synthesis of 3a,7a-dihydro-3H,4H-furo[3,4-c]pyran-1-ones via an intramolecular hetero-Diels-Alder reaction of easily accessible α,β-unsaturated γ-ketoesters was investigated. The reaction was found to proceed in a highly stereoselective way leading to single, cis-configured product isomers. The same diastereomer is formed, independently of the configuration of the enone double bond of the precursor. The respective E- and Z-isomers react either through an endo-E-syn or an exo-Z-syn transition state.     

Structure and absolute configuration of 3-alkylpiperidine alkaloids from an Indonesian sponge of the genus Halichondria

Chemical analysis of an Indonesian sponge sample has provided three new 3-alkylpiperidine alkaloids, tetradehydrohaliclonacyclamine A, its mono-N-oxide derivative, and a 2-epi isomer. The absolute structure of tetradehydrohaliclonacyclamine A has been established by X-ray crystallography from anomalous dispersion effects using Cu radiation, which determined that the absolute configuration is 2S, 3S, 7S, 9S while an HPLC study revealed that the alkaloid is enantiomerically pure.     

Conformational effects on lipase-mediated acylations of 2-substituted cyclohexanols

Lipase-mediated acetylations of trans- and cis-2-substituted cyclohexanols gave the corresponding (1R)-cyclohexyl acetates and (1S)-cyclohexanols in high yields and ee, but c-4-tert-butyl-c-2-ethenyl-r-1-cyclohexanol was unreactive owing to the steric interaction between the axial OH group and the axial H atoms at the 3- and 5-positions. In the cis-isomer the OH group occupies an equatorial position to bind to the lipase, and less bulky axial alkenyl and alkynyl groups might not so much prevent acetylations than an alkyl group.     

Analysis of the decay of picosecond fluorescence in semiconductors: Criteria for the presumption of electroneutrality during the decay of an exponential electron-hole profile

When an exponential profile of electron-hole pairs is photogenerated (in a semiconductor) with a delta-function light pulse, unequal diffusion coefficients of holes and electrons (i.e. D_e≠ D_h) effect deviations from electroneutrality as electrons and holes diffuse into the bulk semiconductor. These deviations will in turn effect errors in the analysis of data (e.g. time resolved fluorescence) when using theory based on the presumption of electroneutrality. We deduce here the experimental conditions required for an effective electroneutrality to be maintained during the course of an experiment. Analyses were carried out using computer simulations without the presumption of electroneutrality and the analytic solution with the presumption of electroneutrality. The differences in the measured fluorescences predicted by the two computations are characterized as a function of a variety of experimental parameters and physical properties: intensity (of the excitation pulse), the absorption of the exciting and emitted light, the the ratio D_h/D_e, bulk dielectric constant of the semiconductor, bulk and surface recombination kinetics. It is shown that a conditon of adequate electroneutrality can be effectively attained when a well defined a minimum number of electron-hole pairs is generated; an upper limit of the number of e^?–k⁺ pairs is also established in order to avoid an intolerable temperature pulse.     

Acoustic method of measuring critical properties of thermally unstable substances

An acoustic method of measuring critical temperatures and pressures of thermally unstable substances is described. In the cell with the liquid under investigation there is a wire probe, 0.02 mm in diameter, which is heated by electric-current pulses with a duration of 0.025 to 0.25 ms. At the moment of liquid boiling-up in the layer that surrounds the probe, one can observe an acoustic wave, which is registered by a ceramic piezoelectric element on a base (PbZrO₃+PbTiO₃). The probe temperature at the moment of boiling-up is determined from its resistance. With an increase of the pressure in the cell with the substance under investigation the amplitude of an acoustic signal decreases. The cessation of boiling-up and disappearance of the acoustic signal show that the critical state is reached. A more precise critical pressure is found by linear extrapolation of the amplitude of the acoustic signal to zero in an amplitude–log (pressure) plot. The paper reports the experimental critical temperatures T_c and the critical pressures p_c of reference substances: n-hexane, n-heptane, n-decane, benzene, and of thermally unstable n-alkanes: docosane and tetracosane, and also the critical curve of the system n-hexadecane–benzene. The critical constants of reference substances were measured by this acoustic method with an error that does not exceed 0.015 T_c and 0.015 p_c.     

Isolation of a Potential Anchoring Motif Based on Proteome Analysis of Escherichia coli and Its Use for Cell Surface Display

For bacterial cell surface display, the target protein needs to be linked to an anchoring motif, and it is essential to choose an appropriate anchoring motif for efficient and stable display of the protein on the cell surface. To isolate a potential anchoring motif that would allow a stable and enhanced display of target proteins on the surface of an Escherichia coli host, we analyzed the outer membrane proteome of E. coli. On the basis of this proteomic analysis, the outer membrane protein X (OmpX), which has a small, monomeric β-barrel structure and is highly expressed, was selected as a potential anchoring motif. The role of OmpX as an anchoring motif for cell surface display was demonstrated using three important industrial enzymes: endoxylanase, lipase, and alkaline phosphatase. Two different positions (Lys¹²², Val¹⁶⁰) in the extracellular loops of OmpX were examined for C-terminal fusion, and the biological activities and localization of the displayed enzymes were analyzed. All three enzymes examined were efficiently displayed on the E. coli cell surface with high activity. These results reveal that the use of OmpX as an anchoring motif is an efficient method to display functional enzymes on the surface of an E. coli host.     

Development of a convenient route for the preparation of the N-Cbz-protected guaninyl synthon required for Boc-mediated PNA synthesis

An efficient, high yielding, chemo- and regioselective, five-step synthetic route to N²-Cbz-guanin-9-yl acetic acid has been developed, which avoids the use of triphosgene. After formation of the N²-Boc protected purine from 2-amino-6-chloropurine, two successive base-controlled alkylations allowed an N⁹-tert-butyl acetate function followed by an N²-Cbz moiety to be installed. The selectivities of these reactions were confirmed through an X-ray crystallographic study of the 6-(2-nitrophenoxy) analogue. Final hydrolytic dechlorination and removal of the Boc and tert-butyl ester protecting groups were accomplished concomitantly under acidic conditions to afford the guanin-9-yl PNA monomer synthon in an overall yield of 53%.     

Catalytic hydrogenation of vinylogous peptides: a route towards γ-peptide foldamers

Catalytic hydrogenation over Pd/C of vinylogous aminoacids and aminoamides has been studied. The configuration of the ethylenic bond has an important effect on the diastereoselectivity. The higher selectivity is observed with the E-vinylogous aminoamides. The conformational preferences of the α,γ-disubstituted γ-peptides have been determined. The 2S,4S-γ-peptide moiety induces a β-like folded structure stabilized by an intramolecular hydrogen bond, whereas the 2S,4R-diastereomer assumes an open structure.     

Enantioselective synthesis of lyxo-(2R,3R,4R)-C18-phytosphingosine using double stereodifferentiation

lyxo-C₁₈-Phytosphingosine can be synthesized by the cis-dihydroxylation of an (E)-allylic trichloroacetamide obtained by an Overman rearrangement. A double stereodifferentiation using AD-mix-β and an enantiomerically enriched (S)-allylic trichloroacetamide allowed the first synthesis of lyxo-(2R,3R,4R)-C₁₈-phytosphingosine with high diastereoselectivity (de=94%) and excellent enantioselectivity (ee=93%). This sphingosine was fully characterized by the physical and spectral data of the corresponding tetraacetate.