Letteratura scientifica selezionata sul tema "Diazo-transfer"

A convenient one-pot approach to the preparation of α-diazo-β-ketosulfones from sulfonyl chlorides is described. It involves the conversion of the sulfonyl chloride to sodium sulfinate, alkylation of the latter with α-haloketones followed by diazo transfer using the ‘sulfonyl-azide-free’ (‘SAFE’) protocol in aqueous medium. The simple and expedient method relies on readily available starting materials and provides facile access to a wide variety of valuable diazo reagents for organic synthesis.

Rates of hydrolysis of 1-diazo-2-indanone and 2-diazo-1-indanone were measured in dilute aqueous perchloric acid solutions using both H2O and D2O as the solvent, and rates of hydrolysis of the latter substrate were measured in dilute aqueous (H2O only) formic acid buffer solutions as well. The data for 1-diazo-2-indanone gave the hydronium ion catalytic coefficient kH+ = 5.7 × 10–3 (mol/L)–1 s–1 and the isotope effect kH+/kD+ = 2.9. The normal direction (kH/kD > 1) of this isotope effect was taken as evidence for a reaction mechanism involving rate-determining hydron transfer from the hydronium ion to the substrate's diazo carbon atom; followed by rapid displacement of diazo nitrogen by a water molecule, giving the observed 1-hydroxy-2-indanone product. The data for 2-diazo-1-indanone, on the other hand, gave a hydronium ion catalytic coefficient two orders of magnitude greater than the value for 1-diazo-2-indanone (kH+ = 5.9 × 10–1 (mol/L)–1 s–1), and an isotope effect near unity (kH+/kD+ = 1.2). It is argued that this isotope effect represents a situation in which diazo carbon hydronation and displacement of diazo nitrogen are each partly rate determining, a conclusion supported by incipient saturation of buffer catalysis in the formic acid buffer solutions. The 100-fold difference in hydronium ion catalytic coefficients for the two substrates is rationalized in terms of differing electron densities on the diazo carbon atoms.Key words: diazo compound hydrolysis, solution kinetics, acid catalysis, solvent isotope effects, buffer catalysis saturation.

The discovery of small organic ligands or biologics capable of modulating biological processes remains one of the biggest challenges in developing new medicines. Different technologies have been implemented over the last decades to ease this process and make it more efficient. In this regard, encoded display technologies have played a major role in the discovery of new antibodies, peptides, and proteins. However, the efficient exploitation of automated high-throughput screening to discover small organic ligands has mainly been limited to big pharmaceutical companies. DNA-Encoded Chemical Libraries (DELs) have emerged as a powerful and cost-effective alternative to solve this issue. The technology has been established during the last 25 years and has become one of the best methods to synthesize and screen libraries of unprecedented size, promising a bright future in the early drug discovery stages. DELs are collections of small molecules individually coupled to oligonucleotide fragments, serving as amplifiable identification barcodes. In the first part of this thesis new DEL designs, displaying molecules capable of targeting challenging therapeutic targets while keeping library-quality at the highest grade, were investigated. A novel single-pharmacophore library, termed AG-DEL, was synthesized. The library was constructed using split-and-pool procedures on single-stranded DNA. The modularity of this library design allowed the creation of different dual-pharmacophore libraries in an encoded self-assembling chemical library format (ESAC 2+1 and ESAC Plus). Furthermore, the new AG-DEL facilitated the use of novel screening methodologies (e.g., photo-crosslinking) to efficiently discover new small organic ligands. DEL synthesis mainly relies on the chemical diversity of building blocks and the efficiency of the chemical reactions to link them. Following this trend, many different groups have made great efforts during the last years to develop new mild and efficient DNA-compatible reactions. One of the most used reactions for DEL synthesis is the amide bond formation, thanks mainly to various reliable reaction protocols and the big commercially available collections of amino acids. Nevertheless, the current availability of DNA-compatible post-functionalization of amino acids is still quite limited due to some restrictions inherent to the presence of the DNA. In the second part of this thesis, a new DNA-compatible diazo-transfer reaction was successfully optimized and implemented. This reaction has shown to be efficient, both in reaction times and reaction yields, as well as to be mild and fully compatible with DNA, as demonstrated by subsequent enzyme-mediated ligation of the oligonucleotide template to a new fragment, and has served for the synthesis of new ESAC Plus libraries within our group. The modulation of protein-protein interactions (PPIs) represents another formidable challenge. These interactions are often characterized by large and flat protein surfaces that are composed of many different interacting groups. Therefore, these interactions are usually targeted using large macrocyclic peptides or antibodies. Notwithstanding this challenge, some examples have been reported during the last years in which small organic ligands or peptidomimetics were specifically designed for targeting this class of proteins. Some of these examples have successfully reached clinical trials and even marketing authorization, showing the critical importance of PPI modulators and indicating broad prospects. In PPI modulation, the discovery of ligands targeting cytokines is even more challenging, due to the small size and particularly flat surface of these proteins. Nevertheless, different small molecule ligands targeting cytokines have been described over the years. Among all these proteins, Interleukin-2 (IL2) represents one of the best examples. IL2 is a pro-inflammatory cytokine, that plays a crucial role in immunity, and different therapeutic approaches using IL2 are increasingly being used for the treatment of a variety of malignancies, like melanoma and renal cell carcinoma. However, the use of IL2 has been limited due to strong side effects related to the high doses of cytokine necessary to achieve a pharmacological effect. Side effects have been linked to the release of pro-inflammatory cytokines as well as to CD25-mediated endothelial damage induced by IL2 binding to endothelial surface receptors, leading to a vascular leak syndrome. The interaction between IL2 and its alpha subunit receptor (IL2Ra or CD25) activates immunosuppressive regulatory T cells (Tregs) and reduces its antitumor activity. Thus, avoiding the formation of the multimeric IL2/IL2R complex can enhance the antitumor response. The last chapter of this thesis was focused on the development of novel DEL-derived IL2 ligands capable of interacting at the CD25 binding domain of IL2. During these studies, a tumor-targeting antibody-IL2 fusion protein, L19-IL2, was used to find ligands masking the IL2 moiety. The ligands were optimized by a medicinal chemistry approach and characterized by fluorescence polarization. Furthermore, the best ligand showed binding at the CD25 binding epitope of IL2, as evidenced by competition experiments using an anti-IL2 antibody. The use of one of the discovered compounds or an affinity matured derivative can allow the generation of a new class of biopharmaceutical-small molecule complexes that localize at the site of the disease and regain activity of the cytokine only at the tumor site.

Thesis (M. S.)--Chemistry and Biochemistry, Georgia Institute of Technology, 2009.
Committee Chair: Oyelere, Adegboyega; Committee Member: Bunz, Uwe; Committee Member: Collard, David; Committee Member: Lobachev, Kirill; Committee Member: Tolbert, Laren.

Nucleophilic additions are an important class of reactions in the preparation of several organic compounds. Metals facilitate nucleophilic additions in many cases. The present work Mechanistic Investigation of Metal Promoted Nucleophilic additions is an attempt to understand the mechanism of nucleophilic additions to imines and carbonyl compounds mediated by the transition metal complexes. Understanding the mechanism of metal promoted nucleophilic additions can facilitate the design and synthesis of more efficient catalysts. Chapter 1 provides a brief introduction to nucleophilic addition. A few named reactions that involve nucleophilic addition are described. An overview of the metal promoted nucleophilic addition reactions and their mechanisms are presented. A short note on the importance of understanding the mechanism of metal promoted nucleophilic addition is included. This section ends with the scope of the present work. Chapter 2 “Mechanistic Investigation of Titanium Mediated Reactions of Imines” deals with two reactions. The first reaction is the formation of reduced amines on reduction of imines. Amines and diamines are synthesized often from imines. A convenient route to such nitrogen containing compounds is through reduction of imines and through reductive coupling of imines respectively. Since both reactions occur in a parallel fashion, during the synthesis of diamines, amines are obtained as side products and vice versa. This problem is acute in the case of titanium based reducing agents. These reducing agents are called low valent titanium reagents because low valent titanium species are generated in situ either from titanium(IV) or titanium(III) reagents. There is no clear understanding of the nature of the low valent titanium involved in the reaction. To rectify this, a mechanistic understanding of this reaction is essential. An attempt was made to probe the mechanism of formation of amines using low valent titanium formed by using two different reducing agents namely phenylsilane and zinc. With the help of isotopic labelling studies, it was found that the mechanism of formation of an amine with phenylsilane involves a direct hydrogen transfer from phenylsilane to an imine. This was verified using deuterium labelled phenylsilane. With zinc, it follows a traditional titanacycle pathway which was verified by quenching with the deuterium oxide. A second reaction that has been probed is the alkylation of imines by Grignard reagents using chiral titanium complexes. Alkylation of imines is one of the suitable routes to prepare chiral amines. Alkylation of imines employing a Grignard reagent with Ti(OiPr)4 can proceed through two different pathways depending on the amount of the Grignard reagent used. Alkylation reaction with one equivalent of Grignard reagent can proceed through a Ti(IV) species and the alkylation reaction with two equivalents of the Grignard reagent can proceed through a Ti(II) species. The reaction proceeding through Ti(IV) is less wasteful as it only requires one equivalent of the Grignard reagent. The two pathways differ from each other in the nature of the transition state where the C-C bond is formed. To verify the favourable pathway, chiral titanium complexes were prepared and alkylation carried out. The alkylation results suggest that one equivalent of Grignard is sufficient to give good yields of the alkylated product and the reaction may proceed through a Ti(IV) promoted path. It was reported in the literature that at least three equivalents of Grignard reagent are required to get good yields of the alkylated product with zirconium complexes. This work suggests a greener alternate to alkylation of imines. Chapter 3 “Asymmetric Transfer Hydrogenation Reaction of Ketones in Water” deals with the synthesis of chiral ruthenium half-sandwich complexes employing a proline diamine ligand which has phenyl, ethyl, benzyl, or hydrogen as a substituent. These complexes were characterized by X-ray diffraction. In addition, all these complexes were obtained as single diastereoisomers. These complexes were used as catalysts for the reduction of a variety of ketones to chiral alcohols in water using sodium formate as a hydride source. Stoichiometric reaction between sodium formate and the catalysts showed the formation of hydride complexes as the active species. Based on the electronic effects observed, the key step is found to be a nucleophilic attack of hydride on the carbonyl carbon of ketones. In the transfer hydrogenation reaction with DCOONa, more of 1-phenylethanol- 1-2H1 was observed with all the ruthenium catalysts suggesting that the hydrogen from sodium formate is transformed into a metal hydride which is subsequently transferred to the ketones to form chiral alcohols. The catalysts were optimized with acetophenone as a model substrate. Only in the case of a catalyst which has a phenyl substituent, silver nitrate was found to enhance the formation of aqua complex which in turn resulted in good yields of the chiral alcohols. Among all the complexes studied, the catalyst bearing a phenyl group induces greatest enantioselectivity. It can also be recycled. Chapter 4 “On the Formation of a Ruthenium-PPh2H Complex Using 1- Phenylethane-1,2-diol” deals with the mechanism of formation of PPh2H from PPh2Cl. This unique transformation involves a ruthenium-cymene dimer, PPh2Cl and 1-phenylethane-1,2- diol. In the attempted synthesis of a ruthenium bisphosphinite complex, using the ruthenium-cymene dimer, chlorodiphenylphosphine and 1-phenylethane-1,2-diol, the formation of [Ru(η6-cymene)Cl2PPh2H] was observed in good yield. Formation of the expected ruthenium bisphosphinite complex was not observed. The reaction was carried out in the absence of 1-phenylethane-1,2-diol resulted in the formation of [Ru(η6- cymene)Cl2PPh2Cl] suggests that the diol acts as a reducing agent. To verify the source of hydrogen in the 1-phenylethane-1,2-diol, deuterated diols were prepared. The reactions with the deuterated diols revealed several interesting aspects of the formation of the Ru-PPh2H complex. Chapter 5 “Mechanistic Studies on the Diazo Transfer Reaction” deals with the synthesis of labelled azides and the labelled azidating reagent to probe the mechanism of the diazo transfer reaction. Azides are important precursors used for a variety of chemical transformations including the celebrated Cu(I) catalyzed click reaction. Azides are also used as protecting groups for amines as they can be conveniently reduced to amines. Azidation of amines usually yield azides, with retention of stereochemistry. There is a possibility that the azide formation can occur through the SNi mechanism with retention of configuration where nitrogen in the starting material will not be retained after forming an azide. The reaction was carried out with 13C and 15N labelled L-valine and L-isoleucine to probe this possibility. The resultant labelled azide has 15N retained in its position. This excluded the SNi pathway. To show where the nucleophilic amine group is attacking the azide, labelled imidazole-1¬sulfonyl azide was synthesized from NaN215N. Reactions were carried out with L-valine (labelled and unlabelled) in the presence of a metal catalyst and with unlabelled L-valine in the absence of catalyst. These results confirm the postulated pathways described in the literature.

The ammonia and diazo technique with CO2-calibration for highly resolving measurement of adiabatic film cooling effectiveness distribution has been developed and improved with respect to high accuracy. Both parts of the technique are based on the analogy between heat and mass transfer. In the ammonia and diazo part a mixture of ammonia and air is injected through the film cooling holes. Downstream of the injection a diazo film is mounted on the experimental surface. Depending on the local ammonia concentrations along the wall the diazo film turns blue. In the CO2-calibration part carbon dioxide is used as a tracer gas. Gas samples are sucked off and analyzed, thus giving the adiabatic film cooling effectiveness at certain locations on the surface. Relating the effectiveness values to the color intensities of the diazo film at the corresponding locations a calibration curve is derived. This calibration can be applied to the whole color distribution of the diazo film resulting in a highly resolved distribution of the adiabatic film cooling effectiveness. The scattering of the measured values along the calibration curve directly indicates the quality of the measurement. The ammonia and diazo technique with CO2-calibration has been applied to injection through a row of holes (α = 35°, p/D = 3) in the flat wall of a wind tnnnel for different blowing rates. The results show a very good suitability of this technique, especially, but not only, if the region around the film cooling holes is of special interest.

The distribution of adiabatic film-cooling effectiveness on the endwall of a large-scale low-speed linear turbine cascade has been measured using a new technique. This technique is based on an established surface-flow visualisation technique, and makes use of the reaction between ammonia gas and a diazo surface coating. A new method of calibration has been developed to relate the result of the reaction to surface concentration of coolant. Using the analogy that exists between heat and mass transfer the distribution of film-cooling effectiveness can then be determined. The complete representation of the film-cooling effectiveness distribution provided by the technique reveals the interaction between the coolant ejected from the endwall and the secondary flow in the turbine blade passage. Over- and under-cooled regions on the endwall are identified, illustrating the need to take these interactions into account in the design process. Modifications to the cooling configuration examined in this paper are proposed as a result of the application of the ammonia and diazo technique.

An experimental study has been conducted to investigate the distribution of the convective heat transfer on the shroud of a high pressure turbine blade in a large scale rotating rig. A continuous thin heater foil technique has been adapted and implemented on the turbine shroud. Thermochromic Liquid Crystals were employed for the surface temperature measurements to derive the experimental heat transfer data. The heat transfer is presented on the shroud top surfaces and the three fins. The experiments were conducted for a variety of Reynolds numbers and flow coefficients. The effects of different inter-shroud gap sizes and reduced fin tip clearance gaps were also investigated. Details of the shroud flow field were obtained using an advanced Ammonia-Diazo surface flow visualisation technique. CFD predictions are compared with the experimental data and used to aid interpretation. Contour maps of the Nusselt number reveal that regions of highest heat transfer are mostly confined to the suction side of the shroud. Peak values exceed the average by as much as 100 percent. It has been found that the interaction between leakage flow through the inter-shroud gaps and the fin tip leakage jets are responsible for this high heat transfer. The inter-shroud gap leakage flow causes a disruption of the boundary layer on the turbine shroud. Furthermore, the development of the large recirculating shroud cavity vortices is severely altered by this leakage flow.

An experimental study has been conducted to investigate the aerothermal performance of a shrouded high pressure turbine blade in a large scale rotating rig. The rotor blade and the associated shroud and casing geometry have been modelled in a large scale low speed turbine rig that was designed to investigate a novel passive shroud cooling methodology. The objective of the present paper is to describe the aerothermal performance of a passive shroud cooling strategy using measured heat transfer and adiabatic cooling effectiveness data. Improved physical understanding of the shroud aerodynamics as reported in the companion paper Lehmann et al. [1] will be used here to support the analysis. Highly resolved experimental heat transfer data was acquired on the shroud, the fins and on the shroud underside with the thin heater film method. The distribution of the adiabatic cooling effectiveness on the rotor shroud was measured with a combination of the Ammonia-Diazo and a foreign gas sampling technique. The measurements are complemented by steady numerical computations of the turbine stage. Due to the impact of vortical flow structures in the over shroud cavities, the Nusselt numbers on the shroud top surfaces were found to be of the same order as on the shroud underside. The passive shroud cooling concept was found to provide quite efficient and uniform cooling to the over-shroud surfaces while the distribution of coolant on the shroud underside was significantly affected by the rotor secondary flow.

The internal heat transfer of turbine blades can be augmented using cyclone cooling, but the consequential impact on the external film cooling may be significant. To determine these effects, the distribution of adiabatic film cooling effectiveness was measured on the surface of a symmetrical blade model containing a cylindrical leading-edge channel. This channel feeds one row, respectively two opposite rows, of eight cooling holes each. Inside this channel two different types and directions of swirl are generated. The resulting adiabatic effectiveness distributions, which are measured using the calibrated ammonia diazo technique, are compared to those measured with a channel flow without swirl (datum configuration). The operating points are defined by blowing ratio (0.6–1.0) and film cooling discharge coefficient (20%–50%). A high full-range resolution over the adiabatic effectiveness is achieved using a weighting average method with multiple experiments per operating point. The lateral-averaged adiabatic effectiveness is presented up to 30 diameters downstream of the cooling holes. These effectiveness values show a high dependency on the configurations and reach values of about 0.3 to 2 times the reference configuration values. This is due to the strong variation of the flow structure inside the cooling holes. PIV-measurements and basic numerical simulations of the channel flow structure and dynamic pressure measurements at the cooling hole exits are carried out to support the results of film cooling effectiveness.