Search results for "SELECTIVITY"
showing 10 items of 1148 documents
Gaseous sensors based on solid proton conductors
1992
Abstract he chemical sensors for different gaseous (alcohol, acetone, ammonia, water vapour) detection at room temperature are developed by using polycrystalline β-alumina and xerogel of antimonic acid hydrate (AAH). The sensitivity and selectivity of sensors depend on the ion-exchange and preparative methods. The possibility of producing different types of potentiometric, amperometric, voltammetric and resistance sensors on the basis of these ion-conducting materials is shown. More success is achieved by producing ammonia-sensitive devices on β-alumina as well as on AAH xerogel.
Exploring the potential of some yeast strains in the stereoselective synthesis of aldol reaction products and its reduced 1,3-dialcohol derivatives
2013
Abstract The behavior of two yeast strains has been studied under different conditions. Both microorganims catalyzed the aldol reaction between activated aldehydes and acetone when a large amount of the latter was present in the reaction medium producing, with moderate stereoselectivity, the aldol product with the R configuration. No reduction of any of the products present in the medium was detected. On the other hand, the carbonyl group of the racemic aldol was reduced to produce chiral 1,3-dialcohol derivatives when water was employed as the only solvent. In this case, the resolution of the racemic starting material was also possible with one of the biocatalysts, and the aldol was recove…
A Novel Approach to Dihydropyrrolones from Iron-Substituted α,β-Unsaturated Imines and Organolithium and Grignard Reagents: Developments, Mechanisms …
2000
Reactions of N-sulfonylimines, derived from Cp(CO)2Fe-substituted (Z)-enals and benzenesulfonamide, with Grignard or organolithium reagents furnish N-sulfonyldihydropyrrolones. When N-sulfinylimines and organometallic reagents are used, unprotected α,β-unsaturated γ-lactams are formed exclusively. Recent mechanistic studies and diastereoselective variations for the synthesis of chiral 5-substituted dihydropyrrolones from iron-substituted azadienes are discussed here.
ChemInform Abstract: Selectively Deprotectable Carbohydrates Based on Regioselective Enzymatic Reactions.
2010
ChemInform Abstract: Regioselective Synthesis of Calixcrowns Derived from p-tert-Butylcalix( 5)arene.
2010
Orbital-Resolved Partial Charge Transfer from the Methoxy Groups of Substituted Pyrenes in Complexes with Tetracyanoquinodimethane—A NEXAFS Study
2012
It is demonstrated that the near-edge X-ray absorption fine structure (NEXAFS) provides a powerful local probe of functional groups in novel charge transfer (CT) compounds and their electronic properties. Microcrystals of tetra-/hexamethoxypyrene as donors with the strong acceptor tetracyano-p-quinodimethane (TMP/HMP-TCNQ) were grown by vapor diffusion. The oxygen and nitrogen K-edge spectra are spectroscopic fingerprints of the functional groups in the donor and acceptor moieties, respectively. The orbital selectivity of the NEXAFS pre-edge resonances allows us to precisely elucidate the participation of specific orbitals in the charge transfer process. Upon complex formation, the intensit…
Rearrangement of 4,5-epoxy-9-trimethylsilyldecalines. Application to the synthesis of the natural eremophilane (-)-aristolochene.
2006
Several 4,5-epoxy-9-trimethylsilyl-eudesmanes and 15-nor-eudesmanes, having different relative stereochemistry and substitution at the oxirane ring, have been prepared starting from (-)-carvone and subjected to acid-promoted rearrangement. The presence of the silicon at C9 favors two different main reaction pathways involving C14-methyl or C1-methylene migration through the stabilization of a C10 carbocation intermediate. Selective 1,2-migration of the bridgehead methyl group takes place with trisubstituted beta-epoxide and tetrasubstituted alpha-epoxide, yielding 4-hydroxy-eremophilane and 15-nor-eremophilane compounds, while the trisubstituted alpha-epoxide suffers successive rearrangemen…
Selective derivatisations of resorcarenes - 2. Multiple regioselective ring closure reactions
1997
Abstract The condensation of the C-pentyl resorcarene 1 with long chain aliphatic diamines 3a-d and excess formaldehyde leads under high dilution conditions to tetrabenzoxazine derivatives 4a-d in which pairs of adjacent oxazine rings are connected by an aliphatic chain. Six new rings are formed per resorcarene molecule during this reaction in a regioselective way. For one example (4a) the chiral cleft-like structure with C2 symmetry was proved by single crystal X-ray analysis. Hydrolysis of the oxazine rings gives the secondary amine derivatives 5a,b with C2v symmetry in high yield.
Reaction of complex ligands: Part 95. Chromium tricarbonyl complexes of polysubstituted naphthohydroquinones: regioselective synthesis via [3+2+1]-be…
2003
Abstract A series of polysubstituted naphthohydroquinoid tricarbonyl chromium complexes were prepared by chromium mediated [3+2+1]-benzannulation of Fischer-type carbene complexes with alkynes. The kinetics and the thermodynamic data of the η6-η6-haptotropic rearrangements were investigated by in-situ NMR spectroscopy. The free activation energies ΔG# range from 23 to 26 kcal mol−1 and only slightly depend on the bulk and the donor/acceptor properties of the substituents. Equilibrium of metal migration was observed for naphthohydroquinone complexes bearing methoxy substituents at the non-hyroquinoid ring (ring A). In complexes bearing methyl groups or only hydrogen substituents at ring A th…