Branched Star-Type Polysilyllithium Compounds: The Effects of beta-Silyl Substitution and of Complexation on Their Molecular Structure

No Abstract     

Electronic effects steer the mechanism of asymmetric hydrogenations of unfunctionalized aryl-substituted alkenes

Density functional theory (PBE and B3LYP) was used to study asymmetric hydrogenations of alkenes catalyzed by an iridium imidazolylidine oxazoline complex. The calculation predicts that the alkene preferentially coordinates to the site trans to the carbene. The coordinated alkene then reacts first with the H2 ligand, then with the hydride to form alkane. Finally, the alkane is released by equilibrating with extrinsic H2 and alkene. Enantioface selectivities for hydrogenations of trisubstituted alkenes seem to be driven primarily by steric interactions with the adamantyl part of the ligand; only the smallest substituents can adopt a site close to it. Application of this theoretical model leads to correct predictions regarding the experimentally observed sense and magnitude of the enantioselectivities.     

A convenient protocol for selective cleavage of 2-hydroxy acid amides. Application to semisynthesis of the cyclic heptapeptide aza HUN-7293

A two-step protocol for the first chemoselective cleavage of 2-hydroxy acid amides has been developed. Mesylation of the model substrate 2-(hydroxypropionylamino)-4-methylpentanoic acid methyl ester (11) followed by treatment with N-ethylthiourea (13) allows cleavage of 2-hydroxy acid amides under smooth conditions. Successful application of this methodology to the open-chain transesterification product 15 (methylester) of the cyclic heptadepsipeptide HUN-7293, a potent inhibitor of inducible cell adhesion molecule expression, delivered the corresponding hexapeptide 18 with unprotected N-terminus in 70-75% yield. This result demonstrates that the protocol developed even works in the presence of an ester and several methylated and unmethylated amide bonds. Finally, a sequence of ligation of methyl D-dehydroglutaminate (20) to the C-terminus of the saponification product 21, followed by the degradation protocol and ring closure, allowed chemical "point mutation" at the DGCN site affording the aza analogue of HUN-7293 (24) in 15% overall yield. To the best of our knowledge this is the first report on chemoselective cleavage of 2-hydroxy acid amides.     

Analysis of cyanobacterial toxins by hydrophilic interaction liquid chromatography-mass spectrometry

The combination of hydrophilic interaction liquid chromatography with electrospray mass spectrometry (HILIC-MS) has been investigated as a tool for the analysis of assorted toxins produced by cyanobacteria. Toxins examined included saxitoxin and its various analogues (1-18), anatoxin-a (ATX-a, 19), cylindrospermopsin (CYN, 20), deoxycylindrospermopsin (doCYN, 21), and microcystins-LR (22) and -RR (23). The saxitoxins could be unequivocally detected in one isocratic analysis using a TSK gel Amide-80 column eluted with 65% B, where eluent A is water and B is a 95% acetonitrile/water solution, both containing 2.0 mM ammonium formate and 3.6 mM formic acid. The analysis of ATX-a, CYN and doCYN required 75% B isocratic. Simultaneous determination of 1-21 was also possible by using gradient elution. HILIC proved to be suitable for the analysis of microcystins, but peak shape was not symmetric and it was concluded that these compounds are best analysed using existing reversed-phase methods. The HILIC-MS method was applied to the analysis of field and cultured samples of Anabaena circinalis and Cylindrospermopsis raciborskii. In general, the method proved quite robust with similar results obtained in two different laboratories using different instrumentation.     

Synthesis and properties of new ditertiary stibines based upon o-, m- or p-xylyl and m- or p-phenylene backbones and their complexes with tungsten, iron and nickel carbonyls

High yield syntheses for 1,2-, 1,3-, and 1,4-xylyl distibines (1,2-C6H4(CH2SbMe2)2, 1,3-C6H4(CH2SbMe2)2, 1,4-C6H4(CH2SbMe2)2, respectively) from Me2SbCl (conveniently made in situ from Me2PhSb and HClgas) and the appropriate di-Grignard are reported. The 1,3- and 1,4-phenylene distibines, 1,3-C6H4(SbMe2)2 and 1,4-C6H4(SbMe2)2, were made similarly. The new ligands have been characterised by mass spectrometry, 1H and 13C[1H] NMR spectroscopy, and by the preparation of methiodide derivatives. The crystal structures of 1,4-C6H4(CH2SbMe2)2 and [1,3-C6H4(CH2SbMe3)2]I2 have been determined. The synthesis of 1,2-C6H4(CH2SbPh2)2 has been achieved similarly in modest yield and the distibine converted into the tetra-iodo-derivative 1,2-C6H4(CH2SbPh2I2)2. The coordination modes available to these ligands have been probed by the synthesis and characterisation of complexes with nickel, iron and tungsten carbonyls. The crystal structure of [[Fe(CO)4]2[micro-1,3-C6H4(CH2SbMe2)2]] has been determined. The spectroscopic properties of these carbonyl derivatives have been compared with those of complexes of other antimony ligands, and in some cases with diphosphine and diarsine complexes, to probe the electronic properties of the new ligands.     

Interpolation of diabatic potential energy surfaces

A method is presented for constructing diabatic potential energy matrices from ab initio quantum chemistry data. The method is similar to that reported previously for single adiabatic potential energy surfaces, but correctly accounts for the nuclear permutation symmetry of diabatic potential energy matrices and other complications that arise from the derivative coupling of electronic states. The method is tested by comparison with an analytic model for the two lowest energy states of H(3).     

Multilayer sol-gel membranes for optical sensing applications: single layer pH and dual layer CO(2) and NH(3) sensors

Organo-silica sol-gel membranes have been prepared and demonstrated in a single layer format for pH measurement and multiple-layer format for both carbon dioxide and ammonia. The sensors are simple and versatile since the same chemistry and membranes are used for each sensor. The sensors use hydroxypyrenetrisulfonic acid (HPTS) as the indicator immobilized in a base-catalyzed sol-gel containing poly(dimethyl)siloxane, aminopropyltriethoxysilane (APTES) and tetraethylorthosilicate (TEOS). This indicator gel is over coated with a hydrophobic sol-gel to reduce cross reactivity to pH when either CO(2) or NH(3) are examined. The gels are very stable and the sensors retain response up to a 12-month period. Sensors can be stored in buffer or dry without loss of function and have response times to that are comparable to literature values.     

Stacking due to ionic transport number mismatch during sample sweeping on microchips

Sample stacking can occur in isoconductive buffer systems as a result of ion transport mismatches that cause changes in buffer conductivity during electrophoresis. Fluorescence imaging was used to examine this effect in the sweeping of hydrophobic dyes with sodium dodecyl sulfate (SDS) on microchips. Imaging revealed the occurrence of a stacking effect in a sodium borate buffer system in which the sample buffer and SDS-containing run buffer had the same initial conductivity. Injected sample plugs were first swept by SDS micelles and the swept band was then stacked at the trailing end of the sample zone. This effect is due to changes in conductivity at both the front and back interfaces of the injected sample plug and can be modeled by moving boundary equations. Maximum signal enhancements of 86-, 160- and 560-fold were obtained for Rhodamine 560, Rhodamine B and Rhodamine 6G, respectively, by the combination of sweeping and stacking within a 1 cm section of microchannel. Based on sample sweeping/stacking and manipulation of the electric field polarity, a method of trapping and concentrating analyte from multiple injections was also demonstrated.     

Role of the A-ring of bryostatin analogues in PKC binding: synthesis and initial biological evaluation of new A-ring-modified bryologs

The syntheses of three newly designed bryostatin analogues are reported. These simplified analogues, which lack the A-ring present in the natural product but possess differing groups at C9, were obtained using a divergent approach from a common intermediate. All three analogues exhibit potent, single-digit nanomolar affinity to protein kinase C.     

Spectroscopy of hydrothermal reactions, 19: pH and salt dependence of decarboxylation of α‐alanine at 280–330°C in an FT‐IR spectroscopy flow reactor

The spontaneous decarboxylation of 0.5 m aqueous α‐alanine solutions as a function of pH (1–9 at 320°C, where neutrality is approximately 6) was determined with a flow reactor at 280–330°C and 275 bar by FT‐IR spectroscopy. The kinetics for the cationic and anionic forms have not been previously reported. The rate constants for the cationic form [CH₃(NH₃⁺)CHCO₂H], the anionic form [CH₃(NH₂)CHCO₂^?], and the zwitterion form [CH₃(NH₃⁺)CHCO₂^?] were obtained and followed the first‐order rate law. The rate of decarboxylation of the zwitterion is three times greater than that of the cationic and anionic forms in the temperature and pH ranges of study. The corresponding Arrhenius parameters were determined and compared with previously reported data. The addition of KCl (1 and 2 m) at the natural pH of α‐alanine resulted in a reduction of the decarboxylation rate, suggesting that the transition state is less polar than the zwitterion and/or that the activity of the zwitterion has been reduced. The α‐alanine solution is therefore somewhat more robust in solutions of high ionic strength, such as seawater, than it is in pure water. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 271–277, 2002