Studies of 9-fluorenyl carbocations. intramolecular hydride migration in a substituted 9-fluorenyl carbocation

The substituted fluorenyl cation, 9-(diphenylmethyl)fluoren-9-yl cation (4), is formed under stable ion conditions (low temperature/strong acid) from its corresponding alcohol 3. This ion is transformed to a substituted diphenyl methyl cation 8 at ambient temperature via an apparent 1,2-hydrogen shift. Irradiation of 9-(diphenylmethyl)fluoren-9-ol in methanol gives products derived from the corresponding cation along with radical-derived products from C-C and C-O homolysis processes. The laser flash photolysis of this alcohol gave a transient corresponding to cation 4. All of the photoproducts are derived from cation 4 or radical pathways. High level MO calculations point to a high barrier (23.8 kcal x mol(-1)) for the 1,2-hydride shift. This barrier is the consequence of the minimum energy conformation of this fluorenyl cation which is less than ideal for the periplanar geometry necessary for this process.     

A phase-switch purification approach for the expedient removal of tagged reagents and scavengers following their application in organic synthesis

In this paper we wish to report on a variety of expedient chemical transformations and purifications achieved via a generic "catch and release" methodology, based on a synthetically inert bipyridyl chelating tag that can be selectively captured with a resin-bound copper(II) species. Utilising this approach we are able to derive many of the same benefits associated with both solid phase synthesis and supported reagent methods.     

Synthesis and application of mono-6-ammonium-6-deoxy-beta-cyclodextrin chloride as chiral selector for capillary electrophoresis

A facile synthetic approach for mono-6-amino-6-deoxy-beta-cyclodextrin (beta-CD-NH2) was proposed. Its hydroxy chloride salt, mono-6-ammonium-6-deoxy-beta-cyclodextrin chloride (beta-CD-NH3Cl) was further prepared and used for the enantioseparation of various anionic and ampholytic analytes by capillary electrophoresis (CE). The effect of background electrolyte (BGE) pH and selector concentration on the enantioseparation was studied. Results showed that beta-CD-NH3Cl displayed powerful chiral resolution ability towards anionic analytes. In addition, baseline separation of a standard mixture consisting of eight acids was achieved within 35 min.     

平面环辛四烯:分子结构操纵实例

环辛四烯为一不平面分子,一般相信,理论中的平面环辛四烯需要将一对不成对电子填入两个简并不成键轨道,因而引起之伪扬-泰勒效应,可使带 D_(8h)对称性之平面环辛四烯非平面化而成“船”形的结构。本文简单介绍加减电子法、带张力环稠合法、键长缩短法和金属螫合法四种可使不平面之环辛四烯成为平面方法。然而,随有机合成方法之不断进展,在将来可能出现更多改变环辛四烯不平面结构的方法。我们在此文章提出化合物98即为一个可使环辛四烯成为平面的例子。98之合成将成为有机合成化学家的一个新的挑战。     

Enantioselective synthesis of (-)-terpestacin and (-)-fusaproliferin: clarification of optical rotational measurements and absolute configurational assignments establishes a homochiral structural series

An enantioselective synthesis of the syncytium formation inhibitor (-)-terpestacin (1, 19 steps, 5.8% yield from the allylation product of (R,R)-pseudoephedrine propionamide, 3) and the fungal metabolite (-)-fusaproliferin (2, 21 steps, 5.3% yield from 3) in their natural configurations is described. The route employs a series of stereoselective enolate alkylation reactions to establish the initial stereogenic center, set the quaternary carbon configuration, close the 15-membered ring, and introduce the side-chain residue with proper stereocontrol. Careful analysis of our synthetic materials alongside natural samples has revealed that several errors were made in the earlier measurements of optical rotation or in the absolute stereochemical assignments of these natural products. Clarifying all discrepancies, we show here that natural terpestacin (1) is levorotatory, not dextrorotatory as originally described, but was correctly assigned as the (1S,11S,15R,23S)-enantiomer. Fusaproliferin (2) is levorotatory, as reported, but is in fact the (1S,11S,15R,23S)-enantiomer and not the antipodal configuration originally assigned.     

Ab initio studies on the mechanism of the size-dependent hydrogen-loss reaction in Mg+(H2O)n

The mechanism of size-dependent intracluster hydrogen loss in the cluster ions Mg(+)(H(2)O)(n), which is switched on around n=6, and off around n=14, was studied by ab initio calculations at the MP2/6-31G* and MP2/6-31G** levels for n=1-6. The reaction proceeds by Mg(+)-assisted breaking of an H-O bond in one of the H(2)O molecules. The reaction barrier is dependent on both the cluster size and the solvation structure. As n increases from 1 to 6, there is a dramatic drop in the reaction barrier, from greater than 70 kcal mol(-1) for n=1 to less than 10 kcal mol(-1) for n=6. In the transition structures, the Mg atom is close to the oxidation state of +2, and H(2)O molecules in the first solvation shell are much more effective in stabilizing the transition structures and lowering the reaction barriers than H(2)O molecules in the other solvation shells. While the reaction barrier for trimer core structures with only three H(2)O molecules in the first shell is greater than 24 kcal mol(-1), even for Mg(+)(H(2)O)(6), it drops considerably for clusters with four-six H(2)O molecules in the first shell. The more highly coordinated complexes have comparable or slightly higher energy than the trimer core structures, and the presence of such high coordination number complexes is the underlying kinetic factor for the switching on of the hydrogen-loss reaction around n=6. For clusters with trimer core structures, the hydrogen loss reaction is much easier when it is preceded by an isomerization step that increases the coordination number around Mg(+). Delocalization of the electron on the singly occupied molecular orbital (SOMO) away from the Mg(+) ion is observed for the hexamer core structure, while at the same time this isomer is the most reactive for the hydrogen-loss reaction, with an energy barrier of only 2.7 kcal mol(-1) at the MP2/6-31G** level.     

Are the electrospray mass spectra of proteins related to their aqueous solution chemistry?

Institute for Environmental Chemistry, National Research Council of Canada, Ottawa, Ontario, Canada The shape of the profile described by the relative abundances of multiply charged ions of proteins in the electrospray mass spectrum can be described by means of one or more Gaussian functions. An aqueous solution equilibrium model of the distribution of multiply charged ions of equine cytochrome c and myoglobin has been shown to match qualitatively the shape of the distribution of these ions in an electrospray mass spectrum. Monotonic functions such as the quadrupole mass spectrometric transmission efficiency may alter the centroid of the profile, but the shape of the ion abundance pattern appears to be controlled by the aqueous solution chemistry of the proteins. NRCC No. 32938.     

Electrospray mass spectrometry: Ethylene glycol as a solvent and its effects on ion desorption

Ethylene glycol solutions of gramicidin S, myoglobin and tetrabutylammonium bromide were analysed by means of electrospray mass spectrometry and their spectra were compared with those of aqueous solutions. The evaporation of water and ethylene glycol droplets, initially at room or elevated temperature, in air at room temperature was modelled. It was found that under conditions where a water droplet's radius would shrink by ～30%, an ethylene glycol droplet shrinks negligibly. Further, droplets that are initially hot (such as those that are ejected from a heated electrospray needle) cool very rapidly owing to evaporation and heat loss to ambient air, and subsequently evaporate much like droplets that are initially at room temperature. For gramicidin S, the ion abundances in ethylene glycol as solvent were ～200 times lower than those in water under room temperature operating conditions. In experiments where the spray probe was heated to ～100°C to reduce the viscosity of ethylene glycol, the gramicidin response difference between the solvents decreased to about a factor of 40. Similar trends were observed for myoglobin and the tetrabutylammonium ion. The gramicidin abundances in ethylene glycol, relative to those in water, are orders of magnitude too large to be accounted for using the conventional solvent evaporation model. It is speculated that decreasing the viscosity increases the velocity of ions drifting in ethylene glycol towards the solution/air interface and increases the total number of analyte ions desorbed at the Taylor cone during electrospray.     

Review of the new edition of ISO 13528

The international standard ISO 13528 is an important reference document for proficiency testing providers and testing laboratories alike, which provides guidance on the use of statistical methods in proficiency testing. With a view to bringing the document in harmony with the ISO/IEC 17043:2010, the first edition of ISO 13528 was revised and the new edition was published in September 2015. This paper attempted to review and discuss the major changes introduced in the new edition of ISO 13528, particularly the part on statistical design with respect to the objective of proficiency testing scheme and the considerations on assigned value determination and performance scoring. Moreover, procedures provided in the new edition of ISO 13528 for qualitative proficiency testing schemes and various robust statistical methods would also be briefly discussed in the paper.     

Are the radical centers in peptide radical cations mobile? The generation, tautomerism, and dissociation of isomeric alpha-carbon-centered triglycine radical cations in the gas phase

The mobility of the radical center in three isomeric triglycine radical cations[G(*)GG](+), [GG(*)G](+), and [GGG(*)](+) has been investigated theoretically via density functional theory (DFT) and experimentally via tandem mass spectrometry. These radical cations were generated by collision-induced dissociations (CIDs) of Cu(II)-containing ternary complexes that contain the tripeptides YGG, GYG, and GGY, respectively (G and Y are the glycine and tyrosine residues, respectively). Dissociative electron transfer within the complexes led to observation of [Y(*)GG](+), [GY(*)G](+), and [GGY(*)](+); CID resulted in cleavage of the tyrosine side chain as p-quinomethide, yielding [G(*)GG](+), [GG(*)G](+), and [GGG(*)](+), respectively. Interconversions between these isomeric triglycine radical cations have relatively high barriers (> or = 44.7 kcal/mol), in support of the thesis that isomerically pure [G(*)GG](+), [GG(*)G](+), and [GGG(*)](+) can be experimentally produced. This is to be contrasted with barriers < 17 kcal/mol that were encountered in the tautomerism of protonated triglycine [Rodriquez C. F. et al. J. Am. Chem. Soc. 2001, 123, 3006-3012]. The CID spectra of [G(*)GG](+), [GG(*)G](+), and [GGG(*)](+) were substantially different, providing experimental proof that initially these ions have distinct structures. DFT calculations showed that direct dissociations are competitive with interconversions followed by dissociation.