Factors affecting reproducibility in the manufacture of glass and siliceous glass open tubular (capillary) columns

Summary Column characteristics affecting the chromatographic behavior of glass and siliceous glass (fused silica) capillary columns include the dimensional uniformity of the column, the physical and chemical characteristics of the column wall and the characteristics of the liquid phase. In the case of the coated column the uniformity and thickness of the liquid phase film are the most important criteria affecting column reproducibility. The paper discusses these factors and their influence on column performance.Presented in part at the Symposium on Standardized Materials for Chromatography, 181 st National American Chemical Society Meeting, Atlanta, Georgia, March 29–April 3, 1981.     

An enantioselective synthesis of (S)-(+)-3-aminomethyl-5-methylhexanoic acid via asymmetric hydrogenation

A concise enantioselective synthesis of (S)-(+)-3-aminomethyl-5-methylhexanoic acid (1, Pregabalin) has been developed. The key step is the asymmetric hydrogenation of a 3-cyano-5-methylhex-3-enoic acid salt 2 with a rhodium Me-DuPHOS catalyst, providing the desired (S)-3-cyano-5-methylhexanoate 3 in very high ee. Subsequent hydrogenation of the nitrile 3 with a heterogeneous nickel catalyst provides Pregabalin 1 in excellent overall yield and purity.     

Evolution and application of the fused silica column

Fused silica is an evolutionary product of continuing efforts to improve our chromatographic capabilities. It offers the normal user new horizons in inertness, chromatographic reproducibility, ease of column installation, and possibilities in on-column trapping. Since this writing, chemically bonded phases in several film thicknesses became available on fused silica columns, and these are especially well suited to on-column injections. The use of fused silica syringe needles to inject directly onto fused silica columns seems particularly interesting. Evolution is, of course, a continuous process and it would appear that we can expect continuing developments in this exciting area.     

Detection of metal ions by glass capillary gas-chromatographic separation of their volatile chelates

The utility of several glass capillary columns to separate some trifluoroacetonates, diethyldithiocarbamates, di(trifluoroethyl) dilithiocarbamates and diisopropyldithiophosphates metal chelates was investigated. The increased sensitivity offered by selective detectors was also explored. Most favorable results were obtained with a short highly inert fused silica column, coated with a non-retentive liquid phase.     

Normal,abnormal and pseudo-abnormal reaction pathways for the imine-peroxyacid reaction

Steric and mesomeric effects have a marked influence upon the formation of oxaziridine (normal pathway) or nitrone (abnormal pathway) products from the imine-peroxyacid reaction; n.m.r. studies of the thermal isomerization of oxaziridines to nitrones provide evidence of a pseudo-abnormal oxidation pathway.     

Collision gas effects in the collision-induced decomposition of protonated and cationized molecules of carbohydrate antibiotics

The collision-induced decomposition (CID) mass spectra of the protonated and cationized molecules of a number of carbohydrate antibiotics of RMM ranging from 700 to 1500 were studied by means of a four-sector mass spectrometer with a floated collision cell. Helium and argon were used as collision gases. This work illustrates that cationized rather than protonated carbohydrate antibiotics give an increased yield of high-mass ions of diagnostic value. Further, when helium is replaced by argon as collision gas, differences in the CID spectra of MH⁺ ions become apparent only for molecules of RMM > 1400 whereas for [M + Na]⁺ ions differences are observed for molecules of RMM as low as 1000. These results have been attributed to the deposition of more internal energy in the precursor ion when argon is used, resulting in increased fragmentation.     

Cyclopropyl alkynes as mechanistic probes to distinguish between vinyl radical and ionic intermediates

[reaction: see text] The reactions of (trans-2-phenylcyclopropyl)ethyne, 1a, (trans,trans-2-methoxy-3-phenylcyclopropyl)ethyne, 1b, and (trans,trans-2-methoxy-1-methyl-3-phenylcyclopropyl)ethyne, 1c, with either aqueous sulfuric acid or tris(trimethylsilyl)silane (or tributyltin hydride) and AIBN have been investigated. Protonation and addition of the silyl (or stannyl) radical occurred at the terminal position of the alkyne giving an alpha-cyclopropyl-substituted vinyl cation or radical, respectively. Under both reaction conditions, 1a yielded products derived from ring opening toward the phenyl substituent. Alkynes 1b and 1c, however, gave different products depending on whether radical or cationic conditions were used. When radical conditions were employed, products derived from regioselective ring opening toward the phenyl substituent were obtained. In contrast, when cationic conditions were employed, products derived from selective ring opening toward the methoxy substituent were isolated. The corresponding alpha-cyclopropyl-substituted vinyllithium derivatives were also synthesized and were found to be stable toward rearrangement. An estimate of the rate constants for ring opening of the alpha-cyclopropylvinyl cations was also made: values of 10(10)-10(12) s(-1) were found for the vinyl cations derived from protonation of the terminal carbon of alkynes 1a-c. Based on these results, cyclopropyl alkynes 1a-c can be classified as hypersensitive mechanistic probes for the detection of vinyl radical or cationic intermediates generated adjacent to the cyclopropyl ring and, in the case of 1b and 1c, the distinction between a radical or cationic intermediate is possible.