The reaction of amides with trifluoromethanesulfonic (triflic) anhydride in the presence of pyridine was thoroughly investigated by NMR spectroscopic techniques. Different pyridinium intermediates were generated from secondary amides, tertiary amides with enolizable protons, and tertiary amides lacking enolizable protons. It was found that the actual triflating reagent is N-(trifluoromethylsulfonyl)pyridinium triflate 11 which is formed by the initial reaction of triflic anhydride with pyridine. The alcoholysis of these intermediates yields O-alkyliminium ethers which can then be easily hydrolyzed under mild acidic conditions to the corresponding esters.Résumé : La réaction entre un amide et l'anhydride trifluorométhanesulfonique (triflique) en présence de pyridine a été examinée minutieusement à l'aide de nombreuses techniques de spectroscopie RMN. Des intermédiaires pyridiniums différents ont été générés à partir d'amides secondaires, d'amides tertiaires avec des protons énolisables et d'amides tertiaires ne possédant pas de protons énolisables. Nous avons aussi démontré que le triflate de N-(trifluorométhylsulfonyl)pyridinium 11, formé lors de la réaction initiale entre l'anhydride triflique et la pyridine, est le véritable agent de triflation. L'alcoolyse de ces intermédiaires donne des éthers O-alkyliminiums qui peuvent être facilement hydrolysés dans des conditions légèrement acides aux esters correspondants. Charette and Grenon 1695Scheme 1. Functional group interconversion involving amides as starting materials.Scheme 2. Postulated intermediates formed in the reaction of amides with triflic anhydride.

Context. The Gaia Data Release 2 (DR2) contains more than half a million sources that are identified as variable stars. Aims. We summarise the processing and results of the identification of variable source candidates of RR Lyrae stars, Cepheids, long-period variables (LPVs), rotation modulation (BY Dra-type) stars, δ Scuti and SX Phoenicis stars, and short-timescale variables. In this release we aim to provide useful but not necessarily complete samples of candidates. Methods. The processed Gaia data consist of the G, G BP , and G RP photometry during the first 22 months of operations as well as positions and parallaxes. Various methods from classical statistics, data mining, and time-series analysis were applied and tailored to the specific properties of Gaia data, as were various visualisation tools to interpret the data. Results. The DR2 variability release contains 228 904 RR Lyrae stars, 11 438 Cepheids, 151 761 LPVs, 147 535 stars with rotation modulation, 8 882 δ Scuti and SX Phoenicis stars, and 3 018 short-timescale variables. These results are distributed over a classification and various Specific Object Studies tables in the Gaia archive, along with the three-band time series and associated statistics for the underlying 550 737 unique sources. We estimate that about half of them are newly identified variables. The variability type completeness varies strongly as a function of sky position as a result of the non-uniform sky coverage and intermediate calibration level of these data. The probabilistic and automated nature of this work implies certain completeness and contamination rates that are quantified so that users can anticipate their effects. This means that even well-known variable sources can be missed or misidentified in the published data. Conclusions. The DR2 variability release only represents a small subset of the processed data. Future releases will include more variable sources and data products; however, DR2 shows the (already) very high quality of the data and great promise for variability studies.

We report detailed analysis of high-resolution spectra of nine high velocity metal-rich dwarfs in the solar neighborhood, selected from the proper motion samples of Grenon. The stars are super metal-rich, and 5 of them have [Fe/H]≥ +0.4, making them the most metal-rich stars currently known. We find that α-elements decrease with increasing metallicity; s-elements are underabundant by about [s-elements/Fe] ≈ −0.3; the Eu II line was measurable for 6 stars, showing [Eu/Fe] ≈ 0.0, except for G161-29 where [Eu/Fe] = +0.50. All calculations followed the same methods used by Castro et al. (1996) [AJ, 111, 2439] for the analysis of the well-known metal-rich prototype µ Leo and the very strong-lined bulge star BW IV-167, for which [Fe/H]=+0.46 and +0.47 were found respectively. While exceeding the [Fe/H] of current bulge samples, the chemistry of these stars has important similarities and differences. The near-solar abundances of the alpha-capture elements places these stars on the metal-rich extension of McWilliam & Rich (1994) [ApJS, 91, 749], but their s-process abundances are much lower than those of the bulge giants. These low s-process values have been interpreted as the hallmark of an ancient stellar population. We are unable to convincingly assign these stars to a known Galactic population and we urge further studies of larger samples.

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Context. A sample of mostly old metal-rich dwarf and turn-off stars with high eccentricity and low maximum height above the Galactic plane has been identified. From their kinematics, it was suggested that the inner disk is their most probable birthplace. Their chemical imprints may therefore reveal important information about the formation and evolution of the still poorly understood inner disk. Aims. To probe the formation history of these stellar populations, a detailed analysis of a sample of very metal-rich stars is carried out. We derive the metallicities, abundances of α elements, ages, and Galactic orbits. Methods. The analysis of 71 metal-rich stars is based on optical high-resolution échelle spectra obtained with the FEROS spectrograph at the ESO 1.52-m Telescope at La Silla, Chile. The metallicities and abundances of C, O, Mg, Si, Ca, and Ti were derived based on LTE detailed analysis, employing the MARCS model atmospheres.Results. We confirm the high metallicity of these stars reaching up to [Fe I/H] = 0.58, and the sample of metal-rich dwarfs can be kinematically subclassified in samples of thick disk, thin disk, and intermediate stellar populations. All sample stars show solar α-Fe ratios, and most of them are old and still quite metal rich. The orbits suggest that the thin disk, thick disk and intermediate populations were formed at Galactocentric distances of ∼ 8 kpc, ∼ 6 kpc, and ∼ 7 kpc, respectively. The mean maximum height of the thick disk subsample of Z max ∼ 380 pc, is lower than for typical thick disk stars.A comparison of α-element abundances of the sample stars with bulge stars shows that the oxygen is compatible with a bulge or inner thick disk origin. Our results suggest that models of radial mixing and dynamical effects of the bar and bar/spiral arms might explain the presence of these old metal-rich dwarf stars in the solar neighbourhood.