Mass spectrometric and gas and high-performance liquid chromatographic behaviour of an impurity in 2,5-hexanedione

The analysis of 2,5-hexanedione, a metabolic compound of several industrial solvents, is normally carried out using gas chromatographic (GC) or GC-mass spectrometric (MS) techniques. This work, with the aim of verifying the possibility of determining the diketone by means of a high-performance liquid chromatographic (HPLC) method with UV detection, illustrates the importance of the choice of a 2,5-hexanedione standard for the determination of the diketone response factor. In some commercial diketone samples the presence of an impurity, which may interfere with the analysis of the target analyte, was ascertained. This impurity showed GC and HPLC behaviour similar to that of 2,5-hexanedione, but gave a very different UV response. This impurity was identified as 3-methylcyclopent-2-enone by means of MS, GC-MS, HPLC-photodiode-array detection, IR and UV spectrometry. The structure was confirmed by comparing the chromatographic, mass and ultraviolet data of the unknown compound with those of a synthesized reference sample. The well known difficulty in determining 2,5-hexanedione by HPLC with UV detection was reconfirmed owing to its low molar absorptivity.     

Hydrogenation of the ansa-Chain of Rifamycins. X-ray crystal structure of (16S)-16,17,18,19-tetrahydrorifamycin S

The catalytic hydrogenation of rifamycin S ( 2 ) over Pd/C, followed by oxidation with K₃[Fe(CN)₆], generates a pair of 16,17,18,19-tetrahydrorifamycins S ( 3/4 ), epimeric at C (16). The use of PtO₂ as catalyst leads to the hydrogenation also of the C(28)?C(29) bond giving, after oxidation by K₃[Fe(CN)₆], a mixture of the epimers (16R)- and (16S)-16,17,18,19,28,29-hexahydrorifamycins S ( 5/6 ). Furthermore, we synthesized the (16R)- and (16S)-3-bromo derivatives 7/8 and (16R)- and (16S)-3-(piperidin-1-yl) derivatives 9/10 . The determination of the X-ray crystal structure of the most abundant epimer 4 of the tetrahydrorifamycins allowed the assignment of the absolute configuration at C(16) of all derivative. A Structure-activity relationship study showed that in general the (16R)-epimers are more potent inhibitors of bacterial RNA polymerase than the (16S)-epimers.     

He2+2: A comparison between Hartree–Fock and density functional techniques

A comparison of Roothaan-Hartree–Fock methods (both restricted and unrestricted) with density functional ones (LCAO -Xα and cellular MS -Xαβ) is made using as test case the He²₂ + molecular ion. It is shown the analogy that exists between RHF and symmetry-adapted LCAO -Xα potential energy curves, as well as between UHF and symmetry-unconstrained LCAO -Xα ones. The influence of symmetry adaptation on the overall behavior of the potential energy curve is also discussed. Finally, the difference in the behavior of the LCAO -Xα and cellular MS -Xαβ calculations is explained as an artifact of the space partitioning in the latter technique. It is concluded that LCAO -Xα method is superior to cellular MS -Xαβ because it requires less effort to reach the same results and that the general behavior is similar to UHF , although the former affords a better equilibrium bond distance and a worse energy barrier than the latter.     

Item selection by latent class-based methods: an application to nursing home evaluation

The evaluation of nursing homes is usually based on the administration of questionnaires made of a large number of polytomous items to their patients. In such a context, the latent class model represents a useful tool for clustering subjects in homogenous groups corresponding to different degrees of impairment of the health conditions. It is known that the performance of model-based clustering and the accuracy of the choice of the number of latent classes may be affected by the presence of irrelevant or noise variables. In this paper, we show the application of an item selection algorithm to a dataset collected within a project, named ULISSE, on the quality-of-life of elderly patients hosted in Italian nursing homes. This algorithm, which is closely related to that proposed by Dean and Raftery in 2010, is aimed at finding the subset of items which provides the best clustering according to the Bayesian Information Criterion. At the same time, it allows us to select the optimal number of latent classes. Given the complexity of the ULISSE study, we perform a validation of the results by means of a sensitivity analysis, with respect to different specifications of the initial subset of items, and of a resampling procedure.     

Asymmetric Alkylation of Cyclic Ketones with Dehydroalanine via H-Bond-Directing Enamine Catalysis: Straightforward Access to Enantiopure Unnatural α-Amino Acids

The growing importance of structurally diverse and functionalized enantiomerically pure unnatural amino acids in the design of drugs, including peptides, has stimulated the development of new synthetic methods. This study reports the challenging direct asymmetric alkylation of cyclic ketones with dehydroalanine derivatives via a conjugate addition reaction for the synthesis of enantiopure ketone-based α-unnatural amino acids. The key to success was the design of a bifunctional primary amine-thiourea catalyst that combines H-bond-directing activation and enamine catalysis. The simultaneous dual activation of the two relatively unreactive partners, confirmed by mass spectrometry studies, results in high reactivity while securing high levels of stereocontrol. A broad substrate scope is accompanied by versatile downstream chemical modifications. The mild reaction conditions and consistently excellent enantioselectivities (>95 % ee in most cases) render this protocol highly practical for the rapid construction of valuable noncanonical enantiopure α-amino-acid building blocks.     

The stucco decorations from St. Lorenzo in Laino (Como, Italy): The materials and the techniques employed by the “Magistri Comacini”

The paper reports a cross-cutting and systematic approach to the analytical study of Baroque stucco decorations by Italian artists known as “Magistri Comacini”. Samples from the Church of St. Lorenzo in Laino (Como, Italy) were analysed using chemical and mineralogical techniques and the results interpreted with the help of art historians in order to enlighten the artistic techniques. The characterisation of the binder, aggregate and organic fractions was carried out through optical microscopy, scanning electron microscopy with microprobe, X-ray powder diffraction, Fourier transform infrared spectroscopy, gas chromatography-mass spectrometry and laser ablation mass spectrometry equipped with plasma source spectrometry. The results on major, minor and trace components ascertained various recipes. In particular, the investigation of the finishing layers corroborated hypotheses of attribution and chronology and stressed the different skills of the artists working in the building, shedding a light on details of the decoration technique.     

Synthesis of tryptamine derivatives via a direct, one-pot reductive alkylation of indoles

An efficient, one-pot reductive alkylation of indoles with N-protected aminoethyl acetals in the presence of TES/TFA is reported. It represents the first general method for the direct synthesis of tryptamine derivatives from indoles and nitrogen-functionalized acetals. This convergent and versatile approach employs safe and inexpensive reagents, proceeds under mild conditions, and tolerates several functional groups. The new procedure was efficiently applied to a gram-scale synthesis of both luzindole, a reference MT2-selective melatonin receptor antagonist, and melatonin.     

On the Best Pinching Constant of Conformal Metrics on \mathbb {S}^{2} with One and Two Conical Singularities

We answer a long-standing open question asked by Thurston (The Geometry and Topology of Three-Manifolds. Princeton University Press, Princeton, 1978) concerning the best pinching constant for conformal metrics on $\mathbb {S}^{2}$ with one and two conical singularities of angles 2π(1+α ₁) and 2π(1+α ₁),2π(1+α ₂) in case α ₁∈(?1,0) and ?1<α ₁<α ₂<0, respectively. The case of one conical singularity is a corollary of a result in Chen and Lin (Commun. Anal. Geom. 6(1):1–19, 1998) concerning the curvature of conformal metrics on ?² with bounded Gaussian curvature 0<a≤K≤b<+∞. The case with two conical singularities is worked out by a generalization of that result.     

The cleavage planes of WC before and after ion implantation

Abstract

Ion implantation hardens but also embrittles the surface of tungsten carbide (WC) Crystals. The embrittlement may limit the useful applications of ion implanted cemented tungsten carbide, thus it has been studied in detail.