A convenient synthesis of pyridazine-3,4-dicarboxylic acid by a hetero diels-alder reaction

A convenient route to prepare unsubstituted pyridazine-3,4-dicarboxylic acid on a preparative scale is described. The synthesis involves a hetero Diels-Alder reaction between a new 1,2-diaza-1,3-diene and ethyl vinyl ether and oxidation of the intermediate 1,4,5,6-tetrahydropyridazine as the key step.     

Rapid and diverse route to natural product-like biaryl ether containing macrocycles

A two-step sequence involving an Ugi four-component reaction and an intramolecular nucleophilic aromatic substitution (S_NAr) has been developed for the rapid access to biaryl-ether containing macrocycles. In the course of this study, we documented that ammonium chloride can promote the Ugi-4CR in non-polar aprotic solvent (toluene) without the interference of an alternative Passerini reaction. Solid phase synthesis of macrocycles by this two-step sequence was also developed using polymer (Wang resin) supported α-(4′-fluoro-3′-nitro)phenethyl isocyanoacetate as one of the inputs.     

A simple copper-catalyzed synthesis of tertiary acyclic amides

The N-arylation of aromatic and aliphatic secondary acyclic amides, known to be poor nucleophiles, has been accomplished using a simple and cheap copper catalytic system. The corresponding tertiary acyclic amides, which can be found in numerous biologically active compounds, have been obtained in good to excellent yields.     

Strained-cyclophane-induced beta-turn template: design, synthesis, and spectroscopic characterization

[structure: see text] Three tetrapeptides incorporating a 14-membered (R(i+1), S(i+2)) cycloisodityrosine at the i + 1 and i + 2 positions were designed and synthesized. Conformational analysis by (1)H NMR and CD spectra as well as molecular modeling indicated that they all adopt a beta-turn conformation. While the CD spectrum of compound 2 is characteristic of the typical type-II beta-turn (maximum at approximately 200 nm and a minimum at approximately 220 nm), that of 1a (atropisomer of 2) is opposite in sign to the expected spectrum of the type-II beta-turn.     

Studies toward soraphen A: an aldol-metathesis avenue to the macrocyclic framework

[reaction: see text] We describe a convergent approach to soraphen A, 1, that involves coupling of two fragments by an aldol condensation-olefin metathesis sequence. This route permits rapid access to congeners of 1.     

Laser-induced breakdown spectroscopy (LIBS) for carbon single shot analysis of micrometer-sized particles

The purpose of this work is to study the ability of the laser-induced breakdown spectroscopy (LIBS) technique to perform in situ (without sample preparation) detection of graphite particles circulating in a gas loop used to simulate the cooling gas circuit of a helium-cooled nuclear reactor. Results obtained with a laboratory scale set up are presented. The experiments were performed in nitrogen with micrometer-sized particles containing carbon (glucose particles and sodium hydrogenocarbonate particles). Statistical shot to shot analysis was used to determine the concentration of the analyte. The variation of LIBS signal as a function of glucose particle diameter showed an underestimation of the signal of particles of diameters larger than 5 μm. This phenomenon is likely to be correlated to an incomplete vaporization in the laser-induced plasma of particles of sizes above 5 μm. Analytical measurements were performed with glucose particles and sodium hydrogenocarbonate particles, and the concentration-based limit of detection of carbon was evaluated to be about 60 μg m⁻³.     

Efficient syntheses of pterulone, pterulone B and related analogues

An efficient synthesis of the three halogenated naturally occurring products, pterulone (2), pterulone B (3) and alcohol 5, and of a wide range of related unnatural analogues has been achieved starting from the two readily available 1-benzoxepine sulfonyl-containing intermediates 6a and 6b. The biological activities of pterulone and some of the synthesized analogues were tested against a wide spectrum of phytopathogenic fungi.     

Total synthesis of mauritines A, B, C, and F: cyclopeptide alkaloids with a 14-membered paracyclophane unit

A unified strategy for the synthesis of mauritines A (5), B (6), C (7), and F (10) has been developed based on a key intramolecular nucleophilic aromatic substitution reaction (S(N)Ar) for the formation of the strained 14-membered paracyclophane. It was demonstrated that the outcome of the cycloetherification is independent of the stereochemistry of the peptide backbone and that both (1R)-16 and (1S)-16 cyclized smoothly to provide the corresponding macrocycle. On the other hand, dehydration of the secondary benzylic alcohol, via the phenylselenide intermediate, is configuration dependent. (1R)-25 underwent the two-step syn-elimination much more easily than (1S)-22. A modified reductive deamination procedure via the diazonium intermediate was developed. A complete assignment of proton and carbon NMR spectroscopy signals for these natural products is reported for the first time.