Chemoselective fluorination of 2-hydroxy-3,4,7,8-tetrahydro-2H-chromen-5(6H)-ones using DAST

In this study, diethylaminosulfur trifluoride (DAST) was successfully applied in monofluorination reactions of some trifluoromethyl substituted 2-hydroxy-2H-chromenones, employing a general, mild, and efficient methodology. The fluorinated compounds (2-fluoro-2H-chromenones) were synthesized as unique products by a chemoselective reaction in 63-81% yield despite the presence of different reactive sites subjected to reactions with DAST.     

Absolute configuration reassignment of two chromanes from Peperomia obtusifolia (Piperaceae) using VCD and DFT calculations

Previous analysis of the ECD spectra of two prenylated benzopyrans isolated from Peperomia obtusifolia, by means of the helicity rule for the chromane chromophore, resulted in the incorrect assignment of their absolute configuration, (S) instead of (R) for a deduced P-helicity of the chromane ring for the (+)-enantiomers. This was discovered by the application of DFT calculations and VCD spectroscopy. Experimental and calculated (B3LYP/6-31G(d)) VCD and IR spectra were compared, and a definitive absolute configuration of (+)-1 and (+)-2 is reassigned directly in solution as (R). The assumption of equatorial positioning of bulky groups, shown here to be invalid for the title molecules, is the underlying cause of the previous incorrect assignment of absolute configuration. Moreover, TDDFT (B3LYP/6-311++G(2d,2p)//B3LYP/6-31G(d)) calculations of ECD spectra have shown that both P- and M-helicity of the heterocyclic ring, for a given absolute configuration, lead to the same sign for the ¹L_b ECD band, thus bringing into question the validity of the empirical ECD helicity rule for chromane molecules.     

Esterification reaction kinetics of acetic acid and n-pentanol catalyzed by sulfated zirconia

This study reports experimental data and kinetic modeling of acetic acid esterification with n-pentanol using sulfated zirconia as a catalyst. Reactions were carried out in an isothermal well-mixed batch reactor at different temperatures (50-80°C), n-pentanol to acid molar ratios (1:1-3:1), and catalyst loadings (5-10 wt% in relation to the total amount of acetic acid). The reaction mechanism regarding the heterogeneous catalysis was evaluated considering pseudo-homogeneous, Eley–Rideal, and Langmuir–Hinshelwood model approaches. The reaction mixture was considered a nonideal solution and the UNIQUAC thermodynamic model was used to take into account the nonidealities in the liquid phase. The results obtained indicated that increases in the temperature and catalyst loading increased the product formation, while changes in the n-pentanol to acetic acid molar ratio showed no significant effect. The estimated enthalpy of the reaction was −8.49 kJ mol⁻¹, suggesting a slightly exothermic reaction. The Eley–Rideal model, with acetic acid adsorbed on the catalyst as the limiting step, was found to be the most significant reaction mechanism.     

Chemical Vapor Deposition for Atomically Dispersed and Nitrogen Coordinated Single Metal Site Catalysts

Atomically dispersed and nitrogen coordinated single metal sites (M-N-C, M=Fe, Co, Ni, Mn) are the popular platinum group-metal (PGM)-free catalysts for many electrochemical reactions. Traditional wet-chemistry catalyst synthesis often requires complex procedures with unsatisfied reproducibility and scalability. Here, we report a facile chemical vapor deposition (CVD) strategy to synthesize the promising M-N-C catalysts. The deposition of gaseous 2-methylimidazole onto M-doped ZnO substrates, followed by an in situ thermal activation, effectively generated single metal sites well dispersed into porous carbon. In particular, an optimal CVD-derived Fe-N-C catalyst exclusively contains atomically dispersed FeN₄ sites with increased Fe loading relative to other catalysts from wet-chemistry synthesis. The catalyst exhibited outstanding oxygen-reduction activity in acidic electrolytes, which was further studied in proton-exchange membrane fuel cells with encouraging performance.     

Stereocontrolled transformation of nitrohexofuranoses into cyclopentylamines via 2-oxabicyclo[2.2.1]heptanes. Part VI: Synthesis and incorporation of the novel polyhydroxylated 5-aminocyclopent-1-enecarboxylic acids into peptides

The first total synthesis of enantiopure methyl (3aR,4S,5S,6R,6aS)-4-benzyloxycarbonylamino-6-hydroxy-2,2-dimethyl-tetrahydro-3aH-cyclopenta[d][1,3]dioxole-5-carboxylate has been carried out according to our recent novel strategy for the transformation of nitrohexofuranoses into cyclopentylamines. This approach is based on an intramolecular cyclization that leads to 2-oxabicyclo[2.2.1]heptane derivatives. E1cb elimination of the methoxy substituent was observed when attempting to incorporate these β-amino acid into peptides. As a result, the synthesis and incorporation of the first polyhydroxylated 5-aminocyclopent-1-enecarboxylic acid into peptides were developed.     

Carboxylic Acids Supported on Silica: A Smooth Acylating Agent for Alcohols

An alternative procedure for the esterification of alcohols by short-chain carboxylic acids supported on silica is presented.     

1H and 13C NMR Chemical Shifts and N-Substituent Effects of Some Unsymmetrically N,N-Disubstituted Acetamides

The ¹H and ¹³C NMR chemical shift assignments of a series of (E) - and (Z)-N,N-Dialkylacetamides [CH₃C(O)NR¹R², with R¹/R²=Me/Et (1), Me/n-Bu (2), Et/n-Bu (3), Et/t-Bu (4), Me/Hydrcxyethyl (5)., Et/Hydroxyethyl (6), Et/Acetylhydroxyethyl (7)] are reported. The ¹H chemical shifts for the N-substituents of the amides 1–7 recorded in benzene-d₆ and in chloroform-d₁ are in agreement with the Hatton and Richards (ASIS) and Paulsen-Todt models, respectively. The ¹³C chemical shifts for the N-substituents of compounds 1–3 were compared with data of the corresponding symmetrical amides, and the results can be explained by the reciprocal steric compression effect of one N-substituent on the other. The validity of this explanation is confirmed by ¹³C spin-lattice relaxation time (T₁) measurements.