Selective Michael-aldol reaction by use of sterically hindered aluminum aryloxides as Lewis acids: an easy approach to cyclobutane amino acids

A formal [2 + 2] cycloaddition of 2-amidoacrylates with monosubstituted donor olefins, including its asymmetric version, is described. The stereoselectivity of this reaction can be modulated by the use of sterically hindered aluminum aryloxides or methylaluminoxane as Lewis acids. The reaction was applied to the synthesis of both stereoisomers of 2-benzyloxycyclobutane-alpha-amino acid, which are protected serine analogues c(4)Ser(OBn).     

Rhodium‐Catalyzed Annulation of ortho‐Alkenyl Anilides with Alkynes: Formation of Unexpected Naphthalene Adducts

o‐Alkenyl N‐triflylanilides underwent rhodium(III)‐catalyzed oxidative annulations with alkynes to produce different types of naphthylamides in a process which involves the cleavage of two C?H bonds. Remarkably, besides formal dehydrogenative (4C+2C) cycloadducts, the reaction also produces variable amounts of isomeric naphthylamides, whose formation requires a formal migration of the alkenyl moiety from the ortho to the meta position of the anilide. The annulation reaction can be efficiently carried out in the absence of external oxidants, such as Cu(OAc)₂.     

Competitive solvation of K+ by C6H6 and H2O in the K+-(C6H6)n-(H2O)m (n = 1–4; m = 1–6) aggregates

The competitive solvation of the potassium ion by benzene and water is investigated at molecular level by means of Molecular Dynamics simulations on the K⁺-(C₆H₆) _n -(H₂O) _m (n = 1–4; m = 1–6) ionic aggregates. The preference of K⁺ to bind C₆H₆ or H₂O is investigated in the range of temperatures in which isomerisation processes are likely by adding water and benzene to the K⁺-(C₆H₆) _n and K⁺-(H₂O) _m aggregates, respectively. Hydrogen bonds and the π-hydrogen bond, in spite of their weakness with respect to the K⁺-π and K⁺-H₂O interactions, play an important role in stabilising different isomers, thus favouring isomerisation processes. Accordingly with experimental information it has been found that K⁺ bind preferably C₆H₆ rather than H₂O and that the fragmentation of C₆H₆ is only observed for aggregates containing four molecules of benzene.     

Electron delocalization in homoconjugated 7,7-diarylnorbornane systems: a computational and experimental study

A joint computational–experimental study has been carried out to analyze the homoconjugative interactions in 7,7‐diarylnorbornane (DPN) derivatives. The experimentally observed new bands in their UV/Vis have been accurately assigned by means of TD‐DFT calculations. Both experimental data and computations show that aromatic homoconjugation in acyclic systems is an effective mechanism for electron delocalization that resembles the situation described for polyphenylenes and polyenes. The effective homoconjugation length in homoconjugated oligomers is in the range of 6–7 aryl rings. The effect of substituents directly attached to the para carbon atom of the DPN moiety have been also studied. We found that the HOMO→LUMO vertical transitions can indeed be modified by the nature of the aromatic substituents in order to provoke dramatic changes in the electronic properties (i.e., in the absorption spectra) of the studied species.     

Rational Design of a Nonbasic Molecular Receptor for Selective NH4+/K+ Complexation in the Gas Phase

A new molecular receptor ( 1 ) for ammonium recognition has been designed and constructed by using only carbon atoms. This molecular receptor can co‐exist in two different isoenergetic conformations but, upon complexation, the conformers are no longer isoenergetic, and a basket‐shaped conformation becomes clearly more stable. The pre‐organised tetrahedral structure of this basket‐shaped molecule favours the complexation of ammonium ions by N? H???π interactions with the four phenyl groups of the host. A similar behaviour is not observed in a similar, but less pre‐organised, reference molecule. ESI‐MS competition experiments show that 1 is able to bind NH₄⁺ over K⁺ selectively. This is the first example of a neutral molecular receptor that shows a remarkable NH₄⁺/K⁺ selectivity. DFT‐calculations provide insight into the nature of host–guest interactions of both 1? NH₄⁺ and 1? K⁺ complexes as well as in the mechanism involved in multiple cation–π interactions and the influence of these interactions on the conformational stability and the selective binding of the host.     

Synthesis of highly functionalised enantiopure bicyclo[3.2.1]- octane systems from carvone

The commercially available monoterpene carvone has been efficiently converted into the tricyclo[3.2.1.0(2.7)]octane and bicyclo[3.2.1]octane systems characteristic of some biologically active compounds. The sequence used for this transformation involves as key features an intramolecular Diels-Alder reaction of a 5-vinyl-1,3-cyclohexadiene and a cyclopropane ring opening.     

Directly suspended droplet microextraction in combination with microvolume UV-vis spectrophotometry for determination of phosphate

A miniaturized methodology for the determination of phosphate in waters has been developed by combining directly suspended droplet microextraction (DSDME) with microvolume spectrophotometry. The method is based on the extraction of the ion pair formed between 12-molybdophosphate and malachite green onto a microdrop of methyl isobutyl ketone and subsequent spectrophotometric determination with no dilution. An enrichment factor of 325 was obtained after 7.5 min of microextraction. The detection limit was 6.1 nM phosphate and the repeatability, expressed as relative standard deviation, was 2.7% (n = 6). The method was successfully applied to the determination of dissolved reactive phosphorus in different freshwater samples.     

Excess molar properties of ternary system (ethanol + water + 1,3-dimethylimidazolium methylsulphate) and its binary mixtures at several temperatures

The density, dynamic viscosity, and refractive index of the ternary system (ethanol + water + 1,3-dimethylimidazolium methylsulphate) at T = 298.15 K and of its binary systems 1,3-dimethylimidazolium methylsulphate with ethanol and with water at several temperatures T = (298.15, 313.15, and 328.15) K and at 0.1 MPa have been measured over the whole composition range. From these physical properties, excess molar volumes, viscosity deviations, refractive index deviations, and excess free energy of activation for the binary systems at the above mentioned temperatures, were calculated and fitted to the Redlich–Kister equation to determine the fitting parameters and the root-mean-square deviations. For the ternary system, the excess properties were calculated and fitted to Cibulka, Singh et al., and Nagata and Sakura equations. The ternary excess properties were predicted from binary contributions using geometrical solution models.     

Superagonistic fluorinated vitamin D3 analogs stabilize helix 12 of the vitamin D receptor

Side chain fluorination is often used to make analogs of 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] resistant to degradation by 24-hydroxylase. The fluorinated nonsteroidal analogs CD578, WU515, and WY1113 have an increased prodifferentiating action on SW480-ADH colon cancer cells, which correlated with stronger induction of vitamin D receptor (VDR)-coactivator interactions and stronger repression of beta-catenin/TCF activity. Cocrystallization of analog CD578 with the zebrafish (z)VDR and an SRC-1 coactivator peptide showed that the fluorine atoms of CD578 make additional contacts with Val444 and Phe448 of activation helix 12 (H12) of the zVDR and with Leu440 of the H11-H12 loop. Consequently, the SRC-1 peptide makes more contacts with the VDR-CD578 complex than with the VDR-1,25(OH)2D3 complex. These data show that fluorination not only affects degradation of an analog but can also have direct effects on H12 stabilization.