Reaction of silyl ketene acetals with epoxides: a new method for the synthesis of γ-butanolides

Titanium tetrachloride promoted reaction of silyl ketene acetals with epoxides, followed by acidic work-up, affords butanolides in moderate/good yields. With epihalohydrins the reaction is regioselective and occurs at the less substituted end of the epoxide; the γ-haloalkyl-γ-butanolides thus obtained can be further transformed into various products.     

Commutativity theorems for s-unital rings with constraints on commutators

Let R be an associative s-unital ring with the polynomial identity \( y^{s}[x^{n},y]=\pm [x,y^{m}]x^t\) or \( y^{s}[x^{n},y]=\pm x^{t}[x,y^{m}]\) , where m, n, s and t are given non-negative integers such that m > 0 or n > 0 and m ≠ n if s = m ? 1 and t = n ? 1. Using similar methods as in our recent papers [1] and [2], we prove here the commutativity of R for various values of m, n, s and t, under appropriate constraints on torsion of commutators.     

Total Synthesis and Bioactivity Mapping of Geodiamolide H

The first total synthesis of the actin-stabilizing marine natural product geodiamolide H was achieved. Solid-phase based peptide assembly paired with scalable stereoselective syntheses of polyketide building blocks and an optimized esterification set the stage for investigating the key ring-closing metathesis. Geodiamolide H and synthetic analogues were characterized for their toxicity and for antiproliferative effects in cellulo, by characterising actin polymerization induction in vitro, and by docking on the F-actin target and property computation in silico, for a better understanding of structure-activity relationships (SAR). A non-natural analogue of geodiamolide H was discovered to be most potent in the series, suggesting significant potential for tool compound design.     

Colorings Of Plane Graphs With No Rainbow Faces

We prove that each n-vertex plane graph with girth g≥4 admits a vertex coloring with at least ⌈n/2⌉+1 colors with no rainbow face, i.e., a face in which all vertices receive distinct colors. This proves a conjecture of Ramamurthi and West. Moreover, we prove for plane graph with girth g≥5 that there is a vertex coloring with at least if g is odd and if g is even. The bounds are tight for all pairs of n and g with g≥4 and n≥5g/2−3. * Supported in part by the Ministry of Science and Technology of Slovenia, Research Project Z1-3129 and by a postdoctoral fellowship of PIMS. ** Institute for Theoretical Computer Science is supported by Ministry of Education of CzechR epublic as project LN00A056.     

Supramolecular catalysis at work: diastereoselective synthesis of a molecular bowl with dynamic inner space

The supramolecular assistance to Pd(0)/Cu(I)-catalyzed cyclotrimerization of stannylated norbornene 7 has been investigated to give molecular bowl 1syn in a stereoselective fashion. Following a divergent strategy, racemic norbornene 7 was synthesized in satisfactory yield. Self-coupling, promoted by Pd(0)/Cu(I) catalysis acting in synergy with CsF, yielded molecular bowl 1syn in a moderate 30% yield. The reaction diastereoselectivity is affected by the concentration of Cu(I) and Cs+: increasing quantities of the cations enhanced the syn/anti ratio of the isolated cyclotrimer from statistical (1:3) to a more desirable (4.5:1) ratio, in favor of the molecular bowl 1syn. 1H NMR spectroscopic studies suggested the coordinating affinity of 1syn toward transition metals Cu(I), Ag(I), and Au(I), to account for the observed templation effect. In particular, the tridentate 1syn has been shown to bind to one Ag(I) cation in the assembly process that is driven with enthalpy (Delta H degrees = -19 +/- 2 kcal/mol, Delta S degrees = -45 eu). The complete coordination was not cooperative, and was hypothesized to be impeded with the adverse entropy. Accordingly, density functional theory (BP86) calculations of 1syn and its mono-, bis-, and tris-Ag(I) complexes suggested that the coordination of one to three silver cations is highly exothermic. The calculations also revealed that the bowl constriction is necessary for the aromatic arms to become preorganized and bind to a silver cation(s) (Delta E approximately 8 kcal/mol). Ultimately, Ag(I) has been shown to assist the diastereoselective formation of 1syn, lending support to the notion of template-directed synthesis.     

Silver(I) mediated folding of a molecular basket

We have investigated Ag(I) mediated folding of a tridentate compound, containing three pyridine flaps tethered to a semirigid scaffold, into a molecular basket, using both experimental and theoretical methods. The basket formation has been shown to be highly favorable in organic media (Delta G degrees = -7.2 kcal/mol), with the assembly process allowing for another ligand to bind preferentially on the outer side.     

A useful synthetic equivalent of an acetone enolate

2-Methoxymethoxy-3-chloropropene derived organometallics act as synthetic equivalents of an acetone enolate. Indium-promoted reaction of this reagent with aldehydes affords aldol adducts in moderate to excellent yields. When the reaction was performed with zinc, aldol products were isolated in protected form as the corresponding MOM-enol ethers.     

Design, synthesis, and conformational dynamics of a gated molecular basket

We have developed a synthesis and examined the conformational behavior and recognition properties of dynamic molecular containers 1-3. As follows from the 1H NMR dilution, diffusion NMR, and vapor pressure osmometry measurements, compound 1 has a low affinity for intermolecular aggregation and is mostly present in monomeric form in dilute chloroform solutions. Inspecting the O-H chemical shift resonances of 1, 3, and model compound 4 as a function of temperature afforded the deltadelta/deltaT coefficients of 17.0, 17.3, and 4.7 ppb K(-1), respectively. In combination with the results from variable temperature 1H NMR and IR measurements, the existence of conformers of 1 and 3 in equilibrium, each having a different extent of hydrogen bonding, was confirmed. Molecular mechanics calculations suggested 1a as the most favorable conformation, with three additional conformers, 1b, 1c, and 1d, populating local energy minima. Further optimization of each of the four conformers using semiempirical PM3 and ab initio (HF/6-31G) methods allowed a determination of their relative free energies and the corresponding Boltzmann population distributions which were heavily weighted toward 1a. A computed composite IR spectrum of a fraction-weighted mixture of the conformers of 1 reproduced the experimentally observed IR spectrum in its structural features, leading to a conclusion that conformer 1a indeed dominates the equilibrium. The egg-shaped cavity of 1 (136.6 angstroms3) is complementary in size, shape, and electrostatic potential to chloroform (74.9 angstroms3). A single-crystal X-ray study of 2 revealed a disordered chloroform molecule positioned inside the cavitand along its C3 axis.     

6alpha-Angeloylplatynecine: a new alkaloid from Senecio nemorensis subsp. fuchsii (C.C. Gmelin) celak

The new alkaloid 6alpha-angeloylplatynecine, together with other four known pyrrolizidine alkaloids was identified from above ground parts of Senecio nemorensis subs. fuchsii.