Synthesis of 3-carbomethoxy-3,4-dialkylcyclohexanones

The uncatalyzed 1,4-addition of phenylmagnesium bromide and furylmagnesium iodide to methyl 5-methoxy-1,5-cyclohexadienylcarboxylate ($\text{1}$), directly followed by alkylation and hydrolysis leads to the corresponding cyclohexanones of type $\text{2}$ (R₁ and R₂ trans) in moderate to high yield.     

Synthesen von Moniliformin,einem Mycotoxin mit Cyclobutendion-Struktur

Syntheses of Moniliformin, a Mycotoxine with a Cyclobutenedione Structure Six different routes to 3-hydroxy-3-cyclobutene-1, 2-dione ( 4 ), the free acid of the mycotoxine Moniliformin (=alkali salt of 4 ) are described. A common feature of all pathways is the synthesis of cyclobutanes having the oxidation level 6. Moniliformin precursors which are easily transformed to 4 by acid catalysed hydrolysis include [2+2]-cycloadducts of ketene with tetraalkoxy-olefins, 3,4,4-trialkoxycyclobutenes, derivatives of polyfluorinated cyclobutenes, the brominated [2+2]-cycloadduct of ethyl vinyl ether and dichloroketene, tetrabromocyclobutanone, [2+2]-photocycloadducts of dichlorovinylenecarbonate with dichloroethylenes, and the dimer of chloroketene. The most convenient synthesis via the [2+2]-cycloadduct of tetraethoxyethylene and ketene ( 14b ) is reported in detail and produces 4 in four simple steps in 57% overall yield. In addition, two new syntheses of squaric acid ( 56 ) are described.     

Description of structures in terms of polyhedrapacking

The structures of alloys can be described in terms of polyhedrapacking. We studiedall structure types found in the binary systemsT ⁵ orT ⁶ withB ³ orB ⁴ (T ⁵: V, Nb, Ta;T ⁶: Cr, Mo, W;B ³: Al, Ga, In, Tl;B ⁴: Si, Ge, Sn, Pb) elements. Most of the structures examined until now could be built up with one or two polyhedra, only in a few cases more than two polyhedra are required. It is found that there are two types: a three-dimensional distribution of discrete polyhedra sharing corners, edges or faces and a layer-like distribution. This model proved valid for all structure types studied. Classification of the structures according to their polyhedrapacking criteria is introduced (Table 1). Table 2 includes the coordination numbers of all atoms in the studied structures.     

Raman spectroscopic analysis of the Maya wall paintings in Ek'Balam, Mexico

Raman spectroscopy has been applied to the examination of wall painting fragments from the archaeological site of Ek'Balam (Yucatán, Mexico). Thirty-three samples have been studied, all originating from room 23 of the Acropolis, and being representative of the painting technique at Ek'Balam during the late Classic Maya period. Several pigments such as haematite, calcite, carbon, cinnabar and indigo were identified in these samples. The latter pigment was presumed to be present as 'Maya blue', which is an intercalation product of indigo and palygorskite clay. The observed Raman spectra are reported and some band assignments have been made. This survey is the first Raman spectroscopic examination of a whole set of pigments in archaeological Maya wall painting fragments.     

Asymmetric Catalysis by Vitamin B12. The Mechanism of the Cob(I)alamin-Catalyzed Isomerization of 1,2-Epoxycyclopentane to (R)-Cyclopent-2-enol

The isomerization of 1,2-epcxycyclopentane ( 1 ) to enantiomerically enriched (R)-cyclopent-2-enol ( 2 ) in protic solvents is catalyzed by cob(I)alamin. The enantiomeric excess (e.e.) of (R)- 2 is usually ca. 60%; it is only slightly dependent on the temperature, but increases with decreasing dielectric constant ε of the solvent. Standard kinetic methods show the reaction to be first order in vitamin B₁₂ and zero order in 1 . The rate constant increases exponentially with increasing ε of the solvent. An Arrhenius plot at ε = 40 gives activation parameters ΔH^≠ = 78 ± 4 kJ·mol^?1 and ΔS^≠ = ?49 ± 1 J·mol^?1·K^?1. The isomerization 1 → 2 proceeds in two steps (Schemes 2 and 7): (i) The epoxide ring is first opened by the proton-assisted fast and irreversible nucleophilic attack of the chiral Co^I catalyst to form diastereoisomeric (1R,2R)- and (1S,2S)-(2-hydroxycyclo-pentyl)cob(III)alamins 6 in a ratio of ca. 4:1 which are the dominant species in the steady state; (ii) The intermediates 6 then decompose in the rate-limiting step to form 2 and recycled catalyst. Experiments with specifically ²H-labeled 1 showed the hydro-cobalt elimination 6 → 2 to be non-stereoselective. It proceeds via reversible Co? C bond homolysis to a free 2-hydroxycyclopentyl radical from which stereoelectronically controlled H-abstraction by Co¹¹ takes place.     

Multivalent ion/polyelectrolyte exchange processes in exponentially growing multilayers

We show, in this paper that multivalent ferrocyanide anions can penetrate into exponentially growing (PGA/PAH)n multilayer films whatever the nature of the last deposited layer. These ions are not able to diffuse out of the film when it is brought in contact with a pure buffer solution. However, the contact of this film with a poly(allylamine) (PAH) or a poly(L-glutamic acid) (PGA) solution leads to the release of ferrocyanide ions from the multilayer. It is shown that the release of ferrocyanide anions, when the film is in contact with a PGA solution, is due to the diffusion of the PGA chains into the film so that an exchange between ferrocyanide ions and PGA chains takes place inside the film. On the other hand, PAH chains do not diffuse into PGA/PAH multilayers. When the film is then brought in contact with a PAH solution, the PAH chains from the solution are expected to strongly interact with the ferrocyanide ions and thus induce a diffusion mechanism of the multivalent anions out of the film, the film/solution interface playing the role of a sink for these ions. This work thus shows that interactions between multivalent ions and exponentially growing films are much more complex than expected at first sight and that polyelectrolyte multilayers must be seen as dynamic entities in which diffusion and exchange processes can take place.     

Role of the spacer in the singlet-singlet energy transfer mechanism (Förster vs Dexter) in cofacial bisporphyrins

The cofacial bisporphyrins H4DPS (DPS = 4,6-bis[5-(2,8,13,17-tetraethyl-3,7,12,18-tetramethylporphyrinyl)]dibenzothiophene), H4DPO (DPO = 4,6-bis[5-(2,8,13,17-tetraethyl-3,7,12,18-tetramethylporphyrinyl)]dibenzofuran), H4DPX (DPX = 4,5-bis[5-(2,8,13,17-tetraethyl-3,7,12,18-tetramethylporphyrinyl)]-9,9-dimethylxanthene), H4DPA (DPA = 1,8-bis[5-(2,8,13,17-tetraethyl-3,7,12,18-tetramethylporphyrinyl)]anthracene), and H4DPB (DPB = 1,8-bis[5-(2,8,13,17-tetraethyl-3,7,12,18-tetramethylporphyrinyl)]biphenylene) have been monometalated by Zn(OAc)2.2H2O and by GaCl3 to explore the singlet-singlet energy transfer from the photoexcited metal porphyrin center to the linked free base porphyrin. The spectroscopic (UV-vis and fluorescence) and photophysical properties (fluorescence lifetimes, tauF, and quantum yields, phiF) have been investigated at 298 and 77 K in degassed 2-MeTHF for the donor-acceptor systems, (Zn)H2DPS, (Zn)H2DPO, (Zn)H2DPA, (Zn)H2DPX, and (Zn)H2DPB, as well as for the bis-zinc complexes, (Zn)2DPS, (Zn)2DPO, (Zn)2DPX, and (Zn)2DPB, respectively, and the monoporphyrin derivatives, H2P, (Zn)P, and (Ga-OMe)P (P2- = 5-phenyl-2,8,13,17-tetraethyl-3,7,12,18-tetramethylporphyrin-dianion). The singlet-singlet energy transfer rate constants (KET) were obtained using KET = (1/tauF -1/tauFo), where tauFo is the fluorescence lifetime of the corresponding bis-zinc(II) systems (or (Zn)P and (Ga-OMe)P) where no energy transfer occurs. The tauF value for three bis-zinc(II) compounds varies from 1.69 to 2.01 ns and is 1.84 (at 298 K) and 3.20 ns (at 77 K) for (Ga-OMe)P. In the donor-acceptor bismacrocycles, depending on the spacer and the temperature, the fluorescence lifetimes decrease down to 50-240 ps. The KET values range from approximately 4 to approximately 21 (ns(-1)) and have been analyzed considering both the F?rster and the Dexter mechanisms. Using the C(meso)-C(meso) distance parameters in the calculations, the F?rster and Dexter mechanisms operate for DPS and DPO, and for DPA, DPX, and DPB spacer systems, respectively. The limit distance where one mechanism dominates over the other is estimated to be around 5-6 A.     

Total synthesis of amaryllidaceae alkaloid buflavine

A concise synthesis of the amaryllidaceae alkaloid buflavine (1) and its regioisomer (2) involving sequential Meyers' biaryl coupling, enecarbamate formation, and hydrogenation followed by ultimate intramolecular reductive amination is presented.     

Complexes of divalent nickel and copper with the Schiff base derived from 2-(2-aminophenyl)benzimidazole and benzaldehyde

Summary The reactions of divalent nickel and copper salts with the Schiff base derived from 2-(2-aminophenyl)benzimidazole and benzaldehyde, L, yield complexes of general formulae [CuL₂X₂] (X=Cl, Br, or ClO₄), [CuL(SO₄)], [CuL(SO₄)] · 4H₂O, and NiLX₂ (X=Cl, Br, or NCS).All the complexes have been characterized by elemental analyses, magnetic measurements, e.s.r. electronic and i.r. spectral studies. The results show that the Schiff base acts as bidentate ligand through the pyridine-like imine nitrogen of imidazole ring and the azomethine nitrogen. Tentative structures of the complexes are suggested.     

Pyrylium-containing polymers as sensory materials for the colorimetric sensing of cyanide in water

A sensory polymeric material for the colorimetric sensing of cyanide in water has been developed based on the reactivity of this anion with the pyrylium cation.