Total synthesis of hyacinthacine A1 and 3-epi-hyacinthacine A1

[reaction: see text] Total synthesis of hyacinthacine A(1) and its epimer at C3 is described. The synthesis includes a stereocontrolled carboazidation of a chiral allylsilane as a key step. C-Si bond oxidation and reduction of the azide, with ring-closure, complete the total synthesis, which establishes the absolute configuration of 3.     

Proprietes Zeolithiques des ferrocyanures mixtes: Isothermes d'adsorption et caracterisation de K2Zn3[Fe(CN)6]2, 9 H2O

K₂Zn₃[Fe(CN)₆]₂, 9 H₂O crystallises in the rhombic system. Lattice parameters are a = 12.941 Å, α = 58.01°. Thermal behaviour of this compound, studied by differential thermal analysis, thermogravimetric analysis and X-ray diffractometry, permitted the determination of the conditions for activation. The adsorption isotherms confirm the zeolitic character of this iron cyanide with a relative important adsorption capacity. The water is released in two steps with a contraction of the lattice.     

An approach based on quantum chemistry calculations and structural analysis of a [2Fe-2S] ferredoxin that reveal a redox-linked switch in the electron-transfer process to the Fd-NADP+ reductase

[2Fe-2S] ferredoxins act as electron carriers in photosynthesis by mediating the transfer of electrons from photosystem I to various enzymes such as ferredoxin:NADP(+):reductase (FNR). We have analyzed by density functional theory the possible variations of the electronic properties of the [2Fe-2S] ferredoxin, from the cyanobacterium Anabaena, depending on the redox-linked structural changes observed by X-ray diffraction at atomic resolution (Morales, R.; et al. Biochemistry 1999, 38, 15764-15773). The present results point out a specific and concerted role of Ser47, Phe65, and Glu94 located at the molecule surface, close to the iron-sulfur cluster. These residues were already known to be crucial for efficient electron transfer to FNR (e.g., Hurley, J. K.; et al. Biochemistry 1997, 36, 11100-11117). Our calculations suggest that the Glu94 carboxylate negative charge regulates the electron charge delocalization between the Ser47 CO group and the Phe65 aromatic ring, depending on the redox state. The Glu94 carboxylate is stabilized by a strong hydrogen bond implicating a hydroxyl-containing side chain (i.e., Ser or Thr) at location 47. We propose that the Phe65 ring acts as an intermediary carrier receiving the reducing electron prior to its transfer from the reduced Fd to FNR, in view of its central role in the Fd-FNR interaction.     

α-Diimines as nitrogen ligands for ruthenium-catalyzed allylation reactions and related (pentamethylcyclopentadienyl) ruthenium complexes

The new Cp*Ru(II) (Cp*: pentamethylcyclopentadienyl) complexes Cp*(dab-R)RuCl, [Cp*(dab-R)(MeCN)Ru][PF₆] (dab-R: RNCH-CHNR; R: iso-propyl, mesityl), and [Cp*(cod)(MeCN)Ru][PF₆], are synthesized in high yields by reacting the corresponding α-diimine or 1,5-cyclooctadiene with [Cp*RuCl]₄ and [Cp*(MeCN)₃Ru][PF₆], respectively. The α-diimine ligands are strongly bonded to the ruthenium centre as shown by the subsequent formation of the alkynyl derivatives Cp*(dab-R)RuCCR′ (R′ = tert-butyl or phenyl) and of the cationic derivatives [Cp*(dab-R)(L)Ru][PF₆] (L = CO, PMe₃). The neutral and cationic α-diimine or 1,5-cyclooctadiene ruthenium complexes are compared as catalyst precursors for the ruthenium-catalyzed allylation of diethyl-sodiomalonate and diethylamine with cinnamyl acetate or ethyl cinnamyl carbonate.     

Preparation by yeast reduction and determination of the sense of chirality of enantiomerically pure ethyl (−)-4,4,4-trichloro-3-hydroxy- and (+)-4,4,4-trifluoro-3-hydroxybutanoate

Reduction of ethyl 4,4,4-trichloro- and 4,4,4-trifluoro-3-oxobutanoate by fermenting baker's yeast (Saccharomyces cerevisiae) on a preparative scale (20–50 g in ca. 3 1 of H₂O) gave 70–80% yields of the trichloro- [(?)-(S)- 1a ] and trifluoro-hydroxyesters [(+)-(R) 2a ] of ca. 85 and 45% ee, respectively. Both, (?)- 1a and (+)- 2a could be obtained in > 98% ee by subsequent crystallization (of(?)- 1 , (+)- 2a or the 3,5-dinitrobenzoate (+)- 2b . The absolute configuration of both hydroxyesters was determined (a) by chemical correlation ((?)- 1a ), (b) from the melting diagrams and mixed melting points (differential-scanning calorimetry Fig. 1) of the dinitrobenzoates of the CF₃-derivative (+)- 2a and its CH₃-analogue 8 , and c) by X-ray analysis of the ester 2f from (+)- 2a and (?)-camphanoyl chloride (Fig. 2 and 3).     

Temperature measurements in microfluidic systems: heat dissipation of negative dielectrophoresis barriers

The manipulation of living biological cells in microfluidic channels by a combination of negative dielectrophoretic barriers and pressure-driven flows is widely employed in lab-on-a-chip systems. However, electric fields in conducting media induce Joule heating. This study investigates if the local temperatures reached under typical experimental conditions in miniaturized systems cause a potential risk for hyperthermic stress or cell damage. Two methods of optical in situ temperature detection have been tested and compared: (i) the exposure of the thermo-dependent fluorescent dye Rhodamine B to heat sources situated in microfluidic channels, and (ii) the use of thermoprecipitating N-alkyl-substituted acrylamide polymers as temperature threshold probes. Two-dimensional images of temperature distributions in the vicinity of active negative dielectrophoresis (nDEP)-barriers have been obtained and local temperature variations of more than 20 degrees C have been observed at the electrode edges. Heat propagation via both buffer and channel walls lead to significant temperature increases within a perimeter of 100 microm and more. These data indicate that power dissipation has to be taken into account when experiments at physiological temperatures are planned.     

Free-radical functionalisation of vinylcyclopropanes

Free-radical mediated cyclopropane ring opening of 2-silylbicyclo[3.1.0]hexane 1 has been carried out leading to the corresponding trisubstituted cyclopentenes 5 and 6 in good yield with complete 1,2-stereocontrol. [3+2]-Annulation has also been performed by trapping the resulting radical with electron-rich and electron-poor olefins, leading to the corresponding polycyclic compounds. Further studies on the functionalisation of dihydropyridines and pyrroles using this methodology is also described.     

Stereoselective Reactions of Phthalimido-Substituted Radicals Derived from (±)-Threonine: A comparison with reactions of N-phthaloyliminium ions

Stereoselective reactions of phthalimido-substituted radicals derived from (±)-threonine with different radical traps are reported (Scheme 3, Table 1). A strong influence of the nature of the radical trap on the stereoselectivity was noticed. Small nucleophilic radical traps gave preferentially the syn products. The observed selectivities are explained with the A^1,3 strain model and depend on steric and electronic effects (Fig. 2). Reactions with electrophilic radical traps such as diphenyl diselenide gave the anti diastereoisomers with moderate stereocontrol, presumably due to stereoelectronic effects. The same stereochemical outcome, i.e., preferential formation of the anti products, was observed for the reactions of the related N-phthaloyliminium ion (Scheme 5, Table 2). The stereochemistry of the ionic reaction is rationalized by a Felkin-Anh model (Fig. 3).     

A Giese reaction for electron-rich alkenes

A general method for the hydroalkylation of electron-rich terminal and non-terminal alkenes such as enol esters, alkenyl sulfides, enol ethers, silyl enol ethers, enamides and enecarbamates has been developed. The reactions are carried out at room temperature under air initiation in the presence of triethylborane acting as a chain transfer reagent and 4-tert-butylcatechol (TBC) as a source of hydrogen atom. The efficacy of the reaction is best explained by very favorable polar effects supporting the chain process and minimizing undesired polar reactions. The stereoselective hydroalkylation of chiral N-(alk-1-en-1-yl)oxazolidin-2-ones takes place with good to excellent diastereocontrol.

Giese reaction not anymore limited to electron poor alkenes! A general method for the radical mediated hydroalkylation of electron rich alkenes including enol ethers, silylenolethers, enamides, and enecarbamates has been developed.