Unifying Scheme for the Biosynthesis of Acyl-Branched Sugars: Extended Substrate Scope of Thiamine-Dependent Enzymes.

Thiamine diphosphate (ThDP) dependent enzymes are useful catalysts for asymmetric C-C bond formation through benzoin-type condensation reactions that result in α-hydroxy ketones. A wide range of aldehydes and ketones can be used as acceptor substrates; however, the donor substrate range is mostly limited to achiral α-keto acids and simple aldehydes. By using a unifying retro-biosynthetic approach towards acyl-branched sugars, we identified a subclass of (myco)bacterial ThDP-dependent enzymes with a greatly extended donor substrate range, namely functionalized chiral α-keto acids with a chain length from C4 to C8 . Highly enantioenriched acyloin products were obtained in good to high yields and several reactions were performed on a preparative scale. The newly introduced functionalized α-keto acids, accessible by known aldolase-catalyzed transformations, substantially broaden the donor substrate range of ThDP-dependent enzymes, thus enabling a more general use of these already valuable catalysts.

Cs8[Fe4S10] and Cs7[Fe4S8], two new sulfido ferrates with different tetrameric anions.

The two new cesium sulfido ferrates Cs(8)[Fe(4)S(10)] and Cs(7)[Fe(4)S(8)] were synthesized at a maximum temperature of 1070 K in corundum crucibles from stoichiometric samples containing elemental Fe and S together with cesium disulfide, Cs(2)S(2). Their crystal structures have been determined by means of single-crystal X-ray diffraction. Cs(8)[Fe(III)(4)S(10)] crystallizes in the triclinic Cs(6)[Ga(4)Se(10)]-type structure and is thus isotypic to the corresponding rubidium salt. The structure exhibits tetramers [Fe(4)S(10)](8-) of edge-sharing tetrahedra, which represent segments of the well-known chain compounds A[FeS(2)]. The monoclinic mixed-valent iron(II/III) sulfido ferrate Cs(7)[Fe(4)S(8)], which is isotypic to the cesium tellurido ferrate, likewise contains oligomeric tetramers of four edge-sharing [FeS(4)] tetrahedra, in this case resulting in only slightly distorted tetrahedral [Fe(4)S(8)](7-) anions with a Fe(4)S(4) cubane core resembling the prominent [Fe(4)(µ(3)-S(4))](+) cluster, e.g., in the active site of ferredoxins. These sulfido ferrate anions are surrounded by 26 Cs cations, which are located at the 8 corners, 6 faces, and 12 edges of a cube. A dense stacking of these cubes, which ultimately results in the overall seven cesium countercations per cluster anion, describes the overall crystal structure completely. According to this arrangement of cluster-centered cubes, a relationship of the packing of Cs cations and cluster anions with the simple cubic packing (α-Po-type structure) can be established by applying the crystallographic group-subgroup formalism. FP-LAPW band-structure calculations applying antiferromagnetic spin ordering of the high-spin Fe ions in the two tetramers predict a small band gap of 1 eV associated with a L → M-CT for Cs(8)[Fe(III)(4)S(10)] and a tiny energy gap of 0.1 eV resulting from a d-d transition for the mixed-valent cluster compound Cs(7)[Fe(II/III)(4)S(8)].

Biomimetic asymmetric synthesis of (R)-GTRI-02 and (3S,4R)-3,4-dihydroxy-3,4-dihydronaphthalen-1(2H)-ones.

The NADPH-dependent tetrahydroxynaphthalene reductase (T4HNR) from Magnaporthe grisea was used for the biomimetic synthesis of (R)-GTRI-02 by stereoselective reduction of 1-(3,6,8-trihydroxy-1-methylnaphthalen-2-yl)ethanone. This also led to the isolation of a (3S,4R)-cis-ketodiol formed by T4HNR-catalyzed reduction of the corresponding hydroxynaphthoquinone. Flaviolin and lawsone also reduced to corresponding cis-ketodiols in good yields.

An inorganic starch-iodine model: the inorganic-organic hybrid compound {(C4H12N2)2[Cu(I)I4](I2)}n.

The dark-blue crystal color of {(C(4)H(12)N(2))(2)[Cu(I)I(4)](I(2))}(n), its mixture of I(-), I(3)(-) and linear I(4)(2-) or linear I(5)(-) polyiodide species in a linear channel arrangement, its channel diameter of approximately 5.5 A and the helical arrangement of the hydrogen bonded {(C(4)H(12)N(2))(2)[Cu(I)I(4)]}(+) supramolecular host around the channels agree with the description of the classical, yet structurally elusive, starch-iodine compound.

Li2B12Si2: the first ternary compound in the system Li/B/Si: synthesis, crystal structure, hardness, spectroscopic investigations, and electronic structure.

We present the synthesis, crystal structure, hardness, IR/Raman and UV/Vis spectra, and FP-LAPW calculations of the electronic structure of Li(2)B(12)Si(2), the first ternary compound in the system Li/B/Si. Yellow, transparent single crystals were synthesized from the elements in tin as solvent at 1500 degrees C in h-BN crucibles in arc-welded Ta ampoules. Li(2)B(12)Si(2) crystallizes orthorhombic in the space group Cmce (no. 64) with a=6.1060(6), b=10.9794(14), c=8.4050(8) A, and Z=4. The crystal structure is characterized by a covalent network of B(12) icosahedra connected by Si atoms and Li atoms located in interstitial spaces. The structure is closely related to that of MgB(12)Si(2) and fulfils the electron-counting rules of Wade and Longuet-Higgins. Measurements of Vickers (H(V)=20.3 GPa) and Knoop microhardness (H(K)=20.4 GPa) revealed that Li(2)B(12)Si(2) is a hard material. The band gap was determined experimentally and calculated by theoretical means. UV/Vis spectra revealed a band gap of 2.27 eV, with which the calculated value of 2.1 eV agrees well. The IR and Raman spectra show the expected oscillations of icosahedral networks. Theoretical investigations of bonding in this structure were carried out with the FP-LAPW method. The results confirm the applicability of simple electron-counting rules and enable some structural specialties to be explained in more detail.

Exploring the chemistry of free CI3+: reactions with the weak Lewis bases PX3 (X = Cl-I), AsI3 and Et2O.

What is the preferred coordination site of CI3+? Recent computational work suggested the iodine atoms of the Lewis acid CI3+ to be more electrophilic than the classically expected carbon atom, e.g. the complex with water is of type I2C-I...OH2+ and not the classically expected I3C-OH2+. If this structure is correct, one may also anticipate reactions of CI3+ as an I+ donor. Thus, we were interested in investigating the chemistry of CI3+ in the room-temperature stable salt [CI3]+[pftb](-) ([pftb](-) = [Al(OC(CF3)3)4]-) with weak nucleophiles that i) mimic water (OEt2) or ii) are electronically deactivated weak nucleophiles (PX3, X = Cl-I; AsI3). One question was: is it possible to obtain iodine-coordinated Lewis acid-base adducts of the CI3+ cation? With Et2O as a base, the cation behaves as a strong Lewis acid and cleaves the ether to give I3C-OEt, C2H4 and [H(Et2O)2]+. By contrast PX3 and AsI3 coordinate to the CI3+ cations and the adducts have classical, carbon-bound ethane-like structures, as proven by X-ray single-crystal diffraction, IR, UV-Vis and NMR spectroscopy. From variable temperature 13C NMR studies, it followed for the I3C-AsI3+ salt that the equilibrium between CI3+ and AsI3 is reversible and temperature dependent in solution. The I3C-PI3+ salt decomposes at room temperature giving PI4+ and C2I4, likely through an iodine coordinated I2C-I[dot dot dot]PI3+ intermediate. Thus CI3+ may also act as an I+ donor. All reactions are in agreement with ab initio quantum chemical calculations at the MP2/TZVPP level and assignments of experimental spectra were aided by quantum chemistry.

Tetrapotassium nonastannide, K(4)Sn(9).

In K(4)Sn(9), which crystallizes with a new structure type, the Sn atoms form isolated Wade nido-[Sn(9)]4- clusters of approximate C(4v) symmetry (monocapped square antiprisms), with Sn-Sn distances ranging from 2.9264 (9) to 3.348 (1) A. The cluster anions are separated by K+ cations and are in a hexagonal close-packed arrangement.