Ernst Egert

Crystalline A-DNA: the X-ray analysis of the fragment d(G-G-T-A-T-A-C-C)

An A-DNA type double helical conformation was observed in the single crystal X-ray structure of the octamer d(G-G-T-A-T-A-C-C), 1, and its 5-bromouracil-containing analogue, 2. The structure of the isomorphous crystals (space group P 61 was solved by a search technique based on packing criteria and R-factor calculations, with use of only low order data. At the present stage of refinement the R factors are 31% for 1 and 28% for 2 at a resolution of 2.25 Å (0.225 nm). The molecules interact through their minor grooves by hydrogen bonding and base to sugar van der Waals contacts. The stable A conformation observed in the crystal may have some structural relevance to promoter regions where the T-A-T-A sequence is frequently found.

Highly regio- and syn-diastereoselective synthesis of 4-hydroxy-1,2-alkadienylcarbamates from α-titanatedl 2-alkynylcarbamates and aldehydes1,2

Abstract N , N- Diisopropyl 2-alkynyl carbamates 9 are deprotonated by n -butyllithium to form lithium compounds 10. After exchange of the cation, titanium reagents 13 add highly diastereoselective to aldehydes with formation of the title compounds 14. In contrast to metallated 2-alkynyl ethers, the reagents 10 and 13 exhibit high γ-regioselectivity in carbonyl addition reactions, due to the chelating carbamoyl group. The syn -configuration of 14 was deduced from the crystal structure of the urethane derivative 21.

Synthesis of highly substituted, diastereomerically and enantiomerically pure 3-acyl-tetrahydrofurans from 4-hydroxy-1-alkenyl-carbamates

Abstract The boron trifluoride promoted condensation of ( Z )- anti -4-hydroxy-1-alkenyl carbamates and aldehydes (or ketones) yields the title compounds with high twofold diastereofacial selectivity. In all, a new flexible strategy is offered for the construction of cis,trans,trans substituted tetrahydrofurans by coupling allylic carbamates and two (different) aldehydes with only two synthetic steps.

Synthesis and rearrangement of dispiro[2.0.3.4]-, dispiro-[3.0.3.3]- and dispiro[2.1.3.3]undecanes. Preferred C4-C5 over C3-C4 and C4-C3 over C5-C6 rearrangements

Abstract The dispiranes 10–12 have been synthesized and rearranged by treatment with acids. With 10, an initial C3-C4 ring enlargement leads to [3.3.3]propellane 21, with 11, an initial C4-C3 ring contraction leads to pentalene 37, and with 12, an initial C4-C5 ring enlargement leads to 42, 43, 44 or 45, depending on the reagent used. The structure of 43 has been determined by crystal structure analysis of the 3,5-dinitrobenzoate 46 derived therefrom. The rearrangement of 10 points to dispirane 47 as potential precursor of (±)-modhephene 48.

Sterically crowded cyclohexanes-2 synthesis, structure and dynamics of hexaspiro[2.0.3.0.2.0.3.0.2.0.3.0]heneicosane

Abstract The synthesis, structure and dynamics of hexaspiro[2.0.3.0.2.0.3.0.- 2.0.3.0]heneicosane 3 are described. 3 adopts a chair conformation in the crystal state but prefers a chair-to-twistboat equilibrium in solution. The activation parameters of the chair-to-chair interconversion of 3 and the closely related hexaspiro[2.0.2.0.2.0.2.0.2.0.2.0]- octadecane ([6]rotane) 4 have been determined by DNMR. Some consequences appertaining to the conformation and dynamics of other fully (cyclo-)alkylated cyclohexanes are discussed.

Reaktionen von Pentaphenylcyclopentadienylnickel-Komplexen mit Thiuramen und Dithiocarbamat: Nickel in den Oxidationsstufen I, II and III

Abstract Tetramethylthiuram monosulfide and tetramethylthiuram disulfide react with [(C5Ph5)Ni(NCCH3)2]BF4 (2) to give the black pentaphenylcyclopentadienyl nickel complex [(C5Ph5)Ni{S2CN(CH3)2}]BF4 (7). Its crystal structure has been determined. It is the first 17-electron cyclopentadienyl nickel(III) compound to be structurally characterized. The formation reaction corresponds formally to an oxidative addition of tetramethylthiuram monosulfide and tetramethylthiuram disulfide to the nickel(II) complex 2. The solid-state magnetic moment of the paramagnetic nickel(III) complex 7 is μeff = 1,77 B.M. The reaction of sodium dimethyldithiocarbamate with [(C4Ph5)Ni(CO)Br] yields the dark-red pentaphenylcyclopentadienyl nickel(II) complex [(C5Ph5)Ni{S2CN(CH3)}] (6). Cyclic voltammetry reveals that the dithiocarbamate complex 6 can be oxidised electrochemically and chemically reversibly at +0.265 V vs. SCE to give the cation 7. The reduction at −1.43 V leads to the nickel(I) complex [(C5Ph5Ni{S2CN(CH3)2}]−. The nickel(III) complex 7 can also be prepared on a preparative scale by oxidation of 6 with ferrocene(1 + )tetrafluoroborate in dichloromethane. The reduction of 7 to give 6 is possible in dichloromethane with tetrakis(dimethylamino)ethylene as the reducing agent.

Stereoselective synthesis of functionalized, tetra- and penta-substituted 1,3-butadienes by allene claisen rearrangement

Abstract syn -Allene carbinol 1a or its esters 11 are transformed to sterically loaded 1,3-alkadien-1-yl carbamates 6 and 15 by Claisen rearrangement with high stereoselectivity. The structures of 6 and 15a were elucidated by single crystal X-ray analyses.

Synthesis of butenolides from α-(phenylthio)-ketones and -esters: crystal structure of an intermediate β-phenylthio-γ-lacton

Alkylation of α-(phenylthio)-ketones or -esters with iodoacetate anion gives 1,4-dicarbonyl compounds which are reduced stereoselectively to β-phenylthio-γ-butyrolactones. Oxidation to sulphoxides and thermolysis provides a general synthesis of β- and γ-substituted Δαβ-butenolides. Treatment of 5,5-dimethyl-4-oxo-3-(phenylthio)-hexanoic acid with NaBH4 gives a single γ-lactone whose PhS and But groups are shown to be cis by an X-ray crystal structure determination.

Preparation and structural investigations of fluorinated tungsten (VI) alkoxides

Abstract The preparation and characterization of compounds of composition Cl6-xW(OCH2CF3)x (x = 1−6) and X4W[OC(CH3) (CF3)2]2 (X = Cl,F) together with 19F NMR spectra are reported. An X-ray structure analysis of Cl4W[OC(CH3)(CF3)2]2 shows a W-O-C angle of 173.6°, which is unusually large for metal alkoxides, and a short W-O distance of 181.9 pm. A discussion on structural aspects and a comparison with related tungsten-nitrogen compounds are presented. Cl4W[OC(CH3) (CF3)2]2 is monoclinic, space group C2/m, with a = 820.3(2), b = 1059.2(2), c = 1071.6(2) pm, β = 99.72(2)°, Z = 2 and was refined to R = 0.031 for 848 observed reflections.

New cytotoxic principles from Datisca glomerata

Datisca glomerata has been systematically fractionated by following cellular toxicity in an effort to identify previously uncharacterized cytotoxic principles. Several new cucurbitacin glycosides, including datiscosides B(8), C(3), D(9), E(4), F(5), G(6), and H(10) and the known compound datiscoside (1), as well as cucurbitacins B(11), D(2), and F(7) have been isolated and characterized. Structures were assigned to the compounds on the basis of their high field 1H n.m.r., 13C n.m.r., high-resolution mass spectra (CI, EI, and FD) and chemical interconversions. The structure of datiscoside C(3) was independently established by single-crystal X-ray analysis at 193 and 293 K.