Electronic ground states of the V2O4+/0/- species from multireference correlation and density functional studies

The molecular and electronic structures of the V2O4+/0/- species are examined by multireference averaged coupled-pair functional (MR-ACPF) and density functional B3LYP calculations. For all three species, new conformers have been found. Shallow potential energy curves imply high mobility of the oxygen atoms in the neutral and anionic species for which antiferromagnetic coupling of the weakly interacting 3dV electrons is found. Good agreement between the MR-ACPF and B3LYP results for the molecular structures and the relative energies of states with different spin multiplicity, as well as for the ionization energy and electron affinity, is observed. For the computation of the height of the transition barriers between different conformers elaborated MR-ACPF calculations are required.     

Selektive Reaktionen an Cytochalasin D

In connection with the total synthesis of cytochalasans the cleavage of the macrocyclic system of cytochalasin D ( 3 ) was studied in order to gain useful relay compounds. Selective scission of the double bond in 19-position was achieved by controlled ozonolysis leading to compound 7 (Scheme 1). Treatment of 3 with OsO₄ and subsequent acetylation gave the tetraacetoxy- and diacetoxy derivatives 8 and 9 , respectively. Sharpless epoxidation of 3 yielded the mono-, di- and the two epimeric triepoxides 10 , 11 , 12 , and 13 , respectively. Further studies concerned the isomerization of the 6(12)-double bond to 6(7)-double bond by an allylic rearrangement. Treatment of 3 with mesylchloride and triethylamine led to 12-hydroxy-, 12-mesyloxy- and 12-chlorozygosporin ( 14 , 16 , and 17 , resp.) (see Scheme 2). Epoxidation of 14 gave a mixture of the two epimeric 6,7-epoxides 21 and 23. Zn-reduction of 18 (the corresponding bromide of 17 ) led to zygosporin G ( 20 ). In order to convert a carbocyclic cytochalasan into a macrocyclic derivative, 3 was converted to 32 (Scheme 2). Treatment of 32 with H₂O₂ in acetic acid/chloroform or with phenylselenylperacid/H₂O₂ yielded the enollactone 33 . Finally, 17,18-secocytochalasin D derivatives were prepared for the synthesis of unnatural analogs of macrolidic cytochalasans. The diol 26 was converted into the ketoaldehydes 38 and 40 and to the corresponding keto-acids 43 and 44 (Scheme 3), which were reduced to the ω-hydroxycarboxylic acids 45 – 48. Treatment of 47 with 2,2^/-dipyridyldisulfide/triphenylphosphine/xylene gave probably the lactone 50 .     

Relation between different variants of the generalized Douglas-Kroll transformation through sixth order

Wolf et al. have recently investigated a generalized Douglas-Kroll transformation. From a general class of unitary transformations that can be used in the Douglas-Kroll transformation, they pick one which is supposed to give, at a given order, an optimal transformed Dirac Hamiltonian. Results were presented through the fifth order. However, no data were given to demonstrate to which extent the so-called "optimal" Douglas-Kroll transformation is superior to other choices. In this work, the Douglas-Kroll transformation is extended to the sixth order for the first time, using computer algebra algorithms to obtain the working equations. It is shown how, at a given order, different variants of the Douglas-Kroll Hamiltonians are related. Various choices of the generalized transformation are examined numerically for the ground states of the one-electron atomic ions with nuclear charges Z=20, 40, 60, 80, 100, and 120. It is shown that compared to the improvement obtained by including the next order, the differences between various choices for the generalized Douglas-Kroll transformation are almost negligible. Results closest to the Dirac eigenvalues are not obtained with the optimal Douglas-Kroll transformation given by Wolf et al., but with the parametrization originally suggested by Douglas and Kroll.     

π-Spin distribution in the radical anion of benzo[2.2]paracyclophane and its relation to those in the radical anions of [2.2](1,4)naphthalenophanes

Radical anions of benzo[2.2]paracyclophane (V) and its 1,2,12,12,14,15,17,18-octadeuterio derivative (V-d₈) in three ethereal solvents (DME, THF and MTHF) and in the temperature range of ?90 to ?50° have been studied by ESR. and ENDOR. spectroscopy. The resulting hyperfine data provide a detailed picture of the π-spin distribution in V · ? which is in full accord with expectation. In particular, it is noteworthy that the naphthalene moiety accommodates almost the entire π-spin population, as may be anticipated by the higher electron affinity of this π-system relative to benzene. The proton coupling constants for V · ? have been compared with those values for the radical anions of anti- and syn-[2.2](1,4)-naphthalenophanes (II and III, respectively) which were obtained under conditions of low frequency electron transfer between the two equivalent naphthalene moieties. Such a comparison corroborates the interpretation of the results reported previously for II · ? and III · ?.     

Approaches to the synthesis of cytochalasans. Part 10. Cuprates as reagents for the formation of C?C bonds

Based upon our novel concept for the total synthesis of cytochalasans, the model lactams 2–9 were treated with Bu₂Cu(CN)Li₂. The results of these conversions vary much from those obtained with Ph₂Cu(CN)Li₂, demonstrating the uncertainty of predictions in cuprate chemistry. The bicyclic compound 20 was prepared in good yield. However, all attempts to convert p-toluenesulfonate 20 into the Ph-substituted derivative 21 , an intermediate for the synthesis of cyiochulusm B(1) , have failed so far.     

A new and versatile one-pot synthesis of indol-2-ones by a novel Ugi-four-component-Heck reaction

A novel one-pot-synthesis of highly substituted indol-2-ones using a combination of Ugi and Heck reaction (U-4CR-Heck) is described. The synthesized indol-2-ones represent an interesting pharmacological scaffold with four potential points of diversity. Thus, this novel reaction-type is amenable to combinatorial high-throughput screening.     

1H- und 13C-NMR.-Spektroskopie der Nonadride

¹H- and ¹³C-NMR. Spectroscopy of the Nonadrides The ¹H- and ¹³C-NMR. spectra of the nonadrides glaucanic acid ( 1 ), glauconic acid ( 2 ), heveadride ( 3 ), byssochlamic acid ( 4 ), scytalidin ( 5 ), rubratoxin A ( 6 ), and rubratoxin B ( 7 ) have been measured. Partial decoupling experiments permitted assignment of the majority of the signals.     

Hexafluoroacetone as Protecting and Activating Reagent. A New Approach to <Emphasis Type="Italic">N</Emphasis>-linked Glycopeptoids

Summary. A preparatively simple synthesis of N-linked glycopeptoids starting from iminodiacetic acid and glycosylamines, using hexafluoroacetone as protecting and activating reagent is described.     

Orbital-free embedding applied to the calculation of induced dipole moments in CO2...X (X = He, Ne, Ar, Kr, Xe, Hg) van der Waals complexes

The orbital-free frozen-density embedding scheme within density-functional theory [T. A. Wesolowski and A. Warshel, J. Phys. Chem. 97, 8050 (1993)] is applied to the calculation of induced dipole moments of the van der Waals complexes CO2...X (X = He, Ne, Ar, Kr, Xe, Hg). The accuracy of the embedding scheme is investigated by comparing to the results of supermolecule Kohn-Sham density-functional theory calculations. The influence of the basis set and the consequences of using orbital-dependent approximations to the exchange-correlation potential in embedding calculations are examined. It is found that in supermolecular Kohn-Sham density-functional calculations, different common approximations to the exchange-correlation potential are not able to describe the induced dipole moments correctly and the reasons for this failure are analyzed. It is shown that the orbital-free embedding scheme is a useful tool for applying different approximations to the exchange-correlation potential in different subsystems and that a physically guided choice of approximations for the different subsystems improves the calculated dipole moments significantly.     

Early–Late Heterometallic Complexes of Gold and Zirconium: Photoluminescence and Reactivity

OH functionalized triarylphosphines were used to assemble zirconocene‐based metalloligands with phosphine donor sites in varying positions. These complexes were subsequently treated with different gold precursors to obtain early–late heterometallic compounds in which the metal atoms exhibit different intermetallic distances. All compounds were fully investigated by spectroscopic techniques, photoluminescence measurements and single crystal X‐ray diffraction. Quantum chemical calculations were also performed. Some compounds show bright emission even at room temperature with quantum yields of up to 19 % (excitation at 350 nm). Furthermore, the reactivity of dimethyl zirconocene derivatives towards gold complexes was investigated, revealing simultaneous ligand exchange and transmetallation reactions.