Te2As2]2-: a planar motif with "conflicting" aromaticity

[K(18-crown-6)]2[Te2As2] is the first four-membered ring Zintl anion of elements from groups XV and XVI. The anion has an unexpected triplet aromatic ground state.     

Intramolecular [2 + 2] photocycloaddition/thermal fragmentation: formally "allowed" and "forbidden"pathways toward 5-8-5 ring systems

The thermal fragmentation of highly functionalized, linear polycyclobutanes with a cis,syn,cis-relative stereochemistry is shown to offer a rapid entry into the dicyclopenta[a,d]cyclooctenyl (5-8-5) ring system. The thermolysis of polyfused cyclobutanes with a cis,syn,cis- or a cis,anti,cis-relationship proceeds in a formally "symmetry-allowed" manner through the intermediacy of a cis,trans-cyclooctadiene. When a bridging tether used to establish the cis,syn,cis-stereochemistry in the intramolecular [2 + 2] photocyclization is present in the thermolysis step, however, the result of a formally "symmetry-forbidden" fragmentation is observed yielding cis,cis-cyclooctadiene-containing 5-8-5 products. In general, the stereochemical observations noted in these fragmentations offer new opportunities for accessing a variety of stereochemical relationships in these 5-8-5 ring systems.     

"Alpha"-, "beta"- and "delta"-anhydrodigitoxigenin

The syntheses of 3β-hydroxy-5β-carda-14, 20:22-dienolide (= «β»-anhydro-), 3β-hydroxy-5β-carda-8:14, 20:22-dienolide (= «α»-anhydro-) and «δ»-anhydro-digitoxigenin (= probably 3β-hydroxy-5β, 14β-carda-8, 20:22-dienolide) by the best ways known to date, have been described. «δ»-Anhydro-digitoxigenin represents the thermodynamically most stable isomer. In this isomer the double bond in position 8 is unaffected by hydrogenation with Pt in acetic acid; with perbenzoic acid an epoxide results from which, on hydrogenation, the double bond can be regenerated in its original position. Analogous reactions are known to occur in the 8:14-epoxides.     

Synthesis of 2"-bromo-8-methylspiro(4H-3,1-benzooxazine-4,1"-cyclopentan)-2(1H)-one and 2-amino-2"-bromo-8-methylspiro(4H-3,1-benzooxazine-4,1"-cyclopentane) from 6-(cyclopent-1-enyl)-2-methylaniline

The reaction of 6-(cyclopent-1-enyl)-N-ethoxycarbonyl-2-methylaniline with Br₂ or its reaction with NH₃ followed by the reaction with Br₂ afforded 2"-bromo-8-methylspiro(4H-3,1-benzooxazine-4,1"-cyclopentan)-2(1H)-one and 2-amino-2"-bromo-8-methylspiro(4H-3,1-benzooxazine-4,1"-cyclopentane), respectively.     

Sandwich complexes based on the "all-metal" Al4 2- aromatic ring

We report on novel sandwichlike structures [Al(4)MAl(4)](q-) (q=0-2 and M=Ti, V, Cr, Zr, Nb, Mo, Hf, Ta and W) based on the recently synthesized all-metal aromatic Al(4)(2-) square ring. The sandwichlike structures have two aromatic tetraaluminum square rings which trap a transition-metal cation from either the first, second, or third row. The stability of the anionic sandwichlike complexes towards electron detachment is discussed, and addition of alkali cations is found to stabilize the 2- charged complexes, preventing spontaneous electron detachment. Once the sandwichlike complexes are formed, the Al(4)(2-) square properties remain nearly unchanged; this fact strongly supports the hypothesis that in these complexes the Al(4)(2-) square rings remain aromatic.     

"Water-stable boron-iodinated dicarbollide dianions [7,8-nido-C2H2B9I9]2- and [7,8-nido-C2H2B9I8H]2-"

The reaction of 3,4,5,7,8,9,10,11,12-I(9)-1,2-closo-C(2)B(10)H(3) with KOH/EtOH gave a mixture of the boron periodinated [1,2,3,4,5,6,9,10,11-I(9)-7,8-nido-C(2)B(9)H(2)](2-) and the highly iodinated on boron [1,2,4,5,6,9,10,11-I(8)-7,8-nido-C(2)B(9)H(3)](2-) in approximately 50% each. Moreover, 3,4,5,6,7,8,9,10,11,12-I(10)-1,2-closo-C(2)B(10)H(2) was reacted with KOH/EtOH to purely produce [1,2,3,4,5,6,9,10,11-I(9)-7,8-nido-C(2)B(9)H(2)](2-). It is the first dinegative dicarbollide stable in water or protic solvent reported in literature.     

Sandwich-like complexes based on "all-metal" (Al 4 2-) aromatic compounds

We report a new sandwich-like structure (Al4TiAl42-) based on the recently synthesized all-metal aromatic Al42- squares. This structure has two aromatic squares (-2 charged each) trapping a titanium +2 cation. These all-metal aromatic squares have been detected by various metallic counterions, with a global charge of -1. Thus, we have also characterized a similar sandwich-like structure interacting with one and two sodium atoms with global charges of -1 and 0. The properties of these compounds strongly support the idea that they may be formed by Al42- aromatic squares.     

Instability of the Al4(2-) "all-metal aromatic" ion and its implications

Al42 - is a prototype structural unit of a new class of "all-metal aromatic" molecules. Without stabilizing counterions this species is unstable with respect to electron autodetachment in the gas phase. We estimated the height of the repulsive Coulomb barrier to approximately 2.7 eV and calculated a lifetime of 9 fs. This is a short lifetime: The only way to study the isolated dianion experimentally is to use electron scattering techniques. Investigations of the validity of bound-state quantum chemical calculations on the isolated species show that the results suffer from significant admixture of continuum states to the bound-state wave function depending on the basis set. Calculations of molecular properties can therefore give essentially arbitrary results for this ill-defined system, as is demonstrated for the energy and nuclear magnetic shieldings. This substantiates that results from calculations on the isolated dianion should be approached with caution.     

Chemistry of iminofurans: IX. Synthesis and cyclization of (2<Emphasis Type="Italic">Z</Emphasis>)-2-{(2<Emphasis Type="Italic">Z</Emphasis>)-2-[2-(3-R-adamantan-1-yl)-2-oxoethylidene]hydrazinyl}-4-(het)aryl-4-oxobut-2-enoic acids

1-(3-R-adamantan-1-yl)-2-[(triphenyl-λ⁵-phosphanylidene)hydrazinylidene]ethanone reacted with 4-aryl(hetaryl)-2,4-dioxobutanoic acids to give 2-{2-[2-(3-R-adamantan-1-yl)-2-oxoethylidene]hydrazinyl}-4-aryl(hetaryl)-4-oxobut-2-enoic acids which were shown to exist in solution as mixtures of Z- and E-isomeric enehydrazine tautomers. The products underwent cyclization to 3-{[2-(3-R-adamantan-1-yl)-2-oxoethylidene]- hydrazinylidene}-5-aryl(hetaryl)furan-2(3H)-ones.     

Theoretical investigation of the "CO in"-"CO out" isomerization in a [2Fe-2S] ferredoxin: free energy profiles and redox states

This paper reports on extensive molecular dynamics simulations (about 40 ns in total) in both the reduced and the oxidized states of Ferredoxin from Cyanobacterium Anabaena PCC7119. These calculations have provided us with the free energy profile of the phi(47) backbone angle which controls the "CO in" to "CO out" transition of Cys46 in the reduced and oxidized Fd7119. Our main motivation has been to identify the time scales involved in the reduction of Fd and single out the amino acid residues crucially affecting the conformational change and, thus, electron transfer. The free energy profiles obtained in this study are relevant to electron transfers in the PSI/Fd7119 and Fd7119/FNR complexes. Our findings based on hydrated ferredoxin simulations are that activated processes are to occur in the protein during electron transfer to and from ferredoxin. The relative stability and the activation barrier of the "CO in" to "CO out" transition can be modulated by the distance between the Ser47 and the Glu94 residues. In our calculations, for short distances, the "CO in" state is favored in the reduced form, whereas for large distances, the "CO out" state becomes increasingly favored. Accordingly, conformational changes in Fd7119 when bound to PSI or FNR can have crucial effects on the kinetics of the electron transfer. Our simulations also show that the hydrogen bond between between Ser47(OG) and Cys46(O) is essential to lock in the "CO out" state. This finding explains why only the Ser47Thr Fd7119 mutant sustains electron transfer activity, as only residues serine and threonine can form a specific hydrogen bond with Cys46(O). Finally, our simulations predict that Phe65 has a large probability of being in close contact with the Cys46(O) at the top of the conformational free energy barrier. This carbonyl/phenyl ring interaction can then facilitate the de-localization of the Fd's electron toward the Pi orbitals of Phe65 aromatic ring which is thought to be crucial to the Fd7119/FNR electron transfer     

Synthesis and properties of <Emphasis Type="Italic">trans</Emphasis>-2-[2-(2-hetaryl)vinyl]benzothiazoles

Electrophilic substitution reactions (formylation and acylation) in the series of 2-[2-(2-furyl)vinyl]- and 2-[2-(2-thienyl)vinyl]benzothiazoles leads to the corresponding derivatives at the α-position of the furan or thiophene ring. The presence of a vinylene bridge weakens deshielding effect of the benzothiazole fragment on π-excessive heterocycles, so that such compounds react at a higher rate and under milder conditions as compared to hetarylbenzothiazoles having no vinylene bridge.     

No-D NMR study of the pathway for n-BuLi "oxidation" of 1,5-cyclooctadiene to dilithium cyclooctatetraene dianion [Li2COT2-

[Reaction: see text] The transformation of 1,5-cyclooctadiene (1) into cyclooctatetraene (3) by way of dianion 2 is an interesting reaction of considerable preparative value. The mechanism was initially suggested to involve lithium hydride loss from 4a/4b, followed by two deprotonations to produce 2. This No-D 1H NMR study indicates the long-term stability of 4a/4b (in the absence of additional n-BuLi) and suggests a different mechanistic sequence, in which 4a/4b is deprotonated a second time prior to LiH ejection.     

Predissociation spectroscopy of the argon-solvated H5O2+ "zundel" cation in the 1000-1900 cm(-1) region

Predissociation spectra of the H5O2+.Ar(1,2) cluster ions are reported in the 1000-1900 cm(-1) region. The weakly bound argon atoms enable investigation of the complex in a linear action mode, and the resulting spectra are much simpler than those reported previously in this region [Asmis et al., Science 299, 1375 (2003) and Fridgen et al., J. Phys. Chem. A 108, 9008 (2004)], which were obtained using infrared multiphoton dissociation of the bare complex. The observed spectrum consists of two relatively narrow bands at 1080 and 1770 cm(-1) that are likely due to excitation of the shared proton and intramolecular bending vibrations of the two water molecules, respectively. The narrow linewidths and relatively small (60 cm(-1)) perturbation introduced by the addition of a second argon atom indicate that the basic "zundel" character of the H5O2+ ion survives upon complexation.     

N-(S)-(1’-氯甲基-2’-烷基)-2-乙基-2-(4S-烷基-4,5-二氢噁唑)丁酰胺的合成

由L-氨基酸不对称合成了4种新型手性化合物(6a,6b,7a,7b),其结构经IR,~1H NMR,~(13)C NMR及MS等证实。