(S,S)‐Lactoyllactic acid and (S,S,S)‐lactoyllactoyllactic acid

The dimeric condensation product of lactic acid, namely (S,S)‐2‐[(2‐hydroxypropanoyl)oxy]propanoic acid, C₆H₁₀O₅, (I), crystallizes with two independent molecules in the asymmetric unit, which both have an essentially planar backbone. The trimeric condensation product, namely (S,S,S)‐3‐hydroxybut‐3‐en‐2‐yl 2‐[(2‐hydroxypropanoyl)oxy]propanoate, C₉H₁₄O₇, (II), has one molecule in the asymmetric unit and consists of two essentially planar parts, with the central C—O bond in a gauche conformation. Both molecules of the dimer are involved in intermolecular hydrogen bonds, forming chains with a C(8) graph set. These chains are connected by D(2) hydrogen bonds to form a two‐dimensional layer. The trimer forms hydrogen‐bonded C(10) and C₂²(6) chains, which together result in a two‐dimensional motif. The Hooft method [Hooft, Straver & Spek (2008). J. Appl. Cryst. 41 , 96–103] was successfully applied to the determination of the absolute structure of (I).     

New ferrocenyl P,S and S,S ligands for asymmetric catalysis

Various new chiral ferrocenyl phosphine-thioethers and thiophosphine-thioethers with planar chirality only have been efficiently synthesized with good overall yields (39–43%) in enantiomerically pure form by a nine-step sequence involving the introduction of the planar chirality by Kagan’s acetal method. These new P,S and S,S ligands have been successfully used in the palladium-catalyzed asymmetric allylic substitution reaction (ee up to 93%).     

Synthesis of α-Oxoketene CyclicO,S and S,S Acetals

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Organische S?uren und S?ureanhydride

Ohne Zusammenfassung     

Section EuNdGa3S7-EuGa4S7 of the ternary system Nd2S3-Ga2S3-EuS

The section EuNdGa₃S₇-EuGa₄S₇ of the ternary system Nd₂S₃-Ga₂S₃-EuS was studied by physicochemical analysis methods (differential thermal, X-ray powder diffraction, and microstructural analyses and microhardness and density measurements). The data obtained were used to construct the state diagram of the section EuNdGa₃S₇-EuGa₄S₇. The section was found to be a quasi-binary section of the ternary system and is of the eutectic type. The coordinates of the eutectic are 64 mol % EuGa₄S₇ and T _melt = 1170 K. A region of solid solutions based on both components was found.     

Electronic structure and reactivity of S—S dications

The electronic structure and reactivity of some S—S dications were studied at the MP2/6-31G* level of theory. The results obtained indicate a stepwise electrophilic addition of disulfonium dication moiety to the double C=C bond to be the preferable mechanism.     

Bond orders and electron delocalization indices for S–N,S–C and S–S bonds in 1,2,3-dithiazole systems

A parametric QTAIM-based (topological) model of bond orders and a modification of the Pauling bond order model are proposed for N,S-containing heterocycles, in particular, for 1,2,3-dithiazoles and 1,2,3-dithiazolium systems, which are prone to the formation of stable radicals and therefore are promising compounds in photovoltaics. These models have been parameterized for covalent S–N, S–C and S–S bonds using the electron delocalization indices. A modified Pauling’s bond order model uses turning radii, that is, the distances within which the potential acting on an electron in a system still tends to return that electron to the atomic basin, and avoids the need to choose the hybridization state of bound atoms arbitrarily.     

(15S,16S,19S,20R,34S)和(15R,16R,19R,20S,34S)氮杂Solamin类似物的合成

以手性四氢吡咯片段3a和3b为起始原料, 采用汇聚式合成策略, 完成氮杂Solamin类似物(threo-trans-erythro)的两个对应的光学异构体2a(15S,16S, 19S,20R, 34S)和2b(15R,16R,19R,20S, 34S)的不对称合成. 化合物的结构以及四氢吡咯片段的立体构型均通过NMR波谱解析的方法得到确证. 这两个非对映异构体具有相近的体外抗肿瘤活性.     

S1←S0 vibronic spectrum and structure of fluoral in the S1 state

The vibronic absorption spectrum of fluoral vapor was studied in the region of the S₁←S₀ electronic transition (313–360 nm). The origin O₀ ⁰⁺) of the transition (29419 cm⁻¹) and a number of fundamental frequencies in the S₀ and S₁ states were determined. The character of intensity distribution in the spectral bands indicates that the electronic excitation leads to significant change of the CF₃ group orientation relative to the molecular frame. Moreover, it was found that the carbonyl fragment of the molecule in the S₁ state has pyramidal structure (in contrast, the carbonyl fragment of the fluoral molecule in the S₀ state is planar). The experimental torsion and inversion energy levels were used for the calculation of internal rotation and inversion potential functions of fluoral molecule in the S₁ state. The potential barriers to internal rotation and inversion were found to be 1270 cm⁻¹ (15.2 kJ mol⁻¹) and 550 cm⁻¹ (6.6 kJ mol⁻¹), respectively. The conformational changes caused by S₁←S₀ electronic excitation in the fluoral molecule are similar to those observed in acetaldehyde and biacetyl molecules and differ from the conformational behavior of hexafluorobiacetyl molecule. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 294–299, February, 1998.     

Mechanisms for proton release during water oxidation in the S2 to S3 and S3 to S4 transitions in photosystem II

The new high-resolution X-ray structure of photosystem II has allowed more detailed studies than before of water oxidation at the oxygen evolving complex (OEC). In the present study the two final S-transitions of water oxidation are studied. The electron coupled proton transfers are followed from the center of the OEC to Asp61, which is considered as the start of the transfer chain through the protein to the lumenal side. It is found that the proton transfers occur in multiple steps. Structures of intermediates and energy diagrams are derived and compared to experimental observations. Since the new experimental structure of the OEC is very similar to the one suggested earlier by density functional calculations, the O-O bond formation step remains essentially the same as the one suggested five years ago. An interesting new result is that the barrier for proton transfer within the OEC actually competes with the O-O bond formation step of being rate-limiting.