Understanding the Fundamental Role of π/π, σ/σ, and σ/π Dispersion Interactions in Shaping Carbon‐Based Materials

Noncovalent interactions involving aromatic rings, such as π‐stacking and CH/π interactions, are central to many areas of modern chemistry. However, recent studies proved that aromaticity is not required for stacking interactions, since similar interaction energies were computed for several aromatic and aliphatic dimers. Herein, the nature and origin of π/π, σ/σ, and σ/π dispersion interactions has been investigated by using dispersion‐corrected density functional theory, energy decomposition analysis, and the recently developed noncovalent interaction (NCI) method. Our analysis shows that π/π and σ/σ stacking interactions are equally important for the benzene and cyclohexane dimers, explaining why both compounds have similar boiling points. Also, similar dispersion forces are found in the benzene???methane and cyclohexane???methane complexes. However, for systems larger than naphthalene, there are enhanced stacking interactions in the aromatic dimers adopting a parallel‐displaced configuration compared to the analogous saturated systems. Although dispersion plays a decisive role in stabilizing all the complexes, the origin of the π/π, σ/σ, and σ/π interactions is different. The NCI method reveals that the dispersion interactions between the hydrogen atoms are responsible for the surprisingly strong aliphatic interactions. Moreover, whereas σ/σ and σ/π interactions are local, the π/π stacking are inherently delocalized, which give rise to a non‐additive effect. These new types of dispersion interactions between saturated groups can be exploited in the rational design of novel carbon materials.     

Conformational analysis of c3′-c8 connected biflavones

Two biflavones isolated from Taxus cuspidata were identified with ginkgetin (1) and sciadopitysin (2) which belong to C3′-C8 connected biflavones. The conformation of 2 in the solid state was determined by X-ray analysis and the conformations of 1 and 2 in the liquid state were discussed using nmr techniques. Complete assignments of ¹H and ¹³C nmr spectra about 1 were made on the basis of COSY, HMQC, HMBC and nOe experiments.     

The Cage Structure of IndanCHF3 is Based on the Cooperative Effects of CH⋅⋅⋅π and CH⋅⋅⋅F Weak Hydrogen Bonds

The structural and energetic features of the C?H???π interaction and the internal dynamics of the CHF₃ group change drastically in going from benzene?CHF₃ to indan?CHF₃, according to the analysis of the rotational spectrum of the latter complex generated in a supersonic expansion.     

Platinum Complexes Containing Pyramidalized Germanium and Tin Dihalide Ligands Bound through σ,σ ME Multiple Bonds

We present the isolation of the first mononuclear dihalogermylene, and mono‐ and dinuclear stannylene complexes of transition metals. These exhibit exceptionally pyramidalized Group 14 centers. Additionally, removal of the halide substituents from the Ge/Sn atom was successfully performed in two ways, halide abstraction and reduction, leading to a variety of unusual structural motifs.     

1,2-Epoxycarotinoide. 2. Mitteilung. Synthese der 1′,2′-Epoxide von β,ψ- und ε,ψ-Carotin (= γ- bzw. δ-Carotin)

The synthesis of the naturally occurring 1′,2′-epoxy-1′,2′-dihydro-β, δ-carotene and 1′,2′-epoxy-1′,2′-dihydro-ε, ψ-carotene is described.     

Shimalactone Biosynthesis Involves Spontaneous Double Bicyclo‐Ring Formation with 8π‐6π Electrocyclization

Shimalactones A and B are neuritogenic polyketides possessing characteristic oxabicyclo[2.2.1]heptane and bicyclo[4.2.0]octadiene ring systems that are produced by the marine fungus Emericella variecolor GF10. We identified a candidate biosynthetic gene cluster and conducted heterologous expression analysis. Expression of ShmA polyketide synthase in Aspergillus oryzae resulted in the production of preshimalactone. Aspergillus oryzae and Saccharomyces cerevisiae transformants expressing ShmA and ShmB produced shimalactones A and B, thus suggesting that the double bicyclo‐ring formation reactions proceed non‐enzymatically from preshimalactone epoxide. DFT calculations strongly support the idea that oxabicyclo‐ring formation and 8π‐6π electrocyclization proceed spontaneously after opening of the preshimalactone epoxide ring through protonation. We confirmed the formation of preshimalactone epoxide in vitro, followed by its non‐enzymatic conversion to shimalactones in the dark.     

Nucleosides. CIX. 2′-Deoxy-ψ-isocytidine, 2 -deoxy-ψ-uridine,and 2′-deoxy-l-methyl-ψ-uridine. Isosteres of deoxycytidine,deoxyuridine and thymidine

2′-Deoxy-ψ-isocytidine (VIIβ), a 2′-deoxy analog of antileukemic ψ-isocytidine and also a C-nucleoside analog of deoxycytidine, was synthesized from ψ-uridine by making use of the newly discovered pyrimidine to pyrimidine transformation reaction [J. Chem. Soc., 14, 537 (1977)]. 2′-Deoxy-ψ-uridine (IIβ) and 2′-deoxy-l-methyl-ψ-uridine (V), both C-nucleoside analogs of deoxyuridine and thymidine, were also synthesized. ψ-Uridine was converted into the 2′-chloro analogs (I) which was reduced with tributyltin hydride to give an α,β-mixture of 2′-deoxy-ψ-uridines. The β-isomer (11β was trimethylsilylated and the product (III) treated with methyl iodide to afford the 1-methyl derivative (IV). After hydrolytic removal of the trimethylsilyl groups from IV, the thymidine analog (V) was obtained in good yield. A crude mixture of II was converted in good yield into an α,β-mixture of 1,3-dimethyl-2 -deoxy-ψ-uridines (VI) by treatment with DMF dimethyl acetal in DMF. Treatment of the β-isomer (VIβ) with guanidine, however, gave the α,β-mixture of 2 -deoxy-ψ-isocytidines (VII). The pure β-isomer (VIIβ) was obtained by thick layer chromatography. The pure α-isomer (VIIα) was obtained when VIα was treated with guanidine. 2 -Deoxy-ψ-isocytidine (VIIβ) and 2 -deoxy-l-methyl-ψ-uridine (V) exhibited inhibitory activity against P815 cells (ID_{5 0} 1.2 μg./ml. and 4.9 μg./ml., respectively) and the thymidine analog V was found to be active against Streptococcus faecium var. duran. J. Heterocyclic Chem., 14, 1119 (1977)     

Nucleophilic Reactions at Tertiary Carbon. Part 2. σ- and π-routes to the 8-hydrindanyl cation

Stereoisomeric ion pairs are implicated as intermediates in the solvolysis of cis and trans-8-hydrindanyl chloride 3 , whereas 4-(cyclopenten-1-yl)butyl tosylate 5 appears to cyclize by way of an unsymmetrically solvated 8-hydrindanyl cation. This follows from the solvolysis products and rates of these compounds in aqueous solvents. The rate and equilibrium constants of the chlorides 3 show that the transition state for the trans-isomer is more stable by 0.5 kcal than the one for the cis-isomer. By inference the intermediates differ by a similar amount of energy. Experimental results are not explained satisfactorily by conformationally isomeric 8-hydrindanyl cations, as suggested in the literature.