Vaporization and thermal decomposition of B,B′,B″-tribromoborazine

Vapor pressure as a function of temperature above solid and liquid (BrBNH)₃ and the thermodynamic parameters of (BrBNH)₃ sublimation and vaporization have been determined using static tensimetry with membrane null-manometer. The thermal decomposition of tribromoborazine has been studied in the saturated and unsaturated vapor regions. Irreversible decomposition occurred at noticeable rates at temperatures higher than 343 K and was accompanied by HBr evolution. In the unsaturated vapor region, thermal decomposition has far lower rates than in the saturated vapor region because of diffusion limitations. The activation energy of condensed-phase thermal decomposition of B,B′,B″-tribromoborazine is 65 ± 3 kJ/mol, and this value proves that tribromoborazine decomposition at low temperatures is not accompanied by opening of the boron-nitrogen cycle.     

Isomerization of B6, B6 − and B6 + clusters

The interconversions between isomers with the same spin multiplicity of neutral B₆ and charged B₆ ^? and B₆ ⁺ clusters have been investigated at the B3LYP/6-311+G* level of theory, including determination of the minimum energy pathways with transition states connecting the corresponding reactants and products. In dynamic calculations, 26 isomers were optimized, including 11 novel isomers. In order to further refine the energies, single-point B3LYP/6-311+G(3df) calculations were carried out on the corresponding B3LYP/6-311+G* geometries of all isomers of B₆, B₆ ^? and B₆ ⁺ and the corresponding isomerization transition states. The stability of each isomer of B₆ (singlet and triplet states), B₆ ^? (doublet state) and B₆ ⁺ (doublet state) was analyzed from both thermodynamic and dynamic viewpoints.     

Structures of Isodoternifolin A and B

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Synthesis and Biological Evaluation of Pyranone Analogues of Territrem B

As a highly selective and potent acetylcholinesterase(AChE)inhibitor,territrem B wasisolated by Ling et al.from a strain of Aspergillus terreus in19791,2.Zhao et alreported mimic preparation of territrem B analogues from triterpenoids and found thatthe enone moiety and aromatic ring played important roles in their syntheticcompounds3,4.To further detect essential pharmacophors for AChE inhibitors,wesimplified the A/B ring system of territrem B and synthesized a number of pyranonepharmacop…     

Enantioselective Total Synthesis of (−)-Cephalotanin B

Cephalotaxus diterpenoids are attractive natural products with intriguing molecular frameworks and promising biological features. As a structurally unusual member, (−)-cephalotanin B possesses an extraordinarily congested heptacyclic skeleton, three lactone units, and nine consecutive stereocenters. Herein, we report an enantioselective total synthesis of (−)-cephalotanin B based on a divergent asymmetric Michael addition reaction, a novel Pauson–Khand/deacyloxylation process discovered in the development of a second-generation stereoselective Pauson–Khand reaction protocol, and an epoxide-opening/elimination/dual-lactonization cascade to construct the challenging propeller-shaped A–B–C ring system as key transformations.     

Asymmetric Synthesis of Perseal B and Viriolin

Perseal B (1) from the leaves of Persea obovatifolia, and viriolin (2) from the leaves of Virola surinamensis belong to 8-O-4' lignan. They show activity against leishmaniasis, a tropical disease transmitted by mosquitoes.     

Synthesis of 5,4″-di-O-succinoylavermectin B1

A reaction of avermectin B₁ with succinic anhydride at ultrahigh pressure (10 kbar) gave previously unavailable 5,4″-di-O-succinoylavermectin B₁, which is of interest as potential antiparasitic agent.

     

Origin of the Bürgi-Dunitz Angle

The Bürgi-Dunitz (BD) angle plays a pivotal role in organic chemistry to rationalize the nucleophilic addition to carbonyl groups. Yet, the origin of the obtuse trajectory of the nucleophile remains incompletely understood. Herein, we quantify the importance of the underlying physical factors quantum chemically. The obtuse BD angle appears to originate from the concerted action of a reduced Pauli repulsion between the nucleophile HOMO and carbonyl π bond, a more stabilizing HOMO-π*-LUMO(C=O) interaction, as well as a more favorable electrostatic attraction.     

The First Total Synthesis of Saurufuran B

First total synthesis of saurufuran B 1, a furanoditerpene, from （E, E）-farnesol 2 and citraconic anhydride 3 through thirteen steps is described. Our work involves two key steps： （1） High regioselective alkylation of 2-（tert-butyldimethylsiloxy）-4-methylfuran 6 with aUylic iodides 5 in the presence of silver wifluoroacetate and （2） Conversion of substituted 7-1actone 7 into furan derivative 8.     

Electrochemical Studies of the Recognition Interaction of Rhodamine B with DNA

The recognition interaction of rhodamine B (RB) with DNA was studied in pH 7.5 Britton-Robinson (B-R) buffer solution by electrochemical techniques. An irreversible oxidation peak at glassy carbon electrode was obtained at 0.92V (vs. SCE). After the addition of DNA into the RB solution, the peak current of RB decreased apparently without the shift of peak potential. The electrochemical parameters such as the charge transfer coefficient α and the electrode reaction standard rate constant ks of RB in the absence and presence of DNA were determined, which did not change, indicating that a non-electroactive complex was formed, so the concentration of RB in the solution decreased and the peak current decreased correspondingly.     

Synthesis of 2-En-l-one-ebelactonyl Benzoates of Territrem B Analogues from Jujubogenin

Two series of territrem B analogues (10a-10c and 18) have been designed and synthesized from jujubogenin 5a which was prepared from jujubogenin glycosides 5b obtained from the leaves of Zizyphusjujuba. The structures of the new compounds were confirmed by 1H-,13C-NMR and MS data. Compounds 10c and 18 showed weak inhibitory effect on AChE at 10^-4 mol/E.     

Divergent Total Syntheses of (−)-Daphnezomines A and B and (+)-Dapholdhamine B

The daphnezomine A-type subfamily of Daphniphyllum alkaloids structurally features a unique aza-adamantane core skeleton and anticipates efficient strategies for completing their syntheses to thoroughly investigate their biological activities. Herein, divergent total syntheses of (−)-daphnezomines A and B and (+)-dapholdhamine B have been accomplished in 16–20 steps from a known epoxide via rapid construction of a common core intermediate. The present work features a Ti-mediated radical cyclization to establish the azabicyclo[3.3.1]nonane ring system, an intramolecular Heck reaction to install the bridgehead all-carbon quaternary stereocenter, a tandem deprotection/reduction/keto amine-carbinolamine tautomerization to furnish the aza-adamantane backbone, and an NIS-promoted 6-endo-trig aminocyclization to assemble the (+)-dapholdhamine B backbone.     

B−B Cleavage and Ring-Expansion of a 1,4,2,3-Diazadiborinine with N-Heterocyclic Carbenes

A 1,4,2,3-diazadiborinine derivative was found to form Lewis adducts with strong two-electron donors such as N-heterocyclic and cyclic (alkyl)(amino)carbenes. Depending on the donor, some of these Lewis pairs are thermally unstable, converting to sole B,N-embedded products upon gentle heating. The products of these reactions, which have been fully characterized by NMR spectroscopy, elemental analysis, and single-crystal X-ray diffraction, were identified as B,N-heterocycles with fused 1,5,2,4-diazadiborepine and 1,4,2-diazaborinine rings. Computational modelling of the reaction mechanism provides insight into the formation of these unique structures, suggesting that a series of B−H, C−N, and B−B bond activation steps are responsible for these “intercalation” reactions between the 1,4,2,3-diazadiborinine and NHCs.     

First Total Synthesis of an Analogue of (±)-Hypargenin B

Hypargenin B, a diterpene with the abietane skeleton, was isolated by Ayhan from the root of an endemic species salvia hypargenia and showed antibacterial activity1. In this diterpene, the junction of A/B ring is trans. In order to provide for studying further the relationship between the structure and bioactivities, we develop a novel route whereby the hypargenin B methyl ether 22 could be obtained in good yield. To our knowledge no total synthetic work has been reported on the title com…     

First synthesis of (±)-cedarmycin B

The synthesis of racemic cedarmycin B,an antibiotic from Streptomyces sp.TP-A0456 was achieved firstly fromγ- butyrolactone.The key step was a Barbier-type addition of 3-bromomethyl-5H-furan-2-one to formaldehyde mediated by zinc, which afforded the soleγ-addition product 4-hydroxymethyl-3-methylene-dihydrofuran-2-one.The final compound was confirmed by ~1H NMR,~(13)C NMR and HRMS.     

Preparation of the C1∼C10 fragment of carbonolide B. A relay approach to carbomycin B

Reaction of methyl 2,3-anhydro-5,6-O-cyclohexylidene-β-D-allofuranoside with 2-methyl-2-propenylmagnesium chloride selectively gave methyl 5,6-O-cyclohexylidene-3-deoxy-3-C-(2-methyl-2-propenyl)-β-D-glucofuranoside, which was converted into the C1∼C10 fragment of carbonolide B by a sequence of reactions involving hydroboration of the prochiral double bond, oxidative cleavage between C5C6, and subsequent stereoselective three-carbon elongation at the C5 position.     

Discovery of Hexagonal Structured Pd–B Nanocrystals

We report on hexagonal close-packed (hcp) palladium (Pd)–boron (B) nanocrystals (NCs) by heavy B doping into face-centered cubic (fcc) Pd NCs. Scanning transmission electron microscopy–electron energy loss spectroscopy and synchrotron powder X-ray diffraction measurements demonstrated that the B atoms are homogeneously distributed inside the hcp Pd lattice. The large paramagnetic susceptibility of Pd is significantly suppressed in Pd–B NCs in good agreement with the reduction of density of states at Fermi energy suggested by X-ray absorption near-edge structure and theoretical calculations.     

Synthesis of New 13-O-Acylavermectin B1 Aglycones

Six new 13-O-acylavermectin B 1 aglycones(3～8) were synthesized from avermectin B1 aglycone and their bioactivities were evaluated against Spodoptera exigua, Spodoptera eridania, Tetranychus urticae and Aphis fabae.