Structural Dynamics and Stereoselectivity of Chiral Benzylideneamine N,C-Chelate Borane Photo-Thermal Isomerization.

New chiral N,C-chelate organoboron compounds based on benzylideneamines (bza) with the general formula of B(bza-R)Mes2 (R=H or Me; Mes=mesityl) are reported. A chiral substituent group R- or S-CH(CH3 )Ph (Ph=phenyl) was introduced to the imine center, which imposed a previously unobserved pseudo- or axial-chirality on the BMes2 , creating distinct diastereomers. NMR spectroscopic studies established that the diastereomers undergo slow exchange in solution at ambient temperature. The chiral N,C-chelate B(bza-R)Mes2 molecules undergo photoisomerization in the same manner as their non-chiral analogues, generating chiral BN-cyclooctatriene (BN-COT) derivatives. Most significantly, by tracking the photoisomerization with circular dichroism (CD) and 1 H NMR spectra along with time-dependent density functional theory (TD-DFT) computational studies, the photoisomerization was established to proceed in a highly stereoselective manner, that is, one diastereomer converts exclusively to the corresponding diastereomer product in the photoreaction.

Atropisomerism in Tertiary Biaryl 2-Amides: A Study of Ar-CO and Ar-Ar' Rotational Barriers.

A rotational barrier study was performed on eight tertiary biaryl 2-amides using variable-temperature (VT) NMR and exchange (EXSY) spectroscopy experiments. Seven out of the eight 2-amido-2'-methylbiphenyls with additional 3- and 6-substitution patterns (1-7) were found to have approximately similar rotational barriers (ΔG⧧Tc = 56.5-67.5 kJ/mol). However, for both 3- and 6-substitution (8), the rotational barrier was found to be significantly higher (ΔG⧧ = 102.6-103.8 kJ/mol). Computational studies performed on all eight compounds gave results in good agreement with the experimental rotational barriers. A transition state in which atropisomerism occurs by a cooperative rotation of the Ar-CO and Ar-Ar' bonds depending on substituent location is proposed.

Triaryl-Boron Functionalized Dinuclear Platinum Complexes Linked by Photoisomerizable Bpe Ligand: Luminescence and Isomerism.

We report herein a series of triarylboron-functionalized Pt(II) dinuclear complexes using a photoactive bis(4-pyridyl)ethylene (bpe) bridging unit. The new Pt(II) complexes display phosphorescence at both room temperature and 77 K. Depending on the conditions under which crystals of the complexes were grown, either the cis- or trans-isomers could be obtained. The trans and cis isomers were found to undergo light-induced isomerization in solution with the trans isomer being the dominant species.

Xylomexicanins I and J: Limonoids with Unusual B/C Rings from Xylocarpus granatum.

Two tetranortriterpenoids with new skeletons, xylomexicanins I and J (1 and 2), were isolated during the investigation of chemical constituents from seeds of the Chinese mangrove, Xylocarpus granatum. Xylomexicanin I (1) is an unprecedented limonoid with bridged B- and C-rings. A biosynthesis pathway for 1 from xylomexicanin F is proposed.

Unusual Fragmentation and Transformation of an N-Heterocyclic Carbene by a Stable Phosphonium-Borane peri-Functionalized Naphthalene.

A 1-phosphonium-8-borane-decorated naphthalene molecule 2 has been found to react with N,N'-dimethylimidazol-2-ylidene (IMe), a popular member of the N-heterocyclic carbene (NHC) family, which converts it into two vinyl-amine fragments one of which is trapped between the phosphonium and borane unit by the formation of a C-C and a B-N bond. The same reactivity was not observed for larger NHC molecules. Control experiments and mechanistic studies have established the involvement of an ylide-borane molecule and an imidazolium salt in addition to IMe carbene in this new transformation of an NHC.

Chemical Constituents of Plants from the Genus Eupatorium (1904-2014).

Eupatorium (family: Compositae), which comprises nearly 1200 species, is distributed throughout tropical America, Europe, Africa, and Asia. Up to now, the reported constituents from the genus Eupatorium involve flavonoids, terpenoids, pyrrolizidine alkaloids, phenylpropanoids, quinonoids, essential oils, and some others, altogether more than 300 compounds. Studies have shown that Eupatorium and its active principles possess a wide range of pharmacological activities, such as cytotoxic, antifungal, insecticidal, antibacterial, anti-inflammatory, and antinociceptive activities. Currently, effective monomeric compounds or active parts have been screened for pharmacological activities from Eupatorium in vivo and in vitro. Increasing amount of data supports application and exploitation for new drug development.

Xylomexicanins C and D, new mexicanolide-type limonoids from Xylocarpus granatum.

Two new limonoids, named xylomexicanins C and D, were isolated from a dichloromethane extract of the seeds of Xylocarpus granatum cultivated in Hainan, China, and their structures were elucidated on the basis of one- and two-dimensional NMR (including 1H, 13C-NMR, DEPT, 1H-1H COSY, HSQC, HMBC, and NOESY) and confirmed by high-resolution mass spectrometry. Xylomexicanin C exhibited antiproliferative activity against human breast carcinoma cells (KT).

Formation of azaborines by photoelimination of B,N-heterocyclic compounds.

Highly fluorescent π-conjugated polycyclic azaborines can be prepared from B,N-heterocyclic compounds with a BR2 -CH2 unit through the elimination of an R-H molecule (see scheme). These clean photoelimination reactions occur both in solution and in polymers doped with the precursors.

PPARγ agonist from Chromolaena odorata.

A phytochemical investigation of Chromolaena odorata resulted in the isolation of five new compounds, 5aα,6,9,9aß,10-pentahydro-10ß-hydroxy-7-methylanthra[1,2-d][1,3]dioxol-5-one (1), 1,2-methylenedioxy-6-methylanthraquinone (2), 3-hydroxy-1,2,4-trimethoxy-6-methylanthraquinone (3), 3-hydroxy-1,2-dimethoxy-6-methylanthraquinone (4), and 7-methoxy-7-epi-medioresinol (5), together with 12 known compounds, odoratin (6), 3ß-acetyloleanolic acid (7), ursolic acid (8), ombuin (9), 4,2'-dihydroxy-4',5',6'-trimethoxychalcone (10), (-)-pinoresinol (11), austrocortinin (12), tianshic acid (13), cleomiscosin D (14), (-)-medioresinol (15), (-)-syringaresinol (16), and cleomiscosin A (17). All the compounds were evaluated for their PPARγ transactivation activity, and compound 6 showed moderate activity with an EC(50) value of 3.10 µM.

Xylomexicanins K-N: limonoids from the leaves and twigs of Xylocarpus granatum.

Six new compounds, xylomexicanins K-N (1-4), granasteroid (5) and 5-methoxy-2-pentylbenzofuran-7-ol (6), along with nine known compounds were isolated from the leaves and twigs of Xylocarpus granatum. Among them, 1 was a biogenetic precursor of 1,8,9-phragmalin limonoid, and 4 represent the first example of degraded A-ring limonoid. The structures of them were elucidated on the basis of one- and two-dimensional NMR spectroscopic data (including 1H, 13C-NMR, DEPT, 1H-1H COSY, HSQC, HMBC, and NOESY) and confirmed by high-resolution mass spectrometry.[Formula: see text].

Novel taxa-4(20),12-diene and 2(3→20)abeotaxane from needles of Taxus canadensis.

A novel 6/8/6-membered taxane with a rare C-12(13)-double bond and rare 2(3→20)abeotaxane were isolated from the needles of Taxus canadensis. Their structures were characterized as 7ß,9α,10ß-triacetoxytaxa-4(20),12-diene-2α,5α,11ß-triol (1) and 2α,7ß,10ß-triacetoxy-5α-hydroxy-2(3→20)abeotaxa-4(20),11-diene-9,13-dione (2) on the basis of 1D and 2D spectroscopic data. 1 is the first example of a natural taxane without substitution at both C-13 and C-14.

Achillinin A, a cytotoxic guaianolide from the flower of Yarrow, Achillea millefolium.

Achillinin A (2ß,3ß-epoxy-1α,4ß,10α-trihydroxyguai-11(13)-en-12,6α-olide, 1), a new guaianolide isolated from the flower of Achillea millefolium, exhibited potential antiproliferative activity to A549, RERF-LC-kj and QG-90 cells with 50% inhibitory concentration (IC(50)) values of 5.8, 10 and 0.31 µM, respectively.

A novel taxane 13-glucoside and other taxanes from the leaves of Taxus cuspidata.

Three new taxanes, 2 α,9 α,10 ß-triacetoxy-13 α-( ß-D-glucopyranosyloxy)taxa-4(20),11-dien-5 α-ol, 5 α,10 ß,13 α-triacetoxytax-11-ene-2 α,7 ß,9 α,20-tetraol, and 5 α,10 ß,13 ß-triacetoxy-2 α,7 ß-dihydroxy-2(3→20) abeotaxa-4(20),11-dien-9-one, were isolated from the leaves of the Japanese yew, Taxus cuspidata. Compound 1 is the first example of a taxane with 13-glycosidic linkage.

Directed Ortho and Remote Metalation of Naphthalene 1,8-Diamide: Complementing SEAr Reactivity for the Synthesis of Substituted Naphthalenes.

Mono- and dianion species of 1,8-naphthalene diamide 2 were generated under sec-BuLi/TMEDA conditions and trapped with a variety of electrophiles to give 2- and 2,7- substituted products 3 and 4. Using Suzuki-Miyaura cross-coupling, mono- and di-iodinated products were converted into the corresponding 2-aryl (5) and 2,7-diaryl (6) products, respectively. The amide-amide rotation barrier of 2 was established by VT NMR, and the structure of fluorenone structure 9, obtained by remote metalation, was secured.

New protein tyrosine phosphatase inhibitors from fungus Aspergillus gorakhpurensis F07ZB1707.

Twelve new compounds, aspergorakhins A-L (1-12) coupled with one known xanthone leptosphaerin D (13), were isolated from the extract of soil-derived fungus Aspergillus gorakhpurensis F07ZB1707. Their structures were elucidated by spectroscopic data analysis including UV, IR, NMR, and HRESIMS. The absolute configurations of 5 and 8-11 were identified using ECD and OR calculations. All compounds were tested by enzyme inhibitory activity assay in vitro. Aspergorakhin A (1) showed selective activities against PTP1B and SHP1 over TCPTP with IC50 values 0.57, 1.19, and 22.97 µM, respectively. Compounds 1 and 2 exhibited modest cytotoxicity against tumor cell lines A549, HeLa, Bel-7402, and SMMC-7721 with IC50 values in the range of 6.75-83.4 µM.

Remarkable reactions and intermediates in titanocene(IV) chemistry: migratory insertion reactions of 2,2-disubstituted-1-alkenes, intramolecular 1,5-σ bond metathesis via ε-agostic interactions, and a rare example of a ß-agostic alkyltitanocene complex.

The compound Cp(2)TiMe(2) reacts with [Ph(3)C][B(C(6)F(5))(4)] in CD(2)Cl(2) at 205 K to give, inter alia, [Cp(2)TiMe(CD(2)Cl(2))][B(C(6)F(5))(4)]. This solvent-separated ion pair reacts in turn with 2,4-dimethyl-1-pentene (DMP) to give a series of cationic species, the first being the alkene complex [Cp(2)TiMe(DMP)](+), which undergoes ready migratory insertion to form the σ-alkyl complex [Cp(2)Ti(CH(2)CMe(2)CH(2)CHMe(2))](+). The latter, which does not contain a ß-hydrogen atom, rearranges rapidly via an unprecedented 1,5-σ bond metathesis reaction involving two isomeric ε-agostic species to give the σ-alkyl species [Cp(2)Ti(CH(2)CHMeCH(2)CMe(3))](+); this does contain a ß-hydrogen atom and, in concurrent processes, eliminates H(2) or 2,4,4-trimethyl-1-pentene (a major product) to form respectively the allylic complex [Cp(2)Ti{η(3)-(CH(2))(2)CCH(2)CMe(3)}](+) (a major product) or the hydride complex [Cp(2)TiH](+). The latter reacts reversibly with free DMP to give the insertion product [Cp(2)Ti(CH(2)CHMeCH(2)CHMe(2))](+) (V, a major product), in which the italicized hydrogen atom engages in a ß-agostic interaction with the metal atom. Compound V is a rare example of both a ß-agostic derivative of a group 4 metallocene and a ß-agostic compound of any metal in which the (1)H resonance of the agostic hydrogen can be identified in the (1)H NMR spectrum (δ -3.43). Interestingly, a NOESY experiment on V indicates slow mutual exchange between the agostic hydrogen atom, the hydrogen atoms on C(1), and those of Me(2). These observations are consistent with the intermediacy of the allylic dihydrogen species [Cp(2)Ti(H(2)){η(3)-(CH(2))(2)CCH(2)CHMe(2)}](+), which loses H(2) to form [Cp(2)Ti{η(3)-(CH(2))(2)CCH(2)CHMe(2)}](+) (a minor product). Support for all steps of the proposed reaction scheme comes from product distributions, from labeling studies utilizing [Cp(2)Ti(CD(3))(CD(2)Cl(2))](+), and from extensive DFT calculations. The observed titanocene-based chemistry stands in stark contrast to that of the analogous zirconium system, in which the unusual but well-characterized cationic methyl alkene complex [Cp(2)ZrMe(DMP)](+) does not undergo migratory insertion and subsequent reactions.

A new taxane with an opened oxetane ring from the needles of Taxus cuspidata.

A new taxoid metabolite was isolated from the MeOH extract of Taxus cuspidata needles. The structure was established as (2alpha,5alpha,7beta,9alpha,10beta,13alpha)-10,13,20-tris(acetyloxy)-1,4,5,7,9-pentahydroxytax-11-en-2-yl benzoate on the basis of spectral analyses including 1H- and 13C-NMR, HMQC, HMBC, and NOESY, and confirmed by HR-FAB-MS.

Multistep Photoisomerization of Dimesitylboron-Functionalized Stilbene Analogues.

Dimesitylboron-functionalized stilbene derivatives have been found to undergo an unusual regioselective photoisomerization upon irradiation at 365 nm. Using NMR to follow the photoreaction, the structures of key reaction intermediates and the final products were established. This photoisomerization occurs in four steps: trans-cis isomerization, Diels-Alder reaction, di-π-methane rearrangement, and ring opening with [1,3]-H migration. This results in the formation of a rare structure with three fused five-membered rings and a six-membered one.

Taxusecone, a novel taxane with an unprecedented 11,12-secotaxane skeleton, from Taxus cuspidata needles.

Taxusecone, 2alpha,7beta,9alpha,10beta-tetraacetoxy-5alpha,12-dihydroxy-11,12-secotax-4(20)-ene-11,13-dione (1), a novel taxane with an unprecedented skeleton, was isolated from the needles of Taxus cuspidata.

Two new 11(15-->1)abeotaxanes with a 2,20-epoxy ring from the needles of Taxus canadensis.

Two 11(15-->1)abeotaxanes having a tetrahydrofuran ring along the carbon atoms C-2, C-3, C-4, C-20 were identified for the first time from the needles of the Canadian yew, Taxus canadensis. The compounds could be identified as 4alpha,10beta,13alpha-triacetoxy-15-benzoyloxy-2alpha,20beta-epoxy-11(15-->1)abeotax-11-ene-5alpha,7beta,9alpha-triol (1) and 4alpha,7beta,9alpha,10beta,15-pentaacetoxy-2alpha,20beta-epoxy-11(15-->1)abeotax-11-ene-5alpha,13alpha-diol (2) on the basis of 1D-, 2D-NMR evidence and high-resolution FABMS analysis. Compound 1 showed weak growth inhibitory activities against T-98 and MM1-CB cells in vitro.