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Marine ladder polyethers have attracted the attention of chemists and biologists because of their potent biological activities. Synthetic chemists have attempted to construct their polyether frameworks by epoxide ring‐opening cascades, as Nakanishi hypothesis describes. However, Baldwin's rules of ring closure state that exo‐selective intramolecular cyclization of epoxy alcohols is preferred over endo‐selective cyclization. Herein, we investigated epoxide ring‐opening cascades of polyepoxy alcohols in [EMIM]BF4/PFTB (1‐ethyl‐3‐methylimidazolium tetrafluoroborate /perfluoro‐tert‐butyl alcohol) and found that all‐endo products were formed via epoxide‐to‐epoxonium ring‐opening cyclizations (not restricted by Baldwin's rules, which only apply to intramolecular hydroxyl‐to‐epoxide cyclizations). We determined that the key factor enabling polyepoxy alcohols to undergo a high proportion of all‐endo‐selective cyclization was inhibition of exo‐selective hydroxyl‐to‐epoxide cyclization starting from the terminal hydroxyl group of a polyepoxy alcohol. By introducing a slow‐release protecting group to the terminal hydroxyl group, we could markedly increase the cyclization yields of polyether fragments with hydrogen atoms at the ring junctions. For the first time, we constructed consecutively fused six‐membered‐ring and fused seven‐, eight‐, and nine‐membered‐ring polyether fragments by epoxide‐to‐epoxonium ring‐opening cyclizations through the addition of a suitable Lewis acid. We also suggest that the biosynthesis of marine ladder polyethers may proceed via epoxide‐to‐epoxonium ring‐opening cyclization of polyepoxide. [ABSTRACT FROM AUTHOR]

Marine ladder polyethers have attracted the attention of chemists and biologists because of their potent biological activities. Synthetic chemists have attempted to construct their polyether frameworks by epoxide ring‐opening cascades, as Nakanishi hypothesis describes. However, Baldwin's rules of ring closure state that exo‐selective intramolecular cyclization of epoxy alcohols is preferred over endo‐selective cyclization. Herein, we investigated epoxide ring‐opening cascades of polyepoxy alcohols in [EMIM]BF4/PFTB (1‐ethyl‐3‐methylimidazolium tetrafluoroborate /perfluoro‐tert‐butyl alcohol) and found that all‐endo products were formed via epoxide‐to‐epoxonium ring‐opening cyclizations (not restricted by Baldwin's rules, which only apply to intramolecular hydroxyl‐to‐epoxide cyclizations). We determined that the key factor enabling polyepoxy alcohols to undergo a high proportion of all‐endo‐selective cyclization was inhibition of exo‐selective hydroxyl‐to‐epoxide cyclization starting from the terminal hydroxyl group of a polyepoxy alcohol. By introducing a slow‐release protecting group to the terminal hydroxyl group, we could markedly increase the cyclization yields of polyether fragments with hydrogen atoms at the ring junctions. For the first time, we constructed consecutively fused six‐membered‐ring and fused seven‐, eight‐, and nine‐membered‐ring polyether fragments by epoxide‐to‐epoxonium ring‐opening cyclizations through the addition of a suitable Lewis acid. We also suggest that the biosynthesis of marine ladder polyethers may proceed via epoxide‐to‐epoxonium ring‐opening cyclization of polyepoxide. [ABSTRACT FROM AUTHOR]

Based on the high signal-to-noise ratio (S/N) and high-dispersion spectroscopic observations, we derived the equivalent width of the Li I (λ = 6707.8 Å) and the lithium abundance for 9 active late-type stars. The correlation between the lithium abundance and the X-ray luminosity (lg L X) indicates that the lithium abundance of these 9 active late-type stars with a higher X-ray luminosity tends to be larger than that of those with lower X-ray luminosity. That is to say that the lithium abundances analogs progressively increase as the activity increases for these late-type stars. Considering both the lithium abundance and the rotational velocity decrease with increasing stellar age for main-sequence stars, and activity increasing with increasing rotation velocity for active late-type stars, like lithium abundance, the rotational velocity and stellar activity are also considered good indicators of age for late-type stars. [ABSTRACT FROM AUTHOR]

The rapid synthesis of marine ladder polyethers from polyepoxide precursors (in analogy with the biosynthetic pathway hypothesized by Nakanishi) is hampered by the fact that the exo‐selective epoxide‐opening cyclization cascade that gives THF‐type polyethers is preferred over the endo‐selective cascade that gives the desired products. We found that perfluoro‐tert‐butanol (PFTB) cooperating with 1‐ethyl‐3‐methylimidazolium tetrafluoroborate ([EMIM]BF4) can promote endo‐selective epoxide‐opening cyclization reactions of trisubstituted epoxy alcohols. Starting from readily accessible homochiral polyepoxy alcohols with a methyl group at all the endo‐cyclization sites, we were able to construct polyethers up to five consecutive fused 6‐, 7‐, and/or 8‐membered rings in one step. Notably, molecules with the 7/7/6/6 and 7/7/6/7/6 polyether frameworks of hemibrevetoxin B and brevenal, respectively, could be synthesized in 40 % and 17 % chemical yields. [ABSTRACT FROM AUTHOR]

The rapid synthesis of marine ladder polyethers from polyepoxide precursors (in analogy with the biosynthetic pathway hypothesized by Nakanishi) is hampered by the fact that the exo‐selective epoxide‐opening cyclization cascade that gives THF‐type polyethers is preferred over the endo‐selective cascade that gives the desired products. We found that perfluoro‐tert‐butanol (PFTB) cooperating with 1‐ethyl‐3‐methylimidazolium tetrafluoroborate ([EMIM]BF4) can promote endo‐selective epoxide‐opening cyclization reactions of trisubstituted epoxy alcohols. Starting from readily accessible homochiral polyepoxy alcohols with a methyl group at all the endo‐cyclization sites, we were able to construct polyethers up to five consecutive fused 6‐, 7‐, and/or 8‐membered rings in one step. Notably, molecules with the 7/7/6/6 and 7/7/6/7/6 polyether frameworks of hemibrevetoxin B and brevenal, respectively, could be synthesized in 40 % and 17 % chemical yields. [ABSTRACT FROM AUTHOR]

Reported herein is the construction of chiral aza‐[n.2.1] skeletons by separable epimers of intramolecular [3+2] cross cycloaddition of cyclopropane 1,1‐diesters with chiral sulfinylimines. The epimer products can be separated by silica gel column chromatography. [ABSTRACT FROM AUTHOR]

Abstract: By using the 2.16 m telescope of Xinglong Observing Station of National Astronomical Observatories and its high-dispersion spectrograph, the high-dispersion spectroscopic observations of six T Tauri-type stars with weak emission lines (i.e., weak-line T Tauri-type stars, abbreviated as WTTS) were carried out. The lithium abundances of these weak-line T Tauri-type stars are calculated and the relationships of the lithium abundances with the rotation periods as well as the amplitudes of light variations of these stars are discussed. It is found by this study that the lithium abundance for the weak-line T Tauri-type stars with fast rotations tends to be less than that of those with slow rotations. However, for all these weak-line T Tauri-type stars, the lithium abundances have no conspicuous correlation with the amplitudes of light variations of these stars in the V waveband. [Copyright &y& Elsevier]

A Lewis acid promoted tandem reaction of intramolecular [3+2] cycloaddition/O−H insertion/ester exchange of cyclopropanes with diazocarbonyls is reported to synthesize fused‐oxa‐[n.2.1] skeletons, which tolerates both aryl and alkyl diazocarbonyl substrates. A wide variety of fused‐oxa‐[n.2.1] skeletons were efficiently constructed. [ABSTRACT FROM AUTHOR]

The asymmetric synthesis of Szántay's amine (+)-2, the pivotal precursor for direct access to (+)-cycloclavine (1), is described for the first time in eleven steps with 19.7% overall yield from the commercially available 4-bromoindole. The strategy features an asymmetric induction by Ellman's sulfinimine and rhodium-catalysed isomerization of the C=C bond. [ABSTRACT FROM AUTHOR]

An enantioselective tandem Michael addition/cyclization reaction between trisubstituted 2-nitro-1,3-enynes and cyclic 1,3-diketones catalyzed by a chiral organosquaramide catalyst in the presence of Ag2CO3 was developed. The chiral functionalized chromene products could be obtained in good yield and in 80-99% enantiomeric excess. The structures and absolute configurations of some of the products were confirmed by X-ray crystallographic analysis. [ABSTRACT FROM AUTHOR]