Concise enantiospecific, stereoselective syntheses of (+)-crispine A and its (-)-antipode

An enantiospecific and stereoselective total synthesis of the natural product (+)-crispine A has been demonstrated employing a Pictet-Spengler bis-cyclization reaction between commercially available (R)-(-)-methyl 2-amino-3-(3,4-dimethoxyphenyl)propanoate and 4-chloro-1,1-dimethoxybutane to preferentially provide the cis tricyclic adduct. Decarboxylation by a convenient two-step protocol provided the enantiopure natural product in three steps with an overall isolated yield of 32% from the amino acid. The unnatural antipode (-)-crispine A was similarly prepared in three steps from the commercially available (S)-(+)-amino acid.     

A simple and efficient method for α-bromination of carbonyl compounds using N-bromosuccinimide in the presence of silica-supported sodium hydrogen sulfate as a heterogeneous catalyst

α-Bromination of carbonyl compounds (cyclic and acyclic ketones, amides and β-ketoesters) has been achieved efficiently by treatment with N-bromosuccinimide (NBS) and catalyzed by silica-supported sodium hydrogen sulfate (NaHSO₄·SiO₂). The products were formed in high yields under mild reaction conditions and in short reaction times.     

Chemoselective deprotection of trityl ethers using silica-supported sodium hydrogen sulfate

A highly selective method for the cleavage of trityl ethers over a wide range of functional groups has been developed using silica-supported sodium hydrogen sulfate (NaHSO₄-SiO₂) as a heterogeneous catalyst. The conversion occurred at room temperature and the yields of the alcohols were found to be excellent.     

Pseudoguaianolides from the Flowers of Parthenium hysterophorus

Two new pseudoguaianolide‐type sesquiterpene lactones, named deacetyltetraneurin A ( 3 ) and hysterone E ( 4 ), were isolated from flowers of the plant Parthenium hysterophorus L., along with the seven known compounds parthenin ( 1 ), coronopilin, tetraneurin A, 8‐β‐hydroxycoronopilin, scopoletin, hysterone D, and conchasin A ( 2 ). The structures of all the compounds were deduced by means of elemental analysis and extensive spectroscopic (1D and 2D NMR) studies, and confirmed for 3 and 4 by X‐ray crystallographic analysis.     

Biflavonoids from Cycas beddomei

Chemical investigation on the constituents of the cones of Cycas beddomei has resulted in the isolation of a new biflavonoid, 2,3-dihydro-4'-O-methyl amentoflavone, along with 2,3,2',3'-tetrahydro hinokiflavone, 2,3,2',3'-tetrahydro amentoflavone, 2,3-dihydro amentoflavone. The last two compounds were not reported earlier from this plant. The structure of the new compound was established by detailed analysis of its spectral (mainly 1D and 2D NMR) data.     

Hypervalent iodine-mediated interaction of aldoximes with activated alkenes including Baylis-Hillman adducts: a new and efficient method for the preparation of nitrile oxides from aldoximes

Treatment of aldoximes with activated alkenes in the presence of diacetoxy iodobenzene (DIB) afforded isoxazolines in high yields. The method has been applied for the preparation of polyfunctional isoxazolines from Baylis-Hillman adducts.     

Origins of selectivity for the [2+2] cycloaddition of alpha,beta-unsaturated ketones within a porous self-assembled organic framework

This article studies the origins of selectivity for the [2+2] cycloadditions of alpha,beta-unsaturated ketones within a porous crystalline host. The host, formed by the self-assembly of a bis-urea macrocycle, contains accessible channels of approximately 6 A diameter and forms stable inclusion complexes with a variety of cyclic and acyclic alpha,beta-unsaturated ketone derivatives. Host 1 crystals provide a robust confined reaction environment for the highly selective [2+2] cycloaddition of 3-methyl-2-cyclopentenone, 2-cyclohexenone, and 2-methyl-2-cyclopentenone, forming their respective exo head-to-tail dimers in high conversion. The products are readily extracted from the self-assembled host and the crystalline host can be efficiently recovered and reused. Molecular modeling studies indicate that the origin of the observed selectivity is due to the excellent match between the size and shape of these guests to dimensions of the host channel and to the preorganization of neighboring enones into favorable reaction geometries. Small substrates, such as acrylic acid and methylvinylketone, were bound by the host and were protected from photoreactions. Larger substrates, such as 4,4-dimethyl-2-cyclohexenone and mesityl oxide, do not undergo selective [2+2] cycloaddition reactions. In an effort to understand these differences in reactivity, we examined these host-guest complexes by thermogravimetric analysis (TGA), NMR, powder X-ray diffraction (PXRD) and molecular modeling.     

Orbit Spaces of Free Involutions on the Product of Two Projective Spaces

Let X be a finitistic space having the mod 2 cohomology algebra of the product of two projective spaces. We study free involutions on X and determine the possible mod 2 cohomology algebra of orbit space of any free involution, using the Leray spectral sequence associated to the Borel fibration ${X \hookrightarrow X_{\mathbb{Z}_2} \longrightarrow B_{\mathbb{Z}_2}}$ . We also give an application of our result to show that if X has the mod 2 cohomology algebra of the product of two real projective spaces (respectively, complex projective spaces), then there does not exist any ${\mathbb{Z}_2}$ -equivariant map from ${\mathbb{S}^k \to X}$ for k ≥ 2 (respectively, k ≥ 3), where ${\mathbb{S}^k}$ is equipped with the antipodal involution.     

Self-assembled phenylethynylene bis-urea macrocycles facilitate the selective photodimerization of coumarin

There is much interest in designing molecular sized containers that influence and facilitate chemical reactions within their nanocavities. On top of the advantages of improved yield and selectivity, the studies of reactions in confinement also give important clues that extend our basic understanding of chemical processes. We report here, the synthesis and self-assembly of an expanded bis-urea macrocycle to give crystals with columnar channels. Constructed from two C-shaped phenylethynylene units and two urea groups, the macrocycle affords a large pore with a diameter of ～9 ?. Despite its increased size, the macrocycles assemble into columns with high fidelity to afford porous crystals. The porosity and accessibility of these channels have been demonstrated by gas adsorption studies and by the uptake of coumarin to afford solid inclusion complexes. Upon UV-irradiation, these inclusion complexes facilitate the conversion of coumarin to its anti-head-to-head (HH) photodimer with high selectivity. This is contrary to what is observed upon the solid-state irradiation of coumarin, which affords photodimers with low selectivity and conversion.     

Oxidative iodination of carbonyl compounds using ammonium iodide and oxone®

A simple, efficient, mild, and regioselective method for oxyiodination of carbonyl compounds has been reported by using NH₄I as the source of iodine and Oxone® as an oxidant. Various carbonyl compounds such as aralkyl ketones, aliphatic ketones (acyclic and cyclic), and β-keto esters proceeded to the respective α-monoiodinated products in moderate to excellent yields. Unsymmetrical aliphatic ketones reacted smoothly yielding a mixture of 1-iodo and 3-iodo ketones with the predominant formation of 1-iodoproduct.