Isomerization of (all‐E)‐Cucurbitaxanthin A

Cucurbitaxanthin A (=(all‐E,3S,5R,6R,3′R)‐3,6‐epoxy‐5,6‐dihydro‐β,β‐carotene‐5,3′‐diol; 1 ) was submitted to thermal isomerization and to I₂‐catalysed photoisomerization. The structure of the main reaction products (9Z)‐ ( 2 ), (9′Z)‐ ( 3 ), (13Z)‐ ( 4 ), and (13′Z)‐cucurbitaxanthin A ( 5 ) was determined by their UV/VIS, CD, ¹H‐NMR, and mass spectra.     

(E/Z)‐Isomerization of 3′‐Epilutein

3′‐Epilutein (=(all‐E,3R,3′S,6′R)‐4′,5′‐didehydro‐5′,6′‐dihydro‐β,β‐carotene‐3,3′‐diol; 1 ), isolated from the flowers of Caltha palustris, was submitted to both thermal isomerization and I₂‐catalyzed photoisomerization. The structures of the main products (9Z)‐ 1 , (9′Z)‐ 1 , (13Z)‐ 1 , (13′Z)‐ 1 , (15Z)‐ 1 , and (9Z,9′Z)‐ 1 were determined based on UV/VIS, CD, ¹H‐NMR, and MS data.     

Preparation and (E/Z)‐Isomerization of the Diastereoisomers of Violaxanthin

Violaxanthin A (=(all‐E,3S,5S,6R,3′S,5′S,6′R)‐5,6 : 5′,6′‐diepoxy‐5,6,5′,6′‐tetrahydro‐β,β‐carotene‐3,3′‐diol =syn,syn‐violaxanthin; 5 ) and violaxanthin B (=(all‐E,3S,5S,6R,3′S,5′R,6′S)‐5,6 : 5′,6′‐diepoxy‐5,6,5′,6′‐tetrahydro‐β,β‐carotene‐3,3′‐diol=syn,anti‐violaxanthin; 6 ) were prepared by epoxidation of zeaxanthin diacetate ( 1 ) with monoperphthalic acid. Violaxanthins 5 and 6 were submitted to thermal isomerization and I₂‐catalyzed photoisomerization. The structure of the main products, i.e., (9Z)‐ 5 , (13Z)‐ 5 , (9Z)‐ 6 , (9′Z)‐ 6 , (13Z)‐ 6 , and (13′Z)‐ 6 , was determined by their UV/VIS, CD, ¹H‐NMR, ¹³C‐NMR, and mass spectra.     

Visible‐Light‐Triggered Molecular Photoswitch Based on Reversible E/Z Isomerization of a 1,2‐Dicyanoethene Derivative

A designed bis(dithienyl) dicyanoethene‐based, strictly E/Z photoswitch (4TCE) operates through state‐selective (E and Z isomer) photoactivation with visible light. The E and Z isomers of 4TCE exhibit remarkably different spectroscopic characteristics, including a large separation (70 nm) in their absorption maxima (λ_max) and a 2.5‐fold increase in molar extinction coefficient from cis to trans. The energetically stable trans form can be completely converted to the cis form within minutes when exposed to white light, whereas the reverse isomerization occurs readily upon irradiation by blue light (λ<480 nm) or completely by thermal conversion at elevated temperatures. These features together with excellent thermal stability and photostability of both isomers make this new E/Z photoswitch a promising building block for photoswitchable materials that operate without the need for UV light.     

Living anionic polymerization of (E)‐1,3‐pentadiene and (Z)‐1,3‐pentadiene isomers

The anionic polymerization of (E)‐1,3‐pentadiene (EP) and (Z)‐1,3‐pentadiene (ZP) together with mixture of the E/Z isomers are investigated, respectively. The kinetic analysis shows that the activation energy for EP (86.17 kJ/mol) is much higher than that for ZP (59.03 kJ/mol). GPC shows that it is the EP rather than the ZP isomer that undergoes anionic living polymerization affording quantitative products of the polymers with well‐controlled molecular weights and narrow molecular weight distributions (1.05 ≤? ≤ 1.09). In addition, THF as polar additive has proved its validity to reduce the molecular weight distribution of poly(ZP) from 1.38 to as low as 1.19. The microstructure and sequence distributions of polypentadiene are characterized by ¹H NMR and quantitative ¹³C NMR. Finally, the distinctive reaction activity of two isomers can be elucidated by two different mechanisms which involve the presence of four forms of zwitterions for EP and the typical [1,5]‐sigmatropic hydrogen‐shift phenomenon for ZP. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 2291–2301     

An Alternative Mechanism for the Cobalt‐Catalyzed Isomerization of Terminal Alkenes to (Z)‐2‐Alkenes

The cobalt‐catalyzed selective isomerization of terminal alkenes to the thermodynamically less‐stable (Z)‐2‐alkenes at ambient temperatures takes place by a new mechanism involving the transfer of a hydrogen atom from a Ph₂PH ligand to the starting material and the formation of a phosphenium complex, which recycles the Ph₂PH complex through a 1,2‐H shift.     

Cobalt‐Catalyzed Isomerization of 1‐Alkenes to (E)‐2‐Alkenes with Dimethylphenylsilylmethylmagnesium Chloride and Its Application to the Stereoselective Synthesis of (E)‐Alkenylsilanes

“Co”axing selectivity into isomerization : Treatment of 1‐alkenes with dimethylphenylsilylmethylmagnesium chloride in the presence of a cobalt‐NHC complex in dioxane at 50 °C or higher provides the corresponding (E)‐2‐alkenes selectively. The isomerization is applicable to the stereoselective synthesis of (E)‐crotylsilanes and (E)‐1‐propenylsilanes from the corresponding homoallylsilanes and allylsilanes, respectively.

     

Ligand‐Tuning of the Excitation Wavelength of a Solid state E/Z Isomerization: [Zn(TA)2(2,2′‐Bipyridyl)] in a Supramolecular Framework

The single‐crystal‐to‐single‐crystal (SCSC) E?Z photoisomerization of TA in the supramolecular solid CECR‐[Zn(TA)₂(bpy)]·H₂O (CECR = C‐ethylcalixresorcinarene, HTA = tiglic acid, and bpy = 2,2′‐bipyridine, is induced by 458 nm light, indicating a red‐shift of the photo‐active wavelength on introduction of the aromatic bpy ligand compared with the previously studied reaction of CECR‐[Zn(TA)₂(H₂O)₂] 4H₂O. Theoretical calculations show that the initial excitation involves the bipyridyl ligand, which acts as an intramolecular photosensitizer for the isomerization process. The reaction is topotactic and illustrated by photodifference maps.     

A highly efficient synthesis of (Z)‐1‐aryl‐2‐silyl‐1‐ stannylethenes and their conversion to (E)‐2‐ arylethenyl‐, (Z)‐2‐(2‐pyridyl)ethenyl‐ and allenyl‐silanes

A Pd(dba)₂–P(OEt)₃ combination allowed the silastannation of arylacetylenes, 1‐hexyne or propargyl alcohols with tributyl(trimethylsilyl)stannane to take place at room temperature, producing (Z)‐2‐silyl‐1‐stannyl‐1‐substituted ethenes in high yields. Novel silyl(stannyl)ethenes were fully characterized by ¹H‐, ¹³C‐, ²⁹Si‐ and ¹¹⁹Sn‐NMR as well as infrared and mass analyses. Treatment of a series of (Z)‐1‐aryl‐2‐silyl‐1‐stannylethenes and (Z)‐1‐(3‐pyridyl)‐2‐silyl‐1‐stannylethene with hydrochloric acid or hydroiodic acid in the presence of tetraethylammonium chloride (TEACl) or tetrabutylammonium iodide (TBAI) led to the exclusive formation of (E)‐trimethyl(2‐arylethenyl)silanes with high stereoselectivity. A similar reaction of (Z)‐1‐(2‐anisyl)‐2‐silyl‐1‐stannylethene also produced E‐type trimethyl[2‐(2‐anisyl)ethenyl]silane, while (Z)‐trimethyl [2‐(2‐pyridyl)ethenyl]silane was produced exclusively from (Z)‐1‐(2‐pyridyl)‐2‐silyl‐1‐stannylethene. Protodestannylation of (Z)‐1‐[hydroxy(phenyl)methyl]‐2‐silyl‐1‐stannylethene with trifluoroacetic acid took place via the β‐elimination of hydroxystannane, providing trimethyl(3‐phenylpropa‐1,2‐dienyl)silane quite easily. The destannylation products were also fully characterized. Copyright © 2007 John Wiley & Sons, Ltd.     

Confirmation of the Absolute (3R,3′S,6′R)‐Configuration of (all‐E)‐3′‐Epilutein

Circular dichroism (CD) spectroscopy was used to distinguish between the isomeric (all‐E)‐configured 3′‐epilutein ( 2 ) and 6′‐epilutein ( 8 ) to establish the absolute configuration of epilutein samples of different (natural and semisynthetic) origin, including samples of 2 obtained from thermally processed sorrel. Thus, the CD data of lutein ( 1 ) and epilutein samples ( 2 ) were compared. Our results unambiguously confirmed the (3R,3′S,6′R)‐configuration of all epilutein samples. Compound 2 was thoroughly characterized, and its ¹³C‐NMR data are published herewith for the first time.     

Enantioselective Nickel‐Catalyzed Intramolecular Allylic Alkenylations Enabled by Reversible Alkenylnickel E/Z Isomerization

Enantioselective nickel‐catalyzed arylative cyclizations of substrates containing a Z ‐allylic phosphate tethered to an alkyne are described. These reactions give multisubstituted chiral aza‐ and carbocycles, and are initiated by the addition of an arylboronic acid to the alkyne, followed by cyclization of the resulting alkenylnickel species onto the allylic phosphate. The reversible E /Z isomerization of the alkenylnickel species is essential for the success of the reactions.     

An Alternative Synthesis of the Dopaminergic Drug 2‐Amino‐1,2,3,4‐tetrahydronaphthalene‐5,6‐diol (5,6‐ADTN)

Racemic 2‐amino‐1,2,3,4‐tetrahydronaphthalene‐5,6‐diol (5,6‐ADTN; 4 ) was synthesized from 5,6‐dimethoxynaphthalene‐2‐carboxylic acid ( 14 ) in four steps (60% overall yield; Scheme). The crucial steps of the synthesis are Birch reduction of 14 to the valuable synthon 15 , Curtius reaction and carbamate formation ( 16 ), hydrogenolysis ( 17 ), and demethylation to the biologically active hydrobromide salt 18 of 4 .     

Diethyl (Z,Z)‐3,3′‐(ethane‐1,2‐diyldiimino)dibut‐2‐enoate

The title compound, C₁₄H₂₄N₂O₄, is an intermediate in the synthesis of the corresponding heterocyclic compounds. The mol­ecule lies about a twofold axis and has a `twist' conformation; it adopts the Z,Z configuration, which is shown to be governed mainly by intramolecular hydrogen bonds.     

Mercuric Acetate Mediated Oxidative Cyclization of (2E, 4E)‐1‐(2‐Hydroxyphenyl)‐5‐phenylpenta‐2,4‐dien‐1‐ones: Synthesis of (Z)‐2‐((E)‐3‐Phenylallylidene)benzofuran‐3(2H)‐ones

(2E,4E)‐1‐(2‐Hydroxyphenyl)‐5‐phenylpenta‐2,4‐dien‐1‐ones 1a , 1b , 1c , 1d , 1e on oxidative cyclization with mercuric acetate in dimethylsulphoxide have provided (Z)‐2‐((E)‐3‐phenylallylidene)benzofuran‐3(2H)‐ones 2a , 2b , 2c , 2d , 2e in good yields.     

Four (E,Z,E)‐1‐(4‐alkoxy­phen­yl)‐6‐(4‐nitro­phen­yl)hexa‐1,3,5‐trienes

The crystal structures of the four E,Z,E isomers of 1‐(4‐alk­oxy­phen­yl)‐6‐(4‐nitro­phen­yl)hexa‐1,3,5‐triene, namely (E,Z,E)‐1‐(4‐methoxy­phen­yl)‐6‐(4‐nitro­phen­yl)hexa‐1,3,5‐triene, C₁₉H₁₇NO₃, (E,Z,E)‐1‐(4‐ethoxy­phen­yl)‐6‐(4‐nitro­phen­yl)hexa‐1,3,5‐triene, C₂₀H₁₉NO₃, (E,Z,E)‐1‐(4‐nitro­phen­yl)‐6‐(4‐n‐propoxyphen­yl)hexa‐1,3,5‐triene, C₂₁H₂₁NO₃, and (E,Z,E)‐1‐(4‐n‐butoxy­phen­yl)‐6‐(4‐nitro­phen­yl)hexa‐1,3,5‐triene, C₂₂H₂₃NO₃, have been determined. Inter­molecular N⋯O dipole inter­actions between the nitro groups are observed for the meth­oxy derivative, while for the eth­oxy derivative, two adjacent mol­ecules are linked at both ends through N⋯O dipole–dipole inter­actions between the N atom of the nitro group and the O atom of the eth­oxy group to form a supra­molecular ring‐like structure. In the crystal structures of the n‐prop­oxy and n‐but­oxy derivatives, the shortest inter­molecular distances are those between the two O atoms of the alk­oxy groups. Thus, the nearest two mol­ecules form an S‐shaped supra­molecular dimer in these crystal structures.