Synthese von optisch aktiven,natürlichen Carotinoiden und strukturell verwandten Naturprodukten. VIII. Synthese von (3S,3′S)-7,8,7′,8′-Tetradehydroastaxanthin und (3S,3′S)-7,8-Didehydroastaxanthin (Asterinsäure)

Synthesis of Optically Active Natural Carotenoids and Structurally Related Compounds. VIII. Synthesis of (3S,3′S)-7,8,7′,8′-Tetradehydroastaxanthin and (3S,3′S)-7,8-Didehydroastaxanthin (Asterinic Acid) The synthesis of all-trans-(3S,3′S)-3,3′-dihydroxy-7,8, 7′,8′-tetradehydro-β, β-carotene-4,4′-dione ( 1 ), of all-trans-(3S,3′S)-3,3′-dihydroxy-7, 8-didehydro-β,β-carotene-4,4′-dione ( 2 ) (asterinic acid = mixture of 1 and 2 ), and of their 9,9′-di-cis- and 9-cis-isomers is reported starting from (4′S)(2E)-5-(4′-hydroxy-2′, 6′,6′-trimethyl-3′-oxo-l′-cyclohexenyl)-3-methyl-2-penten-4-ynal ( 8 ). The absolute configuration (3S,3′S) for both components 1 and 2 of asterinic acid ex Asterias rubens is confirmed on the basis of spectroscopic and direct comparison.     

Die thermische Umlagerung von 2-(Buta-1′,3′-dienyl)-phenolen in 2-Alkyl-2H-chromene; Beispiele für aromatische [1,7a]- und [1,5s]sigmatropische Wasserstoffverschiebungen

2-(1′-cis,3′-cis-)- and 2-(1′-cis,3′-trans-Penta-1′,3′-dienyl)-phenol (cis, cis- 4 and cis, trans- 4 , cf. scheme 1) rearrange thermally at 85–110° via [1,7 a] hydrogen shifts to yield the o-quinomethide 2 (R ? CH₃) which rapidly cyclises to give 2-ethyl-2H-chromene ( 7 ). The trans formation of cis, cis- and cis, trans- 4 into 7 is accompanied by a thermal cis, trans isomerisation of the 3′ double bond in 4. The isomerisation indicates that [1,7 a] hydrogen shifts in 2 compete with the electrocyclic ring closure of 2 . The isomeric phenols, trans, trans- and trans, cis- 4 , are stable at 85–110° but at 190° rearrange also to form 7 . This rearrangement is induced by a thermal cis, trans isomerisation of the 1′ double bond which occurs via [1, 5s] hydrogen shifts. Deuterium labelling experiments show that the chromene 7 is in equilibrium with the o-quinomethide 2 (R ? CH₃), at 210°. Thus, when 2-benzyl-2H-chromene ( 9 ) or 2-(1′-trans,3′-trans,-4′-phenyl-buta1′,3′-dienyl)-phenol (trans, trans- 6 ) is heated in diglyme solution at >200°, an equilibrium mixture of both compounds (～ 55% 9 and 45% 6 ) is obtained.     

Flavonoid Aglycones and Indole Alkaloids from the Roots of Acacia Confusa

From the methanolic extract of the roots of Acacia confusa Merr. (Leguminosae), (‐)‐2,3‐cis‐3,4‐cis‐4′‐methoxy‐3,3′,4,7,8‐pentahydroxyflavan ( 1 ), (‐)‐2,3‐cis‐3,4‐cis‐3,3′,4,4′,7,8‐hexahydroxyflavan ( 2 ), (‐)‐2,3‐trans‐3′,4′,7,8‐tetrahydroxydihydroflavonol ( 3 ), (+)‐catechin ( 4 ), (‐)‐epicatechin ( 5 ), 3′,4′,7,8‐tetrahydroxyflavonol ( 6 ) together with N‐methyltryptamine ( 7 ) and N,N‐dimethyltryptamine ( 8 ) were isolated, and their structures were established by analysis of their spectroscopic data. Among them, compound 1 was a new flavonoid. Additionally, the results of chromatographic bioassay on lettuce seeds indicated that compounds 7 and 8 exhibited significant phytotoxicity at a concentration lower than 14 mM.     

(6′RS,8′RS,2E)- und (6′RS,8′SR,2E)-3-Methyl-3-(2y,2′,6′-trimethyl-7′-oxabicyclo[4.3.0]non-9′-en-8′-yl)-2-propenal ([(5RS,8RS)- und (5RS,8SR)-5,8-Epoxy-5,8-dihydro-ionyloinden]acetaldehyd): Synthese und Röntgenstrukturanalyse

Synthesis and X-Ray Structure of (6′RS,8′RS,2E)- and (6′RS,8′SR,2E)-3-Methyl-3-(2′,2′,6′-trimethyl-7′-oxabicyclo[4.3.0]non-9′-en-8′-yl)-2-propenal ([(5RS,8RS)- and (5RS,8SR)-5,8-Epoxy-5,8-dihydro-ionylidene]acetaldehyde) To check our previous spectroscopic assignments of the structures of trans- and cis-substituted furanoid end groups of carotenoid-5,8-epoxides, we now have synthesized the title compounds. An X-ray structure determination of a single crystal of the trans-isomer (±)- -10A is in agreement with the ¹ H-NMR spectroscopic arguments: isomers with Δδ (H? C(7), H? C(8)) = 0.15–0.22 ppm and J > 1.4 for H? C(7) belong to the cis-series; Δδ in trans-compounds is < 0.07 ppm, and H? C(7) appears as a broad singulett.     

Cyclopentyl derivatives of 8-azahypoxanthine and 8-azaadenine. Carbocyclic analogs of 8-azainosine and 8-azaadenosine

Four types of carboeyclic analogs of 8-azahypoxanthine and 8-azaadenine nucleosides have been prepared. This group of analogs is comprised of derivatives having the (±)-as-3-(hydroxy-methyl)cyclopentyl,(±)-trans-3-hydroxy-cis-4-(hydroxymethyl)cyclopentyl), (±)-trans-2-hydroxy-cis-4-(hydroxymethyl)cyclopentyl, and (±)-trans-2, trans-3-dihydroxy-cis-4-(hydroxymethyl)-cyelopentyl groups at position 3 of 3,6-dihydro-7H-v-triazolo[4,5-d]pyrimidm-7-one and of 7-amino-3H-v-triazolo[4,5-d]pyrimidine. Diazotization of (5-amino-6-chloropyrimidin-4-yl-amino)eyclopentane derivatives and acidic hydrolysis, without isolation of the resulting 7-chloro-3H-v-triazolo[4,5-d]pyrimidines yielded the 8-azahypoxanthine derivatives (III). Treatment of unpurified 7-chloro-3H-v-triazolo[4,5-d]pyrimidines with anhydrous ammonia gave the 8-azaadenine derivatives (IV). The cyclopentane analogs of 8-azaadenylic acid and of 8-aza-adenosine 3′,5′ -cyclic monophosphate were prepared from the 8-azaadenosine analog.     

Reaction of 1,1′-iminobis-2-butanols with sulfuric acid

Treatment of 1,1′-iminobis-2-butanols with 70% w/w sulfuric acid gives cis- and trans-2,6-diethylmorpholines, 3-ethyl-4-methylpyridine and unsaturated 3-ethyl-4-methylpiperidines. Hydrogenation of the unsaturated 3-ethyl-4-methylpiperidines gives cis- and trans-3-ethyl-4-methylpiperidines. With 50% w/w sulfuric acid cis- and trans-2,6-diethylmorpholines and a small amount of cis- and trans-2-ethyl-7-methyl-hexahydro-1,4-oxazepines are obtained.     

Über die absolute Konfiguration der Visnagane

(+)-cis-Khellactone methyl ether ( 4 ) and (?)-trans-khellactone methyl ether ( 6 ) had earlier been assigned the absolute configurations 3′-S; 4′-S and 3′-S; 4′-R, respectively, on the basis of the FREUDENBERG , rule. Both compounds together with their defunctionalised derivatives (?)- 7 and (+)- 8 (=(+)-lomatin), obtained from a mixture of (+)-visnadin ( 1 ) and (+)-samidin ( 2 ), were investigated by the HOREAU method. A conformational analytical study showed that the optical yield should rise in the order 4 < 6 < 7 , 8. This order was found and the α-phenylbutyric acid liberated was always dextrorotatory. The centre 3′ of the khellactones and their derivatives must be R-chiral and not S. Treatment of (?)- 6 with pyridinium perbromide gave (?)-trans-3-bromokhellactone methyl ether ( 11 ) as orthorhombic crystals. The X-ray crystal structure determination was made using the anomalous scattering of the Mo-K α radiation by Br. The result, — centre 3′ R-chiral (fig. g) — showed that the HOREAU method was correct.     

Optisch aktive 4,5-Epoxy-4,5-dihydro-α-ionone und Synthese der stereoisomeren 4,5:4′,5′-Diepoxy-4,5,4′,5′-tetrahydro-ϵ,ϵ-carotine und der sterische Verlauf ihrer Hydrolyse

Optically Active 4,5-Epoxy-4,5-dihydro-α-ionones; Synthesis of the Stereoisomeric 4,5:4′,5′-Diepoxy-4,5,4′,5′-tetrahydro-?,?-carotenes and the Steric Course of their Hydrolysis We prove that epoxidation with peracid of α-ionone, contrary to a recently published statement, predominantly leads to the cis-epoxide. Acid hydrolysis affords a single 4,5-glycol whose structure, established by an X-ray analysis, shows that oxirane opening occurred with inversion at the least substituted position (C(4)). Stable cis-and trans-epoxides are prepared by epoxidation of the C₁₅-phosphonates derived from α-ionone. Both the racemic and optically active form are used for the synthesis of the 4,5:4′,5′-diepoxy-4,5,4′,5′-tetrahydro-?,?-carotenes having the following configuration in the end groups: meso-cis/cis, meso-trans/trans, rac-cis/trans, rac- and (6R, 6′ R)-cis/cis, rac- and (6R, 6′R)-trans/trans, rac- and (6R, 6′R)-cis/trans, and (6R, 6′ R)-cis/?. Acid hydrolysis of the cis/cis-epoxycarotenoids under relatively strong conditions occurs again with inversion at C(4)/C(4′) in case of the cis/cis-epoxycarotenoids, but at C(5)/C(5′) in case of the trans/trans-epoxycarotenoids. An independent synthesis of this 4,5,4′,5′-tetrahydro-?,?-carotene-4,5,4′,5′-tetrol is presented. The irregular results of the oxirane hydrolysis are explained by assumption of neighbouring effects of the lateral chain. 400-Mz-¹H-NMR data are given for each of the stereoisomeric sets. In the visible range of the CD spectra, the (6R, 6R′)-epoxycarotenoids compared with (6R, 6R′)-?,?-carotene exhibit an inversion of the Cotton effects.     

New Monoterpene-Substituted Dihydrochalcones from Piper aduncum

Five New unusual monoterpene-substituted dihydrochalcones, the adunctins A–E (1″S)-1-{2′-hydroxy-4′-methoxy-6′-[4″-methyl-1″-(1?-methylethyl)cyclohex-3″ -en-1″ -yloxy]phenyl}-3-phenylpropan-1-one ( 1 ), (5aR*,8R*,9aR*)-3-phenyl-1-[5′,8′,9′,9′a-tetrahydro-3′-hydroxy-1′-methoxy-8′-(1″-methylethyl)-5′-a-methyldibenzo-[b,d]furan-4′-yl]propan-1-one ( 2 ), (2′R*,4″S*)-1-{6′-hydroxy-4′-methoxy-4″-(1?-methylethyl)spiro[benzo[b]-furan-2′(3′H),1″ -cyclohex-2″ -en]-7′-yl}-3-phenylpropan-1-one ( 3 ), (2′R*,4″R*)-1-{6′-hydroxy-4′-methylethyl-4″-(1?-methylethyl)spiro[benzo[b]furan-2′(3′H),1″-cyclohex-2″-en]-7′-yl}-3-phenypropan-1-one ( 4 ), and (5′aR*,6′S*, 9′R*,9′aS*)-1-[5′a,6′,7′,8′,9′a-hexahydro-3′,6′-methoxy-6′-methyl-9′-(1″-methylethyl)dibenzo[b,d]-furan-4′-yl]-3-phenylpropan-1-one ( 5 ) were isolated from the leaves of Piper aduncum (Piperaceae) by preparative liquid chromatography. In addition, (?)-methyllindaretin ( 6 ), trans-phytol, and α-tocopherol ( = vitamin E) were also isolated and identified. The structures were elucidated by spectroscopic methods, including 1D- and 2D-NMR spectroscopy as well as single-crystal X-ray diffraction analysis. The antibacterial and cytotoxic potentials of the isolates were also investigated.     

Isolation and Identification of Three Major Metabolites of Retinoic Acid from Rat Feces

Following the intraperitoneal administration of high doses of ¹⁴C- and ³H- labelled retinoic acid (1) to rats, three major metabolites and the intact compound were isolated from the feces in microgram amounts by use of column, thin-layer and high-pressure liquid chromatography. Their structures were elucidated by mass spectrometry and Fourier Transform ¹H-NMR. spectroscopy as 2 (all-trans-4-oxoretinoic acid), 3 (7-trans-9-cis-11-trans-13-trans-5′-hydroxy-retinoic acid). Hydroxylation of the 5-methyl group of the cyclohexene ring, oxidation of the cyclohexene ring in position 4 and cis-trans isomerisation of the nonatetraenoic acid side chain were the reactions, which produced these products from retinoic acid. The metabolites 2 and 4 each accounted for about 4% of the radioactivity administered. The metabolite 3 and the parent compound accounted for about 16% and 17% of the dose, respectively.     

Regio‐ and stereospecific assembly of dispiro[indoline‐3,3′‐pyrrolizine‐1′,5′′‐thiazolidines] from simple precursors using a one‐pot procedure: synthesis,spectroscopic and structural characterization,and a proposed mechanism of formation

The synthesis and characterization of three new dispiro[indoline‐3,3′‐pyrrolizine‐1′,5′′‐thiazolidine] compounds are reported, together with the crystal structures of two of them. (3RS,1′SR,2′SR,7a′SR)‐2′‐(4‐Chlorophenyl)‐1‐hexyl‐2′′‐sulfanylidene‐5′,6′,7′,7a′‐tetrahydro‐2′H‐dispiro[indoline‐3,3′‐pyrrolizine‐1′,5′′‐thiazolidine]‐2,4′′‐dione, C₂₈H₃₀ClN₃O₂S₂, (I), (3RS,1′SR,2′SR,7a′SR)‐2′‐(4‐chlorophenyl)‐1‐benzyl‐5‐methyl‐2′′‐sulfanylidene‐5′,6′,7′,7a′‐tetrahydro‐2′H‐dispiro[indoline‐3,3′‐pyrrolizine‐1′,5′′‐thiazolidine]‐2,4′′‐dione, C₃₀H₂₆ClN₃O₂S₂, (II), and (3RS,1′SR,2′SR,7a′SR)‐2′‐(4‐chlorophenyl)‐5‐fluoro‐2′′‐sulfanylidene‐5′,6′,7′,7a′‐tetrahydro‐2′H‐dispiro[indoline‐3,3′‐pyrrolizine‐1′,5′′‐thiazolidine]‐2,4′′‐dione, C₂₂H₁₇ClFN₃O₂S₂, (III), were each isolated as a single regioisomer using a one‐pot reaction involving l ‐proline, a substituted isatin and (Z)‐5‐(4‐chlorobenzylidene)‐2‐sulfanylidenethiazolidin‐4‐one [5‐(4‐chlorobenzylidene)rhodanine]. The compositions of (I)–(III) were established by elemental analysis, complemented by high‐resolution mass spectrometry in the case of (I); their constitutions, including the definition of the regiochemistry, were established using NMR spectroscopy, and the relative configurations at the four stereogenic centres were established using single‐crystal X‐ray structure analysis. A possible reaction mechanism for the formation of (I)–(III) is proposed, based on the detailed stereochemistry. The molecules of (I) are linked into simple chains by a single N—H…N hydrogen bond, those of (II) are linked into a chain of rings by a combination of N—H…O and C—H…S=C hydrogen bonds, and those of (III) are linked into sheets by a combination of N—H…N and N—H…S=C hydrogen bonds.     

Three New Chalcones from the Aerial Parts of Angelica keiskei

Three new chalcones, 3′‐carboxymethyl‐4,2′‐dihydroxy‐4′‐methoxychalcone ( 1 ), (±)‐4,2′,4′‐trihydroxy‐3′‐[(3‐hydroxy‐2,2‐dimethyl‐6‐methylenecyclohexyl)methyl]chalcone ( 2 ), and 2′′‐hydroxyangelichalcone ( 3 ), were isolated from the aerial parts of Angelica keiskei (Umbelliferae) together with five known compounds, artocarmitin A ( 4 ), (+)‐cis‐(3′R,4′R)‐methylkhellactone ( 5 ), (?)‐trans‐(3′R,4′S)‐methylkhellactone ( 6 ), 3,4‐dihydroxanthotoxin ( 7 ), and (Z)‐p‐coumaryl alcohol ( 8 ). The known compounds 4  –  8 were identified from A. keiskei for the first time. The structures of 1  –  3 were elucidated by interpreting spectroscopic data including 1D‐ and 2D‐NMR.     

Synthesis of spiro[indoline‐3,3′‐pyrrolizines] by 1,3‐dipolar reactions between isatins,l‐proline and electron‐deficient alkenes

Two spiro[indoline‐3,3′‐pyrrolizine] derivatives have been synthesized in good yield with high regio‐ and stereospecificity using one‐pot reactions between readily available starting materials, namely l ‐proline, substituted 1H‐indole‐2,3‐diones and electron‐deficient alkenes. The products have been fully characterized by elemental analysis, IR and NMR spectroscopy, mass spectrometry and crystal structure analysis. In (1′RS ,2′RS ,3SR ,7a′SR )‐2′‐benzoyl‐1‐hexyl‐2‐oxo‐1′,2′,5′,6′,7′,7a′‐hexahydrospiro[indoline‐3,3′‐pyrrolizine]‐1′‐carboxylic acid, C₂₈H₃₂N₂O₄, (I), the unsubstituted pyrrole ring and the reduced spiro‐fused pyrrole ring adopt half‐chair and envelope conformations, respectively, while in (1′RS ,2′RS ,3SR ,7a′SR )‐1′,2′‐bis(4‐chlorobenzoyl)‐5,7‐dichloro‐2‐oxo‐1′,2′,5′,6′,7′,7a′‐hexahydrospiro[indoline‐3,3′‐pyrrolizine], which crystallizes as a partial dichloromethane solvate, C₂₈H₂₀Cl₄N₂O₃·0.981CH₂Cl₂, (II), where the solvent component is disordered over three sets of atomic sites, these two rings adopt envelope and half‐chair conformations, respectively. Molecules of (I) are linked by an O—H…·O hydrogen bond to form cyclic R ₆⁶(48) hexamers of (S ₆) symmetry, which are further linked by two C—H…O hydrogen bonds to form a three‐dimensional framework structure. In compound (II), inversion‐related pairs of N—H…O hydrogen bonds link the spiro[indoline‐3,3′‐pyrrolizine] molecules into simple R ₂²(8) dimers.     

Regioselective Transformations of 2′,3′-Seconucleosides into Anhydro Structures and Chiral Crown Ethers

Intramolecular cyclisation of properly protected and activated derivatives of 2′,3′-secouridine ( = 1-{2-hydroxy-1-[2-hydroxy-1-(hydroxymethyl)ethoxy]-ethyl}uracil; 1 ) provided access to the 2,2′-, 2,3′-, 2,5′-, 2′,5′-, 3′,5′-, and 2′,3′-anhydro-2′,3′-secouridines 5, 16, 17, 26, 28 , and 31 , respectively (Schemes 1–3). Reaction of 2′,5′-anhydro-3′-O-(methylsulfonyl)- ( 25 ) and 2′,3′-anhydro-5′-O-(methylsulfonyl)-2′,3′-secouridine ( 32 ) with CH₂CI₂ in the presence of 1,8-diazabicyclo[5.4.0]undec-7-ene generated the N(3)-methylene-bridged bis-uridine structure 37 and 36 , respectively (Scheme 3). Novel chiral 18-crown-6 ethers 40 and 44 , containing a hydroxymethyl and a uracil-1-yl or adenin-9-yl as the pendant groups in a 1,3-cis relationship, were synthesized from 5′-O-(triphenylmethyl)-2′,3′-secouridine ( 2 ) and 5′-O,N⁶-bis(triphenylmethyl)-2′,3′-secoadenosine ( 41 ) on reaction with 3,6,9-trioxaundecane-1,11-diyl bis(4-toluenesulfonate) and detritylation of the thus obtained (triphenylmethoxy) methylcompound 39 and 43 , respectively (Scheme 4).     

The stereochemistry of 1,6,7,12b-Tetrahydro-2H,4H-[1,2] oxazino[3′,4′:1,2]-pyrido[3,4-b]indole and of 1,2,3,6,7,12b-Hexahydro-3-methyl-4H-pyrimido[3′,4′:1,2]pyrido[3,4-6] indole

From an analysis of nmr spectral data, 1,6,7,12b-tetrahydro-2H,4H-[1,3 ]oxazino[3′, 4′ :1,2]-pyrido[ 3,4-b ]indole is shown to exist in solution at room temperature almost entirely in the cis-fused ring conformation with the nitrogen lone pair bisecting the C4 methylene group whereas under the same conditions 1,2,3,6,7,12b-hexahydro-3-methyl-4H-pyrimido[3′,4′:1,2] pyrido-[3,4-b ]indole exists as an approximately 50:50 equilibrium mixture of the cis and trans-fused ring conformations.     

Absolute Konfiguration von Loroxanthin (=(3R, 3′R, 6′R)-β, ϵ-Carotin-3, 19, 3′-triol)

Absolute Configuration of Loroxanthin (=(3R, 3′R, 6′R)-β, ?-Carotene-3, 19, 3′-triol) ‘Loroxanthin’, isolated from Chlorella vulgaris, was separated by HPLC. methods in two major isomers, a mono-cis-loroxanthin and the all-trans-form. Solutions of the pure isomers easily set up again a mixture of the cis/trans-isomers. Extensive ¹H-NMR. spectral measurements at 400 MHz allowed to establish the 3′, 6′-trans-configuration at the ?-end group in both isomers and the (9E)-configuration in the mono-cis-isomer. The absolute configurations at C(3) and C(6′) were deduced from CD. correlations with synthetic (9Z, 3R, 6′R)-β, ?-carotene-3, 19-diol ( 5 ) and (9E, 3R, 6′R)-β, ?-carotene-3, 19-diol ( 6 ), respectively. Thus, all-trans-loroxanthin ( 3 ) is (9Z, 3R, 3′R, 6′R)-β, ?-carotene-3, 19, 3′-triol and its predominant mono-cis-isomer is (9E, 3R, 3′R, 6′R)-β, ?-carotene-3, 19, 3′-triol ( 4 ). Cooccurrence in the same organism and identical chirality at all centers suggest that loroxanthin is biosynthesized from lutein ( 2 ).     

Synthesis,structure and some reactions of 4a',5′,6′,7′,8′,8a'‐hexahydro‐4′H‐spiro[cyclohexane‐1,9′‐[1,2,4]triazolo[5,1‐b]‐quinazolines]

2′‐Substituted 5′,6′,7′,8′‐tetrahydro‐4′H‐spiro[cyclohexane‐1,9′‐[1,2,4]triazolo[5,1‐b]quinazolines] 3a‐d were synthesized by condensation of 3‐substituted 5‐amino‐1,2,4‐triazoles 1a‐d with 2‐cyclohexylidene cyclohexanone 2 in DMF. The compounds 3 were hydrogenated with sodium borohydride in ethanol to give 2′‐substituted cis‐4a',5′,6′,7′,8′,8a'‐hexahydro‐4′H‐spiro[cyclohexane‐1,9′‐[1,2,4]triazolo[5,1‐b]quinazolines] 4a‐d in high yields. The reactions of alkylation, acylation and sulfonylation of the compounds 4 were studied. The structure of the synthesized compounds was determined on the basis of NMR measurements including HSQC, HMBC, NOESY techniques and confirmed by the X‐ray analysis of 6 and 11b . The described synthetic protocols provide rapid access to novel and diversely substituted hydrogenated [1,2,4]triazolo[5,1‐b]quinazolines.     

Effect of the ortho Modification of Azobenzene on the Photoregulatory Efficiency of DNA Hybridization and the Thermal Stability of its cis Form

We synthesized various azobenzenes methylated at their ortho positions with respect to the azo bond for more effective photoregulation of DNA hybridization. Photoregulatory efficiency, evaluated from the change of T_m (ΔT_m) induced by trans–cis isomerization, was significantly improved for all ortho‐modified azobenzenes compared with non‐modified azobenzene due to the more stabilized trans form and the more destabilized cis form. Among the synthesized azobenzenes, 4‐carboxy‐2′,6′ ‐ dimethylazobenzene ( 2′,6′‐Me‐Azo ), in which two ortho positions of the distal benzene ring with respect to carboxyl group were methylated, exhibited the largest ΔT_m, whereas the newly synthesized 2,6‐Me‐Azo (4‐carboxy‐2,6‐dimethylazobenzene), which possesses two methyl groups on the two ortho positions of the other benzene ring, showed moderate improvement of ΔT_m. Both NMR spectroscopic analysis and computer modeling revealed that the two methyl groups on 2′,6′‐Me‐Azo were located near the imino protons of adjacent base pairs; these stabilized the DNA duplex by stacking interactions in the trans form and destabilized the DNA duplex by steric hindrance in the cis form. In addition, the thermal stability of cis‐ 2′,6′‐Me‐Azo was also greatly improved, but not that of cis‐ 2,6‐Me‐Azo . Solvent effects on the half‐life of the cis form demonstrated that cis‐to‐trans isomerization of all the modified azobenzenes proceeded through an inversion route. Improved thermal stability of 2′,6′‐Me‐Azo but not 2,6‐Me‐Azo in the cis form was attributed to the retardation of the inversion process due to steric hindrance between lone pair electrons of the π orbital of the nitrogen atom and the methyl group on the distal benzene ring.     

Trägerfixierte Organometallkomplexe. VI. Charakterisierung und Reaktivität Polysiloxan-gebundener (Ether-phosphan)ruthenium(II)-Komplexe

Supported Organometallic Complexes. VI. Characterization und Reactivity of Polysiloxane-Bound (Ether-phosphane)ruthenium(II) Complexes The ligands PhP(R)CH₂D [R = (CH₃O)₃Si(CH₂)₃; D = CH₂OCH₃ ( 1b ); D = tetrahydrofuryl ( 1c ); D = 1,4-dioxanyl ( 1d )] have been used to synthesize (ether-phosphane)ruthenium(II) complexes, which have been copolymerized with Si(OEt)₄ to yield polysiloxane-bound complexes. The monomers cis,cis,trans-Cl₂Ru(CO)₂(P ～ O)₂ ( 3b ) and HRuCl(CO)(P ～ O)₃ ( 5b ) were treated with NaBH₄ to form cis,cis,trans-H₂Ru(CO)₂(P ～ O)₂ ( 4b ) and H₂Ru(CO)(P ～ O)₃ ( 6b ), respectively (P ～ O = η¹-P coordinated; = η²- coordinated). Addition of Si(OEt)₄ and water leads to a base catalyzed hydrolysis of the silicon alkoxy-functions and a precipitation of the immobilized counterparts 4b ′, 6b ′. The polysiloxane matrix resulting by this new sol gel route has been described under quantitative aspects by ²⁹Si CP-MAS NMR spectroscopy. 4b ′ reacts with carbon monoxide to form Ru(CO)₃(P ～ O)₂ ( 7b ′). Chelated polysiloxane-bound complexes Cl₂Ru( )₂ ( 9c ′, d ′) and Cl₂Ru( )(P ～ O)₂ ( 10b ′, c ′) have been synthesized by the reaction of 1b–c with Cl₂Ru(PPh₃)₃ ( 8 ) followed by a copolymerization with Si(OEt)₄. The polysiloxane-bound complexes 9c ′, d ′ and 10b ′, c ′ react with one equivalent of CO to give Cl₂Ru(CO)( )(P ～ O) ( 12b ′– d ′). Excess CO leads to the all-trans-complexes Cl₂Ru(CO)₂(P ～ O)₂ ( 14b ′– d ′), which are thermally isomerized to cis,cis,trans- 3b ′– d ′. The chemical shift anisotropy of ³¹P in crystalline Cl₂Ru( )₂ ( 9a , R = Ph, D = CH₂OCH₃) has been compared with polysiloxane-bound 9d ′ indicating a non-rigid behavior of the complexes in the matrix.     

Stereochemical Analysis of an Aromatic Triplet Di-π-methane Rearrangement

It is shown that (−)-(S)-N,N-dimethyl-2-(1′-methylallyl)aniline ((−)-(S)- 4 ), on direct irradiation in MeCN at 20°, undergoes in its lowest-lying triplet state an aromatic di-π-methane (ADPM) rearrangement to yield (−)-(1′R,2′R)- and (+)-(1′R,2′S)-N,N-dimethyl-2-(2-methylcyclopropyl)aniline ((−)-trans- and (+)-cis- 7 ) in an initial trans/cis ratio of 4.71 ± 0.14 and in optical yields of 28.8 ± 5.2% and 15 ± 5%, respectively. The ADPM rearrangement of (−)-(S)- 4 to the trans- and cis-configurated products occurs with a preponderance of the path leading to retention of configuration at the pivot atom (C(1′) in the reactant and C(2′) in the products) for (−)-trans- 7 and to inversion of configuration for (+)-cis- 7 , respectively. The results can be rationalized by assuming reaction paths which involve the occurrence of discrete 1,4- and 1,3-diradicals (cf. Schemes 10, 12, and 13). A general analysis of such ADPM rearrangements which allows the classification of these photochemical reactions in terms of borderline cases is presented (Scheme 14). It is found that the optical yields in these ‘step-by-step’ rearrangements are determined by the first step, i.e. by the disrotatory bond formation between C(2) of the aromatic moiety and C(2′) of the allylic side chain leading to the generation of the 1,4-diradicals. Moderation of the optical yields can occur in the ring closure of the 1,3-diradicals to the final products, which may take place with different trans/cis-ratios for the individual 1,3-diradicals. Compounds (−)-trans- 7 as well as (+)-cis- 7 easily undergo the well-known photochemical trans/cis-isomerization. It mainly leads to racemization. However, a small part of the molecules shows trans/cis-isomerization with inversion of configuration at C(1′), which is best explained by a photochemical cleavage of the C(1′)–C(3′) bond.