Zum Mechanismus der Photochemie des Benzfurazans

Mechanistic studies on the photochemical reactions of benzfurazan . From other works it is known that irradiation of benzfurazan ( 1 ) in methanol gives the carbaminacid-ester 4 , whereas in benzene the azepinederivative 3 is obtained (Scheme 1). The compounds 5–8 (Scheme 2) have been proposed as intermediates. In our investigations we detected and characterized by means of UV.- and IR.-spectroscopy the two species 5 and 8 . Irradiation of 1 with 350 nm light at room temperature in a strongly polar solvent (e.g. H₂O) yields exclusively 5 (Fig. 1) with a quantum yield of 0.48. In non polar solvents (e.g. hexane) 5 isomerizes in a second photochemical step to 8 (quantum yield 0.43) (Fig. 3). Thermally, 5 can be converted back to 1 . The rate constant for this reaction at room temperature is 2 · 10^–5s^–1. The transformation 5 → 8 was also investigated at low temperature. There was no direct evidence for any intermediates of the type oxazirene ( 6 ) or nitrene ( 7 ). However, the formation of azepine 3 upon irradiation of 5 in benzene suggests as intermediate the nitrene 7 which could be converted into 8 in a fast thermal reaction (Scheme 3).     

Photoinduzierte Cycloadditionen von aliphatischen 2H-Azirinen. 37. Mitteilung über Photoreaktionen

The purely aliphatic 2,3-dipropyl-2H-azirine ( 1 ) reacts on irradiation with a mercury high-pressure lamp through a Vycor filter with methyl trifluoroacetate or acetone to form 3-oxazolines 3a, b (65%) resp. 4 (14%) (Scheme 1). 9-Azabicyclo[6.1.0]non-1(9)-ene ( 5 ) on irradiation in the presence of the dipolarophiles methyl trifluoroacetate, methyl difluoroacetate, 1,1,1-trifluoro-propanone and acetone behaves in a similar way, whereby the corresponding bicyclic 3-oxazolines 7–10 result in yields of 60–20% (Scheme 2). By analogy with the photochemical behaviour of 3-aryl-2H-azirines it is assumed that nitrile-ylides 2 resp. 6 represent intermediates. In fact irradiation of 2,3-dipropyl-2H-azirine ( 1 , λ_max 239 nm, ? 240) at ?196° with light of wavelength 245 nm in a hydrocarbonglass gives rise to a pronounced maximum at 280 nm, for which an ? of ? 15000 can be estimated. The quantum yield for the formation of nitrile-methylide 2 is 0,8. Irradiation of the dipole 2 at ?196° or warming to ?150° causes the maximum at 280 nm to disappear.     

Photochemische Cycloadditionen von 3-Phenyl-2H-azirinen mit Benzoyl-, Äthoxycarbonyl- und Vinylphosphonaten 34. Mitteilung über Photoreaktionen

The irradiation of the 3-phenyl-2H-azirines 1a–c in the presence of diethyl benzoylphosphonate ( 8 ) in cyclonexane solution, using a mercury high pressure lamp (pyrex filter), yields the diethyl (4, 5-diphenyl-3-oxazolin-5-yl)-phosphonates 9a–c (Scheme 3). In the case of 1b a mixture of two diastereomeric 3-oxazolines, resulting from a regiospecific but non-stereospecific cycloaddition of the benzonitrile-benzylide dipole 2b to the carbonyl group of the phosphonate 8 , was isolated. Benzonitrile-isopropylide ( 2a ), generated from 2,2-dimethyl-3-phenyl-2H-azirine ( 1a ), undergoes a cycloaddition reaction to the ester-carbonyl group of diethyl ethoxycarbonylphosphonate ( 15 ) with the same regiospecificity to give the 3-oxazoline derivative 16 (Scheme 5). The azirines 1a–c , on irradiation in benzene in the presence of diethyl vinylphosphonate ( 17 ) give non-regiospecifically the Δ¹-pyrrolines 13a–c and 14a–c (Scheme 6).     

Photochemische Cycloadditionen von 3-Phenyl-2H-azirinen an Carbonsäurechloride. 35. Mitteilung über Photoreaktionen

Irradiation of 2, 3-diphenyl-2H-azirine ( 1a ) and 1-azido-1-phenyl-propene, the precursor of 2-methyl-3-phenyl-2H-azirine ( 1b ), in benzene, with a high pressure mercury lamp (pyrex filter) in the presence of acid chlorides yields the oxazoles 5a–d (Scheme 2). Photolysis of 2, 2-dimethyl-3-phenyl-2H-azirine ( 1c ) under the same conditions gives after methanolysis the 5-methoxy-2, 2-dimethyl-4-phenyl-3-oxazolines 7a, b, d , while hydrolysis of the reaction mixture leads to the formation of the 1, 2-diketones 8a, c, d (Scheme 4). The suggested reaction path for all these reactions is a 1, 3-dipolar cycloaddition of the photochemically generated benzonitrilemethylides 2 to the carbonyl double bond of the acid chlorides to give the intermediates 4 , followed by either elimination of hydrogen chloride or solvolysis (Schemes 2 and 4). Irradiation of 1c in the presence of acetic acid anhydride leads via the intermediate 9 to the 5-hydroxy-3-oxazoline 10 and the 5-methylidene-3-oxazoline 11 (Scheme 5).     

Photoreaktionen von 1-Alkylbenztriazolen

The irradiation of benzotriazoles (cf. Scheme 2) with light of 225–325 nm in protic and in aromatic solvents was investigated. In aqueous 0.1N H₂SO₄ benzotriazole ( 5 ) and 1-methyl-benzotriazole ( 6 ) yielded 2-amino- and 2-methylaminophenol ( 25 and 26 ), respectively (Scheme 3). In 2-propanol 6 , 5-chloro- and 6-chloro-1-methyl-benzotriazole ( 14 and 15 ) were reduced to N-methylaniline, 4-chloro- and 3-chloro-N-methyl-aniline ( 27 , 28 and 29 ), respectively (Scheme 4). When the benzotriazoles were irradiated in aromatic solvents only C, C coupling products were observed (cf. Scheme 6 and Tables 1–4). It is of importance that 5-chloro-1-methyl-benztriazole ( 14 ) when decomposed photolytically in benzene solution yielded only 4-chloro-2-phenyl-N-methyl-aniline ( 49 ) and its 6-chloro isomer only 5-chloro-2-phenyl-N-methyl-aniline ( 50 ), i.e. the intervention of benzo-1H-azirine intermediates (e.g. 53 , Scheme 8) can be excluded. The substitution patterns which are observed when 6 is irradiated in toluene, anisole, fluoro-, chloro-, bromobenzene and benzonitrile (cf. Table 4) can best be explained by assuming that 6 , after loss of nitrogen, forms a diradical intermediate in the singlet state with highly zwitterionic character. 1-(1′-Alkenyl)-benzotriazoles (cf. Table 7) form on irradiation in cyclohexane solution indoles by intramolecular ring closure of the diradical intermediate and proton shift. After irradiation of 1-decyl-benzotriazole ( 8 ) in a glassy matrix at 77K a 7-line ESR. spectrum characteristic of a triplet radical is observed. This is in agreement with the fact that the lowest lying state of intermediates of type 2 (Scheme 1) should be a triplet state (cf. [21] [26]).     

Exceptional Products of the Desulfurization of N,2-Diaryl-5-(arylimino)- 2,5-dihydro-4-nitroisothiazol-3-amines

The desulfurization of several N,2-diaryl-5-(arylimino)-2,5-dihydro-4-nitroisothiazol-3-amines 5 with Ph₃P led to complex mixtures of products in low yields. For instance, quinoxaline-2-carboxamide 1-oxides of type 6 (Scheme 2) and, in some cases, also 3-nitroquinolines of type 7 (Scheme 5) were isolated. By the desulfurization of the substituted derivatives 5b – e , a rearrangement of the intermediates yielded 6 and 7 with a different substitution pattern from that expected from the starting materials (Scheme 3). The additional formation of two isomeric 1,2,5-oxadiazole-3-carboxamides 8 was observed only in the case of 5d (R¹=R²=F) (Scheme 6). Under the same reaction conditions, the major product of the desulfurization of 5c was the quinoxaline-2-carboxamide 1-oxide 9 (Scheme 7). Reaction mechanisms involving intermediate ketene imines and O transfer from the NO₂ group to the neighboring ketene imine are proposed. The structures of 6a , 6e , 6k , 7b , and 8d were established by X-ray crystallography, while the structure of 9 was elucidated by 2D-NMR spectroscopy and corroborated by X-ray crystallography.     

Tropone Is a Mere Ketone for Cycloadditions to Ketenes

Tropone ( 1 ) reacts with ketenes 2 to yield [8+2] cycloadducts, the γ‐lactones 3 . The concerted [8+2] cycloaddition path is formally symmetry‐allowed, but we established that it is unfavorable. Careful low‐temperature NMR (¹H, ¹³C, and ¹⁹F) spectroscopies of the reaction of diphenyl ketene ( 2b ) or bis(trifluoromethyl) ketene ( 2c ) with tropone ( 1 ) allowed the direct detection of a β‐lactone intermediates 5b , c and novel norcaradiene species 6b , c in head‐to‐head configurations. The [2+2] cycloadducts 5b , c equilibrated with the norcaradienes 6b , c . The β‐lactones 5b and 5c were converted to the γ‐lactones 3b and 3c , respectively, in quantitative yields. The DFT calculations showed that the concerted [8+2] cycloaddition is unfavorable. The first step of the calculated reaction 1 + 2c is a cycloaddition which leads to a dioxetane intermediate. This initial [2+2] cycloadduct is isomerized to the β‐lactone 5c via the first zwitterionic intermediate. The β‐lactone 5c is further isomerized to the product γ‐lactone 3c via the second zwitterion intermediate. Thus, 3c is not formed via the well‐established two‐step mechanism including zwitterionic intermediates but via a five‐step mechanism composed of a [2+2] cycloaddition and subsequent isomerization (Scheme 12).     

Synthese von 2H-Isoindol-4,7 -dionen durch photochemische Cycloaddition von 2,3-Diphenyl-2H-azirin an 1,4-Chinone. 38. Mitteilung über Photoreaktionen

2, 3-Diphenyl-2H-azirine ( 1 ) reacts on irradiation with light of wavelength 290–350 nm with 1,4-benzoquinones 3–6 or with 1,4-naphthoquinones 7–9 forming the yellow to red coloured 1,3-diphenyl-2H-isoindole-4, 7-diones 10–13 (33–43% yield) resp. 1, 3-diphenyl-2H-benzo[f]isoindole-4,9-diones 14–16 (33–36% yield) (Scheme 1). The structures of these hitherto unknown products follow from the analytical and spectral data. The probable formation of the isoindole-diones is depicted in Scheme 2. The intermediate benzonitrile-benzylide ( 2 ), which most certainly arises, adds onto the unsubstituted C, C-double bond of the quinones and not onto the C,O-double bonds. On exclusion of atmospheric oxygen there results from 1 and 2-methyl-1, 4-benzoquinone ( 4 ) a product (probably b ) which hardly absorbs in the region 350–450 nm. The latter, with the agency of atmospheric oxygen (but not 4 ), is converted into 5-methyl-1, 3-diphenyl-2H-isoindole-4, 7-dione ( 11 ). The relative slowness of this oxidation (see Fig. 2) enables an almost complete photochemical transformation of the azirine 1 , which only weakly absorbs above 290 nm. Otherwise 11 , which strongly absorbs above 290 nm, would hinder the photolysis of 1 .     

Photochemie von in 4-Stellung substituierten 5-Methyl-3-phenyl-isoxazolen.44. Mitteilung über Photoreaktionen

Photochemistry of 4-substituted 5-Methyl-3-phenyl-isoxazoles. 4-Trideuterioacetyl-5-methyl-3-phenyl-isoxazole ([CD₃CO]- 27 ), upon irradiation with 254 nm light, was converted into a 1:1 mixture of oxazoles [CD₃CO]- 35 and [CD₃]- 35 (Scheme 13). This isomerization is accompagnied by a slower transformation of ([CD₃CO]- 27 ) into [CD₃]- 27 . Irradiation of the isoxazole derivatives 28, 29, 30 and (E)- 31 yielded only oxazoles 36, 37, 38 and (E), (Z)- 39 ; no 4-acetyl-5-alkoxy-2-phenyl-oxazole, 2-acetyl-3-methyl-5-phenyl-pyrrole or 2-acetyl-4-methoxycarbonyl-3-methyl-5-phenyl-pyrrole, respectively, were formed (Scheme 9 and 10). Similarly (E)- 32 gave a mixture of (E), (Z)- 40 only (Scheme 11). Upon shorter irradiation, the intermediate 2H-azirines (E), (Z)- 41 could be isolated (Scheme 11). Photochemical (E)/(Z)-isomerization of the 2-(trifluoro-ethoxycarbonyl)-1-methyl-vinyl side chain in all the compounds 32, 40 and 41 is fast. At 230° the isoxazoles (E)- and (Z)- 32 are converted into oxazoles (E), (Z)- 40 . The same compounds are also obtained by thermal isomerization of the 2H-azirines (E), (Z)- 41 . The most probable mechanism for the photochemical transformations of the isoxazoles, as exemplified in the case of the isoxazole 27 , is shown in Scheme 13. A benzonitrile-methylide intermediate is postulated for the photochemical conversion of the 2H-azirines into oxazoles. 2H-Azirines are also intermediates in the thermal isoxazole-oxazole rearrangement. It is however not yet clear, if the thermal 2H-azirine-oxazole transformation involves the same transient species as the photochemical reaction. A mechanism for the photochemical isomerization of the 2H-azirine 11 to the oxazole 15 is proposed (Scheme 3).     

Reactions with Carbo(heterylhydrazonoyl) Halides. I. Chemistry of Carbo(3-phenylpyrazol-5-yl-hydrazonoyl) Chlorides

The Carbo(3-phenylpyrazol-5-yl-hydrazonoyl) halides 1a , b react with active methylene compounds to yield the 1-(3-phenylpyrazol-5-yl)-pyrazole derivatives 2a – k (Scheme 1). The acyclic intermediates 3a , b could be isolated from reaction of 1a , b with acetylacetone, thus establishing the substitution mechanism for these reactions. Compounds 1a , b reacted with carbon disulfide, phenyl isothiocyanate, methyl cyanide, and with p-chlorobenzaldehyde to yield the corresponding heterocyclic derivatives 5 – 8 , respectively (Scheme 2). The behaviour of compounds 2 with hydrazine hydrate is reported.     

Zum Mechanismus der Photochemie von 2-Alkylindazolen in wässerigen Lösungen

Mechanistic studies on the photochemistry of 2-alkylindazoles in aqueous solutions. The photochemistry of 2-alkylindazoles 1 in aqueous solutions is rather complex, the relative yields of different products being dependent on the pH-value of the irradiated solution: In neutral or basic solutions (pH > 7) as well as in most of the organic solvents isomerization to 1-alkyl-benzimidazoles 2 takes place. In dilute sulfuric acid (pH 2–4) this reaction is suppressed and the dihydro-azepinones 3 and 4 are formed. Irradiation in strongly acid solutions (pH < 1) yields the o-amino-acetophenones 5 (Scheme 1). The relative quantum yields of the photoproducts 2–5 have been measured as a function of the pH-value of the irradiated solution (Fig. 1). A comparison of these yields with the protonation equilibrium of the indazole in its first excited singlet state (pK = 2.8) suggests that 2 and 3 are both photoproducts of the neutral indazole molecule, whereas 4 as well as 5 are formed from the protonated indazole. The rearrangement of the indazole 1 to the benzimidazole 2 proceeds via an intermediate 6 , which can be produced in high concentrations by monochromatic irradiation of 1 at low temperatures. The thermal reactivity of this intermediate in dilute sulfuric acid could be investigated: At pH 8 the only product is the benzimidazole 2 . With decreasing pH-value increasing amounts of 3 are formed and at pH < 4 the formation of 2 is completely suppressed, the only product being the azepinone 3 . Thus, 3 is a solvolysis product of the intermediate 6 (Scheme 2). The most probable primary product of singlet indazolium is the nitrenium ion 7 . From this intermediate the formation of 5 can proceed in well-known thermal reactions. The formation of 4 is possibly due to a further protonation equilibrium nitrenium-nitrene. The nitrene 7 can be converted into the azepinone 4 via the azirine 8 (Scheme 3). The pK-values of different indazoles and intermediates are listed in the Table.     

Steroids and Sexhormones. Part 265. Limonin,Part VI. Synthesis of a model compound incorporating rings D and E of 14,15-deepoxylimonin

Starting from the known formyl ketene thioacetal 6 , model compound 11 was synthesized. The key intermediates, the epimeric furylmethanols 7a and 7b , were converted into the same dithioortholactone 8b (Scheme 1) and further elaborated into the model compound 11 (Scheme 2), a versatile compound in the synthesis of limonin ( 1 ). The acid catalyzed conversion of the epimers 7a and 7b into 8b may probably involve a hydride-transfer reaction with inversion of configuration at C(17) of alcohol 7a (Scheme 4, row b).     

Additionsreaktionen von 3-Dimethylamino-2,2-dimethyl-2H-azirin an Phenylisocyanat und Diphenylketen

Addition Reaction of 3-Dimethylamino-2,2-dimethyl-2H-azirine with Phenylisocyanate and Diphenylketene 3-Dimethylamino-2,2-dimethyl-2H-azirine ( 1a ) reacts with carbon disulfide and isothiocyanates with splitting of the azirine N(1), C(3)-double bond to give dipolar, fivemembered heterocyclic 1:1 adducts. In some cases, these products can undergo secondary reactions to yield 1:2 and 1:3 adducts. In this paper it is shown that the reaction of 1a with phenylisocyanate also takes place by cleavage of the N(1), C(3)-bond, whereas with diphenylketene N(1), C(2)-splitting is observed. The reaction of 1a and phenylisocyanate in hexane at room temperature yields the 1:3 adduct 2 in addition to the trimeric isocyanate 3 (Scheme 1). A mechanism for the formation of 2 is given in Scheme 5. Hydrolysis experiments with the 1:3 adduct 2 , yielding the hydantoins 4–6 and the ureas 7 and 8 (Schemes 3 and 5), show that the formation of this adduct via the intermediates d , e and f is a reversible reaction. The aminoazirines 1a and 1b undergo an addition reaction with diphenylketene to give the 3-oxazolines 14 (Scheme 8), the structure of which has been established by spectral data and oxidative degradation of 14a to the 3-oxazolin-2-one 15 (R¹ ? R² ? CH₃, Scheme 9).     

Photoinduzierte 1, 3-dipolare Cycloaddition von 3-Phenyl-2H-azirinen an Azodicarbonsäure-diäthylester. 32. Mitteilung über Photoreaktionen

Irradiation of 2, 2-dimethyl-3-phenyl- ( 1a ), 2, 3-diphenyl-2H-azirine ( 1b ) or the azirine-precursors 1-azido-1-phenyl-propene ( 2a ) and 1-azido-1-phenyl-ethylene ( 2b ), respectively, in benzene in the presence of azodicarboxylic acid diethylester, yields the corresponding 1, 2-carbethoxy-3-phenyl-Δ³-1, 2, 4-triazolines 4a–d (Scheme 1). Refluxing 4 ( a, c or d ) in 0, 2–0, 4M aqueous ethanolic potassium hydroxide leads to the formation of the 1-carbethoxy-3-phenyl-Δ²-1, 2, 4-triazolines 6 ( a, c or d ). Under the same conditions 4b is converted to 3, 5-diphenyl-1, 2, 4-triazole ( 7b , Scheme 2). In 10M aqueous potassium hydroxide solution heating of either 4 ( c or d ) or 6 ( c or d ) yields the 3-phenyl-1, 2, 4-triazoles 7 ( c or d ). Photolysis of 1-carbethoxy-5, 5-dimethyl-3-phenyl-Δ²-1, 2, 4-triazoline ( 6a ) in benzene in the presence of oxygen and trifluoroacetic acid methylester gives the 5-methoxy-2, 2-dimethyl-4-phenyl-5-trifluoromethyl-3-oxazoline ( 13 , Scheme 5). 5, 5-Dimethyl-3-phenyl-1, 2, 4-triazole seems to be the intermediate, which on losing nitrogen gives the benzonitrile-isopropylide ( 3a ).     

Photochemische Erzeugung und Reaktionen des Benzonitril-benzylids. 42. Mitteilung über Photoreaktionen

Photochemical Generation and Reactions of Benzonitrile-benzylide The low temperature irradiation of 2,3-diphenyl-2H-azirine ( 1 ) in DMBP-glass at ?196° has been reinvestigated. It was possible to convert 1 nearly quantitatively into the dipolar species benzonitrile-benzylide ( 3 , Φ₃ = 0,78), which exhibits UV.-absorptions at 344 (? = 48000) and 244 nm (? = 28500) (Fig. 1, Tab. 1). Irradiation of 3 with 345 nm light at ?196° resulted in almost complete reconversion to the azirine 1 (Φ = 0,15; Fig. 2). When the solution of 3 in the DMBP-glass was warmed up to about ?160° a quantitative dimerization to 1,3,4, 6-tetraphenyl-2,5-diaza-1,3,5-hexatriene ( 8 ) occurred. This proves that 8 is not only formed by the indirect route 3 + 1 → 7 \documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\longrightarrow }\limits^{hv} $\end{document} 11 → 8 known before (Scheme 1), but also by dimerization of 3 either by direct head to head coupling or via the intermediate e (p. 2675), followed by a fast thermal hydrogen transfer reaction. The occurrence of the dipolar intermediate 11 in the photochemical conversion of the bicyclic compound 7 to 8 could also be demonstrated by low temperature experiments: On irradiation at ?196° 7 gave the cherry red dipolar intermediate 11 (λ_max = 520 nm), which at ?120° isomerizes to 8 . It should be noted, that neither 7 nor 11 are formed by dimerization reactions of 3 . Experiments carried out at room temperature demonstrate, that both processes for the formation of 8 may compete: Irradiation of a solution of 1 (DMBP, c = 8 × 10^?4 to 5 × 10^?3M ) with 350 nm light of high intensity (which does not excite the bicyclic compound 7 ) leads to a relative high photostationary concentration of the dipolar species 3 . Under these conditions the formation of 8 is due to dimerization of 3 (Φ₈ = 0,19). With low light intensity only a very low stationary concentration of 3 can be obtained. Therefore the reaction of 3 with 1 , leading to the bicyclic intermediate 7 , becomes now predominant (Φ_?1 = 1,55, which corresponds with the expected value of 2 × 0,8). Irradiation of 1 at ?130° with 350 nm light of high intensity gives 8 with a quantum yield of 0,44. This is in agreement with the theoretical value Φ₈ = 0,4 for an exclusive formation of 8 by dimerization of 3 . The lower quantum yield for the formation of 8 at room temperature makes probable that under these conditions 3 not only dimerizes to 8 , but also to another, so far unidentified dimer, e.g. 2,3,5,6-Tetraphenyl-2,5-dihydropyrazine. By flash photolysis of a solution of 1 (cyclohexane, c = 10^?4M , 25°) the disappearance of 3 could directly be measured by UV.-spectroscopy: At relative high concentrations (c ≥ 10^?7M ) 3 disappeared according to a second order reaction with the rate constant k = 5 × 10⁷M ^?1S ^?1. At lower concentrations (c ≤ 10^?7M) the rate of disappearance of 3 follows first order kinetics. The rate constant of this pseudo first order reaction ( 3 + 1 → 7 ) has been determined to be 1 → 10⁴M^?1S^?1. Using Padwa's table of relative rates for the cycloaddition of the dipolar species 3 to various dipolarophiles, including the azirine 1 , an absolute rate constant of k ≈ 8 × 10⁸M ^?1S ^?1 for the addition of 3 to the most active dipolarophile fumaronitrile could be estimated. In cyclohexane at room temperature, the diffusion controlled rate constant equals 6,6 × 10⁹M ^?1S ^?1. In Table 1 the UV.-maxima of several nitrile-ylides, among them a purely aliphatic one, are given.     

Photo-oxygenation of Styrenic Estrogens: Structural Analysis of 8,9-Didehydro-, 6,7-Didehydro-, and 9,11-Didehydroestrone Derivatives and Their Reactivity towards Singlet Oxygen

A series of didehydro-3-O-methyl-estrones having a styrenic framework, with the ring-A-conjugated double bond in all three possible positions (8,9-didehydro- ( 6 ),9,11-didehydro- ( 1b ), 6,7-didehydro- ( 9 ), and the 12,18-di-nor-8,9-didehydroestrone analog 11 ), were compared for their reactivity towards singlet oxygen. Under dye-sensitized photo-oxygenation conditions, both, products derived from ene-type reactions with the isolation of a stable hydroperoxide and a fragmentation product, were obtained from 6 (see Scheme 3), while only fragmentation took place for 1b (Scheme 1), Kinetic studies indicated that 6 is more reactive towards ¹O₂ than 1b (β = 9.2·10^?3 mol·1^?1 vs 3.3·10^?2 mol·1^?1, resp.). The observed reactivity, apparently, does not match with ene-type reaction and [2 + 2]cycloaddition being in competition, since the most activated substrate 6 preferentially yields ene-type products and their derivatives. Conformational analysis on the structure of 6 and 1b , both calculated by molecular-mechanic techniques (MMPMI) and determined by X-ray diffraction, show that the allylic H-atoms satisfy the orthogonality rule for ene-type reactions. The product distribution is best rationalized by applying Fukui's rule which takes into account a combination of electronic and geometric factors. Substrates 9 and 11 yielded photo-products arising from ene-type reaction with no stable primary products isolated (Scheme 4). Geometric considerations based on the calculated structures by molecular mechanics are consistent with the observed results.     

Photocyclisierungen von 1, 1'-Polymethylen-di-2-pyridonen. 46. Mitteilung über Photoreaktionen

Photocyclization of 1, 1′-Polymethylene-di-2-pyridones . Benzophenone sensitized irradiation of the four dipyridones 1-4 gave the internal photocyclization products 6 (64%, Scheme 4), 7 (60%, Scheme 5), 8 (Scheme 6), and 11 (26%, Scheme 7), respectively. The decamethylene compound 5 yielded only polymeric material. The primary [2+2] photoproduct 8 from dipyridone 3 (Scheme 6) is relatively unstable. Further irradiation or heating to 65° induced a Cope rearrangement to give compound 9 which, on heating to 137°, was converted into the isomeric compound 10 . This product, as well as the other photoproducts mentioned, are rearranged back to their respective starting materials upon direct irradiation with 254 nm light or by heating to higher temperatures. The various possibilities for cycloadditions of pyridones are discussed as well as the possible factors which are responsible for the highly regioselective photoreactions of the dipyridones 1–4 .     

The Chemistry of Thujone. Part XIV.. Synthesis of biologically active aryl terpenoid analogues

Employing thujone-derived intermediates, a series of achiral ( 9a–d ; Scheme 1) and chiral ( 11b and 11d; Scheme 2) terpene analogues related to the biologically active ‘terpenoid’ hybrids have been prepared. The stereochemistry of the key epoxidation reaction was established by correlation of the product 11b with the previously reported alcohol (R)- 20 of known absolute configuration (Scheme 3).     

Reaktionen von 3-Dimethylamino-2,2-dimethyl-2H-azirin mit Phenolen und Halogenaromaten

Reactions of 3-dimethylamino-2,2-dimethyl-2H-azirine with phenols and aryl halides The reactions of 3-dimethylamino-2,2-dimethyl-2H-azirine ( 1 ) with phenols are described in chap. 1. The azirine 1 reacts with the 2-formyl- and 2-acetylphenols 5 – 8 to yield the N′-methylidene derivatives of 2-amino-N,N-dimethyl-isobutyramide 9 - 12 (Scheme 2, tautomeric form b ). These products are in equilibrium with the tautomeric quinoide forms 9a-12a . Under similar conditions 4-hydroxybenzaldehyde did not react with 1 . Reaction of 1 with 4-hydroxycoumarine ( 13 ) gives the 4-amino-coumarine 14 (Scheme 2). The mechanism of these reactions is analogous to the previously reported one for the reaction of 1 with cyclic enolisable 1,3-diketones [2] [3]. Activated phenols with pK_a-values < 8, e.g. 2- and 4-nitrophenol, 2,4-dinitrophenol and pentachlorophenol, undergo addition reactions with 1 in boiling benzene solution to give the aniline derivatives 15 - 18 (Scheme 3). A reaction mechanism is given in Scheme 3: after protonation of the azirine 1 followed by attack of the phenolate ion at the amidinium-C-atom, the intermediate of type e undergoes a rearrangement to the spiro-Meisenheimer complexes of type f . Ring opening leads to 15 – 18 . A similar reaction is observed for 2,4-dinitro-thiophenol and 1 , giving 2-(N′-(2,4-dinitrophenyl)amino)-N,N-dimethyl-isobutyrothioamide ( 19 ). The azirine 1 reacts with the more acidic 2,4,6-trinitrophenol (picric acid) to yield 3,3,6,6-tetramethylpiperazine-2,5-bis(N,N-dimethyliminium) dipicrate ( 21 , Scheme 4). The methacrylamidinium salt 22 is the only product (97% yield) in the reaction of 8-hydroxy-5,7-dinitroquinoline and 1 in acetonitrile solution. The reaction of 1 with picric acid can be explained in a similar way as the previously reported one with strong acids (cf. Scheme 1, [1] [3] [5]). An alternative mechanism without formation of the 1-aza-allylcation c is postulated in Scheme 5, together with a mechanism which could explain the exclusive formation of 22 in the reaction of 1 with 8-hydroxy-5,7-dinitroquinoline. In chap. 2 a few reactions of the azirine 1 with aryl halides are reported. In the reaction with 2,4-dinitrofluorobenzene it is shown by UV. and NMR., that m , n and o are intermediates (Scheme 6). Working up the reaction mixture with water, hydrogen sulfide or benzylamine leads to the aniline derivatives 17 , 19 and 26 , respectively. With picryl chloride and 8-hydroxy-5,7-dinitroquinoline the azirine 1 undergoes a nucleophilic aromatic substitution to afford the intermediates p and q , which via deprotonation and ring opening give acrylamidine derivatives ( 27 and 29 , Scheme 7 and 8). The steric hindrance in p and q between the aziridine ring and the two groups in o-position could be the reason for the different behaviour of the intermediates n and p or q (cf. Schemes 6 and 8).     

Technische Verfahren zur Synthese von Carotinoiden und verwandten Verbindungen aus 6-Oxo-isophoron. III. Ein neues Konzept für die Synthese der enantiomeren Astaxanthine

Technical Procedures for the Synthesis of Carotenoids and Related Compounds from 6-Oxo-isophorone. III. A New Concept for the Synthesis of the Enantiomeric Astaxanthins A new and efficient concept for the total synthesis of (3S, 3'S)- and (3R, 3'R)-astaxanthin ( 1a and 1c , resp.) in high overall yield and up to 99,2% enantiomeric purity is described. Key intermediates are the (S)- and (R)-acetals 10 and 17 , respectively (Scheme 2). These chiral building blocks were synthesized via three different routes: a) functionalization of the enantiomeric 3-hydroxy-6-oxo-isophorons⁴) 2 and 11 , respectively (Scheme 2); b) optical resolution of 3,4-dihydroxy-compound⁴) 19 (Scheme 3), and c) fermentative reductions of 6-oxo-isophorone derivatives (Schemes 4 and 5). - The absolute configurations of the two intermediates 12 and 13 (Scheme 2) have been confirmed by X-ray analysis. - The final steps leading to the enantiomeric astaxanthins are identical with those described for optically inactive astaxanthin [1].