Laser-induced intermolecular photocycloaddition of 3,4,5,6-tetrachloro-1,2-benzoquinone

An intermolecular [4+2] photocycloaddition was initiated by excitation of the lowest energetic ¹(π—π*) state of o-chloranil (3,4,5,6-tetr     

Intramolecular photocycloaddition reactions of 4-alkenyloxycoumarins

Intramolecular [_π2 + _π2] photocycloaddition reactions are reported for 4-(but-3′-enyloxy) and 4-(pent-4′-enyloxy) coumarins.     

Concise Access to the Skeleton of Protoilludane Sesquiterpenes through a Photochemical Reaction Cascade: Total Synthesis of Atlanticone C

In a single photochemical operation (λ≥350 nm) an easily accessible indanone derivative was converted into a structurally complex precursor of the protoilludane sesquiterpenes. The product (60 % yield) contains all 15 carbon atoms of the skeleton in the required connectivity and was transformed into the natural product atlanticone C (9 steps, 6 % overall yield). In addition, it was shown that other protoilludanes, such as Δ⁶‐protoilludene and paesslerin A, can be prepared in a concise fashion via the photochemical key intermediate. The photochemical reaction cascade comprises an ortho photocycloaddition, a thermal disrotatory ring opening and a regioselective disrotatory [4π] photocyclization.     

Differentiation between regio- and stereoisomers of bicyclo[3,2,0]heptanone-2 derivatives by chemical ionization mass spectrometry

The c.i. (isobutane) mass spectral behaviour was examined for a series of bicyclo[3,2,0]heptanone 2 derivatives, produced by (2π+2π) photocycloaddition reactions. The c.i. (isobutane) data allow unequivocal differentiation between hh- and ht-regioisomers. In some cases, a further assignment of the syn or anti form can also be made on the basis of the intensity of the protonated molecule ion.     

Complex Carbocyclic Skeletons from Aryl Ketones through a Three-Photon Cascade Reaction

Starting from readily available 7-substituted 1-indanones, products with a tetracyclo[5.3.1.0^1,70^4,11]undec-2-ene skeleton were obtained upon irradiation at λ=350 nm (eight examples, 49–67 % yield). The assembly of the structurally complex carbon framework proceeds in a three-photon process comprising an ortho photocycloaddition, a disrotatory [4π] photocyclization, and a di-π-methane rearrangement. The flat aromatic core of the starting material is converted into a functionalized polycyclic hydrocarbon with exit vectors in three dimensions. Ring opening reactions at the central cyclopropane ring were explored, which enable the preparation of tricyclo[5.3.1.0^4,11]undec-2-enes and of tricyclo[6.2.1.0^1,5]undecanes.     

Photochemical generation of benzene-naphthalene biplanemers

A direct biplanemer synthesis could be achieved by a quantitative [4π+4π]photocycloaddition of benzyloxy substituted benzene and naphthalene rings (3d,e ? 4d,e). Since the products do not show a thermal Cope rearrangement—in contrast to the related longicyclic conjugated 9,10-benzotricyclo[4.2.2.2^2,5]dodeca-3,7,9-trienes—but a quantitative reverse reaction, they are interesting systems for an optical switching.     

Complex Carbocyclic Skeletons from Aryl Ketones through a Three‐Photon Cascade Reaction

Starting from readily available 7‐substituted 1‐indanones, products with a tetracyclo[5.3.1.0^1,70^4,11]undec‐2‐ene skeleton were obtained upon irradiation at λ=350 nm (eight examples, 49–67 % yield). The assembly of the structurally complex carbon framework proceeds in a three‐photon process comprising an ortho photocycloaddition, a disrotatory [4π] photocyclization, and a di‐π‐methane rearrangement. The flat aromatic core of the starting material is converted into a functionalized polycyclic hydrocarbon with exit vectors in three dimensions. Ring opening reactions at the central cyclopropane ring were explored, which enable the preparation of tricyclo[5.3.1.0^4,11]undec‐2‐enes and of tricyclo[6.2.1.0^1,5]undecanes.     

Vibrational analyses,vibronic coupling and identification of electronic origins in the lower 1nπ* and 1ππ* states of phenazine and acrid

The spectra of the lowest ¹ nπ2 and ¹ ππ2 singlets of phenazine and acridine were studied in a biphenyl matrix at 2 K. The major findings are: (1) The lowest ππ2 singlets L^a and L^b in phenazine and acridine are clearly located and the vibrational analysis of these states support their assignments. (2) Good evidence is presented that a ¹ nπ2 state is the lowest excited singlet in acridine. The ¹nπ2 origin. The fact that the ¹L^a spectrum of acridine is twenty times broder than the ¹ L^a spectrum of antharacone which is otherwise very similar, is additional evidence that the nπ2 is the lowest excited singlet. (3) Three b^2g and three b^1g out-of-plane modes act as false origins for the long- and short-axis nπ2 spectra of phenazine. (4)Sharp lines in the region of phenazine's second singlet are shown to come from the lower ¹B^3u state and appear to be the same type of vibration interference as described by Wessel. (5) The vibrational analysis of the pure crystal spectra of phenazine published previously has been resigned.     

Investigation of the Property Constants of Both Al and Cu Complexes with Picramine Ca and Using Competition Coordination for the Selective Determination of Trace Amounts of Cu

The reactions between Cu(II) and picramine CA (PCA) and Al(III) and PCA were both sensitive at pH 7. The competition coordination between Cu and the Al–PCA complex was applied to the selective determination of Cu (CCSD) by the spectral correction technique. Results have shown that the complexes formed at pH 7 are Al₂(PCA) and Cu(PCA)₃, their cumulative stability constants are 4.43 × 10⁵ and 2.08 × 10¹⁵, respectively, and their real molar absorptivities are 5.78 × 10³ at 650 nm and 3.19 × 10⁴at 530 nm. At the analysis of the samples, the relative standard deviation (RSD) was 5.0% and the recovery rate of Cu was between 94 and 108%.     

Intramolecular [2+2] Photocycloaddition of Cyclic Enones: Selectivity Control by Lewis Acids and Mechanistic Implications

The intramolecular [2+2] photocycloaddition of 3-alkenyl-2-cycloalkenones was performed in an enantioselective fashion (nine representative examples, 54–86 % yield, 76–96 % ee) upon irradiation at λ=366 nm in the presence of an AlBr₃-activated oxazaborolidine as the Lewis acid. An extensive screening of proline-derived oxazaborolidines showed that the enantioface differentiation depends strongly on the nature of the aryl group at the 3-position of the heterocycle. DFT calculations of the Lewis acid–substrate complex indicate that attractive dispersion forces may be responsible for a change of the binding mode. The catalytic [2+2] photocycloaddition was shown to proceed on the triplet hypersurface with a quantum yield of 0.05. The positive effect of Lewis acids on the outcome of a given intramolecular [2+2] photocycloaddition was illustrated by optimizing the key step in a concise total synthesis of the sesquiterpene (±)-italicene.     

Observation of the fourth positive system of CO in dissociative recombination of vibrationally excited CO+2

The CO fourth positive system obtained by dissociative recombination of CO₂⁺(X̃²π_g) ions has been observed under various conditions of CO₂⁺ vibrational excitation. It is shown that the CO(A¹π) vibrational distribution is directly linked to CO⁺₂ excitation. Implications for planetary airglows are discussed.     

Diastereo‐ and Enantioselective Intramolecular [2+2] Photocycloaddition Reactions of 3‐(ω′‐Alkenyl)‐ and 3‐(ω′‐Alkenyloxy)‐Substituted 5,6‐Dihydro‐1H‐pyridin‐2‐ones

3‐(ω′‐Alkenyl)‐substituted 5,6‐dihydro‐1H‐pyridin‐2‐ones 2 – 4 were prepared as photocycloaddition precursors either by cross‐coupling from 3‐iodo‐5,6‐dihydro‐1H‐pyridin‐2‐one ( 8 ) or—more favorably—from the corresponding α‐(ω′‐alkenyl)‐substituted δ‐valerolactams 9 – 11 by a selenylation/elimination sequence (56–62 % overall yield). 3‐(ω′‐Alkenyloxy)‐substituted 5,6‐dihydro‐1H‐pyridin‐2‐ones 5 and 6 were accessible in 43 and 37 % overall yield from 3‐diazopiperidin‐2‐one ( 15 ) by an α,α‐chloroselenylation reaction at the 3‐position followed by nucleophilic displacement of a chloride ion with an ω‐alkenolate and oxidative elimination of selenoxide. Upon irradiation at λ=254 nm, the precursor compounds underwent a clean intramolecular [2+2] photocycloaddition reaction. Substrates 2 and 5 , tethered by a two‐atom chain, exclusively delivered the respective crossed products 19 and 20 , and substrates 3 , 5 , and 6 , tethered by longer chains, gave the straight products 21 – 23 . The completely regio‐ and diastereoselective photocycloaddition reactions proceeded in 63–83 % yield. Irradiation in the presence of the chiral templates (?)‐ 1 and (+)‐ 31 at ?75 °C in toluene rendered the reactions enantioselective with selectivities varying between 40 and 85 % ee. Truncated template rac‐ 31 was prepared as a noranalogue of the well‐established template 1 in eight steps and 56 % yield from the Kemp triacid ( 24 ). Subsequent resolution delivered the enantiomerically pure templates (?)‐ 31 and (+)‐ 31 . The outcome of the reactions is compared to the results achieved with 4‐substituted 5,6‐dihydro‐1H‐pyridin‐2‐ones and quinolones.     

Design of crystal packings of styryl heterocycles and regularities of [2+2] photocycloaddition in their single crystals 8. Topochemical [2+2] autophotocycloaddition and back reaction in styryl dye of the benzothiazole series

New styryl dye of the 2-benzothiazole series was synthesized. The new dye contains two methoxy groups in the benzene ring and tosylate counterion. The [2+2] photocycloaddition (PCA) of the dye was studied in the polycrystalline film and in single crystal. Two modifications of the dye cocrystallizate with hydroquinone differed in the ratio of components were obtained, and their ability to enter PCA was studied. According to the X-ray diffraction data, molecular cations of the dye form stack packings either of syn-“head-to-tail” type or relatively isolated stacking dimers. In all cases, the ethylene bonds of the adjacent cations are brought together and antiparallel, favoring PCA to form the centrosymmetric rctt-isomer of 1,2,3,4-tetrasubstituted cyclobutane. In two cases, the PCA reaction proceeded as the “single crystal-to-single crystal” transformation. Hydrogen bonds in crystals including hydroquinone molecules strengthen the crystal packing retarding the PCA. The back photoreaction (retro-PCA) was detected: it occurs without single crystal decomposition and results in the accumulation of the initial styryl dye in crystal consisting of the cyclobutane derivative. This is the first example of such a transformation in single crystals.     

Formation of C2(e3πg, C1πg) in the photolysis of CO(X1ϵg+) at 193 nm with an ArF laser

CO(X¹?_g⁺) was irradiated with an ArF laser (193 nm) at room temperature at pressures of 2–20 mbar. C(2 ¹D) atoms were formed by two-photon absorption and observed by the 247.8 nm C(3 ¹P^o→2 ¹S) fluorescence. Fluorescence of C₂ (e ³π_g → a ³π_u, C ¹π_g → A ¹π_g and d ³π_g → a ³π_g) was detected. The emission time dependence led to the conclusion that C₂¹ is formed by recombination of electronically excited carbon atoms.     

Photochemische reaktionen—XXXII: Photochemie von gibberellin C; wege zur funktionalisierung von gibbanen in position 8 und 11

The n→π^* photochemistry of saturated diterpenoid ketones of the gibberellin C type 1 has been investigated. A Norrish type 1 cleavage leading to Δ⁷⁽¹¹⁾-unsaturated 7,8-seco-8-aldehydes 5 and 6, respectively, takes place as the main process. These secoaldehydes upon prolonged irradiation undergo intramolecular “crossed” photocycloaddition to the highly strained oxetanes 7 and 8. The oxetane methylester 8 represents a suitable key compound for proton catalyzed ring opening reactions under nucleophilic attack at C-8. In such a manner a series of 8α,11α-bifunctionalized gibbanes 12–20 have been prepared in good yields and under mild conditions.     

An asymmetric synthesis of cis,anti, cis-tricyclo[5,3,0,02,6]decanes applying γ-hydroxymethyl-γ-butyrolactone as a chiral synthon. First asymmetric total synthesis of (−)-β-bourbonene

The asymmetric total synthesis of cis, anti, cis-tricyclo[5,3,0,0^2,6]decane sesquiterpene β-bourbonene is described. The key (2+2) photocycloaddition was carried out applying the optically pure butenolide derivative $\text{6b}$ as a chiral synthon.     

Enantiodifferentiating [2+2] photocycloaddition of cyclohexenone carboxylic acid with ethylene using 8-phenylmenthyl amine as a chiral template

Enantiodifferentiating [2+2] photocycloaddition of cyclohexenone carboxylic acid with the smallest olefin, ethylene, was accomplished using 8-phenylmenthyl amine as a chiral template. Particularly, we succeeded in enhancing the enantiomeric excess (ee) value by selective excitation of a chiral substrate–template supramolecular complex.     

Synthesis and complexing property of four-bridged crownopaddlanes

A new series of crown compounds crownopaddlanes 3a-c bearing three cyclobutane rings were prepared by means of intramolecular [2+2] photocycloaddition of styrene derivatives. The yield of crownopaddlane 3b possessing five ethereal oxygen atoms was remarkably high 52% with the addition of sodium borofluoride in the photoreaction system. As this template effect suggests, 3b showed extraordinarily high Na⁺-selectivity with high efficiency on the liquid-liquid extraction of alkali metal picrates, though 3a having four ethereal oxygen atoms did not extracted any cations in this system. The high Na⁺-selectivity of 3b was further clarified by the equilibrium stability constants (log K_a) for Na⁺ (5.85) and K⁺ (2.91) in acetonitrile solution. The complexation of 3b to Na⁺ cation was also examined by X-ray crystallography. Crownopaddlane 3c bearing six ethereal oxygen atoms also efficiently and selectively extracted alkali metal cations, compared with conventional 18-crown-6 derivatives.     

Photochemistry of Chlorinated 2-Cycloalkenones

The 6-chloro-2-cyclohexenones 3, 6 and 11 , and the 5-chloro-2-cyclopentenone 15 were newly synthesized. The results obtained with compounds 3 and 15 in photocycloadditions to olefins show that the oxetane vs. cyclobutane product ratio is reduced by the substitution of florine by chlorine in the α′ -position of the enone. No oxetanes are formed in the intramolecular photocycloaddition of 6. Compound 11 does not photoadd to olefins. The newly synthesized 2-chloro-3-cyclohexenones 8 and 9 are also photostable towards light of λ=366 nm, but π-π*-excitation (λ=254 nm) in pentane leads to the formation of 4,4-dimethylcyclohexanone (29) .     

Consequences of a through-bond interaction in tetracyclo-[5.3.0.0.0]-deca-4,8-diene (“Hypostrophene”)

Photoelectron spectroscopy is used to demonstrate the mechanistic consequences of the level ordering in a given molecule on its reactivity, using the recently synthesized hypostrophene, which contains two CC double bonds in a rigid, cisoid conformation, as an example. The inability of this molecule to close photochemically to the saturated analog is traced to the presence of an exceptionally high-lying σ level which is ideally oriented for an effective through-bond coupling of the two π orbitals. Contrary to the norbornadiene case, this through-bond coupling overrides the direct through-space interaction, placing the in-phase combination of the two π orbitals above the out-of-phase combination, and thus converts the _π2_s+_π2_s photocycloaddition from a symmetry-allowed to a symmetry-forbidden reaction.