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Efficient synthesis of medium-sized cyclic molecules containing an (E)-alkene was performed via the highly (E)selective photochemical isomerization of the (Z)-isomer, facilitated by AgNO3-impregnated silica gel.

Linear maltodextrin (LM) was cross-linked by pyromellitic dianhydride to afford LM polymers of different cross-linking degrees. When soaked in water, these cross-linked LM polymers (nanosponges (NSs)), evolved into several phases from sol to suspension, then to flowing gel, and finally to rigid gel with an increase in their content. Enantiodifferentiating photoisomerization of (Z)-cyclooctene (1Z) to chiral (E)-isomer (1E), which was employed as a benchmark reaction to quantitatively assess the environmental-to-molecular chirality transfer process, was performed in aqueous media containing these pyromellitate-crosslinked LM-NSs in different phases. The enantiomeric excess (ee) of 1E obtained was relatively insensitive to the phases at least up to the flowing gel phase, but became highly sensitive in the rigid gel phase, exhibiting an abrupt drop in the early rigid gel phase followed by a rapid recovery in the late rigid gel phase. A comparison with the phase-dependent ee profiles previously reported for similar pyromellitate-crosslinked cyclodextrin (CD)- and cyclic nigerosylnigerose (CNN)-NSs revealed that the chiral void space created around the pyromellitate linker in NS is responsible for the dramatic changes in ee in the rigid gel phase, whereas the inherent host cavity in CD/CNN plays only limited roles in the supramolecular photochirogenesis mediated by the sensitizer-crosslinked NSs. The latter insight allows us to further expand the applicable range of the present concept and methodology by employing a much wider variety of oligosaccharides as well as substrates and sensitizing cross-linkers.

Bioorthogonal ligations have emerged as highly versatile chemical tools for biomedical research. The exceptionally fast reaction between 1,2,4,5-tetrazines and trans-cyclooctenes (TCOs), also known as tetrazine ligation, is frequently used in this regard. Growing numbers of applications for the tetrazine ligation led to an increased demand for TCO compounds, whose commercial availability is still very limited. Reported photochemical procedures for the preparation of TCOs using flow chemistry are straightforward and high yielding but require expensive equipment. Within this contribution, we present the construction and characterization of a low-cost flow photoreactor assembled from readily accessible components. Syntheses of all commonly used trans-cyclooctene derivatives were successfully carried out using the described system. We are convinced that the presented system for photoisomerization will promote access to bioorthogonally reactive TCO derivatives.

Cyclic nigerosylnigerose (CNN), a saucer-shaped cyclic tetrasaccharide with a shallow concave surface, was reacted with pyromellitic dianhydride in 1 : 2 and 1 : 4 ratios to give two CNN-based polymers of different degrees of crosslinking, both of which swelled upon soaking in water, acting as a 'nanosponge' (NS). These NSs evolved several phases from isotropic solution to flowing and rigid gels via suspension by gradually increasing the concentration in water. The CNN-NSs thus prepared effectively mediated the enantiodifferentiating photoisomerization of (Z)-cyclooctene (1Z) to chiral (E)-isomer (1E). The enantiomeric excess (ee) of 1E obtained was a critical function of the solvent composition and the phase evolved at different CNN-NS concentrations in water. In isotropic solution, the enantioselectivity was generally low (-4% to +6% ee) but the chiral sense of 1E was inverted by increasing the methanol content. Interestingly, the product's ee was controlled more dramatically by the phase evolved, as was the case with the cyclodextrin-based nanosponge (CD-NS) reported previously. Thus, the ee of 1E was low in solution and suspension, but suddenly leaped at the phase border of flowing gel and rigid gel to give the highest ee of 22-24%, which are much higher than those obtained with CD-NSs (6-12% ee), revealing the positive roles of the chiral void space formed upon gelation of the crosslinked saccharide polymer.

Pyromellitate-bridged cyclodextrin nanosponges (CDNSs) evolved from sol into gel state upon gradual increase of the concentration from 0.2 to 2000 mg mL-1 in water. The enantiodifferentiating geometrical photoisomerizations of (Z)-cyclooctene and (Z,Z)-1,3-cyclooctadiene sensitized by CDNS at various concentrations were critically affected by the phase transition of CDNS to afford the corresponding (E)- and (E,Z)-isomers in the highest enantiomeric excesses in the gel state.

Isophthalic and terephthalic acid monoesters of cyclic nigerosyl-(1→6) -nigerose (CNN), a cyclic tetrasaccharide composed of four d-glucopyranosyl residues connected by alternating α-1,3- and α-1,6-linkages, were synthesized as novel chiral supramolecular sensitizers for enantiodifferentiating photoisomerization of (Z)-cyclooctene () to planar chiral (E)-isomer (1E). Despite the saucer-shaped shallow cavity of CNN that does not immediately guarantee strong ground-state interactions with, the sensitizer-appended CNNs afforded optically active 1E in such enantiomeric excesses that are much improved than those obtained with an α-cyclodextrin analog and comparable with those obtained with a β-cyclodextrin analog. Interestingly, the enantiomeric excess values obtained were a critical function of temperature and solvent to show an inversion of the product chirality by changing the environmental variants. Nevertheless, all of the differential activation parameters calculated from the temperature-dependent enantiomeric excesses gave an excellent compensatory enthalpy-entropy relationship, indicating an operation of a single enantiodifferentiating mechanism in the present chiral photosensitization with modified CNNs. Chirality 24:921-927, 2012. 2012 Wiley Periodicals, Inc. Copyright

Solvent effect upon asymmetric photosensitization has been investigated in the enantiodifferentiating photoisomerization of cyclooctene(1Z), sensitized by benzoate modified β-cyclodextrin derivatives bearing nitrogen, oxygen or sulfur substituents. The enantiomeric excess (ee) and E/Z ratio of reaction products were susceptible to the concentration of methanol in the aqueous solution, which could switch to the chirality of product unprecedentedly. Further investigation indicated that the conformation of the modified CDs in aqueous methanol solutions with 1Z were highly sensitive to both the substituent(s) on benzoate moiety of the modified CDs and the concentration of methanol. Solvent content represents a new versatile tool to efficiently manipulate the asymmetric photochemical reactions, in which the chirality of products can be switched by simply changing the methanol content of reaction solvent rather than synthesizing the antipodal sensitizers.

Photochemical planar-to-planar chirality transfer was effected by using (R)-[10]paracyclophane-12-carboxylates as a planar-chiral sensitizer and (Z)-cyclooctene and (Z,Z)-1,5-cyclooctadiene as prochiral substrates to give a planar-chiral (E)- and (E,Z)-isomer in up to 44% and 87% enantiomeric excess, respectively, the latter of which being the highest ever reported for a sensitized photochirogenic reaction.

(Chemical Equation Presented) A series of 6-O-benzoyl-β-cyclodextrins (CDs) with methyl, methoxy, methoxycarbonyl, and bromo substituent(s) at the ortho-, meta-, and/or para-position(s) were synthesized as chiral sensitizing hosts for the use in supramole

Described is a scalable procedure for driving photochemical sytheses of trans-cyclooctene derivatives through metal complexation. During photoirradiation, reaction mixtures are continuously pumped through a column of a AgNO3-impregnated silica gel. The trans-cyclooctene derivative is selectively retained by the AgNO3-impregnated silica, but the cis-isomer elutes from the column back to the reaction flask, where it is photoisomerized and recirculated through the column. The method provides access to a range of usefully functionalized derivatives of trans-cyclooctene, including a derivative of 5-aza-trans-cyclooctene that underwent transannular cyclization upon treatment with bromine. The alkene stereochemistry is transferred with high fidelity to the hexahydropyrrolizine framework in the transannular cyclization. Copyright

A simple procedure is reported for the stereoselective debromination of vic-dibromides with nickel boride at ambient temperature. Debromination with concomitant reduction of vic-dibromides to give dihydro products in a one-pot reaction is also reported. α,β-Dibromoketones can also be converted to their corresponding alcohols.

Permethylated 6-O-modified β-cyclodextrins 2a-2d were synthesized as novel photosensitizing hosts with a flexible skeleton. Circular dichroism (CD) and 2D NMR spectral examinations of benzoate 2a revealed that the benzoate moiety is deeply included into its own cavity in aqueous solution. Upon addition of (Z)-cyclooctene (1Z) to a 50% aqueous methanol solution of 2a at 25 °C, the benzoate moiety of 2a was gradually excluded from the cavity as indicated by the CD spectral changes; the Job's plot revealed the formation of a 1:1 complex of 2a with 1Z. The binding constants for the complexation of 1Z by 2a were determined by CD spectral titration in 50% aqueous methanol at various temperatures. The van't Hoff analysis of the obtained data afforded the thermodynamic parameters (ΔH° = -3.1 kJ mol-1, ΔS° = 48.5 J mol-1 K-1), demonstrating the entropy-driven complexation by the permethylated cyclodextrin. This is in sharp contrast to the complexation of 1Z by nonmethylated β-cyclodextrin benzoate that is driven by enthalpy (ΔH° = -31.8 kJ mol-1 and ΔS° = -51.1 J mol-1 K-1). Upon supramolecular photosensitization with 2a-2d, 1Z isomerized to the (E)-isomer (1E) in moderate enantiomeric excesses (ee's), which however displayed significant temperature dependence with accompanying switching of the product's chirality in an extreme case. Such dynamic behavior of ee is very different from that reported for the photosensitization with nonmethylated cyclodextrin benzoate, where the product's ee is controlled by host occupancy. Eyring treatment of the ee obtained at various temperatures (-1 mol -1. The origin of the contrasting behavior of permethylated versus nonmethylated cyclodextrin hosts is inferred to be the conformational flexibility of the former host, which enables the entropy-driven guest complexation in the ground state and the entropy-controlled enantiodifferentiation in the excited state.

The photochemistry of optically active (E)-cyclooctene in cyclopentane was initiated with a lamp at ca. 254 nm and a Nd-YAG laser at 266 nm. The alkene racemizes slightly faster than it isomerizes to (Z)-cyclooctene, suggesting a previously hidden mechanism of E-Z photointerconversion. The photoreaction at 266 nm may be multiphoton in character.

(Martix presented) Catalytic carbonylation of epoxides to β-lactones was effected by a highly active and selective bimetallic catalyst comprised of a chromium(III) porphyrin cation and a cobalt tetracarbonyl anion. The complex is readily synthesized from commercially available compounds in high yield. Carbonylation of numerous linear epoxides, as well as bicyclic epoxides derived from 8- and 12-membered hydrocarbons, proceeded with high activity, selectivity, and yield.

The photosensitized enantiodifferentiating isomerization of (Z)-cyclooctene (1Z) in supercritical carbon dioxide has been performed for the first time. The critical control of the enantiomeric excess (ee) of chiral product, (E)-cyclooctene (1E), and even the switching of the product's chirality were achieved through a small change of pressure particularly in the low-density region near the critical density. Quantitative evaluation of these ee changes was achieved by estimating differential activation volumes between the diastereomeric transition states of the reaction.

Photochemical debromination of dibromohydro cinnamamides gave the carbon-carbon double bond compounds.

Deuterium labelling and 2H NMR mapping demonstrate that cyclooctyl and cyclodecyl mesylates in aqueous EtOH exhibit greatly enhanced or exclusive levels of 1,5-hydride shift, provided a Me3Sn group is β to the migrating hydrogen, and after tin group loss from the formal β-stannyl cation, results in regiospecific, transannular alkene formation.

Thymidine and uridine, as well as calf thymus DNA (ctDNA), have been shown for the first time to function as chiral photosensitizers in aqueous solution, to effect the enantiodifferentiating photoisomerization of (Z)-cyclooctene, giving the chiral (E)-isomer in enantiomeric excesses of up to 15%.

The epoxidation of trans-cyclooctene (trans-1) with the MTO/H2O2, MTO/UHP, and NaY/MTO/H2O2 oxidants leads to a mixture of trans/cis-olefins 1, trans/cis-epoxides 2, and the cis-diol 3. While the oxygen transfer proceeds stereoselectively, the monoperoxo rhenium complex A, which is generated in situ during the catalytic cycle, is responsible for the facile deoxygenation, isomerization, and hydrolysis of the trans-epoxide. In the case of the homogeneous MTO/H2O2 system, rapid decomposition of the catalytically active rhenium species into HReO4 circumvents the formation of such side products. In contrast, for the heterogeneous oxidants MTO/UHP and NaY/MTO/H2O2, the catalytically active rhenium species are sufficiently stabilized and survive long enough to promote the observed side reactions.

Zero-valent palladium precatalysts containing rigid bidentate bis(arylimino)acenaphthene ligands (shown schematically) facilitate the highly stereoselective homogeneous catalytic hydrogenation of alkynes to (Z)- alkenes. Internal, terminal, aryl-substituted, and cyclic alkynes are suitable substrates, as are some enynes, which are chemoselectively hydrogenated to dienes. E=CO2Me; R1, R2 = 4-OCH3, 4-CH3, 2,6-(CH3)2.

Pressure effects upon asymmetric photosensitization have been investigated for the first time in the enantiodifferentiating Z-E photoisomerization of cyclooctene (1), sensitized by chiral aromatic esters (2-7). The product's enantiomeric excess (ee) and E/Z ratio were critical functions of the applied pressure, exhibiting an unprecedented switching of the product chirality. Depending upon the chiral sensitizer employed, the differential activation volume (ΔΔV(+) varies widely from -3.7 to +5.6 cm3 mol-1, which is unexpectedly large for an enantiodifferentiation in the excited state. However, the ΔΔV(+) values obtained do not correlate with the differential activation enthalpy (ΔΔH(+)) or entropy (ΔΔS(+)) obtained from temperature-dependence studies, indicating that pressure and temperature function as independent perturbants for the photoenantio- differentiation process. Further investigations on the pressure dependence of ee at low temperatures enable us to construct the first three-dimensional diagram that correlates the product's ee with pressure and temperature changes. The combined effects of temperature and pressure provide us with a versatile tool for the multidimensional control of asymmetric photochemical reactions, in which we can switch and/or enhance the product chirality at more readily accessible temperatures and pressures, without using antipodal sensitizers.

Aromatic phosphate, phosphonate, phosphinate and phosphine oxide have been used as effective sensitizers for the geometrical photoisomerization of (Z)-cyclooctene (1Z) to its highly strained (E)-isomer (1E). Photosensitizations with phosphate and phosphonate gave moderate photostationary-state E/Z ratios of 0.14-0.17. A comparative study of the fluorescence quenching experiments and kinetic analysis of the product yield demonstrate that the photosensitized isomerization proceeds through a mixed mechanism that involves both singlet and triplet excited states. Triphenylphosphine oxide is not fluorescent and gave a low photostationary-state E/Z ratio of 0.05. Using (-)-menthyl or (-)-bornyl phosphate, phosphonate and phosphinate as chiral sensitizers, the enantiodifferentiating photoisomerization of 1Z was performed to afford optically active 1E in enantiomeric excesses ≤5%.

Highly efficient and unique enantiodifferentiating Z-E photoisomerization of cyclooctene (1) was achieved through singlet photosensitization with chiral polyalkyl benzenepolycarboxylates.Conversion dependence of optical yields and an attempted kinetic resolution of racemic (E)-isomer 1E unequivocally showed that the enantiodifferentiating step is not the initial quenching of chiral sensitizer with enantiomeric 1E but the rotational relaxation of planar 1Z to twisted singlet cyclooctene (1p) within the singlet exciplex with chiral sensitizer.Fluorescence quenching of some benzenetetracarboxylates with 1Z and 1E provides further evidence for the intervention of an exciplex intermediate, showing a new emission at a longer wavelength, and also for the insignificant enantiodifferentiation in the quenching step, affording nearly diffusion-controlled quenching rate constants around 1010 s-1 for 1E.This novel enantiodifferentiating photosensitizing system via exciplex, in combination with low-temperature irradiation, not only affords the highest optical yields up to 53percent but also exhibits unusual temperature-switching behavior of product chirality especially with the o-dicarboxylate sensitizer.The activation parameters obtained in the temperature-dependence study indicate that the temperature-switching phenomenon is attributed to the unequal activation entropies, or frequency factors, for the enantiodifferentiating relaxations of 1Z to (R)- and (S)-1p within the exciplex, for which the dynamic structural changes in the relaxation process may be responsible.

The second-order reaction rates were measured for the MCPBA epoxidation in CH2Cl2 for a series of cyclic olefins including bridgehead olefms and trans-cycloalkenes. As expected, strained bridgehead alkenes and trans-cycloalkenes showed faster reaction rates than nonstrained cis-cycloalkenes. The MM-2 steric energies of alkenes, alkanes, and their corresponding epoxides were calculated to evaluate the strain energy released in each reaction (ΔSE). Plots of log krel vs olefin strain did not show a good correlation. However, the plot of log krel vs ΔSE (which is defined as the steric energy difference between olefin and the corresponding epoxide) showed a good correlation for each set of di- and trisubstituted olefins. This result suggests that ΔSE directly reflects strain energy relief in the transition state. From the slope for the plot log krel vs ΔSE, it was thought that approximately 42% of strain (ΔSE) was released in the transition state for the MCPBA epoxidation. Also, trialkyl-subtituted alkenes were found to be about 50 times more reactive than dialkyl-substituted alkenes in cases where the strain energy relief (ΔSE) is the same. The reaction rate is also plotted versus ionization potential of the olefin, assuming that the major orbital interaction lies between the LUMO of the peracid and the HOMO of the olefin. Although, in some cases, a rough correlation of the reaction rate with the ionization potential of the olefin exists, the frontier orbital interaction is not viewed as the dominant factor since conjugated alkenes, which have higher HOMO energies than simple olefins, are not more reactive in MCPBA epoxidation.

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In the triplet-sensitized photoisomerization with benzene, toluene, or p-xylene, the photostationary E/Z ratio of cyclo-octene, (E/Z)pss, dramatically increases with lowered irradiation temperature (+75 to -123°C) or with increased solvent viscosity, whereas the (E/Z)pss ratio of oct-2-ene stayed unchanged over the temperature range +25 to -78°C. Theoretical examinations indicate that the temperature-dependent (E/Z) value is not a simple function of solvent viscosity but also depends upon the solvent structure. The Stern-Volmer studies further revealed that the variable quenching rate constants for Z and E isomers are responsible for the temperature- and viscosity-dependent photostationary state.

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In the triplet-sensitized photoisomerization, the photostationary trans/cis ratio of cyclooctene doubled upon lowering irradiation temperature from 25 to -78 deg C, whereas the ratio of 2-octene stayed unchanged.

(η2-trans-Cyclooctene)(η2-olefin)Fe(CO) 3 complexes are synthesized by thermal olefin exchange [3, olefin = trans-cyclooctene, from (η2-cis-cyclooctene)2Fe(CO)3 (2) and trans-cyclooctene; 6, olefin = methyl acrylate, from (η2-methyl acrylate)2Fe(CO)3 and trans-cyclooctene] or CO photosubstitution [3, from Fe(CO)5 and excess trans-cyclooctene; 5, olefin = dimethyl fumarate, from (η2-trans-cyclooctene)Fe(CO)4 and dimethyl fumarate or from (η2-dimethyl fumarate)Fe(CO)4 and trans-cyclooctene]. The products are obtained as mixtures of diastereomers, some of which (3a, C2 isomer; 3b, Cs isomer; 5a) are separated by fractional crystallization. The reaction of 2 with (-)-trans-cyclooctene yields (-)-3a. Single-crystal X-ray diffraction analyses of racemic 3a and 5a establish a trigonal-bipyramidal geometry with the C=C units occupying two equatorial positions. Crystals of racemic 3a are monoclinic of space group P21/n with a = 12.069 (2) A?, b = 8.278 (1) A?, c = 18.815 (2) A?, β = 98.386 (9)°, and Z = 4. The unit cell contains two molecules of each 3a enantiomer. The structure was solved and refined to R = 0.034 and Rw = 0.037 by using 4218 independent reflections. Crystals of 5a are orthorhombic of space group P212121 with a = 7.811 (1) A?, b = 13.167 (1) A?, c = 20.432 (2) A?, and Z = 4. The structure was solved and refined to R = 0.037 and Rw = 0.045 by using 5506 independent reflections. All compounds are characterized by elemental analysis and IR (including CO force field parameters of 3a) and 13C NMR spectroscopy. The 13C NMR coordination shifts of the olefinic carbon atoms in 3a and 5a correlate (inversely) with the respective metal-carbon distances. Variable-temperature 13C NMR spectroscopy of 3b reveals three sequential dynamic processes: (a) olefin rotation (ΔH≠ = 8.0 ± 0.5 kcal mol-1, ΔS≠ = -5.1 ± 3 cal mol-1 K-1), (b) eq-CO/ax-CO two-site exchange (ΔH≠ = 13.8 ± 0.4 kcal mol-1, ΔS≠ = 2.8 ± 1.2 cal mol-1 K-1), (c) complete CO scrambling (ΔH≠ = 16.8 ± 0.8 kcal mol-1, ΔS≠ = 10.2 ± 3 cal mol-1 K-1). In the case of 3a the two CO site-exchange processes are indistinguishable. The dynamic behavior of the mixed bis(olefin) complexes 5a and 6 indicates that the two olefins rotate independently, and rotation of both of the olefins appears to be a prerequisite for CO scrambling. In conclusion, these observations are in accord with a sequence of three Berry pseudo-rotational steps, alternately using an olefin, a CO group, and the respective other olefin as the pivotal ligand.

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The photochemistry of cis-bicyclodec-9-ene (1), cis-bicyclodeca-2,9-diene (4), and trans-bicyclodeca-2,9-diene (8) has been examined at 193 nm in pentane solution.The photochemistry of 1 was complicated by the apparent formation of the thermally labile diene 15, which prevented direct determination of the primary photoproducts of 1.Dienes 4 and 8 were found to eliminate acetylene stereoselectively to give mainly the syn-elimination products 17 and 18, respectively.Ring opening of 4 and 8 gave, as primary products, trienes 5, 6, and 16, indicating that the stereospecificity suggested by the Woodward-Hoffmann rules of orbital symmetry was not followed.

The cis-cyclooctene and norbornene complexes of AuCl, AuCl(cis-C8H14) (1) and AuCl(C7H10) (2), respectively, have been prepared from the reaction of AuCl(CO) with the corresponding olefin at room temperature by a CO displacement reaction. Gas volumetric equilibrium data on the reaction between AuCl(CO) and norbornene show that the formation of the olefin complex is favored at atmospheric pressure of carbon monoxide. The constant of the equilibrium AuCl(CO) + norbornene ? AuCl(norbornene) + CO is K = 4 ± 1 at 21.5°C. No CO was found to be displaced from AuCl(CO) with cyanoolefins. The crystal structure of 1 has been solved by X-ray diffraction methods. For 2, only the crystal data and a structure with insufficient resolution were obtained. The crystal data at 190 K are as follows: 1, a = 23.115 (4) A?, b = 5.419 (3) A?, c = 7.356 (2) A?, Z = 4, orthorhombic, space group Pca21, R = 0.037; 2, a = 5.697 (2) A?, b = 20.24 (1) A?, c = 6.620 (2) A?, Z = 4, orthorhombic, space group Ama2, R = 0.085. The gold atom presents the expected linear geometry, considering the olefin as a monodentate ligand.

Treatment of the erythro-β-hydroxyalkyldibenzophosphole oxide (6), derived from (E)-stilbene oxide and lithiodibenzophosphole, with 1,5-diazabicycloundec-5-ene (DBU) in dimethyl sulphoxide (DMSO) affords (Z)-stilbene with high stereospecificity.

The behavior of three medium-sized cycloalkenes cyclooctene (10), cyclodecene (17) and cyclododecene (21) on direct irradiation in pentane and methanol solution has been studied.The results are summarized in Tables 1-3.Irradiation of medium-sized cycloalkenes is a convenient procedure for the preparation of bicyclic products (cf. 13, 14, 19, 20 and 23) through transannular insertion reactions of carbene intermediates (cf. 11, 18, and 22) thought to arise from rearrangement of the 1 state via a 1,2-hydrogen shift.The formation of trans-decalin (20) is in contrast to the reported formation of the cis isomer on base-initiated decomposition of the corresponding tosylhydrazone.None of the three cycloalkenes 10, 17, or 21 underwent competing nucleophilic trapping of the 1 state in methanol, in contrast with other alkenes previously studied.However, cyclododecene (21) afforded the methyl ether 25, which apparently resulted from protonation of the 1(?,?*) state, and the epoxide 26, which is thought to arise from electron transfer to oxygen by the 1 state followed by protonation of the resulting superoxide ion and oxidation of unreacted cyclododecene (21).

It was found that the reaction of five-membered cyclic trithiocarbonates with n-butylithium in tetrahydrofuran is applicable to the preparation of unsymmetrical linear trithiocarbonates and further to the interconversion of cis- and trans olefins.

The use of aromatic carboxylic esters, e.g. methyl benzoate, as sensitizers gave an unusual cis-trans-photoisomerization of cyclo-octene, in which the trans : cis-ratio in the photostationary state was anomalously high, provided the usual triplet sensitization is operating (Table 1).From Stern-Volmer quenching studies and the consequent comparison of the decay ratios for three different sensitizers in two different solvents with those for direct or triplet-sensitized photoisomerizations (Table 4), it is concluded that the present photoisomerization takes place via a singlet mechanism which involves non-vertical singlet energy transfer from an excited aromatic ester to the cyclo-octene moiety in an exciplex resulting in 90 deg twisted cyclo-octene (1p), which in turn decays to ground state cis- or trans-cyclo-octene in the ratio of 0.56 : 0.44.The asymmetric cis-trans-photoisomerization induced by chiral aromatic esters also supports the involvement of an exciplex.