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The naphtholic chromophore is responsible for the photophysical and photochemical properties of cinnamylnaphthols 13, f. Both fluorescence emission and photocyclization occur from the naphtholic singlet excited states. The formation of five- and six- membered ring products (2e and 3e, f) is shown to involve a proton transfer mechanism. The low efficiency of trans to cis isomerization is explained as the result of deactivation of the styrenic triplet via intramolecualr energy transfer. The resulting naphtholic triplets behave as energy sinks.

The attachment of substituents to the styrenic ring of trans -2-cinnamylphenol produces a decrease in the singlet energy of the styrenic moiety. As a consequence, the resulting chromophore becomes responsible for the photophysical and photochemical properties of this type of bichromophoric compounds. Both fluorescence emission and photocyclization occur predominantly from the lowest-lying styrenic singlet. The result is a marked regioselectivity towards 6-membered ring products, via an excited state proton transfer.

Direct irradiation of o-cinnamylphenols 1a–c led to mixtures of the cis-isomers 3a–c, the dihydrobenzofurans 4a–c and the dihdyrobenzopyrans 5–c. Under the same conditions, o-hydroxystilbenes 2a,b gave their cis-isomers 9a,b as single photoproducts. Using 2,4,6-triphenylpyrylium tetrafluoroborate as photosensitizer the reactions of the methyl- or methoxy- substituted derivatives appear to proceed via single electron transfer. Compounds 1b,c underwent oxidative fragmentation to the aldehydes 8b,c, while 2b cyclized to benzofuran 10b.

Triphenylpyrylium (TP + ), and dibenzo[a,d]tropylium (DT + ) cations have been incorporated inside zeolites, and their potential as heterogeneous photosensitizers has been examined using the PET-cyclodimerization of 1,3-cyclohexadiene (CHD) as test reaction. The behaviour of zeolites containing TP + as heterogeneous photosensitizers was compared with the results obtained in homogeneous solution using the soluble TP + tetrafluoroborate salt (TPT). With the cation encapsulated with Y zeolite the reaction proceeded slower, presumably due to light scattering and mobility restrictions associated with the diffusion difficulties in the solid. Accordingly, inside the extra large pore MCM-41 the reaction efficiency was enhanced by a factor of 9, although it remained lower than in the homogeneous system. As in solution, the endo [4+2] adduct was the major product. When a La-exchanged Y zeolite containing TP + was used, the degree of conversion was even lower. This suggests that CHD cyclodimerization must occur via electron transfer to singlet TP + . In the case of DT + within ZSM-5, the selectivity was reversed with respect to TP + : the major product was the anti [2+2] dimer. This was attributed to generation of 3 CHD as the result of back electron transfer at the 1 [CHD + ·/DT·] stage.

The photodeacetalization of 4-(p-isopropylphenyl)-2-nonyl-1,3-dioxolane 1 using 2,4,6-triphenylpyrylium tetrafluoroborate (TPT) as sensitizer has been studied to explore the possibility of achieving a slow sunlight-controlled release of polymer-bound carbonyl functionalized semiochemicals. This reaction has been found to produce mainly n-decanal and p-isopropylbenzaldehyde, together with lower amounts of the corresponding acids. The degree of conversion and the product distribution depended on the presence of water or oxygen and the amount of photosensitizer. The results are justified through generation of the radical cation 1+·, followed by ring opening and trapping of the radical/cationic sites by oxygen or water. The delocalized 1,3-radical cations III and the ozonides IV appear to be involved as key intermediates. Formation of n-nonanal as a minor product is explained through cleavage of the C2-nonyl bond of 1+·.

Photolysis of α,β-diacetoxystilbene (1) adsorbed onto a Fe3+-doped sepiolite (Fe-Sp) leads to benzoic acid (3), 9,10-diacetoxyphenanthrene (5), diphenic anhydride (6), and benzil (7). While compounds 3, 5 and 6 are also observed in the direct irradiation of 1 in hexane solution, diketone 7 is formed under electron transfer conditions using 2,4,6-triphenylpyrylium tetrafluoroborate as photosensitizer. These results strongly suggest that formation of benzil (7) in the photolysis of 1 on Fe-Sp involves the intermediacy of 1+, generated by single electron transfer from excited 1 to the iron active sites of the clay. In contrast to the unsubstituted stilbene, reactivity of 1+ with ground state molecular oxygen has been established by cyclic voltammetry through the current intensity decrease of the oxidation peak of 1 in the presence of oxygen. Finally, the thermodynamic feasibility of a direct electron transfer pathway between excited 1* and oxygen leading to the formation of a dioxetane intermediate is discussed.

Photolysis of phenols 1a-d in chloroform or carbon tetrachloride afforded the corresponding di- or trichloromethyl phenyl ethers (2a-d or 6a-d, respectively). In chloroform, hydroxybenzaldehydes 4a-d or 5 and phenyl formates 3a-d were obtained as minor photoproducts. In carbon tetrachloride, traces of salicyloyl chloride (7), phenyl salicylate (8) and phenyl p-hydroxybenzoate (9) were detected starting from the parent phenol (1a). The obtained results agree with the involvement of an electron transfer mechanism.

Photolysis of 2-phenylpropionic acid ( 1 ) in acetonitrile, methanol or benzene leads to ethylbenzene ( 2 ), 2,3-diphenylbutane ( 3 d,l and meso ), 1-(2-ethylphenyl)-1-phenylethane ( 4 ), 1-(4-ethylphenyl)-1-phenylethane ( 5 ) and acetophenone ( 6 ). In cyclohexane or carbon tetrachloride, solvent derived products are formed. These results involve homolytic cleavage of the C C bond α to the car☐y group, which affords 1-phenylethyl radical (PER) as key intermediate. The α,α coupling of PER in solution to give 3 is nonstereoselective; by contrast, formation of the meso isomer is preferred upon inclusion of 1 within β-cyclodextrin. This is attributed to the coupling of two long-lived PER-CD units

The photooxygenation of trans - and cis -stilbene (2a,b) by means of 2,4,6-triphenylpyrylium tetrafluoroborate (TPT) led to benzaldehyde (5) and diphenylacetaldeyde (6). Under the same conditions trans - and cis -1,2-diphenyloxirane (4a,b) afforded 6 as major product, together with minor amounts of 5, benzophenone (8) and, in the case of 4b, 1,2-diphenylethanone (7). These data support the intermediacy of the oxiranes in the photooxygenation of the corresponding stilbenes and reveal that rearrangement of the radical cations 4a,b+. occurs by two alternative pathways: phenyl migration and H migration. The TPT-photosensitized oxygenation of 5-methylene-10,11-dihydro-5H-dibenzo[a,d]cycloheptene (9) produced mainly dibenzosuberone (10) and dibenzosuberane (11), together with trace amounts of products arising from dehydrogenation of the dimethylene bridge (14,15,16) and solvent-derived products (12,13,18). The formation of 10 and 11 is justified via the radical cation of the 1,1-diphenylethylene moiety, generated by electron transfer from the olefin to the excited sensitizer. The epoxide 20, aldehyde 17 and dioxetane 19 appear to be key intermediates. Formation of the minor compounds 14, 15 and 16 can be explained through the radical cation of the 1,2-diphenylethane substructure.

Dihydrobenzopyrones 1a-c and their enol acetates 3a-e have been submitted to oxidation under single electron transfer (SET) conditions, using three alternative ways of activation: chemical oxidation with cerium(IV) ammonium nitrate (CAN), photochemical oxidation using triphenylpyrylium tetrafluoroborate (TPT) as sensitizer or electrochemical oxidation. The most significant products obtained are diketones 4, hydroxyketones 5, rearranged benzopyrones 6, enones 9 and, in the case of enol acetate 3c, 2-methylchromone (10) and 1,2-diphenylethane (13). These results are rationalized according to three major pathways from the radical cations: i) information of the α-carbonyl radicals I (through deprotonation of the enols 2+. or cleavage of the carbonyl-oxygen bond of their acetates 3+., eventually followed by secondary oxidation to the carbenium ions II, ii) breaking of the bond linking C2 with one of the substituted and iii) ring opening.

Direct irradiation of the title compound 4 in hexane led to 1,3-diphenyl-1,4-pentanedione ( 5 ) in 75 % yield. This is explained through carbonyl-oxygen bond cleavage (process iii), to afford a cyclopropyloxy radical (I) which rearranges to a β-carbonyl radical (II) prior to in cage recombination. In methanol, the solvent addition product 6 was obtained as byproduct. It formation is rationalized by means of the symmetrical cationic intermediate IV, on the basis of deuterium incorporation to C(1) and C(3) in MeOD. Photosensitization by acetone resulted exclusively in trans to cis isomerization, via the diradical III (process i). Finally electron transfer activation by means of cerium(IV) ammonium nitrate (CAN) or excited triphenylpyrylium tetrafluoroborate (TPT) gave rise to chalcone ( 7 ) or the β-functionalized ketones 8 or 9 . These products must arise from a common cationic precursor (V), after cleavage of the central carbon-carbon bond (process i) in the radical cation 4 + . From the preparative point of view, the obtention of diketone 5 in 75 % yield by photolysis of 4 in hexane is exploitable, since it constitutes a synthetic equivalent of the addition of acyl anions or acyl radicals to enones.

Carboxylic acid 1 and its derivatives 2 , 3 treated with triflic acid below −20°C afford the dications 1H 2+2- 3H 2+2. Cyclodehydration above 20°C (or in the gas-phase, CIMS of 3 ) leads to the diprotonated ketone 4H 2+2.

Two diaryl acetylenedicarboxylates 1 a,b have been prepared in moderate yields by direct reaction of acetylenedicarboxylic acid with the corresponding phenols catalyzed by sulfuric or p -toluenesulfonic acids and also through acetylenedicarbonyl chloride. The accompanying by-products have been characterized. Attempted esterification using N,N′-dicyclohexylcarbodiimide as condensing agent gives an asymmetric ester amide 11a and a substituted uracil 12a . Photolysis of the aryl esters 1 a,b in benzene solution affords the benzocoumaran-3-ones 14a,b . In the case of 1b , minor amounts of a polycyclic condensed 4-chromanone 17b have also been observed. These results shows that the nature of the substituent attached at the β-position of the C  C triple bond plays an important role in the cyclization of o -hydroxyaryl ethynyl ketones.

The photochemistry of enol esters 1 is dominated by electrocyclic ring opening ( 1b → 3 + 4 ), 1,3-acyl migration ( 1c , d → 6c , d ) and [2 + 2] dimerization ( 1d → 11). Photosensitization with triphenylpyrylium tetrafluoroborate (TPT) enhances formation of oxidation products ( 1b → 5 , 1c , d → 9c , d + 10c , d ), presumably through the intermediacy of radical cations 1b–d + .

Irradiation of the p -methoxyphenyl and p -methylphenyl esters of 2-butynoic, propynoic, 3-(ethylenedioxy)butanoic, 3,3-dimethoxypropanoic and 3-oxobutanoic acids (1–3) affords the corresponding photo-Fries products 4–6. Compound 5a is converted in part into the acetophenone 7a, by way of a Norrish type II photo-reaction, while compound 6a is reluctant to undergo this process, in spite of the fact that it also possess γ-carbonyl hydrogen atoms. From the preparative point of view, the photorearrangement of the esters 1a-d and 2a,c-d is exploitable, while that of 3a proceeds with a lower yield. The differences found in the photochemical behaviour of 2a and 3a show the sharp influence of the acetal group on the course of the reaction. Compounds 4–6 are representative model compounds valuable as direct chromone precursors; in fact, they can be readily cyclized to the chromones 10 under basic or acidic conditions.

Photocyclization of 2-chloroacetyl-1,2,3,4,5,6-hexahydro-1, 5-methanoazocino[4,3-b]indole ( 5 ) takes place at the indole 4-position to give a 1 ,2 ,3 ,4 , 5 ,6-hexahydro-2 ,11-ethano-1 ,5-methanoazocino [4 , 3-6] indole system. Consequently, the method appears to be unsuitable for constructing the pyrrolidine ring of pentacyclic Strychnos indole alkaloids.

The ethylene acetals of o and p -acetoxyacetophenone 3a,b, prepared from the corresponding hydroxyacetophenones by treatment with acetic anhydride in pyridine and subsequently with ethylene glycol and p -toluenesulphonic acid, give upon irradiation the expected photo-Fries products 4a,b (from 3a) or 6 (from 3b). These compounds are converted in part into diacetylphenols 5a,b as a consequence of a deacetalization, occurring to some extent during the irradiation and further in the chromatographic workup. The yields of the photoproducts and the ortho / para ratio (in the case of 3a) are very similar to those given in the literature for phenyl acetate. By contrast, the acetoxyacetophenones 2a,b do not undergo any appreciable change upon irradiation,showing the deactivating effect of the acetyl side chain. Therefore, it is concluded that cyclic acetals are suitable carbonyl blocking groups in order to circumvent the deactivating effect of acyl substituents on the photo-Fries rearrangement of aryl esters.

Irradiation of the aryl trans -2-butenoates 4b-e affords the corresponding photo-Fries products 1b-e , together with their cis isomers 3b-e . After separation by means of HPLC, the kinetics of the basic cyclization of the trans and cis - o -hydroxyketones 1b-e and 3b-e to the chromanones 2b-e were studied. The mechanistic implications of the obtained data are discussed.

The photolysis of 4-acetoxy-2 H -chromene 1a leads to a mixture of o -acetoxyphenyl vinyl ketone 2a , 4-chromanone 3, chromone 4 , 3-acetoxy-4-chromanone 5, 3-hydroxy-4-chromanone 6 and a cyclobutane dimer with anti head-to-head structure (7). The formation of 2a is explained by a photochemical electrocyclic opening of the pyran ring, followed by cis - trans photoisomerization and transacylation. Compounds 3 and 4 would arise from 1a by way of a photolytic cleavage of the acetyl-oxygen bond and subsequent disproportionation of the resulting radical 8 . The isolation of the 3-substituted chromanones 5 and 6 can be accounted for in terms of a photochemical epoxidation of 1a . This process and the [ 2+2 ] photodimerization are unprecedented in the photochemistry of 2 H -chromenes. Moreover, the photochemical epoxidation of 1a could have biological significance.

The photolysis of the 7-acetoxybenzopyran derivatives 1a-5a has been carried out. Chromene 1a was found to undergo extensive photopolymerization. Chromanone 2a underwent a rather inefficient photo-Fries rearrangement to the 6 - and 8 - acetyl derivatives 2b and 2c . Diacetoxychromene 3a gave the unsaturated ketone 8 as the main product, besides its deacetylation analogue 9 and chromanone 11 . Diacetoxychroman 4a afforded a mixture of four C-acetyl products: 1b, 1c, 2b and 2c , together with Chromene 1a and chromanone 2a. Finally, irradiation of chroman 5a gave rise to a mixture of the two possible photo-Fries products 5b (43%) and 5c (52%). The mechanistic implications of the above results are discussed, with special emphasis on the photoreactivity of the phenyl ester as compared with that of the pyran ring, the enol ester and the benzyl ester moieties. The synthetic applications of these transformations are illustrated with the preparation of eupatoriochromene 1b and encecalin 1e .

— p -Methoxyphenyl salicylate 2 gives 2,2′ - dihydroxy - 5 - methoxybenzophenone 3 upon irradiation, but the yields are low both in hexane and in methanol. p -Methoxyphenyl o -acetoxybenzoate 1 undergoes photo-Fries rearrangement, affording 2′ - acetoxy - 2 - hydroxy - 5 - methoxybenzophenone 4 in good yield. Compound 4 is not stable in silica gel: it isomerizes to 2′ - acetoxy - 2 - hydroxy - 5′ - methoxybenzophenone 5 , that is also the major acetylation product of 3 with acetyl chloride. Starting from 1 , 2-methoxyxanthorne 7 is obtained in 57% overall yield.

The photochemistry of 5-phenyl-, 5-(2',5'-dimethoxyphenyl)-, 5-(2'-acetoxy-5'-methoxyphenyl)- and 5- (2',5'-diacetoxyphenyl)- 2(3H)-furanone ( 1a-d ) has been investigated. Compound 1a yields phenyl vinyl ketone as expected. Similarly, 1b affords the corresponding aryl vinyl ketone but, in this case, photodimerization also occurs. Irradiation of the two o-acetoxyaryl furanones 1c and 1d gives rise to the formation of chromones as the main products. This interesting result can be accounted for in terms of a photochemical opening of the lactonic ring followed by radical addition to the acetoxy group.

A series of carboxylic acid derivatives 1–6 containing a second interacting function have been converted into the corresponding O -protonated species or acylium ions upon treatment with 96% sulfuric acid or oleum, respectively, at room temperature. The resulting stable cations have been spectroscopically characterized and submitted to irradiation. Cis-trans photoisomerization was observed in the ionic speciesderived from trans -crotonic, fumaric and maleic acids, and maleic anhydride. By contrast protonated crotonolactone 5a+ was found to be photostable. Finally, protonated α-bromobutyrolactone 6a+ was clean and efficiently dehydrohalogenated to 5a+ upon irradiation. Since the normal n → π* photoreactions of the uncharged carboxylic acid and their derivatives are inhibited under these conditions, protonation can be envisaged as a chemical selective method for photoprotection of the carboxy group.