Acetic photoaddition

Photoaddition of the enol acetate (48) in hexane resulted in the intramolecular [2 + 2]cycloaddition with the production of a 2 3 mixture of the isomeric tricyclic ketones (49) and (50). (Upon hydrolytic cleavage, the mixture was converted to the compounds (51) and (52) 18,22). 

The oxidative photoaddition of NNP to 3-butenyl chloride and bromide in the presence of perchloric acid gave 2-nitrato-5-azoniaspiro [4,5] decane perchlorate (306) in 38 and 46% yield, respectively165 (equation 146). The yield of the salt is obviously much higher, but it is difficult to extract from aqueous solution. The oxidative photoaddition to 3-butenol and its acetate gives 72-80% of the expected product 305, while that to various benzoate esters gives about 26-33% and that to p-toluenesulphonate gives no product. 

Other synthetic routes to benzazepines involving ring expansion of six-membered heterocycles include the action of diazomethane (77CPB321), sulfonium ylides (77H(7)37> or acyl halides (75T1991) on quaternary 3,4-dihydroisoquinolines that of sulfoxonium ylides on quaternary quinolines (74IJC(B)1238) and the zinc-acetic acid reduction of quaternary 1-acyltetrahydroisoquinolines (77BSF893). Photoaddition of acyl- or aryl-nitrenes to the exocyclic alkene bond of 2-methylene-1,2-dihydroquinolines results in ring expansion to... 

Ultraviolet irradiation of CCXLV, which is obtained by the condensation of diphenylketene and diphenyldiazomethane, in hydroxylic solvents, effects its cleavage into the starting materials in the presence of carboxylic acids or alcohols, photoaddition takes place with the formation of benzilic acid derivatives.167 On the other hand, irradiation of CCXLV in benzene containing acetic acid and/or methanol produces CCXLVI and CCXLVII, respectively. Irradiation in the presence of diphenylmethane under nitrogen atmosphere yields phenylacetic acid. 

A further mechanism for photoaddition that applies to cyclohexenes or cydoheptenes begins with formation of the highly reactive trans isomer of the cycloalkene. In this way 1-methylcydoheptene gives an ether on irradiation in methanol (2.53), and I-methylcydohexene an acetate with acetic acid (2.54). In both cases a... 

Photoadditions that arise by initial excitation of the aromatic compound are not common. Benzvalenes are readily attacked by hydroxylic compounds, and so irradiation of benzene in aqueous solutions of acetic acid, for example, results in the formation of a bicydic product (and an isomer derived from it by subsequent photoisomerizationl as a result of addition to the initially formed valence isomer (3.38). A different kind of photoaddition occurs when benzenes react photochemically with amines cyclohexa-T, 4-dienes are the major products (3.39), accompanied by cyclohexa-1.3-dienes, and unlike many of the photochemical reactions of benzene this does not suffer loss of efficiency in scaling-up. 

Unlike coumarin, chromone (206) undergoes efficient unsensitized photoaddition to tetramethylethylene, cyclopentene, ketene dimethyl acetal, and but-2-yne.180 The major product of such an addition to tetramethylene is the cis-fused cyclobutane derivative (207) the formation of the two minor products (208 and 209) is easily rationalized. Added benzophenone has no visible effect on this cycloaddition, which is therefore believed to involve the attack of triplet chromone on the ground-state alkene. Photoaddition to furo-chromones has also been studied,179 and the photosensitized cyclo-... 

Ketene silyl acetals underwent photoaddition involving single electron transfer with electron-deficient alkenes and sensitizers such as phenanthrene (phen)179. Two examples of the types of products, which were formed regioselectively, are illustrated in Scheme 60. 

Another example involves photoaddition of the chiral acetal 165 to cyclopentenone, affording the photoproduct 166 in a considerable enantiomeric excess90 (Scheme 36). 

The above examples have presented a better induction effect when the chiral auxiliary was located at the enone molecule. Double auxiliary induction has been examined by Scharf and coworkers99. Systematic study on the photoaddition of chiral enones 203 to chiral ketene acetals 204 provides examples of matched (45% de) and mismatched (9% de) double stereo differentiation (Scheme 44). 

The photosensitized 1-4 addition of alcohols to hexenopyranosuloses first reported by B. Fraser Reid and coworkers [58 a] has been developed with other studies on photoadditions of oxycarbinyl species such as polyols, acetals, dioxolanes, aldehydes. A mechanistic study on this photoaddition has been recently detailed [58 b] showing that the important photochemical event is hydrogen abstraction from methanol, for example, to form the hydroxymethyl radical. 

Ohashi et al. [128] found that the yields of ortho photoaddition of acrylonitrile and methacrylonitrile to benzene and that of acrylonitrile to toluene are considerable increased when zinc(II) chloride is present in the solution. This was ascribed to increased electron affinity of (meth)acrylonitrile by complex formation with ZnCl2 and it confirmed the occurrence of charge transfer during ortho photocycloaddition. This was further explored by investigating solvent effects on ortho additions of acceptor olefins and donor arenes [136,139], Irradiation of anisole and acrylonitrile in acetonitrile at 254 nm yielded a mixture of stereoisomers of l-methoxy-8-cyanobicyclo[4.2.0]octa-2,4-diene as a major product. A similar reaction occurred in ethyl acetate. However, irradiation of a mixture of anisole and acrylonitrile in methanol under similar conditions gave the substitution products 4-methoxy-a-methylbenzeneacetonitrile (49%) and 2-methoxy-a-methylbenzeneacetonitrile (10%) solely (Scheme 43). 

The 1,4-photoaddition of aliphatic amines with benzene via photoinduced electron transfer was first reported by Bryce-Smith more than 30 years ago [375-378], In the photoreaction of triethylamine with benzene, the proton transfer from the radical cation of triethylamine to the radical anion of benzene is proposed as a probable pathway (Scheme 113). In the case of tertiary amines, the photoaddition is accelerated by the addition of methanol or acetic acid as a proton source. Similar photoaddition of diethyl ether to benzene takes place assisted by trifluoroacetic acid, where methanol is not affective [379], In these photoreactions, a-hydrogen next to the heteroatom moves to the radical anion of benzene as a proton, followed by radical ccoupling to give 1,4-addition products. Similar photoaddition of amines to the benzene ring has been reported by Ohashi et al. [380,381],... 

Mikami et al. also investigated the addition of ketene silyl acetals. They found that addition of the silyl enol ether of acetone and allylic silanes did not result in the synthesis of substituted l,2-dihydro[60]fullerenes [218a,220], In 1997, Mikami et al. [221] reported the photoaddition of allylic stannanes that leads to monoallylation of C6o (Scheme 13). 

A variety of photoaddition reactions of nucleophiles to electron-rich substrates in the presence of electron acceptors were developed in the early 1970s to 1980s. Arnold and his coworkers reported that the photoaddition of methanol to 1,1-diphenylethene occurs in the presence of methyl p-cyano-benzoate in an anti-Markownikoff manner [33]. Photoaddition of nucleophiles such as water, alcohols, acetic acid, and cyanide anion to electron-rich arylalkenes were also reported [34-37]. The photoaddition reaction proceeds via the mechanism as shown in Scheme 6. 

A recent application of the furan-carbonyl photocycloaddition involved the synthesis of the mycotoxin asteltoxin (147)." Scheme 16 shows the synthetic procedure that began with the photoaddition of 3,4-dimethylfuran and p-benzyloxypropanal to furnish photoaldol (148), which was epoxidized with MCPBA to afford the functionalized product (149) in 50% overall yield. Hydrolysis (THF, 3N HCl) provided the monocyclic hemiacetal which was protected as its hydrazone (150). Chelation-controlled addition of ethylmagnesium bromide to the latent a-hydroxy aldehyde (150) and acetonide formation produced compound (151), which was transformed through routine operations to aldehyde (152). Chelation-controlled addition of the lithium salt of pentadienyl sulfoxide (153) followed by double 2,3-sigma-tropic rearrangement provided (154) as a 3 1 mixture of isomers (Scheme 17). Acid-catalyzed cyclization of (154) (CSA/CH2CI2) gave the bicyclic acetal (155), which was transformed in several steps to ( )-asteltoxin (147). ... 

The photoaddition of methyl acrylate to pyridones (49) gave the three [2 + 2] cycloaddition products (50), (51), and (52).Similar observations have been reported previously.A patent has been lodged which deals with the photoad-dition of alkenes (RCH=CH2 R = H, CN, or MeC02CH2) to the pyridones (53) to yield the adducts (54) and (55). The photocycloaddition of vinyl acetate to the pyridinones (56) affords the head-to-tail adducts (57) preferentially. ... 

In the photoaddition of water (equation 30) or acetic acid (equation 31) to alkynes it seems likely that ionic addition occurs by protonation of the alkyne excited state, particularly in view of the observation that the hydration reaction is speeded up by acid and retarded by base. The sensitized addition of acetic acid to medium-ring cycloalkynes to give enol acetates (equation 32) is strongly reminiscent of the analogous addition to cycloalkenes, which has been shown to go by way of protonation of the highly strained /ra/is-cycloalkene. 

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