Bicyclo photocycloaddition

The enol ether double bond contained within the ds-fused dioxa-bicyclo[3.2.0]heptene photoadducts can also be oxidized, in a completely diastereoselective fashion, with mCPBA. Treatment of intermediate XXII, derived in one step from a Patemo-Buchi reaction between 3,4-dimethylfuran and benzaldehyde, with mCPBA results in the formation of intermediate XXIII. Once again, consecutive photocycloaddition and oxidation reactions furnish a highly oxygenated system that possesses five contiguous stereocenters, one of which is quaternary. Intermediate XXIII is particularly interesting because its constitution and its relative stereochemical relationships bear close homology to a portion of a natural product known as asteltoxin. 

In contrast to the [2-1-2]-photocycloaddition which is a widely used method to generate four-membered rings, Koreeda and Zhang used a thermal rearrangement key step to construct the tricyclic framework of kelsoene [23]. In earlier work, it was shown that upon treatment with base, y-keto-p-toluene-sulfonate rac-32 is converted to a 44/56 mixture of bicyclo[3.1.1 ]heptanone rac-33 and bicyclo[3.2.0]heptanone rac-34 (Scheme 10) [24-26]. 

Several Ar-alkyl-2-oxo-l,2,3,4-tetrahydroazocine-3-carbonitriles and -carboxylates (88) have been obtained in very low yields by an unusual photocycloaddition of 2-pyridinones and acrylonitrile or methyl acrylate (80T81). The major products are the bicyclo[4.2.0] adducts (89). 

Cycloaddition of alkenes to the benzene ring does not occur when both molecules are in their ground electronic states. The reaction can only be brought about by photoexcitation of either of the two addends. Three types of photochemical cycloaddition of alkenes to benzene and its derivatives are presently known. Ortho photocycloaddition, also referred to as 1,2-photocycloaddition or [2 + 2] photocycloaddition, leads to bicyclo[4.2.0]octa-2,4-dienes. Meta photocycloaddition, also referred to as 1,3-photocycloaddition or [2 + 3] photocycloaddition, gives triyclo[3.3.0.02 8]oct-3-enes, also named l,2,2a,2b,4a,4b-hexahydrocyclo-propa[crf]pcn(alcnes. Para photocycloaddition, also referred to as 1,4-photocy-cloaddition or [2 + 4] photocycloaddition, results in bicyclo[2.2.2]octa-2,5-... 

Ortho photocycloaddition was first reported in a U.S. patent [1] dated September 3, 1957. Irradiation of benzonitrile in the presence of various alkenes resulted in the formation of derivatives of l-cyanobicyclo[4.2.0]octa-2,4-diene. The first ortho photocycloaddition to benzene was reported in 1959 by Angus and Bryce-Smith [2], who discovered that benzene and maleic anhydride react to form a stable adduct at 60°C under the influence of ultraviolet radiation. This 1 2 adduct was formed from one molecule of benzene and two molecules of maleic anhydride. Two years later, Bryce-Smith and Lodge [3] found that acetylenes could also be photoadded to benzene. The isolated products were cyclooctatetraenes, formed by ring opening of the primarily formed bicyclo[4.2.0]octa-2,4,7-trienes. Since those early years, hundreds of examples of ortho photocycloadditions of alkenes to the benzene ring and many mechanistic investigations have been reported and they will be discussed in this chapter. 

In Table 5, the ortho adducts are collected, which have been reported to be formed by intermolecular photocycloaddition of benzene and derivatives of benzene to simple alkenes. Ortho photocycloadditions of alkenes to perfluorinated benzenes are separately collected in Table 6. Intramolecular ortho photocycloadditions have been grouped in Table 7. (The many intramolecular ortho photocycloadditions proceeding via the triplet state of the arene moiety are found in Table 4.) All adducts shown in the tables are the primary ortho adducts. Many of these bicyclo[4.2.0]octa-2,4-diene derivatives are, however, unstable and not all of... 

In the formation of tetraenes from bicyclo[4.2.0]octa-2,4-dienes, two bonds are broken. This may occur in one concerted reaction which can be regarded as a retro [2 + 2] cycloaddition. It is also possible that the central bond, being part of a cyclohexadiene system, is the first one to break in a thermal, concerted disrota-tory process that leads to a 1,3,5-cyclooctatriene derivative. Ring opening of the cyclooctatriene then might take place photochemically, again disrotatory, to produce a tetraene. This two-step sequence was first observed by Mirbach et al. [114] in their study of the photocycloaddition of the two parent molecules benzene and ethene. The same explanation for the formation of a tetraene was given by Nuss et al. [160] in their report on the intramolecular ortho photocycloaddition of ( )-6-(2-methoxyphenyl)-5,5-dimethyl-2-hexenenitrile (see Scheme 40). 

Several articles have been published on the intramolecular ortho photocycloaddition of substituted 4-phenoxybut-l-enes [104,105,164-166], In most cases studied, ortho adducts are converted into derivatives of bicyclo[4.2.0]octa-2,7-di-ene. This was also found [113] for ortho adducts derived from ort/ro-methyl- and ortto-methoxy-3-benzyloxyprop-1 -cries. In these cases, the angular products are formed exclusively, which is attributed to steric properties of the cyclooctatriene. 

The photocycloaddition of alkenes to benzene rings can be classified into three types (2 + 2) (ortho), (3 + 2) (meta), and (4 + 2) (para) photocycloaddition, depending on the substituents, the reaction media, and the additives, as shown in Scheme 3. In general, para addition is very limited, and ortho and para adducts are more labile thermally and photochemically than the meta adducts. The (2 + 2) (ortho) photocycloadducts, bicyclo[4.2.0]octa-2,4-dienes 1, give thermally the ring-opened 1,3,5-cyclooctatrienes 4 and their following photochemical ring clo-... 

The intermolecular ortho photocycloaddition of acetylenes to benzene has provided routes to cyclooctatetraenes [78-84], Intramolecular photocycloaddition of alkynes to aromatic rings is also investigated. Morrison et al. reported that photolysis of 6-phenyl-2-hexyne in hexane solution using 254-nm light leads to the formation of bicyclo[6.3.0]undecatetraene (251 and/or 252) [284] (Scheme 70). Pirrung prepared various bicyclo[6.3.0]undecatetraenes (254a-e) by the in-... 

The sesquiterpene (+)-asteriscanolide 1 was first isolated from Asteriscus aquaticus L and characterized by San Feliciano in 1985.1 It has captured the attention of organic chemists mainly because of its uncommon bicyclo[6.3.0]undecane ring system bridged by a butyrolactone fragment. The only prior enantioselective synthesis of 1 has been described by Wender in 1988 featuring an Ni(0)-promoted [4 + 4]-cycloaddition.2 Booker-Milburn and co-workers described the sequential application of intramolecular [2 + 2]-photocycloaddition, Curtius rearrangement, and oxidative fragmentation to produce the 7-desmethyl derivative in 1997.3... 

Cyclopentenones undergo a clean [2 + 2]-photocycloaddition to a variety of alkenes, resulting in products with a bicyclo[3.2.0]heptanone skeleton. The two annelated... 

The crossed intramolecular [2 + 2]-photocycloaddition of allenes to a, 3-unsat-urated y-lactones has been extensively studied by Hiemstra et al. in an approach to racemic solanoedepin A (87). The sensitized irradiation of butenolide 85 in a 9 1 mixture of benzene and acetone, for example, led selectively to the strained photocycloadduct 86 (Scheme 6.31) [89]. The facial diastereoselectivity is determined by the stereogenic center, to which the allene is attached. The carbon atom in exposition to the carbonyl carbon atom is attacked from its re face, forming a bond to the tertiary allene carbon atom, while the P-carbon atom is being connected to the internal allene carbon atom by a si face attack. The method allows facial diaster-eocontrol over three contiguous stereogenic centers in the bicyclo[2.1.1]heptane part of the natural product. 

Properly alkylated bicyclo[4.2.0]-octan-2-ones formed by 2+2 photocycloaddition of alkenes to cyclohexenones cleave to 5, -unsaturated ketones, one example leading to a sesquiterpene synthesis [73]. 

In the preparative application of [2 + 2]-photocycloadditions of cyclic enones to (substituted) alkenes, two factors concerning product formation are of decisive relevance, namely the regioselectivity and the (overall) rate of conversion. Regarding the regioselectivity in the addition to mono- and 1,1-disubstituted alkenes, Corey had shown that the preferred addition mode of cyclohex-2-enone to isobutene or 1,1-dimethoxyethylene was the one leading to—both cis- and trans-fused—bicyclo[4.2.0]octan-2-ones with the substituents on C(7) [8]. In contrast, in the reaction with acrylonitrile, the alternate orientation was observed to occur preferentially. Similar results were also reported by Cantrell for the photocycloaddition of 3-methyl-cyclohex-2-enone to differently substituted alkenes [14]. No significant differences in the overall rates of product formation for the different alkenes were observed in these studies. In order to explain these observed... 

Finally, bicyclo[3.3.0]octenones such as 129 have been found capable of [2+2] photocycloaddition to simple alkenes, the ring forming process occurring with high stereoselectivity from the less hindered surface.200 ... 

Other examples of favored formation of cyclobutanes include photocycloadditions of tri-fluoromethyl-substituted coumarins with dienes,and of 5-(trifluoromethyl)-l,3-dioxin-4-ones, e.g. 3, with alkenes, " - - " as well as intramolecular photocycloadditions, such as those of bicyclo[2.2.2]octadiene and tricyclononadiene derivatives. 

A similar application of a vinylcyclopropane in a stereoselective photoreaction has been repotted by Dtirr and coworkers. Scheme S shows the addition of a bicyclo[3.1.0]hexenol derivative and acetophenone, which afforded the strained 7-oxatricyclo[4.2.0.0]octane and a comparable amount of tetraphe-nylcyclopentadiene. Since the vinylcyclopropane is known to undergo photochemical isomerization and photofragmentation to the cyclopentadiene, it is conceivable that these reactions precede photocycloaddition. If this is the case, the major oxetane (4 1 selectivity) arises from addition of the alkoxy radical to... 

Full details of the effects of acid and solvent polarity on various photocycloaddition reactions of ethylenes and acetylenes to benzene have been published and a mechanistic rationale of the observations has been presented. Thus the largely homopolar concerted meta cycloaddition of ethylenes to benzene, which yields the tricyclo[3.3.0.0 "]oct-3-ene system (36), has little sensitivity to either solvent polarity change or the presence of a proton donor, but the onho process leading to bicyclo[4.2.0]octa-2,4-diene derivatives (37)... 

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