Enones, cyclic, photochemical reactions

Cyclic a,) -unsaturated ketones present a rich array of photochemical reactions, some of which are of considerable synthetic value (see Section 6.4 of Part B). For cyclohex-enones, two prominent reactions are the di-rr-methane rearrangement (path A) and the lumiketone rearrangement (path B). 

Cyclobutane formation via light-induced [2 + 2] cycloaddition is probably one of the best studied photochemical reactions and has been reviewed thoroughly up to 1972 (Houben-Weyl, Vols. 4/5 a and 4/5 b). The most important types of C —C double-bond chromophores undergoing such reactions arc alkenes, 1,3-dienes, styrenes, stilbenes, arenes, hetarenes, cycloalk-2-enones, cyclohexa-2,4(and 2,5)-dienones, 1,4-benzoquinones, and heteroanalogs of these cyclic unsaturated carbonyl compounds. For p notocyciodimerizations see Houben-Weyl, Vol. 4/5 a, p 278 and for mixed [2 + 2] photocycloadditions of these same chromophores to alkenes see Section 1.3.2.3. 

Cyclic enones participate in several other photochemical reactions. Irradiation of neat cyclopentenone417 or cyclohexenone418 leads to formation of dimers. Eaton has shown that the reaction is quenched by piperylene and that there is a strong polar solvent effect on the ratio of head-to-head and head-to-tail dimers formed.511 Similar solvent effects have been noted for the triplet-state photodimerization of isophorone.512 Head-to-head dimers are favored in polar solvents and in neat ketone, while head-to-tail dimers are favored in nonpolar media. 

Diastereoselective [2+2] photocycloaddition of a polymer-supported cyclic chiral enone with ethylene has been reported (Scheme 12.33) [43]. The auxiliary was derived from (-)-8-(p-methoxyphenyl) menthol (87). Protection of the secondary alcohol and demethylation were carried out to give (-)-8-(p-hydroxyphenyl)menthyl acetate (88). An alkyl linker was introduced and finally loaded to poly (ethylene glycol) grafted Wang resin. Deprotection of the alcohol functionality was followed by esterification with cyclohexen-3-one-l-carboxylic acid to provide the chiral enone 89. The photochemical reaction with ethylene was performed by irradiating with light (k > 280 nm). Trifluoroacetic acid (TFA) or aqueous hydrolysis with... 

The primary and fastest photochemical reaction of acyclic and some cyclic a, P-unsaturated ketones is cis-trans isomerisation. This isomerisation is not expected for cyclopent-2-enone or cyclohept-2-enone because of high strain energy, but it has been reported for cis-cyclooct-2-enone and cis-cyclohept-2-enone. It is thought to occur from triplet excited state. 

Koppes, M.J.C.M. and Cerfontain, H., Photochemistry of P,y-enones. 11. Photochemical reactivities of cyclic-a-phenyl P,y-enones. Singlet 1,3-acyl shift, decarbonylation and unquenchable oxa-di-7t-methane reactions upon direct irradiation.. Reel. Trav. Chim. Pays-Bas, 107, 549,1988. 

Acyclic a,jS-unsaturated ketones do not provide much photochemistry, except for facile geometric isomerization about the double bond. Although some also isomerize to /3,y-unsaturated ketones, others, such as mesityl oxide, apparently undergo no net reaction upon absorption of irradiation. Their low reactivity might result from a rapid relaxation to an appreciably more twisted geometry in their triplet states than can be obtained in the cyclic enones. Below are some examples of typical photochemical behavior.426,427... 

There is a striking difference between the photochemical reactivity of oc,(3-unsaturated enones and the corresponding ynones. Whereas many cyclic enones undergo [2+2] cycloaddition to alkenes at the C=C double bond of the enone (probably from the triplet nn state) to yield cyclobutanes, acyclic enones easily deactivate radiationless by rotation about the central C-C single bond. Ynones on the other hand behave much more like alkyl-substituted carbonyl compounds and add to (sterically less encumberd) alkenes to yield oxetanes (Sch. 11) [38,39]. The regioselectivity of the Paterno-Biichi reaction is similar to that of aliphatic or aromatic carbonyl compounds with a preference for primary attack at the less substituted carbon atom (e.g., 41 and 42 from the reaction of but-3-in-2-one 40 with... 

Cyclic enones with ring sizes of six-to-eight carbons can be photochemically induced to undergo [4+2] cycloadditions via isomerization to a strained trans isomer (Schs. 22-24). Irradiation of 2-cycloheptenone 99 leads to [2+2] dimerization of an intermediate r -2-cycloheptenone 100, but if this irradiation is conducted with an excess of cyclopentadiene 32 at —50 °C, a single [4+2] adduct 101 is isolated in very high yield [65,66]. The somewhat less strained nms-2-cyclo-octenone can be generated and trapped by subsequent addition of a cyclopentadiene [67,68]. Extension of this reaction to intramolecular examples has recently been reported [69]. 

The photochemistry of o< -unsaturated ketones has attracted much attention and is still a field fiettd of current interest. /Numerous examples of such photochemical transformations are well-documented for cyclic enones and dienones, including both cycloaddition reactions and rearrangements. For example, cyclopentenones 1 and 2 readily rearrange to... 

Additions to Cyclohexenones and Related Systems - Caldwell and his coworkers have studied the photochemical addition of 1,1-diphenylethene to 4,4-dimethylcyclohex-2-enone. The products from this reaction, carried out in cyclohexane, are shown in Scheme 1. Although other evidence (see reference 9b above and references cited therein) has suggested that an exciplex is not a key interaction in such (2+2)-photocycloaddition reactions Caldwell et al. conclude from their detailed study of this system that a triplet exciplex is involved. Schuster and his coworkers have reported that a variety of cyclic enones (17) -(19) add photochemically to fullerene. The yields of the adducts vary but with some of the less heavily substituted enones the yields can be reasonable as shown by the data under the appropriate structure. Suginome et aO report the synthesis of the cycloadducts (20) by the photochemical addition of various ethenes to the enone (21). The adduct (22) was also synthesized by photochemical addition of methoxycyclohexene to the enone (20). 

Among these reactions, the photochemical cycloadditions of C=C bonds, which can create up to four asymmetric carbons during the photochemical step, are particularly interesting, and numerous synthetic applications of this reaction have been reported. Advances in the understanding of the origin of asymmetric induction, during addition of alkenes with carbonyl derivatives, cyclic enones, and aromatic compounds, will be discussed in detail. 

Hastings, D. J., Weedon, A. C., Origin of the Regioselectivity in the Photochemical Cycloaddition Reactions of Cyclic Enones with Alkenes Chemical Trapping Evidence for the Structures, Mechanism of Formation, and Fates of 1,4 Biradical Intermediates, J. Am. Chem. Soc. 1991, 113, 8525 8527. 

Various photochemical (2 + 2)-cycloadditions of heteroaromatic compounds have been reported in which an enone moiety is incorporated either into the olefinic reagent or into the heteroaromatic compound. Both furan and thiophene have been found to give cycloaddition reactions with maleic anhydride derivatives in the presence of a sensitizer.202 213 The cycloadducts (185 and 186) were formed in high yield, but in the case of 2,5-dimethylthiophene, cyclobutane formation was the minor pathway, as oxetane formation predominated.210 Cyclic enones, such as 2-cyclopenten-l-one and 2-cyclohexen-l-one reacted with furan to afford mixtures of (2 + 2)-cycloadducts (187a, R = H) and (188),... 

Margaretha and co-workers have described the cycloaddition of some cyclohex-2-enones to acrylonitrile. The addition reactions show moderate regioselectivity and form mixtures of exo- and ewJ6)-5-oxobicyclo[4.2.0]-octane-7-carbonitriles. Madhavan and Pitchumani have reported the dimerization of 2-cyclohexenone confined in clay interlayers (cation-exchanged bentonite). The reaction is remarkably regioselective and affords the head-to-head dimer almost exclusively. The cyclic alkene (24) undergoes photochemical addition to enones to afford the adducts (25) and (26) in the yields shown. 

The photochemical reactivity of P-ketoesters is different form that of P-diketones. Irradiation of a P-ketoester in the presence of an alkene produces oxetane via the ketone carbonyl instead of the desired cyclobutane ring system. Therefore, it is necessary to covalently lock the ketoesters as the enol tautomers. To this end, silyl enol ethers, 129 and 132a, and enol acetates, 130 and 132b, were prepared, but these substrates still fail to undergo the desired intramolecular [2 + 2] photocycloaddition with olefins. The only new products observed in these reactions result from the photo-Fries rearrangement of the cyclic enol acetate (130 to 131) and cis-trans isomerization of both acyclic substrates 132a/b. However, tetronates are appropriate substrates for both intermolecular and intramolecular photocycloadditions with olefins. In addition, enol acetates and silyl enol ethers of p-keto esters are known to undergo [2 + 2] photoaddition with cyclic enones (vide infra). 

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