Enones photochemical reaction

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). 

From a preparative point of view, the photochemical [2 + 2] cycloaddition is the most important of the photochemical reactions especially the cycloaddition involving enones. The [2 + 2] cycloaddition is the method of choice for the construction of cyclobutane derivatives as well as cyclobutane units within larger target molecules. 

Both target compounds discussed in this review, kelsoene (1) and preussin (2), provide a fascinating playground for synthetic organic chemists. The construction of the cyclobutane in kelsoene limits the number of methods and invites the application of photochemical reactions as key steps. Indeed, three out of five completed syntheses are based on an intermolecular enone [2+2]-photocycloaddition and one—our own—is based on an intramolecular Cu-catalyzed [2+2]-photocycloaddition. A unique approach is based on a homo-Favorskii rearrangement as the key step. Contrary to that, the pyrrolidine core of preussin offers a plentitude of synthetic alternatives which is reflected by the large number of syntheses completed to date. The photochemical pathway to preussin has remained unique as it is the only route which does not retrosynthetically disconnect the five-membered heterocycle. The photochemical key step is employed for a stereo- and regioselective carbo-hydroxylation of a dihydropyrrole precursor. 

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. 

Photochemical reactions of carbohydrates have been discussed.279 Therefore, only examples applicable to the synthesis of branched sugars are briefly described here. Although non-stereoselectivity and low yield are general defects of photoreactions, photoaddition of an alcohol to the enones 125 and 122a proceeded with the same stereo- and regio-selec-tivities as ionic addition ( see Section II,4,c), and in rather better yields,... 

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. 

Figure 2. Photochemical reaction pathways for enones Refer to Table I for list of substituents.

A few examples of 2 + 2 photocycloadditions of olefins on carbohydrate enones have been recently reported. The main results are described in the preceding review [1] on photochemical reactions of sugars. 

The cyclopropyl enone 31 was photoisomerized to the cyclopropanone 32 at liquid nitrogen temperatures.34) The unstable 32 was characterized on the basis of ir spectra and thermal and photochemical reactions (see Section 4.1.5). When 31 was irradiated at room temperature in 45% acetic acid, the phenol 33 was produced, most probably through 32 and the spiro dienone intermediate 34 (Scheme 5).35>... 

Orfanopoulos et al. studied the photochemical reaction of alkenes, aryalkenes, dienes dienones, and acyclic enones with [60]fullerene to obtain various substituted cyclobutylfullerenes [240,241,243,247], For example, the photocycloaddition of cis- and Irans-1 -(p-mcthoxyphenyl)-1 -propenc 68 to C6o gives only the trans [2 + 2] adducts (Scheme 27), thus the reaction is stereospecific for the most thermodynamically stable cycloadduct. A possible mechanism includes the formation of a common dipolar or biradical intermediate between 3C o and the arylalkene. Subsequent fast rotation of the aryl moiety around the former double bond leads exclusively to the trans-69 [2 + 2] adduct. Irradiation of this product, yielded 90% trans-68,10% cis-68 and cycloreversion products. Thus, a concerted mechanism can be excluded because the photocycloreversion is expected to give the trans-68 as the only product. These results can be explained by the formation of a common dipolar or diradical intermediate. Similarly, cycloreversion products from C6o and tetraalkoxyethylene... 

The 2 + 2 cycloadditions do occur in the excited state so these are photochemical reactions.3 They work best if one component (usually an enone) absorbs the light to form the excited state... 

Despite the fact that the [2 + 2] -photocycloaddition reaction of enones has a history of more than 100 years, it has remained a vital and attractive reaction. The continuing interest and many applications to increasingly more complex targets not only bear testimony to its utility but also contradict the myth that photochemical reactions are nonselective and unpredictable. It would be desirable if this most useful chemistry could also be appreciated in the life sciences industry. The first blockbuster drug to be synthesized via a [2 + 2]-photocycloaddition is yet to be developed. Apart from the conventional evolution of the reaction, which involves an increase in scope and an improvement in its practical execution [154], it is expected that sensitization - as a means of catalyzing photochemical reactions in general [155-157] - will become a dominant factor in the development of catalytic enantioselective [2 + 2]-photo-cycloaddition variants. 

Photochemical [2 + 2]cycloaddition to enones. This reaction has been shown to occur mainly from the less-hindered a-side of steroidal enones.2 Cycloaddition to the cyclopentenone (1) is also stereoselective, but the stereoselectivity and the yields are dependent upon the solvent. The highest yields are generally obtained in an apolar solvent (such as hexane). 

Mariano and co-workers also reported a similar C-Si bond cleavage of the cation radical of dimethyl(trimethylsilylmethyl)amine 13 in the photochemical reaction. Photoirradiation of a mixture of 13 and carvone 14 gives the addition product 15 of the diethylaminomethyl radical to the enone (Scheme 21). ... 

Since a-alkylthio carbocations are formed from a-stannyl sulfides by the oxidation with metallic reagents, we first studied the photochemical reaction of a-stannyl sulfides and conjugated enones. A methanol solution of (phenylthiomethyl)tributylstannane (61A) and 2-cyclohexen-l-one (14d) was irradiated through a UV cut filter hu > 300 nm). As expected, 3-(phenylthiomethyl)cyclohexan-l-one (62), and l-phenylthiomethyl-2-cyclohexen-l-ol (63) were obtained in 53% total yield as a 48 5 mixture. The corresponding silyl derivatives... 

The photochemical reaction of 3,4-methylenedioxyphenoxymethyl-stannane 73b with cyclopentenone 14e and cyclohexenone 14f proceeds smoothly and the aryloxymethylation products were obtained in good yield. In contrast, cycloheptenone 14g and acyclic enones 14h gave only a small amount of the desired product (Table 9 condition... 

Additions to Cyclohexenones and Related Systems - Irradiation of 3-methylcyclohexenone in methanol solution in the presence of the ester (42) results in the synthesis of adduct (43) in moderate yields. This product was used as the starting material for an approach to the synthesis of trichodiene. The enone (44) undergoes photochemical reaction with 2,3-dimethylbut-2-ene in benzene or acetonitrile solution and using 350 nm light.The four products were identified as (45), (46), (47) and (48). The formation of the oxetane (45) follows the conventional route and the cycloadduct (48) arises via the biradical... 

The photochemical reaction between the sulfine (261) and cyclooctyne affords the three products shown in Scheme 8. The enone and (262) are also formed from the thermal reaction of the sulfine and cyclooctyne and the route to these products is proposed to involve 1,3-addition of the sulfine to the cyclooctyne with the formation of (263) as the key intermediate. Fluorenone is proposed to arise from the oxathirane (264) which is formed photochemically from (261). The photochemical fragmentation of dimethyl sulfoxide in the gas phase has been studied and the primary fragments were identified as arising from C-S bond fission. ... 

Related photochemical reactions in which carbon dioxide is eliminated are summarized in Table 10. As well as y-aryl, y-enone substituents effectively promote the reaction. A remarkable example is provided by dihydro-l,2-(6j5,ll/ )-santonine (3). This crystalline solid slowly liquefied on exposure to normal laboratory lighting and then recrystallized. The product, norepimaalienone (4) has lost the elements of carbon dioxide and the stereochemistry of the methyl-bearing carbon is inverted. Simple derivatives of compound 3 underwent the same reaction. The cyclopropanes produced are themselves photosensitive and if more forcing conditions are employed, alkene rather than cyclopropyl products are isolated. ... 

Photochemical Reactions. The addition of allene to a steroidal enone on AI2O3 revealed a reversal of stereochemistry to that observed in solution [41] (Scheme 13). 

Several review articles have dealt with various aspects of cycloaddition reactions. A short review has presented examples to illustrate the use to which tethered alkenes can be put in the synthesis of cyclobutenes. The photochemical (2 + 2)-cycloaddition of enones to ethene has been studied from a theoretical standpoint. A review has highlighted the use of linear templates to control photochemical reactions such as (2 + 2)-photocycloadditions. The stereochemical control of photochemical reactions in clay-intercalated compounds has been discussed. A review has highlighted the interdisciplinary nature of photochemistry. ... 

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