Photochemical cycloaddition reactions enones

Scheme 6.8. Photochemical Cycloaddition Reactions of Enones with Alkenes and Alkynes... 

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. 

The 1980s witnessed a revival of interest in synthetic photochemistry, largely based on the successful application of intramolecular arene-olefin and enone-olefin photochemical cycloaddition reactions to the synthesis of polycyclic natural and unnatural products. The potential application of intramolecular arene-amine addition to the synthesis of alkaloids was suggested in 1973 by Btyce-Smith et al.," based on their observation of intramolecular photochemical 1,4-addition reaction of the (N,A/-dimethylaminoalkyl)benzenes I and 2 to yield 3 and 4, respectively. Intramolecular addition reactions have subsequently been observed for a large number of (aminoal-... 

Maradyn, D.J. and Weedon, A.C., Trapping of triplet 1,4-biradicals with hydrogen selenide in the intramolecular photochemical cycloaddition reaction of 3-(4 -pentenyl)cycloalk-2-enones - verification of the rule of five, /. Am. Chem. Soc., 117, 5359-5360,1995. 

Bauslaugh, P. G., Photochemical cycloaddition reactions of enones to alkenes synthetic applications, Synthesis, 287,1970. 

Although photochemical cycloadditions have gained acceptance in synthetic chemistry, most such reactions are limited to a relatively small scale. The use of a 1000-watt street lamp permits the irradiation of up to 1 mol of substrate in less time than 0.2 mol can be irradiated with the conventional 450-watt lamps. Thus, under optimum conditions, the submitters were able to add ethylene to 3-methylcyclohexenone on a 20-g scale in 48 hr (801) with a 450-watt lamp with the apparatus described here 94 g of this enone was condensed with ethylene in 8 hr (91%). 

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. 

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). 

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... 

Intermolecular.- Reviews have described some of the photochemical (2+2)-cycloaddition reactions of enones used in the synthesis of natural products. Other reports have also focussed on such additions as key steps in the design of natural products. Thus the photoadducts obtained by the cycloaddition of the enone (61) to the alkene (62) have been used as the starting materials for the synthesis of terpenoid intermediates. ... 

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. 

Copper-catalysed Michael addition followed by an aldol reaction with formaldehyde, gives a 1 1 mixture of diastereoisomers of the aldol 86 that can be eliminated to the enone 83. The resulting efficient photochemical cycloaddition gives ketone 82 with total regioselectivity probably because it is intramolecular. 

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. ... 

The scope of photochemical [2 + 2] cycloadditions of enones to alkenes in solution is limited by the failure of most acyclic oc,/ -unsaturated carbonyl compounds to undergo bimolecular reactions in competition with rapid unimolecular cisjtrans photoisomerization. 

A preliminary examination of asymmetric induction in photochemical [2 + 2] cycloaddition reaction of an enone to an alkene employed a chiral auxiliary attached to the alkene component74. The photocycloaddition of 2-cyclopentenone to the optically active ketene acetal 1 led both to oxetanes and cyclobutanes in the ratio 6.5 3.5 with a total yield of 60%. The m-[2C + 2C] addition of the chiral alkene I to 2-cyclopentenone was completely regioselective and gave four diastereomeric head-to-tail cycloadducts 2-4 in the ratio 6 29 33 32. 

The mechanisms of two particular classes of photochemical (2 + 2)-cycloaddition reactions have been studied extensively, viz. (i) of reactions in which either the heterocyclic or the substrate contains an enone moiety,1231,24 which are believed to occur via the excited triplet state of the enone, and (ii) of oxetane formation, which is thought to proceed via n — 7T excitation of the carbonyl group, followed by intersystem crossing to the triplet state and addition to the carbon-carbon double bond in its ground state6 in some cases, however, an oxetane has been reported to be formed from a ground-state ketone and an excited-state olefin (Section III,C,2). 

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),... 

Intermolecular Additions to Cyclopentenones and Related Systems. Mehta and co-workers have described the photochemical addition of 1,2-dichloroethene to the enone (12). This affords the adduct (13), which can be transformed into the protoilludane-type molecule shown in Scheme 4. A further study related to the (2 + 2)-cycloaddition reactions of compounds such as (14) and (15) has determined the crystal structures of these compounds. The... 

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. 

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