Oxa-di-jt-methane rearrangement

Sadeghpoor, R., Ghandi, M., Najafi, H.M., and Farzaneh, F. (1998) The oxa-di-jt-methane rearrangement of p.y-unsaturated ketones induced by the external heavy atom cation effect within a zeolite. Chemical Communications, (3), 329-330. 

Unsaturated ketones undergo a formal 1,2-acyl shift from the lowest excited triplet state to afford cyclopropyl ketones (for some early examples cf. Houben-Weyl, Vol. 4/5b, p 794), a reaction which has been termed oxa-di-Jt-methane rearrangement.Acyclic ketones normally do not undergo this reaction, however, an exception is in systems where the C-C double bond is further conjugated, e.g. to a phenyl group. 

Oxa-di-Jt-methane rearrangement leads to cyclopropane derivatives, compounds that are otherwise difficult to synthesize. The diphenylenal 88, for example, is converted to the cyclopropyl aldehyde 89 by triplet sensitization (Scheme 6.35).660 The photoproduct can be further transformed to other compounds, for example a diphenylvinylcyclopropane derivative 90. 

Acetophenone-sensitized irradiation of (83) affords an 80% yield of the triquinane derivative (84). This product is the key intermediate in a new synthesis of (— )-hirsutene. The tricyclotridecenones shown in Scheme 3 are photoreac-tive. Irradiation with acetone as the sensitizer brings about an oxa-di- jt-methane rearrangement to yield (85) while direct irradiation in benzene gives (86) by a... 

Photochemical oxa-di-jt-methane rearrangement of bicyclo[3.2.1]octa-noid scaffolds affords multifunctional, donor-acceptor cyclopropanes (34). Application of this reaction to bicyclo[2.2.2]octenones has been used as a key step in the total synthesis the hirsutane-type sesquiterpenes (-)-connatusin A (35), (+)-connatusin B (36), and the marine sesquiterpenoid 2-isocyanoallopupukeanane (37). ... 

For this simple substrate the [1, 2] shift product is not observed and this is also a common feature of substrates which geometrically isomerise, thereby dissipating triplet energy which might be otherwise used to effect the oxa-di-Jt-methane rearrangement. 

The oxa-di-Jt-methane rearrangement of p, y-unsaturated carbonyl compounds is most commonly observed in rigid systems where isomerisation of the alkene is inhibited. It occurs from the lowest tc -> n triplet state and can be sensitised by triplet sensitiser. 

Behr etal. studied the photoreaction of tetracyclic trienedione 139, which was prepared by the thermal addition of o-benzoquinone to barrelene (15%). Irradiation of 139 in deoxygenated methylcy-clohexane (MCH) or chloroform with benzophenone at 366 nm afforded the cage compound 140 (25%). The first step of this cycloaddition may be the formation of a biradical by a pathway similar to that of the oxa-di-Jt-methane rearrangement from the lowest (n 71 ) state (Scheme 25). Since the excitation energy of the a-dicarbonyl unit in 139 [EA (biacetyl) = 0.72 eV] is similar to that of 2-cyanonaphthalene, intramolecular excitation energy transfer from dione to diene similar to that from 2-cyanonaphthalene... 

For a review on the oxa-di-jt-methane rearrangement Demuth, M., Synthetic aspects of the oxa-di-Jt-methane rearrangement, in Organic Photochemistry, Vol. 11, Padwa, A., Ed. Marcel Dekker, New York, 1991, pp. 37-109. 

Most recently, oxa-di-Jt-methane rearrangement in aza-bicyclo[2.2.2]oct-2-en-5-one was explored this provides a novel route to pyrrozolidine alkaloids. Thus, sensitized irradiation of 126 and 128 furnished azabicyclo[3.3.0]octanones 127 and 129, respectively (Scheme 24). However, the derivatives 130 did not undergo the oxa-di-Jt-methane reaction. ... 

Greenland, H., Kozyrod, R.P., and Pinhey, J. T., The photochemistry of 2,2-dimethyl- and 2-aryl-2-methylnaphthalen-l(2H)-ones. Substitution requirement for the oxa-di-Jt-methane rearrangement,/. Chem. Soc., Perkin Trans. 1, 2011,1986. 

Lyle, T.A., Mereyala, H.B., Pascuel, A., and Frei, B., Photochemical reactions. Photochemistry of homoconjugated cyclobutanones. 11. Decisive effect of gem-dimethyl substitution on the course of oxa-di-jt-methane rearrangement, Helv. Chim. Acta, 67,774,1984 and references therein. 

Singh, V. and Jagadish, B., Synthesis of exo-annulated bicyclo[2.2.2]octenone and its oxa-di-Jt-methane rearrangement to cisisynxis triquinane, Indian J. Chem., 35B, 401,1996. 

Migration and oxa-di-Jt-methane rearrangements are the two fascinating aspects of P,y-enone photochemistry and studies on both were done in parallel. Oxa-di-Jt-methane reactions, however, received somewhat more attention, perhaps due to their synthetic applications and, as such, have been discussed in several reviews. - Most of the studies on 1,3-shifts were made in conjunction with the ODPM reaction with a view to resolve the singlet—triplet dichotomy. Nevertheless, the Hterature on the... 

Singh, V. and Raju, B.N.S., cycloaddition synthesis and oxa-di-Jt-methane rearrangement... 

However, studies recently carried out in our laboratory have demonstrated that some of the ideas firmly established about the photoreactivity of these compounds are incorrect. Thus, the normal photochemical reactivity of P,y-unsaturated aldehydes reported in books and monographs is decarbonyla-tion. " However, our results demonstrate that many P,y-unsaturated aldehydes undergo the oxa-di-Jt-methane rearrangement even more efficiently than the corresponding methyl ketones, which is contrary to popular belief. Alternative reaction modes, such as 1,3-formyl migration and decarbonylation, that take place via the Tj excited state and, therefore, are not Norrish Type 1 processes, have also been uncovered. These novel photoreactions of aldehydes are discussed in a separate chapter. 

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