Oxa-di-7t-methane

DFT and ab initio calculations have shown that oq/S-unsaturated acyl radicals and a-ketenyl radicals are not resonance forms but are rapidly interconverting isomers separated by a low but not negligible energy barrier of <22 kJmol-1.64 Bi-cyclo[2.2.2]octenones (60) bearing an endo vinyl substituent have been shown to give diquinane products (61) on triplet-sensitized irradiation the reaction has been described as occuring via an oxa-di-7t-methane rearrangement. 

Demuth M (1991) Oxa-di-7t-methane Photoisomerizations. In Trost BM (ed) Comprehensive Organic Synthesis. Pergamon Press, Oxford, chap 2.6... 

The photoisomerization of 40 and 41 shows that oxa-di-7t-methane rearrangements are probably not restricted to. y-unsaturated ketones or aldehydes, in that other carbonyl containing chroraophores can also undergo this type of reaction. 42.43)... 

In a number of compounds there exists the possible choice between an oxa-di-7t-methane and a di-7r-methane rearrangement. Both modes of reaction are found. With 15c [Eq. (7)] only the oxa-di-n-methane product 17 c is obtained and not the corresponding di-Ti-methane product. 20) On sensitized irradiation of 42 with R=H, 44a is formed, which is the 1,2-acyl shift product and not 44b and/or 44c which are the benzo-vinyl-methane rearrangement products. 44) On the other hand, 45 gives only 46, a di-ir-methane reaction in which one of the participating double bonds is part of an a,P-enone, and none of the oxa-di-re-methane or benzo-vinyl methane products 47 and 48, respectively, are formed.46). In the same way 49 only gives 50, and not the oxa-di-Tc-methane or the divinylmethane product. 45) in the reaction of 51 to 52 it has not yet been established whether an oxa-di-Tc- or a di-7i-methane pathway is taken. 46)... 

A most interesting case of the oxa-di-7t-methane rearreuigement, proceeding on direct irradiation, is found in 53 49,so) and its steroid analogue 36. ) The 1,2-acyl shift occurs on direct irradiation, cannot... 

Of di-7c-methane and oxa-di-7t-methane rearrangements reported, that of greatest preparative value involves the photolysis of bicyclo[2,2,2]octadienones. The triplet-state reactions of these latter compounds have been examined for the first time in the absence of benzannelation. Bicyclo-octadienones (42) have available to them four di-7t-methane and two oxa-di-ii-methane modes of rearrangement, and, remarkably, only one di-7c-methane route is followed, to give (43) regiospecifically and in good (40—70 %) yields. 

Photochemical Rearrangements in P,Y-Unsaturated Enones The Oxa-di-7t-methane Rearrangement... 

Paquette and co-workers employed the oxa-di-7t-methane product 134 for the synthesis of the antibiotic pentalenolactone P methyl ester, a highly functionalized sesquiterpene lactone. The oxa-di-Jt-methane reaction of 133 into 134 sets most of the stereochemical requirements of the target in a highly... 

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. 

McClure, C.K., Link, J.S., and Kiesshng, A., The oxa-di-7t-methane photochemical rearrangement of quinuchdinones, 1- and 2-aza-3-carboxybicyclo[2.2.2]oct-2-ene-5-ones Application to the synthesis of pyrrohzidine alkaloids. Abstract no. ORGN 480,221" ACS Meeting, San Diego, CA, 2001. 

Subsequent to the aforementioned photoisomerization, which was later rationalized as a 1,3-acyl sigmatropic shift, photochemical reactions of various types of P,y-enones were examined. It was observed that P,y-unsaturated enones undergo two unique reactions viz. 1,3-acyl sigmatropic shift and oxa-di-7C-methane (ODPM) reaction or 1,2-acyl shift), which are characteristic of their excited states. The direct irradiation of P,y-enones such as 2, in general leads to the formation of another enone 3 as a result of migration of the carbonyl group to the y-carbon and formation of a new 7t-bond between the a- and P-carbons (Scheme 2). Sensitized irradiation, on the other hand, causes a 1,2-acyl shift (or oxa-di-7t-methane reaction), leading to a cyclopropyl ketone of type 4. 

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