Oxa-di-7r-methane rearrangement

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)... 

Mechanism b), the oxa-di-7r-methane mechanism, is in line with CNDO calculations which show that u-orbital interaction between the carbonyl and the p-carbon is favored in the tt—n triplet state. 7) The concerted process, mechanism (c) has been pointed out to be unlikely in that a cyclopropyl ketone triplet is of higher energy than the triplet of the starting ketone. It is reasoned that the rearrangement thus cannot take place entirely in the triplet manifold and a spin inversion to yield an intermediate during the overall reaction is needed. On the other hand, this explanation may be too simple, there is no a priori reason why a reaction of triplet state to ground state should not proceed with synchronous bond breaking and formation. 

The rearrangement of l-methoxybicyclo-oct-5-en-2-one to bicyclo-[3,3,0]octane-2,7-dione is another example of the oxa-di-7r-methane reaction. ... 

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