Di-Jt-methane reaction

Approximately three years ago we initiated a study of the photochemistry of dibenzobarrelene-ll,12-diester derivatives (22, Scheme 9) as part of a general program aimed at investigating the di-jT-methane reaction in the solid state (review, 37). ciganek had shown some 20 years earlier that the dimethyl diester 22a undergoes smooth photorearrangement in solution to afford the dibenzosemibullvalene derivative 23a (38). This reaction is... 

The photoreaction of other bicyclo[2.2.2]octenones, and their congeners with more than one p,y-C-C double bond, have the possibility of dual deactivation pathways for example, ODPM and di-Jt-methane reaction upon excitation have been examined. > > - The benzobicyclo[2.2.2]octenones 122a and 122b are some of the earher examples that were found to follow the ODPM reaction pathway upon acetone-sensitized irradiation that involved the non-benzenoid C-C double bond to give the products 123a and 123b, respectively, albeit in low yield (Scheme 23). The bicyclo[2.2.2]octadienone 124, however, yielded a very small amount of ODPM product. The major product was due to di-Jt-methane reaction. ... 

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. ... 

A variety of compounds containing a P,y-enone chromophore in a complex molecular framework have been synthesized and their photochemical reactions have been examined. In the majority of the chromophoric systems, the P,y-enone group is part of a bicyclo[2.2.2] octane framework in which the oxa-di-ji-methane reaction is most general and efficient. Mehta and Srikrishna reported an oxa-di-Jt-methane reaction in the tricychc compound 131 and its halo analogue that, upon sensitized irradiation, gave 132. It is interesting to note that [jt -l-Jt ]-cycloaddition was not observed. Similarly, the P,y-enone 133 underwent the ODPM reaction to give the compound 134 with a cyclopropane annulated bicyclo [3.3.0] octane framework, in excellent yield (Scheme 25). ... 

The functionally more complex ene-dione 135 also gave an oxa-di-JT-methane reaction product 136 upon direct as well as sensitized irradiation. It is suggested that the ODPM occurs from the lowest triplet state (T,) during direct irradiation. Hart and co-workers examined the photoreaction of the tri- and tetracyclic compounds 137 and 138 in various solvents. Irradiation of 137 in acetone, as well as in acetonitrile, dichloromethane, and cyclohexane, gave the ODPM product 139 in addition to a cage compound as a result of intramolecular -addition. Irradiation of 138 gave the ODPM product... 

Bicyclic[2.2.2]octenones annellated with an oxygen heterocyclic ring also underwent the oxa-di-Jt-methane reaction upon sensitized irradiation. The tricyclic compounds 150 and 152 gave oxa-di-Jt-methane products 151 and 153, respectively, upon sensitized irradiation in acetone (Scheme 28). 

In addition to the above examples, more heavily substituted e do-tricyclic compounds 194 and 195 also underwent oxa-di-n-methane reaction to give 196 and 197, respectively, on sensitized irradiation in acetone (Scheme 36).These intermediates were converted into higher polyquinanes. The dominance of the oxa-di-Jt-methane reaction in 195, even in the presence of the additional carbonyl chromophore, is noteworthy. Bicyclo[2.2.2]octenones annulated with other higher rings have also been reported to show oxa-di-Jt-methane reactivity. ... 

The compound 200 with abicyclo[3.2.2]nonadienone ring system underwent oxa-di-Jt-methane reaction involving the C-C double bond in the ethano-bridge. This gives the product 201 selectively upon... 

The steroidal epoxy compound 205 was reported to undergo rearrangement to the ketone 206, which subsequently underwent an oxa-di-Jt-methane reaction to give the cyclopropyl compound 207 along with its stereoisomer and other compounds (Scheme 39). °°... 

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... 

The existence of the biradicals and the multipHcity of the surfaces on which these are formed have not been demonstrated directly however, experimental results (stereochemistry of the reaction, CIDNP [chemically induced dynamic nuclear polarization], radical trapping experiments, and quantum yield measurements) support their existence. Recently, the mechanism of 1,3-migration and oxa-di-Jt-methane reactions in terms of potential energy surface and decay funnels has been described this also supports the aforementioned mechanistic impHcations. The detailed mechanism, however, depends, in a very subtle way, on the structure of the chromophoric system and the presence of the functional groups. 

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... 

In our opinion, these results have modified some of the ideas firmly estabhshed for many years on the photoreactivity of P,Y-unsaturated systems and demonstrate that a considerable research effort remains necessary in order to rationaHze the photo-behavior of these compounds. The following sections summarize our findings in the area of SET-sensitized di-Jt-methane reactions. 

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