Ji-methane reaction

The di-ji-methane reaction results in a 1,2-shift in a 1,4-diene unit, but such shifts sometimes occur in monoalkenes (2.44, and the mechanism must be different. The substrates are usually tetra-substituted ethylenes, and it is suggested that the reactive excited state is a Rydberg singlet state, which rearranges initially to give a carbene. Support for such a mechanism comes from the structures of products obtained from 1,2-dimethy(cyclohexene (2.45), which are... 

The ketones 56,57, and 58 are some of the monocychc P,y-enones whose photochemical reactions upon sensitized irradiation have been examined. Irradiation of the dienone 56 in ether is reported to give the bicyclic ketone 59 via oxa-di-Ji-methane reaction. Recently, the photoreaction of cyclohexadienone 56 and its derivatives having chiral auxiliaries was examined in alkali ion exchanged Y zeolites. Thus, irradiation of 56 in dry (-)-ephedrine-modified zeoHte gave the ODPM product in 30% ee. Significant... 

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). ... 

Koppes, M.J.C. M. and Cerfontain, H., Photochemical reactivities of cychc-a-phenyl-P,y-enones. Singlet 1,3-acyl shift, decarbonylation and unquenchable oxa-di-Ji-methane reactions upon direct irradiation. Reel. Trav. Chim. Pays-Bas, 107, 549, 1988. 

This process is known as the di-ji-methane (DPM) rearrangement [3]. One of the first examples of this reaction was reported in 1966 by... 

Photochemical reaction of several steroidal P,y-enones have been examined. Thus, the ketones 67 and 70 underwent oxa-di-7i-methane reaction on sensitized irradiation, while 68 and 69 gave ODPM products upon direct irradiation (Scheme 12). The modified steroidal ketone 70 gave the ODPM product 74 on direct irradiation as well It appears that the dihedral angle between the carbonyl and vinyl groups in the enones controls the efficiency of population of the triplet excited state on direct irradiation and, hence 1,3-acyl shift vs. 1,2-acyl shift (oxa-di-Ji-methane) selectivity. ... 

DiaryIcyclohex-i-enones undergo a different photorearrangement to bicydo[3.1.0]hexan-2-ones, in which an aryl substituent migrates from C-4 to C-3 (4.84). This reaction finds a parallel in the di-ir-methane rearrangement of 3-phenylalkenes (see p. 54). It is usually efficient ( = 0.1-0.2), it occurs by way of the (n,ji ) triplet... 

The previously mentioned study of the base-catalyzed decomposition of ethyl-cobaloxime by Brown and Hessley [76] represents an interesting example of a base-catalyzed mode II cleavage. While the ethylene product of this reaction is undoubtedly formed via a mode I ji elimination, the ethane product was originally thought to be formed by a mode III cleavage in analogy with the base-catalyzed formation of methane from methylcobaloxime (see Section 5.2 (c)). However,... 

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