Y-Unsaturated Systems

Details of the photochemical behaviour of the p, - unsaturated enones (186) has been reported. This supplements the report made earlier in note form. The reaction described involves the photoconversion of the enones (186) into the imines (187) and (188) by a path which is thought to involve a 1,5-benzoyl migration. The terminus for this is a pendant phenyl group. Such a process has not previously been described. An 

The photodecarbonylation of the dienones (191a) and (191b) has been examined and shown to be a concerted process. The c/s-isomer (191a) yields initially the triene (192A) while the trans-isomer (191b) affords the triene (192B). These experiments prove conclusively that the decarbonylation is a concerted process.  

5-Acyl migrations occur on the irradiation of the lactones (193, = COMe). 

This treatment yields (193b, = COMe) which is also photoreactive and is 

Adam and his coworkers have studied the oxa-di-ir-methane rearrangement of the norbornenone (209). They approached the problem by the synthesis of precursors of the putative biradicals such as (210). The oxa-di-ir-methane rearrangement of the P,7-unsaturated enones (211, 212) has been reported by Schultz and his coworkers. 

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For more than 30 years, di-7i-methane processes were limited to the three different types of (3,y-unsaturated systems mentioned above. However, recent studies have shown that the rearrangement is applicable to 2-aza-1,4-dienes [13]. Thus, triplet-sensitized irradiation of compound 11 affords the cyclopropylimine 12 and the TV-vinylaziridine 13 (Sch. 6). The photoreaction of 11 represents the first example of a 2-aza-di-7i-methane rearrangement (2-ADPM) that brings about the formation of a heterocyclic product. The reaction has been extended to other 2-azadienes that yield the corresponding cyclopropylimines regioselectively [13b]. 

The results obtained in these studies on the photoreactivity of different 3,y-unsaturated systems under SET-sensitization have opened new lines of research in the area of di-7r-methane photochemistry. However, further studies will be necessary to determine the scope and synthetic potential of these new reactions of the di-7r-methane type via radical-ion intermediates. 

The oxa-di-TT-methane photochemical rearrangement of 3,Y-unsaturated systems normally occurs via a triplet pathway but Fuchs and co-workers have described an example in which direct irradiation (presumably singlet) gives the oxa-di-it-methane product whereas triplet sensitization follows a different path. 

B.y-Unsaturated Systems.- Irradiation of the methyl enone (197) affords the 1,3-acyl migrated compound (198), In addition fragmentation products were obtained. The irradiation of the 1,3-acyl migrated compound (198) does not reform the starting... 

The o, )-unsaturated ester 7 can be transformed reductively into the p,Y-unsaturated system 8 in high yield using magnesium in methanol J Reaction of the chiral crotyl silane 9 with various aldehydes and acetals offers a diastereoselective approach to branched P,Y-unsatuiated esters, as in the example in Scheme 2, and, as indicated, stereospecific allylic rearrangement gave the a,p-unsaturated system. Similar reactions were also described using the C-2 epimer of 9. ... 

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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