Oxa-di-ir-methane rearrangements

The chemistry of the oxa-di-ir-methane rearrangement is covered in Chapter 2.6, so that here, for the sake of completeness, only a few remarks on its more salient features are presented. 

This photochemical transformation, shown in equation (42), is a process that occurs especially frequently in the oxa-di-ir-methane rearrangement. This isomerization may proceed via a Norrish type I reaction. 

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. 

When only one of the two double bonds is part of a ring, there are many possible combinations these have already been reviewed. A recently reported representative example involves the photochemistry of (7), which contains both the aryl and the acetyl chromophores (equation 7). The observed products arise both from the standard di-ir-methane and oxa-di-ir-methane rearrangements. 

The 3-oxa-di-ir-methane rearrangement is far less common. As a symmetrical system, the product should be a vinyl epoxide (equation 23). So far, no 3-oxa-di- ir-methane products have been observed from the divinyl ethers depicted in the equation. It was shown that the solution photochemistry of (21a) derives from its singlet ir, iT -state and parallels that of the unsubstituted divinyl ether (21d) and of furan in the gas phase. All products could be rationalized in terms of the initial formation of a singlet vinyl-vi-nyloxy radical pair (equation 24). Triplet sensitization brings about cis-trans isomerization and consequent deactivation. Had the 3-oxa-di- ir-methane product, i.e. the vinyloxirane (22), been formed, it would have decomposed via Griffin fragmentation as confumed by irradiation of an authentic sample. 

The l-aza-S-oxa-di-ir-methane rearrangement of (25) does not occur. Instead, cis-trans isomerization and photoelimination of methanol are observed for this oxime (equation 21) Finally, the l-oxa-4-aza-di-iT-methane rearrangement of compound (26) also did not take place. This substrate gave rise to the photo-induced l,S-benzoyl migration shown in equation (28). ... 

Interestingly, the dienediones 92 underwent double oxa-di-ir-methane rearrangements upon direct irradiation to give triasteranedione 93 in moderate yields (Scheme 17). The trienone 94 with a bicy-clo[4.2.1]nonane ring system gave 95, wherein the P.y-C-C double bond of the diene moiety participated in the oxa-di-n-methane reaction. Similarly, the trienone 96 also furnished the oxa-di-n-methane product 97 (Scheme 18). ... 

Hancock and co-workers investigated the photoreaction of P,y-enones with a-hydroxy and alkoxy substituents.While the direct irradiation of a-hydroxyenone 27 led to the formation of 1,4-dione 28 via an efficient 1,3-acyl shift followed by tautomerization, photoreaction of 29 gave a complex mixture of products arising from various other reactions in addition to the 1,3-acyl shift product 30 (Scheme 8). Eichenberger and co-workers reported 1,3-acyl shift in ketones 31a and 31b on photoreaction from a ( tnr ) state to give 32a and 32b, respectively, in addition to an oxa-di-ir-methane rearrangement. 

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