Aza-di-jr-methane rearrangement

Perhaps one of the most ubiquitous photochemical reactions undergone by non-conjugated dienes is that encountered in the 1,4-diene system. This is referred to as the all-carbon di-jr-methane reaction. This system is one in which a central tetrahedral carbon atom is flanked by two vinyl groups. The fundamental reaction was discovered and reported in 1967 by Zimmerman and his coworkers ". A detailed account of this reaction and the closely related oxa-di-rr- and the aza-di-jr-methane rearrangements has been published recently ". This review gives the history of how and when the reaction was discovered and its developments in all its guises up to the present time. Others have reviewed other aspects of the reaction This section of this review will be devoted to the last decade of results. 

The extension of the di-jr-methane rearrangement from 1,4-dienes and /3,/-unsaturated ketones to the use of 1-aza-1,4-dienes has been achieved129 (equation 73). 

Vinyl-vinyl bridging reactions are also operative in the barrelene derivatives 324. The reaction arises from the triplet and is brought about by xanthene sensitizaion. The semibullvalenes 325 and 326 are formed in a ratio of 8.3 1 in this rearrangement. Even when hetero-atoms are present, there are some instances when vinyl-vinyl bridging is operative. This is demonstrated by the conversion of 327, R = H, into 328. This reaction mode is in competition with the pyrazino-vinyl bridging path that yields 329, R = H, and 330, R = Others have also examined the reactivity of such pyrazine derivatives. The pyrazino-vinyl system is an example of the aza-di-jr-methane system originally discovered by Armesto and his coworkers and reviewed recently by Zimmerman and Armesto . 

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