Oxa-di-rr-methane rearrangement

Ring contraction of cyclopentenone and cyclohexenone derivatives to cyclobutanes was observed during direct irradiation of various j5,y-unsaturated cyclohexenones. By way of a Nor-rish type I cleavage in n,7i excited singlet (5t) and n,7t excited triplet (T2) states, an acyl/allyl diradical is formed which recombines in the allyl position to form the product of a 1,3-acyl migration,102 while sensitized irradiation leads mainly to oxa-di-rr-methane rearrangement (see Section 4.2.3.). 

The third set of molecules that bring out the uniqueness of zeolites in the context of asymmetric induction is of 2,4-cyclohexadienones (6,6-dimethyl-2,4-cyclohexadienones 38, and 2,2-dimethyl-1,2-dihydronaphthalenones 39) [279,282,287,289,293-295]. The basic chromophore in these molecules is the conjugated dienone that undergoes oxa-di-rr-methane rearrangement to give a bicyclic product (Scheme 21). According to the accepted mechanism, the chirality is introduced into the system at the first step, yielding the diradical intermediate... 

Dimethyl-2,4-cyclohexadienone photorearranges to bicyclic [3.1.0] product via oxa-di-rr-methane rearrangement (Scheme 41). This achiral molecule photorearranges in solution to the chiral bicyclic product as a racemic mixture. This molecule within NaY zeolites in the presence of chiral inductors yields products with respectable ee. The best numbers are 50% (ephedrine as a chiral inductor at — 55°C) and 28% (pseudoephedrine at room temperature) [294]. These numbers are significant when one recognizes that in solution only a racemic mixture is obtained. 

For a recent review of di-rr-methane and oxa-di-rr-methane rearrangements see Hixson et... 

In distinction to the oxa-di-rr-methane rearrangement, the di-n-methane rearrangement occurs in both singlet and triplet excited state. 

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. 

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