Di-jr-methane rearrangements

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). 

The application of the di-jr-methane rearrangement in organic synthesis could be expected to increase. Pattenden and Whybrow145 have applied this rearrangement as a key step in the total synthesis of ( ) desoxytaylorione 341 (Scheme 71). 

The photocyclization reaction of 2-allylanilines, depending on the degree of alkylation at the N atom, has long been a subject of interest. When iV,iV-dialkylanilines 60a-62a were irradiated in methanol, the corresponding 2-cyclopropylanilines 60b-62b were formed in 73, 60 and 47% yield, respectively (equation 17)163. This reaction proceeds in the triplet state via a clean aromatic di-jr -methane rearrangement. However, no reaction occurs when 2-allyl-l-(Ai,Ai-dimethylamino)naphthalene is irradiated in methanol. Thus, this photochemical cyclopropanization of allylated anilines appears to have only a limited scope as preparative method, because it is successful only for a few substrates. 

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. 

Asymmetric induction in the di-jr-methane rearrangement is also of interest and studies on this have examined the influence of chiral esters. Thus the irradiation of 370 yields a cyclooctatetraene 371 and a diastereoisomeric mixture of the semibullvalenes 372 and 373 in a ratio of 60 40 in solution and 20 80 in the solid phase. The position of attachment is vital and the diastereoselectivity shown by the barrelene 374 is very poor and yields a 1 1 mixture of products . Further examples of the control exercised when dibenzobarrelenes are inadiated in the crystalline phase have used the derivatives 375 and 376 as the acid salts formed with chiral amines. Irradiation affords the products 377 and 378 respectively, obtained after esterification with MeOH, with an ee of >95% . The influence of a chiral crystal lattice on the outcome of the di-jr-methane reaction of achiral 379 has been studied. The irradiation in the crystalline phase gives two chiral di-TT-rnethane products 380 and 381. The former of these is racemic but the latter is obtained in high enantiomeric excess which, under the best conditions, i.e. at —20 °C, approaches 100%. The irradiation of ciystals of 345, as an ethanol complex, affords 382 with an ee of 94%. Carrying out the inadiation at temperatures lower than ambient enhances the specificity of the reaction . 

The photochemical cycloaddition of 4-methyl-l,2,4-triazoline-3,5-dione to the dibenzo-cyclooctatetraene 289 yields 3.5% of the cycloadduct 290, together with 36% of 291, the product of a di-jr-methane rearrangement (equation 153. Anthrasteroids 293 (R = H, Ac or COPh) are produced in an oxidative rearrangement when the phenyltriazoline-dione adduct of 292 is treated with boron trifluoride etherate. The 5,7-diene system of ergosteryl acetate 294 can be protected by cycloadduct formation, allowing selective hydrogenation of the 22,23-double bond " ... 

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