Dienes oxidative rearrangement

A simple two-step protocol for the generation of a terminal diene is to add allyl magnesium bromide to an aldehyde or a ketone and subsequent acid or base catalysed dehydration (equation 34)72. Cheng and coworkers used this sequence for the synthesis of some indole natural products (equation 35)72a. Regiospecific dienones can be prepared by 1,2-addition of vinyllithium to a,/l-unsaturated carbonyl compounds and oxidative rearrangement of the resulting dienols with pyridinium dichromate (equation 36)73. 

The acid-catalysed decomposition of the adducts (291) and (292 R = 0) derived from the parent steroidal 5,7-dienes furnishes a novel route to 4,4-dimethylcholesta-5,7,14(15)-trien-3-one (90%) and cholesta-4,6,8( 14)-trien-3-one respectively.165 It has also been noted that reaction of the adduct (292 R = /3-OAc,H) with boron trifluoride etherate gives an oxidative rearrangement to the anthrasteroid (293)... 

Oxidative rearrangements of dienes are related to the dienone hmol rearrangement, which is discussed in Volume 3, ChiqMer 3.S. The examples discussed here ate limited to cyclohexadenes, and the driving force for the rearrangement is aromatizatbn. 

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 " ... 

The (4n+2) r-electrocyclization of l//-azepines is analogous to the oxepin-benzene oxide rearrangement. It yields 7-azabicyclo[4.1.0]hepta-2,4-dienes (dihydrobenzazirines) and occurs with electron-accepting N-substituents [13]. An example is the conversion of 7 into 8 at -70°C in a proportion of 9 1. A trimethylene chain bridging the 2,7-positions favours the formation of dihydrobenzazirine (9), but a tetramethylene bridge that of azepine (10) ... 

The dehydrogenation of 2 -hydroxychalcones and flavanones to flavones, of l-(2-hydroxyaryl)alk-2-en-l-ones and chroman-4-ones to chromones and of thiochroman-4-ones to thiochromones can be accomplished using iodine in hot DMSO <97HCM223>. E-3-Styrylchromones result from the oxidative rearrangement of 5-aryl-l-(2-hydroxyphenyl)penta-2,4-dien-l-ones with thallium(lll) nitrate <97LA2065>. Dimethyldioxirane converts flavanones into flavones by way of the 2-hydroxyflavanone. This approach enables flavans to be converted to a variety of flavonoids <97TL4651>. 

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