Linearly Conjugated Dienones

2 Linearly Conjugated Dienones - The quinonemethide (169) has been prepared by flash vacuum pyrolysis. Irradiation at 7.6 K of the quinonemethide (169) at A, 340 nm results in ring closure and the formation of the 4,6-dimethylbenzoxete (170). 

Irradiation through Pyrex of crystalline mixtures of the pyrone (171) and maleimide (172) results in the formation of the (2 + 2) cycloadduct (173) which is different from the solution phase behaviour when products arising from a (2 + 4)-cycloaddition mode are obtained. The (2 + 4)-cycloadduct is unstable under the experimental conditions and rearranges into the (2 + 2)-cycloadduct 

2 Linearly Conjugated Dienones - The photochemical reactivity of linearly conjugated dienones has been studied for many years. The ring opening reaction to afford ketenes has synthetic utility. This type of reaction has again been 

2 Linearly Conjugated Dienones. - Two reaction modes have been detected following a study of the photoreactivity of 4,6-dimethyl-oc-pyrone in a matrix. Irradiation with A 315 nm brings about both ring-opening to yield a ketene and valence isomerization to produce l,5-dimethyl-2-oxa-3-oxobicyclo[2.2.0]-hex-5-ene.  

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Other linearly conjugated dienones also are reduced to unconjugated... 

Application of the Corey reaction to linear conjugated dienones gives the y,(5-methylene-a,/9-unsaturated ketones. ... 

The photolysis of dienone (52) to the linearly conjugated dienone (56) involves a four-photon sequence and nicely illustrates the involvement of spiro intermediates<39,43> (Table 7.3). 

The photorearrangement of a dienone was noted<4) as early as 1830 in a study of the sesquiterpene a-santonin (1). However, the structure and stereochemistry of the various photoproducts were not conclusively established until 1965.(6) Upon irradiation in neutral media, a-santonin (1) undergoes rapid rearrangement to the cyclopropyl ketone, lumisantonin (2). However, if the irradiation is not terminated after a short period of time the lumisantonin itself rearranges into a linearly conjugated dienone (3). The dienone (3) can be isolated from the photolysis of either (1) or (2) in benzene or ether. In nucleophilic solvents (alcohol or water) the dienone (3) is also photo-chemically active and is further converted into an ester or an acid (photo-santonic acid) (4). 

The rearrangement of spirocyclic hicyclohexenones into spirocyclic linear-conjugated dienones (Chart 6) poses another intriguing mechanistic problem. As may be concluded from the specific conversions of the di-... 

Linearly conjugated dienones may be completely reduced to saturated alcohols using excess lithium in liquid ammonia. In variously substituted dienones, the less-substituted double bond is often selectively reduced under these conditions. For example, treatment of a steroidal 14,16-dien-20-one with lithium in liquid ammonia (with or without 1-propanol) leads mainly to reduction of the 16,17 double bond (Scheme 13), - and the less-substituted double bond of cross-conjugated steroidal dienones, - - - - santonin, or related substrates is selectively reduced under these conditions (Scheme 13). ° ... 

Linearly-conjugated Dienones.- The only observed reaction of the dienones (246) from either the triplet or the singlet state was trans-cis isomerization around the C-7,C-8 bond The isomeric compound (247) also undergoes this isomerization as well as trans-cis isomerism of the C-9,C-10 bond. The diene (248) is also photo-reactive giving the product (249) by a 1,5-hydrogen transfer. Irradiation of (249) yields (250) by a Norrish type II process. 

Linearly Conjugated Dienones.- A review dealing with the... 

Linearly Conjugated Dienones.—Z-is-Isomerization of the 7 3 dodecadienoate mixture (199a and b) was achieved by sensitized (E > 222 J mol-1)... 

Dyson, N. H, J. A. Edwards, and J. H. Fried Steroids CCXIX. The Methyle-nation of Unsaturated Ketones. Part III. The Addition of Dimethyloxosulfonium Methylide to Linear Conjugated Dienones. Tetrahedron Letters (London) 17, 1841 (1966). 

The first photochemical reaction of a cyclohexadienone chromophore in santonin was noted. a-Santonin undergoes photochemical rearrangement in an non-nucleophilic media to give the cyclopropyl ketone lumisantonin, which is itself photochemically labile and yields the linearly conjugated dienone mazdasantonin. 

Irradiation of trienone 96a, which bears an additional double bond conjugated with the 4,5-double bond of the dienone, in glacial acetic acid yielded exclusively the azulene linearly conjugated dienone 97 in 54% yield (Scheme 17). Under similar conditions, trienone 96b yielded lactone 98 in 85% yield. Thus, it seems that the 6,7-double bond does not interfere in the photochemical rearrangement. 

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