Bicyclo hexenone

Elaborate mechanistic schemes have been suggested for the principal rearrangements of cyclohexenone, 2,5-cyclohexadienone, and bicyclo-hexenone systems induced by w - tt excitation which are compatible with the experimental data outlined above. In essence, these mechanisms are based on the common concept that the complicated structural changes are initiated in an electronically excited state. For the appreciably complex ketones considered, reaction initiation in a vibrationally excited ground state produced by adiabatic ir n demotion is expected to be readily suppressed in solution by collisional deactivation. It has been pointed out that by this general concept the rearrangements provide a decay path for electronically excited states which allows transfer of minimal amounts of enei to the environment in each step. 

The carbethoxy dienone 158 is isomerized smoothly to the bicyclo-hexenone product 160 in either dioxane or aqueous acetic acid, and no hydroxyketone formation could be detected. The noteworthy reluctance to incorporate solvent under acidic conditions in which hydroxyketone formation normally predominates has been ascribed to additional stabihzation of the cyclopropane bonds in the intermediate 159 towards hydrolytic cleavage (see Chart 26). 

In this reaction, photolysis of the substrate 138 gives the bicyclo[3.1.0]hexenone (143). Though the reaction is formally the same (note the conversion of 133 to 134... 

Bicyclo[4.3.0]nonenes, thanks to their frequent appearance in natural products, are other important targets for novel annulation methodology. A six-membered ring-annulation to cyclopentenones has yet to be developed, the main reason for this being that, until very recently, the levels of enantioselectivity in catalytic 1,4-additions to 2-cyclopentenone were too low for a synthetically useful procedure. However, a highly enantioselective annulation of a five-membered ring to 2-cyclo-hexenone has been developed (Scheme 7.26) [80]. 

Liao and Wei took advantage of the possibility of photochemically rearranging cyclohexa-2,4-dienones into bicyclo[3.1.0]hexenones in their approach to synthetically useful cyclo-pentenones such as 135 [6, 160-162]. This approach was based on the acid-catalyzed cyclopropane ring-opening of bicyclo[3.1.0]hexenones such as 134, as generated photochemically from non-dimerizing ortfio-quinone monoketals such as 133 (Figure 33) [165]. 

In 1968, Qriffiths and Hatt established that although dienylketene (282) (generated as the only photochemical product from cyclohexadienone 281) cyclizes thermally to (281), other thermal cyclizations compete, such as the foimaticxi of the bicyclo[3.1.0]hexenone (283) or solvent (amines or alcohols) addition product (284).The thermal formadm of bicyclo[3.1.0]hexenones (283) from ketenes (282) can be (diotochemically reversed. Thus 2,4-cyclohexadienones (287) can be obtained from 2,S-cyclohexa-dienones (285).The stereospecificity of the thermal ring opening of the bicyclo[3.1.0]hexenones to dienylketenes has been established. 

CTOSs-conjngated Dienones.—Photochemical rearrangement of dienones of the type shown in (227) is a well known process and leads to bicyclo[3.1.0]hexenones. These compounds are also photoreactive and ring-open to give zwitterions [e.g., (228)]. Schultz et al." have used this rearrangement process and have successfully trapjjed the zwitterion (228) [from (227)] by an intramolecular thermal reaction with the azide group of the side-chain to afford the adduct (229). The cyclohexadienone (230) is photochemically rear-... 

Photochemical ring opening of linearly conjugated cyclohexadienones affords dienylketenes (145), which react in one of the following ways recycli-zation to the original or to a stereoisomeric cyclohexadienone, formation of bicyclo[3.l.0]hexenones (146), or addition of a protic nucleophile to yield substituted hexadienecarboxylic acids (147) (Quinkert et al., 1979). 

BHT, pulse radiolysis of 1102, 1103 Bicyclocalix[4]arenes 1431 Bicyclo[4.1.0]hexenones, photoformation of 1048... 

The photorearrangements of 3-methoxycyclohexa-2,5-dienones 4 gave a diastereomeric mixture of bicyclo[3.1.0]hexenones 5a-e in excellent yields (67-81 While relatively... 

Albini and co-workers have described the photochemical behaviour of bicyclo[3.1.0]hexenones such as (55). The irradiation brings about the ringopening of the three-membered ring, and the resultant zwitterions are trapped by the adjacent hydroxyl group to afford (56). Several examples were described. 

Over-irradiation products [72e], formed by light-induced bicyclo[3.1.0]-hexenone-phenol rearrangement, have hampered the preparative use of the bicyclization of ortho-cpaaol acetates. 

Both bicyclo[3,l,l]heptenones and bicyclo[3,l,0]hexenones show bathochromic hypochromic u.v. spectral shifts relative to the corresponding bicyclo[3,2,l]octenone and monocyclic models. These shifts have been accounted for in terms of extension of the chromophore by the small ring, rather than by any distortion of the enone geometry induced by the presence of the small ring. 

Treatment of the tricyclic ketone (165) with borontrifluoride etherate gave over 90% of a mixture of double bond isomers of the bicyclic ketone (166). The tricyclic ketone was prepared by intramolecular photocyclization of the 4-allylated cyclo-pentenone (167X rather than by the more conventional cyclization of a vinyl cyclo-hexenone. Cyclization of (167) in this fashion is a further example of the preference of 1,5-dienes for photocyclization to give bicyclo[2,l,l]hexane skeletons rather than bicy do [2,2,0] hexanes. 

Photochemistry of 1-dehydrotestosterone acetate and its methyl derivatives begins with conversion of 2, 5-cyclohexadienones (XVI) into a bicyclo [3.1.0] hexenone (XVII) and then to new cyclohexadienone (XVIII) of assigned... 

We found that this method properly predicts the reactivity of a number of solid-state reactions. For example, note Equation 75.3 and the reactivity of the bicyclo[3.1.0]hexenones 15a and 15b, which were in agreement with the Inert Gas model. 

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