Bicyclo octenones formation

As indicated in Scheme 57, the formation of the secondary bicyclo[3.3.0]octenone product is thought to occur via an in situ transannular closure of the initially formed cyclooctadienone 185. In some cases, cycloctenediones and the [5 + 2]-cycloadducts are observed as minor products (Equations (33) and (34)). 

Selectivity for the cyclopentenone formation is improved by an intramolecular version. For example, 1,6-hepta-diyne (256, X = CH2) reacts in benzene with 2molarequiv. of Bu Me2SiH to give two bicyclo[3.3.0]octenones, 257 and 258, in 14% and 63% yield, respectively, at 95 °G under GO (20 atm) pressure.When benzene is replaced by acetonitrile, 258 becomes a sole product (60% yield). On the other hand, the reaction at 25 °G gives normal silylformylation product 259 as a major component (59%) with the concomitant formation of 258 (20%) (Scheme 12). ... 

Recently, the reaction of masked ortho-benzoquinone [92] with C60 was tested [93]. The [4+2] cycloaddition reaction of such electron-deficient dienes with fullerenes resulted in the formation of highly functionalized bicyclo [2.2.2] octenone-fused fullerenes. The reactants were generated in situ by the oxidation of the readily available 2-methoxy phenols with hypervalent iodine agents. For the several different masked ortho-benzoquinones that were tested, it was found that the yield of the cycloadducts depends on the nature of the starting materials and the reaction conditions. Other Diels-Alder reactions of such electron-deficient dienes with electron-poor fullerenes involved tropones [94], 1,3-butadienes substituted with electron-withdrawing groups [95], and 2-pyrone [96]. 

If rapid bond formation at the stage of the Norrish I fragments competes with decarbonylation, 1,2-acyl shift products are formed as the major components from direct photolysis. In case of the cyclobuteno-annulated bicyclo[2.2.2]octenone 21, 1,2-acyl shift results in the cyclo-butanone 22, whereas ODPM rearrangement (photolysis in acetone) results in the cyclobuteno-diquinane 23 in 44% yield (Sch. 21) [49]. 

While the condensation of enamine 37 with methyl OY7 s-2-butenoate, followed by acid hydrolysis and sodium borohydride reduction affords lactone 38 with reasonable efficiency, the cyclodehydrative ring contraction of this intermediate with PPA gives a mixture of bicyclo[3.3.0]octenones in abysmal (< 5 %) yield.66 To circumvent this difficulty and enable the large scale production of 39,2-carbo-methoxy-4,4-dimethylcyclohexanone was initially transformed to tram diacid 40 under Favoiskii conditions (Scheme 14). Conversion to the diacid chloride and condensation with lithium dimethylcuprate resulted in formation of the diacetyl derivative. In basic solution, the latter is reported to experience epimerization and aldol cyclization with dehydration in 82 % yield. With hydrogen and palladium on charcoal, the essentially quantitative production of 39 was achieved.66 ... 

---