Cycloaddition 1.3- cyclohexanedione

An useful intermediate for the preparation of 1,4-cyclohexanediones is obtained by cycloaddition of 6-isopropyl-3-methyl-2-cyclohexenone to a cyclobutanediol derivative (4.59) 470>. 

Yoshida and coworkers [63, 64] studied the oxidative cycloaddition of cyclic 1,3-dione (1,3-cyclopentanedione and some 1,3-cyclohexanediones) and olefins in various solvents and electrolytes. The best results were obtained in acetonitrile containing tetraethylammonium tosylate as electrolyte (97% yield with 5,5-dimethyl-l,3-cyclohexadione and styrene) (Scheme 45). 

Cycloaddition Reactions with Other Nucleophiles The anodic two-electron oxidation of catechol affords o-quinone that may react with the enolates of 4-hydroxycoumarine or 5,5-dimethyl-1,3-cyclohexanedione (dimedone). The resulting adducts undergo a second anodic oxidation leading to benzofuran derivatives in good yields (90-95%) (Scheme 53) [75, 76]. 

Condensation of hydroxylamine rac-10 with 1,4-cyclohexanedione monoethylene acetal provided nitrone rac-9, which was treated with ethyl acrylate to give 74% of a 8-9 1 mixture of isoxazolidine rac-8 and its diastereomer. Reduction of this mixture with hydrogen afforded 8-9 1 of a mixture of lactam rac-1 and its diastereomer (Scheme 7). After crystallization, X-ray analysis of the major product confirmed that its stereochemistry is the one required for carrying out the synthesis of FR901483. The excellent stereoselectivity observed in the 1,3-dipolar cycloaddition of nitrone rac-9 was attributed to the substitution pattern in the stereogenic center adjacent to the nitrogen atom. These substituents stabilize one chair conformer relative to the other and thus allow a good... 

Isoflavans and Isoflavylium Salts.—Cycloaddition of dimethylsulphoxonium methylide (183) to cyclohexanediones (or their enols) (182) gave good yields of 3,4,5,6,7,8-hexahydro-3-hydroxy-2//-l-benzopyran-5-ones (184). When the O-acetate (185) of this was pyrolysed at 150 °C, a 2 1 mixture of 2H- (186) and 4//-benzopyrans (187) was obtained. ""... 

As shown in Scheme 12.25, aldehyde 114, the precursor for preparing the appropriate a,(3-unsaturated iminium salt, was obtained quantitatively via SOs-pyridine oxidation of E, E-famesol. An a,(3-unsaturated iminium salt was generated via heating 114 at 90 °C in the presence of piperidine and AC2O [24,64], and the subsequent formal cycloaddition with 5-methyl-1,3-cyclohexanedione 115 was accomplished on multigram scale to give the desired adduct 116 as a 1 1 mixture of two diastereomers in 70% yield. 

Tangutorine To prepare aza-cycloaddition substrate 378, tryptamine 371 was diprotected and brominated so that an ensuing Heck coupling would provide ester 377 in 82% yield (Scheme 12.93). Further synthetic manipulations, including condensation with 1,3-cyclohexanedione 106,... 

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