[16691-79-5] · C21H16O5 · 2,5-Bis(methoxycarbonyl)-3,4-diphenylcyclopentadienone · (MW 348.35)
(reagent for cycloaddition reactions1)
Form Supplied in: orange solid; may contain Diels-Alder dimer as impurity.1
Analysis of Reagent Purity: 1H NMR.1c
Preparative Method: equimolar amounts of benzil and dimethyl 2-propanone-1,3-dicarboxylate in MeOH or EtOH with OH- are allowed to react at 25 °C for 1 day to give the intermediate dimethyl 3,4-diphenyl-4-hydroxycyclopent-2-enone-2,5-dicarboxylate in 70-96% yield. Treatment of the intermediate with acid in benzene1a or in Ac2O1c with heating gives the dienone in 98% yield containing 3% of the Diels-Alder dimer.1c
As with other cyclopentadienones,2 the reagent reacts readily in cycloaddition reactions with a variety of dienophiles. Dimerization upon heating to 200 °C has been reported,1a but the monomeric reagent is stable at rt. Reactions with dienophiles are typically conducted at temperatures between 20-120 °C. Reactions have been run neat or in inert solvents such as benzene and 1,2-dichloroethane. In the presence of excess dienophile, the reactions are allowed to proceed until the characteristic orange color of the dienone is discharged.
Reactions with dienophiles generally give [4 + 2] cycloaddition (Diels-Alder) products. Additions to substituted styrenes occur with modest to good endo stereoselectivity (eq 1), but cycloadditions with electron deficient dienophiles such as Maleic Anhydride give only the endo product.3 Cycloadditions with trans-stilbene, Dimethyl Maleate, and Dimethyl Fumarate show that the reactions are stereospecific.4 Kinetic studies of reactions with substituted styrenes gave negative ρ values,3,5 leading to the classification of
reverse electron demand cycloadditions. Cycloadditions with alkenes generally give bicyclic products such as shown in eq 1, but the first-formed products from cycloadditions with alkynes such as Dimethyl Acetylenedicarboxylate decarbonylate to give aromatic products (eq 2).1a,3
Because the dienone is monomeric and highly reactive in [4 + 2] cycloadditions, it has been used to trap intermediate alkene products that are unstable to the reaction conditions. For example, oxidation of the benzadiyne equivalent precursor in eq 3 in the presence of the dienone gives the substituted anthracene shown.6
Mixtures of [4 + 6] and [4 + 2] cycloadducts are obtained in reactions of the dienone with oxepin7 (eq 4), a 1H-azepin,8 and 1,3,5,7-cyclooctatetraene.9 Reaction of the dienone with tropone gives [4 + 6] addition products.3
Dipolar cycloadditions as in reactions with diazoalkanes1b and with nitrile N-oxides10 give [2 + 3] cycloadducts (eq 5).
Wayne State University, Detroit, MI, USA