4-Cyclopentene-l ,3-dione

The photo-induced exo selectivity was observed in other classic Diels-Alder reactions. Data relating to some exo adducts obtained by reacting cyclopentadiene or cyclohexadiene with 2-methyl-1,4-benzoquinone, 5-hydroxynaphtho-quinone, 4-cyclopentene-l,3-dione and maleic anhydride are given in Scheme 4.13. The presence and amount of EtsN plays a decisive role in reversing the endo selectivity. The possibility that the prevalence of exo adduct is due to isomerization of endo adduct under photolytic conditions was rejected by control experiments, at least for less reactive dienophiles. 

Utilizing an alternate mode of Diels-Alder reactivity, Harman has examined the cycloaddition reactions of 4,5-T -Os(II)pentaammine-3-vinylpyrrole complexes with suitably activated dienophiles <96JA7117>. For instance, cycloaddition of the p-vinylpyrrole complex 58 with 4-cyclopentene-l,3-dione, followed by DDQ oxidation affords 59, possessing the fused-ring indole skeleton of the marine cytotoxic agent, herbindole B. 

The similar insertion of 93 into 1,2-diphenylcyclopropenone results in cy-clobutenones or o- and p-alkoxyphenols [49]. In the reaction of 93 with 1-alkynylcyclobutenols, 2-alkenyl-4-cyclopentene-l,3-diones are obtained via alkyl shift-ring expansion [50]. (Scheme 33)... 

As compounds exhibiting enhancing effects on CL reactions, a variety of phenols, e.g., firefly luciferin and 6-hydroxybenzothiazole derivatives [12,13], 4-iodophe-nol [14], 4-(4-hydroxyphenyl)thiazole [15], 2-(4,-hydroxy-3 -methoxy-benzyli-dene)-4-cyclopentene-l,3-dione (KIH-201) [16], and 2-(4-hydroxyphenyl)-4,5-diphenylimidazole (HDI) and 2-(4-hydroxyphenyl)-4,5-di(2-pyridyl)imidazole (HPI)[17] (Fig. 6A), and phenylboronic acid derivatives, e.g., 4-phenylylboronic acid [18], 4-iodophenylboronic acid [19], and4-[4,5-di(2-pyridyl)-l //-imidazol-2-yl]phenylboronic acid (DPPA) [20] (Fig. 6B), in the luminol/hydrogen peroxide/peroxidase system are well known. Rhodamine B and quinine are used as sensitizers in the CL-emitting reaction between cerium (IV) and thiol compounds. This CL reaction was successfully applied to the sensitive determination of various thiol drugs [21-32],... 

Figure 6 Representative (A) phenol-type and (B) phenylboronic acid-type enhancers for luminol/hydrogen peroxide/peroxidase system. KIH-201, 2-(4 -hydroxy-3 -methoxy-benzylidene)-4-cyclopentene-l,3-dione HDI, 2-(4-hydroxyphenyl)-4,5-diphenylimida-zole HPI, 2-(4-hydroxyphenyl)-4,5-di(2-pyridyl)imidazole DPPA, 4-[4,5-di(2-pyridyl)-1 //iruidazol 2 yl phenylboronic acid).

The CD induced in 2-(2,4,6-cycloheptatrien-l-ylidine)-4-cyclopentene-l,3-dione upon its inclusion in 6-cyclodextrin was used to obtain full assignment of the observed absorption bands [49]. It was concluded that the low energy band system was composed of three electronic transitions, two of which were polarized perpendicular, and one parallel, with respect to the long molecular axis of the solute. The higher energy band system was found to also consist of three allowed electronic transitions, but two of these were polarized parallel, and one perpendicular, to the long axis of the solute molecule. These deductions were supported by a series of CNDO/S-CI calculations performed on the system. A similar approach was taken to assign the polarizations of the various transitions associated with 2-thioxo-l,3-benzodithiole and 2-selenoxo-1,3-benzodithiole [50]. 

Materials. (Bicyclo[2.2.1]hepta-2,5-diene) rhodium(I) chloride dimer and silver hexafluorophosphate were purchased from Strem Chemical the rhodium complex was recrystallized from acetone prior to use. All silanes (Aldrich) were distilled under a nitrogen atmosphere, freeze-thaw degassed and stored under a nitrogen atmosphere. 4-cyclopentene-l,3-dione (CPDK) from Aldrich was recrystallized from diethyl ether and stored in the dark under a nitrogen atmosphere at -40°C. All solvents were distilled under nitrogen from appropriate drying agents immediately prior to use. 

The reactivity of the methylidyne cluster 45, formed along with 46 from Ru3( T-H)( r-COMe)(CO)io and the diphosphine ligand 4,5-bis(diphenyl-phosphino)-4-cyclopenten-l,3-dione, has been reported. Mild thermolysis... 

Similarly Ni catalysts are useful substitutes to Pd for the selective olefin reduction in an a,/8-unsaturated ketone. Finely divided Ni (and partially deactivated by washing it with a 0.1% MeOH solution of acetic acid) in a mixture 5% HCCl3/EtOH is efficient at RT and atm P. Neutral catalysts, U—Ni—N, obtained by refluxing precipitated Ni with isopropanol, also shows selectivity, mesityl oxide being reduced to methyl-1 pentanone-2 . 4-Cyclopentene-l,3-dione, 5, can be converted into 1,3-cyclo-pentanedione, 6, by hydrogenation over partially deactivated neutral Ni catalyst ... 

In accord with this model, groups larger than methyl generally provide lower asymmetric induction. In the case of acetylenic ketones, the acetylene also occupies the equatorial position. Thus, the reagent does not appear to discriminate by purely steric factors. In fact, it is proposed that there is an unfavorable h/tt interaction between the axial oxygen of the 1, l -bi-2-naphthalenol moiety and an unsaturated system if it is in the axial position. One exception to this generalization is in the reduction of 4-cyclopentene-l,3-dione in which the alkene portion presumably occupies the axial position. In this case there is said to be a favorable n/tr interaction. 

In connection with this search, the chemical behavior of the carbon-centered radical was also examined [118]. The similar hydroxycyclobutenone 140 bearing Barton s ester at C-4 was photolyzed (W-lamp) to again give a 5-endo-cyclized product, 4-cyclopentene-l,3-dione 145, prior to enol-keto tautomerization (Scheme 27). 

Scheme 27 Ring expansion to 4-cyclopentene-l,3-dione induced by carbon-centered radical [118]...

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