Tropone cycloaddition reactions

Whereas tropones usually act as dienes in cycloaddition reactions (Section 5.4), tricarbonyl (tropone) iron 59 displays a reactivity that is almost identical to that of a normal enone. High pressure cycloadditions of 59 with 1-oxygen substituted dienes 60 gave the desired cycloadducts 61 in good to excellent yields (Equation 5.9). The subsequent decomplexation of the cycloadducts has been accomplished by treatment with CAN [20]. 

The study of high pressure cycloaddition reactions of tropone (125) with maleic anhydride and norbornene allowed the reaction activation volumes to be measured and showed that they are large, negative and solvent-dependent (Scheme 5.17) [43a]. 

Several unusual cycloaddition reactions of 9 with unsaturated ketones should be mentioned in conclusion the heterocumulene generated photolytically from 7 undergoes [8 + 2]-cycloaddition with tropone to form 33 (40%) the structure of the product has been unequivocally established by X-ray structure analysis 22,23). Once again, the affinity of phosphorus for oxygen is manifested an entirely analogous cycloaddition reaction is known for diphenylketene 26). 

Cycloaddition reactions using tropone or another cyclic triene as the 6ji partner have been abundantly described in the literature. It has been found that virtually all metal-free [6 + 4] cycloadditions of cyclic trienes afford predominantly exo adducts. This has been rationalized by consideration of the HOMO-LUMO interactions between the diene and triene partners. An unfavorable repulsive secondary orbital interaction between the remaining lobes of the diene HOMO and those of the triene LUMO develops during an endo approach. The exo transition state is devoid of this interaction (Figure 9). 

Mahon and colleagues297 studied the cycloaddition reactions of substituted cis-1,2-isopropylidenedioxycyclohexadienes. The reaction of tropone (471) with cyclohexadiene 472, for example, afforded the expected exo cycloadduct 473 with good yield (equation 140). 

Kato et al. (119) explored reactions of fulvenes with a variety of mesoionic heterocycles. Unfortunately, reactions of miinchnone 38 with several fulvenes afforded complex mixtures in each case, and no identifiable products were reported, although Friedrichsen and co-workers (120-122) previously reported the reaction between mtinchnones and fulvenes to give cycloadducts. Kato et al. (123) also studied the cycloaddition reactions of tropone with several mesoionic heterocycles. Despite heroic efforts, the reaction of tropone with miinchnone 38 was complex and could not be unraveled. However, as described later, the reaction of tropone with isomtlnchnones was successful. Wu et al. (124) effected the cycloaddition between a miinchnone and fullerene-60 (Ceo) to give the corresponding dihydropyrrole in excellent yield. 

The 1,3-dipolar cycloaddition reactions of several other mesoionic heterocycles have been investigated since Potts review (1). Kato et al. (113,114,123) found that the l,3-thiazohum-5-olate (358) ring system affords low yields or complex reaction mixtures with benzocyclopropene (113), benzocyclobutadiene (114), and tropone (123). Likewise, Vedejs and Wilde (209) isolated in low yields a cycloadduct of 358d with thiopivaldehyde, along with a ring-opened thiamide. Also, 1,3-thiazo-lium-5-olate 359 reacts with thiopivaldehyde to give 360 (209). 

The bicyclic ketone (44), obtained from the Fe2(CO)9-promoted [3 + 4] cycloaddition reaction of a,a,a, a -tetrabromoacetone and 2-isopropylfuran followed by Zn-Cu couple reduction, has been converted to the naturally occurring troponoid, /3-thujaplicin (46) (75JOC806). Hydrogenation of (44), ether cleavage, bromination and dehydrobromination gave the tropone (45), an intermediate easily converted into the tropolone (46) by a standard procedure (Scheme 10). A related [3 + 4] cycloaddition reaction of oxyallyl metallic with furan has been used to assemble the antibiotic C-nucleosides (78JA2561). 

The kinetics of high-pressure cycloaddition reactions of tropone with dienophiles and enophiles have been reviewed.254 The Diels-Alder reaction of 9,10-dimethylanthracene with acrylonitrile has been investigated at high pressure in acetonitrile and in ethereal solutions of lithium perchlorate.255 The combination of high pressure and a solution of lithium perchlorate in diethyl ether is an excellent reaction rate accelerator in 4 + 2-cycloaddition reactions.256... 

An extensive review of 6 + 4-cycloaddition reactions has been published.287 The use of a precatalyst in the chromium(0)-promoted 6 + 4-cycloaddition reaction of cyclohcpta-1.3.5-tricnc with acyclic dienes ensures that only stoichiometric quantities of the metal are involved in the cycloaddition (Scheme 59).288 The first example of an aqueous 6 + 4-cycloaddition of tropone with the water-soluble diene 1 -(/l-D-glucopyranosyloxyjbuta-1,3-dicnc (152) yields 2 - (glucopyrano sy] o x y) b i cy clo-... 

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]. 

Besides the common oleftnic dipolarophiles, other unsaturated systems have been evaluated in cycloaddition reactions of zwitterionic TMM-Pd complexes, including polyenes and acetylenes. While acyclic electron-poor dienes generally gave mixtures of five- and seven-membered rings [48], a limited number of selective [3 + 4] and [3 + 6] cycloaddition reactions have been achieved with cyclic polyenic substrates as illustrated by formation of cycloadducts 41 and 42 from pyrone [49] and tropone [50], respectively (Scheme 16). On the other hand, activated alkynes have failed to produce the corresponding cyclopentene derivatives [51]. 

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