Tetracyclone Diels-Alder reaction

A further method for the synthesis of o-diaroylbenzenes has been described by Ried and Bdnninghausen. Tetracyclone (145), a potent diene for Diels-Alder reactions, together with dibenzoylacetylene (146) gives 147. This method has proved of value in a number of other cases and is also applicable when the cyclopentadienone derivative is dimeric ° as in the case of 2,5-dialkyl-3,4-diarylcyclopentadienones. °... 

In the synthesis of the bisdienophilic phthalocyanine 102, a Diels-Alder reaction of a reactive isobenzofuran (generated by the tetracyclone route) is involved (97CB801). For further work see [95JCS(CC)2449 98ZN(B)1069],... 

This double Claisen-Schmidt reaction takes place under the influence of ethano-lic potassium hydroxide (Expt 7.7) and presumably proceeds in the stepwise manner (cf. formulation in Section 6.12.2, p. 1032). The four aryl groups in tetra-cyclopentadienone effectively stabilise the cyclopentadienone system, which otherwise has only a transient existence and readily undergoes dimerisation by way of a diene-dienophile interaction (Diels-Alder reaction, Section 7.6). The use of tetracyclone as a dienophile for the preparation of 3,4,5,6-tetraphenyl-dihydrophthalic anhydride is noted on p. 1121. 

Conjugated diynes can function as dienophiles in Diels-Alder reactions, and adducts have been obtained in which one or both of the triple bonds participates. Most of the studies have involved the use of substituted cyclopentadienones, e.g. tetraphenylcyclopentadienone ( tetracyclone ), which form adducts that undergo decarbonylation to give aromatic hydrocarbons. Butadiyne itself reacts with tetracyclone to give hexaphenylquaterphenyl (221) while diphenylbutadiyne gives the mono(222)- and di(223)-adduct . Bis(4-biphenylyl)butadiyne gives the monoadduct 224 . 

In the attempted synthesis of twisted polycycle 1,2,3,4-tetraphenylfluorenoM, 9-g/i]quinoline, R.A. Pascal Jr. et al. used the Combes quinoline synthesis to assemble the azaaceanthrene core. Oxidation with DDQ was followed by a Diels-Alder reaction with tetracyclone (tetraphenylcyclopentadienone) to afford the corresponding cycloadduct. However, the last decarbonylation step of the sequence failed to work even under forcing conditions, presumably due to steric hindrance. 

Similarly, in the Diels-Alder reaction between 1,1-dimethylallene and tetracyclone 6.200 giving the product 6.408 which loses carbon monoxide to give the triene 6.409, attack has taken place on the double bond carrying the methyl groups, but orthogonal to them 6.410, avoiding the steric clash which would have taken place had it attacked the less substituted double bond 6.411.896 However, this result may be an anomaly, perhaps because phencyclone is so large, because maleic and fumaric esters predominantly attack the less-substituted double bond in [2 + 2] reactions (see p. 282). 

For example, 3-perfluoroalkyl-4-ethoxycarbonylfuranes 48 have been prepared from bicyclohepta-2,5-diene derivatives 49 by sequence involving cycloaddition of acetylene to tetracyclone 50 and retro Diels-Alder reaction of the cycloadduct. ... 

Further evidence for the intermediacy of cyclopropenes in the cycloheptatrienyli-dene-arylcarbene interconversion comes from the trapping of the cyclopropene by Diels-Alder reactions with cyclopentadiene, furan, butadiene, and tetracyclone (see also p. 97). 

Higher homologues of linear acenes have been synthesized using the isobenzofuran cycloaddition method as illustrated in the following schemes (Schemes 16.26 and 16.27) [29]. The cycloadduct of tetracyclone shown in Scheme 16.26 (in the box) is a synthetic equivalent of benzodifuran, and on heating, it releases 1,2,3,4-tetraphenylbenzene by a retro Diels-Alder reaction generating isobenzofuran in situ, probably in a sequential manner. 

A review of the cycloaddition reactions of o-benzoquinones as carbodiene, heterodiene, dienophile, or heterodienophile has been published. In the Diels-Alder reaction of furans with masked o-benzoquinones (145), the furans unexpectedly behaved as dienophiles to yield cycloadducts (146) (Scheme 56). Masked benzoquinones behave as dienes which undergo Diels-Alder reactions with electron-rich dienophiles such as enol ethers and thienol ethers.The asymmetric Diels-Alder reactions of 5-substituted and 5,6-disubstituted (S)-2-(p-tolylsulflnyl)-l,4-benzoquinones with cyclopentadiene and fran -piperylene show complete regio- and jr-facial selectivities. The hetero-Diels-Alder reactions of o-benzoquinones with tetracyclone produce cyclopenta[I ][l,4]benzodioxinone derivatives in high yield. 

Similarly, Diels-Alder/retro-Diels-Alder sequence was utilized to the synthesis of ethyl 4-(trifluoromethyl)furan-3-carboxylate 92 starting from furan and ethyl trifluorobut-2-ynoate via intermediate products 90 and 91 [78], Cycloadduct 90 was also transformed into 92 by two-step procedure including its Diels-Alder reaction with tetracyclone and thermolysis of adduct 93 [79]. 

When dihydropyrazoline 6 (stereochemistry unknown, but probably exo) was heated in the presence of copper powder to 180°C (or 200°C as stated in experimental) in addition to pyrazole (7) a resinous residue was obtained which was described as a polymer (8) of the then-unknown isobenzofuran 2. The transient existence of 2 was proved by Fieser and Haddadin in 1964 (64JA2081 65CJC1599). Refluxing the Diels-Alder adduct of 1,4-dihydro-1,4-expoxynaphthalene 9 and tetracyclone (10, stereochemistry of the cy-clopentenone moiety unknown see also 71TL2337) in the presence of 9 yields 12 (exo,exo) and 13 (exo,endo there is a misprint in the experimental section of 65CJC1599) in 91% overall yield (12/13 = 42/58). For the reaction of 2 with 9 see also Meier et al. (97LA663). 

As previously mentioned, short-life cycloalkynes add to reactive dienes such as l,3-diphenylbenzo[c]furan (16) and tetracyclone to give Diels-Alder-type adducts, and the reaction has frequently been used to establish the intermediacy of short-life cyclic acetylenes. The addition reactions have found synthetic applications - > > . The addition of 1-diethylaminobutadienc to cycloalkynes provides an interesting synthetic route for benzo annelation . Intervention of dehydrobullvalene was also confirmed by the formation of DicIs-Alder-type adducts - . At present only azides are used as 1,3 dipoles in the addition reaction with short-life cycloalkynes - - . ... 

Benzyne is reactive as a dienophile and provides many examples of Diels-Alder addition. It is efficiently trapped by anthracene to give triptycene and by tetracyclone to give 1,2,3,4-tetraphenylnaphthalene via decarbonylation of adduct 23, reactions which are commonly used to quantify the formation of benzyne under given conditions. 

A closely related reaction is observed with chloromaleic anhydride (218), which gives the apparent aryne adduct 216 with tetracyclone (151) but stops at the Diels-Alder adduct 219 with other dienes such as cyclopentadiene (146b), butadiene (146e), and anthracene (147). Not only does this result suggest the absence of an elimination-addition mechanism via the aryne (23) in this system. 

MgO. The base is unnecessary for the decomposition of 287, which occurs in its absence, and served only to remove the HI since its strong reducing properties otherwise lead to the exclusive production of tars. Based on these observations it was concluded that the formation of the adduct 285 from the mercury compound 286 does not require an elimination-addition mechanism via the aryne (4) but probably involves an addition-elimination reaction of the halide 287, formed by decomposition of 286, via the Diels-Alder adduct (288). A similar conclusion was reached with regard to the formation of a tetracyclone adduct 289 from the perchloro analog 290. ... 

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