Tropones, Diels-Alder reaction

Fig. 11 Dinucleus BINOL-aluminum complex catalyzed tropone Diels-Alder reaction...

Tropones are non-benzenoid compounds that behave like 47r-components in a Diels Alder reaction. These compounds are of interest because of their synthetic applications based on the Diels Alder reaction, since the cycloadducts can be easily converted into a large variety of compounds. 

Azulene quinones [49b] are compounds related to the family of tropones and are considered to possess great biological and physiological potential. Several polycyclic compounds have been prepared by high pressure (3kbar, PhCl, 130°C, 15h) Diels-Alder reaction of 3-bromo-l,5-azulene quinone (137) and 3-bromo-l,7-azulene quinone (138) with several dienophiles. The cycloadditions were regioselective and afforded cycloadducts in reasonable to good yields (Scheme 5.20). 

Diels-Alder cycloadditions involving norbomene 57 [34], benzonorbomene (83), 7-isopropylidenenorbomadiene and 7-isopropylidenebenzonorbomadiene (84) as dienophiles are characterized as inverse-electron-demand Diels-Alder reactions [161,162], These compounds react with electron-deficient dienes, such as tropone. In the inverse-electron-demand Diels-Alder reaction, orbital interaction between the HOMO of the dienophile and the LUMO of the diene is important. Thus, orbital unsymmetrization of the olefin it orbital of norbomene (57) is assumed to be involved in these top selectivities in the Diels-Alder cycloaddition. 

All Diels-Alder reactions of tropones 51 as dienes with different types of dienophiles shown in Scheme 11 are accelerated by pressure, so that in some cases the desired cycloadducts are only formed at high pressure. An interesting synthetic equivalent of the unreactive acetylene in Diels-Alder syntheses is the oxanorbomadiene derivative 52 (Scheme 11 entry 2). 52 reacts with tropones forming the adducts 53, 54 and 55, which undergo a retro-Diels-Alder reaction leading to 56 and 57, the formal [4+2] cycloadducts of tropones to acetylene. 

SCHEME 11. Diels-Alder reactions of tropones as dienes... 

Dibenzofuro[4,5-c]tropone (8H-dibenzo[a,e]furo[3,4-c]cyclohepten-8-one 368, X = CO) and the corresponding thiepin-5,5-dioxide (8//-dibenzo-[6,/]furo[3,4-retro Diels-Alder route. In refluxing benzene, the intermediate (367, X = CO) can be isolated. Further routes to 368 (X = CO) by retro Diels-Alder reaction exist. The 1-methyl derivative of 368 (X = CO) has been trapped as a maleic anhydride adduct. ... 

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... 

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]. 

This will apply not only to Diels-Alder reactions, but to other Woodward-Hoff-mann symmetry-allowed reactions, as well. For example, the good donor-acceptor pair, cycloheptatriene and tropone, react to give only the allowed ([6 + 4], then [4 + 2]) adduct <517, while cycloheptatriene dimerizes to give not only the analogous adduct, 7, but the forbidden, [6 + 2] adduct, 8, as well as several other unidentified dimers18. ... 

Unsymmetrically substituted tropones wherein the substitution is remote from the actual bonding centers have also been the subject of regioselectivity studies. Modest chemical yields accompanied high regioselectivities in the resultant products isolated from the addition of various dienes to 3- and 4-ethoxycarbonyltropones, (18) and (21). So-called even regioselectivity, which completely parallels the regioselectivity observed in the Diels-Alder reaction, prevailed in these cases. Lower selectivities characterized the reactions of 3- and 4-methoxytropones with the same set of electron-rich diene partners. 

Diek-AUer reactions. The Diels-Alder reaction of cyclopropene with cyclopcnt-adicne has already been mentioned (I, 1040). In fact, cyclopropene is one of the most reactive dienophiles. It reacts with tropone (1) in methylene chloride at 0° to give the 1,4-addition product (2). Similarly, reaction with tropolone (3) affords the adduct (4). 

The pressure dependence of the orbital symmetry-allowed [6 - - 4] cycloaddition of tropone with 1,3-dienes was first studied by le Noble and Ojosipe , who reported extremely small absolute values of and AV. A reinvestigation by Takeshita and his coworkers showed, however, that the activation and reaction volumes of these cycloadditions are of the same order of magnitude as those of Diels-Alder reactions (Scheme l7 entry 1). Dogan confirmed this finding with a study of the reaction between 1,3-butadiene and tropone in which a [6 + 4] cycloaddition competes with a [4 + 2] Diels-Alder reaction. The activation volume of the overall reaction was again found to be highly negative. However, the ratio between the [6+4] and [4+2] cycloadduct turned out to be almost pressure-independent, which means that the ifference between the activation volumes (SAT ) is almost zero and hence the activation volumes of both reactions are of the same value. 

There is a special kind of site-selectivity which has been called periselectivity. When a conjugated system enters into a reaction, a cycloaddition for example, the whole of the conjugated array of electrons may be mobilized, or a large part of them, or only a small part of them. The Woodward-Hoffmann rules limit the total number of electrons (to 6, 10, 14 etc. in all-suprafacial reactions, for example), but they do not tell us which of 6 or 10 electrons would be preferred if both were feasible. Thus in the reaction of cyclopentadiene (355) and tropone (356), mentioned at the beginning of this book, there is a possibility of a Diels-Alder reaction, leading to 354, but, in fact, an equally allowed, ten-electron reaction is actually observed,121 namely the one leading to the adduct (357). The product is probably not thermodynamically much preferred to the... 

Methyl 2-azabicyclo[3.1 -0]hex-3-ene-2-carboxylate (4) also reacts according to a [4+6] cycloaddition mechanism if tropone (26) is used as the electron-deficient triene. The primary product 27 of this [(27t-l-2ff)-l-(27t-f-27r + 27r)] process, containing a doubly bridged 11-membered ring, could not be isolated and immediately underwent an intramolecular Diels-Alder reaction to form the cage compound 28. ... 

Chloro-substituted derivatives 11 and 13 of tropones were prepared in good yields by solvolysis of tetrachlorobicyclo[4.1.0]heptenes 10 and 12, which were obtained in moderate yields by Diels-Alder reaction of tetrachlorocyclopropene with electron-rich 1,3-dienes. ... 

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