Heptafulvenes 8+2 cycloaddition reactions

Liu and colleagues295,296 studied the cycloaddition reactions between electron-deficient 8,8-disubstituted heptafulvenes 466 and electron-rich 6,6-disubstituted fulvenes. The substituted heptafulvene reacted as the trienophile in this case. Only when 6,6-dimethylfulvene (465) and heptafulvenes 466a-b were used as the triene and trienophiles, respectively,... 

Diphenylketene undergoes 2 + 2- or 2 -E 4-cycloaddition reactions with various 1,3-diazabuta-1,3-dienes. The 2 - - 4-, 4 -E 2-, 6 -E 4- and 8 -E 2-cycloaddition reactions of heptafulvenes have been reviewed. ... 

Daub and colleagues studied the [8 + 2] cycloaddition reactions of electron-rich 8-substituted heptafulvenes with a wide variety of acceptor substituted alkenes. 8-Methoxyheptafulvene (534) proved to give the best results, the more electron-rich heptafulvenes being less reactive toward [8 -b 2] cycloaddition reactions and more prone to oxidative dimerization . The reactions of 8-methoxyheptafulvene with acceptor substituted polyenophiles 535 can in principle produce up to 8 diastereomers. The reactions proved, however, highly regioselective, the exo and site selectivities being moderate to good, and afforded mixtures of 536, 537 and 538 (equation 155, Table 31). ... 

Another example of the trimethylenemethane type of compound is provided by the tricarbonyliron complex of heptafulvene. This complex undergoes a cycloaddition reaction with methyl acetylenedicarboxylate and the product can be converted into 1,2-dicarbomethyoxyazulene [Eq. (163) (Kerber and Ehntholt, 1973)]. 

Morita and coworkers [76] described a [8+2] cycloaddition of 8-oxoheptafulvene 299, a heptafulvene ketene, with cycloheptatriene iron carbonyl 300 to give tricyclo[8.5.0.0 ]pentadeca-3,5,7,ll,13-pentaene-2-one Fe(CO)3 301 (Scheme 7.66). Usually, ketenes undergo [2+2] cycloaddition reactions (see above), and the observation mentioned here is a special case. 

A rich cycloaddition chemistry with phosphaalkynes 9 may be expected from cross-conjugated carbocyclic compounds of the heptafulvene type (in the present case, tropone (41)). Thermal [rtf - and [jt a ]-cycloadditions are allowed by symmetry. Thus, the thermal reaction of tropone (41) with an excess of the phosphaalkynes 9 a, 9 e (without a solvent) furnishes the diphosphatetracyclic products 43a,b [45]. The initiating Diels-Alder reaction to 42 (there are P-NMR spectroscopic indications for its intermediate formation) is accordingly followed by a [2 -t- 2 + 2]-cycloaddition (ashomo-Diels-Alder reaction) with a second equivalent of the phosphaalkyne to yield 43 (Scheme 6-9). The observed peri- and regioselectivities of the two reaction steps can be confirmed by frontier orbital calculations [43],... 

J With enamines. l-Oxa-2-azulenones (as tethered heptafulvenes) with enamines undergo [8+2] cycloaddition as well (85HOU(5/2c)127, p. 218 98SL950). This way, in the case of heterocyclic enamines (such as 40), azuleno[l,2]-fused azoles and azines are formed (Schemes 13 and 14) The reactions of oxaazulenone 37 with the isomer mixtures of enamines 40a or 40b yield mixtures of isomeric dihydrothiophenes 41 and 42 that are dehydrogenated by 2,3-dichloro-5,6-dicyano-p-benzoqui-none (DDQ) to yield thiophenes 43 and 44, respectively (83CL1721). Whereas the latter [l,2-c]-fused compound is unstable because of its o-quinonoid structure, thiophenes 43a and b are demethoxycarbonylated to yield substances 45a and b. 

Oxoheptafulvene (197), prepared by in situ dehydrochlorination of cyclo-heptatriene-l-carboxylic acid chloride, apparently undergoes [8 + 2]-cycloaddition with benzophenone to give (198), but this possibly arises via the [2 -t- 2]-adduct (199). On reaction of the heptafulvene with cyclopentadiene the [2 -h 2]-adduct is observed, which rearranges thermally to (2(X)), perhaps via (201) or two consecutive [1,5]-shifts. ... 

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