Barrelenes, Diels-Alder

Barrelene was obtained via a double Diels-Alder reaction from a-pyrone with methyl acrylate (H.E. Zimmerman, I969A). The primarily forming bicyclic lactone decarboxylates in the heat, and the resulting cyclohexadiene rapidly undergoes another Diels-Alder cyclization. Standard reactions have then been used to eliminate the methoxycarbonyl groups and to introduce C—C double bonds. Irradiation of barrelene produces semibullvalene and cyclooctatetraene (H.E. Zimmerman. 1969B). 

Synthetic applications of other decarbonylation reactions are found in the conversion of cyclooctatetraene to barrelene 250), with the photodecarbonyla-tion of a Diels-Alder adduct as key step (2.31) and the preparation of tetrathioesters from 1,3-dithioles (2.32) 251). The most remarcable application of such a reaction up to date is the synthesis of tetra t.butyltetrahedrane from a tricyclic ketone precursor (2.33) 252). 

The asymmetric Diels-Alder cycloadditions of enantiopure (5)-5-(/ -tolylsulfinyl)-1,4-benzoquinones with Dane s diene under thermal and Lewis acid conditions produce tetracyclic quinones after spontaneous elimination of the sulfinyl group.The Diels-Alder reaction of barrelene with o-benzoquinone produces tetracyclo[6.2.2.2 .0 ]tetradeca-9,ll,13-triene-4,5-dione. Under kinetic control, the Diels-Alder cycloaddition of 2,3-dicyano-p-benzoquinone (98) with cyclopentadiene in MeOH produces the single cycloadduct (99) (Scheme 38). ... 

Norbornadiene adds typical dienophiles in a homo-Diels-Alder reaction whereby, for example, the adduct 86 is obtained with tetra-cyanoethylene. Dimethyl acetylenedicarboxylate and dicyanoacetyl-ene add to barrelene (bicyclo[2.2.2]octa-2,5,7-triene) in a similar way. °... 

Aromatic compounds can participate in both [2+2] and [4+2] photocycloaddition reactions with uracil derivatives to give either benzocyclobutane or ethenoquinazoline (barrelene) derivatives, which can then undergo a number of subsequent photochemical reactions. The products obtained are dependent upon the reaction conditions, and thus the photocycloaddition reaction between naphthalenes 470 and l,3-dimethyl-5-fluorouracil 471 in cyclohexane gave 4a-fluoro-5,10-ethenobenzo[/]quinazolines 472 as products as a result of a [4+2] photocycloaddition (photo-Diels-Alder) reaction <2002TL3113, 2003H(61)377>. 

Arsenin, antimonin and bismin react readily with dienophiles in the Diels-Alder reaction, and the reaction with hexafluoro-2-butyne and other acetylenic dienophiles has been used to trap these heterocycles as barrelenes (115), in particular the unstable bismin (equation 21). The reactivity increases with increasing size of the heteroatom and this may be related to decreased aromatic character of the heterobenzenes with increasing size of the heteroatom. In fact, bismin and antimonin are so reactive that at low temperatures (< -10 °C and < - 50 °C respectively) both exist as Diels-Alder dimers (116 equation 20). 

Diels-Alder type of cycloreversions are frequently observed among the azide adducts from barrelenes. The triazoline reaction product from phenyl azide and tetrafluorobenzobarrelene has never been isolated because of the ease with which it undergoes cycloreversion (Scheme 140).166 A similar reaction occurs with 1,4-diphosphabarrelene (see Scheme 23)167,168 as well as with the ethylazidoformate adduct of tetrafluorobenzodihydrobarrelene.169... 

Barrelene, benzobarrelene and dibenzobarrelene derivatives are readily synthesized by Diels-Alder reactions of arenes with alkenes. For barrelene and related derivatives, the DPM rearrangement proceeds solely from the triplet state, giving semibullva-lenes (Scheme 4.1) [3, 4]. 

Other heterodienophiles include diethyl azodicarboxylate, tosyl cyanide and phosphaalkynes. Barrelene, benzobarrelene and 2,3-naphthobarrelene react at 100-110 °C (sealed tube) with diethyl azodicarboxylate to give as the main product the corresponding homo-Diels-Alder adduct 27. ... 

The reactions of the strained benzene derivative 25 with DCA were originally studied with the aim of synthesizing planar cyclooctatetraene derivatives which are interesting compounds with respect to the question of their antiaromaticity [56]. The cyclooctatetraene derivative, 29 (Scheme 2.9) synthesized by photolysis of the barrelene derivative 26, however, turned out to be non-planar [57]. But the synthesis and the reactions of barrelene derivative 26 are interesting with respect to the utility of high pressure to control the course of reaction. The reaction of 25 with DCA at 1 bar and 127 °C produces the (1 1) Diels-Alder adduct 26 (yield, 49 %) and the unexpected dark blue (2 1) adduct 27 (yield, 14 %). At high pressure (9 kbar, 83 °C) 27 is the major product even after the low conversion of 36 % of the... 

Oxidative bisdecarboxylation. An efficient recent synthesis of Nenitzescu s hydrocarbon (3) involves a Diels-Alder reaction followed by bisdecarboxylation with this Ni complex. Pb(OAc)4 is unsatisfactory for the latter step. The product (3) was also used for preparation of barrelene (4) in 24% yield from (1). 

Cyclooctatetraene (COT), a An non-aromatic hydrocarbon, is only one of the many (CH)s compounds, but it is a central character. The other isomers, many of which interconvert with COT by thermal and photochemical pathways, include bicyclo[4.2.0]octa-2,3,7-triene (BOT), semibullvalene (SB), barrelene (B), tricyclo[3.3.0.0 ]octa-3,7-diene (TOD), the cyclobutadiene dimers (CBD), tetracyclo[4.2.0.0. " 0 ]oct-7-ene (TOE), cubane (C), tetracyclo[4.2.0.0. 0 ] octene - the intramolecular Diels-Alder isomer of BOT (IDA), tetracy-... 

Syntheses have been reported of barrelene (505) and of Nenitzescu s hydrocarbon (503) on a reasonable scale and without some of the difficulties that attend the previously published routes." " The preparation of (503) is of central importance, it being obtained by the oxidative bisdecarboxylation of the cyclo-octatetraene-maleic anhydride Diels-Alder adduct (502) with dicarbonylbis(triphenylphosphine)-nickel in diglyme at reflux. Cycloaddition of (503) to 2,5-dimethyI-3,5-diphenyl-cyclopentadienone in hot benzene afforded (504) and its endo-isomer. Photolysis of (504) yielded (505) in 24% yield from cyclo-octatetraene. The thermally labile benzene-maleic anhydride [4 + 2] cycloadduct (506) was obtained in similar fashion from (502). A review of the mechanisms of pyrolysis of (503) and a number of isomers and benzo-fused (CH) derivatives (collectively termed Nenitzescu s hydrocarbons) has appeared." ... 

Other Octacarbonyidicobalt-Catalyzed Reactions. A retro-Diels-Alder reaction catalyzed by Co2(CO)8 was observed in which a barrelene derivative loses a C2H2 fragment (eq 30). An analogous cleavage accompanies the cyclopentenone synthesis (see below) when norbomadiene reacts in certain solvents with alkynehexacarbonyldicobalts, as shown by the formation of dicarbonylcyclopentadienylcobalt, whereas a Diels-Alder addition catalyzed by a cobalt carbonyl species is involved when the same reaction is applied to cyclohexadiene. ... 

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