Dienes-cyclobutene with aromatic rings

The first example illustrates how a 1,4-dehydroaromatic system with cyclohexane ring having two double bonds may be also disconnected according to a retro-Diels-Alder to give a diene and an acetylene as the dienophile [25]. The second example makes clear that even an aromatic double bond may be -in some instances-involved in a retrosynthetic pericyclic disconnection [26]. In the synthetic direction, the polycyclisation involves a conrotatory electrocyclic cyclobutene ring opening, (16 15) followed by an intramolecular Diels-Alder addition (see Scheme... 

Vinyl- and 3-vinyl-thiophenes behave as a diene on reaction with dienophiles to give six-membered ring-fused thiophenes. Cycloaddition of the cyclobutene 842 with 841 or with 844 followed by aromatization gives benzo[/ ]thio-phene-fused benzocyclobutenedione 843 or 845, respectively (Scheme 129) <1996J(P1)497>. Heterohelicene 847 is synthesized by the reaction of 846 with 1,4-benzoquinone <2001JA11899>. 

The application of thermolytic reactions (e.g. Diels-Alder and reverse Diels-Alder processes) for the synthesis of fluoro-organic compounds has been reviewed. The cycloaddition of the meso-ionic dithiolone or thiazolone compounds (209 X = S or NPh) with the Diels-Alder adduct of cyclo-octatetraene and dimethyl acetylenedicarboxylate occurs at the cyclobutene ring to give the exo-adducts (210). Pyrolysis of compounds (210) yields dimethyl phthalate and 7,9-diheteratricyclo[4,2, 1,0 ]non-3-en-8-ones (211 X = S or NPh) by retro-[4-t- 2] addition. The similar additions of (209) and related compounds to dimethyl 7-oxabicyclo[2,2,l]-hepta-2,5-diene-2,3-dicarboxylate afforded [4 -1-2] adducts which on pyrolysis suffered double thermal fragmentation e.g. loss of XCO and 3,4-dimethoxycarbonyl-furan) to afford five-membered aromatic heterocyclics. ... 

Spiro[3,4]octa-l,5,7-triene (249) has been prepared by modification of the carboxylic acid (250). The triene was characterized chemically by reduction to the saturated hydrocarbon and by its Diels-Alder reaction with JV-phenyltriazoline-3,5-dione. Any spiroconjugative interaction in the triene is not shown on its u.v. spectrum, which has at 262 nm, the same as observed for the diene (251). The triene is unstable, rearranging at — 5°C (t = 90 min) to 6-vinylfulvene and a dihydropentalene derivative. The ring-opening of (249) to 6-vinylfulvene is unusually rapid for a 3,3-disubstituted cyclobutene derivative, and other similarly strained compounds do not show this reactivity. The ring-opening may well proceed via a biradical intermediate with stabilization of the zwitterionic form of the biradical by formation of the aromatic cyclopentadienyl anion. 

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