Chiral-o-quinodimethanes

A review of Diels-Alder reactions of fullerenes with acyclic and cyclic dienes has been presented. The addition of substituted pyrimidine o-quinodimethanes (75) to [60]fullerenes yields novel organofullerenes (76) bearing a pyrimidine nucleus covalently attached to the Ceo cage (Scheme 26). The Diels-Alder dimerization of cyclopenta[/]phenanthrene (77) with isobenzindene (78) yields the dimer (79) in 85% yield (Scheme 27). Further evidence has been supplied to support an early reorganization of the r-network in the dimerization of 2-methoxycarbonylbuta-1,3-diene. The Lewis acid-catalysed Diels-Alder reactions of acrylate derivatives of new carbohydrate-based chiral auxiliaries with cyclohexadiene show excellent endo. exo... 

Several reviews have dealt with Diels-Alder reactions of o-quinodimethanes, 2" emphasizing the more powerful, selective and thus extensively used intramolecular versions (cf. Scheme 135) introduced in 1971. 2 The most recent survey also tackled the issue of asymmetric induction by means of a chiral auxiliary group attached to the o-quinodimethane precursor.This section aims at a complementary coverage of intermolecular Diels-Alder reactions of o-quinodimethanes, underlining synthetically relevant aspects. 

A synthesis of (+)-estradiol from an alkyl 1,3-dihydrobenzo[c]thiophene-2,2-dioxide involving o-quinodimethane formation by thermal extrusion of SO2 and subsequent cycloaddition has been achieved in an overall yield of 50% (ref. 143). Thus a chiral cyclopentanone component (ref. 144) was used to alkylate the appropriate benzothiophene dioxide and the required tetracyclic stucture obtained directly with avoidance of the customary hydrogenation. It was found desirable at the alkylation stage to enhance deprotonation at Cl by having a cyano group in the benzenoid ring. The ( anotetracycle (R CN) was reacted... 

Next, we investigated an asymmetric Diels-Alder reaction of o-quinodimethanes generated in situ from benzocyclobutenols utilizing a tartaric acid ester as a chiral auxiliary. The Diels-Alder reaction of o-quinodimethane with olefins is a useful method to construct tetrahydronaph-thalene frameworks bearing up to four stereocenters, which are key intermediates for the synthesis of polycyclic... 

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