Reaction conditions: (a) THPO(CH2)3MgBr; (b) PhCH2NH2; (c) H30+; (d) DEAD/PPh3; (e) FhLi; (f) heat in air

Scheme 16 Synthesis of tetrahydroquinolinequinone in analogy with the xanthone core [94]

Finally noted in this type of ring transformation is construction of porphyrin-quinone architectures. This was performed by introduction of a porphyrin residue at C-3 by the coupling reaction of 12 with some bro-moporphyrins (cf. Scheme 2) and the prescribed conversion to quinone derivatives, fascinating as potential anticancer agents [95,96].

The electrocyclic reaction in which unsaturated substituents participate at C-2 of cyclobutenedione provides a somewhat different cyclization mode. The thermal rearrangements of 2-dienylcyclobutenones 88 and 3-(o-vinylphenyl)-cyclobutenediones 92 underwent well-precedented 4n-6n electrocyclic reactions, but within the diene moiety to phenolic intermediates 90 and 94. These were allowed to react intramolecularly to give benzofurans 91 and naphthofu-

ranones 95, respectively [97,98]. This work represents a new aspect of squaric acid chemistry (Scheme 17).

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