Scheme 17 Thermal rearrangements of 2-dienylcyclobutenones and 3-(o-vinylphenyl)-cyclobutenediones [97,98]

When the substituents of o-vinylphenyl and isobutenyl groups were placed at C-2 and C-3, respectively, such as 96, thermolysis (70 °C) preferred tandem 8n-6n electrocyclic reactions between these substituents to give a tetracyclic cyclobutenone 98. This underwent the precedented ring transformation/oxidation and, ultimately, photofragmentation with expulsion of isobutylene to give angular furoquinone 99, for example, by the use of 2-lithiofuran. In the case of an alkynyl group at C-3, thermolysis gave an alkenylidenefuran [99-101] (Scheme 18).

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