Photochemische Umwandlungen. XVIII Photochemische Isomerisierung annellierter Oxanorbornadiene Vorläufige Mitteilung

Photochemical cycloaddition reactions in anellated oxanorbornadiene derivatives have been investigated. In the case of 7a the tetracyclic isomer 8a can be isolated which adds dimethylacetylenedicarboxylate in the manner of a bis-homo-furan system. On thermal activation 8a yields the thiepino-oxepin derivative 9a. In contrast, on direct photoexcitation of the benzoxanorbornadiene the primary photoproduct cannot be identified, the reaction leading directly to the benz[d]oxepin.