Exemplifying with the 4.5-dicarbomethoxy oxepin 6a the authors describe an oxepin synthesis from furanes and acetylenic dienophiles via Diels-Alder reaction (4a), photochemical oxanorbornadiene-oxaquadricyclane transformation (4a 5a), and thermal 3σ 3π opening of the highly strained oxaquadricyclane 5a. With dimethylacetylenedicarboxylate, methylpropiolate, maleic anhydride, and di-methoxycarbonyl-oxanorbornadiene (4a) 5a yields the 1:1 adducts 19a, 19b, 22, 23 and 26 (unstable) by strictly stereospecific addition to the α-positions of the oxygen bridge. With the same dienophiles the oxepin 6a reacts only through its valence-tautomeric benzene-oxide form 7a giving stereospecifically 27, 29, 30 and 31. No definite conclusions are drawn with regard to the mechanistic implications of the photostep 4a 5a, the thermal 3σ 3π-transformation 5a 6a/7a, and the bishomofurane cycloaddition reactions. Scope and limitations of this oxepin synthesis are briefly discussed.

Photochemische Umwandlungen, 45. Mitteilung [1] die 3σ → 3π‐route zu oxepin‐derivaten

A practical and general Diels-Alder reaction of pentafulvenes with arynes.

D3-trishomocubanetrione synthesis and optical resolution

Vinylogous Heptafulvenes from Vinylogous Fulvenes

Vinyloge Heptafulvene aus vinylogen Fulvenen

The benzonorbornadiene derivatives 4, 5, 6 and 19 give upon direct photoexcitation the benzo-tricyclo [3.2.0.02, 7] heptenes 14, 17, 21 and 23 in yields up to 60%. Sensitization by acetone improves the yields considerably. The spectral data of the substrates 4, 5, 6, 7 and 19, of the photoproducts 14, 17, 21 and 23, as well as of the dihydrobenzonorbornadienes 13, 16 and 22, are presented.

P. Würsch

Papers

Photochemische Umwandlungen. XXXIV [1]. Photochemische Isomerisierung substituierter Benzonorbornadiene