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

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.