Hcrrn Professor Dr. A . Liittringhuus zum 65. Geburtstag gewidmet (24. VII. 71) Summary. Exemplifying with the 4.5-dicarbomethoxy oxepin 6 a the authors describe an oxepin synthesis from fdranes and acetylenic dienophiles via Diels-Alder reaction (4a), photochemical oxanorbornadiene-oxaquadricyclane transformation (4a + 5a), and thermal 30 + 372 opening of thc highly strained oxaquadricyclane 5 a. With dimethylacetylenedicarboxylate, methylpropiolate, nialeic anhydride, and di-methoxycarbonyl-oxanorbornadiene (4a) 5a yields the 1:l adducts 19a, 19b, 22, 23 and 26 (unstable) by strictly stereospecific addition to the cc-positions of the oxygen bridge. With the same dienophiles the oxepin 6 a reacts only through its valence-tautomeric benzene-oxide form 7 a giving stereospecifically 27, 29, 30 and 31. No definitc conclusions are drawn with regard t o the mechanistic implications of the photostep 4 a + 5a, the thermal 30 --f 3n-transformation 5a --f 6 a/7a, and the bishomofurane cycloaddition reactions. Scopc