The Effect of Pressure on the Cycloadditions of Cyanoacetylene to Furan Derivatives – [2.2](2,5)Furanoparacyclophane, [8](2,5)Furanophane, and Furan

At 1 bar and 160°C the reaction of cyanoacetylene (1) with [2.2](2,5)furanoparacyclophane (3) produced the unexpected “ring‐enlarged” ketones 6–11. In the reaction of 1 with [8](2,5)furanophane (4) comparable products 21 and 22 were observed, in addition to the products 19 and 20 expected from a consecutive Diels‐Alder addition, Alder‐Rickert cleavage process and the Diels‐Alder addition of 1,4‐dicyano‐1,3‐cyclobutadiene (2a) to 4, respectively. In the reaction of 1 with the parent furan 5 only the (2:1) and (1:2) Diels‐Alder adducts 23, 25, 26, and 27 were found. High‐pressure experiments and the reactivity of 2‐cyano‐7‐oxabicyclo‐[2.2.1]hepta‐2,5‐diene (24), which was prepared independently by flow‐thermolysis of the (1:2) Diels‐Alder adducts 26 and 27, provide evidence that the (2:1) adducts 20, 23, and 13 are probably formed by a sequence of Diels‐Alder and [2 + 2] cycloadditions rather than by the reverse sequence starting with [2 + 2] cyclodimerization of 1 followed by Diels‐Alder reaction with cyclobutadiene 2a as postulated by the analogy to the trimerization of 1 and the cycloaddition of 1 to paracyclophane. The high‐pressure experiments led us to propose a new mechanism of formation of the “ringenlarged” ketones 6–11.