The effect of pressure on the cycloadditions of cyanoacetylene to furan derivatives - [2.2](2,5)furanoparacyclophane, [8](2,5)furanophane, and furan

Article abstract of DOI:10.1002/jlac.199719970119

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 "ring-enlarged" ketones 6-11. VCH Verlagsgesellschaft mbH, 1997.