Complex Reactions and Synthetic Applications

8) illustrates that the effect of pressure and that of a catalyst can be complementary [55]. At 160 °C and 1 bar the reaction of 20 with DCA leads to the dicyano-substituted furane (22) (20 %) and oxepin (24) (1 %). In a kinetically controlled reaction of 20 with DCA at 9 kbar and 20 °C the thermally unstable Diels-Aldcr adduct 21, the precursor of 22, is formed as the only product which undergoes a smooth retro Diels-Alder reaction at 1 bar and 20 °C producing the starting materials 20 and DCA. At 160 C the retro Diels-Alder reaction of 21 leading to 22 and acetylene is obviously competitive with the non-productive reaction leading to 20 

5-diene) is not thermally stable and undergoes a retro Diels-Alder reaction, producing benzonitrile and ethene [58, 59]. The reaction is highly accelerated by pressure and occurs at 9 kbar and 50 °C. Under these conditions the primary Diels-Alder adduct is stable and can be isolated in good yields. A similar effect of pressure was observed in the trimerization of cyanoacetylene leading to 1,2,3- and 

2-dicyanocyclobutadiene followed by Diels-Alder cycloaddition of cyanacetylene to the cyclobutadiene producing the Dewar-benzene derivatives which aromatize to the observed benzene derivatives via orbital-symmetry forbidden electrocyclic ringopening. 

In the past decade the utility of high pressure has been demonstrated for many syntheses involving Diels-Alder reactions as key steps (sec Chapters 9 and 11) We will describe here only few selected examples in order to illustrate the utility of high pressure in organic syntheses. 

An interesting example is the pressure-induced reaction of buckminsterfullerene Cfio with 1,3,5-cycloheptatriene [60]. Generally C o reacts as an electron-deficient dienophile or dipolarophile in numerous Diels-Alder or 1,3-dipolar cycloadditions and 1,3,5-cycloheptatriene as a diene. The reaction with C(,o is a rare example where both adducts derived from the norcaradiene as well as the cycloheptatriene are observed. 

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