If there is an aryl substituent on the acetylene terminus of enyne‐allenes, then its reaction mode may be changed from the Myers‐Saito cyclization to a novel C2–C6 cyclization resulting in a net intramolecular Diels‐Alder or ene reaction. As a consequence, the thermal cyclization of readily accessible acyclic enyne‐allenes can be utilized for the synthesis of complex benzofulvene and benzofluorene derivatives. Kinetic results of the C2–C6 cyclization reaction indicate a two‐step reaction pathway with a benzofulvene biradical intermediate.
The nature of the substituent at the alkyne terminus of enyne‐allenes is critical for the switch from the Myers–Saito C2–C7 cyclization to the C2–C6 cyclization (see reaction scheme below). Quantum‐chemical calculations show the influence of steric (R=tBu) and electronic effects (R=Ph) on the regioselectivity of the thermal reaction; for R=NH2 a reduction in activation energy to below 18 kcal mol−1 and a modified reaction mechanism for the C2–C6 cyclization are postulated. R1=R2=H.
During the last few years interest in the thermal biradical cyclizations ['] of enediynes, enyne cumulenes, and enyne allenes has increased dramatically. This trend is due to the fascinating mechanisms of these systems operative in natural antitumor antibiotics[21 as well as their potential in the synthesis of carbocyclic ~ystems.[~1 Consequently, numerous derivatives of enyne allenes with various substituents and substitution patterns, which on heating generally furnish the expected Myers-Saito cyclization products,r1b. dl have already been studied.r41 However, we found quite unexpectedly that the simple attachment of an aryl group to the alkyne terminus of the enyne allenes redirects the reaction course to a novel C2-C6 cyclization, giving rise to formal ener5] and Diels-Alder products (Scheme
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