An Unprecedented Biradical Cyclization as an Alternative Pathway to the Myers–Saito Cycloaromatization in the Thermal Reactions of Enyne Allenes

Abstract

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 (63 %)

Twisted Arenes

“…Copyright (1971) American Chemical Society 38 K.K. Wang the Schmittel cascade cyclization reactions [38][39][40] leading to 37a. Similarly, 37b and 37c were obtained from benzannulated enediynes 32b and 32c, respectively.…”

Fluoro‐Ene Reaction Versus [2+2] Cycloaddition in the Thermal C2–C6 Cyclization of Enyne‐Allenes: An Experimental and Theoretical Investigation

“…With reference to the diradical, the barrier for a [1,5]‐fluorine shift is quite large (18–22 kcal mol –1 ) compared with that of a hydrogen shift (1.8 kcal mol –1 in EA7 ), whereas cyclobutene formation shows a barrier of 16.4 kcal mol –1 (29.9 kcal mol –1 with respect to EA7 ). This explains why two structurally rather similar enyne‐allenes, that is, EA2 and EA2′ , furnish upon thermolysis two constitutionally isomeric products, that is, 1 H ‐cyclobuta[ a ]indene CB2 in a stepwise [2+2] cycloaddition9,27 and benzofulvene BF2′ 3a,8a by a stepwise ene reaction, respectively.…”

Fast Synthesis of Benzofluorenes by Selenium‐Mediated Carbocyclizations

“…[9] Intramolecular [4+2] cycloadditions of 2-propynyl diarylacetylenes [10] or radical cycloaromatizations of nonconjugated benzotriynes [11] can lead to benzo[b]fluorene derivatives; additionally, palladium-mediated arylations to benzo [b]fluorenes have been investigated.…”