Synthese von Triafulven‐Vorstufen für <i>Retro</i>‐<i>Diels</i>‐<i>Alder</i>‐Reaktionen

Abstract

Synthesis of Triafulvene Precursors for Retro ‐ Diels ‐ Alder Reactions Triafulvene precursors exo − 15 and endo − 15 have been prepared by addition of dibromocarbene to benzobarrelene 12 followed by a lithium‐halogen exchange, methylation, and elimination of HBr ( 12→13→14→15 ), (Scheme 2) . Gas‐phase pyrolysis of exo / endo ‐mixtures of 15 above 400° gave minor amounts of naphthalene ( 16 ), traces of a hydrocarbon C 4 H 4 identified by MS (presumably triafulvene 1 ) and predominantly (36%) the isomerization product 17 (Scheme 3) . In a second synthetic approach the well‐known cycloheptatriene‐norcaradiene equilibrium of type 26⇆27 has been utilised to prepare various endo ‐ trans ‐3‐(X‐methyl) tricyclo[3.2.2.0 2,4 ]nona‐6,8‐dienes 31 (Scheme 5) . However, numerous elimination experiments 31→9 failed so far. The structure of two rearrangement products 33 and 34 (Scheme 6) has been elucidated.