Reactivity of cyclopentenyl anion analogous heterocycles: 1,5‐electrocyclization of 2‐oxa‐, 2‐thia‐, 2‐aza‐ and 2‐phosphabicyclo[3.2.0]hept‐3‐ene. A sigmatropic [1,3] carbon shift

Carbonyl ylide-like intermediates are involved in the 1,5-electrocyclization of the bicyclo[3.2.0]heptenes 3a–c. The activation barriers analyzed by the time- and temperature-dependence of the exo ⇌ endo isomerization of specifically deuterated derivatives or of the racemization of optically active derivatives turned out to be higher by Δ ΔG≠ ≥ 11 kcal/mol than those determined for the corresponding bicyclo [3.1.0]hexenes 1a–c. This result can be considered as an evidence for the electrocyclic nature of these ring openings due to the diminished Walsh character of cyclobutane bonds compared to cyclopropane bonds. A stereochemical analysis of the fragmentation of 2-oxabicyclo[3.2.0]heptene 3a to furan and ethene leads to the conclusion that a sigmatropic [1.3] carbon shift proceeding with inversion of the migarting carbon followed by stereospecific retro-Diels-Alder reaction is the major pathway for this reaction similar to the rearrangement and fragmentation of the corresponding carbocycle 3e.