REACTIVITY OF CYCLOPENTENYL ANION ANALOGOUS HETEROCYCLES - 1,5-ELECTROCYCLIZATION OF 2-OXABICYCLO[3.2.0]HEPT-3-ENE, 2-THIABICYCLO[3.2.0]HEPT-3-ENE, 2-AZABICYCLO[3.2.0]HEPT-3-ENE AND 2-PHOSPHABICYCLO[3.2.0]HEPT-3-ENE - A SIGMATROPIC [1,3]-CARBON SHIFT

Abstract

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 exe reversible arrow endo isomerization of specifically deuterated derivatives or of the racemization of optically active derivatives turned out to be higher by Delta Delta G greater than or equal to 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.