Stereospecific ring contractions of 7-chlorobicyclo[3.2.0]hept-2-en-6-ones and equilibration studies of 7-alkylbicyclo[3.2.0]hept-2-en-6-ones

Abstract

7-Alkyl-7-endo-chlorobicyclo[3.2.0]hept-2-en-6-ones (1b–d) undergo ring contraction readily and stereospecifically with aqueous base. Because of conformational factors, the 7-exo-chloro-epimers (2b–d) show an increasing reluctance to undergo ring contraction as the size of the alkyl group increases, and in the case of (2c and d) 2-alkyltropones are also formed. Equilibration studies show that for 7-alkylbicyclo[3.2.0]hept-2-en-6-ones, the large alkyl group prefers the endo-configuration, whereas in the 2,3-dihydro-derivatives it prefers the exo-configuration; however in both equilibria, energy differences between endo- and exo-epimers are small.