Contraction de cycle a partir de la tosyloxy-8 bicyclo[4.2.0]octanone-7 cis

Cis 8-endo tosyloxybicyclo[4.2.0]oct-3-en-7-one treated with H₄AlLi undergoes a stereospecific ring contraction to give endo 7-formyl bicyclo[4.1.0]hept-3-ene and endo 7-hydroxymethyl bicyclo[4.1.0]hept-3-ène. On the contrary, the same compound treated with MeONa in ether or methanol (conditions of the Favorski rearrangement) gives, beside solvolysis products when CH₃OH is solvent, the two epimeric esters 7-endo and 7-exo carbomethoxybicyclo[4.1.0]hept-3-ène. Several pathways are postulated for the rearrangement.