Effect of β-Substituents on the Regioselectivity of the Diazomethane Ring Expansion of α-Methyl-α-methoxycyclobutanones to Cyclopentanones

The ring expansion of 22 differently β-substituted α-methyl-α-methoxycyclobutanones by diazomethane was studied. Migration of the less-substituted α-carbon was favored with the single exception of a very sterically hindered β-benzyloxy β-substituent.