Substituted on the same carbon by an electron donating group and an adjacent multiple bond, cyclopropanes provide building
blocks of unprecedented synthetic potential. They are readily available along different routes involving: cyclopropanone hemiacetals,
oxaspiropentanes, alkylidenecyclopropanes, 1-heterosubstituted lithiocyclopropanes, α-enone silyl enol ethers, 1,3-dichloroacetone
and 1-hydroxycyclopropylcarbonyl derivatives as main sources.
First of all, they undergo acid-induced C3→C4 ring expansion to four-membered rings, in particular to the 2-vinylcyclobutanone system, which is an efficient precursor
of C5, C6 and C8 homologous rings by subsequent acid- and base-induced, thermal, or photolytic ring enlargements. On the other hand, they
undergo thermal C3→C5 ring expansion providing cyclopentanone enol ethers or derivatives with high chemo-, regio- and stereoselectivity.