Ring expansion and ring contraction

In chemistry, ring expansion and ring contraction are chemical reactions that increase or decrease, respectively, the number of atoms in a ring of a cyclic compound.

Ring expansion reactions

Examples of ring expansion reactions are:

• the Beckmann rearrangement
• the Dowd-Beckwith ring expansion reaction.
• the Tiffeneau-Demjanov rearrangement
• the vinylcyclopropane-cyclopentene rearrangement
• the Buchner ring expansion

Ring contraction reactions

Examples of ring contraction reactions are:

• the Favorskii rearrangement
• the Demjanov rearrangement
• Ring contractions of cyclic ketones and cycloalkenes brought about by hypervalent iodine ^[1] for example the reaction of cyclohexene with iodosobenzene and boron trifluoride to cyclopentanecarbaldehyde:

References

1. ^ Hypervalent Iodine–Mediated Ring Contraction Reactions Luiz F. Silva, Jr Molecules 2006, 11, 421-434 Online article