opened by attack from an outside group. Ring opening as a means of ring expansion can also be applied to larger systems to give access to even larger ring syscyclization. The Grob fragmentation can be applied as an example of such an expansion. Like the pinacol type migration the Grob fragmentation relies on an electron donating group to promote the bond migration and encourage the leaving group to be expelled. In this case the electron donating group can be a pseudo electron donating group which is capable of eliminating and donating an electron pair into the carbon with the breaking bond. Working with two smaller rings can allow for elaboration of two parts of the molecule separately before working with the expanded ring. The Dowd-Beckwith ring expansion is also capable of adding several carbons to a ring at a time,e of adding several carbons to a ring at a time, and is a useful tool for making large rings. While it proceeds through an intermediate bicycle the final cyclization and ring opening take place within the same radical reaction. This expansion is useful because it allows the expansion of a beta-ketoester to a large cyclic ketone which can easily be elaborated using either the cyclic ketone or the exocyclic ester. Heteroatom insertion reactions Heteroatom additions to rings can occur through ring expansions if not they are not done through de-novo ring synthesis. These introductions are primarily ring expansions because they often take place through migration/insertion pathways similar to those mentioned above for carbon. Examples include high impact applications of the Beckmann rearrangement (for introduction of nitrogen into codeine) and the Baeyer-Villiger oxidation (introduction of oxygen to cage-annulated ethers) in synthesis. Both occur with the expulsion of a leaving group as the alkyl group migrates onto the exocyclic heteroatom, which is strikingly similar to the pinacol type rearrangement. Ring contraction reactions Ring contractions are useful for making smaller, more strained rings from larger rings. The impetus for making these rings comes from the difficulty associated with making a fully elaborated small ring when such a ring could more easily be made from an elaborated larger ring, from which an atom can be excised, or that the original larger scaffold is more accessible. Ring contractions are easily characterized simply by the reactive intermediate which performs the contraction. The standard intermediates are anionic, cationic, and carbenoid. Anionic contractions The Favorskii rearrangement is the