refers specifically to a vicinal dihydroxide rearrangement, there are other pinacol type rearrangements that proceed through the same general mechanism such as the Tiffeneau-Demjanov rearrangement. These "semipinacol rearrangements" occur under milder conditions and are thus preferable in complex syntheses. These reactions are useful beyond simply expanding a ring because the exocyclic group attacked may also have other functionality appended to it besides the leaving group. The group to which the endocyclic bond migrates can also be selectively added to the ring based on the functionality already present, for example 1,2 addition into a cyclic ketone. Carbon insertion through opening of a bicycle Carbon introduction through the opening of a bicyclic system is another way to introduce either a single carbon or several at a time to a ring. The single carbon introduction often goes through a cyclopropane containing bicyclic intermediate, which is subsequently opened to give the expanded ring. The expansion can occur either through an electrocylic ring opening or by an induced cleavage of the shared bond. These reactions are differentiated in terms of how the carbon is added to form the cyclopropane ring. A Simmons-Smith like reaction with an alkene containing ring can be used to introduce a single carbon. Other methods exist to cyclopropantate other functionalities, such as the Buchner ring expansion fornucleophihlic attack (A) and neighboring group donation (B).The formation of the cyclopropane ring in a single carbon expansion allows the introduction of additional functionality along with the ring expansion, provided the additional functionality is appended to the carbon being inserted. The target of cyclopropanation also varies depending on the conditions used. The Simmons-Smith reaction adds to alkenes while the Buchner ring expansion allows addition to typically unreactive arenes. The Buchner ring expansion is useful because it gives cycloheptatrienes as the ring opened products, which are found in the core of some natural products, such as azulenes. An important consideration in these ring expansions is the ring opening to an expanded ring and not an exocyclic group on the original ring or an unopenable product. The Buchner ring expansion is encouraged to open to the desired product by placing electron withdrawing groups on the carbon added. In order to perform the ring opening on saturated bicyclic molecules the cyclopropane must be introduced such that a neighboring group can facilitate the expansion or the ring must be opened by attackate the expansion or the ring must be