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 . The bond migration steps in the Baeyer-Villiger oxidation ( A ) and the Beckmann rearrangement ( B ).
32.
Hyperconjugation can be used for rationalizing a variety of other chemical phenomena, including the anomeric effect, the gauche effect, the rotational barrier of ethane, the beta-silicon effect, the vibrational frequency of exocyclic carbonyl groups, and the relative stability of substituted carbocations and substituted carbon centred steric hindrance.
33.
To determine if the sugar is ? or ?, the structure is drawn in a Fischer projection; if the endocyclic oxygen ( O5 ) and exocyclic oxygen ( O1 ) are " cis ", the sugar is ?; if they are " trans ", the sugar is ?.
34.
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.
35.
For example, the invariant core residues G5, G8, G12 and C3 in the minimal hammerhead ribozyme were each observed to be so fragile that changing even a single exocyclic functional group on any one of these nucleotides results in a dramatic reduction or abolition of catalytic activity, yet few of these appeared to form hydrogen bonds involving the Watson-Crick faces of these nucleotide bases in any of the minimal hammerhead structures, apart from a G-5 interaction in the product structure.
