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RESEARCH PRODUCT

Recent Advances in Asymmetric [3,3]-Sigmatropic Rearrangements.

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The synthesis of new complex structures is still a challenge in preparative organic chemistry. Focusing on the generation of defined stereogenic centers, the [3,3]-sigmatropic rearrangements are known as reliable reactions. Always, a highly ordered transition state must be passed through, which allows the shift of chiral information from the reactant into the nascent product. Generally, the complete [1,3]- and, frequently, the [1,4]-chirality transfer enables one to predict the configuration of the new centers. This review focuses on Claisen and Cope rearrangements, which adopt the chiral information via a so termed asymmetric induction. This means, that the directing chiral subunit is placed outside of the six centers of the rearrangement system being reorganized during the course of the [3,3]-sigmatropic reaction. Reviewing the literature since 1995, enantioselective Claisen rearrangements have been widely investigated. The unique sense of the reaction allows the conversion of an easily accessible C atom-heteroatom bond into a new C-C bond making this rearrangement useful for constructing complex molecules. In contrast, the Cope rearrangement is reversible. One crucial requirement is to force the process to completion with respect to the desired product. Hence enantioselective Cope rearrangements' are always included as one step in a reaction cascade to guarantee the unique sense of the process. Analyzing such reactions in more detail, the chirality-inducing step is run prior to the Cope rearrangement. Thus, the [3,3]-sigma-tropic rearrangement is conducted under the well-known [1,3]-chirality transfer conditions.