Tandem Sequences Involving Michael Additions and Sigmatropic Rearrangements

Abstract

Claisen and related [3,3]-sigmatropic rearrangements have been well described as useful synthetic organic tools. Frequently, the [3,3]-sigmatropic rearrangements are combined with other reactions in a tandem sequence leading to the formation of several bonds in only one synthetic step. This review presents the most relevant advances in tandem sequences combining a conjugate Michael-type addition and a [3,3]-sigmatropic rearrangement. Relevant examples of the use of different types of nucleophiles initiating the tandem sequences, along with detailed commentary illustrating the use of the title sequences in the preparation of synthetically useful building blocks, are presented. Examples of catalytic methodologies based on these tandem sequences are also provided. Finally, the asymmetric version of an inverse tandem [3,3]-rearrangement/Michael addition sequence oriented to the synthesis of optically active γ-substituted δ-amino acids is detailed. 1 Introduction 2 Michael Addition/[3,3]-Rearrangement Sequences 2.1 C-Nucleophiles 2.2 N-Nucleophiles 2.3 P-Nucleophiles 2.4 O-Nucleophiles 3 [3,3]-Rearrangement/Michael Addition Sequences 4 Conclusion

Direct allylic C–H alkylation of enol silyl ethers enabled by photoredox–Brønsted base hybrid catalysis

“…On the other hand, the allylic sp 3 -hybridized carbons of the enol silyl ethers are potential reaction sites as the electron-rich enol moiety contributes to decreasing bond-dissociation enthalpy of the allylic C–H bonds. In particular, selective bond formation at the allylic carbons 4 with preservation of the enol silyl ether component offers an opportunity to harness the reactivity of the resulting functionalized enol silyl ethers for the conventional polar reactions, enabling access to α,β-difunctionalized carbonyl compounds 5–7 . However, despite their potential synthetic utility, only a few catalytic systems are available for direct allylic C–H functionalization of enol silyl ethers or their analogs, which rely on transition metal catalysis and synergistic photoredox-thiol catalysis 8,9 .…”