Short Enantioselective Formal Synthesis of (-)-Platencin

Recent Advances in the Construction of Quaternary Stereocenters via Palladium-Catalyzed Decarboxylative Asymmetric Allylic Alkylation

Palladium-catalyzed decarboxylative asymmetric allylic alkylation (DAAA) provides an efficient and powerful strategy to construct quaternary stereocenters, which are widely present in biologically active natural products and approved drugs. In this short review, we summarize recent developments (since 2018) in the facile synthesis of quaternary stereocenters via DAAA methods. Several representative examples of the use of DAAA strategies for the total synthesis of complex natural products further demonstrate its synthetic potential in the realm of organic and medicinal chemistry.

1 Introduction

2 Construction of Quaternary Stereocenters via Palladium Catalyzed DAAA

3 Construction of Quaternary Stereocenters via Pd-Catalyzed Interceptive DAAA

4 Application of DAAA in Natural Product Synthesis

5 Conclusion

A Low-Cost and Scalable Synthesis of a Cyclohexanone-Related Bifunctional Building Block

A practical route to 2-(2-(2-methyl-1,3-dioxolan-2-yl)ethyl)cyclohexan-1-one was developed, featuring the use of inexpensive starting materials/reagents and readily attainable reaction conditions. The overall transformation was achieved in 53% yield with one chromatographic purification via NaOH-mediated aldol condensation, ethylene glycol protection of the ketone group in the presence of HC(OEt)₃/concd HCl, saturation of the C=C bond and the benzene ring with Al-Ni alloy in aqueous KOH, and oxidation of the intermediate cyclohexanol with aqueous NaClO/TEMPO/KBr.

Synthesis of (+)-Panamonon B, 7-epi-Panamonon B, and Their (Z)-Isomers

(+)-Panamonon B was synthesized with the key quaternary center (of a predefined absolute configuration) installed using Stoltz asymmetric allylation. The C-5 ketone functionality and the cross-conjugated enone moiety in the side chain were introduced via a photosensitized [2+4] cycloaddition of singlet oxygen to diene silyl enol ether and an aldol condensation under the conditions of Sugiura, respectively. The ¹H and ¹³C NMR of the synthetic and natural samples were fully consistent with each other. However, because two samples showed opposite signs for optical rotations, they must be antipodes to one another. The synthesis also provided valuable chances to observe unexpected, yet rather intriguing, phenomena such as a bulky substituent in an axial position of a cyclohexane ring and (E)-and (Z)-isomers with opposite signs for optical rotations despite their identical stereogenic centers. The rare occurrence of a bulky substituent in an axial position of a cyclohexane ring is rationalized as a consequence of the presence of a quaternary center and formation of the five-membered lactone fused to the six-membered ring, while the so far unnoticed influence of C═C geometry on optical rotation is shown to be consistent with the information encapsulated in several discrete pairs of similar compounds retrieved from the literature.

Recent Advances in Enantioselective Pd-Catalyzed Allylic Substitution: From Design to Applications

This Review compiles the evolution, mechanistic understanding, and more recent advances in enantioselective Pd-catalyzed allylic substitution and decarboxylative and oxidative allylic substitutions. For each reaction, the catalytic data, as well as examples of their application to the synthesis of more complex molecules, are collected. Sections in which we discuss key mechanistic aspects for high selectivity and a comparison with other metals (with advantages and disadvantages) are also included. For Pd-catalyzed asymmetric allylic substitution, the catalytic data are grouped according to the type of nucleophile employed. Because of the prominent position of the use of stabilized carbon nucleophiles and heteronucleophiles, many chiral ligands have been developed. To better compare the results, they are presented grouped by ligand types. Pd-catalyzed asymmetric decarboxylative reactions are mainly promoted by PHOX or Trost ligands, which justifies organizing this section in chronological order. For asymmetric oxidative allylic substitution the results are grouped according to the type of nucleophile used.

The Allylic Alkylation of Ketone Enolates

The palladium-catalyzed allylic alkylation of non-stabilized ketone enolates was thought for a long time to be not as efficient as the analogous reactions of stabilized enolates, e. g. of malonates and β-ketoesters. The field has experienced a rapid development during the last two decades, with a range of new, highly efficient protocols evolved. In this review, the early developments as well as current methods and applications of palladium-catalyzed ketone enolate allylations will be discussed.

Probing Trends in Enantioinduction via Substrate Design: Palladium-Catalyzed Decarboxylative Allylic Alkylation of α-Enaminones

Herein, we report the palladium-catalyzed decarboxylative asymmetric allylic alkylation of α-enaminones. In addition to serving as valuable synthetic building blocks, we exploit the α-enaminone scaffold and its derivatives as probes to highlight structural and electronic factors that govern enantioselectivity in this asymmetric alkylation reaction. Utilizing the (S)-t-BuPHOX ligand in a variety of nonpolar solvents, the alkylated products are obtained in up to 99% yield and 99% enantiomeric excess.