Total Synthesis of Mesembrine - The Construction of Quaternary Stereocenters by Gold-Catalyzed Diyne Desymmetrization

Palladium-catalyzed functionalizations of acidic and non-acidic C(sp3)-H bonds - recent advances

A tremendous upsurge has been seen in the recent decade for the proximal and remote functionalization of activated and unactivated substrates via palladium redox pathways. This feature article discusses some of the recent reports on direct as well as indirect C(sp3)-H functionalization via cross-coupling reactions under palladium catalysis. Activated substrates (possessing acidic C(sp3)-H) including enones, ketones, aldehydes, silylenol ethers, esters, silyl ketene acetals, amides, cyano, α-amino esters, and O-carbamates, capable of undergoing cross-coupling reactions at the α-, β-, γ-, δ- and ε-positions, will be discussed. To overcome the challenging task of achieving regioselectivity, a variety of innovative modifications have been reported. The reports of C-H activations based on directing group, and as native functionality have been illustrated at the β-, γ- and δ-positions. Substrates such as α-amino esters, carbonyls, carboxylic acids and their derivatives, afford site-selective C(sp3)-H functionalization via varied-sized reactive metallacycles and are a unique class of substrates whose C(sp3)-H functionalizations were earlier considered as very difficult.

Gold-Catalyzed Synthesis of Small Rings

Three- and four-membered rings, widespread motifs in nature and medicinal chemistry, have fascinated chemists ever since their discovery. However, due to energetic considerations, small rings are often difficult to assemble. In this regard, homogeneous gold catalysis has emerged as a powerful tool to construct these highly strained carbocycles. This review aims to provide a comprehensive summary of all the major advances and discoveries made in the gold-catalyzed synthesis of cyclopropanes, cyclopropenes, cyclobutanes, cyclobutenes, and their corresponding heterocyclic or heterosubstituted analogs.

Palladium-Mediated Remote Functionalization in γ- and ε-Arylations and Alkenylations of Unblocked Cyclic Enones

We report herein an extensive investigation of simple and regioselective endo- as well as exo-γ-arylations of silyl-dienol ethers of unblocked cyclic enones with the utilization of palladium-catalyzed, modified Kuwajima-Urabe conditions. We have also successfully explored a new exo-ε-arylation of silyl-trienol ethers of π-extended cyclic enones. In addition, we also report, herein, exclusive γ- and ε-alkenylation of silyl-dienol and silyl-trienol ethers of cyclic enones.

Mesembrine: The archetypal psycho-active Sceletium alkaloid

(-)-Mesembrine is a chiral alkaloid that features an aryloctahydroindole skeleton and is most commonly found in species of the succulent genus Sceletium. Several Sceletium species are used by various ethnic groups in South Africa to manage disorders of the central nervous system. Binding assays have revealed that mesembrine is a more potent inhibitor of the serotonin transporter (SERT) than fluoxetine (Prozac) which has prompted the commercialization of mesembrine-containing consumer products. The congested all carbon quaternary stereocenter present at the bridgehead of mesembrine has rendered it a compound of interest for research in synthetic chemistry, which has assisted in the absolute configuration of the naturally occurring isomer to be assigned. Accordingly, this review will cover the recent literature pertaining to the distribution, structural elucidation, chemotaxonomy, biosynthesis, organic synthesis, as well as the biological activities of (-)-mesembrine. Recent synthetic procedures of the non-natural enantiomer as well as the racemate are also considered.

Construction of Acyclic Quaternary Carbon Stereocenters by Catalytic Asymmetric Hydroalkynylation of Unactivated Alkenes

Quaternary carbon stereocenters are common structural motifs in organic synthesis. The construction of these stereocenters in a catalytic and enantioselective manner remains a prominent synthetic challenge. In particular, methods for the synthesis of alkyne-substituted quaternary carbon stereocenters are very rare. Previous catalytic systems for hydroalkynylation of alkenes create tertiary stereocenters. We describe here an iridium catalyzed asymmetric hydroalkynylation of nonactivated trisubstituted alkene. The hydroalkynylation of β,γ-unsaturated amides occurs with high regio- and enantioselectivities to afford alkyne-substituted acyclic quaternary carbon stereocenters. Computational and experimental data suggest that the enantioselectivity is not only determined by the facial selectivity of the alkene but also by an alkene isomerization process. This strategy provides an efficient method to access alkyne-substituted acyclic quaternary carbon stereocenters with minimally functionalized starting materials.