Generation of all carbon quaternary stereocenters at the C-3 carbon of piperidinones and pyrrolidinones and its application in natural product total synthesis

Chemoselective Intramolecular Morita-Baylis-Hillman Reaction; Acrylamide and Ketone as Sluggish Reacting Partners on a Labile Framework

Chemoselectivity is an important issue frequently encountered while working over labile precursors. Carbonyl compounds with a heteroatom at the β carbon are sensitive precursors because they are prone to elimination under different conditions. Morita-Baylis-Hillman (MBH) reaction, although a widespread method for C-C bond formation, has its own limitations. Acrylamide and ketone are such limitations of the MBH reaction. Using them together for an intramolecular MBH (IMBH) reaction on a labile framework prone to elimination is a significant 2-fold synthetic challenge. A highly chemoselective IMBH reaction on such precursors has been established using 1,4-diazabicyclo[2.2.2]octane (DABCO) as a promoter. The protocol leads to quick access to a diversely substituted and functionalized piperidone framework in high yields. Various substitution patterns in the form of 34 successful examples have been studied. A diastereoselective version and tolerance to various functional and protecting groups are the added advantages of the developed methodology. A tertiary carbon at the β position of ketone, however, led to complete reversal of selectivity and gave only the elimination product. Control experiments toward a better understanding of the substitution pattern, role of catalyst, and mechanistic study have been carried out. As an application of the IMBH adduct, a one-step allylic rearrangement for the dihydropyridone framework has also been demonstrated.

Chiral Aminoalcohol-Derived δ-Lactams Provide Easy Access to Piperidines and Acyclic Five-Carbon Building Blocks Bearing a Tertiary and a Quaternary Stereocenter

A procedure for the synthesis of enantiopure piperidines and acyclic building blocks (5-aminopentanols, O-protected 5-hydroxypentanenitriles) containing a tertiary and a quaternary stereocenter has been developed. Starting from a phenylglycinol- or aminoindanol-derived δ-lactam bearing an alkyl substituent at the α-position of the N,O-acetal carbon, easily accessible by a cyclocondensation reaction, the stereoselective dialkylation at the carbonyl α-position generates the quaternary stereocenter and the subsequent two-step reductive removal of the chiral inductor provides enantiopure 3,3,5-trisubstituted piperidines. Alternatively, the simultaneous reductive opening of the oxazolidine and piperidone rings of the dialkylated lactams followed by reductive or oxidative cleavage of the chiral inductor opens access to chiral 2,2,4-trisubstituted 5-amino-1-pentanols or 2,4,4-trisubstituted 5-hydroxypentanenitriles.

Finding activity through rigidity: syntheses of natural products containing tricyclic bridgehead carbon centers

Covering: up to 2022Tricyclic bridgehead carbon centers (TBCCs) are a synthetically challenging substructure found in many complex natural products. Here we review the syntheses of ten representative families of TBCC-containing isolates, with the goal of outlining the strategies and tactics used to install these centers, including a discussion of the evolution of the successful synthetic design. We provide a summary of common strategies to inform future synthetic endeavors.

Regio- and Enantioselective Catalytic δ-C-H Amidation of Dioxazolones Enabled by Open-Shell Copper-Nitrenoid Transfer

Controlling regio- and enantioselectivity in C-H functionalization reactions is of paramount importance due to their versatile synthetic utilities. Herein, we describe a new approach for the asymmetric δ-C(sp³)-H amidation catalysis of dioxazolones using a Cu(I) precursor with a chiral bisoxazoline ligand to access six-membered lactams with high to excellent regio- and enantioselectivity (up to >19:1 rr and >99:1 er). Combined experimental and computational mechanistic studies unveiled that the open-shell character of the postulated Cu-nitrenoids enables the regioselective hydrogen atom abstraction and subsequent enantio-determining radical rebound of the resulting carbon radical intermediates. The synthetic utility of this asymmetric cyclization was demonstrated in the diastereoselective introduction of additional functional groups into the chiral δ-lactam skeleton as well as in the rapid access to biorelevant azacyclic compounds.

Biocatalytic stereoselective synthesis of pyrrolidine-2,3-diones containing all-carbon quaternary stereocenters

Highly functionalized pyrrolidine-2,3-diones can be synthesized efficiently and stereoselectively under mild conditions using a biocatalytic approach. The reaction led to the formation of new all-carbon quaternary stereocenters from Myceliophthora thermophila laccase (Novozym 51003) catalyzed oxidation of catechols to ortho-quinones and subsequent 1,4-addition with 3-hydroxy-1,5-dihydro-2H-pyrrol-2-ones. The reaction was conducted with various substituents on both reactants, resulting in 13 products in moderate to good yields (42-91%). The same 15 reactions were also tested with K₃Fe(CN)₆ as a catalyst, but here only one reaction resulted in a product (60% yield).

Nickel-Catalyzed Enantioselective Reductive Alkyl-Carbamoylation of Internal Alkenes

Herein, we leverage the Ni-catalyzed enantioselective reductive dicarbofunctionalization of internal alkenes with alkyl iodides to enable the synthesis of chiral pyrrolidinones bearing vicinal stereogenic centers. The application of newly developed 1-Nap Quinim is critical for formation of two contiguous stereocenters in high yield, enantioselectivity, and diastereoselectivity. This catalytic system also improves both the yield and enantioselectivity in the synthesis of α,α-dialkylated γ-lactams. Computational studies reveal that the enantiodetermining step proceeds with a carbamoyl-NiI intermediate that is reduced by the Mn reductant prior to intramolecular migratory insertion. The presence of the t-butyl group of the Quinim ligand leads to an unfavorable distortion of the substrate in the TS that leads to the minor enantiomer. Calculations also support an improvement in enantioselectivity with 1-Nap Quinim compared to p-tol Quinim.

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

Cyclocarbopalladation/Stille Cascade: Stereoselective Access to Quaternary Functionalized Carbons

Exploration of the 4-exo-dig cyclocarbopalladation in the discovery of new and original scaffolds afforded some unexpected results. The search for a way to produce seven-membered ring systems led to polycyclic molecules bearing a tetrasubstituted carbon. The triple bond that substitutes the cyclohexene ring on the starting compound is crucial for a high stereoselectivity. This observation has been confirmed by the reaction of a nonsubstituted cyclohexene ring resulting in poor stereoselectivity and low yields.

Quaternary stereocentres via catalytic enantioconvergent nucleophilic substitution reactions of tertiary alkyl halides

The development of efficient methods, particularly catalytic and enantioselective processes, for the construction of all-carbon quaternary stereocentres is an important (and difficult) challenge in organic synthesis, due to the occurrence of this motif in a range of bioactive molecules. One conceptually straightforward and potentially versatile approach is the catalytic enantioconvergent substitution reaction of a readily available racemic tertiary alkyl electrophile by an organometallic nucleophile; however, examples of such processes are rare. In this report, we demonstrate that a nickel-based chiral catalyst achieves enantioconvergent couplings of a variety of tertiary electrophiles (cyclic and acyclic α-halocarbonyl compounds) with alkenylmetal nucleophiles to form quaternary stereocentres with good yield and enantioselectivity under mild conditions in the presence of a range of functional groups. These couplings, which likely proceed via a radical pathway, provide access to an array of useful families of organic compounds, including intermediates in the total synthesis of two natural products, (−)-eburnamonine and madindoline A.

Straightforward access to densely substituted chiral succinimides through enantioselective organocatalyzed Michael addition of α-alkyl-cyclic ketones to maleimides

A simple, cheap and unchanged organocatalytic system provides efficient access to densely substituted chiral succinimides bearing QCC–TCC vicinal stereocenters from unreactive α-alkyl cyclic ketones and maleimides.

Axial-to-Central Chirality Transfer for Construction of Quaternary Stereocenters via Dearomatization of BINOLs

All-carbon quaternary stereocenters are versatile building blocks, and their asymmetric construction has attracted much attention. Herein, we disclose an axial-to-central chirality transfer strategy for the synthesis of chiral quaternary stereocenters via dearomatization of (S)-BINOLs. The reaction proceeded smoothly with a wide range of propargyl carbonates to afford chiral spiro-compounds in high yields with excellent enantioselectivities. In addition, the strategy was extended to kinetic resolution of rac-BINOLs albeit with moderate s value.

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.