Asymmetric construction of all-carbon quaternary stereocenters in the total synthesis of natural products

Biocatalytic Synthesis of Nitrile-Bearing All-Carbon Quaternary Stereocenters

The synthesis of all-carbon quaternary stereocenters containing nitriles is a very important and challenging subject in organic chemistry. We used a biocatalytic approach under mild conditions to obtain new derivatives of these scaffolds by oxidation of catechols by Myceliophthora thermophila laccase (Novozym 51003) to afford o-quinones and 1,4-addition of a series of carbon nucleophiles containing tertiary alkyle nitriles to these intermediates. Using this approach, α-cyano carbonyls bearing a quaternary stereocenter were also prepared. Finally, the yields for the prepared compounds were 72-94%.

Phosphine-Catalyzed Synthesis and Cytotoxic Evaluation of Michael Adducts of the Sesquiterpene Lactone Arglabin

A general method for chemo- and diastereoselective modification of anticancer natural product arglabin with nitrogen- and carbon-centered pronucleophiles under the influence of nucleophilic phosphine catalysts was developed. The locked s-cis-geometry of α-methylene-γ-butyrolactone moiety of arglabin favors for the additional stabilization of the zwitterionic intermediate by electrostatic interaction between phosphonium and enolate oxygen centers, leading to the unprecedentedly high efficiency of the phosphine-catalyzed Michael additions to this sesquiterpene lactone. Using n-Bu3P as the catalyst, pyrazole, phthalimide, 2-oxazolidinone, 4-quinazolinone, uracil, thymine, cytosine, and adenine adducts of arglabin were obtained. The n-Bu3P-catalyzed reaction of arglabin with active methylene compounds resulted in the predominant formation of bisadducts bearing a new quaternary carbon center. All synthesized Michael adducts and previously obtained phosphorylated arglabin derivatives were evaluated in vitro against eleven cancer and two normal cell lines, and the results were compared to those of natural arglabin and its dimethylamino hydrochloride salt currently used as anticancer drugs. 2-Oxazolidinone, uracil, diethyl malonate, dibenzyl phosphonate, and diethyl cyanomethylphosphonate derivatives of arglabin exhibited more potent antiproliferative activity towards several cancer cell lines and lower cytotoxicity towards normal cell lines in comparison to the reference compounds, indicating the feasibility of the developed methodology for the design of novel anticancer drugs with better therapeutic potential.

Cross-coupling of organic fluorides with allenes: a silyl-radical-relay pathway for the construction of α-alkynyl-substituted all-carbon quaternary centres

Controlling the transformation of versatile and reactive allenes is a considerable challenge. Herein, we report an efficient silylboronate-mediated cross-coupling reaction of organic fluorides with allenes to construct a series of sterically demanding α-ethynyl-containing all-carbon quaternary centers (ACQCs), using catalyst-free silyl-radical-relay reactions to selectively functionalize highly inert C-F bonds in organic fluorides. The key to the success of this transformation lies in the radical rearrangement of an in situ-generated allenyl radical to form a bulky tertiary propargyl radical; however, the transformation does not show efficiency when using the propargyl isomer directly. This unique reaction enables the cross-coupling of a tertiary carbon radical center with a C(sp2)-F bond or a benzylic C(sp3)-F bond. α-Ethynyl-containing ACQCs with (hetero)aromatic substituents and benzyl were efficiently synthesized in a single step using electronically and sterically diverse organic fluorides and allenes. The practical utility of this protocol is showcased by the late-stage functionalization of bioactive molecules and the modification of a liquid crystalline material.

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).

Lithium Enolate with a Lithium-Alkyne Interaction in the Enantioselective Construction of Quaternary Carbon Centers: Concise Synthesis of (+)-Goniomitine

We report a method for direct enantioselective alkylation of 3-alkynoic and 2,3-alkendioic acids that form quaternary stereogenic centers, and application of this method to the total enantioselective synthesis of a complex alkaloid (+)-goniomitine. The methods were effective in the alkylation of both 3-alkynoic acids, 2,3-alkendioic acids substrates with a broad range of heterocyclic and functionalized alkyl group substituents. Accompanying crystallographic studies provide mechanistic insight into the structure of well-defined chiral aggregates, highlighting cation-π interactions between lithium and alkyne groups.

Oxalates as Activating Groups for Tertiary Alcohols in Photoredox-Catalyzed gem-Difluoroallylation To Construct All-Carbon Quaternary Centers

Construction of challenging and important all-carbon quaternary centers has received growing attention. Herein, with oxalates as activating groups for tertiary alcohols, we report photoredox-catalyzed gem-difluoroallylation to construct all-carbon quaternary centers enabled by efficient tertiary radical addition to α-trifluoromethyl alkenes. This transformation shows good functional group tolerance for both α-trifluoromethyl alkenes and oxalates. Moreover, this strategy is also successfully applied to the synthesis of monofluoralkenes from the corresponding electron-rich gem-difluoroalkenes and cesium tertiary alkyl oxalates under modified conditions.

Total Synthesis of the Alleged Structure of (+)-Fimbricalyxoid A

An enantioselective total synthesis of the alleged structure of (+)-fimbricalyxoid A is reported. The synthetic strategy features a pyridine-N-oxidate-mediated S_N2' reaction to introduce an oxygen functionality at position C3 of the A-ring and a sequential three-step process via the cleavage of the C-O bonds and hemiketalization to form the 3,20-oxybridge. With this strategy, the target molecule was synthesized in 19% overall yield and 12 steps from our previously synthesized cis-fused octahydrophenanthrene (+)-6.

Stereospecific Construction of Quaternary Carbon Stereocenters from Quaternary Carbon Stereocenters

Organoaluminum species promote a smooth nucleophilic substitution at the quaternary carbon stereocenter of stereodefined polysubstituted cyclopropyl methyl phosphate with a complete inversion of configuration, even when more reactive functional groups are present. The regio- and diastereoselectivity of the substitution is attributed to the existence of a bicyclobutonium intermediate.

A Three-Step Process to Facilitate the Enantioselective Assembly of Cis-Fused Octahydrophenanthrenes with a Quaternary Stereocenter

A three-step process for the enantioselective assembly of cis-fused octahydrophenanthrenes with a quaternary stereocenter is reported. This synthetic strategy relies on a regioselective γ-alkylation, a one-pot sequence of asymmetric hydrogenation and oxidation, and an intramolecular enolate arylation to facilitate the rapid and enantioselective construction of cis-fused octahydrophenanthrene scaffolds with an arylated all-carbon quaternary stereocenter concisely and efficiently.

Rh(iii)-Catalyzed C–C coupling of unactivated C(sp3)–H bonds with iodonium ylides for accessing all-carbon quaternary centers

Rhodium-catalyzed inert C(sp3)–H activation/carbene insertion has been realized, leading to the construction of all-carbon quaternary centers.

Asymmetric Bromoaminocyclization and Desymmetrization of Cyclohexa-1,4-dienes through Anion Phase-Transfer Catalysis

The catalytic enantioselective desymmetrizing bromoaminocyclization of prochiral cyclohexa-1,4-dienes has been achieved by using chiral anion phase-transfer catalysis, providing a range of enantioenriched cis-3a-arylhydroindoles bearing an all-carbon quaternary stereocenter in good yields (up to 78%) and excellent enantioselectivities (up to 97% ee). Furthermore, the potential application of this methodology to natural product total synthesis was demonstrated by the asymmetric synthesis of (+)-Mesembrane.

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.

Nickel-catalyzed reductive benzylation of tertiary alkyl halides with benzyl chlorides and chloroformates

We report a Ni-catalyzed cross-electrophile coupling of benzyl chlorides and chloroformates with unactivated tertiary alkyl halides to forge the challenging benzylated all C(sp³)-quaternary carbon centers.

Asymmetric Synthesis of 1-Tetralones Bearing A Remote Quaternary Stereocenter through Rh-Catalyzed C-C Activation of Cyclopentanones

Herein, we describe the preparation of 1-tetralones bearing a remote quaternary stereocenter in a highly enantioselective manner. A sequence of Pd-catalyzed asymmetric 1,4-addition and Rh-catalyzed enantiospecific C-C/C-H activation delivers diverse 1-tetralones with a C4 quaternary stereocenter, which are prepared in good overall yields and high enantioselectivity.

Enantioselective synthesis of mixed 3,3'-bisindoles via a phosphine-catalyzed umpolung γ-addition of 3'-indolyl-3-oxindoles to allenoates

An enantioselective umpolung γ-addition reaction of 3'-indolyl-3-oxindoles to allenoates catalyzed by amino acid-derived bifunctional phosphine catalysts has been developed. A wide range of chiral mixed 3,3'-bisindole scaffolds containing an all-carbon quaternary stereogenic center were obtained in high yields and with excellent enantioselectivities. 3,3'-Bisindoles are valuable synthetic intermediates, the employment of which led to formal total syntheses of (+)-Chimonanthine, (+)-Folicanthine and (-)-Calycanthine, as well as facile creation of useful pyrrolidinoindoline core structure.

Lewis-Acid-Mediated Thiocyano Semipinacol Rearrangement of Allylic Alcohols for Construction of α-Quaternary Center β-Thiocyano Carbonyls

An electrophilic thiocyano semipinacol rearrangement of allylic alcohols has been achieved for the first time by using N-thiocyano-dibenzenesulfonimide (NTSI). This approach provides a direct, simple, and efficient strategy for the formation of thiocyano carbonyl compounds with moderate to excellent yields. Meanwhile, an all-carbon quaternary center was rapidly constructed. In addition, an asymmetric version of this tandem reaction was preliminarily investigated.

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.

Electrochemical Semipinacol Rearrangements of Allylic Alcohols: Construction of All-Carbon Quaternary Stereocenters

The first examples of electrochemical trifluoromethylation and sulfonylation/semipinacol rearrangements of allylic alcohols were developed using cheap and stable RSO₂Na (R = CF₃, Ph) as reagents. Various β-trifluoromethyl and sulfonated ketones were obtained in moderate to excellent yields. This strategy provides a facile, direct, and complementary approach to construct all-carbon quaternary stereocenters. In addition, the reaction has the advantages of being chemical oxidant-free and metal-free and has safe and mild reaction conditions.

Chemoselective direct reductive trifluoromethylation of amides: a flexible access to functionalized α-trifluoromethylamines

A one-pot method is reported for the direct transformation of secondary amides to α-trifluoromethylamines.

Recent advances on the total synthesis of alkaloids in mainland China

Alkaloids are a large family of natural products that mostly contain basic nitrogen atoms. Because of their intriguing structures and important functions, they have long been popular targets for synthetic organic chemists. China's chemists have made significant progress in the area of alkaloid synthesis over past decades. In this article, selected total syntheses of alkaloids from research groups in mainland China during the period 2011–16 are highlighted.

Gold-catalyzed diastereoselective domino dearomatization/ipso-cyclization/aza-Michael sequence: a facile access to diverse fused azaspiro tetracyclic scaffolds

A facile and diversity-oriented access to complex tetracyclic benzo[e]pyrrolo[2,3-c]indole-2,4,7(5H)-triones through a post-Ugi gold(i)-catalyzed domino dearomatization/ipso-cyclization/aza-Michael sequence is elaborated.

Enantioselective synthesis of tetrahydrocyclopenta[b]indole bearing a chiral quaternary carbon center via Pd(ii)–SPRIX-catalyzed C–H activation

Pd(ii)–SPRIX promotes the first highly enantioselective cyclization of 3-alkenylindoles into the corresponding indoles bearing a chiral quaternary carbon center via C–H activation and an assisting effect by the allyl substituent.