A Local-Desymmetrization-Based Divergent Synthesis of Quinine and Quinidine

Organocatalyst-mediated five-pot synthesis of (-)-quinine

In this work, the enantioselective total synthesis of (-)-quinine has been accomplished in a pot-economical manner using five reaction vessels. In the first pot, reactions involve the diphenylprolinol silyl ether-mediated Michael reaction, aza-Henry reaction, hemiaminalization, and elimination of HNO₂ (five reactions), affording a chiral tetrahydropyridine with excellent enantioselectivity. In the second pot, five reactions proceed with excellent diastereoselectivity to afford a trisubstituted piperidine with the desired stereochemistry. A further five reactions are carried out in the last one-pot sequence.

Symmetry-Driven Total Synthesis of Myrioneurinol

We report a total synthesis of the Myrioneuron alkaloid myrioneurinol enabled by the recognition of hidden symmetry within its polycyclic structure. Our approach traces myrioneurinol's complex framework back to a symmetrical diketone precursor, a double reductive amination of which forges its central piperidine unit. By employing an inexpensive chiral amine in this key desymmetrizing event, four stereocenters of the natural product including the core quaternary stereocenter are set in an absolute sense, providing the first asymmetric entry to this target. Other noteworthy strategic maneuvers include utilizing a bicyclic alkene as a latent cis-1,3-bis(hydroxymethyl) synthon and a topologically controlled alkene hydrogenation. Overall, our synthesis proceeds in 18 steps and ∼1% yield from commercial materials.

Cu-Catalyzed Asymmetric 1,3-Dipolar Cycloaddition of N-2,2,2-Trifluoroethylisatin Ketimines Enables the Desymmetrization of N-Arylmaleimides: Access to Enantioenriched F3C-Containing Octahydropyrrolo[3,4-c]pyrroles

With a Cu(OTf)₂/chiral ferrocenyl P,N-ligand complex as a catalyst, the enantioselective desymmetrization of N-arylmaleimides was successfully realized by taking advantage of the asymmetric 1,3-dipolar cycloaddition reaction of N-2,2,2-trifluoroethylisatin ketimines. A series of structurally diverse F₃C-containing octahydropyrrolo[3,4-c]pyrroles, bearing four contiguous carbon stereocenters and one stereogenic chiral C-N axial bond, were obtained with excellent results (≤99% yield, >20:1 dr, and 99% ee).

Early and Late Steps of Quinine Biosynthesis

The enzymatic basis for quinine 1 biosynthesis was investigated. Transcriptomic data from the producing plant led to the discovery of three enzymes involved in the early and late steps of the pathway. A medium-chain alcohol dehydrogenase (CpDCS) and an esterase (CpDCE) yielded the biosynthetic intermediate dihydrocorynantheal 2 from strictosidine aglycone 3. Additionally, the discovery of an O-methyltransferase specific for 6′-hydroxycinchoninone 4 suggested the final step order to be cinchoninone 16/17 hydroxylation, methylation, and keto-reduction.

Total Synthesis of (-)-Batrachotoxinin A: A Local-Desymmetrization Approach

An enantioselective total synthesis of (-)-batrachotoxinin A is accomplished based on a key photoredox coupling reaction and the subsequent local-desymmetrization operation. After the expedient assembly of the highly oxidized steroid skeleton, a delicate sequence of redox manipulations was carried out to deliver a late-stage intermediate on gram scale-and ultimately (-)-batrachotoxinin A in an efficient manner.

Enantioselective total synthesis of the unnatural enantiomer of quinine

A practical enantioselective total synthesis of the unnatural (+)-quinine and (-)-9-epi-quinine enantiomers, which are important organocatalysts, is reported. The key transformation is a successive organocatalytic formal aza [3 + 3] cycloaddition/Strecker-type cyanation reaction to form an optically active tetrasubstituted piperidine derivative. This organocatalytic reaction proceeded in high yield and gave excellent enantiomeric excess with only 0.5 mol% catalyst loading. In addition, an imidate group, derived from a cyano group, was incorporated in the strategy for site-selective modification of the C4-alkyl chiral piperidine ring of quinine. Furthermore, an efficient coupling between the quinuclidine precursor and dihydroquinoline unit was achieved on a gram scale. The 15-step (LLS) synthetic protocol provided both (+)-quinine and (-)-9-epi-quinine, each with 16% overall yield.

Indole Alkaloid Synthesis Facilitated by Photoredox Catalytic Radical Cascade Reactions

The monoterpene indole alkaloids, containing over 3000 known members and more than 40 structural types, represent one of the largest natural product families that have proven to be an important drug source. Their complex chemical structures and significant biological activities have rendered these alkaloids attractive targets in the synthetic community for decades. While chemists have developed many synthetic methodologies and tactics toward this end, general strategies allowing divergent access to a large variety of structural types and members of monoterpene indole alkaloids are still limited and highly desirable. Photoredox catalysis has emerged in recent years as a powerful tool to realize chemical transformations via single electron transfer (SET) processes that would otherwise be inaccessible. In particular, when the radical species generated by the visible light photoinduced approach is involved in well-designed cascade reactions, the formation of multiple chemical bonds and the assembly of structurally complex molecules would be secured in a green and economic manner. This protocol might serve to remodel the way of thinking for the preparation of useful pharmaceuticals and complex natural products. Due to a long-standing interest in the synthesis of diverse indole alkaloids, our group previously developed a cyclopropanation strategy ( Qin , Y. Acc. Chem. Res. 2011 , 44 , 447 ) that was versatile to access several intriguing indole alkaloid molecules. With an idea of developing more general synthetic approaches to as many members of various indole alkaloids as possible, we recently disclosed new radical cascade reactions enabled by photoredox catalysis, leading to the collective asymmetric total synthesis of 42 monoterpene indole alkaloids belonging to 7 structural types. Several important discoveries deserve to be highlighted. First, the use of photocatalytic technology allowed us to achieve an unusual reaction pathway that reversed the conventional reactivity between two nucleophilic amine and enamine groups. Second, a crucial nitrogen-centered radical, directly generated from a sulfonamide N-H bond, triggered three types of cascade reactions to deliver indole alkaloid cores with manifold functionalities and controllable diastereoselectivities. Moreover, expansion of this catalytic, scalable, and general methodology permitted the total synthesis of a large collection of indole alkaloids. In this Account, we wish to provide a complete picture of our studies concerning the original synthetic design, method development, and applications in total synthesis. It is anticipated that the visible-light-driven cascade strategy will find further utility in the realm of natural product synthesis.