Asymmetric synthesis of tetrabenazine and dihydrotetrabenazine

Chiral phosphoric acid-catalyzed enantioselective phosphinylation of 3,4-dihydroisoquinolines with diarylphosphine oxides

Chiral phosphorous-containing compounds are playing a more and more significant role in several different research fields. Here, we show a chiral phosphoric acid-catalyzed enantioselective phosphinylation of 3,4-dihydroisoquinolines with diarylphosphine oxides for the efficient and practical construction of a family of chiral α-amino diarylphosphine oxides with a diverse range of functional groups. The phosphine products are suitable for transforming to several kinds of chiral (thio)ureas, which might be employed as chiral ligands or catalysts with potential applications in asymmetric catalysis. Control and NMR tracking experiments show that the reaction proceeds via the tert-butyl 1-(tert-butoxy)-3,4-dihydroiso-quinoline-2(1H)-carboxylate intermediate, followed by C-P bond formation. Furthermore, computational studies elucidated that the hydrogen bonding strength between the phosphonate and isoquinolinium determines the stereoselectivity of the phosphinylation reaction.

Stereoselective Synthesis of Benzo[a]quinolizidines via Aerobic DDQ-Catalyzed Allylation and Reductive Cyclization

Stereoselective synthesis of C₄-substituted benzo[a]quinolizidines via redox-controlled catalytic C-C-bond-forming reactions was carried out. Aerobic DDQ-catalyzed allylation of N-Cbz tetrahydroisoquinolines efficiently provided α-allylated products 5, which were transformed to enones 6 via cross-metathesis reactions using the second-generation Hoveyda-Grubbs catalyst. Palladium-catalyzed hydrogenation of 6 prompted alkene reduction, protecting group removal, and intramolecular reductive amination in one step to afford the desired benzo[a]quinolizidines 7 as single diastereomers.

Synthesis of Tetrabenazine and Its Derivatives, Pursuing Efficiency and Selectivity

Tetrabenazine is a US Food and Drug Administration (FDA)-approved drug that exhibits a dopamine depleting effect and is used for the treatment of chorea in Huntington’s disease. Mechanistically, tetrabenazine binds and inhibits vesicular monoamine transporter type 2, which is responsible for importing neurotransmitters from the cytosol to the vesicles in neuronal cells. This transportation contributes to the release of neurotransmitters inside the cell to the synaptic cleft, resulting in dopaminergic signal transmission. The highly potent inhibitory activity of tetrabenazine has led to its advanced applications and in-depth investigation of prodrug design and metabolite drug discovery. In addition, the synthesis of enantiomerically pure tetrabenazine has been pursued. After a series of research studies, tetrabenazine derivatives such as valbenazine and deutetrabenazine have been approved by the US FDA. In addition, radioisotopically labeled tetrabenazine permits the early diagnosis of Parkinson’s disease, which is difficult to treat during the later stages of this disease. These applications were made possible by the synthetic efforts aimed toward the efficient and asymmetric synthesis of tetrabenazine. In this review, various syntheses of tetrabenazine and its derivatives have been summarized.

Rapid construction of tetrahydropyridine scaffolds via formal imino Diels-Alder reactions of Schiff bases and Nazarov reagents

Described is a one-flask, two-step method for the synthesis of highly functionalized piperidines. The process involves formal [4 + 2] cycloadditions of Schiff bases and Nazarov reagents, followed by facile elaborations of the initial cycloadducts. Notably, these aza-annulations are facilitated by protic solvents and proceed smoothly under ambient conditions, without other additives. The synthetic utility of this annulation protocol is further showcased through a concise, convergent synthesis of (±)-tetrabenazine.

Asymmetric Synthesis of Isoquinoline Alkaloids: 2004-2015

In the past decade, the asymmetric synthesis of chiral nonracemic isoquinoline alkaloids, a family of natural products showing a wide range of structural diversity and biological and pharmaceutical activity, has been based either on continuation or improvement of known traditional methods or on new, recently developed, strategies. Both diastereoselective and enantioselective catalytic methods have been applied. This review describes the stereochemically modified traditional syntheses (the Pictet-Spengler, the Bischler-Napieralski, and the Pomeranz-Fritsch-Bobbitt) along with strategies based on closing of the nitrogen-containing ring B of the isoquinoline core by the formation of bonds between C₁-N₂, N₂-C₃, C₁-N₂/N₂-C₃, and C₁-N₂/C₄-C_4a atoms. Methods involving introduction of substituents at the C1 carbon of isoquinoline core along with syntheses applying various biocatalytic techniques have also been reviewed.

Redox-Annulation of Cyclic Amines and β-Ketoaldehydes

Benzo[a]quinolizine-2-one derivatives are readily assembled from 1,2,3,4-tetrahydroisoquinoline and β-ketoaldehydes by means of a new intramolecular redox-Mannich process. These reactions are promoted by simple acetic acid and are thought to involve azomethine ylides as reactive intermediates.

Synthesis of (±)-Tetrabenazine by Visible Light Photoredox Catalysis

(±)-Tetrabenazine was synthesized in six steps from commercially available compounds. The key cyclization substrate was assembled rapidly via Baylis-Hillman and aza-Michael reactions. Annulation of the final ring was achieved through visible light photocatalysis, wherein carbon-carbon bond formation was driven by the oxidation of a tertiary amine. Solvent played a critical role in the photoredox cyclization outcome, whereas methanol led to a mixed ketal, acetonitrile/water (10:1) gave direct cyclization to (±)-tetrabenazine and occurred more rapidly.

PET radioligands for the vesicular transporters for monoamines and acetylcholine

The vesicular transporters for the monoamine and acetylcholine have been successfully targeted for the development of radioligands for human brain imaging. The vesicular monoamine transporter type 2 ligands are based on the structure of tetrabenazine, a known clinically used drug. In contrast, the radioligands for vesicular acetylcholine transporter are based on vesamicol, a toxic xenobiotic. The similarities and differences in the development of these two classes of radioligands are discussed.

A ring-closing metathesis-based approach to the synthesis of (+)-tetrabenazine

A modular stereoselective synthesis of the vesicular monoamine transport inhibitors (+)-tetrabenazine ((+)-1) and (+)-α-dihydrotetrabenazine ((+)-2) has been developed. The approach is based on amine 4 and acid 5 as the key building blocks, which were elaborated into macrolactam 3 by amide coupling and a subsequent highly E-selective RCM reaction. Macrolactam 3 could be converted into tetrabenazine in three known steps.

Preparation and evaluation of tetrabenazine enantiomers and all eight stereoisomers of dihydrotetrabenazine as VMAT2 inhibitors

Tetrabenazine (TBZ) ((±)-1) and dihydrotetrabenazines (DHTBZ) are potent inhibitors of VMAT2. Herein, a practical chemical resolution of (±)-1 and stereoselective synthesis of all eight DHTBZ stereoisomers are described. The result of VMAT2 binding assay revealed that (+)-1 (Ki=4.47 nM) was 8000-fold more potent than (-)-1 (Ki=36,400 nM). Among all eight DHTBZ stereoisomers, (2R,3R,11bR)-DHTBZ ((+)-2: Ki=3.96 nM) showed the greatest affinity for VMAT2. The (3R,11bR)-configuration appeared to play a key role for VMAT2 binding. In summary, (+)-1, (+)-2, and their derivatives warrant further studies in order to develop more potent and safer drugs for the treatment of chorea associated with Huntington's disease and other hyperkinetic disorders.

Synthesis and biological evaluation of 3-alkyl-dihydrotetrabenazine derivatives as vesicular monoamine transporter-2 (VMAT2) ligands

In the search of new probes for in vivo brain imaging of vesicular monoamine transporter type 2 (VMAT2), we have developed an efficient synthesis of a novel series of 3-alkyl-dihydrotetrabenazine (DTBZ) derivatives. The affinity of VMAT2 was evaluated by an in vitro inhibitory binding assay using [(125)I]-iodovinyl-TBZ or [(18)F](+)-FP-DTBZ as radioligands in rat striatal tissue homogenates. New DTBZ derivatives exhibited moderate to good binding affinity to VMAT2. Among these new ligands, compound 4b showed the best affinity for VMAT2 (K(i)=5.98 nM) and may be a useful lead compound for future structure-activity studies.

Preparation and Characterization of Tetrabenazine Enantiomers against Vesicular Monoamine Transporter 2

As a clinical medication for the treatment of hyperkinetic movement disorders, in conditions such as Huntington's disease, tetrabenazine (TBZ) has been always used in its racemic form. To establish whether or not its beneficial therapeutic actions are enantiospecific, a practical total synthetic route was developed to yield each enantiomeric form to allow their chemical and pharmacological characterization. We briefly summarize the total synthesis of TBZ and report a detailed procedure for resolution of TBZ into its enantiomers, (+)-TBZ and (-)-TBZ. This allowed determination of the optical rotation and absolute configurations of each TBZ enantiomer, based on X-ray crystallographic analysis, together with characterization of their inhibitory action at the vesicular monoamine transporter 2, where (+)-TBZ proved three-fold more active than (-)-TBZ.