Integrated Batch and Continuous Flow Process for the Synthesis of Goniothalamin

Rapid plugged flow synthesis of nucleoside analogues via Suzuki-Miyaura coupling and heck Alkenylation of 5-Iodo-2'-deoxyuridine (or cytidine)

Nucleosides modification via conventional cross-coupling has been performed using different catalytic systems and found to take place via long reaction times. However, since the pandemic, nucleoside-based antivirals and vaccines have received widespread attention and the requirement for rapid modification and synthesis of these moieties has become a major objective for researchers. To address this challenge, we describe the development of a rapid flow-based cross-coupling synthesis protocol for a variety of C5-pyrimidine substituted nucleosides. The protocol allows for facile access to multiple nucleoside analogues in very good yields in a few minutes compared to conventional batch chemistry. To highlight the utility of our approach, the synthesis of an anti-HSV drug, BVDU was also achieved in an efficient manner using our new protocol.

Graphical abstract

Supplementary Information

The online version contains supplementary material available at 10.1007/s41981-023-00265-1.

Development of a Fully Continuous-Flow Approach Towards Asymmetric Total Synthesis of Tetrahydroprotoberberine Natural Alkaloids

Continuous flow synthetic technologies had been widely applied in the total synthesis in the past few decades. Fully continuous flow synthesis is still extremely focused on multi-step synthesis of complex natural pharmaceutical molecules. Thus, the development of fully continuous flow total synthesis of natural products is in demand but challenging. Herein, we demonstrated the first fully continuous flow approach towards asymmetric total synthesis of natural tetrahydroprotoberberine alkaloids, (-)-isocanadine, (-)-tetrahydropseudocoptisine, (-)-stylopine and (-)-nandinine. This method features a concise linear sequence involving four chemical transformations and three on-line work-up processing in an integrated flow platform, without any intermediate purification. The overall yield and enantioselectivity of this four-step continuous flow chemistry were up to 50 % and 92 %ee, respectively, in a total residence time of 32.5 min, corresponding to a throughput of 145 mg/h.

Annulation-retro-Claisen cascade of bifunctional peroxides for the synthesis of lactone natural products

A new and highly efficient annulation-retro-Claisen cascade, which involves the [4 + 1] or [5 + 1] annulation of α-benzoylacetates with bielectrophilic peroxides and a subsequent debenzoylation process under mild basic conditions, has been developed for the rapid construction of valuable tetrahydrofuran- and dihydropyran-2-carboxylates in good yields. By employing the new reaction, the unified total synthesis of γ- and δ-lactone natural products such as (±)-tanikolide, (±)-goniothalamins, (±)-7-epi-goniodiol, and (±)-plakolide A has been accomplished in 4-7 steps.

Recent advances in continuous flow synthesis of heterocycles

In the current scenario, flow chemistry is emerging as a significant technology in the field of organic synthesis. This miniaturized protocol including microreactors facilitates excellent heat transfer, low solvent wastage, lesser reaction time, a safer environment for reagent handling and appreciable yields of desired products. Thus, this "enabling technology" has a great scope in the synthesis and preparation of a variety of heterocycles that require toxic reagents as starting materials. This review discusses the recent advances (2020-2021) in continuous flow strategy for synthesis and derivatization of variety of heterocyclic entities, of different ring size, using different approaches. This also highlights the advantages of different combined techniques like Microwave assisted heating, electrochemical flow cell, LED light source, NMR and FT-IR analysis, etc., that enables utilization of various mechanisms and real-time monitoring of reactions leading to improved results.

Gram-Scale Domino Synthesis in Batch and Flow Mode of Azetidinium Salts

Azetidinium salts are important motifs in organic synthesis but are difficult to obtain due to extremely long synthetic protocols. Herein, a rapid continuous-flow process for the on-demand synthesis of azetidinium salts is described. In particular, the nucleophilic addition of secondary amines and the subsequent intramolecular N-cyclization have been investigated in batch and continuous-flow modes, exploring the effects of solvent type, temperature, reaction time, and amine substituent on the synthesis of azetidinium salts. This has enabled us to quickly identify optimal reaction conditions and obtain microkinetic parameters, verifying that the use of a flow reactor leads to a reduction of the activation energy for the epichlorohydrin aminolysis due to the better control of mass and heat transfer during reaction. This confirms the key role of continuous-flow technologies to affect the kinetics of a reaction and make synthetic protocols ultrarapid and more efficient.

Flow-Assisted Synthesis of Alkyl Citrate Natural Products

The development of a flow-assisted synthesis of alkyl citrate natural products is described. The flow route harnesses a number of steps including the generation of ketene silyl acetal, a formal [2 + 2] cycloaddition, and a methanolysis cascade to efficiently generate a highly substituted, and stereodefined tetrahydrofuran intermediate. A heterogeneous pseudo-Finkelstein reaction and zinc-mediated elimination furnish a key alkene alkyl citrate fragment in high yield over a multistep sequence that provides direct entry to compounds such as (-)-CJ-13982 (1), (-)-CJ-13,981 (2), L-731,120 (3), and related natural products. The flow methodology developed in this study enables a new machine-assisted approach toward the efficient and scalable synthesis of the alkyl citrate family of natural products.

Efficient synthesis of diverse C-3 monodesmosidic saponins by a continuous microfluidic glycosylation/batch deprotection method

Triterpene and steroid saponins have various pharmacological activities but the synthesis of C-3 monodesmosidic saponins remains challenging. Herein, a series of C-3 glycosyl monodesmosidic saponins was synthesized via the microfluidic glycosylation of triterpenoids or steroids at the C-3 position, without the formation of orthoester byproducts, and subsequent deprotection of the benzoyl (Bz) group. This microfluidic glycosylation/batch deprotection sequence enabled the efficient synthesis of C-3 saponins with fewer purification steps and a shorter reaction time than conventional batch synthesis and stepwise microfluidic glycosylation. Furthermore, this system minimized the consumption of the imidate donor. Using this reaction system, 18 different C-3 saponins and 13 different C-28-benzyl-C-3 saponins, including 8 new compounds, were synthesized from various sugars and triterpenes or steroids. Our synthetic approach is expected to be suitable for further expanding the C-3 saponin library for pharmacological studies.

Development of Continuous Flow Systems to Access Secondary Amines Through Previously Incompatible Biocatalytic Cascades

A key aim of biocatalysis is to mimic the ability of eukaryotic cells to carry out multistep cascades in a controlled and selective way. As biocatalytic cascades get more complex, reactions become unattainable under typical batch conditions. Here a number of continuous flow systems were used to overcome batch incompatibility, thus allowing for successful biocatalytic cascades. As proof-of-principle, reactive carbonyl intermediates were generated in situ using alcohol oxidases, then passed directly to a series of packed-bed modules containing different aminating biocatalysts which accordingly produced a range of structurally distinct amines. The method was expanded to employ a batch incompatible sequential amination cascade via an oxidase/transaminase/imine reductase sequence, introducing different amine reagents at each step without cross-reactivity. The combined approaches allowed for the biocatalytic synthesis of the natural product 4O-methylnorbelladine.

Development of Continuous Flow Systems to Access Secondary Amines Through Previously Incompatible Biocatalytic Cascades*

A key aim of biocatalysis is to mimic the ability of eukaryotic cells to carry out multistep cascades in a controlled and selective way. As biocatalytic cascades get more complex, reactions become unattainable under typical batch conditions. Here a number of continuous flow systems were used to overcome batch incompatibility, thus allowing for successful biocatalytic cascades. As proof-of-principle, reactive carbonyl intermediates were generated in situ using alcohol oxidases, then passed directly to a series of packed-bed modules containing different aminating biocatalysts which accordingly produced a range of structurally distinct amines. The method was expanded to employ a batch incompatible sequential amination cascade via an oxidase/transaminase/imine reductase sequence, introducing different amine reagents at each step without cross-reactivity. The combined approaches allowed for the biocatalytic synthesis of the natural product 4O-methylnorbelladine.