Regioselective alkylation of the exocyclic nitrogen of adenine and adenosine by the Mitsunobu reaction

Recent Advances in the Mitsunobu and Related Reactions: A Review from 2010 to 2024

This review discusses recent progress in the most significant synthetic approaches involving transformations under the Mitsunobu reaction. The Mitsunobu reaction entails the "redox" condensation of an acidic pronucleophile 'Nu-H' and an electrophilic primary or secondary alcohol, facilitated by stoichiometric amounts of phosphines and azodicarboxylate reagents. Widely utilized for dehydrative oxidation-reduction condensation, this reaction shows synthetic utility through its tolerance of a broad range of acidic pronucleophiles, including carboxylic acids, pro-imides, hydroxamates, phenols, thiols, fluorinated alcohols, oximes, thioamides, pyridinium and imidazolium salts, pyrimidine bases, α-ketoesters, and trimethylmethane tricarboxylate, thereby yielding a variety of functional and potentially biologically active compounds. The purpose of this review is to focus on recent advances and applications of Mitsunobu reaction chemistry, particularly from 2010 to 2024. In addition to discussing newer reagents that facilitate purification, we will describe contemporary applications of this chemistry, especially concerning the synthesis of potential biological compounds and their precursors. This focus review of the Mitsunobu reaction summarizes its origins, the current understanding of its mechanism, and recent improvements and applications. We aim for this work to serve as a useful resource for scientists working in this research domain.

Synthesis of N6-dA Damaged DNAs to Probe the Replication Ability of Human Translesion Polymerases

The DNA adducts formed by the alkenylbenzene natural products, safrole (SF) and methyleugenol (MEG) are primarily attributed to their reported carcinogenic properties. Herein, we report a concise strategy to access N6-Ac-SF/MEG-dA phosphoramidites, which were selectively incorporated into DNA oligonucleotides by solid-phase DNA synthesis. The replication studies using human polymerases hpolκ and hpolη showed that both polymerases replicate these adducts error-free, which indicates that these polymerases do not contribute to the adduct-induced mutagenicity.

Scaffold hopping from indoles to indazoles yields dual MCL-1/BCL-2 inhibitors from MCL-1 selective leads

Overexpression of the anti-apoptotic BCL-2 proteins is associated with the development and progression of a range of cancers. Venetoclax, an FDA-approved BCL-2 inhibitor, is fast becoming the standard-of-care for acute myeloid leukemia and chronic lymphocytic leukemia. However, the median survival offered by venetoclax is only 18 months (as part of a combination therapy regimen), and one of the primary culprits for this is the concomitant upregulation of sister anti-apoptotic proteins, in particular MCL-1 (and BCL-xL), which provides an escape route that manifests as venetoclax resistance. Since inhibition of BCL-xL leads to thrombocytopenia, we believe that a dual MCL-1/BCL-2 inhibitor may provide an enhanced therapeutic effect relative to a selective BCL-2 inhibitor. Beginning with a carboxylic acid-containing literature compound that is a potent inhibitor of MCL-1 and a moderate inhibitor of BCL-2, we herein describe our efforts to develop dual inhibitors of MCL-1 and BCL-2 by scaffold hopping from an indole core to an indazole framework. Subsequently, further elaboration of our novel N2-substituted, indazole-3-carboxylic acid lead into a family of indazole-3-acylsulfonamides resulted in improved inhibition of both MCL-1 and BCL-2, possibly through occupation of the p4 pocket, with minimal or no inhibition of BCL-xL.

Ex Vivo Feedback Control of Neurotransmission Using a Photocaged Adenosine A1 Receptor Agonist

We report the design, synthesis, and validation of the novel compound photocaged N⁶-cyclopentyladenosine (cCPA) to achieve precisely localized and timed release of the parent adenosine A1 receptor agonist CPA using 405 nm light. G_i protein-coupled A₁ receptors (A₁Rs) modulate neurotransmission via pre- and post-synaptic routes. The dynamics of the CPA-mediated effect on neurotransmission, characterized by fast activation and slow recovery, make it possible to implement a closed-loop control paradigm. The strength of neurotransmission is monitored as the amplitude of stimulus-evoked local field potentials. It is used for feedback control of light to release CPA. This system makes it possible to regulate neurotransmission to a pre-defined level in acute hippocampal brain slices incubated with 3 µM cCPA. This novel approach of closed-loop photopharmacology holds therapeutic potential for fine-tuned control of neurotransmission in diseases associated with neuronal hyperexcitability.

Synthesis of Triazole-Linked SAM-Adenosine Conjugates: Functionalization of Adenosine at N-1 or N-6 Position without Protecting Groups

More than 150 RNA chemical modifications have been identified to date. Among them, methylation of adenosine at the N-6 position (m⁶A) is crucial for RNA metabolism, stability and other important biological events. In particular, this is the most abundant mark found in mRNA in mammalian cells. The presence of a methyl group at the N-1 position of adenosine (m¹A) is mostly found in ncRNA and mRNA and is mainly responsible for stability and translation fidelity. These modifications are installed by m⁶A and m¹A RNA methyltransferases (RNA MTases), respectively. In human, deregulation of m⁶A RNA MTases activity is associated with many diseases including cancer. To date, the molecular mechanism involved in the methyl transfer, in particular substrate recognition, remains unclear. We report the synthesis of new SAM-adenosine conjugates containing a triazole linker branched at the N-1 or N-6 position of adenosine. Our methodology does not require protecting groups for the functionalization of adenosine at these two positions. The molecules described here were designed as potential bisubstrate analogues for m⁶A and m¹A RNA MTases that could be further employed for structural studies. This is the first report of compounds mimicking the transition state of the methylation reaction catalyzed by m¹A RNA MTases.

Pronounced Inhibition Shift from HIV Reverse Transcriptase to Herpetic DNA Polymerases by Increasing the Flexibility of α-Carboxy Nucleoside Phosphonates

Alpha-carboxynucleoside phosphonates (α-CNPs) are novel viral DNA polymerase inhibitors that do not need metabolic conversion for enzyme inhibition. The prototype contains a cyclopentyl linker between nucleobase and α-carboxyphosphonate and preferentially (50- to 100-fold) inhibits HIV-1 RT compared with herpetic DNA polymerases. A synthesis methodology involving three steps has been developed for the synthesis of a series of novel α-CNPs, including a Rh(II)-catalyzed O-H insertion that connects the carboxyphosphonate group to a linker moiety and an attachment of a nucleobase to the other end of the linker by a Mitsunobu reaction followed by final deprotection. Replacing the cyclopentyl moiety in the prototype α-CNPs by a more flexible entity results in a selectivity shift of ∼ 100-fold in favor of the herpetic DNA polymerases when compared to selectivity for HIV-1 RT. The nature of the kinetic interaction of the acyclic α-CNPs against the herpetic DNA polymerases differs from the nature of the nucleobase-specific kinetic interaction of the cyclopentyl α-CNPs against HIV RT.

The Mitsunobu reaction in the 21st century

Recent advancements in the condensation of alcohols with pronucleophiles by the Mitsunobu reaction are described.

N6-substituted adenosines. Cytokinin and antitumor activities

A series of N6-adenosine derivatives were synthesized by alkylation of N6-acetyl-2′,3′,5′-tri-O-acetyladenosine (1) with alkyl halides and alcohols. It was shown that propargyl derivative 2a is a good substrate for copper(I) catalyzed Huisgen [3+2] cycloaddition with azides. This click-reaction can be used for preparation of the libraries of 1,2,3-triazolyl modified adenosines. Biological activities of N6-adenosines were studied in two plant and six human cancer cell assays. The remarkable parallel between cytokinin and cytotoxic activities was found. The most cytokinin active compounds 3c–3e at the same time appeared to be the most potent cytotoxic agents.

Synthesis of New Racemic α,α-Diaminocarboxylic Ester Derivatives

New racemic methyl or ethyl α-aminoglycinate derivatives were synthesized by N-alkylation of amines (aniline, 4-methylaniline, 2-methylaniline, 2,4-dimethoxyaniline, 2-nitroaniline, 4-chloro-2-fluoroaniline, 2-naphthylamine, benzylamine, N,N-dibenzylamine, and cyclohexylamine) with methyl or ethyl α-azidoglycinate.