Radical Route for the Alkylation of Purine Nucleosides at C6 via Minisci Reaction

Regioselective Homolytic C2-H Borylation of Unprotected Adenosine and Adenine Derivatives via Minisci Reaction

A Minisci-type borylation of unprotected adenosine, adenine nucleotide, and adenosine analogues was successfully achieved through photocatalysis or thermal activation. Despite the challenges posed by the presence of two potential reactive sites (C2 and C8) in the purine motif, the unique nucleophilic amine-ligated boryl radicals effortlessly achieved excellent C2 site selectivity and simultaneously avoided the formation of multifunctionalized products. This protocol proved effective for the late-stage borylation of some important biomolecules as well as a few antiviral and antitumor drug molecules, such as AMP, cAMP, Vidarabine, Cordycepin, Tenofovir, Adefovir, GS-441524, etc. Theoretical calculations shed light on the site selectivity, revealing that the free energy barriers for the C2-Minisci addition are further lowered through the chelation of additive Mg2+ to N3 and furyl oxygen. This phenomenon has been confirmed by an IGMH analysis. Preliminary antitumor evaluation, derivation of the C2-borylated adenosine to other analogues with high-value functionalities, along with the CuAAC click reactions, suggest the potential application of this methodology in drug molecular optimization studies and chemical biology.

Copper Catalyzed Three-Component Ullmann C-S Coupling in PEG for the Synthesis of 6-Aryl/alkylthio-purines

To tackle the environmental unfriendly issue in existing synthesis strategies for 6-substitued thiopurine derivatives, such as poor step economy, frequent use of malodorous organic sulfur starting materials, toxic organic solvents, and equivalent dosage of base, we have developed a CuI-catalyzed base-free three-component Ullmann C-S coupling synthetic strategy, featured using inorganic salt Na2S as the sulfur source and nontoxic PEG-600 as the solvent. The newly developed strategy is particularly effective for the synthesis of 6-arylthiopurines. The high catalytic efficiency in PEG-600 can be rationalized by the high soluble ability of CuI catalyst, likely due to the presence of multiple oxygen coordination sites in PEG.

Terminal C(sp3)-H borylation through intermolecular radical sampling

Hydrogen atom transfer (HAT) processes can overcome the strong bond dissociation energies (BDEs) of inert C(sp3)-H bonds and thereby convert feedstock alkanes into value-added fine chemicals. Nevertheless, the high reactivity of HAT reagents, coupled with the small differences among various C(sp3)-H bond strengths, renders site-selective transformations of straight-chain alkanes a great challenge. Here, we present a photocatalytic intermolecular radical sampling process for the iron-catalyzed borylation of terminal C(sp3)-H bonds in substrates with small steric hindrance, including unbranched alkanes. Mechanistic investigations have revealed that the reaction proceeds through a reversible HAT process, followed by a selective borylation of carbon radicals. A boron-sulfoxide complex may contribute to the high terminal regioselectivity observed.

Photocatalyst-free visible-light-induced highly selective acylation of purine nucleosides at the C6 position

A protocol for visible-light-induced C-H acylation selectively at the C6 position of purine nucleosides with aldehydes under photocatalyst-free conditions was established herein. This protocol allows the green, mild, and efficient functionalization of various purine nucleosides with a broad range of alkyl and aryl aldehydes.

Silver-catalyzed cyclization of α-imino-oxy acids to fused tetralone derivatives

A silver-catalyzed intramolecular radical relay cyclization of α-imino-oxy acids under mild conditions has been described. This reaction offers facile access to a diverse range of fused tetralone derivatives with exquisite stereoselectivity in moderate to good yields (40-98%). Experimental studies show that the reaction undergoes a decarboxylation and acetone fragmentation/1,5-hydrogen atom transfer (HAT)/cyclization process.

Metal-free direct C-6-H alkylation of purines and purine nucleosides enabled by oxidative homolysis of 4-alkyl-1,4-dihydropyridines at room temperature

Herein we report the application of 4-alkyl-1,4-dihydropyridines (DHPs), which are easily prepared from inexpensive aldehydes in one step, for the direct site-specific C-H alkylation of purines and purine nucleosides. Despite there being three active C(sp²)-H bonds (C-2-H, C-6-H, and C-8-H) in the structure, the reactions still show high regioselectivity at the purinyl C-6-H position. Importantly, the reactions successfully avoid the use of transition metal catalysts and additional acids. Meanwhile, the protocols are not sensitive to moisture and require only persulfate as an oxidant. Besides, this method displays broad functional group compatibility and is easy to scale up. Notably, pharmaceutical purines, e.g. the natural product 6-hydroxymethyl nebularine isolated from basidiomycetes, can be smoothly prepared using this protocol.

Regiospecificity C(sp2)-C(sp3) Bond Construction between Purines and Alkenes to Synthesize C6-Alkylpurines and Purine Nucleosides Using O2 as the Oxidant

A highly site-selective and Markovnikov-type radical C⁶-H alkylation of purines with alkenes is achieved, allowing fast construction of the C(sp²)-C(sp³) bond at the C-6-position of purines and purine nucleosides using O₂ as a green oxidant and alkenes as cheap alkylation reagents. The route was also a radical route to synthesize C⁶-alkyl-N⁷-substituted purines with potential steric hindrance between C⁶-alkyl groups and N⁷-substituted groups. This reaction is easily scaled up and has excellent functional group compatibility and broad substrate scopes. Moreover, the unstable intermediate was also separated, which was the key evidence for the reaction mechanism.

Direct Regioselective C-H Cyanation of Purines

A direct regioselective C-H cyanation of purines was developed through a sequential triflic anhydride activation, nucleophilic cyanation with TMSCN, followed by a process of base-mediated elimination of triflous acid (CF₃SO₂H). In most cases, the direct C-H cyanation occurred on the electron-rich imidazole motif of purines, affording 8-cyanated purine derivatives in moderate to excellent yields. Various functional groups, including allyl, alkynyl, ketone, ester, nitro et al. were tolerated and acted as a C8 directing group. The electron-donating 6-diethylamino, as C2-directing group substituent, can switch the regioselectivity of purine from 8- to 2-position, enabling the synthesis of 8- and 2-cyano 6-dialkylaminopurines from corresponding 6-chloropurine in different reaction order. Further functional manipulations of the cyano group allow the conversions of 8-cyanopurines to corresponding purine amides, imidates, imidothioates, imidamides, oxazolines, and isothiazoles.

Direct Minisci-Type C–H Amidation of Purine Bases

A method for the C–H carboxyamidation of purines has been developed that is capable of directly installing primary, secondary, and tertiary amides. Previous Minisci-type investigations on purines were limited to alkylations and arylations. Herein, we present the first method for the direct C–H amidation of a wide range of purines: xanthine, guanine, and adenine structures, including guanosine- and adenosine-type nucleosides. The Minisci-type reaction is also metal-free, cheap, operationally simple, scalable, and applicable to late-stage functionalizations of biologically important molecules.

The practical acetylation of nucleosides using acetic anhydride/acetic acid as a reusable solvent

The highly practical acetylation of free nucleosides is achieved using acetic anhydride/acetic acid as a reusable solvent and acetylating regent. A series of nucleosides, including ribosyl, deoxyribosyl, arabinosyl, acyclic and pyranosyl, and many clinical drugs were acetylated efficiently, even on large scale (200 g).

Synthesis of 2'-deoxyguanosine from 2'-deoxyadenosine via C2 nitration

An efficient synthetic method has been developed for the synthesis of 2'-deoxyguanosine from the more commercially available 2'-deoxyadenosine via late-stage C2 nitration in 48.7% total yield by a 5-step synthetic procedure. Crucially, 2'-deoxyadenosine was fully protected by bennzoyl groups and then nitrated at C2 by tetrabutylammonium nitrate/trifluoroacetic anhydride. The resulting 2-NO₂ moiety was converted into 2-NH₂ by Ni-catalyzed hydrogenolysis. Finally, 2'-deoxyguanosine was obtained from the diaminopurine intermediate by deaminase-catalyzed reaction. Furthermore, the 2-NO₂ moiety also appeared to be a versatile handle to introduce a variety of functional groups, resulting in a divergent access to 2-substituted 2'-deoxyadenosine analogues.

Organic-acid catalysed Minisci-type arylation of heterocycles with aryl acyl peroxides

A metal-free method for the Minisci-type arylation of heterocycles with aryl acyl peroxides has been reported. This strategy enables the rapid and simple synthesis of a series of Minisci-type adducts...

Synthesis of C6-Substituted Purine Nucleoside Analogues via Late-Stage Photoredox/Nickel Dual Catalytic Cross-Coupling

Nucleoside analogues have been and continue to be extremely important compounds in drug discovery. Despite the significant effort dedicated to their synthesis, medicinal chemistry campaigns around these structures are often hampered by synthetic challenges. We describe a strategy for the functionalization of purine nucleosides via photoredox and nickel-catalyzed sp²-sp³ cross-coupling. The conditions described herein allow for coupling of unprotected nucleosides with readily available alkyl bromides, providing opportunities for their application to parallel medicinal chemistry.

Metal free decarboxylative aminoxylation of carboxylic acids using a biphasic solvent system

The smooth oxidative radical decarboxylation of carboxylic acids with TEMPO and other derivatives as radical scavengers is reported. The key to success was the use of a two-phase solvent system to avoid otherwise predominant side reactions such as the oxidation of TEMPO by persulfate and enabled the selective formation of synthetically useful alkoxyamines. The method does not require transition metals and was successfully used in a new synthetic approach for the antidepressant indatraline.

Decarboxylative C–H alkylation of heteroarenes by copper catalysis

A copper-catalyzed decarboxylative C–H alkylation of heteroarenes with alkyl carboxylic acids has been realized.

Phenylene-bridged bis(benzimidazolium) (BBIm2+): a dicationic organic photoredox catalyst

A dicationic photoredox catalyst composed of phenylene-bridged bis(benzimidazolium) (BBIm2+) was designed, synthesised and demonstrated to promote the photochemical decarboxylative hydroxylation and dimerisation of carboxylic acids. The catalytic activity of BBIm2+ was higher than that for a monocation analogue, suggesting that the dicationic nature of BBIm2+ plays a key role in these decarboxylative reactions. The rate constant for the decay of the triplet-triplet absorption of the excited BBIm2+ increased with increasing concentration of the carboxylate anion with a saturated dependence, suggesting that photoinduced electron transfer occurs within the ion pair complex composed of the triplet excited state of BBIm2+ and a carboxylate anion.

Direct radical alkylation and acylation of 2H-indazoles using substituted Hantzsch esters as radical reservoirs

A platform approach for the direct synthesis of 3-substituted 2H-indazole derivatives has been developed using a Ag(i)/Na₂S₂O₈ system.

Diacetyl as a "traceless" visible light photosensitizer in metal-free cross-dehydrogenative coupling reactions

Minisci alkylation is of prime importance for its applicability in functionalizing diverse heteroarenes, which are core structures in many bioactive compounds. In alkyl radical generation processes, precious metal catalysts, high temperatures and excessive oxidants are generally involved, which lead to sustainability and safety concerns. Herein we report a new strategy using diacetyl (2,3-butanedione) as an abundant, visible light-sensitive and "traceless" hydrogen atom abstractor to achieve metal-free cross-dehydrogenative Minisci alkylation under mild conditions. Mechanistic studies supported hydrogen atom transfer (HAT) between an activated C(sp3)-H substrate and diacetyl. Moreover, with the assistance of di-tert-butyl peroxide (DTBP), the scope of the reaction could be extended to strong aliphatic C-H bonds via diacetyl-mediated energy transfer. The robustness of this strategy was demonstrated by functionalizing complex molecules such as quinine, fasudil, nicotine, menthol and alanine derivatives.

Regioselective C–H and N–H functionalization of purine derivatives and analogues: a synthetic and mechanistic perspective

This perspective provides a systematic and concise overview of the recent development in C–H/N–H bond functionalization in purine derivatives and analogues.

Metal-free oxidative decarbonylative alkylation of chromones using aliphatic aldehydes

A decarbonylative alkylation of chromones via radical conjugate addition under metal-free conditions was developed using aliphatic aldehydes as alkylating reagents.

Metal-free C-H alkylation of heteroarenes with alkyltrifluoroborates: a general protocol for 1°, 2° and 3° alkylation

A photoredox-catalyzed C-H functionalization of heteroarenes using a variety of primary, secondary, and tertiary alkyltrifluoroborates is reported. Using Fukuzumi's organophotocatalyst and a mild oxidant, conditions amenable for functionalizing complex heteroaromatics are described, providing a valuable tool for late-stage derivatization. The reported method addresses the three major limitations of previously reported photoredox-mediated Minisci reactions: (1) use of superstoichiometric amounts of a radical precursor, (2) capricious regioselectivity, and (3) incorporation of expensive photocatalysts. Additionally, a number of unprecedented, complex alkyl radicals are used, thereby increasing the chemical space accessible to Minisci chemistry. To showcase the application in late-stage functionalization, quinine and camptothecin analogues were synthesized. Finally, NMR studies were conducted to provide a rationalization for the heteroaryl activation that permits the use of a single equivalent of radical precursor and also leads to enhanced regioselectivity. Thus, by 1H and 13C NMR a distinct heteroaryl species was observed in the presence of acid catalyst and BF3.