Sulfoxonium ylides for direct methylation of N-heterocycles in water

The direct methylation of N-heterocycles is an important transformation for the advancement of pharmaceuticals, agrochemicals, functional materials, and other chemical entities. Herein, the unprecedented C(sp2)–H methylation of iminoamido N-heterocycles as nucleoside base analogues is described. Notably, trimethylsulfoxonium salt was employed as a methylating agent under aqueous conditions. A wide substrate scope and excellent level of functional group tolerance were attained. Moreover, this method can be readily applied to the site-selective methylation of azauracil nucleosides. The feasibility of gram-scale reactions and various transformations of the products highlight the synthetic potential of the developed method. Combined deuterium-labeling experiments aided the elucidation of a plausible reaction mechanism.


Introduction
Methylation is one of vital processes in biological systems because DNA and histone methylation is responsible for gene expression without affecting the gene sequence [1][2] . For example, the C(sp 2 )-H methylation of uracil of deoxyuridine-5'-monophosphate (dUMP) by thymidylate synthase is a well-known metabolic process for DNA biosynthesis and RNA post-translational modi cation [3][4] . In addition, methylation of cysteine can affect signaling pathways by disrupting ubiquitin-chain sensing in NF-κB activation 5 . Methylation is also an important modi cation in medicinal chemistry and drug discovery, as physical and pharmacological properties of lead candidates can be favorably or adversely affected with the mere addition of a methyl group 6 . To address this challenge, number of synthetic strategies have been explored to install methyl and alkyl groups on N-heterocyclic molecules [7][8][9] . In particular, alkylation beyond monoazines, i.e., that of diazines and triazines, has emerged as a hot topic in this area due to the relevance of a number of bioactive molecules [10][11][12][13][14] .
Since the landmark discovery by Johnson, Corey, and Chaykovsky in the 1960s 37-39 , sulfoxonium ylides have been widely employed as methylene surrogates in epoxidations, cyclopropanations, and aziridinations via intermolecular nucleophilic addition into electron-de cient π-unsaturated compounds, such as carbonyls, α,β-unsaturated compounds, and imines [40][41][42][43] . However, there are no reports on the direct alkylation of N-heterocyclic compounds using alkyl sulfoxonium ylides. Moreover, previous reports related to sulfoxonium ylides predominantly rely on the use of organic solvents, which preclude them as environmentally benign processes [44][45][46][47][48] . Herein, we describe the unprecedented C(sp 2 )-H alkylation of iminoamido diazines and triazines as nucleoside base analogues with alkyl sulfoxonium salts. Of particular importance is the unconventional mechanistic pathway for the imino alkylation in aqueous media, which can provide novel insights into Corey-Chaykovsky reagents (Fig. 1c). Moreover, the amenability of this protocol to the pharmaceutical industry is attributed to the use of H 2 O as the greenest solvent and KOH as an economical reagent.

Results
Investigation of reaction conditions. Optimization of reaction conditions commenced with the coupling between pyrazinone 1a and trimethylsulfoxonium iodide (2a), as shown in Table 1. The formation of C3methylated pyrazinone 3a was achieved with the use of KO t Bu in THF at 100 o C (entry 1). This reaction could likewise be performed in protic solvents, such as MeOH (entry 3). Surprisingly, aqueous media accelerated the coupling reaction between 1a and 2a to afford 3a in 84% yield within 2 h (entry 4). It should be noted that the aziridine compound generated from the imine functionality by the Corey-Chaykovsky aziridination was not observed in all cases, suggesting that the reaction pathways of the Corey-Chaykovsky reaction and the C(sp 2 )-H methylation reported herein are distinct. More importantly, this reaction was successful with the use of KOH, a cost-e cient base, to provide 3a in 91% yield (entry 5). Screening of other bases yielded inferior results (entries 6 and 7). Control experiments revealed that the loading amount of KOH and reaction temperature were crucial for increasing the yield of 3a (entries 8 and 9) (see Table 1 in the Supplementary Files).
Substrate scope with respect to pyrazinones. With the optimized reaction conditions in hand, the substrate scope of pyrazinones was examined ( Table 2). A wide range of N-alkylated and N-benzylated pyrazinones 1b-1f were found to be suitable substrates for this transformation, providing the corresponding C3-methylated pyrazinones 3b-3f in high yields. Substrates 1c-1f produced the corresponding products in low yields in H 2 O, presumably due to the presence of hydrophobic substituents. After careful screening of reaction conditions, it was established that protic solvents, such as EtOH and i-PrOH, resulted in higher levels of conversion. Notably, N-tetrahydropyranyl and N-benzyl groups are readily removable via conventional protocols for further elaboration of the free-(NH)-amido group. In addition, we observed the successful methylation of N-arylated pyrazinones 1g-1l. Halogensubstituted and electron-rich N-aryl groups (1g and 1j) displayed good reactivity, whereas electronde cient N-aryl groups (1h, 1i, 1k and 1l) were relatively less reactive. The electronic effect on this transformation was likewise observed for the C4-arylated pyrazinones 1m-1p. The tolerance of cyano and bromo groups presents valuable opportunities for further versatile synthetic transformation.
Moreover, tri-substituted pyrazinone adduct 3q was formed smoothly in 73% yield. Furthermore, the siteselective methylation of 1r-1t was successfully achieved under the standard reaction conditions, affording the corresponding products 3r-3t in high yields. (see Table 2 in the Supplementary Files) Substrate scope with respect to sulfoxonium salts. Having established the broad scope of pyrazinones, the scope of sulfoxonium salts 2b-2e was then evaluated. In the case of ethylation and propylation, trialkyl sulfoxonium chlorides were employed as alkylating agents to afford the corresponding products 3u (70%) and 3v (77%). The current protocol could be implemented using Johnson's sulfoxonium salts 49 2d and 2e to furnish cyclopropylated pyrazinone 3w (45%) and methylated pyrazinone 3a (35%), respectively.
The reaction also readily proceeded with N-arylated quinoxalinones 4g-4k, furnishing the corresponding products 5g-5k. Notably, the electronic property of the quinoxalinone or N-aryl rings had a profound effect on this transformation. For example, the methylation of highly electron-de cient quinoxalinone derivatives 4f, 4h and 4k was less e cient. Finally, pyrido-pyrazinone 4l also participated in the methylation reaction to provide 5l in 78% yield. (see Table 3 in the Supplementary Files) Substrate scope with respect to azauracils and azauracil nucleosides. Azauracil, an azapyrimidinone analogue of uracil, is of interest in medicinal chemistry due to its potential to inhibit various species of microorganisms. The ribonucleosides of 6-azauracil have been shown to display diverse biological pro les such as antiviral, antitumor, and antifungal activities [50][51][52][53] . Based on the methylation of uracil in nature, we envisioned the direct alkylation of 6-azauracils and 6-azauracil nucleosides (Scheme 4). N-Substituted azauracil 6a was smoothly alkylated with 2a-2c to afford the corresponding products 7a-7c in satisfactory yields. In addition, triazinones 6b-6e were compatible with this process. It is noteworthy that hydroxyl and 2-pyranosyl groups in 7e and 7g were also tolerable. Direct C(sp 2 )-H methylation of 2'deoxy-6-azauridine nucleoside 6f was successfully achieved by using K 3 PO 4 as a base, providing 7h in 94% yield. Moreover, azauracil ribonucleoside 6g was also methylated with 2a to afford 7i in 91% yield.
Mechanistic investigation and proposed reaction mechanism. To garner mechanistic insights into this process, a series of deuterium-labeling experiments were performed (Fig. 2a). Treatment of 1a with KOH in the presence of D 2 O resulted in 45% and 94% deuterium incorporation at the C3 and C6 positions, respectively. Partial deuteration (20%) on the N-Me group was also observed. These results indicate that a hydroxide anion possibly undergo nucleophilic addition onto imino functionality of pyrazinone 1a to generate a nitrogen anion intermediate, which can undergo a deuteration/deprotonation equilibrium reaction at the C3 and C6 positions (see the Supporting Information for the proposed mechanism). The reaction of 1a with 2a in the presence of D 2 O under otherwise identical reaction conditions provided 93% deuteration at the benzylic CH 3 moiety as well as 73% deuteration at the C6 position, suggesting C3methylation by sulfoxonium ylide on 1a followed by subsequent deproto-deuteration at the benzylic position. Benzylic deuteration was further con rmed by the reaction of 3a in aqueous KOH solution. To evaluate the unique reactivity of pyrazinones as cyclic iminoamides, control experiments were performed with N-aryl 2-pyridone and 2H-benzo[b] [1,4]oxazin-2-one. No formation of methylated adducts under the standard reaction conditions was observed, revealing that the cyclic iminoamide backbone is crucial for the C(sp 2 )-H methylation (see the Supporting Information for the details). Based on the deuteriumlabeling experiment results and previous literature related to sulfoxonium ylides, 8 we propose a plausible reaction mechanism (Fig. 2b) Synthetic applications. To illustrate the synthetic potential of the developed protocol, gram-scale experiments were rst performed (Fig. 3a). The reaction of pyrazinone 1a (2 g, 10.8 mmol) with 2a afforded 3a in 90% yield. In addition, a gram-scale reaction of quinoxalinone 4d was also successfully performed, providing 5d in 95% yield. It is noteworthy that both products 3a and 5d were isolated by simple ltration instead of column chromatography, indicating the capability of the developed method in process chemistry. Next, various transformations of the synthesized C3-methylated products were performed (Fig 3b). The Ni(II)-catalyzed cross-coupling reaction between 3a and benzyl alcohol gave 8a in 82% yield. Benzylic oxidation of 3a with SeO 2 followed by reductive amination with 1-adamantylamine provided 8b in 40% yield. In addition, benzylic bromination of 5a and subsequent O-nucleophilic substitution with estrone furnished 8c in 50% yield. To demonstrate the utility of the N-protecting group, the tetrahydropyranoyl group on 3d was removed under acidic conditions to provide the free-(NH)pyrazinone adduct in 90% yield, which further reacted with arylsulfonyl chloride to deliver 8d in 60% yield.
Synthesis of tetra-substituted pyrazine. To further demonstrate the utility of the developed methodology, the sequential transformation of N-benzylated pyrazinone 9a was performed (Fig. 4). Pyrazinone 9a was smoothly methylated with 2a to provide 9b in 82% yield. Removal of the benzyl group of 9b followed by treatment with PhPOCl 2 afforded chloropyrazine 9c in 79% yield. Finally, Suzuki cross-coupling reaction between 9c and 2-thiophene boronic acid under Pd(II) catalysis provided tetra-substituted pyrazine 9d in 70% yield.

Discussion
In conclusion, we presented an unprecedented protocol for the direct C-H alkylation of N-heterocycles, including pyrazinones, quinoxalinones, and azauracils, using sulfoxonium salts as the alkylating agents. The use of aqueous solvent in the coupling reaction and the selective methylation of azauracil nucleosides under the developed reaction conditions are noteworthy. The synthetic applicability of this method was demonstrated by successful gram-scale reactions and valuable synthetic transformations of the methylated products, as well as by the synthesis of a tetra-substituted pyrazine via sequential transformations. The ndings of this study are expected to be of signi cant interest to organic and medicinal chemists because of the synthetic simplicity, broad substrate scope, high e ciency, excellent chemoselectivity, and environmentally benign nature of the developed methodology.

Methods
General methods. For the synthetic procedure and 1 H, 13      Synthesis of tetra-substituted pyrazine.

Supplementary Files
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