Tuning Cross-Coupling Approaches to C3 Modification of 3-Deazapurines.

A general approach to C3 modification of purine scaffold through various types of cross-coupling reactions has been established. Tuning substrate electronics and reaction conditions resulted in the development of highly efficient sp2-sp, sp2-sp2, and sp2-sp3 cross-coupling conditions for modification of 3-deazaadenine to access C3-modified adenine and hypoxanthine scaffolds. The optimized methodologies to access the corresponding 3-deazaadenosine phosphoramidites for solid-phase DNA synthesis have been demonstrated.

Introduction

Purines are important building blocks used effectively across various disciplines. Functionalization and modification of purines have been of interest for synthetic organic chemistry, medicinal chemistry, and toxicology as reflected by the half-century application and development of purine-based anticancer or antiviral agents.[1−4] Purines are also of interest as detection probes[5−9] and as mechanistic probes for understanding cellular responses to DNA and RNA modification.[10,11] Approaching these highly functionalized heterocycles often requires tuning of existing or development of new synthetic routes. Overcoming challenges associated with the reactivity of 3-deazaadenine derivatives, in particular, has been approached through de novo construction of 3-deaza-3-substituted purine backbone or functionalization of nucleosides. Several modifications, such as 3-methyl, the related protected 3-hydroxymethyl, and methylenethiol or nitro groups were reported.[12−15] 3-Deaza-3-fluoroadenosines had to be synthesized stepwise, but other 3-halo derivatives could also be obtained through direct halogenation of the nonsubstituted 3-deazaadenosines.[16−22] The use of cross-coupling reactions in C3 modification of purines is, however, limited. Examples of palladium-catalyzed methylations of 3-deaza-3-bromoadenosine analogues through Kumada coupling have been demonstrated.[17,20] Recently, the modification of adenosine nucleosides through Sonogashira cross-coupling has been reported, providing access to 3-deaza-3-alkyne adenosine derivatives.[8,9,23] However, the use of nucleosides as substrates for cross-coupling reactions can be challenging due to the presence of potentially labile glycosidic bonds or nucleophilic functional groups that have to be protected to achieve the compatibility with many organometallic reagents, as reflected by the reported examples of Negishi and Kumada reactions for the introduction of alkyl or benzyl substituents.[24−27] Given the importance of modified nucleobases itself, the method that would allow a direct access to C-3 modified purine scaffolds would be therefore of interest. Here, we describe a general strategy that allows the deliberate synthesis of various 3-deaza-3-substituted purine analogues through metal-catalyzed cross-coupling reactions.

Results and Discussion

En route to 3-deazaadenine analogues of purines, we first investigated the reactivity of the readily available 3-deazaadenine 1 (Scheme ) in cross-coupling reactions.[15] In direct attempts to utilize it as a cross-coupling partner, we were able to obtain the 3-iodo-3-deazaadenine 2 and observe its acceptable reactivity under Suzuki cross-coupling conditions using the Pd(OAc)2/3,3′,3″-phosphanetriyltris(benzenesulfonic acid) trisodium salt (TPPTS) catalyst system, a general method developed for aqueous coupling reactions of halogenated purine/pyrimidine nucleobases and nucleosides.[4,26,28] The reactions of 2 with phenylboronic acid and trans-2-phenylvinylboronic acid afforded compounds 3a and 3b with 65 and 58% yield, respectively (Scheme ).

Scheme 1

Synthesis of Compounds 1–4

Despite the sufficient results obtained for Suzuki cross-coupling of 2 with model aryl/vinylboronic acids, this condition failed in the attempt to produce an alkyl analogue with model ethylboronic acid, similarly as reported by other authors.[26] The reactivity of 2 can be hampered by its low solubility in organic solvents and the presence of active nucleophilic nitrogens. When the amino group at C6 was protected as an isobutyryl amide (the established purine protecting group), the more soluble N-(7-bromo-1H-imidazo[4,5-c]pyridin-4-yl)isobutyramide 4 was obtained (Scheme ).[15] Various Suzuki coupling conditions of 4 with phenyl- and isobutylboronic acids were investigated, involving metal catalysts, such as Pd(PPh3)4, Pd2(dba)3, PddppfCl2, and NiCl2(dme); ligands, such as PCy3, (t-Bu)2PMeHBF4, and SPhos; and bases, such as K2CO3, Cs2CO3, K3PO4, and KOt-Bu in ethereal or alcoholic solvents.[29−33] Typically, the unreacted starting material was recovered from the reaction mixtures, highlighting the lack of reactivity of 4. Also, the liability of amide protecting group prohibited the use of aqueous coupling conditions utilizing bases such as carbonates.

We have also evaluated the possibility of using a commercial C6-chloro (5) and the known C6-azide[34] (5b) purines as starting materials amenable for further amination and reduction, respectively, to access 3-deazaadenine. However, attempts to install a halide at C3 (compound 6, Scheme ) have failed, precluding further evaluation.

As the electron-withdrawing groups at C6 (chloride, amide, or azide) were shutting off the reactivity of C3 position, we hypothesized that the use of electronically more neutral protecting groups will promote the solubility of 1 and retain its reactivity toward cross-coupling reactions. Unfavorably, the direct attempts to react 5 with bisbenzylamines to produce the desired cross-coupling partner of type 13 (Scheme ) failed even under harsh conditions (e.g., 140 °C, neat or dimethylformamide (DMF)/K2CO3), reflecting the evident low reactivity of this substrate toward direct displacement of chlorine atom.[12,34] We decided to investigate the catalytic aminations with bis(4-methoxybenzylamine) ((PMB)2NH) on 5. In parallel, the catalytic amination of PMB-protected analogue was investigated, considering the reported challenges for modifications of unprotected imidazoles.[35] It should be noted that PMB protection of 5 (PMBCl, K2CO3, N,N-dimethylacetamide (DMA), 45 °C, 24 h, 96% yield) yielded a ∼1.6:1 mixture of N9- and N7-PMB-protected products that was used directly.[36] Several literature conditions applicable for chloroarenes, 6-halopurines, and their nucleosides were investigated.[37−42] RuPhos and its Pd-bound precatalyst in the presence of NaOt-Bu in tetrahydrofuran (THF)[41] have been shown to be particularly successful for the reactions of nonactivated heretoaryl chlorides with secondary amines. Under these conditions, up to 19% of N9-PMB-protected product (identical to 13-H, Experimental Section) was obtained, albeit with higher catalyst loading (6 mol %) and prolonged reaction time (43 h). However, a significant decomposition was observed. The use of LiHMDS instead of NaOt-Bu led to the exclusive decomposition of both protected and nonprotected 5. Next, Pd(OAc)2 was tested in combination with ligands, such as BINAP, SPhos, and XPhos, in the presence of Cs2CO3 or NaOt-Bu in solvents, such as toluene, THF, dioxane, and DMF.[37−40,42] Significant decomposition was observed in the majority of cases, but no product formation. Some level of success was achieved with the combination of Pd(OAc)2 (0.1 equiv) and SPhos (0.2 equiv) in the presence of NaOt-Bu in toluene (110 °C, 22 h). 13-H was isolated in 21% yield, reflecting the low reactivity of heteroaryl chlorides with secondary amines.[40,41] For practical purposes, we have not investigated this reaction further.

Scheme 2

Synthesis of Compound 13

To overcome hurdles posed by the limited reactivity of the tested substrates, we decided to construct the fully protected 3-deazaadenine analogue from the cheap, available precursors, using a similar route as reported in the literature.[15,43] The highly regioselective sequential substitutions of 4- and 2-chloro atoms of the known derivative 9(43) with 4-methoxybenzylamine (PMBNH2) followed by (PMB)2NH produced compound 11 (Experimental Section), and upon further reduction of the nitro group and cyclization, we were able to obtain the target 3-deazaadenine precursor (13) with the exclusive N9-PMB protection and 26% overall yield in six steps from a commercial 3-nitropyridine-2,4-diol 7 (Scheme ). The length of the synthetic route is similar to that of 1(15) but offers access to the fully protected soluble starting material amenable to functionalization.

Gratifyingly, a variety of different coupling conditions involving 13 were successfully demonstrated, as shown in Table , including the examples of reactions such as Suzuki[43,44] (entries 1–5), Heck[45] (entries 6 and 7), Sonogashira[8,23] (entry 8), Kumada[46] (entries 9 and 10), Negishi (entries 11–14),[47] and the nickel-catalyzed reductive couplings of alkyl halides recently developed by the Weix group (entry 15).[48]

Table 1

Transformation Scope for the Metal-Catalyzed Couplings of 13

entry	substrate	cond.a	product	yield (%)
1	PhB(OH)2	A	14a, R = Ph	91
2	PhCH=CH2B(OH)2	A	14b, R = (E)-styryl	79
3	Ph(CH2)2B(OH)2	B	14c, R = (CH2)2Ph	35
4	EtB(OH)2	B	14d, R = Et	31
5	i-BuB(OH)2	B	14e, R = i-Bu	29
6	PhCH=CH2	C	14b, R = (E)-styryl	53
7	CH2=CHCO2Me	C	14f, R = (E)-acrylate	57b
8	PhC≡CH	D	14g, R = C≡CPh	89
9	Al(Me)3	E	14h, R = Me	87
10	BnMgCl	E	14i, R = Bn	45
11	Ph(CH2)2ZnX	F	14c, R = (CH2)2Ph	77
12	EtZnX	F	14d, R = Et	76
13	i-BuZnX	F	14e, R = i-Bu	85
14	i-PrZnX	F	14j, R = i-Pr	86c
15	Br(CH2)3CO2Et	G	14k, R = (CH2)3CO2Et	32b

Condition A: Ar or vinylB(OH)2 (1.5 equiv), Cs2CO3 (2 equiv), Pd(PPh3)4 (0.1 equiv), dioxane/H2O (2/1), Ar, 90 °C, 6–16 h; condition B: AlkylB(OH)2 (1.1–1.5 equiv), K2CO3 (3 equiv), Pd(PPh3)4 (0.1 equiv), dry dioxane, Ar, 90 °C, 72 h; condition C: styrene or methyl acrylate (5 equiv), Et3N (7 equiv), PPh3 (0.5 equiv), Pd(OAc)2 (0.1 equiv), dry CH3CN, Ar, 90 °C, 36 h; condition D: phenylacetylene (1.5 equiv), Pd(PPh3)2Cl2 (0.1 equiv), CuI (0.1 equiv), dry DMF/Et3N (2/1), Ar, 80 °C, 24 h; condition E: Al(Me)3 or BnMgCl solution (3–3.3 equiv), Pd(PPh3)4 (0.1 equiv), dry THF, Ar, 75 °C, 4 h; condition F: ZnCl2 (0.5 M in THF, 5.7 equiv), AlkylMgX solution (X = Br or Cl, 8.6 equiv), Ar, rt, 1 h, then 13 (1 equiv), PddppfCl2 (0.1 equiv), Ar, rt, 21–29 h; condition G: Br(CH2)3CO2Et (1.1 equiv), Zn (2 equiv), 4,4′-dimethoxy-2,2′-bipyridine (0.1 equiv), NiI2 (0.13 equiv), pyridine (0.1 equiv), NaI (0.25 equiv), dry DMA, Ar, rt, 5 min, then 60 °C, 20 h.

Yield isolated after treatment of the crude mixture with N-iodosuccinimide (NIS) and separation of 13-I (for details, see Experimental Section).

Contains ∼20% of n-propyl impurity.

The Suzuki couplings could be accomplished with aromatic, alkenyl, and alkyl boronic acids (entries 1–5). Both electron-rich and electron-deficient alkenes were tolerated under the Heck conditions (entries 6 and 7). Sonogashira coupling with phenylacetylene proceeded with high efficiency (89%, entry 8).

The formation of alkyl derivatives, with the challenging substrates containing β-hydrogens, such as alkylboronic acids or ethyl 4-bromobutyrate, was less efficient (29–35%, entries 3–5 and 15) and accompanied by a significant hydrodebromination reaction, indicating the extent of the undesired β-H elimination. This byproduct (13-H) could be subjected to halogenation, for example, by the treatment of crude reaction mixtures with N-bromosuccinimide (NBS)/NIS, facilitating a chromatographic separation of the cross-coupled product 14, while the haloderivative 13/13-I could be recovered (see Experimental Section). The iodo analogue 13-I obtained this way offered no selectivity enhancement for alkylations using conditions B or F. Propitiously, simple alkyl derivatives could be obtained with improved yields (45–87%) through Kumada-type couplings with substrates such as trimethylaluminum and benzylmagnesium chloride (entries 9 and 10). Ultimately, modified Negishi conditions allowed the access to a variety of primary alkyl derivatives with notably higher yields than those obtained through Suzuki protocol (entries 11–13 vs 3–5). In the case of challenging isopropyl derivative, we observed up to 20% of the known isopropyl to n-propyl isomerization (entry 14).[49] Interestingly, our Negishi protocol provided satisfying and consistent results only when an excess of Grignard reagent against an overstoichiometric amount of zinc chloride was used, which helped to improve the solubility of the intermediate organozinc species. This observation seems to be in accord with the works by the Organ group that unveiled the mechanisms involving organozinc reagents in Negishi cross-couplings.[50,51]

Clearly, the fully protected nucleobase 13 allows the use of a broad scope of coupling conditions with not only the formation of saturated and unsaturated C–C bonds, but also the introduction of functional groups, to access the C3-modified 3-deazaadenine derivatives. These results pinpoint appropriateness of the PMB as a protecting group with suitable electronic and chemical properties that allowed to achieve the desired reactivity of 13 among the other tested 3-deaza-3-haloadenine substrates.

We further aimed to elaborate the products to the protected phosphoramidites to verify their compatibility with producing building blocks for solid-phase DNA synthesis. The route to phosphoramidites is exemplified by the synthesis of compound 20i (Scheme ). We successfully optimized the procedure for complete deprotection of benzyl derivative 14i affording modified purine 15i.[52] The known problem of selectivity for purine glycosylations prompted us to use the tetramethylsuccinimide directing/protecting group, developed by the McLaughlin group,[53] to increase the selectivity toward the desired N9 β-glycosylation. The crude salt 15i was converted accordingly to the corresponding imide 16i through the reported two-step procedure using tetramethylsuccinic anhydride (Me4SA).[53] The imide 16i was subjected to glycosylation with 1-(α)-chloro-3,5-di-O-(p-toluoyl)-2-deoxy-d-ribose, affording the desired N9 β-nucleoside 17i in 39% yield. The configuration of nucleoside 17i was unambiguously confirmed by the nuclear Overhauser enhancement spectroscopy (NOESY) experiment (Figure S1, Supporting Information). It was then elaborated to the corresponding phosphoramidite 20i through standard methods (Scheme ). Analogously, two other phosphoramidites 20d and 20e were prepared from 14d and 14e, ethyl and isobutyl analogues, respectively. All three phosphoramidites were successfully used in solid-phase DNA synthesis, and further studies are in progress. Interestingly, the preparation of the corresponding phosphoramidite 20a from phenyl derivative 14a could not be achieved due to the destabilizing effect of the Ph group on the glycosidic bond (see Experimental Section). We have also considered the possibility of the direct functionalization of nucleosides by an attempt to prepare a bromo analogue of 17i through the halogenation of the reported nonmodified 3-deazaadenine phosphoramidite.[13] However, the reactions with NBS in CH2Cl2 or DMF were unsuccessful, likely due to the electronegative effect of the tetramethylsuccinimide group.

Scheme 3

Synthesis of Phosphoramidite 20i

Additionally, the modified 3-deazaadenines could be readily converted to the corresponding 3-deazahypoxanthines, purine analogues of interest for medicinal chemistry,[36,54] upon treatment with NaNO2 as exemplified by the reaction of crude salt 15c to produce 21c in 57% yield based on 14c (Scheme ).[55,56] This transformation further demonstrates the potential of our synthetic approach toward C3-modified purine scaffold.

Scheme 4

Synthesis of 3-Deazahypoxanthine 21c

Conclusions

This study illustrates the merit of a new fully protected 3-bromo-3-deazaadenine derivative developed by us as a building block for the deliberate synthesis of 3-substituted 3-deazaadenine and adenosine derivatives through metal-catalyzed cross-coupling reactions and selective N9 β-glycosylations. It should be expected that both modified purines and their nucleosides will be of interest for applications related to chemical biology, DNA studies, and medicinal chemistry.

Experimental Section

General Methods

All reagents were used as received unless otherwise stated from Sigma-Aldrich, TCI America, Alfa Aesar, AK Scientific, and Chem-Impex International. 4-Chloro-1H-imidazo[4,5-c]pyridine was obtained from MolBridge. Bis(4-methoxybenzyl)amine and TPPTS were obtained from Ark Pharm. 1-(α)-Chloro-3,5-di-O-(p-toluoyl)-2-deoxy-d-ribose was purchased from Berry & Associates. Anhydrous methanol, pyridine, and N,N-dimethylacetamide (DMA) were obtained from Sigma-Aldrich. N,N-dimethylformamide (DMF) and 1,4-dioxane were dried and stored over CaH2 before use. Dry acetonitrile, dichloromethane, and tetrahydrofuran were dispensed from automated Innovative Technology Pure-Solv 400 Solvent Purification System. Analytical thin-layer chromatography was carried out on commercial SiliCycle SiliaPlate 0.2 mm F254 plates. Preparative silica chromatography was performed using SiliCycle SiliaFlash F60 40–63 μm (230–400 mesh). 1H, 13C, and 31P NMR spectra were recorded at room temperature (rt) with a Varian Unity Inova 400 MHz spectrometer. CDCl3, dimethyl sulfoxide (DMSO)-d6, CD3OD, acetone-d6, and D2O were used as solvents and referenced to the corresponding residual solvent peaks (7.26 and 77.16 ppm for CDCl3, respectively; 2.50 and 39.52 ppm for DMSO-d6, respectively; 3.31 and 49.0 ppm for CD3OD, respectively; 2.05 and 29.84 ppm for acetone-d6, respectively; and 4.79 ppm for D2O).[57] The following abbreviations are used to indicate the multiplicity: s: singlet; d: doublet; t: triplet; q: quartet; m: multiplet; b: broad signal; app: approximate. The coupling constants are expressed in hertz (Hz). The multiplicity of carbon atoms was determined by DEPT-135 experiment; Cq: quaternary carbon. Mass spectrometry (MS) data (electrospray ionization, ESI) were obtained using a Thermo Finnigan LCQ Advantage Ion-Trap Mass Spectrometer. High-resolution (HR) MS data (ESI) were obtained using a Thermo Fisher Orbitrap Elite Hybrid Ion Trap-Orbitrap Mass Spectrometer in Chemical Advanced Resolution Methods (ChARM) Laboratory at Michigan Technological University.

Synthesis and Characterization Data

Compounds 1,[15]4,[15,22] and 5b(34) were prepared according to the reported procedures.

7-Iodo-1H-imidazo[4,5-c]pyridin-4-amine (2)

To a suspension of amine 1 (536 mg, ca. 4 mmol) in dry DMF (8 mL), N-iodosuccinimide (NIS) (1.125 g, 5 mmol) was added in one portion. A slightly exothermic reaction occurred, and the resulting dark solution was stirred at room temperature for 24 h. After 24 h, the solution was added dropwise to EtOAc (100 mL) with vigorous stirring, and the resulting solid was filtered and washed with EtOAc. The solid was then suspended in chloroform (50 mL) and vigorously stirred under reflux condenser at 70 °C overnight. After filtration and drying in air, 2 was obtained as a dark brown solid in 27% yield (0.27 g). HRMS (ESI): m/z [M + H]+ calcd for C6H4N4I: 260.96373; found: 260.96323. MS (ESI): m/z, 261.2 for [M + H]+. 1H NMR (400 MHz, DMSO-d6): δ, 8.49 (s, 1H), 8.44 (bs, 2H), 7.97 (s, 1H) ppm. We were not able to obtain satisfactory 13C NMR data in DMSO-d6.

7-Phenyl-1H-imidazo[4,5-c]pyridin-4-amine (3a)

This compound was synthesized according to the reported general procedures.[4,26] To a vial equipped with septa, a mixture of deionized water and acetonitrile (2:1, 9 mL) was added and bubbled with argon for 10 min. 2 (150 mg, 0.58 mmol), phenylboronic acid (88 mg, 0.72 mmol), palladium(II) acetate (7 mg, 0.029 mmol), TPPTS (41 mg, 0.072 mmol), and anhydrous sodium carbonate (183 mg, 1.7 mmol) were added, and the headspace of the vial was flushed with argon for 1 min. The mixture was vigorously stirred at 100 °C for 7 h. The mixture was then cooled and diluted with ethyl acetate (50 mL) and brine (10 mL). The phases were separated, and the aqueous phase was extracted with additional ethyl acetate (3 × 30 mL). Combined organic extracts were dried with MgSO4. After filtration and concentration, the residue was purified by column chromatography on silica gel, eluting with 0–10% MeOH in CH2Cl2 to obtain 3a as a yellow solid in 65% yield (79 mg). HRMS (ESI): m/z [M + H]+ calcd for C12H11N4: 211.09838; found: 211.09680. MS (ESI): m/z, 211.3 for [M + H]+. 1H NMR (400 MHz, DMSO-d6): δ, 8.20 (s, 1H), 7.85 (s, 2H), 7.73 (b, 2H), 7.48–7.44 (m, 2H), 7.34–7.30 (m, 1H), 6.46 (b, 2H) ppm. 13C NMR (100 MHz, DMSO-d6): δ, 141.3, 137.5, 136.1, 128.7, 127.3, 126.5 ppm.

(E)-7-Styryl-1H-imidazo[4,5-c]pyridin-4-amine (3b)

This compound was synthesized in a similar fashion to 3a. To a vial equipped with septa, a mixture of deionized water and acetonitrile (2:1, 6 mL) was added and bubbled with argon for 10 min. 2 (100 mg, 0.38 mmol), trans-2-phenylvinylboronic acid (71 mg, 0.48 mmol), palladium(II) acetate (5 mg, 0.019 mmol), TPPTS (28 mg, 0.048 mmol), and anhydrous sodium carbonate (122 mg, 1.2 mmol) were added, and the headspace of the vial was flushed with argon for 1 min. The mixture was vigorously stirred at 100 °C for 16 h. The mixture was then cooled and diluted with ethyl acetate (40 mL) and brine (10 mL). The phases were separated, and the aqueous phase was extracted with additional ethyl acetate (3 × 20 mL). Combined organic extracts were dried with MgSO4. After filtration and concentration, the residue was purified by column chromatography on silica gel, eluting with 0–10% MeOH in CH2Cl2 to obtain 3b as a cream solid in 58% yield (52 mg). HRMS (ESI): m/z [M + H]+ calcd for C14H13N4: 237.11403; found: 211.11269. 1H NMR (400 MHz, DMSO-d6): δ, 8.28 (s, 1H), 7.89 (s, 1H), 7.58–7.56 (m, 2H), 7.38–7.31 (m, 4H), 7.23–7.20 (m, 1H), 6.52 (b, 2H) ppm. 13C NMR (100 MHz, DMSO-d6): δ, 141.1, 139.7, 138.2, 128.6, 126.8, 125.8, 124.0 ppm.

Synthesis of 13

Compounds 8 and 9 were prepared by modification of the reported procedures.[43]

5-Bromo-3-nitropyridine-2,3-diol (8)

To the suspension of 3-nitropyridine-2,4-diol 7 (10 g, 64.1 mmol) in glacial acetic acid (100 mL), bromine (4.6 mL, 90 mmol) was added in one portion, and the mixture was stirred at 70 °C for 2.5 h. The reaction mixture was then cooled and poured onto 300 mL of crushed ice, stirred vigorously for 30 min, and chilled in the freezer for 1 h. The precipitated solids were filtered on paper, washed with a small volume of ice-cold water several times and then with ethanol (3 × 15 mL), and dried in air. 8 was obtained as a yellow solid in 74% yield (11.2 g). 1H NMR (400 MHz, DMSO-d6): δ, 7.86 (s, 1H) ppm. 13C NMR (100 MHz, DMSO-d6): δ, 159.1, 155.6, 138.6, 128.2, 94.0 ppm.

5-Bromo-2,4-dichloro-3-nitropyridine (9)

A suspension of 8 (11.1 g, 47.4 mmol) in POCl3 (100 mL) was stirred at 90 °C for 24 h under argon, an excess pressure being released for several times within the first few hours. Then, POCl3 was carefully distilled off under vacuum. The oily residue was diluted with Et2O (100 mL), cooled on an ice bath, and crushed ice (200 mL) was carefully added (exothermic reaction). The phases were separated, aqueous phase was additionally extracted with Et2O (2 × 50 mL), and the combined organics were washed with brine and dried with MgSO4. After filtration and concentration, the residue was purified by column chromatography on silica gel, eluting with 0–10% EtOAc in hexanes to obtain 9 as a yellow oil that slowly solidified (9.27 g, 72% yield). 1H NMR (400 MHz, CDCl3): δ, 8.68 (s, 1H) ppm. 13C NMR (100 MHz, CDCl3): δ, 152.1, 141.5, 137.8, 121.3 ppm.

5-Bromo-2-chloro-N-(4-methoxybenzyl)-3-nitropyridin-4-amine (10)

To the solution of 9 (9.25 g, 34.3 mmol) in dry DMF (50 mL) at 0 °C under argon, 4-methoxybenzylamine (4.7 mL, 36 mmol) was added in one portion, followed by dropwise addition of Et3N (5.5 mL, 36 mmol) over 5 min. The mixture was allowed to warm up to room temperature and stirred for 24 h. The mixture was then diluted with EtOAc (300 mL), washed with brine (4 × 100 mL), and dried with MgSO4. After filtration, the solution was concentrated in vacuo to give a yellow solid that was triturated with hexanes to give 10 as a dark orange solid (12.1 g, 95% yield). HRMS (ESI): m/z [M + H]+ calcd for C13H1279Br35ClN3O3: 371.97507; found: 371.97488. 1H NMR (400 MHz, CDCl3): δ, 8.23 (s, 1H), 7.24–7.22 (d, J = 8.4, 2H), 6.93–6.91 (d, J = 8.4, 2H), 5.24 (bs, 1H), 4.22–4.21 (d, J = 4.8, 2H), 3.82 (s, 3H) ppm. 13C NMR (100 MHz, CDCl3): δ, 160.1 (Cq), 149.3, 143.4 (Cq), 143.3 (Cq), 129.7, 127.9 (Cq), 114.8, 109.4 (Cq), 55.5, 47.0 (CH2) ppm.

5-Bromo-N2,N2,N4-tris(4-methoxybenzyl)-3-nitropyridine-2,4-diamine (11)

A 250 mL Schlenk tube was charged with 10 (12.1 g, 32.6 mmol), bis(4-methoxybenzyl)amine (10.9 g, 42.4 mmol), n-butanol (100 mL), and Et3N (6 mL, 43.1 mmol). The mixture was heated at 130 °C for 24 h. It was cooled, diluted with EtOAc (400 mL), washed with brine (200 mL), and dried with MgSO4. After filtration, the solution was concentrated to give a dark residue that was purified by column chromatography on silica gel, eluting with 0–15% EtOAc in hexanes to obtain 11 as a red foam (17.6 g, 92% yield). HRMS (ESI): m/z [M + H]+ calcd for C29H3079BrN4O5: 593.13999; found: 593.13894. 1H NMR (400 MHz, CDCl3): δ, 8.09 (s, 1H), 7.21–7.18 (d, J = 8.8, 2H), 7.08–7.05 (d, J = 8.8, 4H), 6.88–6.86 (d, J = 8.8, 2H), 6.82–6.80 (d, J = 8.8, 4H), 6.14–6.12 (t, J = 4.8, 1H), 4.46–4.45 (d, J = 4.8, 2H), 4.29 (s, 4H), 3.782 (s, 3H), 3.778 (s, 6H) ppm. 13C NMR (100 MHz, CDCl3): δ, 159.6 (Cq), 159.0 (Cq), 155.5 (Cq), 151.1, 146.5 (Cq), 129.7, 129.5 (Cq), 129.4, 129.2 (Cq), 126.0 (Cq), 114.5, 113.9, 99.6 (Cq), 55.4, 55.3, 53.7 (CH2), 49.2 (CH2) ppm.

5-Bromo-N2,N2,N4-tris(4-methoxybenzyl)pyridine-2,3,4-triamine (12)

A 1 L round-bottom flask was charged with 11 (16.7 g, 28.2 mmol), ethanol (600 mL), and deionized water (300 mL), followed by Na2S2O4 (9.81 g, 56.4 mmol, 2 equiv). The mixture was heated at 85 °C under reflux condenser. Additional sodium hydrosulfite was added at 30 min intervals (overall, 4 × 2 equiv) over 2.5 h. The mixture was then concentrated in vacuo to remove most of ethanol and diluted with EtOAc (200 mL). It was carefully neutralized with solid NaHCO3 (effervescence!) with vigorous stirring. The organic layer was separated, the aqueous phase was additionally extracted with EtOAc (2 × 150 mL), and the combined organics were washed with brine (100 mL) and dried with MgSO4. After filtration and concentration, the residue was purified by column chromatography on silica gel, eluting with 0–60% EtOAc in hexanes, and the product was additionally triturated with hexanes containing some EtOAc to obtain 12 as a cream solid in 59% yield (9.4 g). This compound is somewhat unstable and should be used within few days. HRMS (ESI): m/z [M + H]+ calcd for C29H3279BrN4O3: 563.1658; found: 563.16396. 1H NMR (400 MHz, CDCl3): δ, 7.77 (s, 1H), 7.22–7.20 (d, J = 8.8, 2H), 7.10–7.08 (d, J = 8.8, 4H), 6.84–6.82 (d, J = 8.8, 2H), 6.81–6.79 (d, J = 8.8, 4H), 4.25–4.24 (d, J = 6.8, 2H), 4.14 (s, 4H), 4.00 (bs, 2H), 3.78 (s, 9H) ppm. 13C NMR (100 MHz, CDCl3): δ, 159.1 (Cq), 158.8 (Cq), 150.2 (Cq), 139.8 (Cq), 138.3, 131.6 (Cq), 130.7 (Cq), 130.1, 129.9 (Cq), 129.2, 114.1, 113.7, 110.8 (Cq), 55.36, 55.34, 53.5 (CH2), 48.7 (CH2) ppm.

7-Bromo-N,N,1-tris(4-methoxybenzyl)-1H-imidazo-[4,5-c]pyridin-4-amine (13)

A 250 mL Schlenk tube was charged with 12 (9.4 g, 16.7 mmol) and trimethyl orthoformate (150 mL). The mixture was heated at 110 °C for 24 h and concentrated in vacuo. The remaining residue was dissolved in the smallest possible volume of EtOAc, and hexanes was carefully added dropwise with stirring to cause slight turbidity. The mixture was chilled in the freezer at −20 °C overnight, and the resulting precipitate was filtered, washed with hexanes, and dried in air to give 13 as an off-white to cream solid in 94% yield (8.96 g). HRMS (ESI): m/z [M + H]+ calcd for C30H3081BrN4O3: 575.1481; found: 575.14530. 1H NMR (399.8 MHz, CDCl3): δ, 7.97 (s, 1H), 7.62 (s, 1H), 7.22–7.20 (d, J = 8.8, 4H), 7.13–7.10 (d, J = 8.8, 2H), 6.90–6.87 (d, J = 8.8, 2H), 6.84–6.82 (d, J = 8.8, 4H), 5.65 (s, 2H), 5.13 (s, 4H), 3.80 (s, 3H), 3.78 (s, 6H) ppm. 13C NMR (100 MHz, CDCl3): δ, 159.6 (Cq), 158.7 (Cq), 151.6 (Cq), 143.1, 141.0, 136.6 (Cq), 131.2 (Cq), 129.5 (Cq), 129.2, 128.5, 128.4 (Cq), 114.5, 113.9, 89.0 (Cq), 55.44, 55.38, 49.8 (CH2), 49.0 (CH2) ppm.

Cross-Coupling Reactions of 13

General Conditions A[43]

A solution of 13 (0.25–3.5 mmol, 1 equiv) in 2:1 dioxane/deionized water mixture (ca. 12 mL/mmol) was placed in a Schlenk tube or a vial equipped with septa. The mixture was bubbled with argon for ca. 1 min/mL. Aryl or vinylboronic acid (1.5 equiv), tetrakis(triphenylphosphine)palladium (0) (0.1 equiv), and anhydrous cesium carbonate (2 equiv) were added, and the mixture was bubbled with argon for few more minutes and heated at 90 °C for 6–16 h with vigorous stirring. It was cooled and diluted with equal volumes of CH2Cl2 and 1:1 water/brine, the phases were separated, and the aqueous phase was extracted twice with CH2Cl2. The combined organic extracts were dried with MgSO4. After filtration and concentration, the products were purified by column chromatography on silica gel eluting with 0–20% EtOAc in hexanes.

General Conditions B[44]

To a flame-dried vial equipped with septa, dry dioxane (ca. 7 mL/mmol) was added and bubbled with argon for ca. 1 min/mL. Compound 13 (0.25–3.5 mmol, 1 equiv), alkylboronic acid (1.1–1.5 equiv), tetrakis(triphenylphosphine)palladium(0) (0.1 equiv), and anhydrous potassium carbonate (3 equiv) were added. The mixture was bubbled with argon for few more minutes and heated at 90 °C for 72 h with vigorous stirring. It was cooled and filtered through Celite and washed with CH2Cl2. After concentration, the products were purified by column chromatography on silica gel eluting with 0–20% EtOAc in hexanes.

General Conditions C[45]

To a flame-dried vial equipped with septa, dry acetonitrile (3.5 mL) was added and bubbled with argon for ca. 1 min/mL. Compound 13 (0.25 mmol, 1 equiv), triphenylphosphine (0.5 equiv), and palladium(II) acetate (0.1 equiv) were added, followed by an olefin (5 equiv) and triethylamine (7 equiv), and the mixture was bubbled with argon for few more minutes and heated at 90 °C for 36 h with vigorous stirring. It was cooled, diluted with CH2Cl2 (50 mL), washed with aqueous NH4Cl (2 × 25 mL), and dried with MgSO4. After filtration and concentration, the products were purified by column chromatography on silica gel eluting with 0–40% EtOAc in hexanes. The procedure for separation of dehalogenated byproduct: the crude mixture obtained after the extractive workup was redissolved in CH2Cl2 (10 mL), and NIS (ca. 0.6 equiv) was added. After stirring at room temperature for 2 h, the mixture was diluted with CH2Cl2 (50 mL), washed with brine containing sodium thiosulfate (25 mL), and dried with MgSO4. After filtration and concentration, the products were purified by column chromatography on silica gel eluting with 0–40% EtOAc in hexanes. 13-I eluted first.

General Conditions D[23]

To a flame-dried vial equipped with septa, dry DMF (5 mL) and triethylamine (2.5 mL) were added and bubbled with argon for ca. 1 min/mL. Compound 13 (0.25 mmol, 1 equiv), CuI (0.1 equiv), and bis(triphenylphosphine)palladium(II) dichloride (0.1 equiv) were added, followed by phenylacetylene (1.5 equiv). The mixture was bubbled with argon for few more minutes and heated at 80 °C for 24 h with vigorous stirring. It was cooled, quenched with saturated aqueous NH4Cl (10 mL), diluted with EtOAc (10 mL), washed with brine (3 × 10 mL), and dried with MgSO4. After filtration and concentration, the products were purified by column chromatography on silica gel eluting with 0–20% EtOAc in hexanes.

General Conditions E[46]

To a flame-dried vial equipped with septa, dry THF (6 mL) was added and bubbled with argon for ca. 1 min/mL. Compound 13 (0.25 mmol, 1 equiv) and tetrakis(triphenylphosphine)palladium(0) (0.1 equiv) were added, followed by trimethylaluminum or benzylmagnesium chloride solutions (3–3.3 equiv), and the mixture was stirred at room temperature for 15 min and then heated at 80 °C for 4 h with vigorous stirring. It was cooled, carefully quenched with Et2O (30 mL), washed with saturated aqueous NH4Cl (30 mL), and dried with MgSO4. After filtration and concentration, the products were purified by column chromatography on silica gel eluting with 0–40% EtOAc in hexanes.

General Conditions F[58]

To a flame-dried vial equipped with septa, was added a solution of ZnCl2 (0.5 M in THF, 5.7 equiv), followed by dropwise addition of Grignard reagent solution (8.6 equiv), and the mixture was stirred under argon for 1 h. Then, compound 13 (0.175–0.7 mmol, 1 equiv) and [1,1′-bis(diphenylphosphino)ferrocene]-dichloropalladium(II) (0.1 equiv) were added, and the mixture was stirred at room temperature under argon for overall 21–29 h. It was carefully quenched with Et2O (30–50 mL), washed with equal volume of saturated aqueous NH4Cl, and dried with MgSO4. After filtration and concentration, the products were purified by column chromatography on silica gel eluting with 0–20% EtOAc in hexanes.

General Conditions G[48]

To a flame-dried vial equipped with septa, were added sequentially NiI2 (0.065 mmol, 0.13 equiv), 4,4′-dimethoxy-2,2′-bipyridine (0.05 mmol, 0.1 equiv), NaI (0.125 mmol, 0.25 equiv), 13 (0.5 mmol, 1 equiv), dry DMA (2 mL), pyridine (5 μL), ethyl 4-bromobutyrate (0.55 mmol, 1.1 equiv), and Zn powder (1 mmol, 2 equiv). The headspace of the vial was flushed with argon for 1 min. The reaction mixture was stirred at room temperature for 5 min and then heated at 60 °C for 20 h with vigorous stirring. It was cooled, diluted with EtOAc (50 mL), washed with brine (3 × 25 mL), and dried with MgSO4. The crude mixtures were subjected to the same procedure of dehalogenated byproduct separation as described under general conditions C. The products were purified by column chromatography on silica gel eluting with 0–40% EtOAc in hexanes. 13-I eluted first.

Characterization Data for Compounds from Table

N,N,1-Tris(4-methoxybenzyl)-7-phenyl-1H-imidazo[4,5-c]pyridin-4-amine (14a)

General conditions A, 6 h, 335 mg, 91%. Yellowish syrup. HRMS (ESI): m/z [M + H]+ calcd for C36H35N4O3: 571.27093; found: 571.26842. 1H NMR (400 MHz, CDCl3): δ, 7.78 (s, 1H), 7.68 (s, 1H), 7.36–7.34 (m, 3H), 7.29–7.27 (d, J = 8.8, 4H), 7.27–7.25 (m, 2H), 6.87–6.85 (d, J = 8.8, 2H), 6.71–6.69 (d, J = 8.8, 2H), 6.56–6.54 (d, J = 8.8, 2H), 5.23 (s, 4H), 4.94 (s, 2H), 3.80 (s, 6H), 3.76 (s, 3H) ppm. 13C NMR (100 MHz, CDCl3): δ, 159.6 (Cq), 158.7 (Cq), 151.6 (Cq), 141.8, 141.0, 137.7 (Cq), 136.7 (Cq), 131.6 (Cq), 130.4, 129.3, 128.2, 128.10, 128.06 (Cq), 128.0 (Cq), 127.4, 114.1, 113.9, 113.8 (Cq), 55.40, 55.38, 49.64 (CH2), 49.55 (CH2) ppm.

(E)-N,N,1-Tris(4-methoxybenzyl)-7-styryl-1H-imidazo[4,5-c]pyridin-4-amine (14b)

General conditions A, 16 h, 123 mg, 79%. General conditions C, 36 h, 83 mg, 53%. Tan solid. HRMS (ESI): m/z [M + H]+ calcd for C38H37N4O3: 597.28658; found: 597.28453. 1H NMR (400 MHz, CDCl3): δ, 8.10 (s, 1H). 7.69 (s, 1H), 7.32–7.23 (m, 8H), 7.20–7.16 (d, J = 15.6, 1H), 7.06–7.04 (d, J = 8.8, 2H), 6.92–6.89 (d, J = 8.8, 2H), 6.86–6.82 (m, 5H), 5.46 (s, 2H), 5.22 (s, 4H), 3.804 (s, 3H), 3.796 (s, 6H) ppm. 13C NMR (100 MHz, CDCl3): δ, 159.6 (Cq), 158.7 (Cq), 151.7 (Cq), 141.0, 139.6, 138.1 (Cq), 137.8 (Cq), 131.5 (Cq), 129.3, 129.0, 128.8, 128.6 (Cq), 127.8 (Cq), 127.6, 127.4, 126.4, 122.2, 114.7, 113.9, 110.8 (Cq), 55.5, 55.4, 49.8 (CH2), 49.7 (CH2) ppm.

N,N,1-Tris(4-methoxybenzyl)-7-phenethyl-1H-imidazo[4,5-c]pyridin-4-amine (14c)

General conditions B, 72 h, 55 mg, 35%. General conditions F, 29 h, 327 mg, 77%. Colorless syrup. HRMS (ESI): m/z [M + H]+ calcd for C38H39N4O3: 599.30224; found: 599.29971. 1H NMR (400 MHz, CDCl3): δ, 7.76 (s, 1H), 7.63 (s, 1H), 7.31 −7.20 (m, 3H), 7.26–7.24 (d, J = 8.8, 4H), 7.10–7.08 (d, J = 8.8, 4H), 6.94–6.92 (d, J = 8.8, 2H), 6.87–6.84 (m, 2H), 6.86–6.83 (d, J = 8.8, 2H), 5.34 (s, 2H), 5.18 (s, 4H), 3.80 (s, 6H), 3.79 (s, 3H), 3.02–2.88 (m, 4H) ppm. 13C NMR (100 MHz, CDCl3): δ, 159.6 (Cq), 158.6 (Cq), 151.4 (Cq), 141.4, 141.3 (Cq), 141.0, 138.8 (Cq), 131.7 (Cq), 129.3, 128.59, 128.56 (Cq), 128.48, 127.6, 126.3, 114.7, 113.8, 110.9 (Cq), 55.44, 55.36, 49.7 (CH2), 49.4 (CH2), 38.5 (CH2), 30.7 (CH2) ppm. For other conditions tested, see Supporting Information.

N,N,1-Tris(4-methoxybenzyl)-1H-imidazo[4,5-c]pyridin-4-amine (13-H)

General conditions B, 72 h, 34 mg, 30%. Yellowish syrup. HRMS (ESI): m/z [M + H]+ calcd for C30H31N4O3: 495.23964; found: 495.23722. 1H NMR (400 MHz, CDCl3): δ, 7.96–7.94 (d, J = 5.6, 1H), 7.70 (s, 1H), 7.25–7.23 (d, J = 8.8, 4H), 7.16–7.14 (d, J = 8.8, 2H), 6.90–6.87 (d, J = 8.8, 2H), 6.84–6.82 (d, J = 8.8, 4H), 6.64–6.62 (d, J = 5.6, 1H), 5.21 (s, 2H), 5.17 (s, 4H), 3.80 (s, 3H), 3.78 (s, 6H) ppm. 13C NMR (100 MHz, CDCl3): δ, 159.8 (Cq), 158.6 (Cq), 152.3 (Cq), 141.1, 140.5 (Cq), 139.2, 131.5 (Cq), 129.2, 129.0, 127.9 (Cq), 127.3 (Cq), 114.6, 113.9, 96.2, 55.45, 55.36, 49.4 (CH2), 48.6 (CH2) ppm.

7-Iodo-N,N,1-tris(4-methoxybenzyl)-1H-imidazo[4,5-c]pyridin-4-amine (13-I)

General conditions C, 36 h, 50 mg, 31%. General conditions G, 20 h, 110 mg, 36%. Yellowish foam. HRMS (ESI): m/z [M + H]+ calcd for C30H30IN4O3: 621.13629; found: 621.13419. 1H NMR (400 MHz, CDCl3): δ, 8.17 (s, 1H), 7.62 (s, 1H), 7.23–7.20 (d, J = 8.8, 4H), 7.08–7.06 (d, J = 8.8, 2H), 6.90–6.88 (d, J = 8.8, 2H), 6.84–6.82 (d, J = 8.8, 4H), 5.67 (s, 2H), 5.15 (s, 4H), 3.80 (s, 3H), 3.79 (s, 6H) ppm. 13C NMR (100 MHz, CDCl3): δ, 159.6 (Cq), 158.7 (Cq), 152.3 (Cq), 149.2, 141.4, 138.5 (Cq), 131.2 (Cq), 129.8 (Cq), 129.2, 128.5, 128.4 (Cq), 114.5, 113.9, 56.0 (Cq), 55.44, 55.38, 49.8 (CH2), 48.5 (CH2) ppm.

7-Ethyl-N,N,1-tris(4-methoxybenzyl)-1H-imidazo[4,5-c]pyridin-4-amine (14d)

General conditions B, 72 h, 284 mg, 31%. General conditions F, 25 h, 69 mg, 76%. Colorless syrup. HRMS (ESI): m/z [M + H]+ calcd for C32H35N4O3: 523.27093; found: 523.26894. 1H NMR (400 MHz, CDCl3): δ, 7.73 (s, 1H), 7.64 (s, 1H), 7.24–7.21 (d, J = 8.8, 4H), 6.99–6.97 (d, J = 8.8, 2H), 6.88–6.86 (d, J = 8.8, 2H), 6.83–6.81 (d, J = 8.8, 4H), 5.44 (s, 2H), 5.15 (s, 4H), 3.79 (s, 3H), 3.78 (s, 6H), 2.77–2.71 (q, J = 7.6, 2H), 1.25–1.21 (t, J = 7.6, 3H) ppm. 13C NMR (100 MHz, CDCl3): δ, 159.6 (Cq), 158.6 (Cq), 151.3 (Cq), 141.0, 140.5, 138.8 (Cq), 131.8 (Cq), 129.3, 128.8 (Cq), 128.6 (Cq), 127.6, 114.6, 113.8, 113.2 (Cq), 55.5, 55.4, 49.7 (CH2), 49.6 (CH2), 22.0 (CH2), 16.2 ppm.

7-Isobutyl-N,N,1-tris(4-methoxybenzyl)-1H-imidazo[4,5-c]pyridin-4-amine (14e)

General conditions B, 72 h, 658 mg, 29%. General conditions F, 21 h, 327 mg, 85%. Yellowish syrup. HRMS (ESI): m/z [M + H]+ calcd for C34H39N4O3: 551.30224; found: 551.29987. 1H NMR (400 MHz, CDCl3): δ, 7.67 (s, 1H), 7.63 (s, 1H), 7.24–7.21 (d, J = 8.8, 4H), 6.98–6.95 (d, J = 8.8, 2H), 6.88–6.86 (d, J = 8.8, 2H), 6.84–6.82 (d, J = 8.8, 4H), 5.40 (s, 2H), 5.15 (s, 4H), 3.79 (s, 3H), 3.78 (s, 6H), 2.51–2.49 (d, J = 7.2, 2H), 1.84–1.74 (septet, J = 6.4, 1H), 0.95–0.94 (d, J = 6.4, 6H) ppm. 13C NMR (100 MHz, CDCl3): δ, 159.6 (Cq), 158.6 (Cq), 151.4 (Cq), 142.3, 140.9, 139.0 (Cq), 131.8 (Cq), 129.3, 128.8 (Cq), 128.7 (Cq), 127.6, 114.7, 113.8, 110.6 (Cq), 55.5, 55.4, 49.6 (CH2), 49.5 (CH2), 38.2 (CH2), 30.7, 22.4 ppm.

Methyl (E)-3-(4-(Bis(4-methoxybenzyl)amino)-1-(4-methoxybenzyl)-1H-imidazo[4,5-c]pyridin-7-yl)acrylate (14f)

General conditions C, 36 h, 86 mg, 57%. Yellowish syrup. HRMS (ESI): m/z [M + H]+ calcd for C34H35N4O5: 579.26077; found: 579.25862. 1H NMR (400 MHz, CDCl3): δ, 8.23 (s, 1H), 8.05–8.01 (d, J = 15.6, 1H), 7.69 (s, 1H), 7.24–7.22 (d, J = 8.8, 4H), 7.12–7.09 (d, J = 8.8, 2H), 6.90–6.88 (d, J = 8.8, 2H), 6.85–6.83 (d, J = 8.8, 4H), 6.26–6.22 (d, J = 15.6, 1H), 5.44 (s, 2H), 5.23 (s, 4H), 3.79–3.78 (overlapped s, 12H) ppm. 13C NMR (100 MHz, CDCl3): δ, 167.8 (Cq), 159.8 (Cq), 158.8 (Cq), 152.8 (Cq), 141.3, 138.8, 138.0 (Cq), 130.9 (Cq), 129.4, 129.2, 128.2 (Cq), 127.6 (Cq), 127.3 (Cq), 114.6, 114.5, 114.0, 107.6 (Cq), 55.41, 55.36, 51.6, 50.4 (CH2), 49.7 (CH2) ppm.

N,N,1-Tris(4-methoxybenzyl)-7-(phenylethynyl)-1H-imidazo[4,5-c]pyridin-4-amine (14g)

General conditions D, 20 h, 138 mg, 89%. Brown syrup. HRMS (ESI): m/z [M + H]+ calcd for C38H35N4O3: 595.27093; found: 595.26894. 1H NMR (400 MHz, CDCl3): δ, 8.22 (s, 1H), 7.67 (s, 1H), 7.39–7.37 (m, 2H), 7.33–7.30 (m, 3H), 7.26–7.24 (d, J = 8.4, 2H), 7.25–7.23 (d, J = 8.4, 4H), 6.89–6.87 (d, J = 8.4, 2H), 6.86–6.84 (d, J = 8.4, 4H), 5.75 (s, 2H), 5.21 (s, 4H), 3.80 (s, 9H) ppm. 13C NMR (100 MHz, CDCl3): δ, 159.6 (Cq), 158.8 (Cq), 151.9 (Cq), 146.7, 140.3, 138.3 (Cq), 131.1, 129.3, 128.9, 128.8 (Cq), 128.5, 128.0, 127.0 (Cq), 123.7 (Cq), 114.5, 113.9, 93.6 (Cq), 85.1 (Cq), 55.44, 55.38, 49.6 (CH2), 48.5 (CH2) ppm.

N,N,1-Tris(4-methoxybenzyl)-7-methyl-1H-imidazo[4,5-c]pyridin-4-amine (14h)

General conditions E, 4 h, 115 mg, 87%. Yellow syrup. HRMS (ESI): m/z [M + H]+ calcd for C31H33N4O3: 509.25528; found: 509.25341. 1H NMR (400 MHz, CDCl3): δ, 7.66 (s, 1H), 7.65 (s, 1H), 7.24–7.21 (d, J = 8.4, 4H), 7.00–6.97 (d, J = 8.4, 2H), 6.88–6.86 (d, J = 8.4, 2H), 6.83–6.81 (d, J = 8.4, 2H), 5.45 (s, 2H), 5.15 (s, 4H), 3.79 (s, 3H), 3.78 (s, 6H), 2.35 (s, 3H) ppm. 13C NMR (100 MHz, CDCl3): δ, 159.5 (Cq), 158.6 (Cq), 151.4 (Cq), 141.5, 140.8, 139.5 (Cq), 131.8 (Cq), 129.3, 129.0, 128.5 (Cq), 127.5, 114.6, 113.8, 106.6 (Cq), 55.44, 55.36, 49.7 (CH2), 49.3 (CH2), 15.1 ppm.

7-Benzyl-N,N,1-tris(4-methoxybenzyl)-1H-imidazo[4,5-c]pyridin-4-amine (14i)

General conditions E, 4 h, 69 mg, 45%. Brown syrup. HRMS (ESI): m/z [M + H]+ calcd for C37H37N4O3: 585.286588; found: 585.28433. 1H NMR (400 MHz, CDCl3): δ, 7.76 (s, 1H), 7.59 (s, 1H), 7.32–7.23 (m, 7H), 7.09–7.08 (d, J = 8.8, 2H), 6.87–6.82 (m, 8H), 5.22 (s, 4H), 5.05 (s, 2H), 4.00 (s, 2H), 3.80 (s, 9H) ppm. 13C NMR (100 MHz, CDCl3): δ, 159.5 (Cq), 158.6 (Cq), 151.8 (Cq), 142.9, 141.4 (Cq), 140.9, 139.2 (Cq), 131.6 (Cq), 129.3, 129.0 (Cq), 128.9 (Cq), 128.8, 128.0, 127.2, 126.5, 114.6, 113.8, 108.5 (Cq), 55.42, 55.35, 49.7 (CH2), 49.0 (CH2), 35.0 (CH2) ppm.

7-Isopropyl-N,N,1-tris(4-methoxybenzyl)-1H-imidazo[4,5-c]pyridin-4-amine (14j)

General conditions F, 27 h, 80 mg, 86%. Contains ∼20% of n-propyl impurity. Colorless syrup. HRMS (ESI): m/z [M + H]+ calcd for C33H37N4O3: 537.28658; found: 537.28573. 1H NMR (400 MHz, CDCl3): δ, 7.89 (s, 1H), 7.63 (s, 1H), 7.25–7.22 (d, J = 8.8, 4H), 6.98–6.96 (d, J = 8.8, 2H), 6.88–6.86 (d, J = 8.8, 2H), 6.84–6.82 (d, J = 8.8, 4H), 5.44 (s, 2H), 5.16 (s, 4H), 3.79 (s, 3H), 3.78 (s, 6H), 3.30–3.20 (septet, J = 6.8, 1H), 1.25–1.24 (d, J = 6.4, 6H) ppm. 13C NMR (100 MHz, CDCl3): δ, 159.6 (Cq), 158.6 (Cq), 151.0 (Cq), 141.4, 138.1 (Cq), 138.0, 131.8 (Cq), 129.3, 128.7 (Cq), 128.6 (Cq), 127.6, 118.3 (Cq), 114.6, 113.8, 55.45, 55.36, 50.0 (CH2), 49.7 (CH2), 26.3, 24.3 ppm.

Ethyl 4-(4-(Bis(4-methoxybenzyl)amino)-1-(4-methoxybenzyl)-1H-imidazo[4,5-c]pyridin-7-yl)butanoate (14k)

General conditions G, 20 h, 100 mg, 32%. Colorless syrup. HRMS (ESI): m/z [M + H]+ calcd for C36H41N4O5: 609.30773; found: 609.30580. 1H NMR (400 MHz, CDCl3): δ, 7.69 (s, 1H), 7.65 (s, 1H), 7.24–7.22 (d, J = 8.8, 4H), 6.98–6.96 (d, J = 8.8, 2H), 6.87–6.85 (d, J = 8.8, 2H), 6.84–6.82 (d, J = 8.8, 4H), 5.47 (s, 2H), 5.16 (s, 4H), 4.14–4.08 (q, J = 7.2, 2H), 3.78 (overlapped s, 9H), 2.71–2.68 (m, 2H), 2.37–2.33 (t, J = 7.2, 2H), 1.91–1.86 (m, 2H), 1.26–1.22 (t, J = 7.2, 3H) ppm. 13C NMR (100 MHz, CDCl3): δ, 173.5 (Cq), 159.5 (Cq), 158.6 (Cq), 151.4 (Cq), 141.6, 141.1, 138.7 (Cq), 131.7 (Cq), 129.3, 128.8 (Cq), 128.7 (Cq), 127.5, 114.6, 113.8, 110.6 (Cq), 60.5 (CH2), 55.42, 55.36, 49.6 (CH2), 49.4 (CH2), 33.7 (CH2), 28.4 (CH2), 27.4 (CH2), 14.4 ppm.

Synthesis of Phosphoramidites 20i, 20d, and 20e and Attempted Synthesis of 20a

7-Benzyl-1H-imidazo[4,5-c]pyridin-4-amine, Trifluoroacetic Acid (TFA) Salt (15i)

This compound was synthesized according to the modified general procedure.[52] A solution of 14i (589 mg, 1 mmol) in TFA (6 mL) was placed in a 100 mL pressure tube, anisole (0.65 mL, 6 equiv) was added, and the mixture was heated at 70 °C for 24 h with vigorous stirring. It was cooled and concentrated under reduced pressure. The residue was coevaporated with toluene (3 × 25 mL) to obtain 15i (ca. 0.34 g, quantitative) as an oil that was taken into next step without further purification.

1-(7-Benzyl-1H-imidazo[4,5-c]pyridin-4-yl)-3,3,4,4-tetramethylpyrrolidine-2,5-dione (16i)

This compound was synthesized according to the reported general procedure.[53] A solution of 15i (0.34 g, 1 mmol) in pyridine (10 mL) was placed in a 100 mL Schlenk flask. Tetramethylsuccinic anhydride (M4SA)[59] (0.312 g, 2 mmol) and DBU (0.6 mL, 4 mmol) were added, and the mixture was heated at 120 °C under argon for 20 h with vigorous stirring. It was cooled and concentrated under reduced pressure. The residue was coevaporated with toluene (3 × 25 mL). The product was purified by column chromatography on silica gel eluting with 0–5% MeOH in CH2Cl2 to give a cream foam. It was taken in SOCl2 (10 mL), heated at 80 °C for 2 h, cooled and concentrated under reduced pressure. The residue was coevaporated with EtOAc (3 × 25 mL). The product was purified by column chromatography on silica gel eluting with 0–5% MeOH in CH2Cl2 to give 16i as a cream foam in 57% yield (0.208 g). HRMS (ESI): m/z [M + H]+ calcd for C21H23N4O2: 363.18212; found: 363.18045. 1H NMR (400 MHz, CD3OD): δ, 8.36 (s, 1H), 8.12 (s, 1H), 7.29–7.20 (m, 5H), 4.35 (s, 2H), 1.37 (bs, 12H) ppm. 13C NMR (100 MHz, CD3OD): δ, 183.4, 146.2, 141.8, 139.7, 137.5, 129.8, 127.7, 124.7, 118.3, 35.5, 21.8 ppm.

(2R,3S,5R)-5-(7-Benzyl-4-(3,3,4,4-tetramethyl-2,5-dioxopyrrolidin-1-yl)-1H-imidazo[4,5-c]pyridin-1-yl)-2-(((4-methylbenzoyl)oxy)methyl)tetrahydrofuran-3-yl 4-Methylbenzoate (17i)

This compound was synthesized according to the reported general procedure.[53] To a solution of 16i (0.19 g, 0.52 mmol) in dry CH3CN (25 mL) in a 100 mL round-bottom flask, 60% NaH in oil (42 mg, 1.05 mmol) was added, and the mixture was vigorously stirred at room temperature under argon for 45 min. 1-(α)-Chloro-3,5-di-O-(p-toluoyl)-2-deoxy-d-ribose (0.368 g, 0.94 mmol) was subsequently added in one portion, and the stirring was continued at room temperature for 4.5 h. The mixture was filtered through Celite and concentrated. The product was purified by column chromatography on silica gel eluting with 50% EtOAc in hexanes. Compound 17i was obtained as a yellowish syrup in 39% yield (0.145 g). HRMS (ESI): m/z [M + H]+ calcd for C42H43N4O7: 715.31322; found: 715.31184. 1H NMR (400 MHz, CDCl3): δ, 8.32 (s, 1H), 8.19 (s, 1H), 7.92–7.90 (d, J = 8.4, 2H), 7.87–7.85 (d, J = 8.4, 2H), 7.34–7.32 (d, J = 8.4, 2H), 7.92–7.90 (d, J = 8.4, 2H), 7.14–7.11 (m, 3H), 7.06–7.04 (m, 2H), 6.20–6.17 (dd, J1 = 5.2, J2 = 9.2, 1H), 5.54–5.53 (m, 1H), 4.60–4.49 (m, 4H), 4.38–4.34 (AB/2, J = 16.8, 1H), 2.49 (s, 3H), 2.46–2.42 (m, 1H), 2.40 (s, 3H), 2.06–2.01 (m, 1H), 1.39 (bs, 1H) ppm. 13C NMR (100 MHz, CDCl3): δ, 181.6 (Cq), 166.2 (Cq), 165.8 (Cq), 144.8 (Cq), 144.6, 144.4 (Cq), 142.3, 139.6 (Cq), 138.7 (Cq), 138.6 (Cq), 137.5 (Cq), 130.0, 129.8, 129.54, 129.47, 129.2, 128.1, 127.1, 126.6 (Cq), 126.5 (Cq), 120.0 (Cq), 85.8, 82.7, 74.8, 64.3 (CH2), 48.1 (Cq), 39.4 (CH2), 35.8 (CH2), 21.9, 21.8, 21.7, 21.5 ppm. For the critical NOESY correlations, see Figure S1 and copies of spectra (Supporting Information).

1-(7-Benzyl-1-((2R,4S,5R)-4-hydroxy-5-(hydroxylmethyl)tetrahydrofuran-2-yl)-1H-imidazo[4,5-c]pyridin-4-yl)-3,3,4,4-tetramethylpyrrolidine-2,5-dione (18i)

This compound was synthesized according to the general procedure.[53] To a solution of 17i (0.14 g, 0.20 mmol) in dry MeOH (4 mL) in a 25 mL round-bottom flask, NaOMe (32 mg, 0.60 mmol) was added. The mixture was stirred at room temperature under argon for 2 h and concentrated in vacuo. The product was purified by column chromatography on silica gel eluting with 0–10% MeOH in dichloromethane to obtain 18i as an off-white solid in 66% yield (62 mg). HRMS (ESI): m/z [M + H]+ calcd for C26H31N4O5: 479.22948; found: 479.22931. 1H NMR (400 MHz, CD3OD): δ, 8.69 (s, 1H), 8.14 (s, 1H), 7.32–7.14 (m, 5H), 6.35–6.32 (dd, J1 = 6.4, J2 = 12.8, 1H), 4.61–4.57 (AB/2, J = 16.8, 1H), 4.47–4.30 (AB/2, J = 16.8, 1H), 4.46–4.44 (m, 1H), 3.95–3.92 (app q, J = 3.6, 1H), 3.73–3.69 (app dd, J1 = 3.6, J2 = 12.0, 1H), 3.66–3.62 (app dd, J1 = 3.6, J2 = 12.0, 1H), 2.52–2.46 (ddd, J1 = J2 = 6.0, J3 = 12.8, 1H), 2.14–2.08 (ddd, J1 = 4.4, J2 = 6.0, J3 = 10.4, 1H), 1.37 (bs, 12H) ppm. 13C NMR (100 MHz, CD3OD): δ, 183.4 (Cq), 145.9 (Cq), 144.3, 141.0 (Cq), 140.1 (Cq), 138.6 (Cq), 138.4 (Cq), 130.1, 129.5, 127.9, 123.5 (Cq), 89.2, 87.1, 71.7, 62.6 (CH2), 48.9 (Cq), 42.0 (CH2), 35.9 (CH2), 21.88, 21.84 ppm.

1-(7-Benzyl-1-((2R,4S,5R)-5-((bis(4-methoxyphenyl)-(phenyl)methoxy)methyl)-4-hydroxytetrahydrofuran-2-yl)-1H-imidazo[4,5-c]pyridin-4-yl)-3,3,4,4-tetramethylpyrrolidine-2,5-dione (19i)

This compound was synthesized according to the reported general procedure.[53] To a solution of 18i (56 mg, 0.12 mmol) in pyridine (2 mL), 4,4′-dimethoxytrityl chloride (DMT-Cl) (155 mg, 0.457 mmol) was added in one portion, and the mixture was stirred at room temperature under argon for 5 h (TLC showed complete conversion). MeOH (0.2 mL) was then added, and the mixture was concentrated in vacuo. The product was purified by column chromatography on silica gel (neutralized with triethylamine) eluting with 5–20% acetone in dichloromethane to obtain 19i as a colorless solid in 69% yield (63 mg). HRMS (ESI): m/z [M + H]+ calcd for C47H49N4O7: 781.36017; found: 781.35738. 1H NMR (400 MHz, CDCl3): δ, 8.24 (s, 1H), 8.10 (s, 1H), 7.38–7.10 (m, 14H), 6.82–6.80 (d, J = 8.4, 4H), 6.16–6.12 (dd, J1 = 6.4, J2 = 12.4, 1H), 4.50–4.46 (AB/2, J = 16.8, 1H), 4.43–4.38 (m, 1H), 4.36–4.32 (AB/2, J = 16.8, 1H), 4.01–3.98 (app q, J = 4.4, 1H), 3.78 (s, 6H), 3.36–3.32 (app dd, J1 = 4.4, J2 = 10.4, 1H), 3.28–3.24 (app dd, J1 = 4.8, J2 = 10.4, 1H), 2.6 (bs, 1H), 2.28–2.22 (ddd, J1 = J2 = 6.0, J3 = 13.2, 1H), 1.90–1.84 (ddd, J1 = J2 = 5.6, J3 = 9.6, 1H), 1.39–1.38 (overlapped s, 12H) ppm. 13C NMR (100 MHz, CDCl3): δ, 181.7 (Cq), 158.7 (Cq), 144.5 (Cq), 144.1, 142.6, 139.4 (Cq), 138.8 (Cq), 138.2 (Cq), 137.6 (Cq), 135.63 (Cq), 135.55 (Cq), 130.12, 130.09, 129.1, 128.6, 128.13, 128.10, 127.11, 127.07, 120.6 (Cq), 113.4, 86.8 (Cq), 85.7, 85.2, 72.0, 63.7 (CH2), 55.3, 48.0 (Cq), 41.4 (CH2), 35.6 (CH2), 21.8, 21.4 ppm.

(2R,3S,5R)-5-(7-Benzyl-4-(3,3,4,4-tetramethyl-2,5-dioxopyrrolidin-1-yl)-1H-imidazo[4,5-c]pyridin-1-yl)-2-((bis(4-methoxyphenyl)(phenyl)methoxy)methyl)tetrahydrofuran-3-yl (2-Cyanoethyl) Diisopropylphosphoramidite (20i)

This compound was synthesized according to the reported general procedure.[60] To a solution of 19i (62 mg, 0.080 mmol) in dry CH2Cl2 (2 mL) containing N,N-diisopropylethylamine (DIEA) (70 μL, 0.4 mmol), 2-cyanoethyl-N,N-diisopropyl chlorophosphoramidite (40 μL, 0.175 mmol) was added in one portion, and the mixture was stirred at room temperature under argon for 3 h (TLC showed complete conversion). The mixture was concentrated in vacuo at rt, and the product was purified by column chromatography on silica gel (neutralized with triethylamine) eluting with 50% EtOAc in hexanes containing 1% triethylamine, to obtain 20i as a mixture of diastereomers (colorless solid, R ∼ 0.5 in 5% acetone/CH2Cl2, two close spots visible on TLC) in 49% yield (38 mg). HRMS (ESI): m/z [M + H]+ calcd for C56H66N6O8P: 981.46801; found: 981.46481. 1H NMR (400 MHz, acetone-d6): δ, 8.40, 8.39 (s, 1H), 8.21, 8.19 (s, 1H), 7.45–7.18 (m, 14H), 6.84–6.80 (m, 4H), 6.40–6.35 (dd, J1 = 7.0, J2 = 12.8, 1H), 4.68–4.78 (m, 2H), 4.56–4.52, 4.53–4.49 (AB/2, J = 16.4, 1H), 4.28–4.26, 4.24–4.21 (app q, J = 4.4, 1H), 3.763, 3.757 (s, 6H), 3.67–3.58 (m, 2H), 3.36–3.32 (m, 2H), 2.91–2.78 (m, 3H), 2.76–2.73 (t, J = 6.0, 1H), 2.68–2.65 (t, J = 6.0, 1H), 1.35, 1.31 (bs, 12H), 1.21–1.19 (m, 6H), 1.15–1.14, 1.13–1.11 (d, J = 6.8, 6H) ppm. 13C NMR (100 MHz, acetone-d6): δ, 181.7, 159.6, 145.9, 144.36, 144.27, 144.02, 143.97, 140.4, 140.08, 140.05, 139.2, 138.8, 136.64, 136.58, 136.52, 130.96, 130.93, 129.7, 129.34, 129.31, 128.97, 128.91, 128.6, 127.6, 127.50, 127.47, 122.3, 122.2, 119.0, 114.0, 87.22, 87.18, 86.6, 86.4, 86.3, 86.2, 74.4, 74.0, 73.8, 64.3, 64.3, 59.7, 59.6, 59.4, 55.5, 55.0, 48.3, 44.06, 43.99, 43.94, 43.86, 40.4, 40.3, 35.5, 24.94, 24.86, 24.82, 22.0, 21.7, 21.4, 20.8, 20.7 ppm. 31P NMR (162 MHz, acetone-d6): δ, 149.8, 149.6 ppm.

7-Ethyl-1H-imidazo[4,5-c]pyridin-4-amine, Trifluoroacetic Acid Salt (15d)

This compound was synthesized in a similar fashion to 15i. A solution of 14d (750 mg, 1.4 mmol) in TFA (10 mL) was placed in a 100 mL pressure tube, anisole (ca. 1 mL, 6 equiv) was added, and the mixture was heated at 70 °C for 24 h with vigorous stirring. It was cooled and concentrated under reduced pressure. The residue was coevaporated with toluene (3 × 20 mL) to obtain 15d (ca. 0.4 g, quantitative) as an oil that was taken into next step without further purification.

1-(7-Ethyl-1H-imidazo[4,5-c]pyridin-4-yl)-3,3,4,4-tetramethylpyrrolidine-2,5-dione (16d)

This compound was synthesized in a similar fashion to 16i. A solution of 15d (0.4 g, 1.4 mmol) in pyridine (15 mL) was placed in a 100 mL Schlenk flask. Tetramethylsuccinic anhydride (M4SA) (0.5 g, 3.2 mmol) and DBU (1 mL, 6.7 mmol) were added, and the mixture was heated at 120 °C under argon for 22 h with vigorous stirring. It was cooled and concentrated under reduced pressure. The residue was coevaporated with toluene (3 × 20 mL). The product was purified by column chromatography on silica gel eluting with 0–5% MeOH in CH2Cl2 to give a cream solid. It was taken in SOCl2 (12 mL), heated at 80 °C for 2 h, cooled, and concentrated under reduced pressure. The residue was coevaporated with EtOAc (3 × 20 mL). The product was purified by column chromatography on silica gel eluting with 0–8% MeOH in CH2Cl2 to give 16d as a cream solid in 88% yield (0.37 g). HRMS (ESI): m/z [M + H]+ calcd for C16H21N4O2: 301.16647; found: 301.16501. 1H NMR (400 MHz, CD3OD): δ, 8.52 (s, 1H), 8.24 (s, 1H), 3.07–3.01 (q, J = 7.2, 2H), 1.41–1.37 (t, J = 7.2, 3H), 1.37 (bs, 12H) ppm. 13C NMR (100 MHz, CD3OD): δ, 182.9, 146.9, 143.5, 139.6, 135.4, 135.1, 128.7, 48.9, 23.2, 21.8, 14.5 ppm.

(2R,3S,5R)-5-(7-Ethyl-4-(3,3,4,4-tetramethyl-2,5-dioxopyrrolidin-1-yl)-1H-imidazo[4,5-c]pyridin-1-yl)-2-(((4-methylbenzoyl)oxy)methyl)tetrahydrofuran-3-yl 4-Methylbenzoate (17d)

This compound was synthesized in a similar fashion to 17i. To a solution of 16i (0.35 g, 1.17 mmol) in dry CH3CN (60 mL) in a 250 mL round-bottom flask, 60% NaH in oil (100 mg, 2.5 mmol) was added, and the mixture was vigorously stirred at room temperature under argon for 45 min. 1-(α)-Chloro-3,5-di-O-(p-toluoyl)-2-deoxy-d-ribose (0.817 g, 2.1 mmol) was subsequently added portionwise over 1 h, and the stirring was continued at room temperature for overall 5 h. The mixture was filtered through Celite and concentrated. The product was purified by column chromatography on silica gel eluting with 0–50% EtOAc in hexanes and concentrated repeatedly with dichloromethane. Compound 17d was obtained as a yellowish syrup in 65% yield (0.494 g). HRMS (ESI): m/z [M + H]+ calcd for C37H41N4O7: 653.29757; found: 653.29654. 1H NMR (400 MHz, CDCl3): δ, 8.27 (s, 1H), 8.25 (s, 1H), 7.92–7.90 (d, J = 8.0, 2H), 7.88–7.86 (d, J = 8.0, 2H), 7.30–7.28 (d, J = 8.4, 2H), 7.24–7.22 (d, J = 8.4, 2H), 6.60–6.57 (dd, J1 = 6.4, J2 = 12.8, 1H), 5.72–5.68 (m, 1H), 4.67–4.59 (m, 3H), 3.16–3.00 (m, 2H), 2.82–2.79 (dd, J1 = 4.4, J2 = 6.8, 2H), 2.44 (s, 3H), 2.40 (s, 3H), 1.43–1.40 (t, J = 7.6, 3H), 1.37 (bs, 1H) ppm. 13C NMR (100 MHz, CDCl3): δ, 181.7 (Cq), 166.3 (Cq), 165.9 (Cq), 144.9 (Cq), 144.4 (Cq), 142.6, 142.0, 138.9 (Cq), 137.6 (Cq), 137.3 (Cq), 130.0, 129.9, 129.8, 129.5, 126.6 (Cq), 126.4 (Cq), 124.2 (Cq), 85.5, 82.7, 74.6, 64.0 (CH2), 48.0 (Cq), 39.7 (CH2), 22.9 (CH2), 21.9, 21.8, 21.7, 21.5, 15.4 ppm. For the critical NOESY correlations, see Figure S2 and copies of spectra (Supporting Information).

1-(7-Ethyl-1-((2R,4S,5R)-4-hydroxy-5-(hydroxymethyl)tetrahydrofuran-2-yl)-1H-imidazo[4,5-c]pyridin-4-yl)-3,3,4,4-tetramethylpyrrolidine-2,5-dione (18d)

This compound was synthesized in a similar fashion to 18i. To a solution of 17d (0.46 g, 0.7 mmol) in dry MeOH (6 mL) in a 25 mL round-bottom flask, NaOMe (114 mg, 2.11 mmol) was added. The mixture was stirred at room temperature under argon for 2 h and concentrated in vacuo. The product was purified by column chromatography on silica gel eluting with 0–10% MeOH in dichloromethane to obtain 18d as an off-white solid in 74% yield (218 mg). HRMS (ESI): m/z [M + H]+ calcd for C21H29N4O5: 417.21383; found: 417.21265. 1H NMR (400 MHz, CD3OD): δ, 8.74 (s, 1H), 8.20 (s, 1H), 6.63–6.60 (dd, J1 = 6.4, J2 = 12.8, 1H), 4.58–4.55 (m, 1H), 4.06–4.03 (app q, J = 3.6, 1H), 3.78–3.65 (m, 2H), 3.23–3.12 (app septet, J = 3.6, 2H), 2.78–2.72 (m, 1H), 2.60–2.54 (ddd, J1 = 3.6, J2 = 6.0, J3 = 10.4, 1H), 1.44–1.40 (t, J = 7.6, 3H), 1.37 (bs, 12H) ppm. 13C NMR (100 MHz, CD3OD): δ, 183.4 (Cq), 145.8 (Cq), 142.4, 140.4 (Cq), 138.4 (Cq), 137.8 (Cq), 127.2 (Cq), 89.4, 87.2, 72.1, 62.7 (CH2), 48.8 (Cq), 42.2 (CH2), 23.5 (CH2), 21.9, 21.8, 15.7 ppm.

1-(1-((2R,4S,5R)-5-((Bis(4-methoxyphenyl)(phenyl)methoxy)methyl)-4-hydroxytetrahydrofuran-2-yl)-7-ethyl-1H-imidazo[4,5-c]pyridin-4-yl)-3,3,4,4-tetramethylpyrrolidine-2,5-dione (19d)

This compound was synthesized in a similar fashion to 19i. To a solution of 18d (190 mg, 0.457 mmol) in pyridine (3 mL), 4,4′-dimethoxytrityl chloride (DMT-Cl) (217 mg, 0.639 mmol) was added in one portion, and the mixture was stirred at room temperature under argon for 4 h. The mixture was concentrated in vacuo. The product was purified by column chromatography on silica gel (neutralized with triethylamine) eluting with 0–20% acetone in dichloromethane to obtain 19d as a colorless solid in 33% yield (110 mg). HRMS (ESI): m/z [M + H]+ calcd for C42H47N4O7: 719.34452; found: 719.34247. 1H NMR (400 MHz, CDCl3): δ, 8.21 (s, 1H), 8.16 (s, 1H), 7.40–7.38 (d, J = 7.2, 2H), 7.30–7.20 (m, 7H), 6.82–6.80 (d, J = 9.2, 4H), 6.42–6.39 (dd, J1 = 6.4, J2 = 12.4, 1H), 4.54–4.48 (m, 1H), 4.11–4.08 (app q, J = 4.4, 1H), 3.77 (s, 6H), 3.38–3.34 (app dd, J1 = 4.4, J2 = 10.4, 1H), 3.32–3.29 (app dd, J1 = 4.8, J2 = 10.4, 1H), 3.17 (bs, 1H), 3.03–3.00 (app dd, J1 = 3.2, J2 = 7.6, 1H), 2.99–2.96 (app dd, J1 = 3.2, J2 = 7.6, 1H), 2.46–2.44 (app t, J = 6.0, 2H), 1.38–1.37 (overlapped s, 12H), 1.36–1.32 (t, J = 7.2, 3H) ppm. 13C NMR (100 MHz, CDCl3): δ, 181.8 (Cq), 158.7 (Cq), 144.5 (Cq), 142.6, 141.9, 138.9 (Cq), 137.3 (Cq), 137.2 (Cq), 135.63 (Cq), 135.58 (Cq), 130.1, 128.13, 128.06, 127.1, 124.6 (Cq), 113.4, 86.8 (Cq), 86.0, 85.2, 71.9, 63.7 (CH2), 55.3, 48.0 (Cq), 41.7 (CH2), 22.7 (CH2), 21.8, 21.4, 15.3 ppm.

(2R,3S,5R)-2-((Bis(4-methoxyphenyl)(phenyl)methoxy)methyl)-5-(7-ethyl-4-(3,3,4,4-tetramethyl-2,5-dioxopyrrolidin-1-yl)-1H-imidazo[4,5-c]pyridin-1-yl)tetrahydrofuran-3-yl (2-Cyanoethyl) Diisopropylphosphoramidite (20d)

This compound was synthesized in a similar fashion to 20i. To a solution of 19d (105 mg, 0.146 mmol) in dry CH2Cl2 (4 mL) containing N,N-diisopropylethylamine (DIEA) (127 μL, 0.731 mmol), 2-cyanoethyl-N,N-diisopropyl chlorophosphoramidite (40 μL, 0.175 mmol) was added in one portion, and the mixture was stirred at room temperature under argon for 4 h (TLC showed complete conversion). The mixture was concentrated in vacuo at rt, and the product was purified by column chromatography on silica gel (neutralized with triethylamine) eluting with 40–60% EtOAc in hexanes containing 1% triethylamine, to obtain 20d as a mixture of diastereomers (white foam, Rf ∼0.5 in 5% acetone/CH2Cl2, two close spots visible on TLC) in 67% yield (90 mg). HRMS (ESI): m/z [M + H]+ calcd for C51H64N6O8P: 919.45236; found: 919.45047. 1H NMR (400 MHz, CDCl3): δ, 8.25 (s, 1H), 8.23, 8.19 (s, 1H), 7.40–7.38 (m, 2H), 7.30–7.20 (m, 7H), 6.83–6.79 (m, 4H), 6.49–6.46 (dd, J1 = 6.8, J2 = 13.6, 1H), 4.70–4.59 (m, 1H), 4.32–4.24 (m, 1H), 3.78, 3.77 (s, 6H), 3.70–3.53 (m, 3H), 3.32–3.31 (m, 1H), 3.13–2.99 (m, 2H), 2.62–2.59 (t, J = 6.4, 1H), 2.57–2.55 (m, 1H), 2.44–2.41 (t, J = 6.4, 1H), 1.38, 1.37 (bs, 12H), 1.20–1.17 (m, 9H), 1.13–1.12 (d, J = 6.8, 3H) ppm. 13C NMR (100 MHz, CDCl3): δ, 181.7, 158.7, 144.5, 142.5, 142.4, 139.0, 137.4, 135.63, 135.56, 130.2, 128.3, 128.2, 128.1, 127.1, 124.5, 124.4, 117.5, 113.4, 86.7, 85.8, 85.5, 85.4, 74.1, 73.9, 63.4, 63.2, 58.4, 58.2, 55.38, 55.36, 48.0, 43.61, 43.55, 43.4, 41.0, 24.7, 24.6, 22.9, 21.8, 21.5, 15.6 ppm. 31P NMR (162 MHz, CDCl3): δ, 150.8, 150.2 ppm.

7-Isobutyl-1H-imidazo[4,5-c]pyridin-4-amine, Trifluoroacetic Acid Salt (15e)

This compound was synthesized in a similar fashion to 15i. A solution of 14e (1.3 g, 2.4 mmol) in TFA (20 mL) was placed in a 100 mL pressure tube, anisole (1.6 mL, 6 equiv) was added, and the mixture was heated at 70 °C for 24 h with vigorous stirring. It was cooled and concentrated under reduced pressure. The residue was coevaporated with toluene (3 × 20 mL) to obtain 15e (ca. 0.8 g, quantitative) as an oil that was taken into next step without further purification.

1-(7-Isobutyl-1H-imidazo[4,5-c]pyridin-4-yl)-3,3,4,4-tetramethylpyrrolidine-2,5-dione (16e)

This compound was synthesized in a similar fashion to 16i. A solution of 15e (ca. 0.8 g, 2.4 mmol) in pyridine (20 mL) was placed in a 100 mL Schlenk flask. Tetramethylsuccinic anhydride (M4SA) (0.8 g, 5.1 mmol) and DBU (1.5 mL, 10.0 mmol) were added, and the mixture was heated at 120 °C under argon for 22 h with vigorous stirring. It was cooled and concentrated under reduced pressure. The residue was coevaporated with toluene (3 × 20 mL). The product was purified by column chromatography on silica gel eluting with 0–5% MeOH in CH2Cl2 to give a cream foam. It was taken in SOCl2 (20 mL), heated at 80 °C for 2 h, cooled, and concentrated under reduced pressure. The residue was coevaporated with EtOAc (3 × 20 mL). The product was purified by column chromatography on silica gel eluting with 0–8% MeOH in CH2Cl2 to give 16e as a cream foam in 79% yield (0.61 g). HRMS (ESI): m/z [M + H]+ calcd for C18H25N4O2: 329.19777; found: 329.19608. 1H NMR (400 MHz, CD3OD): δ, 8.43 (s, 1H), 8.16 (s, 1H), 2.87–2.85 (d, J = 7.6, 2H), 2.11–2.05 (app septet, J = 6.8, 1H), 1.37 (bs, 12H), 0.99–0.97 (d, J = 6.4, 6H) ppm. 13C NMR (100 MHz, CD3OD): δ, 183.2, 146.4, 143.7, 141.6, 135.8, 135.4, 126.0, 38.9, 30.4, 22.6, 21.9 ppm.

(2R,3S,5R)-5-(7-Isobutyl-4-(3,3,4,4-tetramethyl-2,5-dioxopyrrolidin-1-yl)-1H-imidazo[4,5-c]pyridin-1-yl)-2-(((4-methylbenzoyl)oxy)methyl)tetrahydrofuran-3-yl 4-Methylbenzoate (17e)

This compound was synthesized in a similar fashion to 17i. To a solution of 16e (0.585 g, 1.78 mmol) in dry CH3CN (60 mL) in a 250 mL round-bottom flask, 60% NaH in oil (143 mg, 3.57 mmol) was added, and the mixture was vigorously stirred at room temperature under argon for 35 min. 1-(α)-Chloro-3,5-di-O-(p-toluoyl)-2-deoxy-d-ribose (1.25 g, 3.21 mmol) was subsequently added portionwise over 1 h, and the stirring was continued at room temperature for overall 4.5 h. The mixture was filtered through Celite and concentrated. The product was purified by column chromatography on silica gel eluting with 0–50% EtOAc in hexanes and concentrated repeatedly with dichloromethane. Compound 17e was obtained as a yellowish solid in 38% yield (0.465 g). HRMS (ESI): m/z [M + H]+ calcd for C39H45N4O7: 681.32887; found: 681.32697. 1H NMR (400 MHz, CDCl3): δ, 8.27 (s, 1H), 8.20 (s, 1H), 7.97–7.95 (d, J = 8.0, 2H), 7.92–7.90 (d, J = 8.0, 2H), 7.30–7.28 (d, J = 8.4, 2H), 7.27–7.25 (d, J = 8.4, 2H), 6.59–6.55 (dd, J1 = 5.2, J2 = 8.8, 1H), 5.73–5.72 (m, 1H), 4.70–4.61 (m, 3H), 2.98–2.92 (dd, J1 = 7.2, J2 = 14.4, 1H), 2.84–2.72 (m, 3H), 2.45 (s, 3H), 2.42 (s, 3H), 1.98–1.88 (app septet, J = 6.4, 1H), 1.38 (bs, 1H), 1.05–1.03 (d, J = 6.8, 6H) ppm. 13C NMR (100 MHz, CDCl3): δ, 181.6 (Cq), 166.3 (Cq), 165.9 (Cq), 144.9 (Cq), 144.4 (Cq), 144.1, 142.2, 139.2 (Cq), 137.7 (Cq), 137.4 (Cq), 129.9, 129.8, 129.54, 129.47, 126.6 (Cq), 126.3 (Cq), 121.8 (Cq), 85.4, 82.9, 74.9, 64.2 (CH2), 48.0 (Cq), 39.9 (CH2), 39.1 (CH2), 30.2, 22.5, 21.9, 21.8, 21.7, 21.5 ppm. For the critical NOESY correlations, see Figure S3 and copies of spectra (Supporting Information).

1-(1-((2R,4S,5R)-4-Hydroxy-5-(hydroxymethyl)tetrahydrofuran-2-yl)-7-isobutyl-1H-imidazo[4,5-c]pyridin-4-yl)-3,3,4,4-tetramethylpyrrolidine-2,5-dione (18e)

This compound was synthesized in a similar fashion to 18i. To a solution of 17e (0.46 g, 0.68 mmol) in dry MeOH (7 mL) in a 25 mL round-bottom flask, NaOMe (110 mg, 2.03 mmol) was added. The mixture was stirred at room temperature under argon for 2 h and concentrated in vacuo. The product was purified by column chromatography on silica gel eluting with 0–10% MeOH in dichloromethane to obtain 18e as an off-white foam in 85% yield (256 mg). HRMS (ESI): m/z [M + H]+ calcd for C23H33N4O5: 445.24512; found: 445.24381. 1H NMR (400 MHz, CD3OD): δ, 8.74 (s, 1H), 8.14 (s, 1H), 6.58–6.55 (dd, J1 = 6.4, J2 = 12.8, 1H), 4.58–4.55 (dt, J1 = 3.2, J2 = 6.8, 1H), 4.06–4.04 (app q, J = 3.2, 1H), 3.77–3.73 (app dd, J1 = 3.6, J2 = 12.0, 1H), 3.72–3.68 (app dd, J1 = 3.6, J2 = 12.0, 1H), 3.06–3.01 (app dd, J1 = 6.8, J2 = 14.4, 1H), 2.92–2.86 (app dd, J1 = 7.2, J2 = 14.0, 1H), 2.76–2.69 (ddd, J1 = 5.6, J2 = 6.8, J3 = 13.2, 1H), 2.56–2.51 (ddd, J1 = 3.6, J2 = 6.0, J3 = 9.6, 1H), 2.04–1.94 (app septet, J = 6.8, 1H), 1.37 (bs, 12H), 1.05–1.03 (d, J = 6.8, 6H) ppm. 13C NMR (100 MHz, CD3OD): δ, 183.4 (Cq), 146.0 (Cq), 144.0, 140.7 (Cq), 138.5 (Cq), 137.9 (Cq), 124.6 (Cq), 89.4, 87.0, 72.3, 62.8 (CH2), 48.9 (Cq), 42.3 (CH2), 39.4 (CH2), 31.1, 22.6, 22.5, 21.9, 21.8 ppm.

1-(1-((2R,4S,5R)-5-((Bis(4-methoxyphenyl)(phenyl)methoxy)methyl)-4-hydroxytetrahydrofuran-2-yl)-7-isobutyl-1H-imidazo[4,5-c]pyridin-4-yl)-3,3,4,4-tetramethylpyrrolidine-2,5-dione (19e)

This compound was synthesized in a similar fashion to 19i. To a solution of 18e (225 mg, 0.507 mmol) in dry pyridine (4 mL), 4,4′-dimethoxytrityl chloride (DMT-Cl) (240 mg, 0.709 mmol) was added in one portion, and the mixture was stirred at room temperature under argon for 4 h. TLC analysis indicated that a lot of starting material remained and then additional DMT-Cl (0.120 g, 0.355 mmol) and cat. 4-dimethylaminopyridine (DMAP) (10 mg) were added. After 14 h, more DMT-Cl (0.120 g, 0.350 mmol), triethylamine (85 μL, 0.608 mmol), and cat. DMAP (10 mg) were added. After 8 h, another portion as above was added, and after 16 h, TLC showed no further conversion (overall 42 h). The mixture was quenched with 200 μL of MeOH, concentrated in vacuo, the residue was taken in AcOEt and filtered, and the filtrate was concentrated in vacuo. The product was purified by column chromatography on silica gel (neutralized with triethylamine) eluting with 0–25% acetone in dichloromethane to obtain 19e as a colorless foam in 46% yield (175 mg). Further elution with 10% MeOH in dichloromethane yielded the recovered starting material 18e as a colorless syrup in 45% yield (102 mg). HRMS (ESI): m/z [M + H]+ calcd for C44H51N4O7: 747.37582; found: 747.37412. 1H NMR (400 MHz, CDCl3): δ, 8.17 (s, 1H), 8.14 (s, 1H), 7.40–7.38 (d, J = 8.8, 2H), 7.30–7.20 (m, 7H), 6.82–6.80 (d, J = 8.8, 4H), 6.42–6.38 (dd, J1 = 6.4, J2 = 12.8, 1H), 4.52–4.51 (m, 1H), 4.10–4.07 (app q, J = 4.4, 1H), 3.77 (s, 6H), 3.38–3.35 (app dd, J1 = 4.4, J2 = 10.4, 1H), 3.33–3.29 (app dd, J1 = 4.8, J2 = 10.4, 1H), 2.88–2.83 (app dd, J1 = 7.2, J2 = 14.4, 1H), 2.75–2.70 (app dd, J1 = 6.4, J2 = 14.0, 1H), 2.46–2.41 (m, 2H), 1.90–1.84 (app quintet, J = 6.4, 1H), 1.37–1.36 (overlapped s, 12H), 1.01–0.99 (dd, J1 = 2.4, J2 = 6.4, 6H) ppm. 13C NMR (100 MHz, CDCl3): δ, 181.7 (Cq), 158.7 (Cq), 144.5 (Cq), 143.6, 142.7, 139.2 (Cq), 137.5 (Cq), 137.4 (Cq), 135.63 (Cq), 135.58 (Cq), 130.1, 128.2, 128.1, 127.1, 122.2 (Cq), 113.4, 86.8 (Cq), 86.0, 85.0, 72.0, 63.7 (CH2), 55.3, 48.0 (Cq), 41.8 (CH2), 38.8 (CH2), 30.1, 22.5, 22.4, 21.8, 21.4 ppm.

(2R,3S,5R)-2-((Bis(4-methoxyphenyl)(phenyl)methoxy)methyl)-5-(7-isobutyl-4-(3,3,4,4-tetramethyl-2,5-dioxopyrrolidin-1-yl)-1H-imidazo[4,5-c]pyridin-1-yl)tetrahydrofuran-3-yl (2-Cyanoethyl) Diisopropylphosphoramidite (20e)

This compound was synthesized in a similar fashion to 20i. To a solution of 19e (105 mg, 0.146 mmol) in dry CH2Cl2 (5 mL) containing N,N-diisopropylethylamine (DIEA) (185 μL, 1.07 mmol), 2-cyanoethyl-N,N-diisopropyl chlorophosphoramidite (105 μL, 0.472 mmol) was added in one portion, and the mixture was stirred at room temperature under argon for 4 h (TLC showed complete conversion). The mixture was concentrated in vacuo at rt, and the product was purified by column chromatography on silica gel (neutralized with triethylamine) eluting with 40–60% EtOAc in hexanes containing 1% triethylamine, to obtain 20e as a mixture of diastereomers (white foam, Rf ∼0.5 in 50% EtOAc in hexanes, two close spots visible on TLC) in 59% yield (120 mg). HRMS (ESI): m/z [M + H]+ calcd for C53H68N6O8P: 947.48366; found: 947.48314. 1H NMR (400 MHz, acetone-d6): δ, 8.43, 8.41 (s, 1H), 8.15 (s, 1H), 7.46–7.41 (m, 2H), 7.33–7.19 (m, 7H), 6.85–6.81 (m, 4H), 6.62–6.58 (dd, J1 = 6.4, J2 = 13.2, 1H), 4.86–4.81 (m, 1H), 4.37–4.29 (m, 1H), 3.96–3.82 (m, 1H), 3.77, 3.76 (s, 6H), 3.74–3.66 (m, 2H), 3.39–3.34 (m, 2H), 3.13–3.09 (m, 1H), 3.03–2.96 (m, 1H), 2.94–2.87 (m, 1H), 2.81 (m, 3H), 2.79–2.76 (t, J = 6.4, 1H), 2.68–2.64 (t, J = 6.0, 1H), 1.36, 1.35, 1.30 (bs, 12H), 1.23–1.21, 1.22–1.20, 1.17–1.16 (d, J = 6.8, 12H), 1.08–1.06 (d, J = 6.8, 3H), 1.05–1.03 (dd, J1 = 1.6, J2 = 6.4, 3H) ppm. 13C NMR (100 MHz, acetone-d6): δ, 181.8, 159.6, 145.9, 144.1, 144.0, 139.9, 138.7, 136.6, 136.5, 131.0, 130.9, 129.0, 128.9, 128.6, 127.6, 123.32, 123.28, 119.0, 114.0, 87.2, 86.7, 86.1, 86.0, 75.0, 74.9, 74.5, 74.3, 64.5, 59.6, 59.4, 55.5, 48.4, 44.1, 44.0, 40.4, 39.0, 25.0, 24.9, 24.8, 22.9, 22.5, 22.0, 21.7, 21.4, 20.8 ppm. 31P NMR (162 MHz, acetone-d6): δ, 149.9, 149.7 ppm.

7-Phenyl-1H-imidazo[4,5-c]pyridin-4-amine, Trifluoroacetic Acid Salt (15a, 3a*TFA)

This compound was synthesized in a similar fashion to 15i. A solution of 14a (2.09 g, 3.65 mmol) in TFA (20 mL) was placed in a 100 mL pressure tube, anisole (2.4 mL, 6 equiv) was added, and the mixture was heated at 70 °C for 24 h with vigorous stirring. It was cooled and concentrated under reduced pressure. The residue was coevaporated with toluene (3 × 25 mL) to obtain a gray-brown powder. It was taken in toluene (10 mL) and filtered, washed with toluene (2 × 5 mL) and hexanes (2 × 5 mL), and dried in air to obtain 15a as a free-flowing, light gray powder, 1.13 g (quantitative). MS (ESI): m/z, 211.3 for [M + H]+. 1H NMR (400 MHz, DMSO-d6): δ, 8.60 (bs, 2H), 8.52 (s, 1H), 7.82 (s, 1H), 7.70–7.69 (m, 2H), 7.56–7.53 (m, 2H), 7.49–7.46 (m, 1H) ppm. 13C NMR (100 MHz, DMSO-d6): δ, 159.4, 159.0, 158.7, 158.3, 147.5, 144.3, 132.7, 129.1, 128.5, 128.0, 126.7, 115.2 ppm.

3,3,4,4-Tetramethyl-1-(7-phenyl-1H-imidazo[4,5-c]pyridin-4-yl)pyrrolidine-2,5-dione (16a)

This compound was synthesized in a similar fashion to 16i. A solution of 15a (0.34 g, 1 mmol) in pyridine (10 mL) was placed in a 100 mL Schlenk flask. Tetramethylsuccinic anhydride (M4SA) (0.546 g, 3.5 mmol) and DBU (1.05 mL, 7 mmol) were added, and the mixture was heated at 120 °C under argon for 20 h with vigorous stirring. It was cooled and concentrated under reduced pressure. The residue was coevaporated with toluene (3 × 25 mL). The product was purified by column chromatography on silica gel eluting with 0–5% MeOH in CH2Cl2 to give a colorless syrup. It was taken in SOCl2 (12 mL), heated at 80 °C for 2 h, cooled, and concentrated under reduced pressure. The residue was coevaporated with EtOAc (3 × 25 mL). The product was purified by column chromatography on silica gel eluting with 0–5% MeOH in CH2Cl2 to give 16a as a light brown foam in 92% yield (0.56 g). HRMS (ESI): m/z [M + H]+ calcd for C20H21N4O2: 349.16647; found: 349.16505. 1H NMR (400 MHz, CD3OD): δ, 8.75 (s, 1H), 8.52 (s, 1H), 7.76–7.75 (m, 2H), 7.61–7.51 (m, 3H), 1.40 (bs, 12H) ppm. 13C NMR (100 MHz, CD3OD): δ, 182.5, 147.9, 142.4, 139.6, 135.4, 134.4, 134.3, 130.5, 129.8, 127.0, 49.1, 21.8 ppm.

((2R,3S,5R)-3-((4-Methylbenzoyl)oxy)-5-(7-phenyl-4-(3,3,4,4-tetramethyl-2,5-dioxopyrrolid-in-1-yl)-1H-imidazo[4,5-c]pyridin-1-yl)tetrahydrofuran-2-yl)methyl 4-Methylbenzoate (17a)

This compound was synthesized in a similar fashion to 17i. To a solution of 16a (0.51 g, 1.46 mmol) in dry CH3CN (60 mL) in a 100 mL round-bottom flask, 60% NaH in oil (117 mg, 2.93 mmol) was added, and the mixture was vigorously stirred at room temperature under argon for 45 min. 1-(α)-Chloro-3,5-di-O-(p-toluoyl)-2-deoxy-d-ribose (0.368 g, 0.94 mmol) was subsequently added portionwise over 1 h, and the stirring was continued at room temperature for overall 5 h. The mixture was filtered through Celite and concentrated. The product was purified by column chromatography on silica gel eluting with 0–50% EtOAc in hexanes. Compound 17a was obtained as a yellowish syrup in 64% yield (0.64 g). HRMS (ESI): m/z [M + H]+ calcd for C41H40N4O7: 701.29757; found: 701.29626. 1H NMR (400 MHz, CDCl3): δ, 8.33 (s, 1H), 8.31 (s, 1H), 7.86–7.84 (d, J = 8.0, 2H), 7.75–7.73 (d, J = 8.0, 2H), 7.46 (bs, 4H), 7.32–7.28 (m, 1H), 7.31–7.29 (d, J = 8.0, 2H), 7.24–7.22 (d, J = 8.0, 2H), 5.96–5.93 (dd, J1 = 5.2, J2 = 8.8, 1H), 5.46–5.44 (m, 1H), 4.62–4.53 (m, 2H), 4.38–4.36 (m, 1H), 2.48 (s, 3H), 2.40 (s, 3H), 2.30–2.16 (m, 2H), 1.40 (bs, 12H) ppm. 13C NMR (100 MHz, CDCl3): δ, 181.7 (Cq), 166.2 (Cq), 165.6 (Cq), 144.7 (Cq), 144.5 (Cq), 142.9, 142.7, 138.4 (Cq), 138.0 (Cq), 137.4 (Cq), 134.8 (Cq), 130.0, 129.7, 129.6, 129.3, 129.0, 128.8, 126.6 (Cq), 126.4 (Cq), 123.5 (Cq), 85.9, 82.8, 74.9, 64.2 (CH2), 48.1 (Cq), 40.1 (CH2), 21.9, 21.8, 21.6 ppm. For the critical NOESY correlations, see Figure S4 and copies of spectra (Supporting Information).

1-(1-((2R,4S,5R)-4-Hydroxy-5-(hydroxymethyl)tetrahydrofuran-2-yl)-7-phenyl-1H-imidazo[4,5-c]pyridin-4-yl)-3,3,4,4-tetramethylpyrrolidine-2,5-dione (18a)

This compound was synthesized in a similar fashion to 18i. To a solution of 17a (0.44 g, 0.63 mmol) in dry MeOH (6 mL) in a 25 mL round-bottom flask, NaOMe (102 mg, 1.89 mmol) was added, and the mixture was stirred at room temperature under argon for 2 h. It was concentrated in vacuo, and the product was purified by column chromatography on silica gel eluting with 0–10% MeOH in CH2Cl2 to obtain 18a as a white foam in 58% yield (0.17 g). HRMS (ESI): m/z [M + H]+ calcd for C25H29N4O5: 465.21383; found: 465.21228. 1H NMR (400 MHz, CD3OD): δ, 8.85 (s, 1H), 8.19 (s, 1H), 7.56 (m, 5H), 5.97–5.94 (t, J = 6.0, 1H), 4.37–4.33 (m, 1H), 3.78–3.65 (m, 3H), 2.35–2.28 (m, 1H), 1.96–1.90 (ddd, J1 = 4.4, J2 = 6.0, J3 = 10.8, 1H), 1.39 (bs, 12H) ppm. 13C NMR (100 MHz, CD3OD): δ, 183.4 (Cq), 146.5 (Cq), 143.1, 139.4 (Cq), 138.7 (Cq), 138.6 (Cq), 135.6 (Cq), 130.9, 130.2, 129.9, 126.0 (Cq), 88.9, 87.1, 71.2, 62.3 (CH2), 49.0 (Cq), 42.9 (CH2), 21.9, 21.8 ppm.

The attempted tritylation of 18a was carried forward in a similar fashion to 18i. To a solution of 18a (0.15 g, 0.323 mmol) in pyridine (3 mL) under Ar, 4,4′-dimethoxytrityl chloride (DMT-Cl) (0.153 g, 0.453 mmol) was added in one portion, and the mixture was stirred at room temperature for 4.5 h. TLC analysis indicated that some starting material remained and then additional DMT-Cl was added (0.076 g, 0.225 mmol). After 3.5 h, the starting material still remained and then triethylamine (45 μL, 0.323 mmol), cat. DMAP (10 mg), and additional DMT-Cl (44 mg, 0.129 mmol) were added. After 8 h, most of the starting material was consumed, the mixture was concentrated in vacuo, and the residue was coevaporated with toluene (3 × 10 mL) to remove residual pyridine. A TLC analysis indicated that an extensive deglycosylation occurred, indicating the instability of the compound, which may be ascribed to the electronic character of phenyl group at the position 3, as suggested by other reports.[15] Deglycosylation was also observed to occur upon mild heating of the TLC plate to remove pyridine before analysis.

Synthesis of 3-Deazahypoxanthine 21c

7-Phenethyl-1H-imidazo[4,5-c]pyridin-4-amine, Trifluoroacetic Acid Salt (15c)

This compound was synthesized in a similar fashion to 15i. A solution of 14c (0.327 g, 0.55 mmol) in TFA (8 mL) was placed in a 100 mL pressure tube, anisole (0.36 mL, 6 equiv) was added, and the mixture was heated at 70 °C for 21 h with vigorous stirring. It was cooled and concentrated under reduced pressure. The residue was coevaporated with toluene (3 × 15 mL) to obtain 15c (ca. 0.2 g, quantitative) as a syrup that was taken into next step without further purification.

7-Phenethyl-1,5-dihydro-4H-imidazo[4,5-c]pyridin-4-one (21c)

This compound was synthesized according to the general procedure.[56] To a solution of crude 15c (ca. 0.2 g, 0.55 mmol) in acetone/H2O/AcOH (4/2/2 mL) was added NaNO2 in one portion (0.755 g, 10.9 mmol) and stirred at room temperature for 5 h in a closed vessel. The pressure was carefully released, and the mixture was carefully neutralized with aq NaHCO3 and diluted with Et2O (50 mL). The phases were shaken, and the precipitated solid was filtered, washed with water and Et2O, and dried in air to obtain 21c as a light yellow solid (75 mg, 57% based on 14c). HRMS (ESI): m/z [M + H]+ calcd for C14H14N3O: 240.11369; found: 240.11281. MS (ESI): m/z, 240.3 for [M + H]+, 262.2 for [M + Na]+, 501.2 for [2M + Na]+; 238.3 for [M – H]−. 1H NMR (400 MHz, DMSO-d6): δ, 10.9 (bs, 1H), 8.09 (s, 1H), 7.26–7.16 (m, 5H), 6.79 (s, 1H), 2.93–2.89 (m, 4H) ppm. 13C NMR (100 MHz, DMSO-d6): δ, 156.1, 141.6, 141.2, 128.4, 128.2, 125.8, 124.9, 110.7, 35.1, 29.7 ppm.

Purification of Salts 15

TFA salts 15 were used for synthetic purposes typically without further purification. Pure amines could be also isolated as water-soluble HCl salts as exemplified below. Free amines could be obtained by neutralization of their aqueous solutions with Na2CO3, followed by extraction with AcOEt.

7-Phenethyl-1H-imidazo[4,5-c]pyridin-4-amine, Hydrochloric Acid Salt (15c*HCl)

A solution of 14c (0.173 g, 0.29 mmol) in EtOH/H2O/HCl conc. (8/8/4 mL) was heated under reflux condenser at 100 °C for 23 h. It was cooled, and EtOH was removed under reduced pressure. It was diluted with aq HCl and washed with Et2O (40 mL, discarded) to remove a precipitated oil. The residue was concentrated and dried under vacuum to obtain 15c*HCl (ca. 0.08 g, quantitative) as an off-white solid. HRMS (ESI): m/z [M + H]+ calcd for C14H15N4: 239.12968; found: 239.12930. MS (ESI): m/z, 239.3 for [M + H]+; 237.3 for [M – H]−, 273.0 for [M + Cl]−. 1H NMR (400 MHz, D2O): δ, 8.35 (s, 1H), 7.28–7.19 (m, 4H), 7.11–7.09 (m, 2H), 3.09–3.05 (m, 2H), 2.99–2.96 (m, 2H) ppm. 13C NMR (100 MHz, D2O): δ, 146.6, 143.6, 140.6, 128.8, 128.6, 127.0, 126.5, 123.8, 114.7, 34.6, 29.3 ppm.