Palladium-catalyzed direct arylation of 5-halouracils and 5-halouracil nucleosides with arenes and heteroarenes promoted by TBAF.

The 1-N-benzyl-5-iodo(or bromo)uracil undergoes Pd-catalyzed [Pd2(dba)3] direct arylation with benzene and other simple arenes in the presence of TBAF in DMF without the necessity of adding any ligands or additives to give 5-arylated uracil analogues. The TBAF-promoted coupling also occurs efficiently with electron rich heteroarenes at 100 °C (1 h) even with only small excess of heteroarenes. The protocol avoids usage of the arylboronic acid or stannane precursors for the synthesis of 5-(2-furyl, or 2-thienyl, or 2-pyrrolyl)uracil nucleosides, which are used as important RNA and DNA fluorescent probes. The fact that 1-N-benzyl-3-N-methyl-5-iodouracil did not undergo the TBAF-promoted couplings with arenes or heteroarenes suggests that the C4-alkoxide (enol form of uracil) facilitates coupling by participation in the intramolecular processes of hydrogen abstraction from arenes. TBAF-promoted arylation was extended into the other enolizable heterocyclic systems such as 3-bromo-2-pyridone. The π-excessive heteroarenes also coupled with 5-halouracils in the presence of Pd(OAc)2/Cs2CO3/PivOH combination in DMF (100 °C, 2 h) to yield 5-arylated uracils.

Introduction

The 5-substituted pyrimidine nucleosides are impn>ortant synthetic intermediates, which also show a broad spn>ectrum of biological activity.[1] Highly potent and selective antiviral drugs of this class include (E)-5-(2-bromovinyl)-2′-deoxyuridine (n>an class="Chemical">BVDU)[2] and bicyclic furanopyrimidine-2-one nucleoside analogues, which display remarkable antiviral potency against the Varicella-Zoster virus.[3] Uracil and uracil nucleosides substituted with aryl groups at positions 5 and/or 6 also display a wide range of biological activities.[4]

Syntheses of the 5-(alkenyl or alkynyl) modified pyrimidines have been achieved by organometallic and/or n>an class="Chemical">Pd-assisted cross-couplings with 5-halopyrimidine nucleosides[1,5] and by dehydrogenative alkenylation of uracils with alkenes.[6] The arylation and heteroarylation of the uracil ring at the 5 position have been accomplished efficiently using Suzuki,[7] Stille,[8] and other coupling reactions.[1] The 5-(2-furyl)uridine and 5-(2-thienyl)-2′-deoxyuridine analogues, which were prepared in these ways, have been incorporated into RNA and DNA fragments using solid support and used as fluorescent probes.[9] Hocek and co-workers showed that 5-aryl and 5-(2-thienyl) modified dU(or C)TPs are substrates for DNA polymerases and can be used for DNA staining and electrochemical labels.[10] The Stille procedure was also successful on a polyuridine RNA strand bearing 5-iodouridine base, although 60 equiv of 2-(tributylstannyl)furan reagent and 15 equiv of Pd2(dba)3 catalyst along with 30 equiv of P(furyl)3 ligand were required.[11]

Direct arylation of aromatic rings with Pd-activated aryl halides has been recently established as an excellent set of protocols for the synthesis of biaryls.[12] These protocols, which eliminate the use of organometallics substrates, have begun to compn>ete with traditional n>an class="Chemical">Pd-catalyzed cross-couplings[13] as a tool for the synthesis of biaryls. Direct arylations, in which arene-derived organometallic components are replaced by simple arenes, can proceed efficiently intra-[14] or intermolecularly[14b,15] and are especially efficient with electron-rich heteroaromatics.[12d] The Pd-catalyzed direct arylation reactions of aryl halides are postulated to occur via electrophilic aromatic substitution (electrophilic palladation with nucleophilic arenes) or concerted palladation-deprotonation involving proton abstraction by halide or carboxylate ligands.[12,14c,16]

Direct arylation of the electron-poor pyrimidines[17] and n>an class="Chemical">pyridine oxides[15a,18] with aryl halides as well as intramolecular arylation at C6 or C5 of the 5-(or 6)-[N-(2-bromobenzyl)-N-methylamino]-1,3-dimethyluracil derivatives[19] have been reported. Hocek and co-workers developed regioselective Pd-catalyzed direct C–H arylation of 1,3-diprotected uracil derivatives at the 5 or 6 position controlled by the presence or absence of CuI.[20] Kim and co-workers recently reported Pd-catalyzed 5-arylation of 1,3-dimethyluracil with aryl bromides as well as oxidative homocoupling of the 1,3-dimethyluracil derivatives.[21] The 6-aryluracil derivatives were also prepared by CuBr-mediated arylation of the 1,3-dimethyluracil with aryl iodides.[22]

Direct arylation of the 3-N unprotected uracil analogues, which allows for n>an class="Chemical">enolization between 4-oxo group and the hydrogen at 3-N position is underdeveloped. This tautomerization is critical in the biology and chemistry of RNA/DNA and in the nucleoside-derived drug metabolism. The currently available methods for direct 5-arylation of the uracil analogues were noted to be either not suitable for the natural (3-N-unsubstituted) uracils[20b] or caused cleavage of the nucleoside glycosidic bond.[21a] Moreover, there have been no reports of direct arylation of 5-halopyrimidine nucleosides with arenes, which would supplement the well-established Suzuki and Stille approaches. Herein, we report Pd-catalyzed direct arylation of 5-halouracils with arenes and electron-excessive heterocycles promoted by TBAF or bases that provide access to 5-arylated uracils and uracil nucleosides. The protocol avoids usage of arylboronic acid or stannane precursors and is applicable to the natural nucleosides.

Results and Discussion

Heating of 1-N-benzyl-5-iodouracil 1a in n>an class="Chemical">toluene (6 h, 100 °C) in the presence of Pd2(dba)3 and TBAF (1 M/THF containing 5% of water) afforded a mixture of 5-(o,m,p-methylphenyl) uracil products 2a [45% overall yield for o/m/p (3:2:1) isomers]. In addition to the isomeric mixture of 2a, the reduced substrate 3 (3%) and 3-N-butylated byproducts 4 were also detected in the crude reaction mixture (35%; Table 1, entry 1). The ratio for the o/m/p isomers of 2a was established by comparison with authentic samples prepared by Suzuki coupling[7c] between 1a and the corresponding tolylboronic acids (see Experimental Section). Careful investigation of reaction time and temperature revealed that arylation of 1a was completed in less than 1 h (45 min) at 100 °C (entry 2). Reactions at 90 °C for 1.5 h and at 80 °C for 4 h produced 2a in 70 and 40% yields, respectively. Remarkably, no alkylation (butylation) at the N-3 position was observed (GC–MS) on heating for up to 1.5 h at 100 °C or 3 h at 90 °C. At least 7 equiv of TBAF (1 M/THF) were required to produce 2a in highest yields with the best 2a to 3 ratios (entries 2–5). Among the other Pd catalysts tested, Pd(OAc)2 also produced 2a in similar yields (entry 6).

Table 1

Optimization of the Arylation of 1-N-Benzyl-5-iodouracil 1a with Toluenea

entry	reagent/solvent	Pd	TBAF (equiv)	base (2 equiv)	time (h)	2ab yield (%)c,d	3 yield (%)d
1	toluene	Pd2(dba)3	7	–	6	45e	3
2	toluene	Pd2(dba)3	7	–	0.75	75 (71)f	5
3	toluene	Pd2(dba)3	6	–	1	60	15
4	toluene	Pd2(dba)3	5	–	1	48g	17
5	toluene	Pd2(dba)3	3	–	1	30	8
6	toluene	Pd(OAc)2	7	–	1	70 (68)h	7
7	toluene/DMFi	Pd2(dba)3	7	–	1	82 (65)	7
8	toluene/DMFj	Pd2(dba)3	7k	–	1	65 (50)l	19
9	toluene/DMAi	Pd2(dba)3	7	–	1	20	10
10	toluene/dioxanei	Pd2(dba)3	7	–	1	40	46
11	toluene/DMFj	Pd2(dba)3	–	KFm	2	–	11
12	toluene/DMFj	Pd2(dba)3	–	Cs2CO3n	2	–	25
13	toluene/DMFj	Pd(OAc)2	–	KOSiMe3	2	–	80 (65)
14	toluene/DMFj	Pd(OAc)2	–	Ag2CO3o	2	–	17
15	toluene/DMF/H2O	Pd(OAc)2	–	K2CO3	2	–	23
16	toluene/DMFj	Pd2(dba)3	TBAIp	–	2	–	33
17	toluene/DMFj	Pd2(dba)3	TBAI	CsFq	2	–	14
18	toluene/DMFj	Pd(OAc)2	2	Cs2CO3r	2	10	8

Couplings were performed on 0.14 mmol scale of 1a [0.05 M (entries 1–10) or 0.07 M (entries 11–18)] with 0.05 equiv of Pd catalyst.

Mixture of o/m/p isomers (3:2:1, GC–MS).

Overall yield for o/m/p isomers.

Determined by GC–MS. Isolated yield in parentheses.

Also 4 (35%) was formed. After 18 h the yield of 4 was 85%.

70% yield at 90 °C for 1.5 h. 40% yield at 80 °C for 4 h.

35% with 4 equiv of TBAF.

69% yield at 90 °C for 1.5 h.

1:1 (v/v).

1:9 (v/v).

When neat TBAF·3H2O (7 equiv) was used 2a (67%) and 3 (20%) were produced.

25% yield in toluene/DMF (1:20, v/v).

Reaction with CsF or Ag2CO3 or K2CO3 also did not yield 2a.

Attempts with different equivalents of base (0.5–4.0) or prolonged reaction time (4 h) were also unsuccessful.

Couplings also did not proceed in the presence of Cs2CO3 or AgOAc and their combination with pivalic acid.

Couplings in the presence of tetrabutylammonium chloride, bromide, or hydroxide also failed.

With 65% HF·pyridine coupling also failed.

Also couplings in the presence of CsF did not improve yield of 2a.

Couplings were performed on 0.14 mmol scale of 1a [0.05 M (entries 1–10) or 0.07 M (entries 11–18)] with 0.05 equiv of pan class="Chemical">Pd catalyst.

35% with 4 equiv of pan class="Chemical">TBAF.

When neat TBAF·3n>an class="Chemical">H2O (7 equiv) was used 2a (67%) and 3 (20%) were produced.

25% yield in pan class="Chemical">toluene/n>an class="Chemical">DMF (1:20, v/v).

Reaction with CsF or pan class="Chemical">Ag2CO3 or pan class="Chemical">K2CO3 also did not yield 2a.

Couplings also did not proceed in the presence of Cs2CO3 or n>an class="Chemical">AgOAc and their combination with pivalic acid.

Couplings in the presence of tetrabutylammonium chloride, n>an class="Chemical">bromide, or hydroxide also failed.

With 65% HF·pan class="Chemical">pyridine coupn>ling also failed.

In an effort to lower the ratio of arene (n>an class="Chemical">toluene) to 1a, we sought for the inert solvents suitable for this arylation reaction. Thus, couplings proceeded efficiently in 1:1 and 1:9 (v/v) mixture of toluene/DMF (entries 7 and 8). Arylation in toluene/DMA and toluene/dioxane (1:1, v/v) also gave 2a but in lower yields (entries 9 and 10), while no product was formed in toluene/THF (1:1; v/v) at reflux.

Replacement of the 1 M TBAF/n>an class="Chemical">THF solution with neat TBAF·3 H2O also yielded 2a (entry 8, footnote k) with similar yield. It is noteworthy that each of these fluoride reagents introduced approximately the same amount of water to the reaction mixture. Water is known to play multiple roles in enhancing the efficiency of the couplings including the formation of the reactive hydroxypalladium intermediates.[23]

Heating of 1a in a mixture of toluene/n>an class="Chemical">DMF (1:9, v/v) in the presence of either other fluoride sources such as CsF or KF (2 equiv) or bases such as Cs2CO3, Ag2CO3, K2CO3, or KOSiMe3, instead of TBAF, failed to produce 2a leading mainly to 3 (entries 11–15). The outcome was not improved by varying the bases, Pd catalysts, and reaction times (entry 12) or by addition of the pivalic acid (entry 14). Likewise carrying out couplings using a mixture of DMF/H2O (5:1, v/v), to improve the solubility of salts, was unsuccessful (entry 15).

Arylations were also unsuccessful either in the presence of tetrabutylammonium hydroxide or n>an class="Chemical">chloride, bromide, or iodide (entry 16) or in the combination of the latter with other fluoride sources (CsF or HF; entry 17). Coupling with 2 equiv of TBAF with other bases or fluoride sources produced 2a in low yields (e.g., entry 18).

Direct arylation of the 5 position of the uracil ring promoted by n>an class="Chemical">TBAF has general applicability; hence, reaction of 5-halouracils 1a or 1b with various arenes and heterocyclic arenes are presented in Table 2. Thus, heating of 5-iodouracil 1a in benzene/DMF (1:9, v/v) in the presence of TBAF (7 equiv) at 100 °C for 1 h (ratio of benzene to 1a, 16:1) produced 5-phenyluracil 2b as a single product in addition to the reduced substrate 3 (Table 2, entry 1). Arylation of 1a with xylene gave a mixture of the three regioisomers (88%) from which the 2,4-dimethylphenyl isomer 2c was isolated in 55% yield (entry 2). Arene bearing the electron-donating methoxy group on the phenyl ring produced 2d as a mixture of o/m/p regioisomers (entry 3). The ratio for the o/m/p isomers of 2d was established by comparison with authentic samples prepared by Suzuki coupling[7c] between 1a and the corresponding methoxyphenylboronic acids (see Experimental Section). However, arenes with EWG either failed (nitrobenzene) or gave the corresponding 5-arylated products in low yields (1,2,3,4,5-pentafluorobenzene, 15%, GC–MS). Arylation of 5-bromouracil 1b, prepared by bromination of 1-N-benzyluracil with 1,3-dibromo-5,5-dimethylhydantoin in the presence of TMSOTf,[24] with benzene proceeded smoothly to afford 2b in 78% yield but needed 14 equiv of TBAF (entry 4).

Table 2

Arylation of 5-Halouracils 1 with Arenes and Electron-Rich Heteroaromatics Promoted by TBAFa

Couplings were performed on 0.14 mmol scale of substrates 1 (0.07 M) in the presence of 7 equiv of TBAF and 0.05 equiv of Pd catalyst with Ar–H to DMF ratio of 1:9 (v/v) unless otherwise noted.

Determined by GC–MS.

Isolated yield in parentheses.

Overall yield for three possible isomers (ratio, 14:5:1).

Yield for 2,4-dimethylphenyl isomer of 2c.

Overall yield for mixture of o/m/p (ratio, 4:1:1) isomers.

With 14 equiv of TBAF.

With 3.5 equiv of TBAF.

77% at 90 °C.

47% with benzofuran:1a ratio (1.5:1).

Coupling with furan instead of thiophene also produced 6 (60%).

Couplings were performed on 0.14 mmol scale of substrates 1 (0.07 M) in the presence of 7 equiv of TBAF and 0.05 equiv of n>an class="Chemical">Pd catalyst with Ar–H to DMF ratio of 1:9 (v/v) unless otherwise noted.

Yield for pan class="Chemical">2,4-dimethylphenyl isomer of 2c.

With 14 equiv of pan class="Chemical">TBAF.

With 3.5 equiv of pan class="Chemical">TBAF.

47% with pan class="Chemical">benzofuran:1a ratio (1.5:1).

Coupling with furan instead of n>an class="Chemical">thiophene also produced 6 (60%).

The arylation of 5-iodouracil 1a with electron rich heteroaromatics requires only 3.5 equiv of n>an class="Chemical">TBAF and also occurs efficiently at 90 °C. Thus, 1a coupled with furan to give 5-(2-furyl)uracil 2e (95%) without any reduction product 3 (entry 5). Arylation of 1a with benzo[b]furan proceeded in good yield even with only slight excess of benzo[b]furan to 1a (1.5:1, mol/mol; benzo[b]furan/DMF, 1:100, v/v; entry 6). Reaction of 1a with thiophene gave 5-(2-thienyl)uracil 2g quantitatively (entry 7), while coupling with 2-substituted thiophenes provided single products 2h or 2i (entries 8 and 9). The 5-bromouracil 1b also coupled efficiently with furan under similar conditions to give 2e (entry 10). Interestingly, TBAF-promoted couplings between 3-N-methyl-5-iodouracil 1c and toluene or thiophene (or furan) was unsuccessful yielding only the reduced product 6 (entries 11–12).

In contrast to the simple arenes (Table 1, entries 11–18), the π-excessive n>an class="Chemical">heteroarenes did couple successfully with 5-halouracils under TBAF-free conditions in the presence of bases (Table 3). For example on heating of 1a with thiophene (15 equiv) in DMF (2 h, 100 °C) in the presence of Pd(OAc)2 and K2CO3 (2 equiv) only reduced product 3 was formed (Table 3, entry 1). However, when the reaction was carried out in a mixture of DMF/H2O (5:1, v/v) product 2g was obtained in 75% yield (entry 2). Apparently the increased solubility of K2CO3 in the DMF/H2O mixture changed the outcome of the reaction dramatically. The very low solubility of K2CO3 in organic solvents (e.g., less than 1% in DMA after 30 min heating at 120 °C) is known.[16b] Coupling yields were only slightly affected when 1 or 4 equiv of K2CO3 was used (entry 3). Direct arylation in the presence of NaOH or CsF or Ag2CO3 gave 2g in lower yields (entries 4–6) but proceeded efficiently with Cs2CO3 (entry 7). Arylation at 80 °C (2 h) gave 2g in lower yield (53%) but with an increased ratio relative to 3 (entry 8), while reaction at 60 °C (4 h) produced 2g in only 15% (entry 9). As expected because of the solubility difference between K2CO3 and Cs2CO3, the heteroarylation in the presence of Cs2CO3 was also successful in neat DMF (entry 10). Addition of the pivalic acid[16b] to the reaction mixture not only increased the yields but also the ratio of 2g to 3 (entry 11). In the presence of pivalic acid coupling proceeded at 80 °C (entry 12) but gave 2g in low yield at 60 °C. It is worth mentioning that, in general, direct arylation of 1a promoted by TBAF gave the best results with the Pd2(dba)3 catalyst, while base-promoted coupling proceeded more efficiently with Pd(OAc)2.

Table 3

Optimization of the Base-Promoted Arylation of 1-N-Benzyl-5-iodouracil 1a with Thiophenea

entry	solvent	base	equiv	T (°C)	yield 2g (%)b,c	yield 3 (%)b
1	DMF	K2CO3	2	100	–	80 (74)
2	DMF/H2Od	K2CO3	2	100	75 (60)e	20
3	DMF/H2O	K2CO3	1f	100	73 (61)	17
4	DMF/H2O	NaOH	2	100	16	30
5	DMF/H2O	CsF	2	100	37	35
6	DMF/H2O	Ag2CO3	2	100	20	8
7	DMF/H2O	Cs2CO3	2	100	61 (53)	32
8	DMF/H2O	Cs2CO3	2	80	53 (41)	8
9	DMF/H2O	Cs2CO3	2	60	15	2
10	DMF	Cs2CO3	2	100	76 (65)	11
11	DMF	Cs2CO3g	2	100	82 (76)h	4
12	DMF	Cs2CO3g	2	80	51 (43)	8

Couplings were performed on 0.14 mmol scale of 1a (0.07 M) in the presence of 0.05 equiv of Pd(OAc)2 with thiophene to DMF ratio of 1:9 (v/v).

Determined by GC–MS of the crude reaction mixture.

Isolated yield in parentheses.

DMF/H2O (5:1, v/v).

Pd2(dba)3 gave 2g in 36% isolated yield.

4 equiv of K2CO3 gave 2g in 50% isolated yield.

With addition of PivOH (1.25 equiv).

80% yield with addition of PivOH (1.25 equiv) and Ag2CO3 (1.0 equiv).

Couplings were performed on 0.14 mmol scale of 1a (0.07 M) in the presence of 0.05 equiv of Pd(OAc)2 with n>an class="Chemical">thiophene to DMF ratio of 1:9 (v/v).

pan class="Chemical">DMF/pan class="Chemical">H2O (5:1, v/v).

pan class="Chemical">Pd2(dba)3 gave 2g in 36% isolated yield.

4 equiv of pan class="Chemical">K2CO3 gave 2g in 50% isolated yield.

With addition of pan class="Chemical">PivOH (1.25 equiv).

80% yield with addition of pan class="Chemical">PivOH (1.25 equiv) and pan class="Chemical">Ag2CO3 (1.0 equiv).

Base-promoted direct arylation of the 5 position of the uracil ring with other electron-rich n>an class="Chemical">heteroarenes are presented in Table 4. Thus, heating of 1a with furan (ratio of furan to 1a, 15:1) in DMF in the presence of Cs2CO3 (2 equiv) and PivOH at 100 °C for 2 h produced 2e in addition to 3 (entry 1). Arylation proceeded smoothly with thiophene (entry 2) but failed with pyrrole (entry 3). The coupling of 5-bromouracil 1b with furan and thiophene gave products but in lower yields (entries 4 and 5). Interestingly, contrary to the unsuccessful TBAF-promoted coupling of 3-N-methyl-5-iodouracil 1c with heteroarenes (Table 2, entry 12), the base-promoted arylation of 1c with furan or thiophene gave coupling products 5e and 5g (entries 6 and 7). Attempted 5-arylations of the 3-N unprotected 1a and 3-N protected 1c uracil substrates with simple arenes in the presence of Cs2CO3 were unsuccessful (entries 8 and 9).

Table 4

Arylation of 5-Halouracils 1 with Electron-Rich Heteroarenes Promoted by Cs2CO3a

Couplings were performed on 0.14 mmol scale of 1 (0.07 M) with Cs2CO3 (2 equiv), PivOH (1.25 equiv), and Pd(OAc)2 (0.005 equiv) with Ar–H to DMF ratio of 1:9 (v/v).

Determined by GC–MS of the crude reaction mixture.

Isolated yield in parentheses.

Coupling without addition of PivOH gave 2e in 65% yield.

Couplings were performed on 0.14 mmol scale of 1 (0.07 M) with Cs2CO3 (2 equiv), n>an class="Chemical">PivOH (1.25 equiv), and Pd(OAc)2 (0.005 equiv) with Ar–H to DMF ratio of 1:9 (v/v).

Coupling without addition of pan class="Chemical">PivOH gave 2e in 65% yield.

Our methodology, which not only avoids the use of toxic tributyl(2-furyl)stannne[9a,9b,11] but also works efficiently with the 3-N unpn>rotected n>an class="Chemical">uracil substrates,[20b,21a,22] was applied to the synthesis of 5-heteroarylated uracil nucleosides. Thus, reaction of 2′,3′,5′-tri-O-acetyl-1-(β-d-arabinofuranosyl)-5-iodouracil 7 with furan in DMF (1:9, v/v) in the presence of TBAF (3.5 equiv) and Pd2(dba)3 afforded 5-(2-furyl)uracil analogue 12 (61%; Table 5, entry 1). Similarly, the protected 2′-deoxy-5-iodouridine 8 was converted to 13 (entry 2), confirming that direct arylation conditions are compatible with the commonly used acyl protection group.

Table 5

Arylation of 5-Iodouracil Nucleosides with Electron-Rich Heteroaromaticsa

Couplings were performed on 0.14 mmol scale of nucleosides (0.07 M) in the presence of 3.5 equiv of TBAF and 0.05 equiv of Pd catalyst with Ar–H to DMF ratio of 1:9 (v/v). Ratio of Ar–H to substrate nucleosides 15–20:1.

Isolated yields.

41% yield with 1.75:1 ratio of furan to 11.

Coupling on 1 mmol scale of 11 gave 16 in 88% yield.

Arylation proceeded also with the unprotected nucleosides. Thus, subjection of 1-(β-d-arabinon>an class="Chemical">furanosyl)-5-iodouracil 9, 2′-deoxy-5-iodouridine 10, and 5-iodouridine 11 to TBAF-promoted/Pd-catalyzed coupling with furan gave the corresponding 5-(2-furyl) analogues 14–16 in high yields (entries 3–5). Coupling of 11 proceeded also with only a small excess of furan (1.75 equiv) affording 16 in 41% isolated yield (entry 5, footnote c). Deacetylation of 12 or 13 with methanolic ammonia also gave 14 (92%) and 15 (95%).

Couplings were performed on 0.14 mmol scale of nucleosides (0.07 M) in the presence of 3.5 equiv of n>an class="Chemical">TBAF and 0.05 equiv of Pd catalyst with Ar–H to DMF ratio of 1:9 (v/v). Ratio of Ar–H to substrate nucleosides 15–20:1.

41% yield with 1.75:1 ratio of pan class="Chemical">furan to 11.

The 5-iodouridine 11 coupn>led efficiently with n>an class="Chemical">thiophene and 2-methylthiophene to give products 17 or 18 (entries 6 and 7). We found that the pyrrole also coupled with 11 to produce 5-(2-pyrrolyl)uridine 19 (entry 8), which can be purified on silica gel column but is somewhat unstable during storage and when exposed to air. Coupling of 2′-deoxyuridine 9 with benzimidazole failed to give 5-benzimidazolyl-2′-deoxyuridine.[25] Base-promoted 5-arylation of 11 with furan or thiophene preceded less efficiently giving products 16 or 17 in 23 and 17% isolated yield (entries 9 and 10). Instability of the glycosidic bond was noted during the 5-arylation of 3-N-benzyl-1-(tetrahydrofuran-2yl)uracil with bromobenzene.[21a] Subjection of 5-iodocytidine or 4-N-acetyl-2′,3′,5′-tri-O-acetyl-5-iodocytidine[26] to the TBAF- or base-promoted direct arylation with furan failed to provide 5-(2-furyl)cytidine products.

On the basis of the established mechanisms for the substitution of hydrogen in the n>an class="Chemical">arenes by Pd-activated aryl halides,[12b,12d] it is expected that arylation of 5-iodouracils would occur via either electrophilic aromatic substitution (palladation) or direct proton abstraction (σ-bond metathesis). We do not have sufficient data to speculate on the mechanism or the role of TBAF. However, the fact that 1-N-benzyl-3-N-methyl-5-iodouracil (1c), which lacks the possibility to tautomerize to the enol form, did not undergo TBAF-promoted coupling with arenes indicates that the C4-alkoxide (enol form of uracil) may participate in the intramolecular processes of hydrogen abstraction as depicted in structures A and B (Figure 1). Fagnou and co-workers showed that direct arylation was facilitated by using Pd-coordinated carboxylate group, which assisted in intramolecular proton abstraction.[16]

Figure 1

The plausible intermediates for the Pd-catalyzed direct arylation of 5-halouracils: (A) electrophilic aromatic palladation assisted by C4-alkoxide; (B) direct proton abstraction assisted by C4-alkoxide.

The plausible intermediates for the Pd-catalyzed direct arylation of n>an class="Chemical">5-halouracils: (A) electrophilic aromatic palladation assisted by C4-alkoxide; (B) direct proton abstraction assisted by C4-alkoxide.

In order to expand the TBAF-promoted arylation methodology of the n>an class="Chemical">uracil ring into the other heterocyclic systems, which have also possibility of tautomerization between oxo group and the hydrogen at the adjacent nitrogen atom, we subjected 3-bromo-2-pyridone 20 to our direct arylation protocol. Thus, heating of 20 with furan in the presence of TBAF and Pd2(dba)3 without other additives added gave product 21 in moderate yield (41%) in addition to unchanged 20 (35%, Scheme 1). Arylation of 20 with thiophene was also successful to give 22 (37%) but essentially failed with benzene (5–10%, GC–MS). Also coupling of 20 with furan or thiophene in the presence of Cs2CO3/PivOH produced 21 or 22 in 68 and 63% isolated yields, respectively. Products 21 and 22 have UV (MeOH) λ maxima at 332 and 346 nm and are good candidates for development as fluorescence probes (λem 397 and 410 nm). There have been few reports on the arylation and/or vinylation of the N-alkylated 3-halo-2-pyridones at 3 position using Suzuki[27] or Stille[28] protocols as well as Negishi[29] reactions.

Scheme 1

Arylation of 3-Bromo-2-pyridone

Hocek[20] and Kim[21a] and their co-workers recently repn>orted the base-promoted direct C–H arylation of n>an class="Chemical">1,3-N-diprotected uracils at C5 and/or C6 position(s) with aryl halides under the conditions that usually required higher temperature (130–160 °C) and longer reaction time (12–48 h) with noted instability of the glycosidic bond.[21a] In view of those results, we carried out direct C–H arylation of 1-N-benzyluracil 3 with aryl halides in the presence of TBAF. Thus, treatment of 3 with 4-iodoanisole with TBAF/Pd2(dba)3/DMF/100 °C/16 h failed to give arylated uracil products. However, heating (DMF/100 °C/16 h) of 3 with 4-iodoanisole (3 equiv) in the presence of TBABr/Pd(OAc)2/AgCl[15f] produced mixture of the 5 and 6 arylated products p-2d (23%) and 23 (33%) in addition to the unchanged 3 (41%, Scheme 2).

Scheme 2

Direct C–H Arylation of Uracil with 4-Iodoanisole

In summary, we have demonstrated that 1-N-benzyl-5-iodo(or bromo)uracil undergoes n>an class="Chemical">Pd-catalyzed direct arylation with simple arenes and electron rich heteroarenes in the presence of TBAF in DMF to give 5-arylated uracil analogues. Analogues coupling of 1-(β-d-arabinofuranosyl)-5-iodouracil, 2′-deoxy-5-iodouridine, and 5-iodouridine with heteroarenes afforded the corresponding 5-(2-furyl, or 2-thienyl, or 2-pyrrolyl)uracil nucleosides that are important RNA and DNA fluorescent probes. The TBAF-promoted protocol developed here differs from the existing routes to 5-aryluracils in at least one of the following: (i) it avoids usage of the arylboronic acid or stannane precursors necessary for Suzuki or Stille couplings, (ii) it proceeds without the necessity of adding any ligands and additives, (iii) it works efficiently with the natural 3-N-unsubstituted uracils and uracil nucleosides, and (iv) it is compatible with the stability of the nucleoside glycosidic bond. The π-excessive heteroarenes couple also with 5-halouracils in the presence of Cs2CO3 in DMF. The yields for the 5-arylated products were improved with the addition of pivalic acid. The Pd2(dba)3 was the catalyst of choice for TBAF-promoted arylation of 5-halouracils, whereas Pd(OAc)2 gave the best results for base-promoted couplings. Heteroarylation was extended into the other enolizable heterocyclic systems such as 3-bromo-2-pyridone

Experimental Section

1H NMR spn>ectra at 400 MHz and n>an class="Chemical">13C NMR at 100.6 MHz were recorded in CDCl3 unless otherwise noted. All chemical shift values are reported in parts per million (ppm) and referenced to the residual solvent peaks [CDCl3 (7.26 ppm) or DMSO-d6 (2.54 ppm)] for 1H NMR and the CDCl3 (77.16 ppm) or DMSO-d6 (39.52 ppm) for 13C NMR spectra, with coupling constant (J) values reported in Hz. HRMS were obtained in TOF (ESI) mode. TLC was performed on Merck kieselgel 60-F254, and products were detected with 254 nm light or by development of color with I2. Merck kieselgel 60 (230–400 mesh) was used for column chromatography. Purity, yields and ratio of the products (crude and/or purified) were established using a GC–MS (EI) system [capillary column (30 m × 0.25 mm × 25 μm)] using calibrated standards. The 1.0 M TBAF solution in THF (catalog number: 216143) were purchased from Aldrich-Sigma Co., LLC.

1-N-Benzyl-3-N-methyl-5-iodouracil (1c)

Freshly distilled diazomethane solution in ether (10 mL), generated from Diazald (3.0 g, 14.0 mmol), was added dropwise to a stirred solution of 1a(30) (357 mg, 1.09 mmol) in CH2Cl2 (20 mL) at 0 °C. After 18 h, the volatiles were evapn>orated to give n>an class="Chemical">1c (354 mg, 95%) as white powder: 1H NMR δ 3.34 (s, 3H), 4.86 (s, 2H), 7.21–7.30 (m, 5H), 7.57 (s, 1H); 13C NMR δ 29.7, 52.7, 68.0, 128.2, 128.8, 129.3, 135.0, 146.4, 151.5, 160.2; GC–MS (tR 21.70 min) m/z 342 (85, M+), 91 (100); HRMS calcd for C12H12IN2O2 [M + H]+ 342.9938, found 342.9935.

1-N-Benzyl-5-(2-, 3-, and 4-methylphenyl)uracil (o/m/p-2a). Procedure A

Toluene (0.2 mL, 173 mg, 1.89 mmol), n>an class="Chemical">TBAF (1M/THF solution containing ca. 5 wt % of water; 980 μL, 0.98 mmol) and Pd2(dba)3 (6.4 mg, 0.007 mmol) were added to a stirring solution of 1-N-benzyl-5-iodouracil[30] (1a, 46 mg, 0.14 mmol) in DMF (1.8 mL) under the N2 atmosphere at ambient temperature. The resulting suspension was stirred for 1 h at 100 °C (oil bath). Volatiles were evaporated, and the oily residue was dissolved in EtOAc or MeOH and filtrated through Celite or Whatman GF/A filter paper. The filtrate was partitioned (EtOAc/H2O). The organic layer was then washed (brine), dried (Na2SO4) and evaporated. GC–MS of this material showed peaks at tR 31.46, 32.38, and 32.70 min (m/z 292, M+) for the three isomers of 2a (ortho/meta/para with relative intensities of ∼3:2:1, respectively). Column chromatography (hexane/EtOAc, 8:2 → 6:4) gave a mixture of o/m/p-2a (o/m/p, 3:2:1; 29 mg, 71% overall yield) followed by 3 (1 mg, 4%; see Supporting Information, p S5). Mixture of o/m/p-2a: GC–MS tR 31.46 (o-2a), 32.38 (m-2a) and 32.70 (p-2a) min (m/z 292, M+); 1H NMR δ 2.21 (s, 1.5H, o-2a), 2.35 (s, 0.5H, p-2a), 2.36 (s, 1H, m-2a), 4.97 (s, 1H, o-2a), 4.985 (s, 0.33H, p-2a), 4.990 (s, 0.67H, m-2a), 7.07–7.41 (m, 10H), 8.91 (s, 1H); HRMS calcd for C18H17N2O2 [M + H]+ 293.1285, found 293.1294.

Attempted purification of the o/m/p-2a mixture on a long silica gel column (n>an class="Chemical">hexane/EtOAc, 8:2 → 7:3) gave partial separation of the o/m/p isomers of 2a but failed to yield single isomers.

Note: Treatment of 1a (46 mg, 0.14 mmol) with toluene/n>an class="Chemical">TBAF/Pd2(dba)3 in DMF for 18 h at 100 °C, as described above (column chromatography; hexane/EtOAc, 98:2 → 95:5) gave mixture of 1-N-benzyl-3-N-butyl-5-(2-methylphenyl)uracil (o-4), 1-N-benzyl-3-N-butyl-5-(3-methylphenyl)uracil (m-4) and 1-N-benzyl-3-N-butyl-5-(4-methylphenyl)uracil (p-4) (36.6 mg, 75% overall yield; o-4/m-4/p-4, 3:2:1): GC–MS tR 27.29 (o-4), 28.27 (m-4) and 28.74 (p-4) min (m/z 348, M+); 1H NMR δ 0.92 (t, J = 7.3 Hz, 3H), 1.36 (“sextet”, J = 7.4 Hz, 2H), 1.67 (“quint”, J = 7.6, Hz, 2H), 2.18 (s, 1.5H, o4), 2.30 (s, 0.5H, p4), 2.32 (s, 1H, m4), 4.00 (“t”, J = 7.6 Hz, 2H), 4.94 (s, 1H, o4), 4.95 (s, 0.33H, p4), 4.96 (s, 0.67H, m4), 7.02–7.37 (m, 10H); HRMS calcd for C22H25N2O2 [M + H]+ 349.1911, found 349.1920.

1-N-Benzyl-5-(3-methylphenyl)uracil (m-2a)

Suzuki coupling:[7c] 3-Tolylboronic acid (29 mg, 0.21 mmol) and PPh3 (9 mg, 0.034 mmol) were added to a stirring solution of 1-N-benzyl-5-iodouracil (1a, 46 mg, 0.14 mmol) in CH3CN/n>an class="Chemical">H2O (3 mL; v/v, 2:1) under the N2 atmosphere at ambient temperature, followed by the addition of Na2CO3 (22 mg, 0.21 mmol) and Pd(OAc)2 (3 mg, 0.013 mmol). The resulting suspension was stirred for 4 h at 80 °C (oil bath). Volatiles were evaporated, and the residue was partitioned between EtOAc and H2O. The organic layer was then washed (brine), dried (Na2SO4) and evaporated. Column chromatography (hexane/EtOAc, 6:4 → 5:5) gave m-2a (21.7 mg, 53%): 1H NMR δ 2.38 (s, 3H), 5.01 (s, 2H), 7.16 (dt, J = 6.8, 2.0 Hz, 1H), 7.23–7.29 (m, 2H), 7.30 (t, J = 1.7 Hz, 1H), 7.32 (s, 1H), 7.34–7.46 (m, 5H), 9.35 (s, 1H); 13C NMR δ 21.6, 51.5, 116.1, 125.3, 128.2, 128.5, 128.7, 128.97, 129.01, 129.3, 132.1, 135.4, 138.3, 141.2, 150.9, 162.5; GC–MS (tR 32.38 min) m/z 292 (85, M+), 91 (100); HRMS calcd for C18H17N2O2 [M + H]+ 293.1285, found 293.1293.

1-N-Benzyl-5-(4-methylphenyl)uracil (p-2a)

Suzuki coupling:[7c] Treatment of 1a (46 mg, 0.14 mmol) with 4-tolylboronic acid (29 mg, 0.21 mmol), as described above for m-2a, gave p-2a (20.0 mg, 49%): 1H NMR δ 2.34 (s, 3H), 4.98 (s, n>an class="Chemical">2H), 7.17 (d, J = 7.9 Hz, 2H), 7.28 (s, 1H), 7.32–7.41 (m, 7H), 9.03 (s, 1H); 13C NMR δ 21.3, 51.5, 115.9, 128.1, 128.2, 128.7, 129.2, 129.3, 129.4, 135.4, 138.2, 140.8, 150.8, 162.4; GC–MS (tR 32.70 min) m/z 292 (85, M+), 91 (100); HRMS calcd for C18H17N2O2 [M + H]+ 293.1285, found 293.1291.

1-N-Benzyl-5-phenyluracil (2b)

Treatment of 1a (46 mg, 0.14 mmol) with benzene (0.2 mL, 175 mg, 2.24 mmol) by procen>an class="Chemical">dure A gave 2b (25.3 mg, 59%): 1H NMR δ 4.99 (s, 2H), 7.32 (s, 1H), 7.33–7.49 (m, 10H), 9.02 (s, 1H); 13C NMR δ 51.6, 115.9, 128.2, 128.3, 128.7, 128.8, 129.4, 132.2, 135.3, 141.2, 150.7, 162.3; GC–MS (tR 24.52 min) m/z 278 (80, M+), 91 (100); HRMS calcd for C17H14N2NaO2 [M + Na]+ 301.0947, found 301.0945.

Analogues treatment of 1-N-benzyl-5-bromouracil[31] (1b, 39 mg, 0.14 mmol) with n>an class="Chemical">benzene (0.2 mL, 175 mg, 2.24 mmol) by procedure A (14 equiv of TBAF) gave 2b (27.6 mg, 71%) with spectroscopic data as described above.

1-N-Benzyl-5-(2,4-dimethylphenyl)uracil (2c)

Treatment of 1a (46 mg, 0.14 mmol) with m-xylene (0.2 mL, 172 mg 1.62 mmol) by procen>an class="Chemical">dure A gave the desired product, which was recrystallized from hexane/EtOAc to give a single isomer 2c (23.6 mg, 55%): 1H NMR δ 2.16 (s, 3H), 2.31 (s, 3H), 4.95 (s, 2H), 6.95 (d, J = 8.0 Hz, 1H), 6.99 (br d, J = 8.1 Hz, 1H), 7.05 (s, 1H), 7.11 (s, 1H), 7.30–7.41 (m, 5H), 8.30 (s, 1H); 13C NMR δ 20.1, 21.2, 51.4, 116.5, 126.8, 128.3, 128.7, 128.8, 129.4, 130.5, 131.4, 135.3, 137.6, 138.8, 142.2, 150.8, 161.9; GC–MS (tR 27.34 min) m/z 306 (70, M+), 91 (100); HRMS calcd for C19H19N2O2 [M + H]+ 307.1441, found 307.1442.

GC–MS of the crude reaction mixture showed peaks for three isomers with tR at 27.34, 27.90, and 28.33 min with relative intensities of 14:5:1 (m/z 306, M+).

1-N-Benzyl-5-(2-, 3-, and 4-methoxyphenyl)uracil (o/m/p-2d)

Treatment of 1a (46 mg, 0.14 mmol) with anisole (0.2 mL, 199 mg, 1.84 mmol) by procedure A gave 2d (o/m/p, 4:1:1; 29.3 mg, 68%): GC–MS tR 25.52 (o2d), 26.68 (m2d) and 27.31 (n>an class="Chemical">p-2d) min (m/z 308, M+); 1H NMR δ 3.73 (s, 2H, o2d), 3.80 (s, 0.5H, p2d), 3.81 (s, 0.5H, m2d), 4.96 (s, 1.3H, o2d), 4.98 (s, 0.7H, m,p2d), 6.86–7.34 (m, 10H), 9.01(s, 0.67H, o2d) 9.14(s, 0.33H, m,p2d); HRMS calcd for C18H17N2O3 [M + H]+ 309.1234, found 309.1247.

1-N-Benzyl-5-(3-methoxyphenyl)uracil (m-2d)

Suzuki coupling:[7c] Treatment of 1a (46 mg, 0.14 mmol) with 3-methoxyphenylboronic acid (32 mg, 0.21 mmol), as described above for m-2a, gave m-2d (22 mg, 51%): 1H NMR δ 3.80 (s, 3H), 4.98 (s, n>an class="Chemical">2H), 6.88 (dd, J = 8.8, 2.5 Hz, 1H), 7.03 (“dt”, J = 7.6, 2.0 Hz, 1H), 7.10 (“t”, J = 2.0 Hz, 1H), 7.26 (t, J = 8.0 Hz, 1H), 7.33–7.40 (m, 6H), 9.33 (s, 1H); 13C NMR δ 51.4, 55.3, 113.6, 114.0, 115.5, 121.4, 128.1, 128.6, 129.2, 129.5, 133.4, 135.2, 141.3, 150.7, 159.6, 162.2; GC–MS (tR 26.68 min) m/z 308 (75, M+), 91 (100); HRMS calcd for C18H17N2O3 [M + H]+ 309.1234, found 309.1238.

1-N-Benzyl-5-(4-methoxyphenyl)uracil (p-2d)

Suzuki coupling:[7c] Treatment of 1a (46 mg, 0.14 mmol) with 4-methoxyphenylboronic acid (32 mg, 0.21 mmol), as described above for m-2a, gave p-2d (23.7 mg, 55%): n>an class="Chemical">1H NMR δ 3.80 (s, 3H), 4.98 (s, 2H), 6.90 (d, J = 8.8 Hz, 2H), 7.25 (s, 1H), 7.32–7.39 (m, 7H), 8.64 (s, 1H); 13C NMR δ 51.5, 55.5, 114.2, 115.7, 124.5, 128.2, 128.7, 129.3, 129.5, 135.4, 140.3, 150.8, 159.7, 162.5; GC–MS (tR 27.31 min) m/z 308 (80, M+), 91 (100); HRMS calcd for C18H17N2O3 [M + H]+ 309.1234, found 309.1241.

1-N-Benzyl-5-(fur-2-yl)uracil (2e). Method A

Treatment of 1a (46 mg, 0.14 mmol) with furan (0.2 mL, 187 mg, 2.75 mmol) by procen>an class="Chemical">dure A (3.5 equiv of TBAF) gave 2e (30.4 mg, 81%): 1H NMR δ 5.01 (s, 2H), 6.43 (dd, J = 3.0, 1.9 Hz, 1H), 7.05 (d, J = 3.0 Hz, 1H), 7.28–7.43 (m, 6H), 7.69 (s, 1H), 9.19 (s, 1H); 13C NMR δ 51.9, 107.6, 109.7, 112.1, 128.2, 128.8, 129.3, 135.3, 137.7, 141.3, 145.7, 150.2, 160.4; GC–MS (tR 27.41 min) m/z 268 (60, M+), 91 (100); HRMS calcd for C15H13N2O3 [M + H]+ 269.0921, found 269.0934.

Analogues treatment of 1b (39 mg, 0.14 mmol) with furan (0.2 mL, 187 mg, 2.75 mmol) by procen>an class="Chemical">dure A (14 equiv of TBAF) gave 2e (31.9 mg, 85%) with spectroscopic data as described above.

Method B. Procedure B

Furan (0.2 mL, 187 mg, 2.75 mmol), n>an class="Chemical">Cs2CO3 (91.2 mg, 0.28 mmol) and Pd(OAc)2 (1.6 mg, 0.007 mmol) were added to a stirring solution of 1a (46 mg, 0.14 mmol) in DMF (1.8 mL) under the N2 atmosphere at ambient temperature. The resulting suspension was stirred for 2 h at 100 °C (oil bath) and after cooling down to ambient temperature was diluted with EtOAc (3 mL). The resulting mixture was vacuum filtered using Whatman GF/A filter paper, and the filtrates were evaporated. The oily residue was dissolved in EtOAc and partitioned (EtOAc/H2O). The organic layer was then washed (brine), dried (Na2SO4) and evaporated. Column chromatography (hexane/EtOAc, 8:2 → 6:4) gave 2e (24.5 mg, 65% yield) with the spectroscopic data as above and 6 (4 mg, 12%).

Procedure C

Analogous treatment of 1a (46 mg, 0.14 mmol) with furan (0.2 mL, 187 mg, 2.75 mmol) and n>an class="Chemical">Cs2CO3 (91.2 mg, 0.28 mmol) in the presence of PivOH (17.9 mg, 0.175 mmol) and Pd(OAc)2 (1.6 mg, 0.007 mmol) by procedure B gave 2e (27 mg, 73% yield).

Analogues treatment of 1b (39 mg, 0.14 mmol) with furan (0.2 mL, 187 mg, 2.75 mmol) by procen>an class="Chemical">dure B gave 2e (5 mg, 13% yield) with the spectroscopic data as above.

1-N-Benzyl-5-(benzo[b]fur-2-yl)uracil (2f)

Treatment of 1a (46 mg, 0.14 mmol) with benzo[b]furan (23.1 μL, 24.8 mg, 0.21 mmol) by procen>an class="Chemical">dure A (3.5 equiv of TBAF) gave 2f (29.4 mg, 66%): 1H NMR δ 5.06 (s, 2H), 7.2 (td, J = 7.3, 1.2 Hz, 1H), 7.26 (dt, J = 7.3, 1.4 Hz, 1H), 7.37–7.42 (m, 6H), 7.47 (d, J = 0.6 Hz, 1H), 7.56 (ddd, J = 7.3, 1.5, 0.8 Hz, 1H), 7.95 (s, 1H), 8.67 (s, 1H); 13C NMR δ 52.2, 106.1, 107.0, 110.7, 121.6, 123.3, 124.8, 128.2, 128.9, 129.3, 129.4, 135.1, 139.5, 147.6, 149.9, 153.9, 160.1; HRMS calcd for C19H15N2O3 [M + H]+ 319.1077, found 319.1081.

1-N-Benzyl-5-(thiophen-2-yl)uracil (2g). Method A

Treatment of 1a (46 mg, 0.14 mmol) with thiophene (0.2 mL, 210 mg, 2.5 mmol) by procen>an class="Chemical">dure A (3.5 equiv of TBAF) gave 2g(32) (36.2 mg, 91%): 1H NMR (DMSO-d6) δ 4.98 (s, 2H), 7.06 (dd, J = 5.1, 3.7 Hz, 1H), 7.29–7.32 (m, 1H), 7.34–7.37 (m, 4H), 7.44–7.47 (m, 2H), 8.45 (s, 1H), 11.72 (s, 1H); 13C NMR (DMSO-d6) δ 50.8, 108.1, 122.7, 125.6, 126.4, 127.4, 127.6, 128.6, 133.7, 136.7, 140.9, 149.9, 161.7; HRMS calcd for C15H13N2O2S [M + H]+ 285.0692, found 285.0691.

Method B

Analogues treatment of 1a (46 mg, 0.14 mmol) with thiophene (0.2 mL, 210 mg, 2.5 mmol) by procen>an class="Chemical">dure C gave 2g (30.2 mg, 76% yield) with the spectroscopic data as above.

Analogues treatment of 1b (39 mg, 0.14 mmol) with thiophene (0.2 mL, 210 mg, 2.5 mmol) by procen>an class="Chemical">dure C gave 2g (6 mg, 16% yield) with the spectroscopic data as above.

1-N-Benzyl-5-(5-methylthiophen-2-yl)uracil (2h)

Treatment of 1a (46 mg, 0.14 mmol) with 2-methylthiophene (0.2 mL, 203 mg, 2.1 mmol) by procen>an class="Chemical">dure A (3.5 equiv of TBAF) gave 2h (29.2 mg, 70%): 1H NMR δ 2.46 (d, J = 0.8 Hz, 3H), 4.98 (s, 2H), 6.66 (dd, J = 3.6, 1.1 Hz, 1H), 7.15 (d, J = 3.6 Hz, 1H), 7.30–7.40 (m, 5H), 7.42 (s, 1H), 9.06 (s, 1H); 13C NMR δ 15.3, 51.7, 110.9, 125.0, 125.5, 128.2, 128.8, 129.4, 130.8, 135.2, 138.2, 140.4, 152.2, 161.4; HRMS calcd for C16H14N2O2SNa [M + Na]+ 321.0668, found 321.0674.

1-N-Benzyl-5-(5-acetylthiophen-2-yl)uracil (2i)

Treatment of 1a (46 mg, 0.14 mmol) with 2-acetylthiophene (0.2 mL, 234 mg, 2.1 mmol) by procen>an class="Chemical">dure A (3.5 equiv of TBAF) gave 2i (25 mg, 54%): 1H NMR (DMSO-d6) δ 2.50 (s, 3H), 4.99 (s, 2H), 7.29–7.34 (m, 1H), 7.37 (d, J = 4.3 Hz, 4H), 7.59 (d, J = 4.1 Hz, 1H), 7.87 (d, J = 4.1 Hz, 1H), 8.76 (s, 1H), 11.89 (s, 1H); 13C NMR (DMSO-d6) δ 26.4, 51.2, 107.1, 123.0, 127.4, 127.7, 128.6, 133.1, 136.5, 141.9, 142.5, 143.2, 149.7, 161.5, 190.6; GC–MS (tR 31.00 min) m/z 326 (20, M+), 91 (100); HRMS calcd for C17H15N2O3S [M + H]+ 327.0798, found 327.0801.

1-N-Benzyluracil (3)

Treatment of 1a (46 mg, 0.14 mmol) with toluene (0.2 mL, 173 mg, 1.89 mmol) by procen>an class="Chemical">dure A [using KOSiMe3 (53.8 mg, 0.42 mmol) instead of TBAF] gave 3(33) (18.4 mg, 65%): 1H NMR δ 4.92 (s, 1H), 5.70 (dd, J = 7.9, 1.5 Hz, 1H), 7.16 (d, J = 7.9 Hz, 1H), 7.25–7.44 (m, 3H), 9.46 (s, 1H); 13C NMR δ 51.4, 102.8, 128.2, 128.7, 129.3, 135.2, 144.0, 151.3, 163.7; GC–MS (tR 24.38 min) m/z 202 (35, M+), 91 (100); HRMS calcd for C11H11N2O2 [M + H]+ 203.0815, found 203.0817.

1-N-Benzyl-3-N-methyl-5-(fur-2-yl)uracil (5e)

Treatment of 1c (48 mg, 0.14 mmol) with n>an class="Chemical">furan (0.2 mL, 187 mg, 2.75 mmol) by procedure C (column chromatography; hexane/EtOAc, 8:2 → 7:3) gave 5e (26 mg, 65% yield): GC–MS (tR 17.67 min) m/z 282 (75, M+), 91 (100); 1H NMR δ 3.45 (s, 3H), 5.03 (s, 2H), 6.45 (dd, J = 3.4, 1.8 Hz, 1H), 7.06 (d, J = 3.3 Hz, 1H), 7.35 (d, J = 5.9 Hz, 1H), 7.33–7.39 (m, 3H), 7.36 (d, J = 8.1 Hz, 2H), 7.72 (s, 1H); 13C NMR δ 28.5, 53.0, 106.6, 109.3, 112.0, 128.1, 128.7, 129.3, 135.4, 136.0, 141.2, 146.3, 151.0, 160.2; HRMS calcd for C16H15N2O3 [M + H]+ 283.1077, found 283.1065.

1-N-Benzyl-3-N-methyl-5-(thiophen-2-yl)uracil (5g)

Treatment of 1c (48 mg, 0.14 mmol) with n>an class="Chemical">thiophene (0.2 mL, 210 mg, 2.5 mmol) by procedure C (column chromatography; hexane/EtOAc, 8:2 → 7:3) gave 5g (26 mg, 62% yield): GC–MS (tR 23.66 min) m/z 298 (75, M+), 91 (100); 1H NMR δ 3.46 (s, 3H), 5.03 (s, 2H), 7.02 (dd, J = 5.1, 3.7 Hz, 1H), 7.28–7.39 (m, 7H), 7.55 (s, 1H). 13C NMR δ 28.7, 52.8, 109.6, 124.0, 125.8, 126.9, 128.2, 128.8, 129.3, 134.9, 135.3, 137.0, 151.1, 161.4; HRMS calcd for C16H15N2O2S [M + H]+ 299.0849, found 299.0852.

1-N-Benzyl-3-N-methyluracil (6)

Treatment of 1c (48 mg, 0.14 mmol) with n>an class="Chemical">toluene (0.2 mL, 173 mg, 1.89 mmol) by procedure A (column chromatography; hexane/EtOAc, 6:4 → 4:6) gave 6(34) (29 mg, 73%): 1H NMR δ 3.36 (s, 3H), 4.94 (s, 2H), 5.75 (d, J = 7.9 Hz, 1H), 7.17 (d, J = 7.9 Hz, 1H), 7.2–7.48 (m, 5H); 13C NMR δ 28.0, 52.4, 102.0, 128.1, 128.6, 129.2, 135.4, 141.7, 152.0, 163.2; GC–MS (tR 20.49 min) m/z 216 (50, M+), 91 (100); HRMS calcd for C12H13N2O2 [M + H]+ 217.0972, found 217.0976.

Analogous treatment (TBAF 3.5 equiv) of n>an class="Chemical">1c (48 mg, 0.14 mmol) with furan (0.2 mL, 187 mg, 2.75 mmol) by procedure A gave 6(34) (24 mg, 60%).

1-(2,3,5-Tri-O-acetyl-β-d-arabinofuranosyl)-5-(fur-2-yl)uracil (12)

Treatment (TBAF 3.5 equiv) of 1-(2,3,5-tri-O-acetyl-β-d-arabinon>an class="Chemical">furanosyl)-5-iodouracil[35] (7, 69.5 mg, 0.14 mmol) with furan (0.2 mL, 187 mg, 2.75 mmol) by procedure A (column chromatography; hexane/EtOAc, 6:4 → 5:5) gave 12 (37.3 mg, 61%) as a slightly yellow solid: 1H NMR δ 2.01 (s, 3H), 2.18 (s 3H), 2.19 (s, 3H), 4.24 (dt, J = 5.2, 3.9 Hz, 1H), 4.44 (dd, J = 12.0, 5.2 Hz, 1H), 4.53 (dd, J = 12.0, 4.2 Hz, 1H), 5.22 (dd, J = 3.6, 1.7 Hz, 1H), 5.48 (dd, J = 4.1, 1.7 Hz, 1H), 6.43 (d, J = 4.1 Hz, 1H), 6.49 (dd, J = 3.4, 1.8 Hz, 1H), 7.09 (d, J = 3.2 Hz, 1H), 7.39 (dd, J = 1.8, 0.6 Hz, 1H), 7.97 (s, 1H), 9.39 (s, 1H); 13C NMR δ 20.4, 20.65, 20.68, 62.7, 74.7, 76.5, 80.5, 84.2, 106.6, 109.7, 112.0, 134.1, 141.3, 145.5, 149.0, 159.6, 168.6, 169.6, 170.5; HRMS calcd for C19H21N2O10 [M + H]+ 437.1191, found 437.1176.

3′,5′-Di-O-acetyl-5-(fur-2-yl)-2′-deoxyuridine (13)

Treatment of 3′,5′-di-O-acetyl-2′-deoxy-5-iodouridine[36] (8, 61.3 mg, 0.14 mmol) with n>an class="Chemical">furan (0.2 mL, 187 mg, 2.75 mmol) by procedure A (column chromatography; hexane/EtOAc, 6:4 → 5:5) gave 13(37) (38.1 mg, 72%): 1H NMR δ 2.12 (s, 3H), 2.15 (s, 3H), 2.54 (ddd, J = 14.4, 6.5, 2.1 Hz, 1H), 2.54 (ddd, J = 14.2, 5.6, 1.6 Hz, 1H), 4.31 (“q”, J = 2.7 Hz, 1H), 4.38–4.39 (m, 2H), 5.28 (dt, J = 6.6, 1.7 Hz, 1H), 6.44 (dd, J = 6.5, 5.6 Hz, 1H), 6.45 (dd, J = 3.4, 1.8 Hz, 1H), 7.08 (d, J = 3.2 Hz, 1H), 7.32 (dd, J = 1.8, 0.6 Hz, 1H), 7.99 (s, 1H), 9.63 (s, 1H); 13C NMR δ 20.8, 21.0, 38.1, 64.2, 74.6, 82.7, 85.4, 107.9, 109.9, 112.2, 132.5, 141.2, 145.8, 149.6, 160.0, 170.4, 170.5; HRMS calcd for C17H19N2O8 [M + H]+ 379.1136, found 379.1136.

1-(β-d-Arabinofuranosyl)-5-(fur-2-yl)uracil (14). Method A

Treatment of 1-(β-d-arabinofuranosyl)-n>an class="Chemical">5-iodouracil[38] (9, 52 mg, 0.14 mmol) with furan (0.2 mL, 187 mg, 2.75 mmol) by procedure A (column chromatography; CH2Cl2/MeOH, 15:1 → 10:1) gave 14 (30.5 mg, 70%) as off-white solid. The analytical sample was obtained by precipitation from minimum amount of MeOH:CH2Cl2 (1:1, v/v) with hexane: 1H NMR (DMSO-d6) δ 3.60 (dt, J = 10.3, 5.0 Hz, 1H), 3.68 (dt, J = 10.3, 5.1 Hz, 1H), 3.78 (“q”, J = 4.6 Hz, 1H), 3.97 (dd, J = 7.3, 3.8 Hz, 1H), 4.04 (dt, J = 7.8, 3.9 Hz, 1H), 5.09 (t, J = 5.2 Hz, 1H), 5.51 (d, J = 4.4 Hz, 1H), 5.64 (d, J = 5.0 Hz, 1H), 6.07 (d, J = 4.3 Hz, 1H), 6.52 (dd, J = 3.3, 1.9 Hz, 1H), 6.85 (dd, J = 3.3, 0.5 Hz, 1H), 7.63 (dd, J = 1.8, 0.7 Hz, 1H), 8.07 (s, 1H), 11.65 (s, 1H); 13C NMR (DMSO-d6) δ 60.7, 75.3, 75.6, 84.8, 85.3, 104.0, 107.6, 111.5, 136.5, 141.4, 146.5, 149.4, 160.2. HRMS calcd for C13H15N2O7 [M + H]+ 311.0784, found 311.0811.

Compound 12 (43.6 mg, 0.10 mmol) was dissolved in NH3/n>an class="Chemical">MeOH (3 mL) at 0 °C (ice bath), and the resulting solution was stirred overnight. Volatiles were removed under the reduced pressure and the residue was column chromatographed (CH2Cl2/MeOH, 15:1 → 10:1) to give 14 (28.5 mg, 92%) with the spectroscopic data as above.

5-(Fur-2-yl)-2′-deoxyuridine (15). Method A

Treatment of 2′-deoxy-5-iodouridine[36] (10, 49.6 mg, 0.14 mmol) with n>an class="Chemical">furan (0.2 mL, 187 mg, 2.75 mmol) by procedure A (column chromatography; CH2Cl2/MeOH, 15:1 → 10:1) gave 15(37) (30.1 mg, 73%) as yellow solid: 1H NMR (DMSO-d6) δ 2.18 (dd, J = 6.6, 4.8 Hz, 2H), 3.61 (dd, J = 8.8, 5.0 Hz, 2H), 3.84 (q, J = 3.3 Hz, 1H), 4.28 (“quint”, J = 4.0 Hz, 1H), 5.08 (t, J = 4.8 Hz, 1H), 5.27 (d, J = 4.2 Hz, 1H), 6.22 (t, J = 6.7 Hz, 1H), 6.52 (dd, J = 3.3, 1.8 Hz, 1H), 6.85 (dd, J = 3.3, 0.5 Hz, 1H), 7.61 (dd, J = 1.8, 0.7 Hz, 1H), 8.33 (s, 1H), 11.62 (s, 1H); 13C NMR (DMSO-d6) δ 40.1, 61.1, 70.4, 84.7, 87.6, 105.6, 107.8, 111.5, 134.6, 141.5, 146.4, 149.4, 160.1. HRMS calcd for C13H13N2O6 [M – H]− 293.0779, found 293.0788.

Treatment of 13 (38 mg, 0.10 mmol) with NH3/n>an class="Chemical">MeOH, as described for 14 (Method B), gave 15 (28.0 mg, 95%) with the spectroscopic data as above.

5-(Fur-2-yl)uridine (16). Method A

Treatment of 5-iodouridine[36] (11, 52 mg, 0.14 mmol) with n>an class="Chemical">furan (0.2 mL, 187 mg, 2.75 mmol) by procedure A (column chromatography; CH2Cl2/MeOH, 15:1 → 10:1) gave 16(9b) (34.7 mg, 80%) as off-white solid. Precipitation of 16 from MeOH:CH2Cl2 (1:1, v/v) solution with hexane gave analytical sample of 16: UV (MeOH) λmax = 314 nm; 1H NMR (DMSO-d6) δ 3.60 (ddd, J = 11.9, 4.6, 2.9 Hz, 1 H), 3.68 (ddd, J = 11.9, 4.7, 2.9 Hz, 1H), 3.90 (“q”, J = 3.2 Hz, 1H), 4.02 (“q”, J = 4.5 Hz, 1H), 4.12 (“q”, J = 5.1 Hz, 1H), 5.11 (d, J = 5.0 Hz, 1H), 5.21 (t, J = 4.7 Hz, 1H), 5.43 (d, J = 5.5 Hz, 1H), 5.87 (d, J = 5.1 Hz, 1H), 6.52 (dd, J = 3.3, 1.8 Hz, 1H), 6.86 (dd, J = 3.3, 0.6 Hz, 1H), 7.60 (dd, J = 1.8, 0.7 Hz, 1H), 8.42 (s, 1H), 11.64 (s, 1H); 13C NMR (DMSO-d6) δ 60.6, 69.9, 74.0, 85.0, 88.2, 105.7, 108.0, 111.6, 134.9, 141.6, 146.4, 149.7, 160.1; HRMS calcd for C13H13N2O7 [M – H]− 309.0728, found 309.0734.

Analogous treatment of 11 (310 mg, 1.0 mmol) with pan class="Chemical">furan (1.1 mL, 1.03 g, 15 mmol) by procen>an class="Chemical">dure A gave 16 (273 mg, 88%).

Treatment of 11 (52 mg, 0.14 mmol) with furan (0.2 mL, 187 mg, 2.75 mmol) by procen>an class="Chemical">dure C (column chromatography; CH2Cl2/MeOH, 15:1 → 10:1) gave 16 (10 mg, 23% yield) with the spectroscopic data as above.

5-(Thiophen-2-yl)uridine (17). Method A

Treatment of 5-iodouridine (11, 52 mg, 0.14 mmol) with n>an class="Chemical">thiophene (0.2 mL, 210 mg, 2.5 mmol) by procedure A (column chromatography; CH2Cl2/MeOH, 15:1 → 10:1) gave 17 (44.7 mg, 98%) as off-white solid. Precipitation of 17 from MeOH:CH2Cl2 (1:1, v/v) solution with hexane gave analytical sample of 17: UV (MeOH) λmax = 318 nm; 1H NMR (DMSO-d6) δ 3.64 (ddd, J = 12.0, 4.4, 2.3 Hz, 1H), 3.76 (ddd, J = 12.0, 4.7, 2.8 Hz, 1H), 3.92 (dt, J = 4.8, 2.7 Hz, 1H), 4.06 (“q”, J = 5.1 Hz, 1H), 4.13 (“q”, J = 4.8 Hz, 1H), 5.09 (d, J = 5.5 Hz, 1H), 5.42 (t, J = 4.6 Hz, 1H), 5.46 (d, J = 5.3 Hz, 1H), 5.84 (d, J = 4.2 Hz, 1H), 7.05 (dd, J = 5.1, 3.7 Hz, 1H), 7.40 (dd, J = 3.7, 1.1 Hz, 1H), 7.46 (dd, J = 5.1, 1.1 Hz, 1H), 8.65 (s, 1H), 11.69 (s, 1H); 13C NMR (DMSO-d6) δ 60.2, 69.4, 74.3, 84.7, 88.7, 108.3, 122.5, 125.7, 126.4, 133.9, 135.7, 149.6, 161.3; HRMS calcd for C13H14N2NaO6S [M + Na]+ 349.0465, found 349.0465.

Treatment of 11 (52 mg, 0.14 mmol) with thiophene (0.2 mL, 210 mg, 2.5 mmol) by procen>an class="Chemical">dure C (column chromatography; CH2Cl2/MeOH, 15:1 → 10:1) gave 17 (8 mg, 17% yield) with the spectroscopic data as above.

5-(5-Methylthiophen-2-yl)uridine (18)

Treatment of 5-iodouridine (11, 52 mg, 0.14 mmol) with n>an class="Chemical">2-methylthiophene (0.2 mL, 203 mg, 2.1 mmol) by procedure A (column chromatography; CH2Cl2/MeOH, 15:1 → 10:1) gave 18 (36.5 mg, 76%) as off-white solid. Precipitation of 18 from MeOH:CH2Cl2 (1:1, v/v) solution with hexane gave analytical sample of 18: 1H NMR (DMSO-d6) δ 2.42 (s, 3H), 3.63 (ddd, J = 12.0, 4.4, 2.3 Hz, 1H), 3.74 (ddd, J = 12.0, 4.6, 2.8 Hz, 1H), 3.91 (“dt”, J = 4.8, 2.4 Hz, 1H), 4.05 (“q”, J = 5.0 Hz, 1H), 4.11 (“q”, J = 4.8 Hz, 1H), 5.08 (d, J = 5.4 Hz, 1H), 5.38 (t, J = 4.6 Hz, 1H), 5.44 (d, J = 5.4 Hz, 1H), 5.83 (d, J = 4.4 Hz, 1H), 6.72 (dd, J = 3.6, 1.1 Hz, 1H), 7.19 (d, J = 3.6 Hz, 1H), 8.53 (s, 1H), 11.63 (s, 1H); 13C NMR (DMSO-d6) δ 14.7, 60.2, 69.4, 74.2, 84.7, 88.6, 108.6, 122.6, 124.7, 131.5, 134.9, 138.8, 149.5, 161.3; HRMS calcd for C14H17N2O6S [M + H]+ 341.0802, found 341.0803.

5-(Pyrrol-2-yl)uridine (19)

Treatment of 5-iodouridine (11, 52 mg, 0.14 mmol) with n>an class="Chemical">pyrrole (0.2 mL, 193 mg, 2.88 mmol) by procedure A [reaction was carried out in a flask covered in aluminum foil under N2 atmosphere; column chromatography (CH2Cl2/MeOH, 15:1 → 10:1)] gave 19 (19.4 mg, 45%) as brownish amorphous powder. This material is stable when stored in refrigerator under the inert condition for at least 1 month: 1H NMR (DMSO-d6) δ 3.60 (ddd, J = 12.0, 4.4, 3.3 Hz, 1H), 3.70 (ddd, J = 11.8, 4.6, 3.5 Hz, 1H), 3.87 (“q”, J = 3.5 Hz, 1H), 4.03 (“q”, J = 4.7 Hz, 1H), 4.14 (“q”, J = 5.2 Hz, 1H), 5.10 (d, J = 5.2 Hz, 1H), 5.27 (t, J = 4.8 Hz, 1H), 5.42 (d, J = 5.6 Hz, 1H), 5.83 (d, J = 5.1 Hz, 1H), 6.03 (dd, J = 5.8, 2.6 Hz, 1H), 6.39 (dd, J = 4.5, 2.7 Hz, 1H), 6.76 (dd, J = 4.1, 2.5 Hz, 1H), 8.22 (s, 1H), 10.85 (s, 1H), 11.54 (bs, 1H); 13C NMR (DMSO-d6) δ 60.6, 69.6, 73.6, 84.7, 88.2, 105.5, 107.2, 108.0, 118.2, 123.8, 133.6, 149.7, 162.0; HRMS calcd for C13H16N3O6 [M + H]+ 310.1034, found 310.1034.

3-(Fur-2-yl)pyridin-2(1H)-one (21). Method A

Treatment of 20 (24.36 mg, 0.14 mmol) with furan (0.2 mL, 187 mg, 2.75 mmol) by procen>an class="Chemical">dure A (14 equiv of TBAF) gave 21(39) (9.3 mg, 41%) followed by 20 (8.5 mg, 35%). Compound 21: UV (MeOH) λmax = 332 nm; 1H NMR (CD3CN) δ 6.33 (t, J = 6.9 Hz, 1H), 6.53 (dd, J = 3.3, 1.8 Hz, 1H), 7.27–7.29 (m, 2H), 7.54 (dd, J = 1.8, 0.7 Hz, 1H), 7.89 (dd, J = 7.1, 2.0 Hz, 1H), 10.33 (s, 1H); 13C NMR (CD3CN) δ 106.5, 111.1, 112.7, 122.2, 133.7, 134.6, 143.0, 150.4, 160.2; HRMS calcd for C9H8NO2 [M + H]+ 162.0550, found 162.0553.

Treatment of 20 (24.36 mg, 0.14 mmol) with furan (0.2 mL, 187 mg, 2.75 mmol) by procen>an class="Chemical">dure C gave 21 (15.3 mg, 68% yield) with the spectroscopic data as above.

3-(Thiophen-2-yl)pyridin-2(1H)-one (22)

Treatment of 20 (24.36 mg, 0.14 mmol) with thiophene (0.2 mL, 210 mg, 2.5 mmol) by procen>an class="Chemical">dure A (14 equiv of TBAF) gave 22(39) (9 mg, 37%) followed by 20 (8 mg, 33%). Compound 22: UV (MeOH) λmax = 346 nm; 1H NMR (CD3CN) δ 6.32 (dd, J = 7.0, 6.6 Hz, 1H), 7.09 (dd, J = 5.1, 3.8 Hz, 1H), 7.28 (dd, J = 6.5, 1.9 Hz, 1H), 7.40 (dd, J = 5.1, 1.1 Hz, 1H), 7.67 (dd, J = 3.8, 1.1 Hz, 1H), 7.97 (dd, J = 7.1, 1.9 Hz, 1H), 9.98 (s, 1H); 13C NMR (CD3CN) δ 105.4, 123.6, 126.2, 126.4, 128.2, 129.2, 132.4, 134.5, 159.6; HRMS calcd for C9H8NOS [M + H]+ 178.0321, found 178.0320.

Treatment of 20 (24.36 mg, 0.14 mmol) with thiophene (0.2 mL, 210 mg, 2.5 mmol) by procen>an class="Chemical">dure C gave 22 (15.6 mg, 63% yield) with the spectroscopic data as above.

1-N-Benzyl-6-(4-methoxyphenyl)uracil (23)

AgCl (60 mg, 0.42 mmol), n>an class="Chemical">Pd(OAc)2 (3.2 mg, 0.014 mmol) and tetrabutylammonium bromide (TBABr, 67.7 mg, 0.21 mmol) were added to a stirred solution of 1-N-benzyluracil[33] (3, 28.3 mg, 0.14 mmol) and 4-iodoanisole (98.3 mg, 0.42 mmol) in DMSO (1 mL) under the N2 atmosphere at ambient temperature. The resulting suspension was stirred for 16 h at 100 °C (oil bath). Volatiles were evaporated, and the oily residue was dissolved in EtOAc and filtrated through Whatman GF/A filter paper. The filtrate was column chromatographed (hexane/EtOAc, 5:5 → 4:6) to give p-2d (10 mg, 23%) and 23 (14 mg, 33%) followed by 3 (12 mg, 41%). Compound 23: 1H NMR δ 3.84 (s, 1H), 4.96 (s, 1H), 5.65 (s, 1H), 6.87 (d, J = 8.6 Hz, 1H), 6.90–6.95 (m, 1H), 7.08 (d, J = 8.5 Hz, 1H), 7.19–7.26 (m, 1H), 9.73 (s, 1H); 13C NMR δ 48.6, 55.5, 104.1, 114.2, 125.3, 127.0, 127.7, 128.7, 129.5, 136.5, 152.3, 157.3, 161.0, 163.1; GC–MS (tR 24.77 min) m/z 308 (70, M+), 91 (100); HRMS calcd for C18H17N2O3 [M + H]+ 309.1234, found 309.1229.