Hydrogermylation of 5-Ethynyluracil Nucleosides: Formation of 5-(2-Germylvinyl)uracil and 5-(2-Germylacetyl)uracil Nucleosides

Abstract

A stereoselective radical-mediated hydrogermylation of the protected 5-ethynyluracil nucleosides with trialkyl-, triaryl,- or tris(trimethylsilyl)-germanes gave (Z)-5-(2-germylvinyl)-uridine, 2'-deoxyuridine or ara-uridine as major products. Reaction of the β-triphenylgermyl vinyl radical intermediate with oxygen and fragmentation of the resulting peroxyradical provided also 5-[2-(triphenylgermyl)acetyl] pyrimidine nucleosides in low to moderate yields. Thermal isomerization of the latter in MeOH occurred via a four-centered activated complex and subsequent hydrolysis of the resulting O-germyl substituted enol yielded 5-acetyluracil nucleosides in quantitative yield.

A broad spectrum of biological activity has been described for 5-substituted pyrimidine nucleosides.¹ One especially potent and selective antiviral drug of this class is (E)-5-(2-bromovinyl)-2'-deoxyuridine (BVDU).² Pronounced cytotoxity and significant antiviral activity have been reported for 1-(β-D-arabinofuranosyl)-5-ethenyluracil³ and 5-ethynyl-2'-deoxyuridine.⁴ The synthesis of the numerous 5-substituted pyrimidine nucleosides via Pd-assisted routes have been reviewed.⁵

The (E)-5-[2-(tributylstannyl)vinyl]uracil nucleosides, prepared by coupling of (E)-1,2-bis(tributylstannyl)ethene with 5-iodouracil precursors⁶ or via radical hydrostannylation of 5-ethynyluracil precursors,^7,8 have been developed as convenient substrates for a mild and rapid radiohalogenation via halodestannylation reactions.^6–9 However, the tendency of (E)-5-[2-(tributylstannyl)vinyl]arabinosyl uridine⁸ and 5-trimethylstannyl araU⁹ to protiodestannylation was noted.

The 5-[2-(trimethylsilyl)ethynyl]uracil nucleosides, prepared by Sonogashira coupling reactions between protected 5-iodouracil nucleosides with (trimethylsilyl)acetylene,^5,10 have been hydrogenated to give (Z)-5-[2-(trimethylsilyl)vinyl]uracil products.¹¹ Solvent dependent isomerization of the latter into the E isomer was observed.¹⁰ The (E)-5-[2-(trimethylsilyl)vinyl]-2'-deoxyuridine has been also prepared by direct Pd-catalyzed coupling of (E)-2-(tributylstannyl)-1-(trimethylsilyl)ethene with protected 5-iodo-2'-deoxyuridine.¹² The 5-vinyl silanes were converted to 5-(2-halovinyl)uracil products upon treatment with XeF₂ and metal halides,¹¹ and were also utilized for the radioiodination via iododesilylations reactions^10,12

The chemistry¹³ and biological activity of organogermanium compounds have been reviewed.^14,15 A few biologically active germane-modified nucleoside analogues have been developed. Among them, the 5-trimethylgermyluracil and 1-(2-tetrahydrofuranyl)-5-trimethylgermyluracil exhibit cytotoxicity to melanoma B16 cells.¹⁶ The 1-(2-tetrahydrofuranyl)-6-trialkylgermyl-5-fluorouracil derivatives have caused inhibition of DNA and RNA biosynthesis in Frhk cells almost twice as efficiently as the renowned antitumor drug Ftorafur.¹⁷ The 5-trimethylgermyl-2'-deoxyuridine, which is one of the few known examples of germanium-containing nucleoside analogues, was shown to inhibit HSV-1 replication in vitro and block incorporation of thymidine into DNA of cancer ovarian cells.¹⁸

Pd(0)-catalyzed,^19,20 Lewis acid-promoted,^21,22 radical-mediated,²³ and ultrasound- and microwave-accelerated²⁴ hydrogermylation of alkynes provide vinyl germanes with high regio and stereoselectivity. Germyldesulfonylation protocols has been also developed for the synthesis of vinyl and (α-fluoro)vinyl germanes.²⁵ Herein, we report that hydrogermylation of the 5-ethynyluracil nucleosides with trialkyl-, triaryl,- and tris(trimethylsilyl)germanes in addition to the expected 5-(2-germylvinyl)uracil nucleosides provides also access to 5-(2-germylacetyl)uracil nucleosides which can be converted to 5-acetyluracil products.

Several hydrogermylation approaches for the preparation of 5-(2-germylvinyl)uracil nucleosides of type 2 were initially tested. Thus, treatment of the acetyl protected 1-(β-D-arabinofuranosyl)-5-ethenyluracil²⁶ 1 with Ph₃GeH in the presence of 1,1'-azobis(cyclohexanecarbonitrile) (ACCN), as radical initiator, in toluene at 90 °C (Method A) produced predominantly the Z-vinyl germanes 2a (E/Z, 5:95; Scheme 1). The ¹H NMR spectra established the configuration for E-2a (J = 18.8 Hz) and Z-2a (J = 13.5 Hz) diastereomers. The formation of the Z isomer as the major product was in agreement with the expected²³ anti-addition of germyl radical to the triple bond. Careful purification of the reaction mixture on a silica gel column gave not only pure Z-2a (47%) but also led to the isolation of a new product, whose structure was assigned (vide infra) as 5-[2-(triphenylgermyl)acetyl]uracil (β-germylketone) derivative 4a (12%; Table 1, entry 1). Analogous treatment of 1 with Ph₃GeH without ACCN also yielded 2a and 4a (entry 2).

Scheme 1.

Hydrogermylation of 1-(β-D-arabinofuranosyl)-5-ethenyluracil 1 with germane hydrides

Reagents and conditions: (a) R₃GeH/(ACCN)/toluene/90 °C (Method A) or R₃GeH/Et₃B/THF/−78 °C (Method B) or Pd(PPh₃)₄/THF/rt (Method C); (b) NH₃/MeOH/rt.

Table 1.

Hydrogermylation of 5-ethynyluracil and related nucleosides with germane hydrides

Entry	Substrate	Methoda	Temp (°C)	R3GeH	Vinyl Germanes			Germyl Ketones
					Product	Yieldb (%)	Ratiob (E/Z)	Product	Yieldb (%)
1	1	A	90	Ph3GeH	2ac	47	0:100	4a	12
2	1	Ad	90	Ph3GeH	2a	60	3:97	4a	15
3	1	B	−78 → −60	Ph3GeH	2a	55	0:100	-	-
4	1	B	0	Ph3GeH	2a	28	0:100	4a	19
5	1	C	25	Ph3GeH	2ae	45	100:0	-	-
6	1	B	0 → 25	Me3GeH	2b	40	15:85	-	-
7	1	A	90	(Me3Si)3GeH	2cf	68	0:100	-	-
8	5	B	−78 → 0	Ph3GeH	7a	40	0:100	11a	13
9	5	B	0 →25	Me3GeH	7b	41	45:55	-	--
10	6	B	−78 → 0	Ph3GeH	9a	61	0:100	12a	12
11	6	Ag	100h	Ph3GeH	9ai	33	0:100	12a	17
12	6	B	0 →25	Me3GeH	9b	35	40:60	-	-
13	13	A	90	Ph3GeH	14	27	0:100	15	13
14	13	Ad	100	Ph3GeH	14	34	0:100	15	20
15	13	Ad,j	100	Ph3GeH	14	86	100:0	-	-
16	13	B	0	Ph3GeH	14	52	0:100	-	-

^aMethod A: R₃GeH/ACCN/toluene/90 or 100 °C; Method B: R₃GeH/Et₃B/THF/−78 or 0 °C; Method C: Pd(PPh₃)₄/THF/rt.

^bFor the isolated products.

^cCrude reaction mixture (E/Z-2a, 5:95).

^dWithout ACCN.

^eIn addition to E-2a the α-addition product was formed (~3:2, 73% overall).

^fCrude reaction mixture (E/Z-2c, 4:96).

^gToluene/THF (20:1, v/v) was used as solvent.

^hOil-bath.

ⁱWith toluene/DMF/H₂O (20:1:0.1, v/v/v) as solvents E-9a, Z-9a and 12a (~2:2:1, 40% overall) were obtained.

^jWith addition of 25 µL H₂O (14 equiv.).

The Et₃B-promoted addition²⁷ of Ph₃GeH to 1 in THF at low temperature (Method B) gave Z-2a as the sole isolated product (55%, entry 3). However, analogous hydrogermylation of 1 at higher temperature (0 °C/6 h) afforded Z-2a and 4a (~3:2, entry 4). Hydrogermylation of 1 with Ph₃GeH in the presence of B(C₆F₅)₃²² in CH₂Cl₂ at ambient temperature produced 2a in lower than 10% yields (TLC, ¹H NMR) while prolonged heating (48 h, reflux) produced a complex reaction mixture. The Pd(PPh₃)₄-catalyzed hydrogermylation¹⁹ of 1 with Ph₃GeH in THF afforded a mixture of the expected E-isomer of 2a and the corresponding 5-[1-(triphenylgermyl)ethenyl] regioisomer, produced by addition of germyl radical to α-carbon,¹⁹ in 3:2 ratio and 73% overall yield (entry 5; Method C). The acyl product 4a was not isolated from the reaction catalyzed by Pd(PPh₃)₄.

At low temperature (−78 °C), Et₃B-promoted hydrogermylation²⁷ of 1 with less reactive trimethylgermane failed to give vinyl germane 2b. However, at ambient temperature the hydrogermylation yielded 2b (E/Z, 15:85; entry 6) although in lower stereoselectivity. Moreover, no 5-[2-(trialkylgermyl)acetyl] product 4b was isolated from the reaction mixture. Hydrogermylation of 1 with reactive (Me₃Si)₃GeH (ACCN/toluene/90 °C) stereoselectively produced the 5-[2-(TTMS-germyl)ethenyl]uracil analogue Z-2c (68%) in a shorter reaction time, though, the acyl product 4c was also not isolated (30 min., entry 7). Deacetylation of Z-2a with NH₃/MeOH afforded Z-3a (86%).

The Et₃B-promoted hydrogermylation of the toluoyl protected of 5-ethynyluridine 5 with Ph₃GeH/Et₃B at −78 °C showed a slow conversion to 7a but warming of the reaction mixture to 0 °C afforded vinyl triphenylgermane Z-7a (40%) together with the 5-[2-(triphenylgermyl)acetyl] product 11a (13%, Scheme 2; entry 8). Analogous hydrogermylation of 5 with Me₃GeH/Et₃B at ambient temperature yielded only vinyl trimethylgermane 7b (E/Z, 45:55; entry 9). Deprotection of E/Z-7b with MeONa/MeOH afforded uridine analogue E/Z-8b (71%), confirming stability²⁸ of the C(sp²)-Ge(alkyl)₃ bond in basic conditions.

Scheme 2.

Hydrogermylation of 5-ethenyluridine 5 and 2'-deoxy-5-ethenyluridine 6 with germane hydrides.

Reagents and conditions: (a) Ph₃GeH/(ACCN)/toluene/(THF or DMF)/90 °C or R₃GeH/Et₃B/THF/−78 °C; (b) MeONa/MeOH/rt; (c) NH₃/MeOH/rt.

Hydrogermylation of the toluoyl protected 5-ethynyl-2'-deoxyuridine 6 with Ph₃GeH/Et₃B produced the vinyl triphenylgermane Z-9a (61%) along with the 5-[2-(triphenylgermyl)acetyl] product 12a (12%, entry 10). Treatment of 6 with Ph₃GeH/ACCN in toluene/THF (20:1, v/v) also yielded Z-9a and 12a (entry 11). Interestingly, analogous hydrogermylation of 6 in toluene/DMF with addition of "measured" amount of water (14 equiv.) produced mixture of E/Z isomers of 9a (~1:1) as well as 12a (entry 11, footnote i; DMF or THF were added to increase solubility of 6 in reaction mixture). Deprotection of Z-9a with NH₃/MeOH provided 2'-deoxyuridine analogue Z-10a (65%). Treatment of 6 with Me₃GeH/Et₃B gave E/Z-9b (entry 12) but once again hydrogermylation with alkyl germanes did not yield the germyl ketone product.

Treatment of E/Z-9b (40:60) with N-bromosuccinimide (NBS) followed by deprotection (NH₃/MeOH) afforded a mixture of 5-(2-bromovinyl)-2'-deoxyuridines (E/Z, ~2:3; 70%) illustrating a potential application of vinyl 5-[2-(trimethylgermyl)vinyl]uracil nucleosides towards the synthesis of 5-(2-halovinyl) analogues with possible applications for radiolabeling. Stereoselective halodegermylation of vinyl trialkylgermanes with NBS or NIS with retention of the double-bond geometry is known.^22,28,29 It is also worth mentioning that substitution of the trialkylgermyl group on a sp² carbon^29,30 (as well as sp carbon^30,31) with a halogen proceed not only more easily than the substitution of the corresponding trialkylsilyl group but also with improved stereochemical outcome.

It is noteworthy that hydrogermylation of the alkyl or aryl alkynes usually provides vinyl germanes in high yields, while the formation of the corresponding β-germylketones have not been observed.^23,24,27,32 We reexamined the hydrogermylation of phenylacetylene with Ph₃GeH under conditions described in Method A and B and found no formation of the β-germylketones. We also found that hydrostannylation and hydrosilylation of alkyne 5 with Ph₃SnH and Ph₃SiH under analogous conditions failed to yield 5-[2-(triphenylstannyl,- or silyl)acyl] products of type 11a suggesting that the formation of 5-acyluracil products is selective for germane hydrides. Intrigued by this interesting finding, we have examined the chemistry involving the formation of 5-keto pyrimidine nucleosides (e.g. 4a, 11a or 12a) employing 1-N-benzyl-5-ethynyluracil 13 as a model substrate. Thus, hydrogermylation of the readily available^33,34 13 with Ph₃GeH/ACCN in toluene (90 ° C/2 h) produced the Z-vinyl germane 14 and germyl ketone 15 (~2:1, entry 13; Scheme 3). Analogous treatment of 13 with Ph₃GeH in toluene without ACCN yielded Z-14 and 15 in 34% and 20% yields (entry 14), whereas similar reaction of 13 with Ph₃GeH in "moist" toluene interestingly yielded only the syn-addition product E-14 (86%, entry 15; see also entry 11, footnote i). Hydrogermylation of 13 with Ph₃GeH in the presence of Et₃B/THF at 0 °C produced Z-14 as a sole product (entry 16).

Scheme 3.

Hydrogermylation of the 5-ethenyluracil 13

Reagents and conditions: (a) Ph₃GeH/(ACCN)/toluene/(H₂O)/100 or 90 °C or Ph₃GeH/Et₃B/THF/0 °C

The structures of the (triphenylgermyl)methyl ketone (β-germylketone) products were established by spectroscopic analysis. Thus, ¹H NMR spectrum of 11a confirmed the absence of the vinyl unit whereas two upfield shifted doublets at 3.76 and 3.87 ppm (J = 9.0 Hz) supported the presence of the C(O)CH₂Ge moiety. The peak at 193.3 ppm in the ¹³C NMR spectrum of 11a confirmed the presence of the ketone. The HMBC and NOE correlations also supported the proposed structures (Figure S1 in SI section). The (+)ESI-MS analyses of 15 produced the [M+Na]+ ion as the predominant peak. The (−)ESI-MS/MS product ion spectrum of [M-H]- (m/z 547 [⁷⁴Ge]) showed loss of 43u (CONH) as the most abundant product ion (m/z 504[⁷⁴Ge]).

Formation of the β-germylketones (e.g., 15) probably involves an initial attack of the triphenylgermyl radical at the triple bond of 13 to give a vinylic radical 16 (Figure 1). Abstraction of hydrogen from triphenylgermane in an anti fashion would produce Z-vinyl germane 14 as a major product while the syn addition could produce the E isomer (path a). Reaction of radical 16 with residual oxygen present in the reaction mixture might lead to peroxyl radical 17 which should abstract hydrogen from germane hydride³⁵ to produce hydroperoxide. The latter can be converted to enol 18, probably by radical mechanism, and undergo tautomerization to yield the conjugated β-germylketone 15 (path b).

Figure 1.

A plausible pathway for the formation of β-germylketone 15 during radical hydrogermylation of alkyne 13 with Ph₃GeH

Efforts have been made to further optimize conditions for the preparation of 15 (and 14) from the reaction of 13 with Ph₃GeH. Thus, experiments under Ar vs N₂ vs atmospheric conditions as well as using dry vs reagent-grade toluene vs "moist" toluene (with the added measured amount of H₂O or D₂O) did not improve the yield of 15. In fact, they led mainly to the formation of Z-14 or E/Z-14 and 15 albeit in different yields and ratios. It is noteworthy that treatment of 13 with Ph₃GeH in oxygenated toluene resulted in the recovery of unchanged 13. Optimal conditions for the formation of β-germylketones would require very low rate of initiation and very low and nearly constant concentration of Ph₃GeH and oxygen, which would allow the germyl radical to react with the vinyl group rather than oxygen and the vinylic radical 16 to react with oxygen rather than germane. Still our experiments employing slow addition of Ph₃GeH via syringe-pump over 24 hours did not improve the yield of 15.

During recrystallization of the crude 15 from MeOH, we noticed the formation of a new byproduct whose structure was established as 5-acetyl-1-N-benzyluracil 20 both spectroscopically and by comparison with a sample of 20 that was independently synthesized by acid-catalyzed hydration³⁶ of 13. We found that heating of 15 in MeOH gave the 5-acetyl product 20 quantitatively. Conversion of (triphenylgermyl)methyl ketone 15 to methyl ketone 20 most probably involves intramolecular thermal isomerization which proceedes via a four-centered activated complex. Hydrolysis of the resulting O-germyl substituted enol 19 led to ketone 20 (Figure 2). Analogous thermal rearrangements of the β-silylketones into O-silyl substituted enols have been reported.³⁷ Heating 15 in MeOD or MeOH-d₄ provided quantitatively 1-deuteriomethyl ketone 21, which supports the proposed degradation pathway.

Figure 2.

A plausible pathway for the thermal degradation of β-germylketone 15

In summary, we have demonstrated that radical hydrogermylation of the 5-acetylenic derivatives of protected uracil, uridine, 2'-deoxyuridine, and 1-(β-D-arabinofuranosyl)uracil analogues with triphenylgermane in toluene at elevated temperature provided vinyl germanes in good yields and high Z-stereoselectivity. The E isomers can be formed in the presence of added "measured" amount of water. Hydrogermylation of 5-ethynyluracil nucleosides with triphenylgermane in addition to vinyl germanes produced also 5-[2-(triphenylgermyl)acetyl]uracil nucleosides in yields up to 20%. Thermolysis of the latter in MeOH afforded quantitatively 5-acetyluracil nucleosides via hydrolysis of the O-germyl substituted enols. The Et₃B-promoted hydrogermylation of 5-ethynyluracil nucleosides with trimethylgermane in THF at low temperature gave E/Z mixture of vinyl germanes. Bromodegermylation of vinyl trimethylgermanes with NBS provides access to 5-(2-bromovinyl) analogues.

Experimental Part

¹H (Me₄Si) NMR spectra at 400 MHz and ¹³C (Me₄Si) at 100.6 MHz were determined in CDCl₃ unless otherwise noted. Mass spectra (MS) were obtained with atmospheric pressure chemical ionization (APCI) technique and HRMS in ESI TOF mode unless otherwise noted.

1-(2,3,5-Tri-O-acetyl-β-D-arabinofuranosyl)-5-(E/Z)-[2-(triphenylgermyl)ethenyl]uracil (2a) and 1-(2,3,5-tri-O-acetyl-β-D-arabinofuranosyl)-5-[2-(triphenylgermyl)acetyl]uracil (4a)

Method A. Nucleoside 1²⁶ (50 mg, 0.13 mmol) was added to freshly distilled toluene (6 mL) and the suspension was stirred and degassed with N₂ for 30 min. The mixture was then preheated at 80 °C and Ph₃GeH (50 mg, 0.16 mmol) and ACCN (4 mg, 0.02 mmol) were added. The temperature was increased to 90 °C and the solution was stirred until 1 was completely consumed (TLC; 14 h). The volatiles were removed in vacuo and the residue was chromatographed (hexane/EtOAc, 2:3) to give a separable mixture of Z-2a (43 mg, 47%) and 4a (11 mg, 12%). Z-2a: ¹H NMR δ 1.99 (s, 3H), 2.09 (s, 3H), 2.11 (s, 3H), 3.70 (dd, J = 13.7, 7.7 Hz, 1H), 3.91–3.98 (m, 2H), 4.97 (dd, J = 3.2, 2.0 Hz, 1H), 5.27 (dd, J = 4.1, 1.9 Hz, 1H), 5.71 (d, J = 4.1 Hz, 1H), 6.56 (d, J = 13.5 Hz, 1H), 7.08 (d, J = 1.0 Hz, 1H), 7.36–7.52 (m, 16H), 8.30 (br. s, 1H); ¹³C NMR δ 20.4, 20.6, 20.7, 62.3, 74.6, 76.1, 79.8, 84.4, 113.4, 128.4, 129.1, 131.7, 134.8, 136.38, 136.41, 138.2, 148.6, 161.3, 168.5, 169.4, 170.2; MS m/z 701 (100, MH⁺, ⁷⁴Ge), 699 (71, MH⁺, ⁷²Ge) 698 (51, MH⁺, ⁷⁰Ge). Anal. Calcd for C₃₅H₃₄GeN₂O₉ • 0.25 EtOH (710.81): C, 59.99; H, 5.03; N, 3.94. Found: C, 59.63; H, 4.92; N, 4.00.

¹H NMR of the crude reaction mixture also showed presence (~5%) of the E-2a with the characteristic peaks at δ 6.31 (d, J = 4.0 Hz, H1'), 6.69 (d, J = 18.8 Hz, CH).

Compound 4a had: ¹H NMR δ 1.90 (s, 3H), 2.14 (s, 3H), 2.15 (s, 3H), 3.48 (d, J = 9.3 Hz, 1H), 4.16–4.19 (m, 1H), 4.17 (d, J = 9.3 Hz, 1H), 4.33 (dd, J = 12.1, 4.5 Hz, 1H), 4.44 (dd, J = 12.1, 4.9 Hz, 1H), 5.12 (dd, J = 3.3, 1.6 Hz, 1H), 5.33 (dd, J = 4.1, 1.6 Hz, 1H), 6.23 (d, J = 4.1 Hz, 1H), 7.21–7.55 (m, 15H), 8.08 (s, 1H), 8.49 (br. s, 1H); ¹³C NMR δ 20.4, 20.7, 20.8, 33.0, 62.4, 74.6, 76.6, 80.8, 83.9, 113.3, 128.4, 129.5, 135.1, 135.3, 146.6, 148.8, 160.4, 168.8, 169.6, 170.8, 194.2; HRMS calcd for C₃₅H₃₄ ⁷⁴GeN₂NaO₁₀ [M+Na]⁺ 739.1323, found 739.1311.

Method B. Et₃B (1M/THF; 140 µL, 0.14 mmol) was added to a stirred solution of 1 (50 mg, 0.127 mmol) and Ph₃GeH (43 mg, 0.14 mmol) in anhydrous THF (5 mL) placed in screw-capped glass tube at −78 °C. After 3 h, when TLC showed appearance of less polar spot, the reaction mixture was warmed up to −60 °C and was stirred for 1.5 h. The volatiles were evaporated and the residue was chromatographed (hexane/EtOAc, 2:3) to give Z-2a (49 mg, 55%) as a white powder after crystallization from hexane/Et₂O.

Method C. Ph₃GeH (59 mg, 0.2 mmol) and Pd(PPh₃)₄ (8 mg, 0.08) were added to a stirred suspension of 1 (70 mg, 0.18 mmol) in THF (3 mL) in a flamed dried round bottle flask at rt under N₂. After 5 h, the volatiles were evaporated in vacuo and the residue was chromatographed (hexane/EtOAc, 1:1) to give inseparable mixture of the E-isomer of 2a and the α-addition product (90 mg, 73%; E-2a/α-addition product, 3:2): MS (ESI) m/z 701 (100, MH⁺, ⁷⁴Ge), 699 (70, MH⁺, ⁷²Ge), 697 (50, MH⁺, ⁷⁰Ge). E-2a: ¹H NMR δ 1.88 (s, 3H), 1.98 (s, 3H), 2.14 (s, 3H), 4.23 (m, 2H), 4.35–4.39 (m, 1H), 5.09 (dd, J = 3.4, 1.5 Hz, 1H), 5.43 (dd, J = 3.9, 1.5 Hz, 1H), 6.31 (d, J = 4.0 Hz, 1H), 6.69 (d, J = 18.8 Hz, 1H), 7.33–7.55 (m, 16H), 7.60 (s, 1H), 9.29 (br. s, 1H). The α-addition product 1-(2,3,5-tri-O-acetyl-β-D-arabinofuranosyl)-5-[1-(triphenylgermyl)-ethenyl]uracil: ¹H NMR δ 1.80 (s, 3H), 2.07 (s, 3H), 2.11 (s, 3H), 4.00–4.23 (m, 2H), 4.45–4.49 (m, 1H), 4.98 (dd, J = 3.5, 1.9 Hz, 1H), 5.35 (dd, J = 4.1, 1.5 Hz, 1H), 5.72 (d, J = 2.0 Hz, 1H), 6.17 (d, J = 4.1 Hz, 1H), 6.41 (d, J = 2.0 Hz, 1H), 7.33–7.55 (m, 16H), 8.98 (br. s, 1H).

1-(2,3,5-Tri-O-acetyl-β-D-arabinofuranosyl)-5-(E/Z)-[2-(trimethylgermyl)ethenyl]uracil (2b)

Nucleoside 1 (50 mg, 0.13 mmol) was treated with Me₃GeH (30 mg, 29.6 µL, 0.25 mmol) in dry THF (5 mL) as described in Method B (with injection of Me₃GeH into the reaction mixture via syringe and progressive warming from 0 °C to rt) for 14 h. The volatiles were removed under reduced pressure and the residue was chromatographed (hexane/EtOAc, 2:3) to give E/Z-2b (27 mg, 40%; E/Z, 15:85): ¹H NMR δ 0.26 (s, 7.65H), 0.28 (s, 1.35H), 2.02 (s, 3H), 2.12 (s, 2.55H), 2.15 (s, 0.45H), 2.16 (s, 2.55H), 2.17 (s, 0.45H), 4.19–4.25 (m, 1H), 4.34 (dd, J = 11.9, 6.2 Hz, 0.85H), 4.37–4.45 (m, 0.15H), 4.44 (dd, J = 11.9, 4.2 Hz, 0.85H), 4.52 (dd, J = 11.9, 6.2 Hz, 0.15H), 5.11 (dd, J = 3.8, 1.4 Hz, 0.85H), 5.15 (dd, J = 3.4, 1.6 Hz, 0.15H), 5.44–5.48 (m, 1H), 6.10 (d, J = 13.8 Hz, 0.85H), 6.24 (d, J = 3.8 Hz, 0.85H), 6.33 (d, J = 4.0 Hz, 0.15H), 6.60 (d, J = 18.9 Hz, 0.15H), 6.80 (d, J = 19.0 Hz, 0.15H), 6.98 (dd, J = 13.8, 1.0 Hz, 0.85H), 7.45 (d, J = 1.0 Hz, 0.85H), 7.59 (s, 0.15H), 8.97 (br. s, 0.15H), 9.09 (br. s, 0.85H); ¹³C NMR δ −1.7, −0.2, 20.5, 20.6, 20.8, 20.87, 20.92, 62.7, 63.2, 74.7, 74.8, 76.4, 76.5, 80.4, 80.8, 84.6, 112.9, 114.2, 132.1, 133.6, 134.3, 136.1, 136.4, 137.6, 149.2, 149.6, 161.8, 162.2, 168.6, 168.7, 169.7, 169.8, 170.5; HRMS calcd for C₂₀H₂₈ ⁷⁴GeNaN₂O₉ [M+Na]⁺ 537.0899, found 537.0888.

1-(2,3,5-Tri-O-acetyl-β-D-arabinofuranosyl)-5-(Z)-[2-(tris(trimethylsilyl)germyl)ethenyl]uracil (2c)

Nitrogen gas was bubbled through a heterogeneous mixture of 1 (127 mg, 0.32 mmol) in dry toluene (10 mL) for 30 min. The suspension was pre-heated up to 90 °C (~5 min) and (Me₃Si)₃GeH (115 mg, 123 µL, 0.39 mmol) was added via syringe followed by ACCN (8 mg, 0.04 mmol) dissolved in degassed toluene (1 mL). The reaction mixture was heated at 90 °C for 30 min. Volatiles were evaporated and the residue was chromatographed (hexane/EtOAc, 3:2) to give Z-2c (152 mg, 68%): ¹H NMR δ 0.20 (s, 27H), 2.01 (s, 3H), 2.10 (s, 3H), 2.14 (s, 3H), 4.17–4.24 (m, 1H), 4.34 (dd, J = 12.0, 5.5 Hz, 1H), 4.38 (dd, J = 12.0, 5.2 Hz, 1H), 5.14 (dd, J = 3.8, 1.6 Hz, 1H), 5.47 (dd, J = 3.8, 1.7 Hz, 1H), 6.16 (d, J = 3.9 Hz, 1H), 6.28 (d, J = 13.5 Hz, 1H), 6.90 (dd, J = 13.5, 1.4 Hz, 1H), 7.29 (d, J = 1.4 Hz,1H), 8.27 (br. s, 1H); ¹³C NMR δ 1.95, 20.7, 20.9, 21.0, 63.1, 75.0, 76.5, 80.7, 85.4, 116.1, 133.9, 134.1, 136.2, 149.6, 162.0, 168.9, 169.7, 170.6; HRMS calcd for C₂₆H₄₇ ⁷⁴GeN₂O₉Si₃ [M+H]⁺ 689.1801, found 689.1798.

¹H NMR of the crude reaction mixture showed 4:96 mixture of E/Z isomers of 2c. The E-2c had the characteristic peaks on the ¹H NMR spectrum at: δ 6.36 (d, J = 3.5 Hz, H1'), 6.63 (d, J = 18.7 Hz, CH), 6.86 (d, J = 18.5 Hz, CH).

1-(β-D-Arabinofuranosyl)-5-(Z)-[2-(triphenylgermyl)ethenyl]uracil (3a)

A saturated solution of NH₃/MeOH (3 mL) was added to a suspension of Z-2a (40.0 mg, 0.057 mmol) in MeOH (2 mL) and the reaction mixture stirred for 6 h at 0 °C and then for 2 h at rt. Volatiles were evaporated and the residue was chromatographed (EtOAc/MeOH, 98:2) to give Z-3a (28.2 mg, 86%): ¹H NMR (MeOH-d₄) δ 3.28 (dd, J = 11.3, 4.0 Hz, 1H), 3.37 (dd, J = 11.3, 5.6 Hz, 1H), 3.76 (ddd, J = 5.8, 4.1, 2.1 Hz, 1H), 3.98–4.02 (m, 2H), 5.59 (d, J = 3.3 Hz, 1H), 6.50 (d, J = 13.2 Hz, 1H), 7.30 (d, J = 13.3 Hz, 1H), 7.35–7.51 (m, 16H); ¹³C NMR (MeOH-d₄) δ 62.6, 76.6, 78.4, 87.0, 88.3, 113.8, 129.3, 130.0, 131.5, 136.0, 138.2, 139.9, 140.7, 151.3, 165.0; HRMS calcd for C₂₉H₂₈ ⁷⁴GeNaN₂O₆ [M+Na]⁺ 597.1051, found 597.1076. Anal. Calcd for C₂₉H₂₈GeN₂O₆•CH₃OH•H₂O (623.24): C, 57.81; H, 5.50; N, 4.49. Found: C, 57.84; H, 5.41; N, 4.69.

5-Ethynyl-2',3',5'-tri-O-p-toluoyluridine (5)

The p-toluoyl chloride (86 µL, 101 mg, 0.65 mmol) was added to a stirred solution of 5-ethynyluridine³⁸ (50 mg, 0.19 mmol) in pyridine (5 mL). After 24 h, volatiles were evaporated and the residue was partitioned (NaHCO₃/H₂O//CHCl₃). The organic layer was washed with HCl/H₂O, NaHCO₃/H₂O, brine, and was dried (MgSO₄), and evaporated. The residue was chromatographed (hexane/EtOAc, 7:3 → 6:4) to give 5 (85 mg, 73%): ¹H NMR δ 2.40 (s, 3H), 2.44 (s, 6H, 2 × Me), 2.94 (s, 1H), 4.69–4.76 (m, 2H), 4.78–4.84 (m, 1H), 5.70 ("t", J = 6.0 Hz, 1H), 5.84 (dd, J = 5.9, 3.6 Hz, 1H, H3), 6.37 (d, J = 6.1 Hz, 1H), 7.18 (d, J = 8.0 Hz, 2H), 7.24 (d, J = 8.0 Hz, 2H), 7.31 (d, J = 8.0 Hz, 2H), 7.81 (s, 1H), 7.84 (d, J = 8.2 Hz, 2H), 7.91 (d, J = 8.2 Hz, 2H), 8.02 (d, J = 8.2 Hz, 2H), 8.18 (s, 1H); ¹³C NMR δ 21.8, 21.9, 63.8, 71.5, 73.76, 73.77, 81.4, 82.5, 87.9, 100.6, 125.7, 126.1, 126.5, 129.4, 129.5, 129.7, 129.9, 130.1, 130.2, 143.2, 144.6, 144.8, 144.9, 148.8, 160.6, 165.4, 165.5, 166.3; HRMS calcd for C₃₅H₃₁N₂O₉ [M+H]⁺ 623.2024, found 623.2033.

5-(Z)-[2-(Triphenylgermyl)ethenyl]-2',3',5'-tri-O-p-toluoyluridine (7a) and 5-[2-(triphenylgermyl)acetyl]-2',3',5'-tri-O-p-toluoyluridine (11a)

Nucleoside 5 (49 mg, 0.08 mmol) was treated with Ph₃GeH (26 mg, 0.085 mmol) in dry THF (5 mL) as described in Method B. After 6 h at −78 °C, the reaction mixture was slowly warmed to 0 °C (~24 h). The volatiles were evaporated and the residue was chromatographed (hexane/EtOAc, 1:1) to give a separable mixture of Z-7a (29 mg, 40%) and 11a (10 mg, 13%). Z-7a: ¹H NMR δ 2.40 (s, 6H), 2.42 (s, 3H), 4.34 (dd, J = 12.2, 5.4 Hz, 1H), 4.40 (dd, J = 12.2, 3.4 Hz, 1H), 4.47 (ddd, J = 5.8, 5.4, 3.5 Hz, 1H), 5.38 (dd, J = 6.2, 4.5 Hz, 1H), 5.51 (“t”, J = 6.0 Hz, 1H), 5.52 (d, J = 4.4 Hz, 1H), 6.50 (d, J = 13.6 Hz, 1H), 7.11 (d, J = 0.9 Hz, 1H), 7.16 (d, J = 8.1 Hz, 2H), 7.19 (d, J = 8.0 Hz, 2H), 7.22 (dd, J = 13.5, 0.9 Hz, 1H), 7.24 (d, J = 8.0 Hz, 2H), 7.31–7.55 (m, 15H), 7.80 (d, J = 8.2 Hz, 4H), 7.96 (d, J = 8.2 Hz, 2H), 8.09 (br. s, 1H); ¹³C NMR δ 21.67, 21.69, 21.72, 63.5, 70.5, 73.6, 79.9, 89.8, 115.0, 125.9, 126.0, 126.7, 128.5, 129.1, 129.2, 129.3, 129.7, 129.8, 129.9, 131.4, 134.7, 136.6, 137.0, 138.2, 144.2, 144.4, 144.5, 148.8, 161.4, 165.0, 165.1, 166.1; HRMS calcd for C₅₃H₄₆⁷⁴GeN₂NaO₉ [M+Na]⁺ 951.2307, found 951.2315.

Compound 11a had: UV (MeOH) λ_max = 282 nm; ¹H NMR δ 2.35 (s, 3H), 2.40 (s, 3H), 2.42 (s, 3H), 3.76 (d, J = 9.0 Hz, 1H), 3.87 (d, J = 9.0 Hz, 1H), 4.67–4.75 (m, 3H), 5.66 (dd, J = 5.9, 5.1 Hz, 1H), 5.83 (“t”, J = 5.7 Hz, 1H), 6.01 (d, J = 5.0 Hz, 1H), 7.16–7.55 (m, 21H), 7.83 (d, J = 8.2 Hz, 2H), 7.87 (d, J = 8.2 Hz, 2H), 8.02 (d, J = 8.2 Hz, 2H), 8.05 (s, 1H); ¹³C NMR δ 21.7, 33.0, 63.5, 71.0, 73.9, 80.9, 90.4, 113.7, 125.7, 126.0, 126.6, 128.2, 129.21, 129.24, 129.3, 129.4, 129.8, 129.9, 135.0, 135.1, 144.1, 144.5, 144.7, 146.6, 148.5, 160.2, 165.19, 165.21, 166.3, 193.3; HRMS calcd for C₅₃H₄₇⁷⁴GeN₂O₁₀ [M+H]⁺ 945.2437, found 945.2456.

5-(E/Z)-[2-(Trimethylgermyl)ethenyl]-2',3',5'-tri-O-p-toluoyluridine (7b)

Nucleoside 5 (50.0 mg, 0.08 mmol) was treated with Me₃GeH (19.0 mg, 18.8 µL 0.16 mmol) in dry THF (5 mL) as described in Method B (with injection of Me₃GeH into the reaction mixture via syringe and progressive warming from 0 °C to rt) for 10 h. The volatiles were evaporated and the residue was chromatographed (hexane/EtOAc, 3:2) to give E/Z-7b (24.5 mg, 41%; E/Z, 45:55). ¹H NMR δ 0.12 (s, 4.05H), 0.20 (s, 4.95H), 2.40, 2.43, 2.44 (singlets, 9H), 4.68–4.82 (m, 3H), 5.72 (“t”, J = 6.0 Hz, 0.55H), 5.78 (“t”, J = 6.3 Hz, 0.45H), 5.82 (dd, J = 6.1, 3.9 Hz, 0.55H), 5.88 (dd, J = 5.8, 2.8 Hz, 0.45H), 5.98 (d, J = 13.7 Hz, 0.55H), 6.34 (d, J = 5.9 Hz, 0.55H), 6.37 (d, J = 19.0 Hz, 0.45H), 6.50 (d, J = 6.8 Hz, 0.45H), 6.69 (d, J = 19.0 Hz, 0.45H), 6.72 (dd, J = 13.7, 1.0 Hz, 0.55H), 7.16–7.32 (m, 6H), 7.34 (d, J = 1.0 Hz, 0.55H), 7.54 (s, 0.45H), 7.83–8.04 (m, 6H), 8.24 (br. s, 0.45H), 8.27 (br. s, 0.55H); ¹³C NMR δ −2.0, −0.2, 21.7, 63.7, 64.2, 71.1, 71.5, 73.45, 73.53, 80.7, 81.0, 86.9, 88.0, 114.4, 116.0, 125.65, 125.70, 125.96, 125.97, 126.3, 126.5, 129.24, 129.25, 129.29, 129.4, 129.6, 129.71, 129.73, 129.88, 129.9, 129.95, 130.0, 131.8, 134.1, 134.4, 135.1, 135.8, 138.7, 144.3, 144.5, 144.57, 144.63, 144.64, 144.7, 149.3, 149.7, 161.3, 161.7, 165.3, 165.35, 165.38, 165.5, 166.1; MS (ESI⁺) m/z 765 (100, MNa⁺, ⁷⁴Ge), 763 (71, MNa⁺, ⁷²Ge), 762 (52 MNa⁺, ⁷⁰Ge); Anal. Calcd for C₃₈H₄₀GeN₂O₉ • H₂O (759.39): C, 60.10; H, 5.57; N, 3.69. Found: C, 60.39; H, 5.38; N, 3.87.

5-(E/Z)-[2-(Trimethylgermyl)ethenyl]uridine (8b)

A 0.1 N solution of MeONa in MeOH (2.5 mL) was added to 7b (18.8 mg, 0.025 mmol; E/Z, ~45:55) and the resulting mixture was stirred at rt for 12 h. The reaction mixture was neutralized to pH ~6.2 by addition of Dowex 50WX2-200(H⁺). Dowex resin was filtered off and the filtrate was evaporated and residue partitioned between Et₂O/H₂O. The organic layer was extensively washed with water and the combined aqueous layer was evaporated to yield E/Z-8b (7 mg, 71%; E/Z, ~40:60): ¹H NMR (D₂O) δ 0.27 (s, 5.4H), 0.32 (s, 3.6H), 3.84–3.91 (m, 1H), 3.95 (dd, J = 12.7, 2.7 Hz, 0.6H), 4.05 (dd, J = 12.9, 2.4 Hz, 0.4H), 4.18–4.24 (m, 1H), 4.29 (“t”, J = 5.0 Hz, 0.6H), 4.34 (“t”, J = 5.7 Hz, 0.4H), 4.39–4.44 (m, 1H), 6.00 (d, J = 3.8 Hz, 0.4H), 6.05 (d, J = 5.3 Hz, 0.6H), 6.36 (d, J = 13.6 Hz, 0.6H), 6.70 (d, J = 19.0 Hz, 0.4H), 6.83 (d, J =19.0 Hz, 0.4H), 6.93 (d, J =13.6 Hz, 0.6H), 7.77 (s, 0.6H), 8.20 (s, 0.4H); ¹³C NMR (MeOH-d₄) δ −2.9, −1.2, 60.1, 61.0, 68.9, 69.8, 73.7, 74.1, 84.0, 84.7, 88.8, 89.7, 113.8, 115.7, 132.1, 134.1, 134.7, 137.6, 137.8, 140.8, 151.1, 151.6, 164.6, 165.3; HRMS calcd for C₁₄H₂₃⁷⁴GeN₂O₆ [M+H]⁺ 389.0762, found 389.0775.

1-(2-Deoxy-3,5-di-O-p-toluoyl-β-D-erythro-pentofuranosyl)-5-(Z)-[2-(triphenylgermyl)ethenyl]uracil (9a) and 1-(2-Deoxy-3,5-di-O-p-toluoyl-β-D-erythro-pentofuranosyl)-5-[2-(triphenylgermyl)acetyl]uracil (12a)

Nucleoside 6¹⁰ (44 mg, 0.09 mmol) was treated with Ph₃GeH (30 mg, 0.1 mmol) in dry THF (5 mL) as described in Method B. After 6 h at −78 °C, the reaction mixture was slowly warmed to 0 °C and stirred for 3 h. The volatiles were evaporated and the residue was chromatographed (hexane/EtOAc, 3:2) to give a separable mixture of Z-9a (43 mg, 61%) and 12a (9 mg, 12%). Z-9a: ¹H NMR δ 1.68 (ddd, J = 14.9, 8.1, 7.0 Hz, 1H), 2.31 (ddd, J = 14.5, 5.7, 1.8 Hz, 1H), 2.41 (s, 3H), 2.45 (s, 3H), 4.16 (dd, J = 11.1, 3.5 Hz, 1H), 4.26–4.30 (m, 1H), 4.32 (dd, J = 11.1, 5.0 Hz, 1H), 5.15 (“dt”, J = 6.8, 1.8 Hz, 1H), 5.84 (dd, J = 8.1, 5.7 Hz, 1H), 6.51 (d, J = 13.5 Hz, 1H), 7.11 (d, J = 1.0 Hz, 1H), 7.21–7.56 (m, 20H), 7.88 (d, J = 8.2 Hz, 2H), 7.91 (d, J = 8.2 Hz, 2H); ¹³C NMR δ 21.68, 21.72, 37.3, 63.9, 74.5, 82.3, 85.5, 114.8, 126.4, 126.7, 128.5, 129.2, 129.25, 129.28, 129.6, 129.8, 131.0, 134.8, 135.6, 136.6, 138.8, 144.2, 144.5, 149.3, 161.7, 165.8, 166.0; HRMS calcd for C₄₅H₄₁ ⁷⁴GeN₂O₇ [M+H]⁺ 795.2120, found 795.2131.

Compound 12a had: ¹H NMR δ 2.18–2.27 (m, 1H), 2.36 (s, 3H), 2.45 (s, 3H), 2.64 (ddd, J = 14.3, 5.7, 1.8 Hz, 1H), 3.81 (d, J = 9.1 Hz, 1H), 3.85 (d, J = 9.1 Hz, 1H), 4.53–4.60 (m, 2H), 4.74–4.80 (m, 1H), 5.54 (“d”, J = 6.6 Hz, 1H), 6.16 (dd, J = 8.3, 5.7 Hz, 1H), 7.16 (d, J = 8.0 Hz, 2H), 7.28 (d, J = 8.1 Hz, 2H), 7.32–7.57 (m, 15H), 7.94 (d, J = 8.2 Hz, 2H), 7.95 (d, J = 8.2 Hz, 2H), 8.15 (s, 1H); ¹³C NMR δ 21.70, 21.73, 32.8, 38.4, 63.8, 74.5, 83.2, 86.2, 113.6, 126.3, 126.6, 128.2, 129.25, 129.28, 129.3, 129.8, 135.1, 135.2, 144.1, 144.5, 145.2, 148.8, 160.3, 165.8, 166.2, 193.5; HRMS calcd for C₄₅H₄₁⁷⁴GeN₂O₈ [M+H]⁺ 811.2075, found 811.2063.

Treatment of 6 (50 mg, 0.10 mmol) with Ph₃GeH (36 mg, 0.12 mmol) in toluene (4 mL) as described in Method A [DMF (0.2 mL) and water (25 µL, 25 mg, 1.4 mmol) were added to the pre-heated reaction mixture] gave partially separated E-9a (13 mg, 16%), 12a (6.5 mg, 8%) and Z-9a (13 mg, 16%). Compound E-9a had characteristic peaks for vinylic protons at δ 6.63 (d, J = 18.8 Hz, 1H) and within the envelope of aromatic protons at δ 7.20–7.50 (Cosy).

1-(2-Deoxy-3,5-di-O-p-toluoyl-β-D-erythro-pentofuranosyl)-5-(E/Z)-[2-(trimethylgermyl)ethenyl]uracil (9b)

Nucleoside 6¹⁰ (45.0 mg, 0.092 mmol) was treated with Me₃GeH (21.8 mg, 21.6 µL, 0.18 mmol) in dry THF (5 mL) as described in Method B (with injection of Me₃GeH into the reaction mixture via syringe and progressive warming from 0 °C to rt) for 10 h. The volatiles were evaporated and the residue was chromatographed (hexane/EtOAc, 1:1) to give E/Z-9b (16.5 mg, 35%; E/Z, 40:60): ¹H NMR δ 0.14 (s, 3.6H), 0.21 (s, 5.4H), 2.25–2.34 (m, 1H), 2.42, 2.43, 2.45 (singlets, 6H), 2.78 (ddd, J = 14.2, 5.5, 1.6 Hz, 0.6H), 2.80 (ddd, J = 14.3, 5.1, 1.2 Hz, 0.4H), 4.56–4.61 (m, 1H), 4.65 (dd, J = 12.2, 3.2 Hz, 0.6H), 4.73–4.77 (m, 0.8H), 4.75 (dd, J = 12.2, 3.8 Hz, 0.6H), 4.59 (“dt”, J = 4.9, 1.9 Hz, 0.6H), 4.63 (“d”, J = 6.4 Hz, 0.4H), 6.00 (d, J = 13.7 Hz, 0.6H), 6.40 (dd, J = 8.7, 5.4 Hz, 0.6H), 6.41 (d, J = 19.2 Hz, 0.4H), 6.46 (d, J = 8.9, 5.2 Hz, 0.4H), 6.72 (d, J = 19.0 Hz, 0.4H), 6.78 (dd, J = 13.7, 1.0 Hz, 0.6H), 7.22–7.32 (m, 4H), 7.48 (d, J = 1.0 Hz, 0.6H), 7.67 (s, 0.4H), 7.85–7.99 (m, 4H), 8.51 (br. s, 0.4H), 8.57 (br. s, 0.6H); ¹³C NMR δ −2.0, −0.2, 21.70, 21.73, 38.5, 64.1, 64.4, 74.7, 75.0, 82.9, 83.1, 85.6, 113.9, 115.4, 126.3, 126.4, 126.5, 129.30, 129.34, 129.50, 129.54 129.6, 129.8, 131.9, 133.9, 134.2, 134.9, 135.1, 138.3, 144.4, 144.5, 144.6, 149.2, 149.7, 161.5, 161.9, 166.0, 166.1; HRMS calcd for C₃₀H₃₄⁷⁴GeN₂NaO₇ [M+Na]⁺ 631.1470, found 631.1482.

Note

Treatment of 9b (E/Z, 40:60; 12 mg; 0.02 mmol;) with NBS (5 mg, 0.028 mmol) in CHCl₃/CH₂Cl₂ (1:1.5, v/v; 2.5 mL) for 6 h at 0 °C, followed by deprotections with NH₃/MeOH (0 °C to rt, 12 h) gave 5-(2-bromovinyl)-2'-deoxyuridine^39,40 (E/Z, 2:3; 70% from 9b).

1-(β-D-Erythro-pentofuranosyl)-5-(Z)-[2-(triphenylgermyl)ethenyl]uracil (10a)

A saturated solution of NH₃/MeOH (2 mL) was added to a suspension of Z-9a (33 mg, 0.042 mmol) in MeOH (2 mL) and the resulting mixture was stirred for 24 h at rt. The volatiles were evaporated and the residue was chromatographed (EtOAc) to give Z-10a (15 mg, 65%): ¹H NMR (MeOH-d₄) δ 1.44 (ddd, J = 14.2, 7.9, 6.6 Hz, 1H), 1.87 (ddd, J = 13.6, 5.9, 2.7 Hz, 1H), 3.38 (“d”, J = 4.4 Hz, 2H), 3.70 (“q”, J = 3.8 Hz, 1H), 3.97 (ddd, J = 6.0, 3.3, 2.8 Hz, 1H), 5.81 (dd, J = 8.0, 6.0 Hz, 1H), 6.52 (d, J = 13.3 Hz, 1H), 7.28 (d, J = 1.1Hz, 1H), 7.31 (dd, J = 13.3, 1.1 Hz, 1H), 7.36–7.54 (m, 15H); ¹³C NMR (MeOH-d₄) δ 40.3, 63.0, 72.3, 86.3, 88.6, 115.8, 129.5, 130.2, 131.8, 135.9, 138.1, 138.2, 140.8, 151.6, 164.7; MS (ESI⁺) m/z 581 (100, MNa⁺, ⁷⁴Ge), 579 (70, MNa⁺, ⁷²Ge), 577 (49, MNa⁺, ⁷⁰Ge); Anal. Calcd for C₂₉H₂₈GeN₂O₅ (557.18): C, 62.51; H, 5.07; N, 5.03. Found: C, 62.14; H, 5.28; N, 4.86.

1-N-Benzyl-5-(Z)-[2-(triphenylgermyl)ethenyl]uracil (14)

Alkyne 13^33,34 (50 mg, 0.22 mmol) was dissolved in anhydrous THF (5 mL) and the resulting solution was stirred for 20 min under N₂ at 0 °C. Ph₃GeH (73 mg, 0.24 mmol) and Et₃B (1M/THF 265 uL, 0.265 mmol,) were added and the resulting solution was stirred at 0 °C for 7 h. The volatiles were evaporated and the residue was chromatographed (hexane/EtOAc, 1:1) to give Z-14 (61 mg, 52%) as a white powder: M.p. 202–204 °C (MeOH); UV (MeOH) max 296 nm (ε 11 000), min 255 (ε 3500); ¹H NMR δ 4.03 (s, 2H), 6.30 (d, J = 13.5 Hz, 1H), 6.69 (“d”, J = 7.0 Hz, 2H), 6.84 (s, 1H), 7.09 (“t”, J = 7.5 Hz, 2H), 7.16 (“t”, J = 7.1 Hz, 1H), 7.23–7.35 (m, 10H), 7.37–7.46 (m, 6H), 8.51 (br. s, 1H); ¹³C NMR δ 51.2, 113.7, 128.3, 128.3, 128.4, 128.8, 129.1, 129.5, 134.8, 135.0, 136.7, 138.7, 141.0, 150.1, 162.4; HRMS calcd for C₃₁H₂₆ ⁷⁴GeN₂NaO₂ [M+Na]⁺ 555.1105, found 555.1113.

1-N-Benzyl-5-(E)-[2-(triphenylgermyl)ethenyl]uracil (14)

Ph₃GeH (118.6 mg, 0.38 mmol) and H₂O (25 µL, 25 mg, 1.4 mmol) were added to a stirred solution of 13 (80 mg, 0.35 mmol) in toluene (5 mL) at rt. The resulting mixture was heated at 100 °C for 12 h. The volatiles were evaporated and the residue was chromatographed (CH₂Cl₂/EtOAc, 10:1) to give E-14 (126 mg, 86%) as white solid. Recrystallization (MeOH) gave white crystals: M.p. 185–186 °C; UV (MeOH) max 300 nm (ε 13 900), 251 nm (ε 15 500), min 272 (ε 7100); ¹H NMR δ 4.92 (s, 2H), 6.56 (d, J = 18.7 Hz, 1H), 7.21 (s, 1H), 7.28–7.30 (m, 2H), 7.33 (d, J = 18.7 Hz, 1H), 7.35–7.51 (m, 18H), 8.78 (br. s, 1H); ¹³C NMR δ 51.5, 113.8, 127.2, 127.9, 128.3, 128.6, 129.1, 129.2, 135.1, 135.2, 136.1, 136.9, 141.7, 150.1, 161.8; HRMS calcd for C₃₁H₂₆ ⁷⁴GeN₂NaO₂ [M+Na]⁺ 555.1105, found 555.1111. Anal. Calcd for C₃₁H₂₆GeN₂O₂ (531.19): C, 70.09; H, 4.93; N, 5.27. Found: C, 69.82; H, 4.64; N, 5.35.

1-N-Benzyl-5-[2-(triphenylgermyl)acetyl]uracil (15)

Alkyne 13 (50 mg, 0.22 mmol) was suspended in dry toluene (5 mL) and the suspension was degassed with N₂ for 45 min at ambient temperature. Ph₃GeH (73 mg, 0.24 mmol) and ACCN (10 mg, 0.041 mmol) were added and the suspension was heated at 90 °C for 2 h. The volatiles were evaporated and residue was chromatographed (hexane/EtOAc, 3:2) to give 15 (15 mg, 13%) followed by Z-14 (31 mg, 27%). Compound 15 was recrystallized (MeOH) to give colorless crystals: M.p. 205–207 °C; UV (MeOH) max 294 nm (ε 10 600), min 255 (ε 3800); ¹H NMR δ 3.81 (s, 2H), 4.77 (s, 2H), 7.24–7.54 (m, 20H), 7.79 (br. s, 1H), 7.85 (s, 1H); ¹³C NMR δ 33.1, 52.4, 113.3, 128.3, 128.5, 129.1, 129.3, 129.5, 134.5, 135.2, 135.3, 149.7, 149.9, 160.7, 194.3; HRMS calcd for C₃₁H₂₆ ⁷⁴GeN₂NaO₃ [M+Na]⁺ 571.1054, found 571.1068. Anal. Calcd for C₃₁H₂₆GeN₂O₃ (547.2): C, 68.04; H, 4.79; N, 5.12. Found: C, 68.26; H, 4.60; N, 4.81.

1-N-Benzyl-5-acetyluracil (20)

A solution of 15 (15 mg, 0.03 mmol) in CH₃OH (3 mL) was heated for 12 h at 65 °C. Volatiles were evaporated and the residue was chromatographed (CH₂Cl₂/EtOAc, 2:1) to give 20⁴¹ (6.5 mg, 95%) as a white powder: UV (MeOH) max 290 nm (ε 12 200), 226 nm (ε 8100), min 249 (ε 1400); ¹H NMR δ 2.61 (s, 3H), 5.01 (s, 2H), 7.33–7.41 (m, 5H), 8.28 (s, 1H), 8.59 (br. s, 1H); ¹³C NMR δ 30.6, 52.5, 112.8, 128.3, 129.0, 129.3, 134.3, 150.2, 150.6, 161.2, 193.9; GC-MS (t_R 26.87 min) m/z 244 (20, M⁺), 91 (100; HRMS calcd for C₁₃H₁₂N₂NaO₃ [M+Na]⁺ 267.0740, found 267.0739.

Analogous treatment of 15 (10 mg, 0.02 mmol) using MeOD or MeOH-d₄ instead of MeOH gave 1-N-benzyl-5-(2-deuterioacetyl)uracil (21; 4.2 mg, 94%): GC-MS (t_R 26.87 min) m/z 245 (20, M⁺), 91 (100). ¹H NMR spectrum of 21 corresponded to this of the above 20 with 1/3 reduction of the integrated intensity for the signal from methyl group at 2.61 ppm.