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Synthesis 2009(4): 602-608  
DOI: 10.1055/s-0028-1083331
PAPER
© Georg Thieme Verlag Stuttgart ˙ New York

Synthesis of 2,3,4-Trideoxy-4,4-difluoro-d-ribo-hexopyranose Adenosines

Jun Xua, Xingang Zhanga, Xiao-Long Qiua, Feng-Ling Qing*a,b
a Key Laboratory of Organofluorine Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 354 Fenglin Lu, Shanghai 200032, P. R. of China
b College of Chemistry, Chemical Engineering and Biotechnology, Donghua University, 2999 North Renmin Lu, Shanghai 201620, P. R. of China
Fax: +(86)2164166128; e-Mail: flq@mail.sioc.ac.cn;

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Publication History

Received 3 September 2008
Publication Date:
27 January 2009 (online)

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Abstract

A novel synthetic route to 2,3,4-trideoxy-4,4-difluoro-β-d-ribo-hexopyranose adenosine and 2,3,4-trideoxy-2,3-didehydro-4,4-difluoro-β-d-ribo-hexopyranose adenosine has been developed. The approach highlights the highly regio- and stereoselective palladium-catalyzed glycosylation of Boc-protected pyranose, which was prepared from the oxidation-cyclization of a difluorinated diol. The diol was provided through ozonization and Lindlar reduction of optically pure enynic alcohol.

Key words

nucleosides - fluorinated compounds - palladium-catalyzed glycosylation

The discovery of a large and varied class of natural bioactive hexopyranosyl nucleoside-containing products, such as blasticidin, [¹] gougerotin, [²] hikizimycin, [³] and mildiomycin, [4] has inspired many efforts on the synthesis of hexopyranosyl nucleosides as potential anticancer and antiviral agents. [5] Among these hexopyranosyl nucleosides, 2,3-dideoxypyranosyl nucleosides and 2,3-unsaturated hexopyranosyl nucleosides constitute a distinct class of bioactive compounds. They have been demonstrated to show anticancer and antiviral activities. For example, 2,3-dideoxy-β-d-ribo-hexopyranose adenosine (5) and 2,3-didehydro-β-d-ribo-hexopyranose adenosine (6) were synthesized as a selective inhibitor of protein synthesis and several transplantable animal tumors [6] [7] (Figure  [¹] ). Interestingly, the presence of a double bond at C2-C3 of pyranose ring in 6 can make it adopt a twisted half-chair conformation that is similar to the furanose rings of the naturally occurring nucleosides. [6a] [8] Based on this fact, many furanose ring mimic nucleosides bearing a 2,3-didehydropyranose ring have been used in various biological systems. [9] Thus, there has been a great demand for efficient synthetic methods to access these valuable compounds. On the other hand, special attention has also been paid to the gem-difluoromethylene group (CF2) because the introduction of this group into organic compounds can bring about remarkable changes in physical, chemical, and biological properties. [¹0] The well-known example is Gemcitabine, [¹¹] a gem-difluoromethylenated nucleoside, which has been used for treatment of lung, ovarian, renal, pancreatic, head, and neck cancers. However, to the best of our knowledge, the gem-difluorinated hexopyranosyl nucleosides have never been reported. As part of our continuous study to develop new antiviral and anticancer agents, we designed gem-difluorinated hexopyranosyl nucleosides 1 and 3, in which the hydroxy groups at C4 in 5 and 6 were replaced with a CF2 group based on the idea that the gem-difluoromethylene group (CF2) is the chemical isostere for hydroxy group. [¹²] Herein, we describe an efficient synthesis of target molecules 1 and 3 from a fluorinated building block.

Figure 1 Hexopyranosyl nucleosides 5, 6 and design of 2,3,4-trideoxy-4,4-difluoro-β-d-ribo-hexopyranose adenosines

Very recently, our group has reported the preparation of gem-difluorinated 1,2-disubstituted carbocyclic nucleosides via palladium-catalyzed glycosylation. We found that the nitrogen nucleophilic bases can specifically attack the more electrophilic carbon of intermediate 9 resulting only in γ-substituted product 8 (Scheme  [¹] ). [¹³] Interestingly, O’Doherty’s group reported that palladium-catalyzed glycosylation of hexopyranosyl substrates with various alcohol nucleophiles proceeded smoothly with excellent stereocontrol and the more electrophilic Pd-π-allyl intermediate I (Scheme  [¹] ) is essential to the reaction. [¹4] Employing this strategy, they successfully synthesized hexopyranosyl nucleosides. [7c] Thus, taking all these results together, we planned to install the base moiety of target molecule 1 by palladium-catalyzed N-glycosylation (Scheme  [²] ). We envisaged that the presence of an electron-withdrawing CF2 group in intermediate 11 would direct bases regioselectively and stereoselectively to attack the carbon far away from the CF2 group. Therefore, the palladium-catalyzed glycosylation of the difluorinated pyranose 12 with base would give nucleoside 10, a precursor of 1. Compound 12 could be prepared from diol 13 by oxidation of the primary hydroxyl to aldehyde followed by the simultaneous cyclization. Diol 13 could be easily obtained via ozonization and Lindlar reduction of our reported optically pure alcohol (R)-14 (Scheme  [²] ). [¹5]

Scheme 1

Scheme 2

Thus, the synthesis of the target molecules was started from (R)-14. Treatment of (R)-14 with O3 followed by NaBH4-mediated reduction gave diol 15 in 75% yield. Hydrogenation of diol 15 in the presence of Lindlar catalyst gave (Z)-diol 16 in 83% yield. Selective benzoylation of the primary hydroxy group of diol 16 and subsequent removal of TBS group with TBAF provided the gem-difluorinated­ alcohol 13 in 70% yield (Scheme  [³] ).

Scheme 3

The preparation of lactol 17 was initially carried out by the Giacomelli’s procedure for the selective oxidation of primary alcohol to aldehyde in the presence of trichloro­isocyanuric acid (TCCA) and catalytic TEMPO (Table  [¹] ). [¹6] However, when diol 13 was treated with 1.0 equivalent of TCCA and catalytic TEMPO at room temperature, only a trace amount of the desired lactol 17 was produced and lactone 18 was formed as the major product (42%) (Table  [¹] , entry 1). The oxidation of diol 13 with bis(acetoxyiodo)benzene (BAIB) afforded the desired lactol 17 in 21% yield (anti/syn = 4:1), but lactone 18 was still produced in 32% yield (entry 3). To prepare lactol 17 efficiently, the complete conversion of diol 13 to lactone 18 and then reduction of 18 to lactol 17 were investigated. The peroxidation of diol 13 with 3.0 equivalents of TCCA provided lactone 18 in 47% yield (entry 2). We were pleased to find that lactone 18 was isolated in 76% yield when BAIB was used instead of TCCA (entry 4).

Table 1 Oxidation of Diol 13

Entry Oxidant Equiv Yield (%) of 17 Yield (%) of 18
1 TCCA 1.0 trace 42
2 TCCA 3.0 - 47
3 BAIB 1.0 21a 32
4 BAIB 3.0 - 76
a Ratio of anti/syn = 4:1, determined by ¹9F NMR spectroscopy.

Reduction of lactone 18 with DIBAL-H afforded diol 19 in 70% yield with anti-isomer as the major product (anti/syn = 4:1, determined by ¹9F NMR spectroscopy). Selective benzoylation of the primary hydroxy group of diol 19 provided lactol 17 in 87% yield (Scheme  [4] ).

Scheme 4

With the key lactol 17 in hand, we then paid our attention to the palladium-catalyzed gylcosylation for the installation of base (Scheme  [5] ). Treatment of 17 with (Boc)2O gave compound 20. As we expected, treatment of 20 with 6-chloropurine in the presence of catalytic Pd(PPh3)4 at 60 ˚C in THF gave exclusively γ-substituted diastereoisomers 10 and 21. More gratifyingly, these two diastereo­isomers could be readily separated by flash chromatography and their absolute configurations were elucidated from NOESY experiments based on the known configuration at C5 as derived from (R)-14. It was noteworthy that the ratio of 10:21 was the same to that of syn/anti-20. This result showed that this palladium-catalyzed gylcosylation was completely stereocontrolled. That is to say, syn-20 provided only the β-anomer 10 and anti-20 gave only the α-anomer 21, which is consistent with a π-allylpalladium intermediate.

Scheme 5

Exposure of 10 and 21 to saturated methanolic ammonia at 80 ˚C gave gem-difluorinated 2,3-unsaturated hexopyranose adenosines 3 and 4 in 73 and 80% yield, respectively. The structure of compound 4 was further confirmed by X-ray diffraction (Figure  [²] ). [¹7] Finally, hydrogenation of 3 and 4 in the presence of catalytic Pd/C gave the target molecules 2,3,4-trideoxy-4,4-difluoro-d-ribo-hexopyranose adenosines 1 and 2 in 87 and 90% yield, respectively (Scheme  [6] ).

Figure 2 ORTEP drawing of the X-ray crystallographic structure of compound 4

Scheme 6

In conclusion, we have accomplished the synthesis of 2,3,4-trideoxy-4,4-difluoro-β-d-ribo-hexopyranose adenosines 1, 3 and their α-anomers 2, 4 using palladium-catalyzed glycosylation as a key step. The high regio- and stereoselectivities of such palladium-catalyzed glycosylation could be used as an efficient and practical strategy for the synthesis of other gem-difluorinated hexopyranosyl substrates. Antiviral and cytotoxicity evaluations of 1-4 are currently in progress and will be reported soon.

THF and benzene were distilled from sodium metal. CH2Cl2 was distilled from CaH2. Melting points are uncorrected. Petroleum ether (PE) used refers to the fraction boiling in the range 60-90 ˚C. ¹H and ¹³C NMR spectra were recorded on a Bruker AM300 spectrometer. ¹9F NMR spectra were recorded on a Bruker AM300 spectrometer (CFCl3 as external standard and low field is positive). Chemical shifts (δ) are reported in ppm, and coupling constants (J) are in Hz.

( R )-6-( tert -Butyldimethylsilyloxy)-3,3-difluorohex-4-yne-1,2-diol (15)

Ozone was bubbled through a solution of (R)-14 (4.45 g, 12.6 mmol) in MeOH-CH2Cl2 (70 mL:70 mL) for 45 min at -78 ˚C till a blue color persisted. Then, N2 was bubbled through the solution until the blue color disappeared and NaBH4 (2.35 g, 63.5 mmol) was added. After warming to r.t. and stirring for 1 h, the reaction was quenched with sat. aq NH4Cl (50 mL). The layers were separated and the aqueous phase was extracted with CH2Cl2 (3 × 30 mL). The combined organic layers were washed with brine (50 mL), dried (Na2SO4) and concentrated under vacuo to give the crude product. Purification by flash silica gel column chromatography (PE-EtOAc, 1:1) yielded 15 (2.66 g, 75%) as a clear oil; [α]D ²6 +8.9 (c 1.03, CHCl3).

IR (film): 3400, 2933, 2861, 1213, 1110, 1074, 838, 781 cm-¹.

¹H NMR (300 MHz, CDCl3): δ = 4.40 (t, J = 5.1 Hz, 2 H), 4.02-3.77 (m, 3 H), 2.86 (br, 2 H), 0.90 (s, 9 H), 0.13 (s, 6 H).

¹³C NMR (75.5 MHz, CDCl3): δ = 112.9 (t, J = 236.8 Hz), 88.4 (t, J = 7.0 Hz), 75.5 (t, J = 38.7 Hz), 74.1 (t, J = 27.9 Hz), 61.2 (t, J = 2.9 Hz), 51.3, 25.7, 18.3, -5.3.

¹9F NMR (282 MHz, CDCl3): δ = -94.98 to -95.04 (m, 2 F).

MS (MALDI): m/z = 303.1 [M + Na]+.

HRMS (MALDI): m/z [M + Na]+ calcd for C12H22F2O3Si + Na: 303.1199; found: 303.1202.

( R , Z )-6-( tert -Butyldimethylsilyloxy)-3,3-difluorohex-4-ene-1,2-diol (16)

A suspension of Lindlar catalyst (0.44 g) and diol 15 (2.88 g, 10.3 mmol) in hexane (150 mL) was stirred under H2 for 48 h at r.t. Filtration and removal of the solvent gave the crude product, which was purified by flash silica gel column chromatography (PE-EtOAc, 1:1) to give 16 (2.40 g, 83%) as a clear oil; [α]D ²6 +4.9 (c 0.48, CHCl3).

IR (film): 3400, 2933, 2861, 2257, 1213, 1110, 1074, 838, 781 cm-¹.

¹H NMR (300 MHz, CDCl3): δ = 6.00 (m, 1 H), 5.54 (q, J = 13.8 Hz, 1 H), 4.43 (s, 1 H), 3.95-3.80 (m, 3 H), 0.90 (s, 9 H), 0.08 (s, 6 H).

¹³C NMR (75.5 MHz, CDCl3): δ = 140.1 (t, J = 5.6 Hz), 120.7 (dd, J = 31.9, 25.7 Hz), 117.4 (t, J = 140.4 Hz), 73.7 (t, J = 29.0 Hz), 61.0 (t, J = 4.2 Hz), 59.8 (t, J = 4.8 Hz), 25.9, 18.3, -5.3.

¹9F NMR (282 MHz, CDCl3): δ = -101.76 (dt, J = 260.3, 11.8 Hz, 1 F), -105.23 (dt, J = 258.6, 13.3 Hz, 1 F).

MS (MALDI): m/z = 305.1 [M + Na]+.

HRMS (MALDI): m/z [M + Na]+ calcd for C12H24F2O3Si + Na: 305.1355; found: 305.1367.

( R , Z )-3,3-Difluoro-2,6-dihydroxyhex-4-enyl Benzoate (13)

To a solution of Z-diol 16 (2.40 g, 8.5 mmol) in anhyd CH2Cl2 (100 mL) was slowly added pyridine (14 mL) followed by BzCl (0.85 mL, 0.88 equiv) at -78 ˚C. The mixture was stirred for 1 h at the same temperature, and then another portion of BzCl (0.23 mL, 0.24 equiv) was added. After stirring the mixture for another 1 h, MeOH (10 mL) was added and the mixture was stirred for 30 min. The mixture was washed sequentially with aq 1 N HCl (50 mL), sat. aq NaHCO3 (30 mL), and brine (30 mL). The organic layer was dried (Na2SO4), filtered, and the solvent was removed in vacuo. The residue was purified by flash silica gel column chromatography (PE-EtOAc, 10:1 to 6:1) to give 2.5 g of the product benzoate. The product was dissolved in THF (30 mL) and TBAF (7 mL, 1 M in THF) was added. The mixture was stirred overnight at r.t. After removal of the solvent in vacuo, the residue was purified by flash silica gel column chromatography (PE-EtOAc, 1:1) to give 13 (1.55 g, 70% for two steps); [α]D ²6 +4.8 (c 0.65, CHCl3).

IR (film): 1755, 1726, 1453, 1230, 1070, 711 cm-¹.

¹H NMR (300 MHz, CDCl3): δ = 8.03 (d, J = 1.5 Hz, 2 H), 7.61 (t, J = 1.2 Hz, 1 H), 7.45 (t, J = 7.5 Hz, 2 H), 6.15-6.06 (m, 1 H), 5.72-5.56 (m, 1 H), 4.62-4.46 (m, 2 H), 4.43-4.38 (m, 2 H), 4.27-4.17 (m, 1 H).

¹³C NMR (75.5 MHz, CDCl3): δ = 167.0, 139.0 (t, J = 3.9 Hz), 133.5, 129.7, 129.3, 128.5, 122.0 (t, J = 19.6 Hz), 120.2 (t, J = 183.8 Hz), 72.0 (t, J = 22.5 Hz), 63.8, 58.7.

¹9F NMR (282 MHz, CDCl3): δ = -100.83 (dm, J = 234.1 Hz, 1 F), -105.00 (ddd, J = 228.1, 14.9 Hz, 3.7 Hz, 1 F).

MS (MALDI): m/z = 273.1 [M + H]+.

HRMS (MALDI): m/z [M + Na]+ calcd for C13H14F2O4 + Na: 295.0752; found: 295.0758.

( R )-(3,3-Difluoro-6-oxo-3,6-dihydro-2 H -pyran-2-yl)methyl Benzoate (18)

To a solution of benzoate 13 (106 mg, 0.39 mmol) in anhyd CH2Cl2 (3 mL) were added BAIB (376 mg, 1.20 mmol) and TEMPO (12 mg, 20 mmol%) at 0 ˚C. After stirring the mixture for 3 h at r.t., the reaction was quenched with aq Na2S2O3 (2 mL). The aqueous phase was extracted with CH2Cl2 (3 × 2 mL) and the combined organic layers were washed sequentially with sat. aq NaHCO3 (3 mL), NH4Cl (3 mL), and brine (3 mL). The combined organic layers were dried (Na2SO4), filtered, and the solvent was removed in vacuo. The residue was purified by flash silica gel column chromatography (PE-EtOAc, 4:1) to give 18 (79 mg, 76%) as a clear oil; [α]D ²6 -2.3 (c 1.25, CHCl3).

IR (film): 1755, 1726, 1453, 1230, 1070, 711 cm-¹.

¹H NMR (300 MHz, CDCl3): δ = 8.03 (d, J = 1.5 Hz, 2 H), 7.61 (t, J = 1.2 Hz, 1 H), 7.45 (t, J = 7.5 Hz, 2 H), 6.89-6.82 (m, 1 H), 6.37 (d, J = 10.2 Hz, 1 H), 5.04-4.92 (m, 1 H), 4.82 (dd, J = 12.6, 3.6 Hz, 1 H), 4.68 (q, J = 6.9 Hz, 1 H).

¹³C NMR (75.5 MHz, CDCl3): δ = 165.8, 159.4, 137.1 (dd, J = 26.3, 19.6 Hz), 133.5, 129.7, 129.0, 128.5, 126.7 (t, J = 6.7 Hz), 111.9 (t, J = 182.0 Hz), 60.4 (t, J = 3.9 Hz).

¹9F NMR (282 MHz, CDCl3): δ = -106.82 (dt, J = 291.0, 8.2 Hz, 1 F), -108.75 (dd, J = 271.3, 19.5 Hz, 1 F).

MS (MALDI): m/z = 286.0 [M + NH4]+.

HRMS (MALDI): m/z [M + Na]+ calcd for C13H10F2O4 + Na: 291.0439; found: 291.0443.

(6 R )-5,5-Difluoro-6-(hydroxymethyl)-5,6-dihydro-2 H -pyran-2-ol (19)

To a solution of lactone 18 (122 mg, 0.46 mmol) in anhyd CH2Cl2 (5 mL) was added DIBAL-H (2 mL, 1.0 M in toluene) at -78 ˚C. The mixture was stirred for 1 h at the same temperature, and then MeOH (7 mL) was added. The mixture was warmed to r.t. and filtered. The filtrate was dried (Na2SO4), filtered, and the solvent was removed in vacuo. The residue was purified by flash silica gel column chromatography (PE-EtOAc, 1:2) to give 19 (53 mg, 70%) as a white solid.

IR (film): 3204, 1387, 1158, 1103, 1073, 1053, 1025, 937, 868, 789, 754 cm-¹.

¹H NMR (300 MHz, CD3OD): δ = 6.17 (dd, J = 9.9, 2.7 Hz, 1 H), 5.89 (t, J = 9.0 Hz, 1 H), 5.34-5.28 (m, 1 H), 4.21(dq, J = 7.2, 3.9 Hz, 1 H), 3.83 (dd, J = 12.0, 3.3 Hz, 1 H), 3.70-3.59 (m, 1 H).

¹³C NMR (75.5 MHz, CD3OD): δ (major product) = 135.2 (t, J = 6.7 Hz), 123.0 (dd, J = 23.0, 19.6 Hz), 113.5 (dd, J = 182.6, 175.3 Hz), 87.5, 70.5 (dd, J = 23.0, 18.6 Hz), 58.4 (d, J = 5.1 Hz); δ (minor product) = 138.1 (t, J = 7.3 Hz), 123.6 (t, J = 21.4 Hz), 113.5 (dd, J = 182.6, 175.3 Hz), 91.5, 76.3 (t, J = 19.6 Hz), 58.5 (d, J = 3.9 Hz).

¹9F NMR (282 MHz, CD3OD): δ (major product) = -109.18 (dd, J = 278.6, 21.7 Hz, 1 F), -114.02 (ddd, J = 279.5, 9.0, 3.1 Hz, 1 F); δ (minor product) = -102.34 (dd, J = 274.9, 6.5 Hz, 1 F), -107.60 (ddd, J = 274.1, 11.3, 6.8 Hz, 1 F).

MS (MALDI): m/z = 225.1 [M + CH3COO]-.

[(2 R )-3,3-Difluoro-6-hydroxy-3,6-dihydro-2 H -pyran-2-yl]meth­yl Benzoate (17)

To a solution of diol 19 (70 mg, 0.42 mmol) in anhyd CH2Cl2 (6 mL) was slowly added pyridine (1 mL) followed by BzCl (55 µL) at -78 ˚C. After stirring the mixture for 1 h at the same temperature, MeOH (2 mL) was added and the mixture was stirred for another 30 min. The mixture was washed sequentially with aq 1 N HCl (5 mL), sat. aq NaHCO3 (3 mL), and brine (3 mL). The resultant organic layer was dried (Na2SO4), filtered, and the solvent was removed in vacuo. The residue was purified by flash silica gel column chromatography (PE-EtOAc, 4:1) to give 17 (99 mg, 87%) as a clear oil.

IR (film): 3500, 1725, 1453, 1281, 1103, 1027, 712 cm-¹.

¹H NMR (300 MHz, CDCl3): δ = 8.06 (d, J = 6.0 Hz, 2 H), 7.60 (t, J = 7.2 Hz, 1 H), 7.45 (t, J = 7.8 Hz, 2 H), 6.26-6.19 (m, 1 H), 6.05 (t, J = 9.9 Hz, 1 H), 5.56 (t, J = 3.3 Hz, 0.8 H), 5.49 (t, J = 5.7 Hz, 0.2 H), 4.77-4.55 (m, 3 H).

¹³C NMR (75.5 MHz, CDCl3): δ (major product) = 166.6, 134.2 (t, J = 7.0 Hz), 133.3, 129.4, 128.6, 128.5, 128.4, 124.0 (dd, J = 22.9, 19.2 Hz), 88.1, 68.3 (dd, J = 23.1, 18.3 Hz), 61.2 (d, J = 4.9 Hz); δ (minor product) = 166.6, 136.9 (t, J = 7.0 Hz), 133.4, 130.0, 129.9, 129.7, 129.5, 124.4 (t, J = 22.0 Hz), 91.8, 73.6 (t, J = 42.1 Hz), 61.4 (d, J = 3.6 Hz).

¹9F NMR (282 MHz, CDCl3): δ (major product) = -109.99 (dd, J = 258.9, 20.6 Hz, 1 F), -114.27 (dd, J = 269.5, 9.6 Hz, 1 F); δ (minor product) = -105.15 (dm, J = 271.8 Hz, 1 F), -106.38 (dm, J = 279.2 Hz, 1 F).

MS (MALDI): m/z = 288.2 [M + NH4]+.

HRMS (MALDI): m/z [M+] calcd for C13H12F2O4: 270.0704; found: 270.0709.

[(2 R )-6-( tert -Butoxycarbonyloxy)-3,3-difluoro-3,6-dihydro-2 H -pyran-2-yl]methyl Benzoate (20)

Lactol 17 (100 mg, 0.37 mmol) was dissolved in CH2Cl2 (8 mL) and the solution was cooled to 0 ˚C. A solution of (Boc)2O (92 mg, 0.42 mmol) and DMAP (20 mg) in CH2Cl2 (2 mL) was added to the mixture. The mixture was stirred at r.t. for 4 h. After concentration, the crude product was purified by flash silica gel column chromatography (PE-EtOAc, 10:1) to give 20 (123 mg, 90%) as a clear oil.

IR (film): 2983, 1756, 1729, 1454, 1396, 1372, 1329, 1278, 1257, 1158, 1110, 1068, 953, 849, 712 cm-¹.

¹H NMR (300 MHz, CDCl3): δ = 8.06 (d, J = 6.0 Hz, 2 H), 7.60 (t, J = 7.2 Hz, 1 H), 7.45 (t, J = 7.8 Hz, 2 H), 6.26-6.13 (m, 3 H), 4.80-4.48 (m, 3 H), 1.46 (s, 9 H).

¹³C NMR (75.5 MHz, CDCl3): δ (major product) = 151.6, 133.1, 131.2 (t, J = 7.0 Hz), 128.4, 128.3, 125.8 (dd, J = 23.2, 19.0 Hz), 89.0, 83.5, 70.0 (dd, J = 23.2, 18.3 Hz), 60.6 (d, J = 5.2 Hz); δ (minor product) = 166.0, 133.1, 132.8 (t, J = 7.0 Hz), 129.8, 129.7, 129.6, 125.1 (t, J = 21.4 Hz), 90.0, 83.6, 73.4 (dd, J = 23.2, 19.3 Hz), 61.7 (d, J = 5.2 Hz).

¹9F NMR (282 MHz, CDCl3): δ (major product) = -109.99 (ddd, J = 260.9, 21.6, 2.3 Hz, 1 F), -114.27 (dd, J = 273.0, 8.5 Hz, 1 F); δ (minor product) = -98.24 (dm, J = 265.9 Hz, 1 F), -109.45 (dm, J = 273.8 Hz, 1 F).

MS (MALDI): m/z = 388.2 [M + NH4]+.

HRMS (MALDI): m/z [M+] calcd For C18H20F2O6: 370.1228; found: 370.1245.

[(2 R ,6 R )-6-(6-Chloro-7 H -purin-7-yl)-3,3-difluoro-3,6-dihydro-2 H -pyran-2-yl]methyl Benzoate (10) and [(2 R ,6 S )-6-(6-Chloro-7 H -purin-7-yl)-3,3-difluoro-3,6-dihydro-2 H -pyran-2-yl]methyl Benzoate (21)

To a solution of compound 20 (130 mg, 0.35 mmol) and 6-chloropurine (105 mg, 0.68 mmol) in THF (10 mL) was added Pd(PPh3)4 (20 mg, 5 mmol%) and PPh3 (9 mg, 10 mmol%). The mixture was stirred at 60 ˚C for 5 h and then cooled to r.t. After concentration, the crude product was purified by flash silica gel column chromatography (PE-EtOAc, 1:1) to give 21 (55 mg, 39%) and 10 (14 mg, 10%) as foams.

10

[α]D ²6 +53.7 (c 0.65, CHCl3).

IR (film): 3116, 1724, 1590, 1564, 1397, 1337, 1272, 1158, 1093, 1052, 948, 831, 711, 636, 566 cm-¹.

¹H NMR (300 MHz, CDCl3): δ = 8.76 (s, 1 H), 8.24 (s, 1 H), 8.02 (d, J = 8.1 Hz, 2 H), 7.57 (t, J = 8.1 Hz, 1 H), 7.43 (t, J = 7.2 Hz, 2 H), 6.76 (t, J = 4.8 Hz, 1 H), 6.45-6.37 (m, 2 H), 4.82-4.79 (m, 1 H), 4.59-4.47 (m, 2 H).

¹³C NMR (75.5 MHz, CDCl3): δ = 171.3, 157.7, 156.6 (d, J = 39.9 Hz), 148.0, 138.6, 137.5 (t, J = 6.7 Hz), 136.7, 134.9, 134.5, 133.6, 132.8 (dd, J = 23.6, 20.2 Hz), 117.2 (dd, J = 184.3, 179.2 Hz), 81.9, 80.6 (dd, J = 23.6, 19.6 Hz), 65.8 (d, J = 4.5 Hz).

¹9F NMR (282 MHz, CDCl3): δ = -106.14 (ddd, J = 282.8, 18.0, 7.9 Hz, 1 F), -107.90 (dt, J = 283.1, 4.5 Hz, 1 F).

MS (MALDI): m/z = 407.0 [M + H]+.

HRMS (MALDI): m/z [M + H]+ calcd for C18H14ClF2N4O3: 407.0717; found: 407.0727.

21

[α]D ²6 -47.7 (c 0.17, MeOH).

IR (film): 2926, 1724, 1591, 1565, 1338, 1276, 1197, 1160, 1096, 1053, 947, 832, 711, 636, 565 cm-¹.

¹H NMR (300 MHz, CDCl3): δ = 8.72 (s, 1 H), 8.21 (s, 1 H), 8.06 (d, J = 6.0 Hz, 2 H), 7.60 (t, J = 7.2 Hz, 1 H), 7.45 (t, J = 7.8 Hz, 2 H), 6.71-6.51 (m, 3 H), 4.70-4.56 (m, 2 H), 4.40-4.30 (m, 1 H).

¹³C NMR (75.5 MHz, CDCl3): δ = 165.7, 152.6, 151.5 (d, J = 11.0 Hz), 143.2, 133.3, 131.9, 129.6 (t, J = 6.9 Hz), 129.3, 129.0, 128.3, 128.0 (d, J = 3.2 Hz), 127.8, 112.0 (dd, J = 184.8, 178.1 Hz), 76.0, 70.9 (dd, J = 23.5, 18.9 Hz), 60.1 (d, J = 4.6 Hz).

¹9F NMR (282 MHz, CDCl3): δ = -109.42 (dd, J = 268.7, 16.1 Hz, 1 F), -111.89 (ddd, J = 272.7, 7.3, 3.4 Hz, 1 F).

MS (MALDI): m/z = 407.0 [M + H]+.

HRMS (MALDI): m/z [M + H]+ calcd for C18H14ClF2N4O3: 407.0717; found: 407.0717.

[(2 R ,6 S )-6-(6-Amino-7 H -purin-7-yl)-3,3-difluoro-3,6-dihydro-2 H -pyran-2-yl]methanol (4)

Methanolic ammonia (20 mL) was added to compound 21 (49 mg, 0.12 mmol) and the mixture was stirred overnight at 80 ˚C. After evaporation of the solvent, the crude product was purified by flash silica gel column chromatography (MeOH-EtOAc, 1:10) to give 4 (27 mg, 80%) as a white solid; mp 172-174 ˚C; [α]D ²4 -88.3 (c 0.25, MeOH).

IR (film): 1650, 1601, 1475, 1172, 1151, 1094, 1036, 836 cm-¹.

¹H NMR (300 MHz, CD3OD): δ = 8.23 (s, 1 H), 8.20 (s, 1 H), 6.65-6.38 (m, 3 H), 4.19-4.07 (m, 1 H), 3.97-3.68 (m, 2 H).

¹³C NMR (75.5 MHz, DMSO-d 6): δ = 156.6, 153.5, 149.9, 140.4, 132.8 (t, J = 11.6 Hz), 126.0 (t, J = 29.4 Hz), 119.4, 114.4, 75.3, 73.7 (t, J = 28.4 Hz), 58.1.

¹9F NMR (282 MHz, CD3OD): δ = -110.77 (ddd, J = 256.6, 19.5, 4.8 Hz, 1 F), -113.95 (ddd, J = 268.7, 9.6, 4.5 Hz, 1 F).

MS (MALDI): m/z = 306.2 [M + Na]+.

HRMS (MALDI): m/z [M + Na]+ calcd for C11H11F2N5O2 + Na: 306.0773; found: 306.0774.

Crystal Data [¹7]

C11H11F2N5O2, M = 283.25, orthorhombic, space group P212121, a = 6.3537(8) Å, b = 12.6258(15) Å, c = 15.2441(17) Å, V = 1222.9 (3) ų, Z = 4, Dx = 1.538 mg˙m-³, Absorption coefficient 0.131 mm-¹, F(000) = 584, Crystal size 0.468 × 0.395 × 0.201 mm³, 6951 reflections collected [R(int)] = 0.1288, 1477 unique, wR2 = 0.1291, R = 0.0561 on I values of 1419 diffraction with I > 2σ(I); R = 0.0577, wR2 = 0.1309 for all data and 191 parameters. Unit cell determination and intensity data collection (qmax = 26.49˚) were performed on a Bruker SMART APEX2 at 293 (2) K. The structure was solved by direct method and refined by the full-matrix least-squares on F ².

[(2 R ,6 R )-6-(6-Amino-7 H -purin-7-yl)-3,3-difluoro-3,6-dihydro-2 H -pyran-2-yl]methanol (3)

Using the same conditions as described for compound 4, compound 3 (7 mg, 73%) was prepared as a white solid from compound 10 (14 mg, 0.03 mmol); mp 109-110 ˚C; [α]D ²4 +14.0 (c 0.05, MeOH).

IR (film): 1657, 1602, 1575, 1171, 1146, 1087, 1036, 841 cm-¹.

¹H NMR (300 MHz, CD3OD): δ = 8.24 (s, 1 H), 8.14 (s, 1 H), 6.66 (t, J = 6.0 Hz, 1 H), 6.60 (d, J = 22.3 Hz, 1 H), 6.39 (t, J = 8.7 Hz, 1 H), 4.28-4.19 (m, 1 H), 3.95 (dd, J = 12.0, 2.7 Hz, 1 H), 3.70 (dd, J = 12.3, 4.2 Hz, 1 H).

¹9F NMR (282 MHz, CD3OD): δ = -107.65 (ddd, J = 252.7, 17.2, 8.2 Hz, 1F), -109.1 (dm, J = 262.3 Hz, 1 F).

MS (MALDI): m/z = 284.0 [M + H]+.

HRMS (MALDI): m/z [M + Na]+ calcd for C11H11F2N5O2 + Na: 306.0773; found: 306.0778.

[(2 R ,6 S )-6-(6-Amino-7 H -purin-7-yl)-3,3-difluorotetrahydro-2 H -pyran-2-yl]methanol (2)

A suspension of Pd/C (12 mg) and compound 4 (20 mg, 0.07 mmol) in MeOH-EtOAc (5 mL:5 mL) was stirred under H2 for 26 h at r.t. Filtration and removal of the solvent gave the crude product, which was purified by flash silica gel column chromatography (MeOH-EtOAc = 1:10) to give 2 (18 mg, 90%) as a white solid; mp 234-236 ˚C; [α]D ²4 +30.3 (c 0.22, MeOH).

IR (film): 2928, 1655, 1474, 1172, 1147, 1036, 840 cm-¹.

¹H NMR (300 MHz, CD3OD): δ = 8.24 (s, 1 H), 8.12 (s, 1 H), 6.21 (s, 1 H), 3.94-3.75 (m, 3 H), 2.85-2.76 (m, 1 H), 2.35-2.28 (m, 1 H).

¹³C NMR (75.5 MHz, CD3OD): δ = 167.2, 155.4, 152.0, 148.6, 139.1, 118.4 (t, J = 178.7 Hz), 77.9, 75.0 (t, J = 22.5 Hz), 57.8, 27.9 (t, J = 18.6 Hz), 24.7.

¹9F NMR (282 MHz, CD3OD): δ = -102.42 (dm, J = 210.7 Hz, 1 F), -114.12 (ddd, J = 206.1, 27.3, 13.3 Hz, 1 F).

MS (MALDI): m/z = 286.0 [M + H]+.

HRMS (MALDI): m/z [M + H]+ calcd for C11H14F2N5O2: 286.1110; found: 286.1105.

[(2 R ,6 R )-6-(6-Amino-7 H -purin-7-yl)-3,3-difluorotetrahydro-2 H -pyran-2-yl]methanol (1)

Using the same conditions as described for compound 2, compound 1 (10 mg, 87%) was prepared as a white solid from compound 3 (11 mg, 0.04 mmol); mp 256-258 ˚C; [α]D ²6 -4.4 (c 0.05, MeOH).

IR (film): 2920, 1652, 1475, 1170, 1144, 1033, 840 cm-¹.

¹H NMR (300 MHz, CD3OD): δ = 8.27 (s, 1 H), 8.14 (s, 1 H), 5.94 (d, J = 10.8 Hz, 1 H), 4.10-3.94 (m, 1 H), 3.83 (d, J = 12.6 Hz, 1 H), 3.64 (dd, J = 12.6, 7.8, 1 H), 2.55 (dd, J = 11.7, 5.7 Hz, 1 H), 2.37-2.18 (m, 3 H).

¹9F NMR (282 MHz, CD3OD): δ = -110.60 (d, J = 246.5 Hz, 1 F), -120.10 (dm, J = 174.3 Hz, 1 F).

MS (MALDI): m/z = 330.0 [M + HCOO]-.

HRMS (MALDI): m/z [M + H]+ calcd for C11H14F2N5O2: 286.1110; found: 286.1098.

Acknowledgment

National Natural Science Foundation of China, and Shanghai Municipal Scientific Committee are greatly acknowledged for funding this work.

17

Further details of the crystal structure investigation can be obtained from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge, CB2 1EZ, UK (CCDC deposition No. 694847).

17

Further details of the crystal structure investigation can be obtained from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge, CB2 1EZ, UK (CCDC deposition No. 694847).

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Figure 1 Hexopyranosyl nucleosides 5, 6 and design of 2,3,4-trideoxy-4,4-difluoro-β-d-ribo-hexopyranose adenosines

Scheme 1

Scheme 2

Scheme 3

Scheme 4

Scheme 5

Figure 2 ORTEP drawing of the X-ray crystallographic structure of compound 4

Scheme 6