Enzymatic separation of epimeric 4-C-hydroxymethylated furanosugars: Synthesis of bicyclic nucleosides

Article abstract of DOI:10.3762/bjoc.13.205

Conversion of D-glucose to 4-C-hydroxymethyl-1,2-O-isopropylidene-α-D-ribofuranose, which is a key precursor for the synthesis of different types of bicyclic/spiro nucleosides, led to the formation of an inseparable 1:1 mixture of the desired product and 4-C-hydroxymethyl-1,2-O-isopropylidene-α-D-xylofuranose. A convenient environment friendly Novozyme-435 catalyzed selective acetylation methodology has been developed for the separation of an epimeric mixture of ribo- and xylotrihydroxyfuranosides in quantitative yields. The structure of both the monoacetylated epimers, i.e., 5-O-acetyl-4-C-hydroxymethyl-1,2-O-isopropylidene-α-D-ribo- and xylofuranose obtained by enzymatic acetylation, has been confirmed by an X-ray study on their corresponding 4-C-p-toluenesulfonyloxymethyl derivatives. Furthermore, the two separated epimers were used for the convergent synthesis of two different types of bicyclic nucleosides, which confirms their synthetic utility.

Full text of DOI:10.3762/bjoc.13.205

Enzymatic separation of epimeric 4-C-hydroxymethylated
furanosugars: Synthesis of bicyclic nucleosides
Neha Rana, Manish Kumar, Vinod Khatri, Jyotirmoy Maity and Ashok K. Prasad*
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2017, 13, 2078–2086.
Bioorganic Laboratory, Department of Chemistry, University of Delhi,
Delhi-110 007, India; Phone: 00-91-11-27662486
doi:10.3762/bjoc.13.205
Received: 15 June 2017
Email:
Accepted: 17 September 2017
Published: 05 October 2017
Ashok K. Prasad* - ashokenzyme@gmail.com
* Corresponding author
Associate Editor: S. Flitsch
Keywords:
© 2017 Rana et al.; licensee Beilstein-Institut.
License and terms: see end of document.
bicyclonucleosides; biocatalysis; lipase; Novozyme®-435; separation
of epimers
Abstract
Conversion of D-glucose to 4-C-hydroxymethyl-1,2-O-isopropylidene-α-D-ribofuranose, which is a key precursor for the synthesis
of different types of bicyclic/spiro nucleosides, led to the formation of an inseparable 1:1 mixture of the desired product and
4-C-hydroxymethyl-1,2-O-isopropylidene-α-D-xylofuranose. A convenient environment friendly Novozyme®-435 catalyzed selec-
tive acetylation methodology has been developed for the separation of an epimeric mixture of ribo- and xylotrihydroxyfuranosides
in quantitative yields. The structure of both the monoacetylated epimers, i.e., 5-O-acetyl-4-C-hydroxymethyl-1,2-O-isopropylidene-
α-D-ribo- and xylofuranose obtained by enzymatic acetylation, has been confirmed by an X-ray study on their corresponding
4-C-p-toluenesulfonyloxymethyl derivatives. Furthermore, the two separated epimers were used for the convergent synthesis of two
different types of bicyclic nucleosides, which confirms their synthetic utility.
Introduction
Sugar-modified bicyclic nucleosides have drawn the attention from diacetone-D-glucose led to the formation of an insepa-
of synthetic chemists because of their effect on the conforma- rable 1:1 mixture of the required compound and its C-3 epimer,
tional restriction of the furanose moiety of the nucleoside [1-9]. i.e., 4-C-hydroxymethyl-1,2-O-isopropylidene-α-D-xylofura-
The conformational restriction has led to the enhancement in nose [10].
target selectivity and in vivo stability of the nucleoside-based
drug candidates. One of the important precursors for the Lipases have been used extensively for the selective manipula-
synthesis of different types of bicyclonucleosides is tion of hydroxy groups present in different sugars and sugar
4-C-hydroxymethyl-1,2-O-isopropylidene-α-D-ribofuranose. moieties of synthetic or naturally occurring glycosides, nucleo-
The synthesis of the ribo-trihydroxy sugar derivative starting sides, etc. Gotor et al. [11] have reported a lipase-mediated
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acylation of an equimolecular mixture of D/L-thymidine with (MeCN), diisopropyl ether (DIPE), toluene and dioxane. We
acetonoxime levulinate as acylating agent and Pseudomonas carried out all ten sets of reactions for diastereoselective acety-
cepacia lipase as biocatalyst. Similar applications of lipases lation of epimeric mixtures of ribo- and xylotrihydroxyfurano-
have been reported for the separation of mixtures of arabinofu- sides 3a,b by using vinyl acetate at 30, 35, 40 and 45 °C and at
ranosyl and -pyranosyl nucleosides [12], O-aryl α,β-D-ribofura- 250 rpm in an incubator shaker to evaluate the appropriate
nosides, etc. [13-15]. We herein report for the first time the use lipase and the reaction conditions. Among the two screened
of Novozyme®-435 for the separation of an epimeric mixture of lipases, Novozyme®-435 (10% w/w of the substrate) in
xylo- and ribofuranosides. Separated epimers were further used 2-methyl-THF and MeCN at 35 °C was found to exhibit exclu-
as sugar precursors for the convergent synthesis of two differ- sive selectivity for the transfer of the acetyl group from vinyl
ent types of bicyclic nucleosides which are monomers of acetate to the C-5 position of 4-C-hydroxymethyl-1,2-O-iso-
oxetano- and locked nucleic acids of medicinal importance [16]. propylidene-α-D-ribofuranose (3a) and 4-C-hydroxymethyl-
1,2-O-isopropylidene-α-D-xylofuranose (3b) to afford
Results and Discussion
5-O-acetyl-4-C-hydroxymethyl-1,2-O-isopropylidene-α-D-ribo-
4-C-Hydroxymethyl-1,2-O-isopropylidene-α-D-ribofuranose furanose (4a) and 5-O-acetyl-4-C-hydroxymethyl-1,2-O-iso-
(3a) can be obtained from D-glucose via diacetonylation fol- propylidene-α-D-xylofuranose (4b) in quantitative yields, re-
lowed by selective deprotection of 5,6-isopropylidene protec- spectively (Scheme 2 and Figure 1). Out of the two suitable sol-
tion, sodium periodate oxidation of the vicinal diol and mixed vents identified from the screening test, 2-methyl-THF as an en-
aldol–Cannizaro reaction on the resulted aldehyde 2. However, vironmentally benign solvent was used for further enzymatic
this methodology always leads to the formation of an insepa- separation reactions.
rable 1:1 mixture of 4-C-hydroxymethyl-1,2-O-isopropylidene-
α-D-ribo/xylofuranose (3a,b, Scheme 1) [10].
In a classical enzymatic reaction, a mixture of ribo- and xylotri-
hydroxyfuranosides 3a,b was incubated with Novozyme®-435
Separation of the mixture of 4-C-hydroxymethyl-1,2-O-iso- in 2-methyl-THF using vinyl acetate as acetyl donor in an incu-
propylidene-α-D-ribo/xylofuranose (3a,b) has been achieved bator shaker at 250 rpm and at 35 °C. We followed the progress
using a selective lipase catalyzed reaction. We screened two dif- of the reaction by using analytical TLC. On complete conver-
ferent lipases, i.e., Thermomyces lanuginosus lipase immobi- sion of the starting materials into the corresponding products,
lized on silica (Lipozyme® TL IM) and Candida antarctica the enzyme was filtered off to quench the reaction and the
lipase-B immobilized on polyacrylate (Lewatit), commonly filtrate was concentrated under reduced pressure to afford a
known as Novozyme®-435 in five different organic solvents, colourless oil. The two products formed in the reaction had dif-
viz. 2-methyltetrahydofuran (2-methyl-THF), acetonitrile ferent polarity and were easily separated by column chromatog-
Scheme 1: Formation of a 1:1 epimeric mixture of 3a and 3b.
Scheme 2: Lipase-catalysed separation of a mixture of ribo- and xylotrihydroxyfuranosides.
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Figure 1: Screening of Novozyme®-435 and Lipozyme TL IM in different organic solvents at 35 °C for regioselective acetylation of trihydroxyribo/xylo-
furanose 3a and 3b (none of the reactions yielded any product when performed in the absence of the enzyme).
raphy over silica gel in quantitative yields. The structure eluci- 4-C-p-toluenesulphonyloxymethyl-α-D-xylofuranose (10,
dation of the two products revealed that Novozyme®-435 exhib- Figure 2). The tosyl derivatives 5 and 10 of dihydroxyfurano-
ited exclusive selectivity for the acetylation of C-5 hydroxy sides 4a and 4b were obtained by their tosylation with TsCl-
group of the substrate 3a,b to afford 5-O-acetylated ribo- and pyridine in 94 and 95% yields, respectively (Scheme 3 and
xylofuranose derivatives 4a and 4b, respectively, in 42% and Scheme 4).
46% yields, calculated on the basis of individual share of the
trihydroxy furanoses in the mixture (Scheme 2).
Two tosylated sugar derivatives 5 and 10 have been successful-
ly used for the convergent synthesis of 3′-O,4′-C-methyleneuri-
The exclusive selectivity of the Novozyme®-435 for the acety- dine (9) and 2′-O,4′-C-methylene-xylouridine (14) to illustrate
lation of the C-5 hydroxy group of ribo-/xylotrihydroxyfura- the usefulness of the trihydroxyribo-/xylofuranose sugar deriva-
noses 3a and 3b have been confirmed by X-ray diffraction tives separated by an enzymatic acetylation methodology
studies on the single crystal of their corresponding (Scheme 3 and Scheme 4). The acetylation of the lone hydroxy
4-C-p-toluenesulfonyloxymethyl derivatives, i.e., 5-O-acetyl- group in monotosylated sugar derivative 5 was carried out with
1,2-O-isopropylidene-4-C-p-toluenesulfonyloxymethyl-α-D- acetic anhydride and 4-dimethylaminopyridine (DMAP) in
ribofuranose (5) and 5-O-acetyl-1,2-O-isopropylidene- dichloromethane (DCM) to give 3,5-di-O-acetyl-1,2-O-iso-
Figure 2: ORTEP diagram of tosylated sugar derivatives 5 and 10.
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Scheme 3: Convergent synthesis of 3′-O,4′-C-methyleneuridine.
Scheme 4: Convergent synthesis of 2′-O,4′-C-methylene-xylouridine.
propylidene-4-C-p-toluenesulphonyloxymethyl-α-D-ribofura- dene-4-C-p-toluenesulfonyloxymethyl-α-D-xylofuranose (11)
nose (6) in 98% yield. The glycosyl donor 7a,b was prepared in 98% yield. Acetolysis of compound 11 yielded the glycosyl
from acetolysis of compound 6 with acetic acid/acetic an- donor 12a,b in 95% yield, which on Vorbrüggen coupling with
hydride/sulfuric acid (100:10:0.1) in 93% yield. The uracil under earlier used base-coupling conditions afforded the
Vorbrüggen coupling [17] of 7a,b with uracil in the presence of corresponding acetylated nucleoside 13 in 92% yield. Subse-
N,O-bis(trimethylsilyl)acetamide (BSA) and trimethylsilyltriflu- quent deacetylation of nucleosides 13 followed by concomitant
oromethane sulfonate (TMS-triflate) in acetonitrile yielded the cyclization with 2 M NaOH solution in water/dioxane (1:1)
triacetylated nucleoside 8 in 94% yield. Subsequently, deacetyl- afforded 2′-O,4′-C-methylene-xylouridine (14) in 95% yield
ation of acetoxy groups in nucleoside 8 with 2 M NaOH solu- (Scheme 4) in a similar manner as described in our earlier
tion in water/dioxane (1:1) also led to the concomitant cyliza- report [18]. The structure of compound 2′-O,4′-C-methylene-
tion between suitably placed C-3′-OH and C-1′′-tosyl groups to xylouridine (14) was confirmed by X-ray diffraction studies on
afford 3′-O,4′-C-methyleneuridine (9) in 82% yield (Scheme 3) its single crystal which also confirms the possibility of restric-
in similar manner as described in our earlier report [18].
tion of ring puckering in bicyclic nucleoside and the sugar ring
puckering is locked in N-type conformation (Figure 3).
A similar sequence of reactions were performed for the synthe-
sis of 2′-O,4′-C-methylene-xylouridine (14) from monotosy- The structures of all the synthesized compounds, i.e., 4a, 4b, 5,
lated sugar derivative 10 which, in turn, was obtained from 6, 7a,b, 8–11, 12a,b, 13 and 14 were unambiguously estab-
enzymatic product 4b in 95% yield. Thus, the acetylation of the lished on the basis of their spectral data (1H and 13C NMR
lone hydroxy group of 10 using acetic anhydride and DMAP in spectra, IR spectra and HRMS) analysis. The structure of
dichloromethane afforded 3,5-di-O-acetyl-1,2-O-isopropyli- known compounds 9 [19] and 14 [20] were further confirmed
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Figure 3: ORTEP diagram and preferential N-type sugar ring puckering of 2′-O,4′-C-methylene-xylouridine (14).
on the basis of comparison of its physical and spectral data with Aldrich Co. (USA). Theremomyces lanuginosus lipase
those reported in the literature. The single crystal X-ray diffrac- (Lipozyme TL IM) immobilized on silica was obtained as a gift
tion analysis has been performed on compounds 5, 10 and 14 from Novozymes Inc., Copenhagen, Denmark. For all the
and their detailed crystallographic data have been deposited in lipase-mediated reactions, AR grade organic solvents were
the Cambridge Crystallographic Data Centre with CCDC No. used, which were purchased from SD Fine-Chem Ltd., Mumbai,
1533725, 1533768 and 1532373, respectively.
India. The IR spectra were recorded by using thin film for oils
and by making KBr discs for solid samples. The 1H and
13C NMR spectra were recorded on a JEOL Alpha-400 spec-
Conclusion
A highly efficient and diastereoselective catalyzed Novozyme®- trometer at 400 and 100.6 MHz, respectively, using TMS as
435-catalyzed acetylation methodology has been developed for internal standard. The chemical shift values are on δ scale and
the selective acetylation of one of the two diastereotopic prima- the coupling constants (J) are in Hz. The HRMS analysis was
ry hydroxymethyl groups present in the inseparable epimeric done on a Q-TOF mass spectrometer using ESI positive mode.
mixture of trihydroxyribo/xylofuranose. The biocatalytic selec- The optical rotations were measured using light of 589 nm
tive acetylation of inseparable mixtures of ribo/xylofuranose de- wavelength. Analytical TLCs were performed on precoated
rivatives has resulted in the easy separation of the 5-O-acety- silica-gel 60 F254 plates; the spots were detected either under
lated ribo/xylofuranose derivatives in quantitative yields. The UV light or by charring with a solution of 4% H2SO4 in
sugar precursors separated by the biocatalytic methodology ethanol. Silica gel (100–200 mesh) was used for column chro-
have been used for the convergent synthesis of the bicyclic matography. All solvents were distilled before use. The single
nucleosides, 3′-O,4′-C-methyleneuridine and 2′-O,4′-C-methy- crystal X-ray diffraction data was collected with graphite-
lene-xylouridine in 66% and 77% overall yields, respectively, monochromated Mo Kα radiation (λ = 0.71073 Å) at USIC,
from respective biaocatalytic monoacetylated product. The University of Delhi, Delhi.
crystal structure of one of the synthesized bicyclic nucleosides,
2′-O,4′-C-methylene-xylouridine (14) revealed N-type puck- General procedure for biocatalytic acetylation of ribo- and
ering of the sugar ring of the compound. The developed biocat- xylotrihydroxyfuranosides 3a,b: synthesis of compounds
alytic methodology will have significant impact in the area of 4a,b. Similar as described in [18] to a solution of the mixture of
synthetic carbohydrate and nucleoside chemistry.
compounds 3a,b (2 g, 9.08 mmol) in 2-methyl-THF (40 mL),
vinyl acetate (1.26 mL, 13.6 mmol) was added followed by the
addition of Novozyme®-435 (0.2 g, 10% w/w of the compound
Experimental
The Candida antarctica lipase-B (CAL-B or Novozyme®-435) 3a,b). The reaction mixture was stirred at 35 °C in an incubator
immobilized on polyacrylate was purchased from Sigma- shaker for 1 h and the progress of the reaction was monitored
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periodically by TLC. On completion, the reaction was quenched 5-O-Acetyl-1,2-O-isopropylidene-4-C-p-toluenesulfonyl-
by filtering off the enzyme, the solvent was removed under oxymethyl-α-D-ribofuranose (5). It was obtained as white
reduced pressure and the residue thus obtained was separated by solid (2.98 g, 94% yield). Rf = 0.6 (5.0% methanol in chloro-
column chromatography using methanol in chloroform as form); mp: 94 °C; [α]D28 +11.98 (c 0.1, MeOH); IR (thin film)
gradient solvent system to afford the two monoacetylated sugar νmax: 3483, 2989, 1745, 1458, 1362, 1239, 1190, 1095, 985,
derivatives 4a and 4b.
839 cm−1; 1H NMR (CDCl3, 400 MHz) δ 7.78 (d, J = 8.4 Hz,
2H), 7.32 (d, J = 8.4 Hz, 2H), 5.77 (d, J = 3.8 Hz, 1H), 4.64 (dd,
5-O-Acetyl-4-C-hydroxymethyl-1,2-O-isopropylidene-α-D- J = 5.7 and 4.2 Hz, 1H), 4.38–4.02 (m, 5H), 2.71 (d, J = 6.9 Hz,
ribofuranose (4a). It was obtained as colourless oil (1.0 g, 1H), 2.41 (s, 3H), 1.99 (s, 3H), 1.42 (s, 3H), 1.31 (s, 3H);
42% yield). Rf = 0.4 (5.0% methanol in chloroform); 13C NMR (CDCl3, 100.6 MHz) δ 170.34, 144.92, 132.43,
[α]D18 +9.16 (c 0.1, MeOH); IR (thin film) νmax: 3471, 2946, 129.79, 128.10, 113.81, 104.54, 84.37, 79.31, 73.06, 68.19,
1739, 1383, 1244, 1166, 1046 875 cm−1; 1H NMR (CDCl3, 64.75, 26.20, 21.60, 20.69; HR-ESI-TOF-MS m/z: [M + K]+,
400 MHz) δ 5.86 (d, J = 4.6 Hz, 1H), 4.73–4.70 (m, 1H), calcd. for [C18H24O9SK]+ 455.0773, found: 455.0768.
4.26–4.14 (m, 3H), 3.84 (s, 2H), 3.19 (d, J = 8.4 Hz, 1H), 2.61
(s, 1H), 2.10 (s, 3H), 1.62 (s, 3H), 1.39 (s, 3H); 13C NMR 5-O-Acetyl-1,2-O-isopropylidene-4-C-p-toluenesulfonyl-
(CDCl3, 100.6 MHz) δ 170.73, 113.57, 104.68, 86.94, 79.41, oxymethyl-α-D-xylofuranose (10). It was obtained as white
72.86, 65.73, 62.25, 26.45, 26.22, 20.85; HR-ESI-TOF-MS m/z: solid (3.01 g, 95% yield). Rf = 0.6 (5.0% methanol in chloro-
[M + Na]+ calcd. for [C11H18O7Na]+ 285.0945, found: form); mp: 82 °C; [α]D24 −27.64 (c 0.1, MeOH); IR (thin film)
285.0947,
νmax: 3464, 2928, 1742, 1363, 1176, 1037, 838 cm−1; 1H NMR
(CDCl3, 400 MHz) δ 7.80 (d, J = 8.4 Hz, 2H), 7.36 (d, J = 7.6
5-O-Acetyl-4-C-hydroxymethyl-1,2-O-isopropylidene-α-D- Hz, 2H), 5.91 (d, J = 4.0 Hz, 1H), 4.60 (d, J = 3.6 Hz, 1H),
xylofuranose (4b). It was obtained as colourless oil (1.10 g, 4.30-4.12 (m, 5H), 2.45 (s, 3H), 2.10 (d, J = 5.2 Hz, 1H), 1.97
46% yield). Rf = 0.3 (5.0% methanol in chloroform); (s, 3H), 1.35 (s, 3H), 1.27 (s, 3H); 13C NMR (CDCl3, 100.6
[α]D22 −13.85 (c 0.1, MeOH); IR (thin film) νmax: 3446, 2943, MHz) δ 171.29, 145.11, 132.24, 129.88, 128.12, 112.59,
1739, 1377, 1248, 1164, 1048, 862 cm−1; 1H NMR (CDCl3, 105.32, 86.95, 86.55, 75.88, 67.79, 62.48, 26.10, 25.66, 21.60,
400 MHz) δ 5.93 (d, J = 4.4 Hz, 1H), 4.64–4.63 (m, 1H), 20.68; HR-ESI-TOF-MS: m/z: [M + H]+, calcd. for
4.31–4.19 (m, 3H), 3.77 (d, J = 11.2 Hz, 1H), 3.63 (d, J = 4.8 [C18H25O9S]+ 417.1214, found: 417.1218.
Hz, 1H), 2.82 (s, 1H), 2.15 (s, 1H), 2.11 (s, 3H), 1.53 (s, 3H),
1.30 (s, 3H); 13C NMR (CDCl3, 100.6 MHz) δ 170.76, 113.56, General procedure for acetylation of the lone hydroxy
104.64, 86.87, 79.44, 72.84, 65.70, 62.23, 26.43, 26.20, 20.84; group in compounds 5 and 10: synthesis of compounds 6
HR-ESI-TOF-MS m/z: [M + H]+ calcd. for [C11H19O7]+ and 11. To a solution of compound 5 (3 g, 7.2 mmol) in
263.1125, found 263.1130.
dichloromethane (30 mL) were added DMAP (176 mg, 1.44
mmol) and Ac2O (1.02 mL, 10.8 mmol) and the reaction mix-
General procedure for the tosylation of monoacetylated ture was stirred at 25–30 °C for 3 h. On completion, the mix-
sugar derivatives 4a and 4b: synthesis of compounds 5 and ture was diluted with cold water (25 mL) and extracted with
10. Similar as described in [18] to a stirred solution of com- ethyl acetate (3 × 100 mL). The combined organic layer was
pound 4a (2 g, 7.6 mmol) in pyridine (20 mL), p-toluene- washed with cold water (2 × 50 mL), dried over sodium sulfate
sulfonyl chloride (2.18 g, 11.4 mmol) was added at 0 °C. The and concentrated under reduced pressure. The residue thus ob-
progress of the reaction was monitored by TLC and on comple- tained was purified by column chromatography using ethyl
tion after 2 h, the reaction mixture was neutralized by 10% acetate in petroleum ether as gradient solvent system to afford
ice-cold hydrochloric acid solution (80 mL) and extracted with compound 6 as colourless oil in 98% yield. The similar proce-
chloroform (3 × 100 mL). The combined organic dure has been followed for the synthesis of compound 11,
extract was washed with saturated aqueous NaHCO3 which was obtained as colourless oil using 4b as starting
(2 × 100 mL), water (2 × 100 mL) and dried over anhydrous material in 98% yield. The spectral data and other details
Na2SO4. The solvent was removed under reduced pressure of compound 6 and 11 are given in Supporting Information
and the residue thus obtained was purified by File 1.
silica gel column chromatography using ethyl acetate in petro-
leum ether as gradient solvent system to afford General procedure for the acetolysis of compounds 6 and
the tosylated compound 5. The similar procedure has been 11: synthesis of tetraacetate compounds 7a,b and 12a,b.
followed for the synthesis of compound 10 using 4b as starting Similar as described in [18] acetic anhydride (6.2 mL,
material.
65.43 mmol) and concentrated sulfuric acid (0.03 mL,
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0.65 mmol) was added to a stirred solution of compound 6 (3 g, vent was removed under reduced pressure. The residue thus ob-
6.5 mmol) in acetic acid (37.4 mL, 654.33 mmol) at 0 oC and tained was purified by silica gel column chromatography using
mixture was stirred for 6 h at 30 °C. On completion, the reac- methanol in chloroform as gradient solvent system to afford 9.
tion was quenched by addition of water (200 mL) and the prod- The similar procedure has been followed for the synthesis of
uct was extracted with chloroform (3 × 100 mL). The combined compound 14 using triacetylated nucleoside 13 as starting mate-
organic layer was washed with sodium bicarbonate solution rial.
(2 × 100 mL), with cold water (2 × 100 mL) and then dried over
sodium sulfate. The solvent was removed under reduced pres- 3′-O,4′-C-Methyleneuridine (9). It was obtained as white solid
sure and the residue thus obtained was purified on (0.38 g, 82% yield). Rf = 0.5 (10% methanol in chloroform);
silica gel column chromatography using ethyl acetate in petro- mp: 215–218 °C; [α]D30 –17.50 (c 0.1, MeOH); IR (thin film)
leum ether as gradient solvent system to afford an anomeric νmax: 3371, 2832, 1685, 1461, 1262, 1025, 816 cm−1; 1H NMR
mixture 7a,b as colourless viscous oil in 93% yield. A similar (DMSO-d6, 400 MHz) δ 11.44 (s, 1H), 7.95 (d, J = 8.4 Hz, 1H),
procedure has been followed for the synthesis of compound 6.14 (d, J = 3.1 Hz, 1H), 5.98 (s, 1H), 5.73 (d, J = 8.4 Hz, 1H),
12a,b as colourless viscous oil using 11 as starting material in 5.37 (s, 1H), 4.90 (s, 1H), 4.62 (d, J = 7.6 Hz, 1H), 4.50 (d, J =
95% yield. Detailed spectral data and other details of 2.3 Hz, 1H), 4.13 (d, J = 6.9 Hz, 1H) 3.53 (s, 2H); 13C NMR
compounds 7a,b and 12a,b have been given in Supporting (DMSO-d6, 100.6 MHz) δ 163.72, 151.26, 141.64, 102.90,
Information File 1.
94.81, 91.28, 90.21, 79.22, 75.88, 61.26; HR-ESI-TOF-MS: m/
z: [M + H]+, calcd. for [C10H13N2O6]+ 257.0768, found:
General procedure for the Vorbrüggen coupling of 257.0760.
tetraacetylated sugar derivatives 7a,b and 12a,b: synthesis
of acetylated nucleosides 8 and 13. Similar as described in 2′-O,4′-C-Methylene-xylouridine (14). It was obtained as
[18] to a stirred solution of compound 7a,b (3.0 g, 5.97 mmol) white solid (0.43 g, 95% yield). Rf = 0.5 (10% methanol in
and nucleobase uracil (1.0 g, 8.9 mmol) in anhydrous aceto- chloroform); mp: 117–120 °C; [α]D20 +36.86 (c 0.1, MeOH);
nitrile (25 mL), N,O-bis(trimethylsilyl)acetamide (5.9 mL, IR (thin film) νmax: 3370, 2946, 1680, 1460, 1271, 1022,
23.88 mmol) was added dropwise. The reaction mixture was 755 cm−1; 1H NMR (DMSO-d6, 400 MHz) δ 11.27 (s, 1H),
stirred at reflux for 1 h, and then cooled to 0 °C. Trimethylsilyl- 7.67 (d, J = 8.2 Hz, 1H), 5.69 (d, J = 1.8 Hz, 1H), 5.51–5.48 (m,
trifluoromethane sulfonate (1.8 mL, 10.14 mmol) was added 2H), 5.01 (t, J = 5.3 Hz, 1H), 4.21 (s, 1H), 4.06 (s, 1H), 3.96 (d,
dropwise into the cooled reaction mixture under stirring and the J = 8.2 Hz, 1H), 3.83–3.76 (m, 2H), 3.73 (d, J = 8.2 Hz, 1H);
reaction mixture was refluxed for 4–6 h. The reaction was 13C NMR (DMSO-d6, 100.6 MHz) δ 163.56, 150.26, 141.43,
quenched with a cold saturated aqueous solution of sodium 98.85, 89.72, 88.33, 77.52, 72.86, 72.00 56.86; HR-ESI-TOF-
hydrogen carbonate (200 mL) and the reaction mixture was MS: m/z: [M + H]+, calcd. for [C10H13N2O6]+ 257.0768, found:
extracted with chloroform (3 × 100 mL). The combined organic 257.0769.
phase was washed with saturated aqueous solutions of NaHCO3
(2 × 100 mL), brine (2 × 100 mL) and cold water (2 × 100 mL). X-ray diffraction studies on tosylated sugar derivatives 5
The washed organic phase was then dried over anhydrous and 10 and bicyclic nucleoside 2′-O,4′-C-methylene-xylouri-
Na2SO4. The solvent was removed under reduced pressure dine (14). Single crystal suitable for X-ray diffraction studies
and the residue thus obtained was purified by silica gel column were grown by dissolving the tosylated sugar derivative 5 in tol-
chromatography using methanol in chloroform as gradient sol- uene and the other tosylated sugar derivative 10 and 2′-O,4′-C-
vent system to afford nucleoside 8 as white solid in 94% yield. methylene-xylouridine (14) in methanol/chloroform and
The similar procedure has been followed for the synthesis of allowing slow evaporation of the solutions at room temperature.
compound 13, which was afforded as white solid using 12a,b as The X-ray diffraction data was collected with graphite mono-
starting material in 92% yield. Detailed spectral data and other chromated Mo Kα radiation (λ = 0.71073 Å) at a temperature of
details of compounds 8 and 13 have been given in Supporting 293 K. The structures were solved by direct methods using
Information File 1.
SHELXS-97 and refined by full-matrix least-squqres method on
F2 (SHELXL-97) [20]. All calculations were carried out using
General procedure for the synthesis of bicyclic nucleosides 9 the WinGX package of the crystallographic programs [21]. For
and 14. Similar as described in [18] to a stirred solution of the molecular graphics, the programs DIAMOND-2 [22] and
triacetylated nucleosides 8 (1 g, 1.8 mmol) in dioxane/water Mercury [23,24] were used. Molecular structures have been
(1:1, 8 mL) was added 2 M NaOH solution (8 mL) and the reac- drawn using ORTEP as software as given in Figure 2 and
tion mixture was stirred at 30 °C for 2–10 h. On completion, the Figure 3. The selected bond lengths, bond angles, etc. are given
reaction mixture was neutralized with acetic acid and the sol- in Table 1.
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Beilstein J. Org. Chem. 2017, 13, 2078–2086.
Table 1: Single crystal X-ray diffraction data of tosylated sugar derivatives 5, 10 and 2′-O,4′-C-methylene-xylouridine (14).
compound 10
compound 5
compound 14
empirical formula
formula weight
crystal system
space group
C18H24O9S
416.43
C18H24O9S
416.43
C10H12N2O6
256.22
monoclinic
P21
monoclinic
P21
monoclinic
P21
a = 8.3342(4) Å
α = 90°
a = 11.354 Å
α = 90°
a = 6.1313(3) Å
α = 90°
b = 11.6972(5) Å
β = 95.660(4)°
b = 9.964 Å
β = 98.66°
b = 7.3638(3) Å
β = 93.213(4)°
unit cell dimensions
c = 10.4684(4) Å
γ = 90°
c = 18.512 Å
γ = 90°
c = 11.7021(6) Å
γ = 90°
volume
1015.56(8) Å3
2
2070.4 Å3
2
527.52(4) Å3
2
Z
density
1.362 mg/m3
0.206 mm−1
440
1.362 mg/m3
0.205 mm−1
896
1.613 mg/m3
0.135 mm−1
268
absorption coefficient
F(000)
−9<=h<=6,
−13<=k<=13,
−12<=l<=12
−13<=h<=13,
−11<=k<=11,
−19<=l<=22
−6<=h<=7,
−8<=k<=8,
−10<=l<=13
index ranges
R(int)
0.0130
0.0256
0.0140
GOF on F2
final R indices
I>2sigma(I)
R indices
all data
1.030
1.022
1.030
R1 = 0.0311
wR2 = 0.0720
R1 = 0.0348
wR2 = 0.0738
1533725
R1 = 0.0500
wR2 = 0.1070
R1 = 0.0647
wR2 = 0.1150
1533768
R1 = 0.0288
wR2 = 0.0691
R1 = 0.0298
wR2 = 0.0701
1532373
CCDC
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doi:10.1248/cpb.52.1399
Supporting Information
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doi:10.1021/ar980051p
Supporting Information File 1
Additional analytical data and NMR spectra.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-13-205-S1.pdf]
5. Evers, M. M.; Toonen, L. J. A.; van Roon-Mom, W. M. C.
Adv. Drug Delivery Rev. 2015, 87, 90–103.
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Acknowledgements
We are grateful to the University of Delhi for providing finan-
cial support under DU-DST Purse Grant and under scheme to
strengthen research and development. We are also thankful to
CIF-USIC University of Delhi, Delhi, for providing crystallo-
graphic data and NMR spectral recording facility. N. R. and M.
K. thank CSIR for the award of Senior and Junior/Senior
Research Fellowships, respectively.
9. Medvecky, M.; Istrate, A.; Leumann, C. J. J. Org. Chem. 2015, 80,
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