A new, efficient entry to nucleoside 2',3'-O,O-cyclophosphorothioates

Article abstract of DOI:10.1016/S0040-4039(00)00132-5

The reaction of 5'-protected ribonucleosides with diphenyl H-phosphonate in pyridine furnished rapid formation of the corresponding 2',3'-cyclic H- phosphonates 3, which upon sulfurisation and the subsequent removal of the 5'-protecting group, afforded nucleoside 2',3'-O,O-cyclophosphorothioates 5 in high yields. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)00132-5

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 2227–2229
A new, efficient entry to nucleoside
20,30-O,O-cyclophosphorothioates
Jadwiga Jankowska,a Małgorzata Wenska,a Mariusz Popenda,a Jacek Stawin´ski b and
Adam Kraszewski a,∗
aInstitute of Bioorganic Chemistry, Polish Academy of Sciences, Noskowskiego 12/14, 61-704 Poznan´, Poland
bDepartment of Organic Chemistry, Stockholm University, Arrhenius Laboratory, S-106 91 Stockholm, Sweden
Received 2 November 1999; accepted 14 January 2000
Abstract
The reaction of 50-protected ribonucleosides with diphenyl H-phosphonate in pyridine furnished rapid formation
of the corresponding 20,30-cyclic H-phosphonates 3, which upon sulfurisation and the subsequent removal of the 50-
protecting group, afforded nucleoside 20,30-O,O-cyclophosphorothioates 5 in high yields. © 2000 Elsevier Science
Ltd. All rights reserved.
Keywords: nucleosides; nucleotides; cyclic phosphorothioates; synthesis.
Nucleoside 20,30-cyclic phosphates have, for a long time been in the focus of chemical research,1 not
least due to their synthetic utility2 and the enormous acceleration observed for reactions that involve
phosphorus in five-membered rings.3 Although the biological significance of 20,30-cyclic phosphates per
se is far from clear, the importance of ribonuclease-4 and ribozyme-catalysed5 reactions that involve
cyclic phosphates as intermediates, make this class of phosphorus compounds an indispensable research
tool in mechanistic bioorganic phosphorus chemistry and in molecular biology.
While a variety of nucleoside 30,50-cyclic phosphates have been investigated,6–8 analogues of 20,30-
cyclic phosphates are difficult to prepare and only nucleoside 20,30-O,O-phosphorothioates have received
attention in conjunction with studies of stereochemical aspects of ribonuclease-catalysed reactions.9,10
These compounds are usually prepared via the reactions of 50-O,N-protected ribonucleosides with
thiophosphoryl chloride,10,11 cyclisation of nucleoside 20(30)-phosphorothioate derivatives,12 or by using
salicylic chlorophosphite as a phosphitylating agent.13 Yields of these reactions (in most instances
determined only by UV-spectroscopy) were invariably low (6–10%)10–12 and with the most efficient,
recent method,13 did not exceed 40%.
During our studies on phosphonylation of 20,30-unprotected ribonucleosides with H-phosphonate
monoesters in the presence of pivaloyl chloride we observed rapid formation of equimolar amounts
∗
Corresponding author. Tel: 48-61-852-85-03; fax 48-61-852-05-32; e-mail: akad@ibch.poznan.pl (A. Kraszewski)
0040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)00132-5
tetl 16414

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of symmetrical H-phosphonate diesters and nucleoside 20,30-cyclic H-phosphonates.14 We attempted
to exploit this finding for the development of a new method for the preparation of nucleoside 20,30-
O,O-phosphorothioates using diphenyl H-phosphonate,15 which is a commercially available, inexpensive
phosphonylating reagent.
To this end, nucleoside 1a in pyridine was treated with diphenyl H-phosphonate 2 (1.5 molar equiv.)
(Scheme 1). The 31P NMR spectroscopy revealed rapid formation of cyclic H-phosphonate 3a [ca.
1
3
1:1 mixture of diastereomers; δP=23.22 ppm (ddd, JPH=734.1 Hz, JPH 12.5 and 5.1 Hz) and 27.22
1
3
ppm (ddd, JPH=731.3 Hz, JPH=14.4 and 6.0 Hz)], which upon addition of elemental sulfur (3 molar
equiv.), readily afforded the expected cyclic phosphorothioate 4a [δP=76.74 ppm and 78.76 ppm (dd,
3JPH=12.9 and 8.4 Hz)]. Using this approach the syntheses of 50-O-protected nucleoside 20,30-O,O-
cyclic phosphorothioates of type 4 were performed on a preparative scale (see below) and after removal
of the 50-O-dimethoxytrityl group (80% aqueous acetic acid, 10 min) the respective nucleoside cyclic
phosphorothioates of type 5 were obtained in excellent yields (70–90%).
Scheme 1.
Typical procedure for the preparation of nucleoside 20,30-O,O-cyclophosphorothioates 5: To a stirred
solution of 50-O-dimethoxytrityl nucleoside of type 1 (1 mmol; made anhydrous by repeated evaporation
of added pyridine) in pyridine (10 mL) was added diphenyl H-phosphonate 2 (1.5 molar equiv). After 20
min (TLC analysis), the produced cyclic H-phosphonate of type 3 was oxidised with elemental sulfur
(3 molar equiv.) for 5 min. The solvent was removed by evaporation and the residue was purified
by silica gel column chromatography using a stepwise gradient of methanol (0–10%) in methylene
chloride:triethylamine (95:5, v/v). The 50-protected phosphorothioates 4 obtained (yields 85–95%) were
treated with 80% acetic acid (10 mL) for 10 min, the mixtures were concentrated to dryness, co-
evaporated with added isopropanol, dissolved in 0.1 M triethylammonium bicarbonate buffer (pH 7,
10 mL) and extracted with methylene chloride (3×20 mL). The aqueous phase was freeze-dried to afford
5 (triethylammonium salts) as white amorphous solids. Yields: 5a, 93%; 5b, 88%; 5c, 78%; 5d, 82%.†
In conclusion, the above protocol for the preparation of nucleoside 20,30-O,O-cyclophosphorothioates
of type 5 represents a new, efficient and general entry to this class of compounds. It makes use of readily
available starting materials, involves mild and efficient chemical transformations and does not require
protection of the exocyclic amino groups in the substrates of type 1 nor use of phosphorus-protecting
groups.
†
Compounds 5a–d, obtained as a mixture of diasteroisomers (ratio 1:1 for 5a–c and 2:1 for 5d) gave correct elemental analysis
data and their chemical identity was confirmed by 1H and 31P NMR spectroscopy. Some diagnostic spectral data [compound,
δP (D2O), (3JPH)]: 5a, 75.24 ppm (dd, 11.0 and 6.9 Hz ), 76.60 ppm (dd, 12.4 and 7.3 Hz); 5b, 75.20 ppm (dd, 11.1 and 6.5 Hz),
76.55 ppm (dd, 12.9 and 7.2 Hz); 5c, 74.74 ppm (dd, 10.8, 6.7 Hz), 76.35 ppm (dd, 11.3, 8.8 Hz); 5d, 74.71 ppm (dd, 10.2, 7.4
Hz), 76.22 ppm (t, 10.9 Hz).

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Acknowledgements
Financial support from the State Committee for Scientific Research, Republic of Poland, the Swedish
Natural Science Research Council, and the Swedish Foundation for Strategic Research is gratefully
acknowledged.
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