AU4101396A

AU4101396A - Synthesis of 35s-labeled oligonucleotides with 3h-1,2-benzodithiol-3-1,1-dioxide

Info

Publication number: AU4101396A
Application number: AU41013/96A
Authority: AU
Inventors: Sudhir Agrawal; Radhakrishnan P Iyer; Weitian Tan
Original assignee: Hybridon Inc
Current assignee: Aceragen Inc
Priority date: 1994-11-07
Filing date: 1995-11-06
Publication date: 1996-05-31
Also published as: CN1165508A; WO1996014277A1; JPH10509447A; EP0790965A1

Description

SYNTHESIS OF 35S-LABELED OLIGONUCLEOTIDES WITH 3H-1 ,2-BENZ0DITHI0L-3~1 , 1-

BACKGROUND OF THE INVENTION Field of the Invention

The invention relates to synthesis of ³⁵S-labeled 3H-1,2 benzodithiol-3-one-l,l dioxide (1) and its use in the preparation of site-specifically ³⁵S-labeled oligonucleotides.

Description of the Prior Art

Since Zamecnik and Stephenson, Proc. Natl. Acad. Sci. USA 75, 280-284 (1978) first demonstrated virus replication inhibition by synthetic oligonucleotides, great interest has been generated in oligonucleotides as therapeutic agents. In recent years, the development of oligonucleotides as therapeutic agents and as agents of gene expression modulation has gained great momentum. The greatest development has been in the use of so-called antisense oligonucleotides, which form Watson-Crick duplexes with target rnRNAs. Agrawal, Trends in Biotechnology 10, 152-158 (1992), extensively reviews the development of antisense oligonucleotides as antiviral agents. See also Uhlmann and Peymann, Chem. Rev. 90, 543 (1990).

Various methods have been developed for the synthesis of oligonucleotides for such purposes. See generally, Methods in Molecular Biology, Vol. 20: Protocols for Oligonucleotides and Analogs (S. Agrawal, Ed., Humana Press, 1993); Oligonucleotides and Analogues: A Practical Approach (F. Eckstein, Ed., 1991); Uhlmann and Peyman,

supra. Early synthetic approaches included phosphodiester and phosphotriester chemistries. Khorana et al., J. Molec. Biol. 72, 209 (1972) discloses phosphodiester chemistry for oligonucleotide synthesis. Reese, Tetrahedron 34, 3143-3179 (1978), discloses phosphotriester chemistry for synthesis of oligonucleotides and polynucleotides.

These early approaches have largely given way to the more efficient phosphoramidite and

H-phosphonate approaches to synthesis. Beaucage and Caruthers, Tetrahedron Lett. 22, 1859- 1862 (1981 (reviewed in Beaucage and Iyer, Tetrahedron 48, 2223 ( 1992)), discloses the use of deoxynucleoside phosphoramidites in polynucleotide synthesis. Agrawal and

Zamecnik, U.S. Patent No. 5,149,798 (1992), discloses optimized synthesis of oligonucleotides by the H-phosphonate approach.

Both of these modern approaches have been used to synthesize oligonucleotides having a variety of modified internucleotide linkages. Agrawal and Goodchild,

Tetrahedron Lett. 28, 3539-3542 (1987), report synthesis of oligonucleotide methylphosphonates using phosphoramidite chemistry. Connolly et al., Biochemistry 23, 3443 (1984), discloses synthesis of oligonucleotide phosphorothioates using phosphoramidite chemistry. Jager et al., Biochemistry 27, 7237 (1988), discloses synthesis of oligonucleotide phosphoramidates using phosphoramidite chemistry. Agrawal et al.,

Proc. Natl. Acad. Sci. USA 85, 7079-7083 (1988), discloses synthesis of oligonucleotide phosphoramidates and phosphorothioates using H-phosphonate chemistry.

The use of 3H- 1 ,2 benzodithiol-3-one- 1 , 1 dioxide (1) as a sulfurizing reagent, (Iyer et al., J. Am. Chem. Soc. 112, 1253-1254 (1990)) in conjunction with phosphoramidite chemistry (Beaucage and Caruthers, Tetrahedron Lett. 22, 1859- 1862 (1981) and Beaucage and Iyer, Tetrahedron 48, 2223-231 1 (1992)) is now well established for the routine synthesis and large-scale manufacture of a variety of oligonucleoside phosphorothioates. Use of this reagent for the synthesis of methylphosphonothioates and other analogs has been reported. Padmapriya et al., Antisense Res. ά Dev. 4, 185-199 ( 1994) and Andrad et al., Bioorg. & Med. Chem. Lett. pp. 2017-2022 (1994). For biological studies, ³⁵S- labeled oligonucleoside phosphorothioates are prepared using the alternate chemistry viz.,

H-phosphonate chemistry. Garegg et al., Chem. Scr. 25, 280-282 (1985). It is difficult to achieve site-specific labeling of phosphorothioates using H-phosphonate chemistry, however, and it is inconvenient to carry out preparation of ³⁵S-labeled oligonucleoside phosphorothioate constructs, such as those with (a) mixed ribonucleotide- deoxyribonucleotide population ("hybrid oligos"), (b) heterogeneous backbones, e.g., deoxyribonucleotide-methyl phosphonate ("chimeric oligos") and (c) mixed phosphodiester-phosphorothioate (PO-PS) backbones. In order to ensure stereochemically

"uniform" product, it is desirable to follow the same chemistry both for synthesis and biological evaluation. The disadvantages of using elemental sulfur have also been recognized. Iyer et al., J. Am. Chem. Soc. 112, 1253-1254 (1990) and Iyer et al., J. Org. Chem. 55, 4693-4698 (1990). In vivo pharmacokinetic studies of pharmalogical compounds, e.g., antisense oligonucleotide phosphorothioates (Agrawal et al., Proc. Natl. Acad. Sci. U.S.A. 88, 7595- 7599 (1991)) requires labelling the compounds to enable detection. ³⁵S-labelling is an established and wide-spread technique. In view of the aforementioned difficulties in synthesizing ³⁵S-labeled oligonucleoside phosphorothioate constructs, improved methods are desirable.

SUMMARY OF THE INVENTION The present invention provides new compounds and improved methods for synthesizing ³⁵S-labeled oligonucleoside phosphorothioates. This invention comprises several aspects. In the first aspect, the present invention provides a novel compound useful for synthesizing oligonucleotide phosphorodiioates labelled with ³³S. This compound, ³⁵S-3H-l,2-benzodithiol-3-one-l,l dioxide (1), has the structure

wherein the asterisk denotes the ³⁵S label.

In a second aspect of the invention, a new method of synthesizing ³⁵S-3H-1,2- benzodithiol-3-one-l,l dioxide (1) is provided. An important consequence of this method is that it allows for the preparation of a variety of ³⁵S-labeled oligonucleotide phosphorothioates and thereby facilitates pharmacokinetic studies of these compounds. The method of synthesizing ^3$S-3H-l,2-benzodithiol-3-one-l,l dioxide (1) is depicted in Fig. 2 and comprises first contacting ³⁵S-thiobenzoic acid (4) with thiosalicylic acid (5) to yield the condensation product, ³⁵S-3 H 1 ,2-benzodithiol-3-one (2). ³⁵S-3 H 1 ,2- benzodithiol-3-one (2) is then oxidized to yield the desired product, ³⁵S-3H-1,2- benzodithiol-3-one-l,l dioxide (1).

In the third aspect of the invention, a new method of synthesizing ³⁵S-labelled oligonucleotides is provided. This method comprises contacting ³⁵S-3H-l,2-benzodithiol- 3-one-l,l dioxide (1) with an oligonucleotide susceptable to oxidative sulfurization. The method of ³⁵S labelling an oligonucleotide synthesized via the phosphoramidite method is depicted in Fig. 3. Other methods are contemplated, however, such as oxidative sulfurization of alkyl- and/or aryl-phosphites to yield the corresponding ³⁵S-labelled alkyl- and/or aryl-phosphonothioate.

Those skilled in the art will appreciate that ³⁵S-3H-l,2-benzodithiol-3-one-l,l dioxide (1) can be used for any purpose and in any way that its unlabelled analog, 3H-1,2- benzodithiol-3-one-l,l dioxide, can be used.

The foregoing merely summarizes certain aspects of the present invention and is not intended, nor should it be construed, to limit the invention in any way.

All patents and other references cited in this specification are hereby incorporated by reference in their entirety.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 depicts the synthesis of ³⁵S-thiobenzoic acid (4) from ³⁵S elemental sulfur and thiobenzoic acid.

Figure 2 depicts the synthesis of ³⁵S-3H-l,2-benzodithiol-3-one-l,l dioxide (1) from ³⁵S-thiobenzoic acid (4) and thiosalicylic acid (5) via the intermediate ³⁵S-3H-1,2- benzodithiol-3-one (2).

Figure 3 depicts the ³S labelling of an oligonucleotide synthesized by the phosphoramidate method.

Figure 4 is a RP-ΗPLC profile of ³⁵S-Λ_p-d[TpsT] and ³⁵S-S_p-d[TpsT] by UV detection at λ=260 nm (Panel A) and by flow scintillation Analysis (Panel B).

Figure 5 displays ³⁵S-labelled oligonucleotides synthesized according to the methods of the present invention.

Figure 6 displays and autoradiogram of oligonucleotides SEQ. ID NOs. 5-7 (purified) and SEQ. ID. NO. 8 (crude) subjected to PAGE.

Figure 7 displays an ion-exchange HPLC profile of ³⁵S-labelled SEQ. ID. NO. 5 as detected by UV absorbance at λ=260 nm (Panel A) and by flow scintillation analysis

(Panel B).

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Because of the ever-increasing interest in antisense oligonucleotides as therapeutic agents, there is a need to provide methods whereby the pharmacokinetic properties of these compounds can be tested. It is necessary to determine biodistribution, as well as to determine the half-lives and degradation products. One method of accomplishing these tasks is to label the oligonucleotides with ³⁵S, a common isotopic label used for tracing and detecting biological compounds.

The present invention provides a new compound useful for synthesizing ³⁵S- labelled antisense oligonucleotides, a new method of synthesizing the compound and new methods for ^3SS-labelling oligonucleotides.

The first aspect of the invention comprises a new compound, ³⁵S-3H-1,2- benzodithiol-3-one-l,l dioxide (1), having the following structure

wherein the asterisk indicates the position of the ³⁵S radionucleotide.

The non-radiolabelled analog, 3H-l,2-benzodithiol-3-one-l,l dioxide, is known (e.g., Beaucage, Regan and Iyer U.S. Patent No. 5,003,097 (Beaucage et al. '097) and Iyer et al., J. Am. Chem. Soc. and J. Org. Chem., supra), but the ^3SS-labelled compound has never before been synthesized.

Those skilled in the art will appreciate that ³⁵S-3H-l,2-benzodithiol-3-one-l,l dioxide (1) can be used for the same purposes and in the same manner as its non- radiolabelled counterpart. A second aspect of the invention comprises a new method of synthesizing ³⁵S -3H-l,2-benzodithiol-3-one- 1,1 dioxide. This method is a modification of the method of Beaucage et al. '097, for example. An important benefit of this method is that it enables production of ³⁵S-3H-l,2-benzodithiol-3-one-l,l dioxide (1). In contradistinction to prior art methods, the method of this invention uses a reactant in which the ³⁵S label is easily incorporated. The prior art teaches that the precursor to 3H-l,2-benzodithiol-3-one-l,l dioxide,

3H-l,2-benzodithiol-3-one, can be produced by mixing 2-thiolbenzoic acid and thiolacetic acid in sulfuric acid. E.g., Beaucage et al. '097. To have the ³⁵S in the appropriate position in the final product using this method, it is necessary to incorporate the ³⁵S in the thiolacetic acid. We attempted the preparation of ³⁵S-thiolacetic acid by a high temperature (125°C) exchange reaction between thiolacetic acid and elemental ³⁵S using a reported procedure. Kawamura et al., Chem. Lett. 1231-1234 (1975). The high volatility

(b.p. 81 °C) and vapor pressure of thiolacetic acid posed problems, however, when using ³⁵S with high specific activity (32 mCi/μmol), and the ³⁵S-labelled thiolacetic acid could not be isolated with high specific activity.

To circumvent the difficulty encountered by trying to ³⁵S label thiolacetic acid, we sought a thiol acid with a higher boiling point and lower vapor pressure. The commercially available thiobenzoic acid (4) seemed an ideal candidate. Before using ^3ϊS-4 in the preparation of 1, we validated the use of 4 in the synthesis of 1, by preparing ^3$S-

3H-l,2-benzodithiol-3-one (2), the precursor to 1 (Fig. 2). Although we do not wish to be bound by any theory, and, indeed, this synthetic method does not depend on any theory, presumably 2 is formed (McKibben and McClelland, J. Chem. Soc. 170-173

(1923)) via the intermediate 3:

A longer time (4 hours) was required for completion of the reaction, although the yield

(ca. 60%) was somewhat lower than when thiolacetic acid is use (ca. 80%). A crystallized sample of 2, thus synthesized, was identical in all respects (m.p., Η-NMR and ^l3C-NMR) to that obtained by the reported procedure using thiolacetic acid Iyer et al., J. Am. Chem. Soc. 112, 1253-1254 (1990) and Iyer et al. J. Org. Chem. 55, 4693-4698 (1990). Having demonstrated the feasibility of using 4 in the preparation of 2, ³⁵S-4 was conveniently prepared (Fig. 1) in high radiochemical yield (78%). ³⁵S-4 thus obtained was converted to ³⁵S-2 (Fig. 2), which, when subjected to carefully controlled oxidation, using hydrogen peroxide in trifluoroacetic acid. Iyer et al., J. Am. Chem. Soc. 112, 1253-1254 (1990) and

Iyer et al. J. Org. Chem. 55, 4693-4698 (1990). It is important to avoid use of excess

H₂O₂. The desired product ³⁵S-1 as a white crystalline solid in 30% chemical yield (based on the amount of 5 used) and having a specific activity of 90 μCi/μmol. The reaction mixture should be worked up immediately after its completion to avoid decomposition of ³⁵S-1.

Thus, the synthetic method according to this aspect of the invention comprises first contacting ³⁵S-thiobenzoic acid (4) with thiosalicylic acid (5) to yield the condensation product, ³⁵S-3 H l,2-benzodithiol-3-one (2). This reaction is acid catalyzed. In a preferred embodiment sulfuric acid is used, although any suitably strong acid may be used. ³⁵S-3H 1 ,2-benzodithiol-3-one (2) is then oxidized to yield the desired product, ⁵S-3H- 1 ,2- benzodithiol-3-one-l,l dioxide (1). Any suitably strong oxidizing agent may be used, e.g., hydrogen peroxide and trifluoroacetic acid, trifluoro peroxyacetic acid or other oxidizing agents such as oxone, sodium periodate NaOCl, RuCl₃ and reagents used in the oxidation of sulfide to sulfone. See, e.g., M. Ηudlicky, Oxidation in Organic Chemistry, ACS Monograph 186, 1990. In a preferred embodiment, oxidation is accomplished with hydrogen peroxide and trifluoroacetic acid. This scheme is depicted in Fig. 2.

The third aspect of the present invention comprises a new method for ³⁵S-labelling oligonucleotides. The method can be used to selectively place the ^3$S at any desired internucleoside linkage. Anywhere from one to all internucleoside linkages may be labelled with ³⁵S. The method comprises contacting ³⁵S-3H-l,2-benzodithiol-3-one-l,l dioxide (1) with an oligonucleotide susceptible to oxidative sulfurization. In a preferred embodiment, the oligonucleotide is synthesized by the phosphoramidite method, and ³⁵S- 3H- 1 ,2-benzodithiol-3-one- 1 , 1 dioxide (1) is contacted with the oligonucleotide having one or more β-cyanoethyl phosphotriester internucleoside linkages under standard conditions known in the art. In another preferred embodiment, an oligonucleotide having one or more alkyl- and/or aryl-phosphite internucleotide linkages is contacted with ³⁵S-3H-1,2- benzodithiol-3-one-l,l dioxide (1) to yield the corresponding ³⁵S-labelled alkyl- and/or aryl-phosphonothioate. This reaction, using the non-radiolabeled oxidative sulfurization agent, is taught by Padmapriya et al., supra.

Those of skill in the art will appreciate that ³⁵S-3H-l,2-benzodithiol-3-one-l,l dioxide (1) can be used to ³⁵S-label any compound that is capable of being sulfurized by the unlabeled analog 3H-l,2-benzodithiol-3-one-l,l dioxide. For instance, the present method is capable of ³⁵S labelling carbohydrates, proteins, and any macromolecule into which one can incorporate an ³⁵S label by oxidative thiolation. Thus, RNA can be labelled with ^3SS in a site-specific manner, as can phosphopeptides. Phosphorothioate and sulfur analogs of phospholipids, glycerophospholipids, and phosphocar bohydtrates (e.g., myoinositol phosphates or their conjugates with other macromolecules) can also be labeled with ³⁵S. ³⁵S can be inserted into thiophosphates and thiotriphosphates (e.g., ATP) and then incorporated into any molecule using chemical or enzymatic phosphorylation reactions. The following examples are provided for illustrative purposes only and are not intended, nor should they be construed, to limit the invention in any way.

EXAMPLES Example 1

Synthesis of "S-3H-1 ,2-benzodithiol-3-one-l , 1 dioxide (I) Synthesis of 35S-3H-1.2-benzodithiol-3-one (2)

A solution of ³⁵S (5 mCi in 100 μl of toluene) (Amersham, England) and 6.5 μl (55 μmol) of unlabelled thiobenzoic acid (Aldrich, Milwaukee, WI) were placed in a 1.5 ml Eppendorf tube and the contents heated at 97°C for 5 hours. The solution was evaporated to dryness under argon and 5 mg of thiosalicylic acid (Aldrich, Milwaukee,

WI) was added. The reaction mixture was cooled to 0°C and sulfuric acid (98%, 50 ml,

(J.T. Baker, Phillipsburg, NJ) was added. The mixture was kept at 50°C for 3 hours. The resulting brown reaction mixture was cooled to -78°C and 600 μl of water was added.

The solution was extracted with methylene chloride (4 X 3 ml) (VWR, Westchester, PA) and the organic layer washed with Na,CO₃ (5%, 2 X 2 ml). (EM Science, Gibbstown, NJ).

The organic layer was evaporated to dryness under a stream of argon to give a yellow solid. The material was then dissolved in 3 ml of warm hexane (J.T. Baker, Phillipsburg, NJ). and after centrifugation the supernatant solution was evaporated to dryness under argon to give a pale yellow solid (4 mg, 44% yield). This material could be used in the next step without additional purification and was stored at -20°C until ready to use.

Synthesis of ³⁵S-3H-1.2-benzodithiol-3-one-l.l dioxide (1) from ³⁵S-3H-1.2-benzodithiol- 3-one (2) To a 1.5 ml Eppendorf tube containing 4 mg of ³⁵S-3H-l,2-benzodithiol-3-one (2)

(prepared as described above), both of which had been cooled to 0°C, 25 ml of trifluroacetic acid (Aldrich, Milwaukee, WI) and 12 ml 30% hydrogen peroxide (Aldrich,

Milwaukee, WI) were added. The reaction mixture was warmed to 42°C. After about 2 hours (as monitored by TLC, silica gel, chloroform Iyer et al., J. Org. Chem.), the reaction mixture was cooled to 0°C and 300 ml of water added. A white precipitate of

^3SS-3H-l,2-benzodithiol-3-one-l,l dioxide (1) was immediately formed. The slurry was centrifuged and the precipitate washed with water (2 X 300 μl) and dried in vacuo to give

2 mg of ³ⁱS-3H-l,2-benzodithiol-3-one-l,l dioxide (1) (total activity of 350 μCi, specific activity 90 μCi/μmol).

A solution of ³⁵S-3H-l,2-benzodithiol-3-one-l,l dioxide (1) in anhydrous acetonitrile (2 mg, 90 μCi/μmol in 200 ml acetonitrile) was used for the oxidative sulfurization reaction described below. The solution could be stored at -20°C until ready for use.

Example 2

Synthesis of^iSS-labelled Oligonucleotides

In order to demonstrate the use of ³⁵S-3H-l,2-benzodithiol-3-one-l,l dioxide (1) in the preparation of oligonucleotides, we prepared ³⁵S-d[TpsT] (where the "ps" stands for phosphorothioate internucleoside linkage) on a 0.1 μmol scale in an automated DNA synthesizer using phosphoramidite chemistry. Beaucage and Caruthers, Tetrahedron Lett.

22, 1859-1862 (1981) and Beaucage and Iyer, Tetrahedron 48, 2223-231 1 (1992). To incorporate the ³⁵S label, the synthesis cycle was interrupted after the formation of the internucleotidic phosphite linkage. The CPG was removed from the column and treated with a solution of (15 ml, 90 μCi/μmol, 30 min.) followed by treatment with a solution of "cold" (i.e., non-³⁵S labelled) 3Η-l,2-benzodithiol-3-one-l,l dioxide (2% in acetonitrile,

100 ml, 10 min). A sample of the (TpsT) prepared under the exact conditions employing

"non-radioactive" 1 revealed that the conversion of TpsT was >99%.

The oxidative-sulfurization was quantitative as determined by "trityl assays" conducted during the synthesis. Agrawal, Protocols in Molecular Biology, supra. After cleavage from the CPG and phosphate deprotection with aqueous ammonium hydroxide

(30%, 2 hours, 35°C) the dimer was examined by poly aery lamide gel electrophoresis

(PAGE, 20%). The autoradiographic image was superimposable on its UV-shadowed band. When subjected to reverse-phase HPLC employing a UV detector interfaced with a radiochemical detector, its radioactivity profile was superimposable on the UV-absorbing peaks, corresponding to retention times of Rp-³⁵S-d[TpsT] (retention time = 22.7 min) and

Sp-³⁵S-d[TpsT] (retention time = 24.0 min) (Fig. 4, Panel A). ("Rp" and "Sp" represent the two configurations at the chiral phosphorous center.) Detection by flow scintillation analysis is display in Fig. 4, Panel B. HPLC analysis was done with a Waters column

(Milford, MA) equipped with a photodiode array UV detector interfaced with a

Radiomatic (Meriden Ct., MA) 500 TR v3.00 radiochemical detector using 8NV C,_g 4μ

Radial Pak (Waters, Milford, MA) cartridge column, gradient (100% A to 60% B over 60 minutes) of buffer A (0.1M CH₃CO₂NH₄) and buffer B (80:20, CH₃CN:0.1 M

CH₃CO₂NH_<), flow rate 1.5 ml min.

We then prepared on a 1 μmol scale a variety of oligonucleotides (SEQ ID NOs

1-4) bearing a pre-determined site of incorporation of the ³⁵S-label. As displayed in Fig.

5, all internucleotide linkages are phosphorothioates, except as indicated. The arrows indicate the ³ S-label site. For site specific incorporation of the ³⁵S label, the synthesis cycle was interrupted at the desired point and treated with ³⁵S-3H- 1 ,2-benzodithiol-3-one- 1,1 dioxide (1) (50 μl, 90 μCi/μmol, 30 min) as before. The oxidative-sulfurization was quantitative as determined by "trityl assays" conducted during the synthesis.

After deprotection with ammonium hydroxide (30%, 10 hour, 55°C), the crude oligonucleotides (SEQ ID NOs 1-4) were purified by preparative PAGE, desalted by

Sephadex G-25 chromatography, lyophilized dry and subjected to analytical PAGE (Fig. 6). The oligonucleotides (SEQ ID NOs 1-4) thus obtained had a specific activity of about 23 ~ 25 μCi/μmol.

We subjected ³⁵S-labelled SEQ. ID. NO. 5 to ion-exchange ΗPLC and detected the eluant by UV detection at λ=260 nm (Fig. 7, Panel A) and by flow scintillation analysis

(Fig. 7, Panel B). Ion-exchange ΗPLC was done using a GEN-PAK FAX column (4.6 X 100 mm) at 65°C using a gradient (80% A to 100 % B over 50 min.) of Buffer A (25 raM Tris ΗCL. pΗ 8.5, 10% CΗ₃CN) to Buffer B (25 mM Tris HC1, 2 M LiCl, pH 8.5, % CH₃CN) and a flow rate of 0.5 ml/min.

Claims

We claim:

1. A ³⁵S containing compound having the structure:

wherein the asterisk indicates the ³⁵S.

2. A method of synthesizing the compound of claim 1 comprising contacting thiosalicylic acid with ³⁵S-thiobenzoic acid in acid medium and oxidizing the reaction product.

3. The method of claim 2 wherein oxidation of the reaction product is accomplished with an oxidizing agent selected from the group consisting of trifluoroacetic acid and hydrogen peroxide, trifluoroperoxyacetic acid, oxone, sodium periodate, NaOCl, and RuCl₃.

4. A method of synthesizing the compound of claim 1 comprising oxidizing ³⁵S-3H- l,2-benzodithiol-3-one, which has the structure

wherein the asterisk indicates the position of the ³⁵S.

5. The method of claim 4 wherein the oxidizing agent is a mixture of trifuoroacetic acid and hydrogen peroxide.

6. The method of claim 4 wherein the ³⁵S-3H-l,2-benzodithiol-3-one is synthesized by contacting thiosalicylic acid with ³⁵S-thiobenzoic acid in acid medium.

7. A method of ³⁵S-labelling an oligonucleotide comprising contacting the compound of claim 1 with an oligonucleotide susceptible to oxidative sulfurization.

8. The method according to claim 7 wherein the oligonucleotide has from one to all β-cyanoethyl phosphoramidite internucleoside linkages.

9. The method according to claim 7 wherein the oligonucleotide has from one to all alkyl or aryl phosphite internucleoside linkages.