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synthesis are of obvious importance and widespread interest. Automated synthesis
on solid-supports using phosphoramidite chemistry has been the most successful
method for the large-scale synthesis of oligonucleotide-based drugs.[1] Detritylation,
coupling, oxidation and capping are the four key steps during automated synthesis
of oligonucleotides. During synthesis of phosphorothioate oligonucleotides oxida-
tion of phosphite P(III) into phosphorothioate triester P(V) is accomplished via a
sulfur-transfer step. Elemental sulfur was first used for this step, which is poorly
soluble in organic solvents commonly used for automated synthesis. Also the rate
of conversion of P(III) into P(V) product is not fast and facile with elemental sulfur.
For these reasons, many sulfur-transfer reagents have been developed during past
decade.[2] Among a dozen reagents described in the literature, 3H-1,2-benzo-
dithiol-3-one 1,1-dioxide (Beaucage reagent)[3] and phenylacetyl disulfide (PADS)[4]
are the two most notable and widely used reagents at the present time on large-scale.
Both are soluble in organic solvents and produce efficient and rapid sulfurization
kinetics. Although these two reagents are currently used on industrial scale, they
have certain limitations. For example, Beaucage reagent is expensive, long-term stabi-
lity in solution is not optimal and the cyclic sulfoxide by-product formed during
sulfurization is an efficient oxidizing agent which increases the levels of phospho-
diesters during packed-bed automated synthesis. Sulfurization with PADS is accom-
plished in combination with 3-picoline. Unfortunately the smell of this mixture is
obnoxious and upon standing it changes color to a dark solution. Thus, our inves-
tigation for an alternative sulfur-transfer reagent that is cheap and user friendly
led us to dimethylthiuram disulfide (DTD) as a reagent of choice.
CHEMISTRY
In 1991, Vu and Hirschbein first reported on the application of tetraethylthiuram
disulfide (1) as a sulfur-transfer reagent.[5] We found 1 attractive as a sulfur-transfer
reagent for three reasons. First, ease of synthesis and low cost. Second, proven utility
for small-scale oligonucleotide synthesis and third its atom efficiency.[6] Based on
these criterions, we embarked on an efficient synthesis of 1 and its analogs using a
common approach. Herein, we report one-pot synthesis of disulfides 1–4 in excellent
yield (> 90%; Sch. 1) starting from alkylamine and carbon disulfide which are both
cheap and commercially available on industrial scale. The products are isolated as
stable pale yellow crystalline solid that is highly soluble in organic solvents.a
aPreparation of dimethylthiuram disulfide (4): NaOH (80 g, 2 mol) was dissolved in water
(500 mL) and the solution was cooled to 0ꢀC. THF (200 mL), methylamine (40% in water,
170 mL, 2 mol) and carbon disulfide (120 mL, 2 mol) were added over a period of 30 min to
aq. NaOH solution while stirring at 0ꢀC. Crushed ice (1.5 Kg) was added to the reaction mix-
ture, followed by acetic acid (300 mL). Hydrogen peroxide (30%, 100 mL, 1 mol) was added
gradually while stirring over 15 min while maintaining the reaction temperature below 5ꢀC.
Heptane (800 mL) was added to the reaction mixture and stirred for additional 30 min. The
product was filtered off and washed with aq. acetic acid (2%, 5ꢁ200 mL) and heptanes
(2ꢁ200 mL). The product was air dried until constant weight (ꢂ1 day) to furnish off-white
solid (205 g, 97%). M.p. 98–100ꢀC (dec.)