Biohybrid-Se-S-coupling reactions of an amino acid derived seleninate

Article abstract of DOI:10.3390/molecules18021963

We describe the synthesis of the N-(2-seleninatoethyl) amide of N-Bocphenylalanine, serving here as a peptide model, and its reductive coupling reactions under mild conditions with unprotected thiouridine and glutathione. Selenosulfide products such as these comprise reversibly conjugated bio-components, and can potentially find uses as probes of biological function, such as enzyme inhibitors, delivery systems, or structural mimics.

Full text of DOI:10.3390/molecules18021963

Molecules 2013, 18, 1963-1972; doi:10.3390/molecules18021963
OPEN ACCESS
molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Article
Biohybrid -Se-S- Coupling Reactions of an Amino Acid
Derived Seleninate
Mohannad Abdo, Zhexun Sun and Spencer Knapp *
Department of Chemistry & Chemical Biology, Rutgers, the State University of New Jersey,
610 Taylor Road, Piscataway, NJ 08854, USA
* Author to whom correspondence should be addressed; E-Mail: spencer.knapp@rutgers.edu;
Tel.: +1-732-445-2627; Fax: +1-732-445-5312.
Received: 9 January 2013; in revised form: 28 January 2013 / Accepted: 30 January 2013 /
Published: 4 February 2013
Abstract: We describe the synthesis of the N-(2-seleninatoethyl) amide of N-Boc-
phenylalanine, serving here as a peptide model, and its reductive coupling reactions under mild
conditions with unprotected thiouridine and glutathione. Selenosulfide products such as
these comprise reversibly conjugated bio-components, and can potentially find uses as probes
of biological function, such as enzyme inhibitors, delivery systems, or structural mimics.
Keywords: click reaction; biomimetic; seleninic acid; thiol
1. Introduction
Coupling reactions that join complex partners play a key role in biosynthesis as well as chemical
synthesis. Chemists aspire to replicate in the laboratory the high efficiency and mild conditions
achieved in Nature, and have developed a number of such reactions that succeed in aqueous solution at
neutral pH and ambient temperature [1,2]. Unlike biosynthetic transformations, however, the chemical
coupling reactions can embrace “unnatural” features such as uncommon bonds and unusual structures.
The versatile azide-alkyne cycloaddition [3] exemplifies this feature: neither the azide nor the alkyne
partner nor the triazole product would be considered biologically normal. Additionally, useful coupling
reactions are often compatible with densely functionalized and generally unprotected partners
incorporating, for example, peptide, nucleoside, carbohydrate, or oligomeric substructures [4,5]. The
resulting biohybrid [6] products can feature useful biology-interactive properties such as (selected
from among many recent examples) enzyme inhibitory activity [7–9], resistance to metabolic

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degradation [10], scaffolding [11,12], imaging and labeling [13], diagnostic features [14,15], special
compartmentalization behavior [16], and drug delivery [17–19].
For several years our research group has investigated the redox coupling [20] of seleninic acids
(RSeO2H, 1, Scheme 1) with thiols 2 to give selenosulfides 4 [21–23]. A wide variety of biomimetic
coupling partners can be assembled that incorporate the seleninic acid functionality as part of a
carbohydrate, nucleoside, amino acid, or lipid framework [21]. The coupling reaction itself proceeds
at room temperature in a wide variety of solvents, including water, alcohols, dioxane, THF, ethyl
acetate, dichloromethane, acetonitrile, toluene, and DMF. The general mechanism of this reaction
(Scheme 1) [20,24] proceeds initially through a thioseleninate intermediate 3. Rapid reduction of 3 to
the selenosulfide product 4 occurs in the presence of excess thiol (the byproduct is disulfide), or often
with no obvious reducing agent, in which case the [O] from 3 “disappears” without reappearing as part
of any isolable byproduct, even when close to 100% of the selenium and sulfur atoms are accounted
for [21,23,24]. Despite this incomplete accounting, the conversion to selenosulfide 4 can be efficient
for one-to-one mixtures of the two components. Air, moisture, buffers, and other additives do not seem
to interfere [23]. The typical byproducts of the one-to-one coupling are the symmetrical disulfide and
the symmetrical diselenide [23]. Exemplifying the potential of such couplings in bioorganic chemistry
was the observation that a tyrosine phosphate mimic, in which a seleninate group (i.e., Ar-CH2SeO2H)
replaces phosphate (Ar-OPO3H2), effectively deactivated protein tyrosine phosphatase by forming a
selenosulfide bond with the cysteine residue in the enzyme active site [22].
Scheme 1. The redox coupling reaction of seleninates and thiols.
In order to expand on these studies, we chose to construct the seleninato moiety as a peptide model,
and to examine its redox coupling to the sulfur atom of two relatively complex partners: the
unprotected 4-thiouridine (a nucleoside thioamide), and glutathione (a tripeptide with an active thiol).
2. Results and Discussion
2.1. Synthesis of Seleninic Acid 10
A greatly simplified “peptide” model compound was commercially available as N-(tert-
butoxycarbonyl)-(S)-phenylalanine (5, Scheme 2). Standard amide formation with aminoethanol (6)
mediated by the coupling reagent 1-[3-(dimethylamino)propyl]-3-ethylcarbodiimide hydrochloride
(EDCI) gave the 2-hydroxylethylamide 7. Conversion of 7 to the 2-iodoethylamide 8 occurred without
incident, and then nucleophilic replacement of iodo by the selenocyanate anion led to 9. Treatment of
selenocyanate 9 with a slight excess of dimethyldioxirane (DMDO), our preferred reagent for the
synthesis of seleninates [21], gave the stable seleninic acid 10. Interestingly, in deuteriomethanol
(CD3OD) solution 10 reversibly forms its apparent diasteriomeric seleninate trideuteriomethyl esters
(1:1). Analogous behavior has been observed with nucleoside seleninic acids [25]. The structure of 10

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is thus initially assigned based on the 1H-, 13C-, and 77Se- (1292.9 and 1294.1 ppm) NMR spectra of its
trideuteriomethyl esters in deuteriomethanol solution, as well as the mass spectrum of the derived
seleninate methyl ester with the appropriate Se isotope cluster centered on m/z 440 (the MNa+ ion of
13
the methyl ester). The C-NMR spectra of 10 taken in acetone-d6 or deuteriochloroform solution,
however, showed the expected 12 signals for a single isomer of the seleninic acid (see Experimental).
Scheme 2. Synthesis of the seleninic acid 10 from N-(Boc)phenylalanine (5).
The structure of 10 was additionally confirmed by an independent preparation (Scheme 3). Thus,
2-hydroxyethylamide 7 was converted to the selenoester 11 by use of the selenocarboxylate Mitsunobu
reaction [26]. Oxidation with DMDO as before gave 10 in similar overall yield and purity. Further
structural confirmation of 10 based on the rapid and efficient redox coupling reaction of seleninates
with p-toluenesulfonylhydrazide [27,28] was carried out, leading to the selenosulfonate 12. This
diagnostic reaction of seleninates could qualify in some respects as a potentially useful biohybrid
coupling reaction, although the biomimetic sulfonylhydrazide partners are typically not as complex nor
are they as readily available as the thiols.
Scheme 3. Alternative synthesis of seleninate 10 and confirmation of the structure.

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2.2. Coupling Reactions of Seleninic Acid 10
The redox coupling reaction of seleninate 10 with commercial 4-thiouridine (13) was conveniently
carried out in methanol solution, since both components easily dissolve (Scheme 4). Following our
usual practice, 10 was added neat in one portion to a solution of 13 without any special precautions to
keep the flask oxygen- or moisture-free. Chromatography of the product served to remove any
disulfide and diselenide byproducts, and led to the modified nucleoside selenosulfide 14 in 47% yield.
While selenosulfides can on occasion disproportionate to a mixture of the selenosulfide, diselenide,
and disulfide, 14 was stable to storage. The 4-thioether structure for 14 is indicated by the pronounced
downfield shift of H-6 to 8.52 ppm [29]. Sulfenylations of 4-thiouridine derivatives are known, and lead
analogously to imidothioate S-derivatives (i.e., disulfides) rather than N-substituted thioamides [29].
The coupled product 14 can be viewed as a reversibly modified nucleoside, since the Se-S bond can
potentially be cleaved by reduction [30,31]; 14 can perhaps also serve as a simplified model for a
peptide oligonucleotide hybrid.
Scheme 4. (a) Coupling reaction of seleninate 10 with 4-thiouridine. (b) Coupling reaction
of 10 with glutathione.
The redox coupling reaction of seleninate 10 with commercial glutathione (GSH, 15) was carried
out in 1:1 acetonitrile/water, a solvent mixture that dissolved both reactants and the coupled product,
selenosulfide 16 (Scheme 4). Reverse phase preparative HPLC served to purify 16, which was
1
13
characterized by its H and C-NMR spectra, as well as its protonated molecular ion possessing the
required Se isotope cluster. Purified 16, as for 14, did not disproportionate on storage. Other types of
selenylations of the GSH thiol have been reported recently, among these GSH pseudoglycosylation with
a glycosyl diselenide [32], selenylation of GSH with ebselen [33], selenylation with selenocysteine [34],
and selenylation of GSH with various electrophilic selenoallyl species [35].

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3. Experimental
3.1. General
Organic solvents used for reactions were of reagent grade and were used as received. Flash
chromatography was performed by using silica gel (E. Merck 230–400 mesh) as the stationary phase.
Silica gel 60 F254 pre-coated plates were used for thin layer chromatography, and visualization was
accomplished with UV light (254 nm) and iodine stain. Prep-scale reverse-phase chromatography was
conducted with a Gilson 215 liquid handler/injector fitted with Gilson 333/334 binary HPLC pumps
and UV/vis dual wavelength detector (model 156) and Trilution software. The chromatographies were
carried out on a Waters XBridge Prep BEH 130 C18 5µm OBD 19 × 100mm column (part # 186003587).
The eluent was acetonitrile (HPLC grade) and Millipore water with 0.05% formic acid buffer. The
analytical LC-MS analyses were conducted by using a Waters 2767 sample manager, consisting of a
Waters 2525 binary gradient HPLC connected to a diode array detector and a Waters Micromass ZQ
mass spectrometer with electro-spray ionization. The LC-MS samples were analyzed as solutions in
water or acetonitrile, prepared at 0.15–0.20 mg/mL concentration. The LC-MS chromatography was
carried out on an Atlantis-C18 column (4.6 × 50 mm; 5 µm) with linear gradients of 0.05% formic acid
in acetonitrile and 0.05% formic acid water. High-resolution mass spectrometry (HRMS) was obtained
on Waters LC-TOF mass spectrometer (model LCT-XE Premier) by using electrospray ionization in
1
13
77
positive or negative mode. H, C, and Se NMR spectra were obtained on a Varian UNITY 400 or
500 instrument. Chemical shifts (δ) are reported in parts per million (ppm) and are referenced to the
residual solvent signal. Coupling constants (J) are reported in hertz (Hz). The usual abbreviations are
used to describe multiplicities: s (singlet), d (doublet), t (triplet), q (quartet), br (broad). NMR solvents
and all other commercially available reagents were used as received and without any further purification.
(S)-tert-Butyl (1-((2-Hydroxyethyl)amino)-1-oxo-3-phenylpropan-2-yl)carbamate (7) [36]. A solution
of N-Boc-L-phenylalanine (5, 265.3 mg, 1.0 mmol) in of acetonitrile (8 mL) was stirred at 0 °C and
then was treated sequentially with hydroxylbenzotriazole (184 mg, 1.2 mmol), diisopropylethylamine
(388 mg, 3.0 mmol) and f 2-aminoethanol (6, 79 mg, 1.3 mmol). After 15 min at 0 °C, 1-ethyl-3-(3-
dimethylaminopropyl)carbodiimide (230 mg, 1.2 mmol) was added and the reaction mixture was
stirred for an additional 12 h at room temperature. The reaction was quenched with aqueous saturated
sodium bicarbonate (10 mL). The reaction mixture was extracted with dichloromethane (3 × 10 mL).
The combined organic extract was washed with brine, and then dried over sodium sulfate. The crude
reaction product was purified by silica gel chromatography with 3% methanol in dichloromethane as
1
the eluent to produce 240 mg (78%) of 7 as a colorless oil: H-NMR (500 MHz, CDCl3) δ 7.19–7.32
(m, 5 H), 6.62 (br s, 1 H), 5.30 (s, 1 H), 4.30–4.38 (m, 1 H), 3.50–3.63 (m, 2 H), 3.28–3.37 (m, 2 H),
3.00–3.09 (m, 2 H), 2.91 (br s, 1 H), 1.35 (s, 9 H); 13C-NMR (126 MHz, CDCl3) δ 172.7, 156.0, 137.1,
129.6, 128.7, 127.0, 80.3, 61.5, 56.3, 42.4, 39.2, 28.5; ESI-MS m/z 309.2 MH+.
(S)-tert-Butyl (1-((2-Iodoethyl)amino)-1-oxo-3-phenylpropan-2-yl)carbamate (8) [37]. A solution of 7
(240 mg, 0.785 mmol) in THF (6 mL) was combined with triphenylphosphine (411 mg, 1.57 mmol)
and imidazole (106.8 mg, 1.57 mmol). After 5 min of stirring, iodine (398 mg, 1.57 mmol) was added
and the reaction mixture was allowed to stir for an additional 1 h. The reaction mixture was

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concentrated and dissolved in dichloromethane (20 mL), which solution was washed sequentially with
5% aqueous Na2S2O3 (10 mL) and brine (10 mL), and then dried over sodium sulfate. Concentration
and then chromatography with 3:7 ethyl acetate/hexanes as the eluent gave 248 mg (76%) of iodide 8
as a yellow solid: 1H-NMR (500 MHz, CDCl3) δ 7.20–7.36 (m, 5 H), 6.55 (br s, 1 H), 5.23 (br d, 1 H,
J = 7.0 Hz), 4.29–4.38 (m, 1 H), 3.44–3.62 (m, 2 H), 3.02–3.18 (m, 4 H), 1.40 (s, 9 H); 13C-NMR (126 MHz,
CDCl3) δ 171.6, 155.7, 136.9, 129.5, 128.9, 127.2, 80.4, 56.1, 42.0, 38.9, 28.6, 4.3; ESI-MS m/z 419.1 MH+.
(S)-tert-Butyl (1-Oxo-3-phenyl-1-((2-selenocyanatoethyl)amino)propan-2-yl)carbamate (9). A solution
of iodide 8 (232 mg, 0.555 mmol) in acetone (3 mL) was treated with potassium selenocyanate (120 mg,
0.832 mmol). The reaction mixture was stirred for 3 h, concentrated, and then partitioned between
dichloromethane and aqueous ammonia chloride. The organic extract was dried over sodium sulfate,
concentrated, and then chromatographed with 3% methanol in dichloromethane as the eluent to afford
204 mg (93%) of 9 as a white solid: 1H-NMR (500 MHz, CDCl3) δ 7.18–7. 35 (m, 5 H), 6.55 (bs, 1 H),
5.10 (br d, 1 H, J = 7.0 Hz), 4.30–4.39 (m, 1 H), 3.65–3.74 (m, 1 H), 3.55–3.65 (m, 1 H), 3.10–3.17
13
(m, 1 H), 3.06 (app d, 2 H, J = 6.5 Hz), 2.94–3.03 (m, 1 H), 1.41 (s, 9 H); C-NMR (126 MHz,
CDCl3) δ 172.3, 155.7, 136.8, 129.5, 129.0, 127.4, 101.6, 80.7, 56.1, 40.1, 38.6, 29.1, 28.5; 77Se NMR
(95 MHz, CDCl3) δ 199.7 (vs. PhSeSePh at 460.0 ppm as an external standard); ESI-MS m/z 420.0 MNa+.
(S)-2-(2-((tert-Butoxycarbonyl)amino)-3-phenylpropanamido)ethaneseleninic acid (10). Dimethyldioxirane
(DMDO) was added to a stirred solution of selenocyanate 9 (111 mg, 0.28 mmol) in dichloro-methane
(7 mL) until starting material was consumed according to TLC analysis (total 1.05 mL of a 0.4 M
solution of DMDO in moist acetone). The reaction mixture was concentrated and then chromatographed
on silica gel with 8% methanol in dichloromethane as the eluent to give 77.3 mg (70%) of 10 as a
colorless oil: 1H-NMR (500 MHz, CD3OD, as the two diasteriomeric seleninate—CD3 esters) δ 7.21–7.32
(m, 5 H), 4.22–4.31 (m, 1 H), 3.46–3.64 (m, 2 H), 3.04–3.12 (m, 2 H), 2.83–2.91 (m, 2 H), 1.38 (s, 9 H);
13C-NMR (126 MHz, CD3OD, as the two diasteriomeric seleninate—CD3 esters) δ 174.3 and 174.2,
156.4, 137.4, 129.3, 128.3, 126.7 and 126.6, 79.6, 57.2 and 57.0, 56.1 and 56.2, 38.0, 33.2 and 33.3,
13
27.5; C-NMR (126 MHz, acetone-d6) δ 173.3, 155.7, 138.2, 129.7, 128.5, 126.6, 78.8, 58.2, 56.2,
13
38.4, 33.7, 28.0; C-NMR (126 MHz, CDCl3) δ 173.9, 155.9, 136.9, 129.6, 128.9, 127.2, 80.5, 56.3,
55.9, 38.9, 33.6, 28.6; 77Se NMR (95 MHz, CD3OD, as the diasteriomeric seleninate esters, SeO2CD3)
δ 1294.1, 1292.9 (vs. PhSeSePh at 460.0 ppm as an external standard); ESI-MS m/z 440.4 as the
seleninate methyl ester···Na+.
(S)-Se-(2-(2-((tert-Butoxycarbonyl)amino)-3-phenylpropanamido)ethyl) 2-phenylethaneselenoate (11).
A solution of triphenylphosphine (126 mg, 0.48 mmol) in tetrahydrofuran (1.5 mL) was stirred at
−20 °C. Diisopropyl azodicarboxylate (97.1 mg, 0.48 mmol) was added dropwise and the reaction
mixture was maintained at −20 °C until the white phosphonium intermediate formed. The reaction
mixture was then cooled to −50 °C, and a solution of alcohol 7 (74.0 mg, 0.24 mmol) in tetrahydrofuran
(2 mL) was added dropwise. After 5 min of stirring, a toluene solution (2 mL) of (2-phenyl)-selenoacetic
acid [prepared by heating at reflux a 2 mL toluene solution of 100 mg (0.734 mmol) of phenyl acetic
acid and 117 mg of Woollins’s reagent for 2 h] was added by cannula and the reaction mixture was
allowed to warm from −50 to 23 °C, and then was stirred for an additional 2 h. The solution was

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concentrated and then chromatographed with 3:7 ethyl acetate/hexanes as the eluent to afford 77 mg
(66%) of 11 as a colorless oil: 1H-NMR (500 MHz, CDCl3) δ 7.39–7.30 (m, 4 H), 7.30–7.25 (m, 4 H),
7.17 (d, 2 H, J = 7.0 Hz), 6.24 (bs, 1 H), 5.18 (bs, 1 H), 4.30 (bs, 1 H), 3.83 (s, 2 H), 3.33–3.47 (m,
2 H), 3.01 (br d, 2 H, J = 6.0 Hz), 2.77–2.92 (m, 2 H), 1.41 (s, 9 H); 13C-NMR (126 MHz, CD3OD) δ
200.7, 171.4, 155.8, 136.9, 132.9, 130.2, 129.6, 129.5, 129.0, 128.9, 128.8, 128.0, 127.1, 80.4, 56.2,
77
54.3, 40.1, 39.0, 28.5, 25.1; Se NMR (95 MHz, CDCl3) δ 547.6 (vs. PhSeSePh at 460.0 ppm as an
external standard); ESI-MS m/z 513.1 MNa+.
(S)-2-(2-((tert-Butoxycarbonyl)amino)-3-phenylpropanamido)ethaneseleninic acid (10) by oxidation of
11. Dimethyldioxirane was added to a stirred solution of selenoester 11 (43.4 mg, 0.089 mmol) in
dichloromethane (1 mL) until starting material was consumed according to TLC analysis (total 0.488 mL
of 0.4 M solution of DMDO in moist acetone). The reaction mixture was concentrated and then
chromatographed on silica with 8% methanol in dichloromethane as the eluent to give 22.6 mg (63%)
of 10 as a colorless oil that spectroscopically matched 10 as prepared above.
(S)-Se-(2-(2-((tert-Butoxycarbonyl)amino)-3-phenylpropanamido)ethyl) 4-methylbenzenesulfonoselenoate
(12). A solution of seleninic acid 10 (22.5 mg, 0.056 mmol) in dichoromethane (3 mL) was added
dropwise to a solution of p-toluenesulfonylhydrazide (11.3 mg, 0.061 mmol) in dichloromethane
(1 mL). After 30 min, the reaction mixture was concentrated then and chromatographed on silica with
3:7 ethyl acetate/hexanes as the eluent to give 25.9 (88%) of selenosulfonate 12 as a yellow oil:
1H-NMR (500 MHz, CDCl3) δ 7.75 (d, 2 H, J = 8.0 Hz), 7.35 (d, 2 H, J = 8.0 Hz), 7.29 (t, 2 H,
J = 7.0 Hz), 7.24 (t, 1 H, J = 7.0 Hz), 7.19 (d, 2 H, J = 7.0 Hz), 6.44 (br s, 1 H), 5.08 (br s, 1 H),
4.29–4.36 (m, 1 H), 3.55–3.66 (m, 2 H), 3.19–3.27 (m, 1 H), 3.11–3.19 (m, 1 H), 3.06 (br d, 2 H,
13
J = 6.5 Hz), 2.45 (s, 3 H), 1.40 (s, 9 H); C-NMR (126 MHz, CDCl3) δ 171.9, 155.8, 145.2, 144.1,
136.9, 130.1, 129.5, 128.9, 127.3, 126.8, 82.3, 56.2, 39.4, 38.8, 33.0, 28.5, 21.9; 77Se NMR (95 MHz,
CDCl3) δ 856.9 (vs. PhSeSePh at 460.0 ppm as an external standard); ESI-MS m/z 549.1 MNa+.
tert-Butyl
(1-((2-(((1-((2R,3R,4S,5R)-3,4-Dihydroxy-5-(hydroxymethyl)tetrahydrofuran-2-yl)-2-oxo-
1,2-dihydropyrimidin-4-yl)thio)selanyl)ethyl)amino)-1-oxo-3-phenylpropan-2-yl)carbamate (14). A
solution of 4-thiouridine (13, 11.9 mg, 0.0456 mmol) in methanol (0.5 mL) was stirred at 0 °C and
then was treated with seleninic acid 10 (18.4 mg, 0.0456 mmol) in one portion. After 2 h at 0 °C, the
reaction was concentrated and then chromatographed with 5% methanol in dichloromethane as the
eluent to produce 13.9 mg (47%) of selenosulfide 14 as a colorless oil: Rf 0.28 (5% methanol in
1
dichloromethane); H-NMR (400 MHz, CD3OD) δ 8.52 (d, 1 H, J = 7.2 Hz), 7.27 (br s, 4 H), 7.21
(br s, 1 H), 6.89 (d, 1 H, J = 7.2 Hz), 5.87 (br s, 1 H), 4.27–4.32 (m, 1 H), 4.09–4.21 (m, 3 H), 3.81 and
3.97 (ABq, 2 H, J = 10.0 Hz), 3.70–3.78 (m, 1 H), 3.22–3.30 (m, 1 H), 3.07–3.15 (m, 2 H), 2.95–3.04
13
(m, 1 H), 2.86 (dd, 1 H, J = 13.2, 10.0 Hz), 1.31 (s, 9 H); C-NMR (126 MHz, CD3OD) δ 179.1,
174.7, 157.7, 156.1, 144.3, 138.8, 130.4, 129.5, 127.7, 103.6, 93.4, 85.9, 80.6, 76.6, 69.9, 61.1, 57.9,
39.4, 39.1, 33.5, 28.7; HRMS m/z 631.1367 MH+, calcd for C25H35N4O8SSe 631.1341; NI-HRMS m/z
629.1191 MH−, calcd for C25H33N4O8SSe 629.1184.
(6S,14R,19S)-19-Amino-6-benzyl-14-((carboxymethyl)carbamoyl)-2,2-dimethyl-4,7,16-trioxo-3-oxa-
12-thia-11-selena-5,8,15-triazaicosan-20-oic acid (16). A stirred solution of glutathione (15, 15.2 mg,

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0.0496 mmol) in acetonitrile/water (1:1, 2 mL) was treated with seleninic acid 10 (20.0 mg, 0.0496
mmol) in one portion. After 2 h, the resulting solution was directly purified by reverse-phase
chromatography (gradient 5–60% acetonitrile over 10 min) and then lyophilized to give 19.0 mg (57%)
1
of 16 as a white amorphous powder showing a single peak at 3.28 min by HPLC analysis. H-NMR
(500 MHz, CD3OD) δ 7.17–7.29 (m, 5 H), 4.68 (dd, 1 H, J = 9.5, 4.5 Hz), 4.20–4.28 (m, 1 H), 4.02
(t, 1 H, J = 6.5 Hz), 3.93 (br s, 2 H), 3.52–3.62 (m, 1 H), 3.42–3.52 (m, 1 H), 2.96–3.10 (m, 3 H),
2.80–2.96 (m, 3 H), 2.58 (t, 2 H, J = 6.5 Hz), 2.12–2.29 (m, 2 H), 1.35 (s, 9 H); 13C-NMR (126 MHz,
CD3OD) δ 173.3, 173.2, 171.7, 171.5, 170.4, 156.4, 137.4, 129.2, 128.3, 126.6, 79.5, 56.4, 53.7, 52.4,
40.7, 39.0, 38.3, 33.3, 31.2, 30.0, 27.5, 25.9; ESI-MS m/z 678.1 MH+.
4. Conclusions
Densely functionalized and unprotected coupling partners (4-thiuridine and glutathione) participate
in the seleninic acid–thiol redox coupling reaction with the 2-seleninatoethyl amide 10 under mild and
neutral conditions in protic solvents. The selenosulfide products 14 and 16, while formed in modest
yield, are stable and easily characterized. No obvious impediment prevents analogous coupling
reactions between peptide-based seleninic acids and thiol-bearing peptides, proteins, or oligonucleotides,
and such applications may now be contemplated.
Supplementary Materials
NMR spectra for compounds 8–12, 14, and 16, and HPLC trace for 16.
Supplementary materials can be accessed at: http://www.mdpi.com/1420-3049/18/2/1963/s1.
Acknowledgments
We thank Samos Pharmaceuticals and the Prusoff Foundation for support of this work, and Jian Liu
for assistance with the spectroscopy.
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Sample Availability: Samples of the compounds 7 and 10 are available from the authors.
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