T3P (propylphosphonic anhydride) mediated conversion of N α-protected amino/peptide acids into thioacids

Article abstract of DOI:10.1016/j.tetlet.2012.01.027

A general, mild and an efficient protocol, which makes use of T3P as an acid activator for the synthesis of N^α-protected amino/peptide thioacids from corresponding acids in the presence of finely ground Na ₂S as hydrosulfide ion donor is described. The protocol employed significantly increases the overall efficiency as the yield, reaction duration and purity of even sterically hindered amino acids.

Full text of DOI:10.1016/j.tetlet.2012.01.027

Tetrahedron Letters 53 (2012) 1406–1409
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Tetrahedron Letters
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a
T3P (propylphosphonic anhydride) mediated conversion of N -protected
amino/peptide acids into thioacids
⇑
Chilakapati Madhu, Basavaprabhu, T.M. Vishwanatha, Vommina V. Sureshbabu
Room No. 109, Peptide Research Laboratory, Department of Studies in Chemistry, Central College Campus, Dr. B. R. Ambedkar Veedhi, Bangalore University, Bangalore 560 001, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A general, mild and an efficient protocol, which makes use of T3P as an acid activator for the synthesis of
N -protected amino/peptide thioacids from corresponding acids in the presence of finely ground Na2S as
hydrosulfide ion donor is described. The protocol employed significantly increases the overall efficiency
as the yield, reaction duration and purity of even sterically hindered amino acids.
Ó 2012 Elsevier Ltd. All rights reserved.
a
Received 9 November 2011
Revised 4 January 2012
Accepted 6 January 2012
Available online 14 January 2012
Keywords:
Propylphosphonic anhydride
a
N -Protected amino/peptide thioacids
Dipeptide synthesis
Thioacids are the vital class of molecules with preferential util-
ity in synthetic organic chemistry particularly in the development
Synthesis of thioacids can be generally accomplished by the
coupling of an acid with SH-Fmoc, SH-Trt or SH-Tomb,11 whereas
availability of these reagents itself is a limitation. Further, cleavage
of such groups involves treatment with acid or base which other-
wise is noncompatible to the amine protecting group. Thioacids
can also be prepared using NaSH,12 but yields can be low due to
the formation of diacyldisulfide as the major byproduct. However,
the most common method for thioacid preparation is the activa-
tion of carboxylic acid by N,N0-carbonyldimidazole (CDI), followed
by the treatment with Na2S, H2S, or Li2S in DMF at 0 °C for an hour
or NaSH in H2O at room temperature.13 Recently Lawesson’s re-
agent has also been employed under microwave irradiation.14
Appreciable epimerization was observed for the peptide thioacid
prepared using Lawesson reagent at room temperature. The key
steps of many ligation techniques in the protein synthesis mainly
of various ligation techniques.1
a-Amino thioacids have emerged
as novel building blocks for the construction of both peptides as
well as peptide like adducts.2 Thioacids react with diverse array
of reactants such as isonitriles,3 azide functions,4 alkyl halides,5
and amines.6 They are easily soluble in organic solvents and the
coupling induced by these thioacids proceeds with less epimeriza-
tion than their (oxygen) acid counterparts. Li et al.,7 introduced
peptidyl thioacids for the first time. The active participation of
thioacids in ligation techniques made the synthesis of thioacids
crucial. Solid phase peptide synthesis of large peptides is problem-
atic where it suffers from low yields and purification problems.
Thus, native chemical ligation strategies have been discovered
where peptide thioacid/thioester would be a C-terminal fragment
and it connects to the amino component of another peptide frag-
ment to form a polypeptide.8 Peptide thioacids have been found
to be potent acyl donors which readily undergo oxidative activa-
tion leading to the insertion of an amide bond upon reaction with
N-terminal peptide. Such ligations are conducted with minimal
depend on the accessibility of
a-amino thioacids in general and
peptide thioacids in particular. Thus synthesis of N-protected ami-
no as well as peptide thioacids through a simple, mild, and eco-
nomical route is of paramount importance. Thus we directed our
attention to develop a viable and mild protocol for the direct access
to thioacids employing T3P–Na2S system.
a-epimerization at the C-terminal thioacid. Thus, they allow for
the coupling of N-terminal and C-terminal glycopeptides to obtain
homogeneous glycoproteins.3a Further, thioacids can also be used
along with other functionalities such as isonitrile,3b azide,4 acid
azide,9 sulfonyl azides10 as coupling partners to form amide, imide,
or sulfonamide bonds. This application distinguishes the utility of
thioacids from thioesters for the preparation of peptides.
T3P (propylphosphonicanhydride)15 is a highly reactive cyclic
anhydride. It has been widely employed as coupling agent for the
epimerization free synthesis of peptides16 in the absence of addi-
tives 1-hydroxybenzotriazole (HOBt) or 1-hydroxy-7-azabenzotria-
zole (HOAt) etc. Also it serves as dehydrant in the construction of
various heterocycles including oxadiazoles, substituted pyrimi-
dines, and quinolines, indoles, b-lactams.17 T3P offers several
advantages over other reagents in terms of higher yields, shorter
reaction duration, ease of isolation of the products, minimal side
reactions, inexpensive, and non-toxic nature.
⇑
Corresponding author. Tel.: +91 08 2296 1339/09986312937.
E-mail addresses: hariccb@hotmail.com, hariccb@gmail.com, sureshbabuvom
mina@rediffmail.com (V.V. Sureshbabu).
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2012.01.027

C. Madhu et al. / Tetrahedron Letters 53 (2012) 1406–1409
1407
At first, the reaction was examined in different solvents such as
DCM, EtOAc, THF, and CH3CN. The results are summarized in Table
1. In the reaction of Fmoc-Leu-OH, 1j with Na2S (3.0 equiv) in the
presence of T3P (1.5 equiv) and TEA (2.0 equiv) in dry DCM, trans-
formation of the acid to thioacid was moderate at 0 °C to rt. In THF
and EtOAc the yields were satisfactory. In contrast, when the reac-
tion was performed under the similar conditions in CH3CN, com-
plete consumption of acid was observed after 2 h and thioacid, 2j
was obtained in a 95% yield.20
The reaction was monitored by HPLC analysis of different ali-
quots of crude reaction mixture at different intervals of time. In
the presence of a base such as TEA, the carboxylate ion attacks
the cyclic anhydride ring of T3P. Meanwhile, the hydrosulfide an-
ion generated by Na2S in the solution adds to the more electro-
philic carbonyl carbon of mixed anhydride and results in the
formation of thioacid. After evaporation of the solvent, simple
work-up is sufficient to remove the byproducts liberated during
the reaction process and affords the resultant thioacid in good
yields. Table 2 summarizes the results. In all these preparations,
no further column purification was needed as the protocol resulted
in water soluble byproducts and thioacids could be isolated
through a simple work-up.
The generality of the protocol was demonstrated using simple
as well as bifunctional amino acids which also resulted in thioacids
with good to excellent yields (Table 2). The protocol can be suc-
cessfully applied even to the sterically hindered amino acids Aib,
1g, and Pro, 1c. Interestingly it is to note that Boc-Ser-OH also fur-
nished the corresponding thioacid 2f in a good yield despite the
fact that the oxidation of –OH group of Ser-OH in the presence of
T3P is known.21 This can be due to the absence of DMSO which
is known to assist the oxidation. The reaction takes place under
R
R
T3P, TEA
Na2S
SH
OH
PgHN
PgHN
0 oC, CH3CN
O
O
2a-j
1a-j
a
Scheme 1. Synthesis of N -protected amino thioacids.
Table 1
Screening of solvents for the optimization of reaction
Entry
Solvent
Time (h)
Yielda (%)
1
2
3
4
DCM
EtOAc
THF
4
3
3
2
62
69
75
95
CH3CN
a
Isolated product yield.
In ongoing studies, our group demonstrated the applicability of
T3P for the one pot transformation of an acid to the corresponding
ureas and carbamates through Curtius18 and Lossen rearrange-
ments19, respectively. Herein we report our results on the direct
a-amino thioacids from corresponding carboxylic
acids in the presence of T3P and finely ground Na2S as hydrosulfide
synthesis of
ion donor.
Initially the thioacid preparation was carried out using Fmoc-
Leu-OH as an acid component in the presence of T3P as an acid
activator, TEA as base to assist T3P activation and Na2S being the
hydrosulfide anion donor (Scheme 1). The reaction conditions were
then optimized by fine tuning the equivalents of T3P, TEA, and
Na2S.
Table 2
a
List of N -protected amino thioacids
Entry
Thioacid (2)
M.P.
Yield (%)
93
Entry
6
Thioacid (2)
M.P.
Yield (%)
74
HO
SH
BocHN
SH
BocHN
O
1
Gum
83–84
O
2a
2f
SH
FmocHN
O
SH
2
3
Gum
Gum
89
88
7
8
70–72
Gum
82
87
BocHN
ZN
O
2g
2b
O
SH
SH
ZHN
O
2h
2c
t
SH
BuO
BocHN
O
SH
4
5
Gum
91
90
9
91–92
86–88
84
95
2d
FmocHN
O
2i
SH
SH
ZHN
FmocHN
82–83
10
O
O
2e
2j

1408
C. Madhu et al. / Tetrahedron Letters 53 (2012) 1406–1409
R
O
5a and 5b. The HPLC analysis of the pure peptides 5a and 5b
showed single peaks at different Rt values, that is, at Rt = 18.17
and 18.96 min, respectively. These studies showed that the pre-
pared peptides, 5a and 5b contain a single optically pure isomer
and consequently it was proved that the present protocol was free
from racemization. HOBt has been employed in carbodiimide med-
iated coupling reactions to suppress the racemization.25 Similarly,
the studies carried out by Danishefsky et al.,23 revealed that the
thioacid in the presence of HOBt, forms an –OBt ester which on
acylation with an amino component leads to amide bond forma-
tion. In such couplings also, HOBt markedly reduces the oxazolone
promoted racemization at the C-terminus. The application of T3P
as an activator can be extended to the preparation of peptide thio-
acids without causing racemization.
R
O
T3P, TEA
Na2S
H
N
H
N
PgHN
OH
PgHN
SH
0 oC, CH3CN
O
R'
O
R'
4a-g
3a-g
Scheme 2. Synthesis of peptide thioacids.
Table 3
a
List of N -protected peptide thioacids
Entry
Pg
R
R0
Yield (%)
4a
4b
4c
4d
4e
4f
Fmoc
Fmoc
Boc
Boc
Z
CH2C6H5
CH3
92
85
82
84
88
82
CH2COOC6H5
CH(CH3)CH2CH3
H
CH(CH3)2
CH(CH3)OC6H5
CH(CH3)2
CH2COOC6H5
–(CH2)3–
CH2CH(CH3)2
CH2C6H5
In conclusion, herein we have described a simple, mild, and an
a
alternative protocol for the direct conversion of N -protected
Z
amino/peptide acids into thioacids by employing T3P/Na2S
26
a
system.
A series of carboxylic acids including N -Fmoc/Z/Boc
amino acids have been converted into corresponding thioacids
in excellent yields. Also the protocol can further be extended
even to the large scale preparations as the byproducts released
were innocuous, water soluble and the protocol is operationally
simple.
Fmoc-L-Phg-SH
(2k)
H
N
COOMe
COOMe
FmocHN
DIPEA/HOBT
in DMSO
O
5a
I2, 4 A0 MS
DMSO, rt
H2N
COOMe
Acknowledgments
H
N
Fmoc-D-Phg-SH
FmocHN
We gratefully acknowledge ArchimicaGmbH, Hochst Industrial
Park 65926, Frankfurt-Hoechst, Germany for providing T3P as a gift
sample. Also, we thank the Council of Scientific and Industrial Re-
search (CSIR), New Delhi (Grant No. 01(2323)/09/EMR-II) for the
financial assistance. Madhu C. thanks the UGC, New Delhi. Govt.
of India for the award of fellowship.
(2l)
O
5b
L and D-Phg-SH.
Scheme 3. Racemization study of Fmoc protected
mild conditions and the protocol is compatible with different ami-
no protecting groups Boc, Z, and Fmoc. In the case of Boc-protected
amino acids, an additional equivalent of base was used to avoid de-
crease in the yield which was possibly due to the acidic byproduct
liberated during the reaction.
References and notes
1. (a) Kent, S. B. H. Chem. Soc. Rev. 2009, 38, 338–351; (b) Muir, T. W. Annu. Rev.
Biochem. 2003, 72, 249–289.
2. Barlett, K. N.; Kolakowski, R. V.; Katukojvala, S.; Williams, L. J. Org. Lett. 2006, 8,
823–826.
Encouraged by the above results, we further extended the pro-
tocol for the synthesis of peptidyl thioacids (Scheme 2). The pep-
tide Fmoc-Phe-Ala-OH was chosen as an acid component and
under the optimized condition, as monitored through the HPLC it
showed a gradual disappearance of an acid peak Rt 14.25 and
appearance of a peak correspond to thioacid Rt 18.28, with an
incomplete consumption of acid was observed. Later, fine tuning
the equivalence of T3P from 1.5 to 2.5 equiv served best in enhanc-
ing the yield of thioacid from good to excellent (Table 3).22 The use
of T3P for the activation leads to the formation of water soluble
byproducts. After a simple work-up, the products can be isolated
in quantitative yield with high purity. The protocol is completely
devoid of any column purification.
3. (a) Wu, X.; Stockdill, J. L.; Wang, P.; Danishefsky, S. J. J. Am. Chem. Soc. 2010, 132,
4098–4100; (b) Rao, Yu.; Li, X.; Danishefsky, S. J. J. Am. Chem. Soc. 2009, 131,
12924–12926.
4. (a) Park, S. D.; Oh, J.-H.; Lim, D. Tetrahedron Lett. 2002, 43, 6309–6311; (b) Fazio,
F.; Wong, C.-H. Tetrahedron Lett. 2003, 44, 9083–9085; (c) Mckervey, M. A.;
O’Sullivan, M. B.; Mayers, P. L.; Green, R. H. J. Chem. Soc., Chem. Commun. 1993,
94–96; (d) Shangguna, N.; Katukojvala, S.; Greenberg, R.; Williams, L. J. J. Am.
Chem. Soc. 2003, 125, 7754–7755; (e) Merkx, R.; van Haren, M. J.; Rijkers, Drik T.
S.; Liskamp, R. M. J. J. Org. Chem. 2007, 72, 4574–4577.
5. Fu, X.; Jiang, S.; Li, C.; Xin, J.; Yang, Y.; Ji, R. Bioorg. Med. Chem. Lett. 2007, 17,
465–470.
6. (a) Blake, J. Int. J. Pept. Protein Res. 1981, 17, 273–274; (b) Yamashiro, D.; Blake,
J. Int. J. Pept. Protein Res. 1981, 18, 383–392; (c) Mitin, Y. V.; Zapevalova, N. P. Int.
J. Pept. Protein Res. 1990, 35, 352–356.
7. (a) Blake, J.; Yamashiro, D.; Ramasharma, K.; Li, C. H. Int. J. Pept. Protein Res.
1986, 28, 468–476; (b) Yamashiro, D.; Li, C. H. Int. J. Pept. Protein Res. 1988, 31,
322–334.
a
The possibility of racemization during the synthesis of N -pro-
tected amino/peptide thioacids from corresponding acids via the
present protocol was assessed following the HPLC analysis of the
8. (a) Hackeng, T. M.; Griffin, J. H.; Dawson, P. E. Proc. Natl. Acad. Sci. U.S.A. 1999,
96, 10068–10073; (b) Bang, D.; Chopra, N.; Kent, S. B. H. J. Am. Chem. Soc. 2004,
126, 1377–1383; (c) Bang, D.; Kent, S. B. H. Angew. Chem., Int. Ed. 2004, 43,
2534–2538; (d) Johnson, E. C. B.; Durek, T.; Kent, S. B. H. Angew. Chem., Int. Ed.
2006, 45, 3283–3287.
thioacids derived from the Fmoc protected L- and D-Phg employing
the present protocol and also the HPLC analyses of the diastereo-
meric dipeptides obtained by these derivatives with NH2-Ala-
OMe following known coupling method (Scheme 3).23,24
9. Mhidia, R.; Beziere, N.; Blanpain, A.; Pommery, N.; Melnyk, O. Org. Lett. 2010,
12, 3982–3985.
10. Barlett, K. N.; Kolakowski, R. V.; Kotukojvala, S.; Williams, L. J. Org. Lett. 2006, 8,
823–826.
Racemization study was carried out through the HPLC analyses
of the thioacids, 2k, and 2l synthesized by the present protocol.
These thioacids appear at different Rt values, that is, Rt = 14.2 min.
11. (a) Crich, D.; Bower, A. A. Org. Lett. 2007, 9, 5323–5325; (b) Vetter, S. Synth.
Commun. 1998, 28, 3219–3223.
12. (a) Cronyn, M. W.; Jiu, J. J. Am. Chem. Soc. 1952, 74, 4726; (b) Shigenaga, A.;
Sumikawa, Y.; Tsuda, S.; Sato, S.; Otaka, A. Tetrahedron 2010, 66, 3290–
3296.
(2k, L-isomer) and Rt = 15.1 min. (2l, D-isomer), also the equimolar
13. Assem, N.; Natarajan, A.; Yudin, A. K. J. Am. Chem. Soc. 2010, 132,
10986–10987.
14. Rao, Y.; Li, X.; Nagorny, P.; Hayashida, J.; Danishefsky, S. J. Tetrahedron Lett.
2009, 50, 6684–6686.
mixture of 2k and 2l appears as distinct peaks at Rt = 14.62 and
15.37 min.
When the optically pure 2k and 2l, as outlined in Scheme 3,
15. (a) LlanesGarcía, A. L. Synlett 2007, 1328; (b) Schwarz, M. Synlett 2000, 1369.
were coupled separately with L-Ala-OMe they yielded peptides

C. Madhu et al. / Tetrahedron Letters 53 (2012) 1406–1409
1409
16. (a) Wissmann, H.; Kleiner, H.-J. Angew. Chem., Int. Ed. Engl. 1980, 19, 133; (b)
Escher, R.; Bünning, P. Angew. Chem., Int. Ed. Engl. 1986, 25, 277.
25. (a) Koenig, W.; Geiger, R. Chem. Ber. 1970, 103, 788–798; (b) Koenig, W.;
Geiger, R. Chem. Ber. 1970, 103, 2024–2033.
17. (a) Zumpe, F. L.; Melanie, F.; Schmitz, K.; Lender, A. Tetrahedron Lett. 2007, 48,
1421; (b) Crawforth, J. M.; Paoletti, M. Tetrahedron Lett. 2009, 50, 4916; (c)
Augustine, J. K.; Vairaperumal, V.; Narasimhan, S.; Alagarsamy, P.;
Radhakrishnan, A. Tetrahedron 2009, 65, 9989; (d) Desroses, M.; Wieckowski,
K.; Stevens, M.; Odell, L. R. Tetrahedron Lett. 2011, 52, 4417.
26. Characterization data for selected compounds
Fmoc-Leu-COSH (2j): White solid, mp 88–90 °C. IR (neat, tmax) 1659, 1766,
3243 cmꢁ1.Rf 0.4 (EtOAc/n-hexane, 60:40).Rt 20.41 (30–100% CH3CN,
30 min).1H NMR (CDCl3, 400 MHz) d ppm 1.05 (d, J = 6.2 Hz, 6H), 1.68 (m,
2H), 1.82 (m, 1H), 3.92 (t, J = 4.8 Hz, 1H), 4.3 (t, J = 4.1 Hz, 1H) 4.56 (d,
J = 5.9 Hz), 4.9 (br, 1H), 6.9 (br, 1H), 7.2–7.6 (m, 8H). 13C NMR (CDCl3, 100 MHz)
d ppm 21.8, 22.4, 39.5, 45.1, 64.3, 66.9, 125.9, 126.5, 127.3, 127.8, 142.6, 143.2,
156.9, 198.7.HRMS (ESI) calcd for C21H23NO3S m/z 392.1296 [M+Na]+, Found
392.1285. C, 68.25; H, 6.26; N, 3.80; O, 12.98; S, 8.71.
18. Basavaprabhu; Narendra, N.; Lamani, R. S.; Sureshbabu, V. V. Tetrahedron Lett.
2010, 51, 3002–3004.
19. Vasantha, B.; Hemantha, H. P.; Sureshbabu, V. V. Synthesis 2010, 2990–2996.
20. General for the synthesis of amino thioacid 2a–j: To a solution of acid (1.0 mmol)
and T3P (1.2 mmol) in dry CH3CN (8.0 mL) was added Et3 N (1.5 mmol) at 0 °C
followed by the finely ground Na2S (3.0 mmol) and the reaction mixture was
stirred for 2 to 3 h till the completion of reaction as monitored by TLC. Then the
reaction mixture was concentrated and then diluted with water. The resultant
solution was then acidified with citric acid solution (10%) and the product was
extracted into EtOAc (10 mL). The organic layer was washed with water
(2 ꢀ 10 mL), brine (10 mL) and dried over anhydrous Na2SO4. Solvent was
evaporated in vacuo to obtain aminothioacid in a quantitative yield.
Method for HPLC analysis: Gradient 0.1% TFA, water/acetonitrile (30:70) in
30 min with a flow rate of 0.5 ml/min. k = 254 nm (Fmoc, Z), 211 nm (Boc).
21. 21. Mendt, A.; Scherer, S.; Bhom, C. PCT Int. Appl. WO 2005102978, 2005;
Chem. Abstr. 2005, 143, 440908.
Boc-Phe-COSH (2b): gum, IR (neat, t
max) 1723, 1765, 3349 cmꢁ1.Rf 0.32 (EtOAc/
n-hexane, 60:40).Rt 16.76 (30–100% CH3CN, 30 min).1H NMR (CDCl3, 400 MHz)
d ppm 1.39 (s, 9H), 3.1 (d, J = 6.5 Hz, 2H), 3.85 (t, J = 5.2 Hz), 4.63 (br, 1H), 7.08–
7.26 (m, 5H), 7.9 (br, 1H). 13C NMR (CDCl3, 100 MHz) d ppm 29.3, 36.6, 68.2,
7(8.7, 126.5, 128.4, 128.9, 140.5, 157.1, 198.5.HRMS (ESI) calcd for C14H19NO3S
m/z 304.0983 [M+Na]+, Found 304.0968. C, 59.78; H, 6.80; N, 4.96; O, 17.03; S,
11.43.
Z-Pro-COSH (2c): gum, IR (neat, t
max) 1747, 1762, 3259 cmꢁ1. Rf 0.35 (EtOAc/n-
hexane, 60:40).Rt 14.33 (30–100% CH3CN, 30 min). 1H NMR (CDCl3, 400 MHz) d
ppm 1.53–1.56 (m, 2H), 1.93–2.01 (m, 2H), 3.35 (t, J = 4.8 Hz, 2H), 4.31 (t,
J = 5.4 Hz, 1H), 4.72 (br, 1H), 5.39 (s, 2H), 7.23 (s, 5H). 13C NMR (CDCl3,
100 MHz) d ppm 21.3, 28.5, 48.1, 65.9, 73.5,128.3, 128.7, 129.5, 140.7, 156.9,
22. General procedure for the synthesis of peptide thioacids 4a–f: Peptide thioacids
were prepared using similar experimental conditions as for the preparation of
amino thioacids. The only difference involves the usage of 2.5 mmol of T3P.
23. Wang, P.; Danishefsky, S. J. J. Am. Chem. Soc. 2010, 132, 17045–17048.
24. Coupling method for the preparation of peptide ester using N-protected amino
thioacids 2a–j: To a solution of thioacid (1.2 equiv) in DMSO was added amino
acid ester (1.0 equiv) and 4 Å MS (2 mg). To the resultant reaction mixture, a
stock solution of DIPEA (1.5 equiv)/HOBT (2.0 equiv) in DMSO and I2
(0.6 equiv) in DMSO was added. The reaction mixture was then allowed to
stir at room temperature for 40–50 min till the completion of the reaction as
monitored through TLC. The crude product is then purified through column
chromatography.
197.9. HRMS (ESI) calcd for C13H15NO3S m/z 288.0671 [M+Na], Found
288.0677.C, 58.88; H, 5.73; N, 5.25; O, 18.06; S, 12.08.
Z-Val-Leu-COSH (4e): White solid, mp 115–117 °C. IR (KBr, tmax) 1747, 1762,
3259 cmꢁ1
. Rf 0.25 (EtOAc/n-hexane, 60:40). Rt 20.45 (30–100% CH3CN,
30 min) 1H NMR (CDCl3, 400 MHz) d ppm 0.95 (d, J = 6.1 Hz, 6H), 1.05 (d,
J = 6.4 Hz, 6H), 1.8–1.84 (m, 3H), 2.59 (m, 1H), 4.2 (t, J = 5.2 Hz, 1H), 4.44 (d,
J = 6.9 Hz, 1H), 4.9 (br, 1H), 5.29 (s, 2H), 6.9 (br, 1H), 7.29 (s, 5H), 7.4 (br, 1H).
13C NMR (CDCl3, 100 MHz) d ppm 16.8, 22.3, 23.1, 31.8, 42.2, 59.5, 64.9, 65.2,
127.8, 128.1, 129.3, 140.3, 156.7, 170.8, 198.7. HRMS (ESI) calcd for
C19H28N2O4S m/z 403.1667 [M+Na], Found 403.1669.C, 59.96; H, 7.44; N,
7.39; O, 16.80; S, 8.41.