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27a
General Procedure for the Preparation of Peptide Acids
A
solution of Nα-protected amino acid (1 mmol)
in dry CH2Cl2 (5 mL) was cooled to 0 ˚C,
EDC (1 mmol), HOBt (1.2 mmol), and O,N-bis-TMS-amino acid (1.5 mmol) were added.
The reaction mixture was stirred for 3-4 h (TLC analysis),
and then evaporation of the solvent and acidification with 1 M HCl
furnished pure peptide acid.
27b For the preparation of O,N-bis-TMS-amino
acids, see: Tantry SJ.
Vasanthakumar G.-R.
Sureshbabu VV.
Lett. Pept.
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51
28
General Procedure
for the Synthesis of Amino/Peptide Thioacids
To
a DMF solution of an acid (1.0 mmol), EDC (1.1 equiv) was added
at 0 ˚C under a nitrogen atmosphere. After stirring
for 10 min, finely ground Na2S (3 equiv) was added to
the reaction mixture which was allowed for stir for 3-4
h until the disappearance of the starting material (TLC analysis).
The residue was dissolved in EtOAc (15 mL), and the solution was
then carefully acidified at 0 ˚C to a pH of 3 by
using 1 M KHSO4. The organic layer was then immediately
separated and removed under reduced pressure. The crude product
was triturated with Et2O or recrystallized with THF-H2O
to obtain pure thioacid.
29
Fmoc-Ile-COSH
Yellow
solid; mp 81-83 ˚C. IR (KBr): νmax = 1689,
1739, 2550, 3342 cm-¹. R
f
= 0.39
(EtOAc-n-hexane = 60:40).
RP-HPLC: t
R = 15.2
(60-100% MeCN, 30 min). ESI-HRMS:
m/z calcd for C21H23NO3S:
392.1296 [M + Na]+;
found: 392.1290. ¹H NMR (400 MHz, CDCl3): δ = 0.88
(t, J = 5.6 Hz,
3 H), 0.98 (d, J = 3.8
Hz, 3 H), 1.12-1.24 (m, 2 H), 2.38-2.47 (m, 1
H), 4.28 (d, J = 6.7
Hz, 1 H), 4.37 (d, J = 7.1
Hz, 1 H), 4.61 (d, J = 4.4
Hz, 2 H), 5.91 (br s, 1 H), 7.43 (br s, 1 H), 7.26-7.84
(m, 8 H). ¹³C NMR (100 MHz, CDCl3): δ = 10.8, 14.3,
24.1, 37.0, 46.4, 65.9, 72.8, 125.9, 127.3, 128.6, 128.9, 139.2,
142.6, 155.2, 197.2.
30
Fmoc-Ala-Phe-COSH
White
solid; mp 126-128 ˚C. IR (KBr): νmax = 1681,
1748, 1768, 2549, 3328 cm-¹. R
f
= 0.53
(CHCl3-MeOH = 80:20). RP-HPLC: t
R = 11.4 (60-100% MeCN,
30 min). ESI-HRMS: m/z calcd
for C27H26N2O4S: 497.1511 [M + Na]+; found:
497.1501. ¹H NMR (400 MHz, CDCl3): δ = 1.2
(d, J = 4.8
Hz, 3 H), 2.6 (d, J = 5.6
Hz, 2 H), 2.8 (br s, 1 H), 3.38 (t, J = 7.4
Hz, 1 H), 3.6 (br s, 1 H), 3.9 (t, J = 6.9
Hz, 2 H), 4.1 (m, 1 H), 4.3 (m, 1 H), 6.32 (br s, 1 H), 7.1 (br
s, 1 H), 7.2-7.9 (m, 13 H). ¹³C
NMR (100 MHz, CDCl3): δ = 17.2, 37.4,
46.8, 51.2, 67.9, 69.7, 125.7, 126.8, 127.2, 127.9, 128.4, 128.9,
131.2, 139.1, 141.4, 143.2, 155.7, 172.1, 197.2.
31 Chiral-HPLC analyses were carried
out employing Chiralpak IA, 250 × 4.6
mm; solvent: hexane-EtOH (7:3); flow rate: 1.0 mL/min.