2-Mercaptopyridone 1-oxide-based uronium salts: New peptide coupling reagents

Full text of DOI:10.1021/jo990660q

8936
J . Org. Chem. 1999, 64, 8936-8939
Notes
Ch a r t 1
2-Mer ca p top yr id on e 1-Oxid e-Ba sed
Ur on iu m Sa lts: New P ep tid e Cou p lin g
Rea gen ts1,2
Miguel A. Baile´n, Rafael Chinchilla,
David J . Dodsworth, and Carmen Na´jera*,†
Departamento de Quı´mica Orga´nica, Universidad de
Alicante, Apartado 99, 03080 Alicante, Spain
Received April 20, 1999
Peptide coupling reagents have been important targets
in the last 20 years. The common problems encountered
in the amide synthesis are extensive racemization of the
amino acid components, usually through oxazolone for-
mation, undesired side reactions such as diketopiperazine
cyclization and guanidino or N-carboxyanhydride forma-
tion, and low coupling rates especially under solid-phase
conditions.3 Coupling reagents can be divided into the
following groups: (a) carbodiimide or another coupling
reagent in combination with additives such as 1-hydroxy-
benzotriazole (HOBt, 1)4 (see Chart 1) or 1-hydroxy-7-
azabenzotriazole (HOAt, 2);5 (b) aminium salts 3 derived
from HOBt (e.g. HBTU,6 TBTU,7 HBPyU,8 HBPipU9) or
HOAt5 (e.g. HATU, HAPyTU, HAPyU, HAPipU, HAM-
DU, HAMTU); (c) phosphonium salts 4 derived from
HOBt (e.g. BOP,10 PyBOP11) or HOAt5 (e.g. AOP, PyAOP);
(d) halouronium or halophosphonium salts 5 (e.g. Py-
Clop,12 PyBrop,12 PyClU,12 BroP,13 CIP,14 TFFH,15 BT-
FFH15); (e) reagents forming mixed anhydrides (e.g.
ClCO2Bui,16 PPA,17 BOP-Cl,18 Dpp-Cl19); (f) hydroxamic
acid-type reagents, in combination with a carbodiimide,
such as N-hydroxysuccinimide (HONSu),20 3-hydroxy-3,4-
dihydro-4-oxo-1,2,3-benzotriazine (HODhbt),22 N-hydroxy-
phthalimide (HOPht),22 N-hydroxy-5-norbornene-2,3-di-
† Fax no.: +34-965903549. E-mail: cnajera@ua.es. URL:
www.ua.es/dept.quimorg.
(1) A preliminary account of this work was presented at the 25th
European Peptide Symposium, Budapest, Hungary, August 30-
September 4, 1998, P-047.
(2) Abbreviations used in this paper: AOP, (7-azabenzotriazol-1-
yl)oxytris(dimethylamonino)phosphonium hexafluorophosphate; BOP,
benzotriazol-1-yloxytris(dimethylamino)phosphonium hexafluorophos-
phate; BOP-Cl, bis(2-oxo-3-oxazolidinyl)phosphinic chloride; BroP,
bromotris(dimethylamino)phosphonium hexafluorophosphate; BTFFH,
bis(tetramethylenefluoroformamidinium) hexafluorophosphate; CIP,
2-chloro-1,3-dimethylimidazolidinium hexafluorophosphate; DppCl,
diphenylphosphinic chloride; HAMDU, O-(7-azabenzotriazol-1-yl)-1,3-
dimethyl-1,3-dimethyleneuronium hexafluorophosphate; HAMTU, O-(7-
azabenzotriazol-1-yl)-1,3-dimethyl-1,3-trimethyleneuronium hexaflu-
orophosphate; HAPipU, O-(7-azabenzotriazol-1-yl)-1,1,3,3-bis(tetra-
methylene)uronium hexafluorophosphate; HAPyTU, S-(7-azabenzo-
triazol-1-yl)-1,1,3,3-bis(tetramethylene)thiouronium hexafluorophos-
phate; HAPyU, O-(7-azabenzotriazol-1-yl)-1,1,3,3-bis(tetramethylene)-
uronium hexafluorophosphate; HATU, O-(7-azabenzotriazol-1-yl)-
1,1,3,3-tetramethyluronium hexafluorophosphate; HBPipU, O-(benzotri-
azol-1-yl)-1,1,3,3-bis(pentamethylene)uronium hexafluorophosphate;
HBPyU, O-(benzotriazol-1-yl)-1,1,3,3-bis(tetramethylene)uronium hexa-
fluorophosphate; HBTU, O-(benzotriazol-1-yl)-1,1,3,3-tetramethyluro-
nium hexafluorophosphate; HOTT, S-(1-Oxido-2-pyridinyl)-1,1,3,3-
tetramethylthiouronium hexafluorophosphate; PyAOP, (7-azabenzo-
triazol-1-yl)oxytris(pyrrolidino)phosphonium hexafluorophosphate; Py-
BOP, benzotriazol-1-yloxy-tripyrrolidinophosphonium hexafluorophos-
phate; PyBrop, bromotripyrrolidinophosphonium hexafluorophosphate;
PyCloP, chlorotripyrrolidinophosphonium hexafluorophosphate; Py-
ClU, chloro-1,1,3,3-bis(tetramethylene)formamidinium hexafluorophos-
phate; PPA, propanephosphoric acid anhydride; TBTU, O-(benzotriazol-
1-yl)-1,1,3,3-tetramethyluronium tetrafluoroborate; TDBTU, O-(3,4-
dihydro-4-oxo-1,2,3-benzotriazin-3-yl)-1,1,3,3-tetramethyluronium
tetrafluoroborate; TFFH, tetramethylfluoroformamidinium hexafluo-
rophosphate; TOTT, S-(1-oxido-2-pyridinyl)-1,1,3,3-tetramethylthio-
uronium tetrafluoroborate.
(5) (a) Carpino, L. A. J . Am. Chem. Soc. 1993, 115, 4397. (b) Carpino,
L. A.; El-Faham, A.; Minor, C. A.; Albericio, F. J . Chem. Soc., Chem.
Commun. 1994, 201. (c) Carpino, L. A.; El-Faham, A.; Albericio, F.
Tetrahedron Lett. 1994, 35, 2279-2282. (d) Klose, J .; El-Faham, A.;
Henklein, P.; Carpino, L. A.; Bienert, M. Tetrahedron Lett. 1999, 40,
2045.
(6) Dourtoglou, V.; Gross, B.; Lambropoulou, V.; Zioudrou, C.
Synthesis 1984, 572.
(7) Knorr, R.; Trzeciak, A.; Bannwarth, W.; Gillessen, D. Tetrahe-
dron Lett. 1989, 30, 1927.
(8) Chen, S.; Xu, J . Tetrahedron Lett. 1992, 33, 647.
(9) Henklein, P.; Beyermann, M.; Bienert, M.; Knorr, R. In Peptides
1990; Proceedings of the European Peptide Symposium; Giralt, E.,
Andrew, D., Eds.; Escom Science Publishers BV: Leiden, The Neth-
erlands, 1990; pp 67-68.
(10) Castro, B.; Dormoy, J .-R. Tetrahedron Lett. 1973, 3243.
(11) Coste, J .; Le-Nguyen, D.; Castro, B. Tetrahedron Lett. 1990,
31, 205.
(12) Coste, J .; Frerot, E.; J ouin, P.; Castro, B. Tetrahedron Lett.
1991, 32, 1967.
(13) Coste, J .; Dufour, M.-N.; Pantaloni, A.; Castro, B. Tetrahedron
Lett. 1990, 31, 669.
(14) Akaji, K.; Kuriyama, N.; Kiso, Y. Tetrahedron Lett. 1994, 35,
3315.
(15) El-Faham, A. Chem. Lett. 1998, 671.
(16) Vaughan, J . R.; Osato, R. L. J . Am. Chem. Soc. 1951, 73, 5553.
(17) Wissmann, H.; Kleiner, H. J . Angew. Chem., Int. Ed. Engl. 1980,
19, 133.
(18) (a) Diago-Meseguer, J .; Palomo-Coll, A. L.; Ferna´ndez-Lizarbe,
J . R.; Zugaza-Bilbao, A. Synthesis 1980, 547. (b) Tung, R. D.; Dhaon,
M. K.; Rich, D. H. J . Org. Chem. 1986, 51, 3350.
(19) Galpin, I. J .; Mohommed, A. K. A.; Patel, A. Tetrahedron Lett.
1988, 44, 1685.
(3) (a) Humphrey, J . M.; Chamberlain, A. R. Chem. Rev. 1997, 97,
2243. (b) Albericio, F.; Bofill, J . M.; El-Faham, A.; Kates, S. A. J . Org.
Chem. 1998, 63, 9678.
(20) Anderson, G. W.; Zimmerman, J . E.; Callahan, F. H. J . Am.
Chem. Soc. 1963, 85, 3039.
(4) Ko¨nig, W.; Geiger, R. Chem. Ber. 1970, 103, 788.
(21) Ko¨nig, W.; Geiger, R. Chem. Ber. 1970, 103, 2024.
10.1021/jo990660q CCC: $18.00 © 1999 American Chemical Society
Published on Web 11/04/1999

Notes
J . Org. Chem., Vol. 64, No. 24, 1999 8937
Sch em e 1
structure, confirming thus the pyridinium N-oxide struc-
ture for 9.28
The uronium salts obtained 9 were used as new peptide
coupling reagents for the preparation of a series of di-
and tripeptides. Thus, different Boc-, Cbz- and Bz-N-
protected amino acids or dipeptides reacted with ethyl
or methyl ester amino acid hydrochlorides in the presence
of the uronium salts 9 and triethylamine as base in
acetonitrile as solvent, affording the peptides with the
yields shown in Table 1. The results obtained show, in
most cases, few differences in the final yield when 9a or
9b was used. Only a better solubility in acetonitrile was
observed when 9b was used. Furthermore, acetonitrile
seems to be the solvent of choice, as other solvents such
as dichloromethane or DMF lowered considerably the
final yields. After workup, the corresponding crude
peptides were isolated free of any impurity (1H NMR, 300
MHz).
When difficult couplings using R,R-dialkyl amino acids
such as R-aminoisobutyric acid (Aib) were performed
(Table 1, entries 13-18), the uronium salt 9b afforded
generally higher yields than 9a (Table 1, compare entries
15 and 16, and entries 17 and 18). The yields of these
obtained Aib-containing peptides are, in general, higher
than when using other previously employed coupling
reagents such as CIP or PyBroP,14 specially with N-Boc-
protected amino acids (Table 1, entries 13-16), which are
particularly prone to N-carboxyanhydride formation.32
Moreover, in these difficult cases yields using 9b were
not very different than when using other known high-
priced uronium salts. For example, when HATU was
employed as coupling agent, the dipeptide Cbz-Val-Aib-
OMe was obtained in 89% yield, whereas using 9b the
yield was 80% (Table 1, entry 18). In addition, hindered
N-methylated amino acids, such as Cbz-MeVal-OH,
which usually proves resistant in coupling reactions,33
afforded good isolated yields of the corresponding pep-
tides when coupled with Val-OMe using 9a or 9b (Table
1, entries 19 and 20).
carboxylic acid imide (HONB),23 and N-hydroxy-1,2-
dihydro-2-oxopyridine (HOPyr, 6);24 (g) hydroxamic acid-
based uronium salts such as O-(1,2-dihydro-2-oxo-1-
pyridyl)-1,1,3,3-tetramethyluronium tetrafluoroborate
(TPTU, 7).7 In relation with this latter reagent, we
envisaged that 2-mercaptopyridine 1-oxide (8) could
represent, from an economical point of view, an interest-
ing alternative to HOPyr (6) and other N-hydroxy deriva-
tives for the preparation of peptide coupling reagents.
2-Mercaptopyridine 1-oxide (8) has been mainly used for
the preparation of Barton thiohydroxamate esters, ad-
equate precursors of radicals by photolysis.25 In connec-
tion with our project on the development of new cheaper
reagents for large-scale peptide couplings, we have found
that effectively compound 8 is an appropriate starting
material for the preparation of a new class of uronium
salts 926 related to HOBt and HOAt as well as to
hydroxamic acid-type reagents.
The uronium salts 9 were prepared by the one-pot
phosgene-free procedure outlined in Scheme 1. Thus, the
reaction of N,N,N′,N′-tetramethylurea (10) with oxalyl
chloride in the presence of a catalytic amount of DMF
afforded the chlorouronium salt. This crude compound
was treated with potassium hexafluorophosphate or
sodium tetrafluoroborate yielding the corresponding salts
11a or 11b, which reacted with N-hydroxy-2-pyridineth-
ione (8) in the presence of triethylamine to give the
uronium salts 9a (HOTT) or 9b (TOTT), in 75 and 57%
isolated overall yields, respectively. The chlorouronium
salts 11 could also be prepared by reaction of the urea
with the more expensive triphosgene, but no difference
in the final overall yields of 9 was observed. In addition,
the bidentate nucleophilic character of thiopyridone 8
would allow two possible structures for the resulting final
product. However, the disappearance of the typical CdS
signal in the 13C NMR spectra of the final uronium salts,
and the presence of a deshielded signal at δ ) 8.52 ppm
Using Young’s test29 (the coupling of Bz-Leu-OH and
Gly-OEt‚HCl, Table 1, entries 9 and 10) and the Anteu-
nis’ test30 (the coupling of Cbz-Gly-Phe-OH and Val-
OMe‚HCl, Table 1, entries 11 and 12), the extent of
racemization with the two reagents was examined. It is
interesting that, in the case of 9b, only 3.7% racemization
was observed using the Young test (Table 1, entry 10), a
lower value than when using other HOBt-based uronium
salts such as BOP (20%), PyBOP (15%), or HBTU
(12.7%),8,11 or even HOAt-derived salts such as HATU,
which afforded a 20% racemization. In addition, when
Anteunis’ test was performed using the HOPyr (6)-
derived TPTU (7) as coupling reagent, which is chemi-
cally related to reagents 9, a higher yield of the corre-
sponding tripeptide was obtained using 7 (98%) but also
a higher degree of epimerization was shown (6.5:1
diastereomer ratio using 7, 10:1 diastereomer ratio using
1
corresponding to an aromatic H6 in the H NMR spec-
tra,27 ruled out the alternative thiopyridone-derived
(22) Nefkens, G. H. L.; Tesser, G. F. J . Am. Chem. Soc. 1961, 83,
1263.
(23) Fujino, M.; Kobayashi, S.; Obayashi, M.; Fukuda, T.; Shina-
gawa, S.; Nishimura, O. Chem. Pharm. Bull. 1974, 22, 1867.
(24) Ko¨nig, W.; Geiger, R. Chem. Ber. 1973, 106, 3626.
(25) (a) Barton, D. H. R. In Half a Century of Free Radical
Chemistry; Cambridge University Press: Cambridge, U.K., 1993. (b)
Crich, D.; Quintere, L. Chem. Rev. 1989, 89, 1413. (c) Barton, D. H.
R.; Zard, S. Z. Pure Appl. Chem. 1986, 58, 675.
(26) During the preparation of this article, the synthesis of the
hexafluorophosphate derivative 9a (HOTT) using phosgene, together
with its applications to the preparation of hindered Barton esters was
described: Garner, P.; Anderson, J . T.; Dey, S.; Youngs, W. J .; Galat,
K. J . Org. Chem. 1998, 63, 5732.
1
9, H NMR 500 MHz; see Table 1, entries 11 and 12).
The uronium salts 9 were also employed as amide-
(28) The X-ray structure of 9a has been reported.26
(29) Williams, M. W.; Young, G. T. J . Chem. Soc. 1963, 881.
(30) Van der Auwers, C.; Vandamme, S.; Anteunis, M. J . O. J . Pept.
Protein Res. 1987, 29, 464.
(31) Fre´rot, E.; Coste, J .; Pantaloni, A.; Dufour, M.-N.; J ouin, P.
Tetrahedron 1991, 47, 259.
(32) Fre´rot, E.; Coste, J .; Poncet, J .; J ouin, P.; Castro, B. Tetrahedron
Lett. 1992, 33, 2815
(33) Coste, J .; Fre´rot, E.; J ouin, P. J . Org. Chem. 1994, 59, 2437.
(27) Hartung, J .; Kneuer, R.; Schwarz, M.; Svoboda, I.; Fueb, H. Eur.
J . Org. Chem. 1999, 97, 7.

8938 J . Org. Chem., Vol. 64, No. 24, 1999
Ta ble 1. P r ep a r ed P ep tid es Usin g Com p ou n d s 9 a s Cou p lin g Rea gen ts
Notes
25 b
entry
reagents
t (h)
peptides
yields (%)a
mpb (°C)
[R]D
(c, solvent)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
9a
9b
9a
9b
9a
9b
9a
9b
9a
9b
9a
9b
9a
9b
9a
9b
9a
9b
9a
9b
3
Boc-Gly-Gly-OMe
89
87
96
97
81
78
85
85
71
69
68
71
58
44
35
67
60
80
80i
82i
207-209
9
9
9
9
9
4
4
4
9
Boc-Gly-Ala-OMe
Boc-Ala-Ala-OMe
Cbz-Val-Val-OMec
Bz-Leu-Gly-OEtd
Cbz-Gly-Phe-Val-OMeg
Boc-Aib-Val-OMe
Boc-Val-Aib-OMe
Cbz-Val-Aib-OMeh
Cbz-MeVal-Val-OMe
168-170
109-110
110-111
-16.0 (1, AcOEt)
-50.2 (1, EtOH)
-18.7 (1, EtOH)
-22.1 (1, EtOH)
-29.3 (3.1, EtOH)e
-31.5 (3.1, EtOH)f
151-152
155-156
110-111
148-149
88-89
19.5 (1, EtOH)
-17.0 (1, EtOH)
-23.0 (1, EtOH)
-78.8 (1, EtOH)j
Isolated yield based on the starting amino acids. Measured from the crude peptide without further purification. c Lit.11 107-109 °C;
a
b
20
d
20
[R]D -21 (c 1, EtOH). Young’s test: lit.29 mp ) 156.7-157 °C; [R]D -34 (c 3.1, EtOH). e 7% of epimerization. f 3.7% epimerization.
g
h
Anteunis’ test30 (underline representing the created bond): 10:1 diastereomer ratio (1H NMR, 500 MHz). Lit.31 mp ) 83-84 °C; [R]D
i
j
20
-24 (c 1, EtOH). Isolated yields after flash chromatography (silica gel). Lit.32 [R]D -90.0 (c 1, EtOH).
Ta ble 2. Com p ou n d s 9 a s Am id a tion Rea gen ts
Exp er im en ta l Section
amide
Gen er a l Meth od s. N-Hydroxy-2-pyridinethione (8) can be
obtained in a cheaper way in 85% yield from an aqueous 40%
solution of sodium 2-pyridinethiol 1-oxide (Fluka) after acidifica-
tion with concentrated HCl, following a reported procedure.34
All other reagents were commercially available and used without
further purification. The prepared peptides and amides were
characterized by NMR (1H and 13C).
entry
acid
amine
reagent
yields (%)a,b
1
2
3
4
5
6
7
8
9
PhCO2H
PhCO2H
PhCO2H
PhCO2H
PhCO2H
PhCO2H
PhCO2H
PhCO2H
C10H21CO2H
n-BuNH2
n-BuNH2
i-PrNH2
t-BuNH2
C6H11NH2
C6H11NH2
Piperidine
Piperidine
n-BuNH2
i-PrNH2
9a
9b
9a
9a
9a
9b
9a
9b
9a
9a
9a
9a
90
87
83
45
95
90
86
86
84
93
84
89
Syn th esis of S-(1-Oxid o-2-p yr id in yl)-1,1,3,3-tetr a m eth yl-
u r on iu m Hexa flu or op h osp h a te a n d Tetr a flu or obor a te
(9a , 9b). To a solution of 1,1,3,3-tetramethylurea (4.8 mL, 40
mmol) and DMF (0.3 mL) in CH2Cl2 (40 mL) was added dropwise
oxalyl chloride (4.2 mL, 48 mmol) at room temperature. The
solution was stirred for 1 h at room temperature and then
refluxed for 4 h. The solvent was evaporated (15 Torr), and the
resulting solid was stirred with portions of CH2Cl2 (2 × 10 mL)
followed by evaporation of the organics (15 Torr) after each
treatment. The obtained crude chlorouronium salt was dissolved
in MeCN (40 mL), and KPF6 (for 9a , 8.8 g, 48 mmol) or NaBF4
(for 9b, 5.27 g, 48 mmol) was added. The mixture was stirred at
room temperature for 24 h, and to the resulting suspension was
added N-hydroxy-2-pyridinethione (8) (5.1 g, 40 mmol). Triethyl-
amine (6.7 mL, 48 mmol) was added dropwise keeping the
temperature below 25 °C, and the resulting suspension was
stirred at room temperature for 5 h and at 45 °C for 1 h. The
solution was filtered through a plug of Celite, the solvents were
evaporated (15 Torr) and the uronium salts 9a ,b were obtained
after crystallization with MeOH/2-propanol (9a , 11 g, 75%; 9b,
7.1 g, 57%).
10
11
12
C
10H21CO2H
C10H21CO2H
10H21CO2H
t-BuNH2
PhNH2
C
Isolated yield of the pure (1H NMR, 300 MHz) crude amide
a
based on the starting acid. bNonoptimized yields.
forming reagents in the reaction between carboxylic acids
and amines (see Table 2). Thus, aromatic (Table 2, entries
1-8) and aliphatic (Table 2, entries 9-12) carboxylic
acids were condensed with differently substituted amines
in the presence of reagents 9 in DMF as solvent, gener-
ally with similar high yields in both cases. The final crude
amides showed pure by 1H NMR (300 MHz) analysis after
the usual work up.
S-(1-Oxido-2-pyridinyl)-1,1,3,3-tetramethyluronium Hexaflu-
or op h osp h a te (9a ): mp 115-117 °C; IR (KBr) 3066, 1618, 842
We conclude that the reagents 9a (HOTT) and 9b
(TOTT), which can be easily prepared in a one-pot
procedure, without using the very toxic phosgene, could
be promising reagents for peptide coupling and amidation
reactions. The simplicity of the synthesis and the lower
price of the starting material, compared for example with
HOBt, HOAt, or HOPyr, could make these thiopyridone-
derived reagents very competitive with other presently
being used coupling reagents. Specially TOTT (9b) can
be recommended, not only because of its generally better
performance in difficult couplings but also from an
economical point of view due to the much lower price of
the starting tetrafluoroborate salt.
cm-1 1H NMR (CD3CN, 300 MHz) δ 8.52 (1 H, m), 7.87 (1 H,
;
m), 7.58-7.50 (2 H, m), 3.22 (12 H, s); 13C NMR (CD3CN, 75
MHz) δ 171.7, 140.8, 131.2, 128.0, 127.1, 44.0. Anal. Calcd for
C10H16N3OSPF6: C, 32.34; H, 4.35; N, 11.32. Found: C, 32.49;
H, 4.74; N, 11.19.
S-(1-Oxid o-2-p yr id in yl)-1,1,3,3-tetr a m eth ylu r on iu m Tet-
r a flu or obor a te (9b): mp 107-108 °C; IR (KBr) 3098, 1623,
1041 cm-1; 1H NMR (CD3CN, 300 MHz) δ 8.52 (1 H, m), 7.87 (1
H, m), 7.61-7.50 (2 H, m), 3.22 (12 H, s); 13C NMR (CD3CN, 75
MHz) δ 171.7, 140.8, 131.2, 128.0, 127.1, 44.0. Anal. Calcd for
C
10H16N3OSBF4: C, 38.33; H, 5.15; N, 13.42; S, 8.63. Found: C,
38.44; H, 5.14; N, 13.23.
(34) Barton, D. H. R.; MacKinnon, J .; Perchet, R. N.; Tse, C.-L. Org.
Synth. 1997, 75, 124.

Notes
P ep tid e Cou p lin g Rea ction s. Gen er a l P r oced u r e. A
J . Org. Chem., Vol. 64, No. 24, 1999 8939
sponding amine (1 mmol) was added, and the resulting mixture
was stirred at room temperature for 4 h. Saturated NaCl was
added, and the mixture was extracted with AcOEt. The organic
layers were washed with 2 N HCl, saturated NaHCO3 and water.
The organics were dried (Na2SO4), filtered, and evaporated (15
Torr) affording the corresponding pure amides (see Table 2).
solution of the N-protected amino acid (1 mmol), the amino acid
ester hydrochloride (1 mmol), triethylamine (0.28 mL, 2 mmol),
and 9a or 9b (1 mmol) in MeCN (10 mL) was stirred at room
temperature until disappearance of the free esterified amino acid
(TLC, ninhydrin). Saturated NaCl was added, and the mixture
was extracted with AcOEt. The organic layers were washed with
2 N HCl, saturated NaHCO3, and water. The organics were dried
(Na2SO4), filtered, and evaporated (15 Torr) affording the
corresponding peptides (see Table 1).
Ack n ow led gm en t. We are grateful to the Spanish
Ministerio de Educacio´n y Ciencia (Grant PB97-0123)
for financial support.
Am id a tion Rea ction s. Gen er a l P r oced u r e. A solution of
the carboxylic acid (1 mmol) and 9a or 9b (1 mmol) in DMF (10
mL) was stirred at room temperature for 30 min. The corre-
J O990660Q