Synthesis of 2-(4,6-Dimethoxy-1,3,5-triazin-2-yloxyimino) derivatives: Application in solution peptide synthesis

Article abstract of DOI:10.3390/molecules15129403

A new class of 1,3,5-triazinyloxyimino derivatives were prepared, characterized and tested for reactivity in solution peptide synthesis. The new triazinyloxyimino derivatives failed to activate the carboxyl group during formation of peptide bonds, but gav

Full text of DOI:10.3390/molecules15129403

Molecules 2010, 15, 9403-9417; doi:10.3390/molecules15129403
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molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Article
Synthesis of 2-(4,6-Dimethoxy-1,3,5-triazin-2-yloxyimino)
Derivatives: Application in Solution Peptide Synthesis
Tarfah I. Al-Warhi 1,*, Hassan M.A. AL-Hazimi 2, Ayman El-Faham 2,3 and
Fernando Albericio 4,5,6
1
Women Students-Medical Studies and Sciences Sections, Chemistry Department, College of
Science, King Saud University, P.O. Box 22452, Riyadh 11495, Saudi Arabia
2
Chemistry Department, College of Science, King Saud University, P.O. Box 2455, Riyadh 11451,
Saudi Arabia
3
Chemistry Department, Faculty of Science, Alexandria University, 426 Ibrahimia, 21321
Alexandria, Egypt
4
Institute for Research in Biomedicine, Barcelona Science Park, Baldiri Reixac 10, Barcelona
08028, Spain
5
CIBER-BBN, Networking Centre on Bioengineering, Biomaterials and Nanomedicine, Barcelona
Science Park, Baldiri Reixac 10, Barcelona 08028, Spain
6
Department of Organic Chemistry, University of Barcelona, Martí i Franqués 1-11, Barcelona
08028, Spain
* Author to whom correspondence should be addressed; E-Mail: tarfah-w@hotmail.com.
Received: 29 November 2010; in revised form: 14 December 2010 / Accepted: 15 December 2010 /
Published: 20 December 2010
Abstract: A new class of 1,3,5-triazinyloxyimino derivatives were prepared, characterized
and tested for reactivity in solution peptide synthesis. The new triazinyloxyimino
derivatives failed to activate the carboxyl group during formation of peptide bonds, but
gave the corresponding N-triazinyl amino acid derivatives as a major product. The oxyma
(ethyl 2-cyano-2-(hydroxyimino)acetate) uronium salt was superior to other uronium salts
in terms of racemization, while 2-chloro-4,6-dimethoxy-1,3,5-triazine (CDMT, 9) gave the
best results.
Keywords: coupling reagents; triazinyloximino derivatives; solution phase peptide
synthesis; uronium salt; racemization

Molecules 2010, 15
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1. Introduction
For many years, 1-hydroxybenzotriazole (HOBt, 1, Figure 1) has been used as a coupling reagent in
peptide synthesis, especially in combination with dicyclohexylcarbodiimide (DCC, 2, Figure 1) [1-4].
Later, other benzotriazole derivatives such as 6-chloro-1-hydroxybenzotriazole (6-Cl-HOBt, 3, Figure
1) and 1-hydroxy-7-azabenzotriazole (HOAt, 4, Figure 1) [5,6] have been introduced and shown more
efficient that the parent (HOBt, 1).
Figure 1. Structure of additives and DCC.
N
N
N
N
N
N
N
N
N
Cl
N
N
OH
1, HOBt
C
OH
3, 6-Cl-HOBt
N
OH
2, DCC
4, HOAt
During the last decade, iminium/uronium and phosphonium salts of 1-hydroxybenzotriazole and 1-
hydroxy-7-azabenzotriazole, 5 and 6 (Figure 2) have shown the supremacy of these kind of derivatives
over carbodiimides in the presence of an additive [5-33].
Despite their widespread use, these reagents are not without ptoblems. The explosive properties of
1-hydroxybenzotriazoles are often not referenced in literature. Sometime, Material Safety Data Sheets
warn that 1-hydroxybenzotriazoles may be unstable, with a relatively high sensitivity to friction and
sparks, but in most cases, there is no mention of how sensitive such substances are to heating under
confinement and no warning is given with respect to their ability to propagate a deflagration or
a detonation.
More recently a new class of coupling reagents such as 7 and 8 (Figure 2) based on changes to the
carbon skeleton structure [34-36] gave more marked increases in coupling efficiency, as well as
reduced racemization levels during the coupling step. Particularly noteworthy are the morpholino
derivatives 8, because the oxygen in the carbon skeleton increases the solubility as well as the
reactivity, which reduces the racemization during the coupling step [34-36].
Figure 2. Structure of uronium/iminium salts.
O
N
O
N
O
N
N
N
N
N
N
N
N
N
N
X
Me
X
X
X
Me
O
P(NR2)3
PF6
N
N
N
Me2N
NMe2
N
Me
O
Me
PF6
PF6
PF6
5a, X = CH
5b, X = N
7a, X = CH
7b, X = N
6a, X = CH
6b, X = N
8a, X = CH
8b, X = N

Molecules 2010, 15
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Although 2-chloro-4,6-dimethoxy-1,3,5-triazine (CDMT, 9) has been successfully applied for the
preparation of peptides, the mechanism of its participation in coupling reactions remains unknown
[37,38]. Furthermore, in order to be successfully stored the reagent itself must be of high purity, the
formation of gaseous products may generate a rapid pressure increase in the container, and
accordingly, precautions should be taken to avoid the serious risk of blowout of toxic gases.
Successful activation of carboxylic acids by means of CDMT (9) confirmed the feasibility of a
multistep process proceeding via triazinylammonium salts such as 10 and (DMTMM, 11) (Figure 3)
formed in situ in the presence of the appropriate amine [37,38].
Figure 3. Structure of triazinylammonium salts.
O
R''
N
+
R'
Cl-
R'''
R
N
Cl
N
N
N
N
MeO
N
OMe
R
N
11, DMTMM
10
The related triazine, 4-(4,6-dimethoxy-1,3,5-triazin-2-yl)-4-methylmorpholinium chloride
(DMTMM, 11), formed by a simple reaction of CDMT (9) with N-methylmorpholine (NMM), has
been found applications in amidation [38-42], esterification [42], glycosidation [43,44] and
phosphonylation [45] methodology. The present work presents the synthesis and application of a new
family of 1,3,5-triazine derivatives and their comparison with CDMT (9) as well as the uronium based
type coupling reagents [46].
2. Results and Discussion
The new triazinyloxyimino derivatives 12a-c were prepared from the reaction of the oxime
derivatives 13a-c with CDMT (9) in the presence of triethylamine. The potassium salts were used in
case of the synthesis of malononitrile and diethylmalonate derivatives (Scheme 1).
Scheme 1. Synthesis of triazinyloxyimino derivatives.
4'
5'
6'
3'
X
Y
X
Y
Et3N
Cl
N
Et3N
13a,c
N
O
N
O
O
N
N
N
N
2
OH
OH
O
2
N
N
N
N
6
X
Y
or
MeO
OMe
4
6
4
MeO
N
OMe
13b
(CDMT, 9)
MeO
N
OMe
N
OK
(DMTOPy, 12d)
13a, X = CN, Y = COOEt
(DMTOC,12a)
13b, X = Y = CN
(DMTODC, 12b)
(DMTOPyC, 12c)
4'
5'
3'
2'
13c, X = CN, Y =
6'
N

Molecules 2010, 15
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The pyridinyl derivative was prepared using the same conditions to afford 12d. All the products
12a-d have been identified on the basis of their spectroscopic data (see Experimental). In order to have
a good comparison to the triazinyloxyimino coupling reagents, some of the related uronium-type
coupling reagents were prepared according to the reported method [36] as shown in Scheme 2.
Scheme 2. Synthesis of uronium salts.
X
R2
R2
R1
N
N
X
N
Et3N/DCM
or KON-R
N
i) (COCl)2
ii) PF6
R1
Y
R1
R3
O
Cl
R4
O
+
R2
PF6
HO N
Y
N
N
R3
N
R3
R4
15a-f
PF6
R4
15a (HOTU), R1 = R2 = R3 = R4 = CH3; X = CN, Y = COOEt
15b (COMU), R1 = R2 = CH3, R3, R4 = CH2CH2OCH2CH2; X = CN, Y = COOEt
15c (HTODC), R1 = R2 = R3 = R4 = CH3; X = CN, Y = CN
15d (HDMODC), R1 = R2 = CH3, R3, R4 = CH2CH2OCH2CH2; X = Y = CN
15e (HTOPC), R1 = R2 = R3 = R4 = CH3; X = CN, Y = 2-pyridyl
15f (HTOPT), R1 = R2 = R3 = R4 = CH3; X,Y = COCH=CH-CH=CH
CH3
CH3
N
N
CH3
PF6
CH3
PF6
Cl
Cl
N
N
H3C
CH3
O
(DCMH, 14a)
(TCFH, 14b)
The uronium salt coupling reagents were prepared by reaction of the chloro salts DCMH 14a or
TCFH 14b with the oxime derivatives in the presence of triethylamine in DCM or with the potassium
salt of the oxime derivatives in acetonitrile to give the oxyimino uronium base coupling reagents 15a-f
(Scheme 2). To investigate the retention of configuration induced for the new coupling reagents,
several previously studied model peptide system 16a,b and 17a-c (Figure 4) were examined [36].
These models involve stepwise coupling and (2+1) segment coupling as well.
Figure 4. Model of peptide used in racemization studies.
Z-Phe-Ala-OMe
Z-Phe-Val-OMe
16a
16b
Z-Gly-Phe-Ala-OMe
17a
Z-Gly-Phe-Val-OMe
Z-Gly-Val-Val-OMe
17c
17b
Results comparing the coupling of Z-Phe-OH to H-Ala-OMe.HCl (18a) with different coupling
agents to afford the Z-Phe-Ala-OMe (16a) are indicated in Table 1. All the uronium type coupling
reagents (HOTU, 15a), (HTOPC, 15e), and (HTOPT, 15f), used gave less than 1% racemization
according to the NMR data but CDMT (9) gave about 15% of the racemized product (Table 1).

Molecules 2010, 15
Table 1. Yield (%) and Racemization (%) of Z-Phe-Ala-OMe (16a) using the Uronium
9407
Salts 15a,e,f and CDMT (9).
Coupling Reagent
(HOTU, 15a)
(HTOPC, 15e)
(HTOPT, 15f)
(CDMT, 9)
Yield %
100
54
66
55
DL %
<1
<1
<1
15
CH3 DL (d) at δ = 1.20 ppm; CH3 LL (d) at δ = 1.32 ppm.
The other triazinyloxyimino derivative (DMTOC, 12a) failed to give the expected product Z-Phe-
Ala-OMe (16a), due to the low activation for the carboxylic group and fast attack by the N-terminal of
the amino acid which led to formation of the N-triazinyl amino acid derivative methyl 2-(4,6-
dimethoxy-1,3,5-triazin-2-ylamino)propanoate (19a) in 77-85% yield, as confirmed by NMR data
(Scheme 3).
Scheme 3. Coupling of Z-Phe-OH with H-Ala-OMe.HCl (18a) using DMTOC (12a) and CDMT (9).
O
H
NMM
N
N
OCH3
+
Z-Phe-OH
H-Ala-OMe.HCl
H3CO
12a (DMTOC), 24hr, at rt
N
N
18a
19a
NMM
9 (CDMT)
OCH3
(15a,e,f)
or
Z-Phe-Ala-OMe
16a
Another example, the coupling of Z-Phe-OH to H-Val-OMe.HCl (18b) to afford the dipeptide Z-
Phe-Val-OMe (16b) showed the same behavior for the uronium salts and triazinyloxyimino derivatives
(Scheme 4, Table 2).
Scheme 4. Coupling of Z-Phe-OH with H-Val-OMe.HCl (18b) using triazinyloxyimino
derivatives and CDMT (9).
NMM
Z-Phe-OH
H-Val-OMe.HCl
+
HN
COOCH3
OCH3
X
18b
N
N
N
N
NMM
9 (CDMT)
(15a,e,f)
or
H3CO
N
MeO
N
OMe
19b
12a (DMTOC), X = ON=CCNCOOC2H5
12c (DMTOPyC), X = ON=CCN-2-pyridyl
Z-Phe-Val-OMe
16b
12d (DMTOPy), X = ONCOCH=CHCH=CH

Molecules 2010, 15
Table 2. Yield (%) and Racemization (%) of Z-Phe-Val-OMe (16b) using the Uronium
9408
Salts 15a,e,f and CDMT (9).
Coupling Reagent
(HOTU, 15a)
(HTOPC, 15e)
(HTOPT, 15f)
(CDMT, 9)
Yield %
DL %
<1
<1
<1
2.0
83
89
68
69
CH3 DL (dd) at δ = 0.66 ppm; CH3 LL (dd) at δ = 0.82 ppm.
The desired dipeptide product was not formed in case of the triazinyloxyimino derivatives
(DMTOC, 12a), (DMTOPyC, 12c), and (DMTOPy, 12d) but rather the side product methyl 2-(4,6-
dimethoxy-1,3,5-triazin-2-ylamino)-3-methylbutanoate (19b), due to the reaction of amino acid ester
with the coupling reagents as indicated by NMR. For the rather non-sensitive case of segment coupling
of Z-Gly-Phe-OH to H-Ala-OMe.HCl (18a), which leads to the tripeptide Z-Gly-Phe-Ala-OMe (17a),
the Oxyma (ethyl 2-cyano-2-(hydroxyimino)acetate) derivatives gave the best results compared with
others as observed from the NMR data (Scheme 5, Table 3).
Scheme 5. Coupling of Z-Gly-Phe-OH with H-Ala-OMe.HCl (18a) using
triazinyloxyimino derivatives and CDMT (9).
O
H
N
NMM
X
Z-Gly-Phe-OH
H-Ala-OMe.HCl
N
OCH3
+
H3CO
18a
N
N
N
N
N
19a
NMM
9 (CDMT)
OCH3
MeO
OMe
or (15a,e,f)
12a (DMTOC), X = ON=CCNCOOC2H5
12c (DMTOPyC), X = ON=CCN-2-pyridyl
12d (DMTOPy), X = ONCOCH=CHCH=CH
Z-Gly-Phe-Ala-OMe
17a
Table 3. Yield (%) and Racemization of (%) Z-Gly-Phe-Ala-OMe (17a) using the
Uronium Salts 15a,e,f and CDMT (9).
Coupling Reagent
(HOTU, 15a)
(HTOPC, 15e)
(HTOPT, 15f)
(CDMT, 9)
Yield %
63
DL %
<1
19
19.0
12.8
30
76
gummy
CH3 DL (d) at δ = 1.19 ppm; CH3 LL (d) at δ = 1.32 ppm.
The rest of the triazinyloxyimino derivatives (DMTOC, 12a), (DMTOPyC, 12c), and (DMTOPy,
12d) did not afford the desired product, giving the side product methyl 2-(4,6-dimethoxy-1,3,5-triazin-
2-ylamino)propanoate (19a) (Scheme 5) as a major product, as revealed from its NMR spectral data.
In the more sensitive cases of the formation of Z-Gly-Phe-Val-OMe (17b) and Z-Gly-Val-Val-OMe
(17c) the same results were obtained. The best results obtained were when the Oxyma derivative
(HOTU, 15a) was using as a coupling reagent, while CDMT (9) gave the highest racemization level

Molecules 2010, 15
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and the triazine coupling reagents failed in the formation of the target product, giving the same side
product 19b, which was obtained from the coupling of Z-Phe-OH with H-Val-OMe.HCl (18b) as
observed from the HPLC and NMR data (Table 4 and 5).
Table 4. Yield (%) and Racemization of (%) Z-Gly-Phe-Val-OMe (17b) using the
Uronium Salts 15a,e,f and CDMT (9).*
Coupling Reagent
(HOTU, 15a)
(HTOPC, 15e)
(HTOPT, 15f)
(CDMT, 9)
Yield %
DL %
< 0.1
26.19
6.15
80
81
78
63
50.0
* The racemization percent was detected by reverse phase HPLC, using a SunFire C18 column
(Waters) (5 m, 4.6 × 150 mm), linear gradient 10-90 0.036% TFA in CH3CN/0.045% TFA in H2O
over 8 min tR LL is 7.43 min, tR DL is 7.61 min as indicated from and authentic samples.
Table 5. Yield (%) and Racemization (%) of Z-Gly-Val-Val-OMe (17c) using the
Uronium Salts 15a,e,f and CDMT (9).*
Coupling Reagent
(HOTU, 15a)
(HTOPC, 15e)
(HTOPT, 15f)
(CDMT, 9)
Yield %
81
DL %
< 0.1
17.5
≈100
95
6.66
60
47.3
* The percent racemization was detected by reverse phase HPLC, using a SunFire C18 column
(Waters) (5 m, 4.6 × 150 mm), linear gradient 10-90 0.036% TFA in CH3CN/0.045% TFA in H2O
over 8 min tR LL is 6.90 min, tR DL is 7.07 min as indicated by authentic samples.
3. Experimental
3.1. General
All chemicals were used without further purification. Solvents are redistilled before use. N-
protected amino acids and esters were purchased from Novabiochem. Melting points are uncorrected
and were determined on a Gallenkamp hot stage. A Perkin Elmer Spectrum 1000 FT-IR Spectrometer
was used for recording infrared (IR) spectra of the prepared compounds as KBr pellets or in
spectroscopic grade dichloromethane. 1H- and 13C-NMR spectra of compounds were run on JEOL 400
MHz NMR spectrometer, in CD3COCD3, CDCl3 or DMSO-d6 at room temperature using TMS as
internal standard. The instruments are located at King Saud University, College of Science, Chemistry
Department. For analytical separations, characterization and determination of racemization, a
reverse-phase Waters 2695 HPLC separation module was used (Barcelona Science Park, University of
Barcelona, Spain) equipped with a SunFire C18 column (Waters, 5 m, 4.6 × 150 mm), 10-90% linear
gradient of 0.036% TFA in CH3CN/0.045% TFA in H2O and coupled to a Waters 2998 PDA-UV
detector. Chromatograms were processed with Empower software. In the detection of the percent
racemization as indicated by authentic samples. Peptide mass was detected by means of an HPLC-

Molecules 2010, 15
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PDA system as described above, coupled to a Water Micromass ZQ mass detector, using the
MassLynx 4.1 software. Elemental analyses were performed on a Perkin-Elmer 2400 elemental
analyzer, and the values found were within ±0.3% of the theoretical values. Oxime derivatives 13a-c
were prepared following the literature procedures [47,48].
3.2. General Synthesis of Triazine Coupling Reagents 12a-d from Oxime Derivatives
2-Chloro-4,6-dimethoxy-1,3,5-triazine (9, 20 mmol) was added to a solution of oxime (20 mmol)
and triethylamine (20 mmol) in dichloromethane (DCM, 200 mL) at 0 ºC. The reaction mixture was
stirred at the same temperature for 1 h and then at room temperature for 5-8 h. DCM (400 mL) was
added, and then the reaction mixture was washed twice with saturated aqueous NaCl (200 mL each).
Finally, the organic solvent was dried with anhydrous Na2SO4, filtered, and the solvent was removed
under reduced pressure. The crude product was recrystallized from DCM/hexane.
Ethyl-2-cyano-2-(4,6-dimethoxy-1,3,5-triazin-2-yloxyimino)acetate (DMTOC, 12a). White powder,
yield 4.87 g (86%), m.p. 110 ºC; IR (KBr): 2986.68 (aliph. CH), 2363.98, 2344.92 (C≡N), 1758.25
1
(C=O, ester), 1608.79 (C=N), 1286.58 (N-O), 1084.42 (C-O-C) cm-1; H-NMR (CDCl3): δ 1.44 (t,
J = 7.2 Hz, 3H, CH3), 4.11 (s, 6H, 2 OCH3), 4.50 (q, J = 7.2 Hz, 2H, CH2) ppm; 13C-NMR (CDCl3): δ
14.19 (CH3), 56.36 (2 OCH3), 64.77 (CH2), 106.95 (C≡N), 131.58 (C=N), 156.88 (C=O), 172.96 (C-
2), 174.18 (C-4,6) ppm; Elemental analysis, calculated for C10H11N5O5 (281.22): C, 42.71; H, 3.94; N,
24.90. Found: C, 42.46; H, 3.82; N, 24.66.
(4,6-Dimethoxy-1,3,5-triazin-2-yloxy)carbonimidoyl dicyanide (DMTODC, 12b). Snow white powder,
yield 3.11 g (66%), m.p. 74 ºC; IR (KBr): 2948.10 (aliph. CH), 2363.80, 2344.87 (C≡N), 1558.96
1
(C=N), 1467.87 (Ar. C=N), 1361.80 (Ar. C-N), 1222.97 (N-O), 1042.73 (C-O-C) cm-1; H-NMR
(CDCl3): δ 3.37 (s, 6H, 2 OCH3) ppm; 13C-NMR (CDCl3): δ 56.58 (2 OCH3), 105.11, 108.19 (2 C≡N),
114.80 (C=N), 172.34 (C-2), 174.21 (C-4,6) ppm; Elemental analysis, calculated for C8H6N6O3
(234.17): C, 41.03; H, 2.58; N, 35.89. Found: C, 41.16; H, 2.49; N, 35.68.
N-(4,6-Dimethoxy-1,3,5-triazin-2-yloxy)picolinimidoyl cyanide (DMTOPyC, 12c). Pink cotton solid,
yield 6.29 g (≈100%) from (10 mmol of starting material), m.p. 198 ºC; IR (KBr): 3468.01 (Ar. C-H),
2924.86 (aliph. CH), 2364.64, 2344.88 (C≡N), 1606.27 (C=N, C=C), 1560.18 (asym. C-O-C), 1361.76
(sym. C-O-C) cm-1; 1H-NMR (CDCl3): δ 4.13 (s, 6H, 2 OCH3), 7.52 (m, 1H, H-5'), 7.86 (dt, 1H, H-4'),
13
8.22 (d, J = 7.32 Hz, 1H, H-3'), 8.82 (d, J = 4.4 Hz, 1H, H-6') ppm; C-NMR (CDCl3): δ 56.11 (2
OCH3), 108.21 (C≡N), 122.26 (C-3'), 126.74 (C-5'), 137.21 (C-4'), 139.89 (C-6'), 147.42 (C-2'),
150.43 (C=N), 173.26 (C-2), 174.04 (C-4,6) ppm; Elemental analysis, calculated for C12H10N6O3
(286.25): C, 50.35; H, 3.52; N, 29.36. Found: C, 50.59; H, 3.42; N, 29.29.
1-(4,6-Dimethoxy-1,3,5-triazin-2-yloxy)pyridin-2(1H)-one (DMTOPy, 12d). The product was obtained
as a light tan powder, yield 4.28 g (85%), m.p. (104-106 ºC); IR (KBr): 3091.57 (Ar. C-H), 2953.80
(aliph. CH), 1678.35 (C=O), 1608.62 (C=N, C=C), 1558.30 (N-O), 1369.99, 1349.16 (Ar. C-N),
1280.22 (asym. C-O-C), 1084.36 (sym. C-O-C) cm-1; 1H-NMR (CDCl3): δ 3.96 (s, 6H, 2 OCH3), 6.25

Molecules 2010, 15
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(dt, 1H, H-5'), 6.75 (dd, 1H, H-3'), 7.42 (1H, H-4'), 7.53 (dd, H-6') ppm; 13C-NMR (CDCl3): δ 56.10 (2
OCH3), 105.39 (C-5'), 123.27 (C-3'), 135.45 (C-6'), 139.79 (C-4'), 157.23 (C=O), 173.96 (C-2), 174.11
(C-4,6) ppm; Elemental analysis, calculated for C10H10N4O4 (250.21): C, 48.00; H, 4.03; N, 22.39.
Found: C, 48.22; H, 4.09; N, 22.45.
3.3. General Synthesis of Oximo-Uronium Type Coupling Reagents 15a-f
To a solution of the oxime potassium salts of 13a-c (20 mmol) in acetonitrile (ACN) (50 mL) was
added to the chloro salts 14a,b (20 mmol) at 0 ºC. The reaction mixture was stirred at this temperature
30 min and stirred at room temperature for 6 h. Filter and wash with acetonitrile. The solvent was
concentrated to small volume (1/4) under reduced pressure, and then dry ether was added to afford the
product as a white solid in pure state [46].
O-[(Cyano(ethoxycarbonyl)methylidene)amino]-1,1,3,3-tetramethyluronium
hexafluorophosphate
(HOTU, 15a). Yield 6.32g (82%) using the potassium salt of the oxime, m.p. (135-137 ºC) (dec). The
triethylamine/oxime combinations gave a lower yield (69%); IR (KBr): 3003.32 (N-CH3), 2964.16
1
(aliph. CH), 2345.36 (C≡N, =N+), 1771.32 (C=O), 1702.40 (C=N), 1266.93 (N-O) cm-1; H-NMR
13
(acetone-d6):  1.37 (t, 3H, CH3), 3.37 (s, 12H, 4 CH3), 4.82 (q, 2H, CH2) ppm; C-NMR (acetone-
d6):  13.46 (CH3), 40.71 (4 CH3), 64.56 (CH2), 106.78 (C≡N), 135.09 (C=N), 156.11 (C=O), 161.43
(C+) ppm; Elemental Analysis, calculated for C10H17F6N4O3P (386.23): C, 31.10; H, 4.44; N, 14.51.
Found: C, 31.33; H, 4.40; N, 14.36.
1-[(1-(Cyano-2-ethoxy-2-oxoethylideneaminooxy)dimethylaminomorpholinomethylene)]methanamin-
ium hexafluorophosphate (COMU, 15b). Yield 7.6g (89%), m.p. (143-144 ºC); 1H-NMR (acetone-d6):
 1.38 (t, 3H, CH3), 3.41 (s, 6H, 2 CH3), 3.82 (t, 4H, 2 CH2), 3.89 (t, 4H, 2 CH2), 4.48 (q, 2H, CH2)
13
ppm; C-NMR (acetone-d6):  13.48 (CH3), 40.70 (2 CH3), 49.94 (2 CH2), 64.59 (2 CH2), 66.04
(CH2), 106.76 (C≡N), 135.03 (C=N), 156.14 (C=O), 160.61 (C+) ppm; Elemental Analysis, calculated
for C12H19F6N4O4P (428.27): C, 33.65; H, 4.47; N, 13.08. Found: C, 33.92; H, 4.41; N, 13.24.
O-[(Dicyanomethylidene)amino]-1,1,3,3-tetramethyluronium hexafluorophosphate (HTODC, 15c).
1
Yield 5.0 g (74%), m.p. (180-181 ºC) (dec); H-NMR (acetone-d6):  3.27 (s, 12H, 4 CH3) ppm;
13C-NMR (acetone-d6):  40.80 (4 CH3), 105.10 (C≡N), 108.21 (C≡N), 119.65 (C=N), 160.67 (C+)
ppm; Elemental Analysis, calculated for C8H12F6N5OP (339.18): C, 28.33; H, 3.57; N, 20.65. Found:
C, 28.56; H, 3.64; N, 20.80.
1-[(1-(Dicyanomethyleneaminooxy)dimethylaminomorpholinomethylene)]methanaminium hexafluoro-
1
phosphate (HDMODC, 15d). Yield 5.7 g (75%), m.p. (118-119 ºC); H-NMR (acetone-d6):  3.41 (s,
13
6H, 2 CH3), 3.87-3.88 (m, 8H, 4 CH2) ppm; C-NMR (acetone-d6):  40.97 (2 CH3), 49.93 (2 CH2),
65.92 (2 CH2), 105.13 (C≡N), 108.15 (C≡N), 119.84 (C=N), 159.77 (C+) ppm; Elemental Analysis,
calculated for C10H14F6N5O2P (381.21): C, 31.51; H, 3.70; N, 18.37. Found: C, 31.77; H, 3.56;
N, 18.52.

Molecules 2010, 15
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N-[(Cyano(pyridine-2-yl)methyleneaminooxy)(dimethylamino)methylene]-N-methylmethanaminium
hexafluorophosphate (HTOPC, 15e). Yield 6.2 g (83%), m.p. (169-171 ºC); IR (KBr): 3069.24
(N-CH3), 2966.06 (aliph. CH), 2345.51 (C≡N, =N+), 1692.23 (C=N, C=C), 1273.86 (N-O) cm-1;
1H-NMR (DMSO-d6): δ 3.21 (s, 12H, 4 CH3), 7.74 (t, 1H, H-5), 8.07 (t, 1H, H-4), 8.14 (d,
13
J = 8.08 Hz, 1H, H-3), 8.85(d, J = 5.12 Hz, 1H, H-6) ppm; C-NMR (DMSO-d6): δ 40.27 (4 CH3),
108.54 (C≡N), 122.74 (C-3), 128.23 (C-5), 138.62 (C-4), 142.07 (C-2), 146.70 (C-6), 150.88 (C=N),
161.17 (C+) ppm; Elemental Analysis, calculated for C12H16F6N5OP (391.21): C, 36.84; H, 4.12; N,
17.90. Found: C, 37.06; H, 4.22; N, 18.12.
O-(2-Pyridone)-1,1,3,3-tetramethyluronium hexafluorophosphate (HTOPT, 15f). Yield 6.4 g (82%),
m.p. (166-176 ºC); IR (KBr): 3096.77 (N-CH3), 2958.98 (aliph. CH), 2345.56 (=N+), 1709.64 (C=O),
1
1672.42 (C=C, C=N), 1272.58 (N-O) cm-1; H-NMR (DMSO-d6): δ 3.11 (s, 12H, 4 CH3), 6.52 (t,
J = 7.36 Hz, 1H, H-5), 6.78 (d, J = 8.8, 1H, H-3), 7.65 (t, J = 7.32 Hz, 1H, H-4), 8.42 (d, J = 7.32 Hz,
13
1H, H-6) ppm; C-NMR (DMSO-d6): δ 40.27 (4 CH3), 107.02 (C-5), 121.98 (C-3), 135.90 (C-4),
142.05 (C-6), 156.56 (C=O), 162.12 (C+) ppm; Elemental Analysis, calculated for C14H18F6N5O2P
(391.21), C, 38.81; H, 4.19; N, 16.16. Found: C, 39.03; H, 4.31; N, 16.34.
3.4. Synthesis of 16a,b (1+1)
The coupling reagents 15a,e,f (0.25 mmol) were added to a mixture of Z-Phe-OH (0.25 mmol), the
appropriate amino acid 18a,b (0.25 mmol) and N-methylmorpholine (0.75 mmol), in dry acetonitrile
(5 mL) at 0 ºC. The reaction mixture was stirred at the same temperature for 1 h and then at room
temperature for 2 h. Ethyl acetate (EtOAc, 50 mL) was added and the mixture was subsequently
washed with 10% aqueous HCl (v/v), saturated aqueous Na2CO3 and saturated aqueous NaCl solution
(2 × 10 mL each). Finally, the organic solvent was dried with anhydrous Na2SO4, filtered, and the
solvent was removed under reduced pressure. The gummy residue obtained 16a,b (DL < 1%) were
dried under vacuum. Both products were obtaind when the coupling chlorotriazine derivative 9 was
used (15% racemization). Using the triazine coupling reagent 12a,c,d instead of 15a,e,f and following
the different procedure whereby Z-Phe-OH was activated with 12a,c,d for 1 hour at room temperature
and then added to a solution of 18a,b in the presence of NMM as a base in acetonitrile, the reaction
mixture was stirred at the same temperature for 24 h gave compounds 19a,b.
Z-Phe-Ala-OMe (16a). IR (KBr): 3303.71 (NH), 2951.65, 2926.12 (aliph. CH), 1743.34 (C=O, ester),
1694.14 (C=O, urethane), 1652.91 (C=O, amide) cm-1; 1H-NMR (CDCl3): δ 1.31 (d, 3H, CH3), 3.06 (d,
2H, CH2-C), 3.72 (s, 3H, OCH3), 4.47-4.51 (m, 2H, 2 CH), 5.07 (s, 2H, CH2-O), 5.38 (1H, NH,
13
amide), 6.42 (1H, NH, urethane), 7.25-7.32 (m, 10H, Ar. H) ppm; C-NMR (CDCl3): δ 18.36 (CH3),
38.64 (CH2-C), 48.23 (CH2-O), 52.57 (OCH3), 56.10 (CH, Phe), 67.16 (CH, Ala), 129.43 (C=O,
amide), 170.43 (C=O, urethane), 172.88 (C=O, ester) ppm.
Z-Phe-Val-OMe (16b). IR (NaCl/DCM): 3310.29 (NH), 2963.93 (aliph. CH), 1741.07 (C=O, ester),
1
1704.99 (C=O, urethane), 1661.22 (C=O, amide) cm-1; H-NMR (CDCl3): δ 0.80-0.84 (dd, 6H, 2
CH3), 2.02-2.12 (m, 1H, CH(CH3)2), 3.05-3.07 (m, 2H, CH2-C), 3.68 (s, 3H, OCH3), 4.45-4.46 (m, 2H,

Molecules 2010, 15
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2 CH), 5.08 (s, 2H, CH2-O), 5.42 (1H, NH, amide), 6.36 (1H, NH, urethane), 7.24-7.32 (m, 10H, Ar.
H) ppm.
3.5. General Method for Synthesis of 17a-c (2+1) Using Different Coupling Reagents
The coupling reagents 15a,e,f (0.25 mmol) were added to a mixture of Z-Gly-Phe-OH (0.25 mmol),
18a,b (0.25 mmol) and N-methylmorpholine (0.75 mmol), in dry ACN (5 mL) at 0 ºC. The reaction
mixture was stirred at the same temperature for 1 h and then at room temperature for 24 h. Ethyl
acetate (EtOAc, 50 mL) was added and the mixture was subsequently washed with 10% aqueous HCl
(v/v), saturated aqueous Na2CO3 and saturated aqueous NaCl solution (2 × 10 mL each). Finally, the
organic solvent was dried with anhydrous Na2SO4, filtered, and the solvent was removed under
reduced pressure. The gummy residue obtained 17a,b (DL < 1% in the case of using 15a) was dried
under vacuum. Both products were obtained when the coupling chlorotriazine 9 was used. Using the
triazine coupling reagent 12a,c,d instead of 15a,e,f and following the different procedure whereby
Z-Gly-Phe-OH was activated with 12a,c,d for 1 h at 0 ºC and then added to a solution of 18a,b in the
presence of NMM as a base in acetonitrile, the reaction mixture was stirred at the same temperature for
1 h, then at room temperature for 24 h gave compounds 19a,b.
Z-Gly-Phe-Ala-OMe (17a). IR (NaCl/DCM): 3434.81, 3272.55 (NH), 2950.43 (aliph. CH), 1735.95
1
(C=O, ester), 1718.13 (C=O, urethane), 1685.96, 1647.73 (2 C=O, amide) cm-1; H-NMR (CDCl3): δ
1.30 (d, 3H, CH3), 3.03 (d, 2H, CH2-N), 3.67 (s, 3H, OCH3), 3.79-3.85 (m, 2H, CH2-C), 4.43-4.46 (m,
1H, CH, Phe), 4.68-4.70 (m, 1H, CH, Ala), 5.08 (s, 2H, CH2-O), 5.62, 6.71 (2H, 2 NH, amide), 6.91
(1H, NH, urethane), 7.14-7.32 (m, 10H, Ar. H) ppm.
Z-Gly-Phe-Val-OMe (17b). IR (NaCl/DCM): 3298.25 (br. NH), 2961.81 (aliph. CH), 1719.76 (br.
C=O, ester & urethane), 1654.46 (2 C=O, amide) cm-1; 1H-NMR (CDCl3: δ 0.77-0.82 (dd, 6H, 2 CH3),
2.02-2.12 (m, 1H, CH(CH3)2), 2.90-3.10 (m, 2H, CH2-N), 3.62 (s, 3H, OCH3), 3.60-3.90 (m, 2H, CH2-
C), 4.35-4.49 (m, 1H, CH, Gly), 4.75-4.85 (m, 1H, CH, Val), 5.06 (s, 2H, CH2-O), 5.85, 6.95 (2H, 2
NH, amide), 7.12-7.20 (m, 1H, NH, urethane), 7.25-7.36 (m, 10H, Ar. H) ppm.
Z-Gly-Val-Val-OMe (17c). Prepared following the same procedure above for 17a,b by replacing
Z-Gly-Phe-OH with Z-Gly-Val-OH, and the amino acid 18b was used. The gummy residue of 17c
obtained when the coupling reagents 15a,e,f (DL < 0.1% in the case of using 15a) was dried under
vacuum. The product was obtained (47.3% racemization) when the coupling chlorotriazine 9 was used.
Using the triazine coupling reagent 12a,c,d instead of 15a,e,f and following the different procedure
whereby Z-Gly-Val-OH was activated with 12a,c,d for 1 hour at 0 ºC and then added to a solution of
18b in the presence of NMM as a base in acetonitrile, the reaction mixture was stirred at room
temperature for 24 h to give compound 19b.
Z-Gly-Val-Val-OMe (17c). IR (NaCl/DCM): 3411.60, 3303.57 (NH), 2967.43 (aliph. CH), 1733.86
(br. C=O, ester & urethane), 1651.45 (2 C=O, amide) cm-1; 1H-NMR (CDCl3): δ 0.88-0.90 (dd, 12H, 4
CH3), 2.06-2.14 (m, 2H, 2 CH(CH3)2), 3.70 (s, 3H, OCH3), 3.90 (2H, CH2-N), 4.38 (1H, CH, Gly),

Molecules 2010, 15
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4.48-4.51 (m, 1H, CH, Val), 5.10 (s, 2H, CH2-O), 5.68, 6.74 (2H, 2 NH, amide), 6.82 (1H, NH,
urethane), 7.25-7.34 (m, 5H, Ar. H) ppm.
Methyl-2-(4,6-dimethoxy-1,3,5-triazin-2-ylamino)propanoate (19a). m.p. 98-100 ºC, 1H-NMR
(CDCl3): δ 1.48 (d, 3H, CH3), 3.72 (s, 3H, CH3 ester), 3.93 (s, 6H, 2 OCH3), 4.70 (q, 1H, CH), 5.93 (d,
1H, NH) ppm; Elemental analysis calculated for C9H14N4O4: C, 44.63; H, 5.83; N, 23.13. Found: C,
44.91; H, 6.00; N, 23.36.
Methyl-2-(4,6-dimethoxy-1,3,5-triazin-2-ylamino)-3-methylbutanoate (19b). m.p. 103-105 ºC,
1H-NMR (CDCl3): δ 0.96 (t, 6H, 2 CH3), 2.21-2.22 (m, 1H, CH), 3.72 (s, 3H, CH3 ester), 3.91 (s, 6H,
2 OCH3), 4.67 (q, 1H, CH), 5.84 (d, 1H, NH) ppm; Elemental analysis calculated for C11H18N4O4: C,
48.88; H, 6.71; N, 20.73. Found: C, 49.12; H, 6.89; N, 21.01.
4. Conclusions
In conclusion, herein a new family of 1,3,5-triazinyloxyimino derivatives 12a-d was compared with
uronium salts. The morpholino and tetramethyluronium salts are more reactive than triazinyloxyimino
derivatives for activation of the carboxylic group and formation of peptide bonds. The new
triazinyloxyimino derivatives 12a-d failed to give the expected products, due to the low activation for
the carboxylic group and fast attack by the N-terminal of the amino acid, which led to formation of the
corresponding N-triazinyl amnio acid derivatives as confirmed by NMR and HPLC. Oxyma
(ethyl 2-cyano-2-(hydroxyimino)acetate) uronium salts were confirmed to be superior to other oxime
derivatives, HOAt (4) and HOBt (1), in terms of both coupling yield and retention of configuration.
Finally, the new triazinyloxyimino derivatives may be useful in the preparation of biologically active
N-triazinyl amino acids as well as N-triazinyl peptides which is now going to be tested in our
laboratory.
Acknowledgements
We are grateful to Research Centre at College of Science, King Saud University, for supporting
this work.
References
1. Rich, D.H.; Singh, J. The Peptides: Analysis, Biology; Gross, E., Meienhofer, J., Eds.; Academic
Press: New York, NY, USA, 1979; Volume 1, pp. 241-261.
2. Knorr, R.; Trzeciak, A.; Bannwarth, W.; Gillessen, D. New coupling reagents in peptide
chemistry. Tetrahedron Lett. 1989, 30, 1927-1930.
3. König, W.; Geiger, R. A new method for the synthesis of peptides: activation of the carboxyl
group with dicyclohexylcarbodiimide and 3-hydroxy-4-oxo-3,4-dihydro-1,2,3-benzotriazine.
Chem. Ber. 1970, 103, 2034-2040.

Molecules 2010, 15
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4. König, W.; Geiger, R. A new method for the synthesis of peptides: activation of the carboxyl
group with dicyclohexylcarbodiimide using 1-hydroxy-benzotriazoles as additives. Chem. Ber.
1970, 103, 788-789.
5. Carpino, L.A. 1-Hydroxy-7-azabenzotriazole. An efficient peptide coupling additive. J. Am.
Chem. Soc. 1993, 115, 4397-4398.
6. Carpino, L.A.; Imazumi, H.; El-Faham, A.; Ferrer, F.J.; Zhang, C.; Lee, Y.; Foxman, B.M.;
Henklein, P.; Hanay, C.; Gge, C.M.; Wenschuh, H.; Klose, J.; Beyermann, M.; Bienert, M. The
Uronium/ Guanidinium Peptide Coupling Reagents: Finally the True Uronium Salts. Angew.
Chem. Int. Ed. 2002, 41, 441-445.
7. Albericio, F.; Bofill, J.M.; El-Faham, A.; Kates, S.A. Use of Onium Salt-Based Coupling
Reagents in Peptide Synthesis. J. Org. Chem. 1998, 63, 9678-9683.
8. Wijkmans, J.C.H.M.; Kruijtzer, J.A.W.; van der Marel, G.A.; van Boom, J.H.; Bloemhoff, W.
CF3-NO2-PyBOP: A new and highly efficient coupling reagent for N-methyl amino acids.
Tetrahedron Lett. 1995, 36, 4643-4646.
9. Kehler, J.; Püsch, A.; Dahl, O. Improved Syntheses of 1-Hydroxy-4-nitro-6-trifluoromethyl-
benzotriazole and 1-Hydroxy-4,6-dinitrobenzotriazole. Acta Chem. Scan. 1996, 50, 1171-1173.
10. Carpino, L.A.; El-Faham, A.; Albericio, F. Racemization Studies During Solid Phase Peptide
Synthesis Using Azabenzotriazole-Based Coupling Reagents. Tetrahedron Lett. 1994, 35,
2279-2282.
11. Carpino, L.A.; El-Faham, A.; Minor, C.H.; Albericio, F. Advantageous applications of
azabenzotriazole (triazolopyridine)-based coupling reagents to solid-phase peptide synthesis. J.
Chem. Soc., Chem. Commun 1994, 201-203.
12. Quibell, M.; Packman, L.C.; Johnson, T. Identification of coupling conditions proceeding with
low C-terminal epimerization during the assembly of fully protected backbone-substituted peptide
segments. J. Chem. Soc. Perkin. Trans. 1996, 1, 1219-1225.
13. Carpino, L.A.; El-Faham, A. The Diisopropylcarbodiimide/1-Hydroxy-7-azabenzotriazole
System: Segment Coupling and Stepwise Peptide Assembly. Tetrahedron 1999, 55, 6813-6830.
14. Carpino, L.A.; Imazumi, H.; Foxman, B.M.; Vela, M.J.; Henklein, P.; El-Faham, A.; Klose, J.;
Bienert, M. Comparison of the Effects of 5- and 6-HOAt on Model Peptide Coupling Reactions
Relative to the Cases for the 4- and 7-Isomers. Org. Lett. 2000, 2, 2253-2256.
15. Angell, Y.M.; Gacia, C.E.; Rich, D.H. Comparative studies of the coupling of N-methylated,
sterically hindered amino acids during solid-phase peptide synthesis. Tetrahedron Lett. 1994, 35,
5981-5984.
16. Gawne, G.; Kenner, G.; Sheppard, R.C. Acyloxyphosphonium salts as acylating agents. Synthesis
of peptides. J. Am. Chem. Soc. 1969, 91, 5669-5671.
17. Castro, B.; Dormoy, J.R. Azidotris(dimethylamino)phosphonium hexafluorophosphate. Excellent
reagent for peptide coupling. Bull. Soc. Chim. Fr. 1973, 3359-3361.
18. Castro, B.; Dormoy, J.R.; Evin, G.; Selve, C. Peptide coupling reagents. IV. N-
[Oxytris(dimethylamino)phosphonium]benzotriazole hexafluorophosphate. Tetrahedron Lett.
1975, 14, 1219-1222.
19. Castro, B.; Dormoy, J.R.; Evin, G.; Selve, C. Peptide coupling reagents. Part VII. Mechanism of
the formation of active esters of hydroxybenzotriazole in the reaction of carboxylate ions on the

Molecules 2010, 15
9416
BOP reagent for peptide coupling. A comparison with Itoh's reagent. J. Chem. Res. (S) 1977,
182-185.
20. Coste, J.; Le-Nguyen, D.; Castro, B. PyBOP: A new peptide coupling reagent devoid of toxic
by-product. Tetrahedron Lett. 1990, 31, 205-208.
21. Dykstra, R.R. Encyclopedia of Reagents for Organic Synthesis; Paquette, L.A., Ed.; John Wiley
& Sons Ltd.: Chichester, UK, 1995; Volume 4, pp. 2668-2675.
22. Carpino, L.A.; El-Faham, A. Effect of Tertiary Bases on O-Benzotriazolyluronium Salt-Induced
Peptide Segment Coupling. J. Org. Chem. 1994, 59, 695-698.
23. Carpino, L.A.; Ionescu, D.; El-Faham, A. Peptide Coupling in the Presence of Highly Hindered
Tertiary Amines. J. Org. Chem. 1996, 61, 2460-2465.
24. Kates, S.A.; Diekmann, E.; El-Faham, A.; Herman, L.W.; Ionescu, D.; McGuinness, B.F.; Triolo,
S.A.; Albericio, F.; Carpino, L.A. Techniques in Protein Chemistry VII; Marshak, D.R., Ed.;
Academic Press: New York, NY, USA, 1996; pp. 515-523.
25. Ehrlich, A.; Heyne, H.U.; Winter, R.; Beyermann, M.; Haber, H.; Carpino, L.A.; Bienert, M.
Cyclization of all-L-Pentapeptides by Means of 1-Hydroxy-7-azabenzotriazole-Derived Uronium
and Phosphonium Reagents. J. Org. Chem. 1996, 61, 8831-8838.
26. Jou, G.; Gozalez, I.; Albericio, F.; Lloyd, P.W.; Giralt, E. Total Synthesis of Dehydrodidemnin B.
Use of Uronium and Phosphonium Salt Coupling Reagents in Peptide Synthesis in Solution. J.
Org. Chem. 1997, 62, 354-366.
27. Han, Y.; Albericio, F.; Barany, G. Occurrence and Minimization of Cysteine Racemization during
Stepwise Solid-Phase Peptide Synthesis. J. Org. Chem. 1997, 62, 4307-4312.
28. Albericio, F.; Cases, M.; Alsina, J.; Triolo, S.A.; Carpino, L.A.; Kates, S.A. On the use of
PyAOP, a phosphonium salt derived from HOAt, in solid-phase peptide synthesis. Tetrahedron
Lett. 1997, 38, 4853-4856.
29. Kim, S.; Chang, A.; Ko, Y.K. Benzotriazol-1-yl diethyl phosphate. A new convenient coupling
reagent for the synthesis of amides and peptides. Tetrahedron Lett. 1985, 26, 1341-1349.
30. Fan, C.X.; Hao, X.L.; Ye, Y.H. A novel organophosphorus compound as a coupling reagent for
peptide synthesis. Synth. Commun 1996, 26, 1455-1460.
31. Carpino, L.A.; Xia, J.; El-Faham, A. 3-Hydroxy-4-oxo-3,4-dihydro-5-azabenzo-1,2,3-triazene. J.
Org. Chem. 2004, 69, 54-61.
32. Carpino, L.A.; Xia, J.; Zhang, C.; El-Faham, A. Organophoshorus and Nitro-substituted Sulfone
Esters of 1-Hydroxy-7-azabenzotriazole as Highly Efficient Fast-Acting Peptide Coupling
Reagents. J. Org. Chem. 2004, 69, 69-71.
33. Carpino, L.A.; El-Faham, A.; Albericio, F. Efficiency in Peptide Coupling: 1-Hydroxy-7-
azabenzotriazole vs. 3,4-Dihydro-3-hydroxy-4-oxo-1,2,3-benzotriazine. J. Org. Chem. 1995, 60,
3561-3564.
34. El-Faham, A.; Khattab, Sh.N.; Abdul-Ghani, M.; Albericio, F. Design and Synthesis of New
Immonium-Type Coupling Reagents. Eur. J. Org. Chem. 2006, 6, 1563-1573.
35. El-Faham, A.; Albericio, F. Novel Proton Acceptor Immonium-Type Coupling Reagents:
Application in Solution and Solid Phase Peptide Synthesis. Org. Lett. 2007, 9, 4475-4477.
36. El-Faham, A.; Albericio, F. Morpholine-Based Immonium and Halogeno amidinium Salts as
Coupling Reagents in Peptide Synthesis. J. Org. Chem. 2008, 73, 2731-2737.

Molecules 2010, 15
9417
37. Kamiński, Z.J. Triazine-based condensing reagents. Biopolymers (Peptide Science) 2000, 55,
140-164.
38. Kamiński, Z.J.; Paneth, P.; Rudzinski, J. A Study on the Activation of Carboxylic Acids by
Means of 2-Chloro-4,6-dimethoxy-1,3,5-triazine and 2-Chloro-4,6-diphenoxy-1,3,5-triazine. J.
Org. Chem. 1998, 63, 4248-4255.
39. Kunishima, M.; Kawachi, C.; Iwasaki, F.; Terao, K.; Tani, S. Synthesis and characterization of 4-
(4,6-dimethoxy-1,3,5-triazin-2-yl)-4-methylmorpholinium chloride. Tetrahedron Lett. 1999, 40,
5327-5330.
40. Kunishima, M.; Kawachi, C.; Morita, J.; Hioki, K.; Terao, K.; Tani, S. Formation of
carboxamides by direct condensation of carboxylic acids and amines in alcohols using a new
alcohol- and water-soluble condensing agent: DMT-MM. Tetrahedron 2001, 57, 1551-1558.
41. Kjell, D.P.; Hallberg, D.W.; Kalbfleisch, J.M.; McCurry, C.K.; Semo, M.J.; Sheldo, E.M.; Spitler,
J.T.; Wang, M. Determination of the Source of the N-Methyl Impurity in the Synthesis of
Pemetrexed Disodium Heptahydrate. Org. Proc. Res. Dev. 2005, 9, 738-742.
42. Kunishima, M.; Kawachi, C.; Morita, J.; Terao, K.; Iwasaki, F.; Tani, S. 4-(4,6-dimethoxy-1,3,5-
triazin-2-yl)-4-methyl-morpholinium chloride: an efficient condensing agent leading to the
formation of amides and esters. Tetrahedron 1999, 55, 13159-13170.
43. Tanaka, T.; Noguchi, M.; Kobayashi, A.; Shoda, S.I. A novel glycosyl donor for chemo-
enzymatic oligosaccharide synthesis: 4,6-dimethoxy-1,3,5-triazin-2-yl glycoside. Chem. Commun
2008,
2016-2018.
44. Paoline, I.; Nuti, F.; de la Cruz Pozo Carrero, M.; Barbetti, F.; Kolesinska, B.; Kamiński, Z.J.;
Chelli, M.; Papini, A.M. A convenient microwave-assisted synthesis of N-glycosyl amino acids.
Tetrahedron Lett. 2007, 48, 2901-2904.
45. Wozniak, L.A.; Gora, M.; Stec, W.J. Chemoselective Activation of Nucleoside 3'-O-
Methylphosphonothioates with 1,3,5-Triazinyl Morpholinium Salts. J. Org. Chem. 2007, 72,
8584-8587.
46. El-Faham, A.; Subirós-Funosas, R.; Prohens, R.; Albericio, F. COMU, a Safer and More Effective
Replacement for Benzotriazole-based Uronium Coupling Reagents. Chem. Eur. J. 2009, 15,
9404-9416.
47. Kitamura, M.; Chiba, S.; Narasaka, K. Synthesis of Primary Amines and N-Methylamines by the
Electrophilic Amination of Grignard Reagents with 2-Imidazolidinone O-Sulfonyloxime. Bull.
Chem. Soc. Jpn. 2003, 76, 1063-1070.
48. Izdebski, J. New Reagents Suppressing Racemization in Peptide Synthesis by the DCC Method.
Polish J. Chem. 1979, 53, 1049-1057.
Sample Availability: Samples of the compounds triazinyloximino derivatives are available from
the authors.
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