morpholino derivatives (nos. 2, 4, 5, 7, and 9) result in
improved control of racemization when compared to their
tetramethylamino counterparts (nos. 1, 3, 6, and 8). Interest-
ingly, morpholino derivatives perform especially well with
just 1 equiv of base. Thus, yields are similar or even slightly
better than those obtained when 2 equiv of base and/or
tetramethylamino derivatives are used.
To compare the relative coupling rates for various reagents
and the effect of the oxygen as a proton acceptor, reaction
of Fmoc-Val-OH with H-Val-NH2 in the presence of 1 or 2
equiv of DIEA was carried out. Table 2 indicates that the
Table 2. Extent of Conversion in the Preparation of
Fmoc-Val-Val-NH2 Using HATU/HDMA and DIEA (1 Equiv/2
Equiv) as a Base in DMF (Calculated by HPLC)
Figure 2. HPLC for the crude pentapeptide prepared in solution.
HATU
yield (%)
HDMA
yield (%)
In a more demanding example, H-Tyr-Aib-Aib-Phe-Leu-
NH2 was manually solid-phase assembled on Fmoc-Rink
Amide-AM-resin using amino acid/activator (3 equiv) and
DIEA (6 equiv) for a 30 min coupling, except for Aib-Aib,
which required 1 h. Yields for coupling of Fmoc-Aib-OH
onto H-Aib-Phe-Leu-NH-Rink Amide-AM-resin were de-
termined by reverse-phase HPLC analysis (integration of
pentapeptide vs tetrapeptide) after cleavage of the pep-
tide from the resin. The best results were obtained with
6-HDMCB (7c) (99% coupling) and HDMA (7a) (98%
coupling) and HDMB (7b) (89% coupling) over HATU (1)
(83% coupling) and HBTU (2) (47% coupling).
In conclusion, the novel proton acceptor coupling reagents
showed superiority to those described previously.9 The
oxygen in the carbocation moiety confers more solubility,
to the reagent, allowing us to carry out the reaction at
higher concentration and facilitating the removal of side
products when chemistry is carried out in solution. Further-
more, it enhances coupling yields and decreases racemiza-
tion, allowing the use of just 1 equiv of base. Results obtained
with HMDB and 6-HDMCB, which contain the less reactive
HOBt and Cl-HOBt, match those obtained with HATU,
which contains the most effective and expensive HOAt.
time (min)
2 equiv
1 equiv
2 equiv
1 equiv
5
10
20
30
60
83.0
87.6
90.5
92.5
93.0
94.0
70.0
76.0
80.0
82.0
82.0
83.0
94.8
95.2
96.4
98.0
99.0
99.0
80.0
85.0
90.0
93.5
95.5
96.0
120
morpholino derivative (HDMA) showed a better coupling
rate in both cases (1 and 2 equiv of DIEA). This indicates
that the oxygen atom increases reactivity and acts as a proton
acceptor during the coupling step.
To demonstrate the effectiveness of the oxygen on the
carbon skeleton of the new reagents in the solution phase,
the pentapeptide H-Tyr-Gly-Gly-Phe-Leu-NH2 was prepared
in solution without isolation and purification of the inter-
mediates using Boc chemistry and HDMB (7b) as a coupling
reagent. The purity of the pentapeptide tested by HPLC
analysis and HPLC-MS was excellent (Figure 2). The HPLC-
MS showed the correct mass for the pentapeptide. No traces
of the urea were detected confirming the high solubility of
these derivatives.
Acknowledgment. This study was partially supported
by CICYT (CTQ2006-03794/BQU), the Generalitat de
Catalunya (2005SGR 00662), Luxembourg Industries Ltd.,
the Institute of Research in Biomedicine, and the Barcelona
Science Park.
(4) Immonium salt derivatives of HOAt, such as HATU and HAPyU,
have been shown to be the most efficient in terms of both reactivity and
controlling racemization. This has been interpreted by the electron-
withdrawing influence of the nitrogen atom that effects stabilization of the
leaving group leading to greater reactivity. Second, the nitrogen makes a
classic neighboring group effect feasible which can both speed up the
reactivity and reduce loss of configuration. (a) Carpino, L. A. J. Am. Chem.
Soc. 1993, 115, 4397-4398. (b) Carpino, L. A.; El-Faham, A.; Minor, C.
A.; Albericio, F. J. Chem. Soc., Chem. Commun. 1993, 201-203.
(5) El-Faham, A.; Khattab, S. N.; Abdul-Ghani, M.; Albericio, F. Eur.
J. Org. Chem. 2006, 1563-1573.
Supporting Information Available: Experimental pro-
cedures and characterization material. This material is
OL701817U
(6) Reactions of urea 6 with other secondary amines did not led to
positive results [thiomorpholine, poor coupling reagent; N-methylpiperazine,
the target compound was not obtained].
(9) Interestingly, Kaminski, Papini, and co-workers have described the
N-methylmorpholinium derivative as the reagent of choice of its family of
reagents for their independent triazinylammonium system. Kaminski, Z.
J.; Kolesinska, B.; Kolesinska, J.; Sabatino, G.; Chelli, M.; Rovero, P.;
Blaszczyk, M.; Glowka, M. L.; Papini, A. M. J. Am. Chem. Soc. 2005,
127, 16912-16920.
(7) Patent pending.
(8) Carpino, L. A.; Imazumi, H.; El-Faham, A.; Ferrer, F. J.; Zhang, C.;
Lee, Y.; Foxman, B. M.; Henklein, P.; Hanay, C.; Mugge, C.; Wenschuh,
H.; Klose, J.; Beyermann, M.; Bienert, M. Angew. Chem., Int. Ed. 2002,
41, 442-445.
Org. Lett., Vol. 9, No. 22, 2007
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