Table 1. Yield and Diastereomeric Excess (de) of Z-L-Phg-L-
Pro-NH2 Isolated from the Reactions in Water with or without
the Additives 1, 2, and 3a
Scheme 1. Syntheses of the Oxyma Derivatives, 2 and 3
entry
additive
time (h)
yield (%)
de (%)c
salt-free amino acids or weakly basic conditions. To date,
several attractive features of Oxyma in peptide chemistry
have been reported using a conventional organic solvent
such as DMF. Although peptide bond-forming reactions
can often be performed in water containing organic sol-
vents, no practical peptide coupling additive has been
developed for the synthesis of oligopeptides in water.7
Herein, we report the development of nonracemizable
peptide-forming reactions of R-amino acids with glycero-
acetonide-Oxyma derivative 2 in water.
1
2
3
4
5
1
2
3
3
2
25
95
5
>99
>99
>99
>99
75
2
2
12
2
<10
45
b
ꢀ
a 7 (1.5 equiv), 8 (1.0 equiv), additive (1.2 equiv), EDCI (1.2 equiv),
and NaHCO3 (3 equiv) in H2O (0.2 M concentrations). b No additive was
added. c de was determined via HPLC and 1H NMR analyses.
derivatives 2 and 3 was examined by an established
model study using Z-L-Phg-OH and HCl•H-L-Pro-
NH2.4c In these studies we performed the reactions
with 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide
(EDCI)11 and NaHCO3 (3 equiv) in water solution.
The yields and diastereomeric excess (de) of Z-L-Phg-L-
Pro-NH2 obtained with 1, 2, and 3 are summarized in
Table 1.
Several Oxyma derivatives that increase water solubility
were designed and synthesized. As summarized in Scheme
1, 2,3-dihydroxypropyl 2-cyano-2-(hydroxyimino)acetate
(Glycero-Oxyma, 3) could be synthesized in three steps
from 2-cyanoacetic acid (4) with an excellent overall yield.
The ester 6, which was synthesized in a single step with N,
N-dimethyl phosphoramidodichloridate,8 was subjected
to the hydroxyimination reaction using NaNO2 and
AcOH to furnish (2,2-dimethyl-1,3-dioxolan-4-yl)methyl
2-cyano-2-(hydroxyimino)acetate (Glyceroacetonide-
Oxyma, 2) in 95% yield.9 Hydrolysis of the acetonide
group of 2 yielded 3 in >97% yield. Several attempts at
synthesizing other types of water-soluble Oxyma deriva-
tives could not be achieved due to the fact that many
water solubilizing groups such as the (p-hydroxyphenyl)-
dimethylsulfonium group and dimethylaminoalcohols
were not amenable to the hydroxyimination reactions
(6f2 in Scheme 1). Nonetheless, analyses of water solu-
bility of 2 and 3 revealed that the glycerol and glyceroace-
tonide groups improved the water solubility 2.1 and >5
times greater than that of Oxyma in 0.2 M NaHCO3 water
solution (pH 8.3).10 Thus, new Oxyma derivatives 2 and
3 are completely solubilized even at 0.1 M concentra-
tions in water at a pH of 8.3. The effectiveness of the
coupling yields and degree of racemization of Oxyma
The reaction with Oxyma 1 furnished the desired pro-
duct 9 in 25% yield in 2 h with >99% de (entry 1).12
Although an excellent de was attained, the product yield
with 1 could not be improved even after extended reaction
times. Interestingly, the same reaction with glyceroaceto-
nide-Oxyma 2 provided 9 in 95% yield with greater than
99% de (entry 2). On the other hand, the reaction with
glycero-Oxyma 3 did not provide a useful level of the
product yield, but the de of the product was greater than
99% (entry 3). The product yield of 9 with 3 was not
noticeably improved even after 12 h (entry 4). Due to the
fact that the coupling reaction of 7 and 8 without the
coupling additive (1, 2, or 3) furnished 9 in 45% with only
75% de(entry 5), the high de’s observedin entries 1ꢀ4 were
attributed to the formation of oxime esters of 1, 2, or 3
(which are less racemizable or not at all) in water media at
a pH of <8.3. However, the hydrophilicꢀhydrophobic
balance of the coupling additive seems to be very impor-
tant to achieve a high yielding amide coupling reaction
between 7 and 8 in water.
(7) (a) Galanis, A. S.; Albericio, F.; Grøtli, M. Org. Lett. 2009, 11,
4488–4491. (b) Hojo, K.; Maeda, M.; Tanakamaru, N.; Kawasaki, K.
Protein Pept. Lett. 2006, 13, 189–192. (c) Kouge, K.; Koizumi, T.; Okai,
H.; Kato, T. Bull. Chem. Soc. Jpn. 1987, 60, 2409–2418. (d) Kunz, H.
Angew. Chem., Int. Ed. Engl. 1978, 17, 67–68 and references therein.
(8) Amancha, P. K.; Liu, H.-J.; Wei, T.; Shia, K.-S. Eur. J. Org.
Chem. 2010, 18, 3473–3480.
To understand the scope and limitations of the peptide-
forming reactions with EDCI, glyceroacetonide-Oxyma 2,
(9) Cheng, L. J.; Lightner, D. A. Synthesis 1999, 1, 46–48.
(10) Water solubility of 1, 2, and 3 was measured via the shake-flask
method; 1 (14.90 mg/mL), 2 (31.5 mg/mL), and 3 (80 mg/mL), respectively.
(11) Kurzer, F.; Douraghi-Zadeh, K. Chem. Rev. 1967, 67, 107–152.
(12) The diastereomer was not detected by HPLC analysis (see
Supporting Information).
Org. Lett., Vol. 14, No. 13, 2012
3373