Efficient amidation from carboxylic acids and azides via selenocarboxylates: Application to the coupling of amino acids and peptides with azides

Article abstract of DOI:10.1021/jo061703n

(Chemical Equation Presented) A facile one-pot procedure for the coupling of carboxylic acid and azide via selenocarboxylate and selenatriazoline has been developed and successfully applied to the coupling of amino acids and peptides with azides. Selenocarboxylates are readily prepared by the reaction of the activated carboxylic acids with LiAlHSeH under mild conditions. The Selenocarboxylates formed in situ are used to react directly with azides to form the corresponding amides via a selenatriazoline intermediate. Excellent yields were obtained for electron-deficient azides, and moderate to good yields were obtained for electron-rich azides. The selenocarboxylate/azide amidation reaction is clean and chemoselective. It provides an attractive alternative method to the conventional acylation of amines when an amide bond needs to be formed without going through an amine intermediate.

Full text of DOI:10.1021/jo061703n

Efficient Amidation from Carboxylic Acids and Azides via
Selenocarboxylates: Application to the Coupling of Amino Acids
and Peptides with Azides
Xinghua Wu† and Longqin Hu*,†,‡
Department of Pharmaceutical Chemistry, Ernest Mario School of Pharmacy, Rutgers,
The State UniVersity of New Jersey, Piscataway, New Jersey 08854, and The Cancer Institute
of New Jersey, New Brunswick, New Jersey 08901
longhu@rutgers.edu
ReceiVed August 16, 2006
A facile one-pot procedure for the coupling of carboxylic acid and azide via selenocarboxylate and
selenatriazoline has been developed and successfully applied to the coupling of amino acids and peptides
with azides. Selenocarboxylates are readily prepared by the reaction of the activated carboxylic acids
with LiAlHSeH under mild conditions. The selenocarboxylates formed in situ are used to react directly
with azides to form the corresponding amides via a selenatriazoline intermediate. Excellent yields were
obtained for electron-deficient azides, and moderate to good yields were obtained for electron-rich azides.
The selenocarboxylate/azide amidation reaction is clean and chemoselective. It provides an attractive
alternative method to the conventional acylation of amines when an amide bond needs to be formed
without going through an amine intermediate.
Introduction
bonds in several strategies including the Staudinger-type ligation
involving the acylation of an iminophosphorane,2-7 the ami-
dation of a carboxylic acid with an azide in the presence of a
trialkyl phosphine,8-10 the Williams thio acid/azide amida-
tion,11,12 and the selenocarboxylate/azide amidation.13-15 Re-
cently, the modified Staudinger ligation,6,7 which forms an amide
bond starting from an azide and a phosphine-linked ester/thio
The carboxamide is among the most common functionalities
in synthetic organic compounds and natural products. Many
methods have been developed for amide bond formation with
most involving the reaction between an activated carboxylic acid
as an electrophile and a free amine as a nucleophile.1 However,
there are situations where free amines cannot be used because
of either structural instability or the presence of functional
groups that are incompatible with amines. Azides are neither
basic nor nucleophilic and have been used to directly form amide
ester in the form of R′COO-Cn)1,2-PR2 or R′COS-Cn)1,2
-
PR2, has been successfully used in N-glycosylation,16-21 peptide
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* To whom correspondence should be addressed. Phone: 732-445-5291.
Fax: 732-445-6312.
† Rutgers, The State University of New Jersey.
‡ The Cancer Institute of New Jersey.
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10.1021/jo061703n CCC: $37.00 © 2007 American Chemical Society
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J. Org. Chem. 2007, 72, 765-774
765

Wu and Hu
SCHEME 1. Direct Amidation from Azides Avoiding 1,6-Elimination from 4-Aminobenzyl Derivatives
ligation,22-25 and lactamization.26 The reaction involves a five-
or six-membered intramolecular transacylation of an imino-
phosphorane, the intermediate of the Staudinger reaction,27
followed by hydrolysis to form an amide. The Staudinger
ligation favors aliphatic azides, whereas for aromatic azides,
especially those with electron-withdrawing substituents, the
reactions are very slow and may fail.28 When the Staudinger-
type ligation is incomplete or fails, the iminophosphorane
intermediate is converted to the corresponding amine by
hydrolysis in an aqueous medium or during aqueous workup
without the possibility of recovering the azide starting material.
The amidation reaction of thio acids with azides has recently
attracted broad attention. Although acetylation of organic azides
with thioacetic acid was first reported in 1980,29 it was
postulated as a conventional nucleophilic acylation reaction
between thioacetic acid and the amine formed via the in situ
reduction of the azide by the adventitious hydrogen sulfide,
which is regenerated during the acylation reaction.30 However,
recent results from the Williams’ group suggested a new
mechanism that involves the formation of a thiatriazoline
intermediate followed by a retro-[3+2] cycloaddition to form
the amide product.12 Two mechanistic pathways were proposed
for the formation of the thiatriazoline intermediate: for electron-
deficient azides, the thiatriazoline is formed via a stepwise linear
coupling of the thio acid with the azide followed by intramo-
lecular cyclization, and for electron-rich azides, the thiatriazoline
is formed via a concerted intermolecular [3+2] cycloaddition
reaction between the thio acid and the azide. Several applications
and modifications of this Williams thio acid/azide amidation
have been reported.31-35
In our research, reduction of compounds 1 and 2 in the form
of 4-nitro or 4-azidobenzyl-LG (LG ) a good leaving group)
failed to give the desired amine intermediate 3 that is needed
to couple to an amino acid or peptide via the conventional
amidation procedures. Instead, the amine intermediate 3 under-
went a spontaneous 1,6-elimination to form quinonimine me-
thide 4 followed by polymerization as shown in Scheme 1.36
Thus, the spontaneous 1,6-elimination precludes the use of
conventional amidation procedures that require the amine
intermediate 3. In this case, the Williams thio acid/azide
amidation would provide an attractive alternative. However, the
thio acid/azide amidation gives low yields for electron-rich and
sterically hindered azides and often requires high reactant
concentration and temperature to achieve a satisfactory conver-
sion rate.12 This prompted us to develop a new amidation
method that can be carried out under mild conditions and is
applicable to amino acids and peptides.
Selenium shares some of the same chemical properties as
sulfur, but the larger and more easily polarizable selenium atom
makes it more nucleophilic than sulfur. For example, selenophe-
nolate was found to react with methyl iodide ∼7 times faster
than thiophenolate in a SN2 displacement reaction.37 We thought
that selenocarboxylic acids ought to be more reactive than thio
acids and potentially facilitate the amide formation with azides.
Indeed, a sterically hindered 2-azidopiperidine derivative reacted
with selenoacetic acid to form the corresponding acetamide in
75% yield after 12 h in refluxing chloroform in the presence of
2,6-lutidine, whereas it was entirely unreactive toward thioacetic
acid under the same reaction conditions.13 Although selenocar-
boxylic acids are known to be unstable and readily oxidized to
diacyl diselenides upon exposure to air, their corresponding
alkali metal and trialkylammonium salts are relatively stable,
especially for aromatic selenocarboxylates. We recently pub-
lished two communications on the amidation reaction of various
aromatic azides with selenocarboxylates generated from diacyl
selenides or diacyl diselenides.14,15 The reactions were found
to be highly chemoselective and compatible with a variety of
(17) Saxon, E.; Luchansky, S. J.; Hang, H. C.; Yu, C.; Lee, S. C.;
Bertozzi, C. R. J. Am. Chem. Soc. 2002, 124, 14893-14902.
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67, 4993-4996.
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Chem. Soc. 2003, 125, 5268-5269.
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J. Am. Chem. Soc. 2003, 125, 11790-11791.
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Hiemstra, H.; Maarseven, J. H. v. Angew. Chem., Int. Ed. 2003, 42, 4373-
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Org. Lett. 2005, 7, 1125-1128.
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Org. Lett. 2006, 8, 823-826.
(27) Staudinger, H.; Meyer, J. HelV. Chim. Acta 1919, 2, 635-646.
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C. R. J. Am. Chem. Soc. 2005, 127, 2686-2695.
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(35) Kolawski, R. V.; Shangguan, N.; Williams, L. J. Tetrahedron Lett.
2006, 47, 1163-1166.
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90, 319-326.
766 J. Org. Chem., Vol. 72, No. 3, 2007

Efficient Amidation Via Selenocarboxylates
SCHEME 2. Three-Step One-Pot Synthesis of Amides from Azides and Carboxylic Acids
functional groups, including hydroxyl, carbonyl, carboxyl, and
cyano. The reactions work very efficiently with electron-
deficient azides. For the less reactive electron-rich azides, mild
heating was required to produce satisfactory reaction rates.15
The amidation reactions could also be carried out under aqueous
conditions without significant loss of yields. However, what was
limiting the application of selenocarboxylate/azide amidation
was the lack of a practical method to prepare selenocarboxylates.
The known methods of preparing selenocarboxylates include
the treatment of trimethylsilyl selenocarboxylates with alkali
metal fluorides;38,39 the reaction of acyl chlorides with alkali
metal selenides;40 the reaction of diacyl selenides or diacyl
diselenides with alkali metal hydroxide,41 alkali metal methox-
ide,42 or piperidine;43and the reaction of carboxylic acids with
Woollin’s reagent, [PhP(dSe)Se]2.44 However, all of the above
methods for the preparation of selenocarboxylates have disad-
vantages: trimethylsilyl selenocarboxylic esters are highly
moisture sensitive; alkali metal selenides are poorly soluble in
organic solvents; and starting from diacyl selenides or diacyl
diselenides is not economical as half of the carboxylate
equivalents are wasted. Although Woollin’s reagent can directly
convert carboxylic acids to the corresponding selenocarboxylic
acids in refluxing toluene, such high reaction temperature would
cause racemization of chiral centers in amino acids and peptides.
Herein, we report a practical method for the preparation of
various selenocarboxylates under much milder conditions and
a general one-pot amidation procedure that is applicable to
amino acids and peptides starting from carboxylic acids and
azides via selenocarboxylate and selenatriazoline intermediates.
activated with isopropyl chloroformate in THF at 0 °C in the
presence of N-methylpiperidine to form the corresponding mixed
anhydride. Then, the mixed anhydride solution is added to the
freshly prepared suspension of LiAlHSeH in THF via cannula
under a nitrogen atmosphere. The reaction starts immediately
with the release of CO2 to form the corresponding selenocar-
boxylate in situ. A solution of the azide in organic solvent is
then added to the above selenocarboxylate solution via a syringe.
The reaction is monitored using TLC and/or LC-MS and worked
up after the disappearance of the starting azide, or no further
change of the reaction mixture is observed. Isopropyl chloro-
formate was found to be a better choice over other common
chloroformates (e.g., methyl chloroformate, ethyl chloroformate,
and isobutyl chloroformate) as the resulting mixed anhydride,
acyl isopropyl carbonate, is more stable and relatively more
resistant to hydrolysis.
Application of the One-Pot Selenocarboxylate/Azide Ami-
dation to Amino Acids and Short Peptides. As the electron-
deficient aromatic azides react more readily with selenocarbox-
ylates,14,15 we selected 4-cyanophenyl azide, a representative
electron-deficient aromatic azide, to demonstrate the general
applicability of our one-pot amidation procedure to various
protected amino acids and short peptides. As shown in Table
1, various amino acids all gave excellent yields (∼90%) of the
desired amide products (entries 1-8) with the exception of
Fmoc-glutamine which gave only 70% isolated yield because
of partial dehydration (entry 9). For dipeptides, Boc-Leu-Trp-
OH and Boc-Ser(Ac)-Phe-OH, and the tripeptide, Boc-Asn-Leu-
Trp-OH, the desired amidation products were obtained in 83-
92% yields without significant racemization (entries 10-12,
Table 1). Furthermore, common amino protecting groups such
as Boc, Fmoc, Z, acetyl, and OBzl are well tolerated under the
current conditions. The high yields of the desired amide products
demonstrate that the carboxylic acids were effectively converted
to the corresponding selenocarboxylates in our one-pot amida-
tion procedure.
Results and Discussion
Preparation of Selenocarboxylates and a General One-
Pot Amidation Procedure. We found that Ishihara selenating
reagent, LiAlHSeH, smoothly converted mixed anhydrides to
the corresponding selenocarboxylates under very mild conditions
as shown in Scheme 2. The preparation of LiAlHSeH entails
treating LiAlH4 with a stoichiometric amount of selenium in
THF at 0 °C to give a gray LiAlHSeH suspension.45 LiAlHSeH
reacted readily with the freshly prepared mixed anhydride at 0
°C to give the corresponding selenocarboxylate which can be
used directly for the amidation reaction. Thus, we developed
the following general one-pot procedure for the selenocarboxy-
late/azide amidation reaction. First, the carboxylic acid is
THF is the solvent of choice for the preparation of LiAlHSeH.
The reaction of LiAlH4 with selenium did not occur in
CH2Cl2. For the amidation step (step 3), both protic and aprotic
solvents could be used as cosolvents. Overall, we found that
the selenocarboxylate/azide amidation proceeded faster in polar
solvents. As shown in Table 2, in comparison with the reaction
in neat THF (entry 1), mixing with polar organic solvents
including acetone, methanol, and acetonitrile slightly accelerated
the reaction but there were no significant effects on the overall
yields (entries 2-4). Increasing the ratio of acetonitrile from
25% to 50% shortened the reaction time slightly from 1.5 to
1.0 h (entries 4 vs 5, Table 2). However, further increasing the
concentration of acetonitrile to 75% caused slight reaction
retardation (entries 5 vs 6, Table 2). This could be because of
the decreased solubility of the highly lipophilic azide in the
presence of too much polar solvent leading to heterogeneity of
the reaction mixture. It should also be noted that the presence
of water did not adversely affect the amidation reaction as we
reported previously,15 and a good overall yield of 89% was
(38) Kawahara, Y.; Kato, S.; Kanda, T.; Murai, T.; Ishihara, H. J. Chem.
Soc., Chem. Commun. 1993, 277-278.
(39) Kawahara, Y.; Kato, S.; Kanda, T.; Murai, T. Bull. Chem. Soc. Jpn.
1994, 67, 1881-1885.
(40) Kojima, Y.; Ibi, K.; Kanda, T.; Murai, T.; Kato, S. Bull. Chem.
Soc. Jpn. 1993, 66, 990-992.
(41) Ishihara, H.; Hirabayashi, Y. Chem. Lett. 1976, 5, 203-204.
(42) Kageyama, H.; Takagi, K.; Murai, T.; Kato, S. Z. Naturforsch 1989,
44b, 1519-1523.
(43) Ishilhara, H.; Muto, S.; Kato, S. Synthesis 1986, 128-130.
(44) Fitzmaurice, J. C.; Williams, D. J.; Wood, P. T.; Woollins, J. D.
J. Chem. Soc., Chem. Commun. 1988, 741-743.
(45) Ishihara, H.; Koketsu, M.; Fukuta, Y.; Nada, F. J. Am. Chem. Soc.
2001, 123, 8408-8409.
J. Org. Chem, Vol. 72, No. 3, 2007 767

Wu and Hu
TABLE 1. Three-Step Amidation of 4-Cyanophenyl Azide with Amino Acids and Peptidesa
a General conditions: amino acid or peptide (1.1 mmol), isobutyl chloroformate (1.1 mmol), and N-methylpiperidine (1.1 mmol), THF, 0 °C, 20 min;
LiAlHSeH (1.1 mmol), 5 °C, 30 min; 4-cyanophenyl azide (1.0 mmol), THF, room temperature, 3 h. b Isolated yields based on azides as the limiting
reagents.
TABLE 2. Effect of Solvents on Reaction Time and Yield of the Amidation Reactiona
Entry
Solvent system used in step 3
Reaction time (h)
Yield of 10 (%)b
1
2
3
4
5
6
7
THF
3
85
87
88
89
92
91
89
THF/CH3COCH3 (3:1, v/v)
THF/MeOH (3:1, v/v)
THF/CH3CN (3:1, v/v)
THF/CH3CN (1:1, v/v)
THF/CH3CN (1:3, v/v)
THF/H2O (3:1, v/v)
1.5
1.0
1.5
1.0
2.0
1.5
a General conditions: Z-Phe-OH (1.1 mmol), isobutyl chloroformate (1.1 mmol), and N-methylpiperidine (1.1 mmol), THF, 0 °C, 20 min; LiAlHSeH (1.1
mmol); 4-cyanophenyl azide (1.0 mmol), room temperature. b Isolated yields based on azides as the limiting reagents.
obtained when a mixed solvent of THF and water (3:1, v/v)
was used as the solvent in the third amidation step (entry 7,
Table 2).
Determination of Amino Acid Racemization during the
Three-Step Amidation Process. To determine whether our
conditions caused racemization of the amino acids during the
one-pot procedure, we hydrolyzed compound 10 with 6 N HCl
followed by derivatization with o-phthalaldehyde (OPA) and
NR-tert-butyloxycarbonyl-L-cysteine (NBC). The resulting isoin-
dole derivatives were analyzed by reversed-phase HPLC and
compared with L-phenylalanine and D-phenylalanine isoindole
derivatives. NBC was found to give better separation of the
isoindole derivatives of D- and L-phenylalanines over the other
often used reagent NR-acetyl-L-cysteine (NAC)46 presumably due
768 J. Org. Chem., Vol. 72, No. 3, 2007

Efficient Amidation Via Selenocarboxylates
FIGURE 2. Stabilities of benzeneselenocarboxylate in THF at room
temperature (-b-) and 55 °C (--O--), in acetonitrile at room
temperature (-9-) and 55 °C (--0--), and in DMSO/EtOAc (1:1 v/v)
at room temperature (-2-) as monitored by conversion to N-(p-
toluenesulfonyl)benzamide in an HPLC assay.
FIGURE 1. OPA/NBC derivatization and subsequent HPLC analysis
of D- and L-phenylalaninamides after acid hydrolysis. The chromato-
grams shown are (a) NR-Z-N′-(4-cyanophenyl)-L-phenylalaninamide 10,
(b) NR-Z-L-phenylalanine control, (c) NR-Z-N′-benzyl-L-phenylalanina-
mide control, and (d) DL-phenylalanine standards.
and DMSO/EtOAc (conditions C) as the solvents. The stability
of benzeneselenocarboxylate was measured by monitoring the
amide formation between benzeneselenocarboxylate and 4-tolu-
enesulfonyl azide under the present reaction conditions. The
amide product, N-(4-toluenesulfonyl)benzamide, was then ana-
lyzed to determine the amount of benzeneselenocarboxylate that
remained in solution via an HPLC assay. To ensure fast and
quantitative consumption of all benzeneselenocarboxylate that
remained in solution at a given time point, 2 equiv of
4-toluenesulfonyl azide was used. Complete and quantitative
amidation was achieved in less than 5 min under these
conditions as monitored by HPLC. Such fast product formation
allowed us to accurately measure the amount of benzenesele-
nocarboxylate during our stability study. Figure 2 illustrates the
stability profiles of benzeneselenocarboxylate in THF under our
current conditions as compared to our earlier conditions in
acetonitrile15 and in 50% DMSO/EtOAc.14 The half-life of
benzeneselenocarboxylate was found to be 16 h in THF at
25 °C (kobs ) 3.1 × 10-5 s-1) as compared to 11 h in
acetontrile15 (kobs ) 1.8 × 10-5 s-1) and 25 min in DMSO/
EtOAc14 (kobs ) 4.6 × 10-4 s-1) at 25 °C. When the temperature
was raised to 55 °C, the half-life of benzeneselenocarboxylate
was around 10 h in THF (kobs ) 1.9 × 10-5 s-1) as compared
to 1.4 h in acetonitrile (kobs ) 1.4 × 10-4 s-1). The longer half-
life of the selenocarboxylate under our present conditions might
be due to the presence of trace amounts of reducing agents in
the reaction mixture, which probably delayed the oxidation/
decomposition of selenocarboxylates caused by a trace amount
of oxygen present in the solvent. This, thus, benefited the
amidation reactions that require longer time and mild heating.
to the increased bulkiness of the t-Boc group as compared to
acetyl. As shown in Figure 1, the resulting isoindole derivatives
of D- and L-phenylalanine standards were baseline separated with
retention times of 13.55 and 12.59 min, respectively, in our
reversed-phase HPLC system (chromatogram d, Figure 1). Using
this analytical method, we found that amide 10 prepared using
our one-pot, three-step amidation process contains a small
amount (1.7%) of the D-isomer (chromatogram a, Figure 1),
and the control NR-Z-N′-benzyl-L-phenylalaninamide sample
prepared using the conventional direct coupling of the mixed
anhydride with benzyl amine contains about 1.5% of the
D-isomer (chromatogram c, Figure 1). These results indicate that
any racemization during our one-pot, three-step amidation
process is the result of the first step of carboxylate activation,
i.e., the formation of a mixed anhydride, rather than of the last
two steps of selenocarboxylate generation and amidation. This
is consistent with an earlier report that conventional amidation
methods using mixed anhydrides could lead to a small amount
of racemization during peptide synthesis.47
Stability of the Selenocarboxylates. We previously dem-
onstrated that the stability of the selenocarboxylate affected the
overall yields of amidation for electron-rich azides, although it
did not present a problem for the faster amidation reactions with
electron-deficient azides.14 We were able to improve the
amidation yields for the less reactive electron-rich azides by
increasing the half-life of the selenocarboxylate.15 Therefore,
the stability of selenocarboxylate under the current conditions
was also determined. We used the same model substrate,
benzeneselenocarboxylate, to compare its stability under the
current conditions using THF as the solvent (conditions A) to
that under previous conditions using acetonitrile (conditions B)
Effects of Azides on the Amidation Reaction. After we
confirmed the increased stability of benzeneselenocarboxylate
under our new reaction conditions, a variety of azides were used
to explore the effects of this improved stability on the amidation
reaction and for comparison with previous amidation conditions.
Similar to what we reported previously, electron-deficient azides
were more reactive than electron-rich azides (Table 3). For
electron-deficient azides, 1.1 equiv of benzeneselenocarboxylate
was sufficient to give excellent conversion to the corresponding
(46) Chernobrovkin, M. G.; Anan’eva, I. A.; Shapovalova, E. N.;
Shpigun, O. A. J. Anal. Chem. 2004, 59, 64-72.
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Protein Res. 1992, 40, 559-566.
J. Org. Chem, Vol. 72, No. 3, 2007 769

Wu and Hu
TABLE 3. Amidation of Benzeneselenocarboxylate with Azides under Different Conditions
a General conditions: (A) in THF with benzeneselenocarboxylate prepared by treating the mixed anhydride with LiAlHSeH; (B) in acetonitrile with
benzeneselenocarboxylate prepared by treating diacyl diselenide with 1 equiv of piperidine in the presence of 1 equiv of diisopropylethylamine (ref 16);
(C) in DMSO/EtOAc (1:1, v/v) with benzeneselenocarboxylate prepared by treating dibenzoyl selenide with 1 equiv of KOMe (ref 15). b Isolated yields
based on azides as the limiting reagents.
amides with isolated yields of 81-98% (conditions A, entries
1-6, Table 3). For electron-rich azides that are less reactive,
2.0 equiv of bezeneselenocarboxylate and mild heating were
used to obtain good yields (conditions A, entries 7-12, Table
3). We previously demonstrated that the yield of selenocar-
boxylate/azide amidation correlated well with the stability and
solubility of the selenocarboxylate in the reaction medium.14,15
For highly reactive electron-deficient azides, the reactions were
complete in a very short time. Thus, the increased half-life of
selenocarboxylate did not seem to offer any benefits for electron-
deficient azides as similar overall yields were obtained under
the present conditions (A) as compared to the previous reported
conditions of B and C (entries 1-5, Table 3). For benzoyl azide,
the desired amide 27 was obtained in 81% yield under the
present conditions (A) as compared to the previous 51% yield
using conditions B15 (entry 6, Table 3). For 2,3,4,6-tetra-O-
acetyl-â-D-galactopyranosyl azide, the desired product 28 was
obtained in 71% yield (entry 7, Table 3). 6-[(tert-Butyldiphe-
nylsilyl)oxy]hexyl azide, representing an aliphatic azide, also
reacted with selenocarboxylate to form the desired amide 33 in
62% yield under our current conditions (A, entry 12, Table 3).
For the less reactive 4-methoxyphenyl azide, the corresponding
amide 29 was obtained in 63% yield under our current
conditions (A) compared to the previous 65% yield under
conditions B15 and only 7% yield under conditions C14 (entry
8, Table 3). Similarly, 4-azido-benzyl alcohol gave the desired
770 J. Org. Chem., Vol. 72, No. 3, 2007

Efficient Amidation Via Selenocarboxylates
SCHEME 3. Proposed Mechanisms of Selenocarboxylate/
Azide Amidation
mechanism, path A, could be operative. Bimolecular union of
the terminal nitrogen of the electron-deficient azide with
selenium of the selenocarboxylate followed by an intramolecular
cyclization form selenatriazoline intermediate 35. Alternatively,
a concerted [3+2] cycloaddition, path B, would give the
selenatriazoline intermediate 35 before decomposition of 35 via
a retro-[3+2] cycloaddition to the amide product 9. The
observed solvent effects and electronic effects of the azide
substrates are consistent with these mechanisms. The amidation
reaction proceeded faster in polar solvents, indicating the
involvement of a polar transition state that can be stabilized by
polar solvents. Electron-withdrawing groups on azides help
stabilize the transition state 34 by delocalizing the negative
charge on the nitrogen and facilitate the amidation reaction
through the path A stepwise mechanism. For the electron-rich
azides, the amidation reaction may proceed through the path B
concerted [3+2] cyclization mechanism and is much slower and
requires a longer reaction time at an elevated temperature just
like the coupling of thio acids with azides.
product 30 with an improved yield of 70% compared to the
previous 56%15and 44%14 yields under conditions B and C,
respectively (entry 9, Table 3). Even the more electron-rich
4-aminophenyl azide reacted with the selenocarboxylate afford-
ing 54% yield of N-(4-aminophenyl)benzamide 32 under both
conditions A and B. These observations are consistent with our
earlier findings that the more electron-rich azides would give
poorer amidation yields. The successful amidation of 4-azido-
benzyl acetate to 4-benzamidobenzyl acetate 31 in 68-76%
yield (entry 10, Table 3) suggests that the amidation does not
involve in situ reduction of the azide to the corresponding amine
prior to amide bond formation. Otherwise, the reduction would
have led to the formation of polymerized quinonimine methide
instead of the desired amide as discussed earlier and depicted
in Scheme 1. Under our current conditions, we were able to
apply mild heating to accelerate the amidation process for the
less reactive azides and to improve the amidation yields.
Although both conditions A and B showed comparable yields
for both electron-deficient and electron-rich azides, the easy
adaption of conditions A to the one-pot procedure starting
directly from carboxylic acids provides a straightforward,
practical, and efficient route to the selenocarboxylate/azide
amidation reaction that is applicable to amino acids and peptides.
Mechanisms of Selenocarboxylate/Azide Amidation. Our
experimental results clearly demonstrated that the yields of
selenocarboxylate/azide amidation depend primarily upon the
electronic properties of the azides. Electron-deficient azides react
with selenocarboxylates much faster and give better yields than
electron-rich azides. This is in apparent contrast to the structure-
activity relationship expected of the “in situ reduction and
acylation” mechanism where electron-rich azides would give
more nucleophilic amines and the more nucleophilic amines
should facilitate the nucleophilic acylation and afford higher
yields than electron-deficient azides. Furthermore, when ben-
zeneselenocarboxylate was incubated with 4-cyanoaniline, there
was no detectable amide formed whereas incubation of ben-
zeneselenocarboxylate with 4-cyanophenyl azide gave 94% yield
of the amide. In addition, we were able to successfully
synthesize 4-benzamidobenzyl acetate 31 from 4-azidobenzyl
acetate, suggesting that there was no in situ reduction of the
azide otherwise the spontaneous decomposition of 4-aminoben-
zyl acetate would lead to the polymerized quinonimine methide
instead of the amide product. On the basis of careful mechanistic
studies, two pathways leading to the formation of a thiatriazoline
intermediate have been identified for the Williams thio acid/
azide amidation.12 Depending on the electronic properties of
the azide, a thiatriazoline intermediate forms via a stepwise or
concerted mechanism. By analogy, selenocarboxylate/azide
amidation likely proceeds through the corresponding selena-
triazoline intermediate 35. As shown in Scheme 3, a stepwise
Conclusion
Selenocarboxylate/azide amidation provides an attractive
alternative method to the conventional nucleophilic acylation
when an amide bond needs to be formed without going through
an amine intermediate. The reaction is highly chemoselective
and clean and can be carried out under aqueous conditions. The
reaction also represents a traceless coupling between a car-
boxylic acid and an azide. Selenocarboxylate/azide amidation
is complementary to the Staudinger ligation, as the Staudinger
ligation favors electron-rich azides and selenocarboxylate/azide
amidation works more effectively on electron-deficient azides.
However, the lack of practical preparation of selenocarboxylates
limited the application of this type of amidation reaction. Here,
we developed a convenient one-pot procedure to form seleno-
carboxylates by reacting the mixed anhydrides of carboxylic
acids with LiAlHSeH under mild conditions. It is effective over
a range of carboxylic acids, including amino acids and peptides.
It overcomes disadvantages of other methods of generating
selenocarboxylates, such as the high temperature required when
Woollin’s reagent44 is used and the waste of the carboxylate
equivalent when a diacyl diselenide or diacyl selenide is used.
This easy-to-handle procedure to form selenocarboxylates
directly from carboxylic acids facilitates the selenocarboxylate/
azide amidation and makes this type of amidation reaction
practical. It should also benefit other reactions related to
selenocarboxylates. In the selenocarboxylate/azide amidation
reaction, excellent yields are obtained for electron-deficient
azides and significantly improved yields are achieved for the
less reactive azides because of the dramatically increased
stability of selenocarboxylates under the present conditions.
Experimental Section
Preparation of LiAlHSeH. LiAlHSeH was prepared essentially
following the literature procedure.45 Briefly, to a suspension of
LiAlH4 (44 mg, 1.1 mmol) in anhydrous THF (10 mL) was added
selenium powder (88 mg, 1.1 mmol) in one portion at 0 °C. The
mixture was stirred at 0 °C for 20 min under a nitrogen atmosphere
and ready to be used.
General Amidation Procedure. To a solution of carboxylic acid
(1.1 mmol) and N-methylpiperidine (0.134 mL, 1.1 mmol) in THF
(10 mL) was added a 1.0 M solution of isopropylchloroformate in
toluene (1.1 mL, 1.1 mmol) at 0 °C under a nitrogen atmosphere.
The resulting mixture was stirred for 20 min at 0 °C. Then, the
J. Org. Chem, Vol. 72, No. 3, 2007 771

Wu and Hu
obtained mixed anhydride solution was slowly added into the
prepared LiAlHSeH solution via cannulation over a period of
5 min. The reaction mixture was stirred for an additional 30 min
below 5 °C under a nitrogen atmosphere. Then, a solution of azide
(1.0 mmol) in THF (1 mL) was added into the above selenocar-
boxylate solution via a syringe. For electron-deficient azides, the
amidation reaction was carried out at 0 °C to room temperature.
For electron-rich azides, 0.5 mmol of the azide was used and the
reaction was carried out at 55 °C. When TLC and/or LC-MS
showed the disappearance of the starting azide or no further change
of the reaction mixture, the reaction mixture was filtered through
a Celite pad that was then rinsed with EtOAc (3 × 25 mL). The
combined organic phase was washed with 5% NaHCO3, water, and
brine and dried over anhydrous Na2SO4. After removal of Na2SO4
through filtration, the filtrate was treated with activated charcoal.
The activated charcoal was filtered off, and the filtrate was then
concentrated to dryness. The crude product was purified by flash
column chromatography (FCC) on silica gel. Yields and physical
and spectroscopic data of all amides are consistent with their
structures.
MHz): δ 10.05 (brs, 1H, -NH), 7.38-7.52 (m, 4H), 4.50 (brs,
-NH), 3.41-3.53 (m, 2H), 1.91-2.28 (m, 4H), 1.49 (s, 9H). 13C
NMR (CDCl3, 50 MHz): δ 171.2, 156.3, 142.6, 133.0, 119.4, 119.0,
106.3, 81.2, 60.7, 47.4, 28.5, 28.3, 24.6. MS (ESI+): m/z (intensity),
316.2 (MH+, 40%), 334.2 (MH+ + H2O, 100%).
NR-tert-Butyloxycarbonyl-N′-(4-cyanophenyl)-L-valinamide (16).
The product (282 mg) was obtained after FCC (CH2Cl2/MeOH,
50:1) in 89% isolated yield. 1H NMR (CDCl3, 200 MHz): δ 9.35
(brs, 1H, -NH), 7.39-7.52 (m, 4H), 5.40 (d, -NH), 4.09-4.16
(m, 1H), 2.06-2.16 (m, 1H), 1.43 (s, 9H), 1.01 (d, 6H, J ) 7.0
Hz). 13C NMR (CDCl3, 50 MHz): δ 171.5, 156.8, 142.0, 133.0,
119.5, 118.9, 106.9, 80.9, 61.2, 30.8, 28.4, 19.6, 18.5. MS (ESI+):
m/z (intensity), 318.2 (MH+, 100%).
NR-(9-Fluorenyloxycarbonyl)-N′-(4-cyanophenyl)-L-glutami-
namide (17). The product (328 mg) was obtained after FCC (CH2-
Cl2/MeOH, 20:1) in 70% isolated yield. 1H NMR (DMSO-d6, 200
MHz): δ 10.54 (s, 1H, -NH), 7.72-7.91 (m, 8H), 7.30-7.46 (m,
4H), 6.83 (s, 1H, -NH), 4.09-4.32 (m, 4H), 2.17-2.21 (m, 2H),
1.83-1.99 (m, 2H). 13C NMR (DMSO-d6, 50 MHz): δ 173.4,
171.7, 156.1, 143.8, 143.1, 140.7, 133.3, 127.7, 127.1, 125.3, 120.1,
119.3, 119.1, 105.1, 65.8, 55.4, 46.6, 31.5, 27.3. MS (ESI+): m/z
(intensity), 469.2 (MH+, 100%), 491.2 (M + Na+, 50%).
NR-Benzyloxycarbonyl-N′-(4-cyanophenyl)-L-phenylalanina-
mide (10). The product (340 mg) was obtained after FCC (CH2-
1
Cl2/MeOH, 10:1) in 85% isolated yield. H NMR (CDCl3, 200
NR-(9-Fluorenyloxycarbonyl)-N′-(4-cyanophenyl)-L-tryptopha-
namide (18). The product (458 mg) was obtained after FCC
MHz): δ 8.80 (brs, 1H, -NH), 7.16-7.48 (m, 14H), 5.82 (d, -NH,
J ) 8.0 Hz), 5.07 (s, 2H), 4.66 (q, 1H, J ) 8.0 Hz), 3.13 (dd, 2H,
J ) 6.6 Hz, J ) 7.4 Hz). 13C NMR (CDCl3, 50 MHz): δ 170.2,
156.7, 141.5, 136.0, 135.8, 133.2, 129.3, 129.0, 128.7, 128.5, 128.1,
127.9, 127.4, 119.8, 107.3, 67.6, 57.4, 38.4. MS (ESI+): m/z
(intensity), 400.2 (MH+, 100%), 441.2 (MH+ + CH3CN, 15%).
O3-Benzyl-NR-benzyloxycarbonyl-N′-(4-cyanophenyl)-L-seri-
namide (11). The product (384 mg) was obtained after FCC (CH2-
1
(hexane/CH2Cl2/MeOH, 10:10:1) in 87% isolated yield. H NMR
(CDCl3, 200 MHz): δ 8.38 (brs, 1H, -NH), 8.10 (s, 1H, NH),
7.78 (d, 2H, J ) 7.4 Hz), 7.28-7.61 (m, 11H), 7.17 (t, 2H, J )
8.0 Hz), 7.05 (t, 2H, J ) 8.0 Hz), 6.95 (brs, 1H, NH), 4.68 (m,
1H), 4.34 (d, 2H, J ) 6.8 Hz), 4.17 (t, 1H, J ) 7.4 Hz), 3.26-3.31
(m, 2H). 13C NMR (CDCl3, 50 MHz): δ 170.4, 156.5, 143.5, 143.4,
141.3, 136.1, 133.0, 127.8, 127.1, 127.0, 124.9, 123.2, 122.5, 120.1,
120.0, 119.7, 118.7, 118.4, 111.4, 109.9, 107.1, 67.4, 56.4, 46.9,
28.2. MS (ESI+): m/z (intensity), 527.2 (MH+, 100%), 549.2 (M
+ Na+, 20%).
1
Cl2/MeOH, 50:1) in 89% isolated yield. H NMR (CDCl3, 200
MHz): δ 8.85 (brs, 1H, -NH), 7.45-7.55 (m, 4H), 7.23-7.32
(m, 10H), 5.92 (d, 1H, J ) 6.8 Hz), 5.11 (s, 2H), 4.47-4.60 (m,
3H), 3.92-3.97 (m, 1H), 3.62-3.70 (m, 1H). 13C NMR (CDCl3,
50 MHz): δ 16.8.7, 156.3, 141.4, 136.9, 135.7, 133.0, 128.5, 128.4,
128.2, 128.1, 127.9, 127.7, 119.6, 118.7, 107.1, 73.5, 69.3, 67.3,
55.0. MS (ESI+): m/z (intensity), 430.2 (MH+, 100%), 452.2 (M
+ Na+, 20%).
NR-tert-Butyloxycarbonyl-L-leucyl-N′-(4-cyanophenyl)-L-tryp-
tophanamide (19). The product (450 mg) was obtained after FCC
1
(CH2Cl2/MeOH, 20:1) in 87% isolated yield. H NMR (CD3OD,
200 MHz): δ 7.59 (d, 2H, J ) 8.4 Hz), δ 7.43-7.50 (m, 3H),
7.21 (d, 1H, J ) 8.0 Hz), 7.01 (s, 1H), 6.85-6.97 (m, 2H), 4.67
(m, 1H), 3.95 (t, 1H), 3.22-3.25 (m, 2H), 1.40-1.60 (m, 1H), 1.35
(t, 2H, J ) 6.6 Hz), 1.25 (s, 9H), 0.80 (dd, 6H, J ) 8.2 Hz). 13C
NMR (CD3OD, 50 MHz): δ 175.7, 172.7, 158.2, 143.8, 138.0,
134.0, 128.7, 124.7, 122.5, 121.2, 119.9, 119.8, 119.3, 112.3, 110.2,
107.8, 80.9, 56.1, 55.1, 41.6, 28.6, 25.8, 23.3, 21.8. MS (ESI+):
m/z (intensity), 540.3 (M + Na+, 100%), 581.3 (M + Na+ + CH3-
CN, 37%).
O3-Acetyl-NR-tert-butyloxycarbonyl-D,L-seryl-N′-(4-cyanophe-
nyl)-L-phenylalaninamide (20). The product (454 mg) was
obtained after FCC (CH2Cl2/MeOH, 20:1) in 92% isolated yield.
1H NMR (CD3OD, 200 MHz): δ 7.68 (d, 2H, J ) 8.4 Hz), 7.55
(d, 2H, J ) 8.4 Hz), 7.14 (brs, 5H), 4.67 (dd, 1H, J ) 8.4 Hz, J )
8.4 Hz), 4.14-4.20 (m, 1H), 4.00-4.09 (m, 2H), 2.88-3.18 (m,
2H), 1.89 (s, 3H), 1.36 (s, 9H). 13C NMR (CD3OD, 50 MHz): δ
172.2, 171.8, 157.6, 143.7, 137.9, 137.7, 134.1, 130.3, 129.5, 127.9,
121.2, 119.7, 107.8, 81.0, 64.7, 56.5, 55.1, 38.9, 28.6, 20.7. MS
(ESI+): m/z (intensity), 517.3 (M + Na+, 100%), 558.2 (M + Na+
+ CH3CN, 40%).
NR-tert-Butyloxycarbonyl-L-asparaginyl-L-leucyl-N′-(4-cy-
anophenyl)-L-tryptophanamide (21). The product (523 mg) was
obtained after FCC (CH2Cl2/MeOH, 20:1) in 83% isolated yield.
1H NMR (CD3OD, 200 MHz): δ 7.78 (d, 2H, J ) 8.8 Hz), 7.52
(d, 3H, J ) 8.8 Hz), 7.34 (d, 1H, J ) 8.8 Hz), 7.05 (s, 1H), 6.85-
7.00 (m, 2H), 4.59-4.68 (m, 1H), 4.30-4.37 (m, 1H), 4.05 (t, 1H,
J ) 7.2 Hz), 3.19-3.4 (m, 2H), 2.48-3.13 (m, 2H), 1.47-1.60
(m, 3H), 1.33 (s, 9H), 0.70 (dd, 6H, J ) 8.8 Hz). 13C NMR (CD3-
OD, 50 MHz): δ 175.0, 174.9, 172.9, 157.5, 144.0, 138.0, 134.0,
128.7, 124.6, 122.4, 121.3, 119.8, 119.3, 112.3, 111.3, 107.8, 80.9,
NR-Benzyloxycarbonyl-N′-(4-cyanophenyl)-L-glutamina-
mide (12). The product (342 mg) was obtained after FCC (CH2-
Cl2/MeOH, 20:1) in 90% isolated yield. 1H NMR (DMSO-d6, 200
MHz): δ 10.55 (s, 1H), 7.70-7.85 (m, 4H), 7.36 (brs, 5H), 6.85
(s, 1H), 5.04 (s, 2H), 4.11-4.20 (m, 1H), 2.15-2.21 (m, 2H), 1.70-
1.99 (m, 2H). 13C NMR (DMSO-d6, 50 MHz): δ 174.2, 172.4,
156.8, 143.9, 137.7, 134.0, 129.1, 128.9, 128.5, 120.1, 119.8, 105.9,
66.3, 56.1, 32.1, 28.0. MS (ESI+): m/z (intensity), 381.2 (MH+,
100%), 444.4 (M + Na+ + CH3CN, 15%).
NR-Benzyloxycarbonyl-N′-(4-cyanophenyl)-L-methionina-
mide (13). The product (337 mg) was obtained after FCC (CH2-
1
Cl2/MeOH, 40:1) in 88% isolated yield. H NMR (CDCl3, 200
MHz): δ 9.02 (brs, 1H, -NH), 7.48-7.61 (m, 4H), 7.35 (brs, 5H),
5.78 (d, J ) 8.0 Hz), 5.14 (s, 2H), 4.51-4.62 (m, 1H), 2.62 (t, 2H,
J ) 6.6 Hz), 1.98-2.26 (m, 5H). 13C NMR (CDCl3, 50 MHz): δ
170.3, 156.9, 141.7, 135.8, 133.3, 128.8, 128.6, 128.1, 119.8, 118.8,
107.4, 67.7, 54.9, 30.9, 30.3, 15.4. MS (ESI+): m/z (intensity), 384.1
(MH+, 100%).
O5-Benzyl-NR-benzyloxycarbonyl-N′-(4-cyanophenyl)-L-glutam-
ic Amide (14). The product (394 mg) was obtained after FCC (CH2-
1
Cl2/MeOH, 50:1) in 90% isolated yield. H NMR (CDCl3, 200
MHz): δ 9.30 (brs, 1H, -NH), 7.51-7.64 (m, 4H), 7.36 (brs, 5H),
5.61 (d, -NH, J ) 6.6 Hz), 5.13 (s, 2H), 4.37-4.40 (m, 1H), 2.45-
2.65 (m, 2H), 1.98-2.29 (m, 2H), 1.49 (s, 9H). 13C NMR (CDCl3,
50 MHz): δ 173.3, 170.7, 156.5, 142.0, 135.6, 133.2, 128.7, 128.5,
128.3, 119.7, 118.9, 107.2, 81.0, 66.9, 54.7, 30.7, 28.4, 27.5. MS
(ESI+): m/z (intensity), 438.2 (MH+, 100%).
NR-tert-Butyloxycarbonyl-N′-(4-cyanophenyl)-L-prolina-
mide (15). The product (285 mg) was obtained after FCC (CH2-
1
Cl2/MeOH, 50:1) in 89% isolated yield. H NMR (CDCl3, 200
772 J. Org. Chem., Vol. 72, No. 3, 2007

Efficient Amidation Via Selenocarboxylates
56.8, 54.5, 52.3, 40.8, 37.8, 28.6, 28.2, 25.6, 23.9, 21.6. MS
(ESI+): m/z (intensity), 632.3 (MH+, 10%), 654.3 (M + Na+,
100%).
(95% yield based on the recovery). 1H NMR (acetone-d6, 200
MHz): δ 9.40 (brs, NH), 7.97 (dd, 2H, J ) 7.8 Hz, J ) 1.4 Hz),
7.72 (d, 2H, J ) 8.6 Hz), 7.42-7.59 (m, 3H), 6.91 (dd, 2H, J )
6.8 Hz, J ) 2.4 Hz), 3.78 (s, 3H, -OCH3). 13C NMR (acetone-d6,
50 MHz): δ 165.7, 156.9, 136.3, 133.3, 131.9, 129.0, 125.0, 122.4,
114.4, 55.5. MS (ESI+): m/z (intensity), 228.1 (MH+, 100%), 269.1
(MH+ + CH3CN, 5%).
N-(4-Nitrophenyl)benzamide (22).48 The product (237 mg) was
obtained after FCC (hexane/EtOAc, 5:1) in 98% isolated yield. 1H
NMR (DMSO-d6, 200 MHz): δ 10.82 (s, -NH), 8.27 (d, 2H, J )
7.4 Hz), 8.08 (d, 2H, J ) 8.4 Hz), 7.99 (d, 2H, J ) 8.4 Hz), 7.52-
7.65 (m, 3H). 13C NMR (DMSO-d6, 50 MHz): δ 167.1, 146.3,
143.3, 135.0, 132.9, 129.3, 128.7, 125.5, 120.6. MS (ESI+): m/z
(intensity), 243.1 (MH+, 100%), 284.1 (MH+ + CH3CN, 60%).
N-(4-Cyanophenyl)benzamide (23).49 The product (209 mg)
was obtained after FCC (hexane/EtOAc, 4:1) in 94% isolated yield.
1H NMR (CDCl3, 200 MHz): δ 8.13 (s, 1H, -NH), 7.70-7.92
(m, 4H), 7.48-7.70 (m, 5H). 13C NMR (CDCl3, 50 MHz): δ 165.9,
142.1, 134.2, 133.5, 132.6, 129.1, 127.2, 120.0, 118.9, 107.5. MS
(ESI+): m/z (intensity), 223.1 (MH+, 100%), 264.1 (MH+ + CH3-
CN, 70%).
N-(4-Hydroxymethylphenyl)benzamide (30). The product (80
mg) was obtained after FCC (hexane/EtOAc, 2:1) in 70% isolated
1
yield (95% yield based on the recovery). H NMR (CD3OD, 200
MHz): δ 7.94 (dd, 2H, J ) 8.4 Hz, J ) 2.0 Hz), 7.67 (dd, 2H,
J ) 7.8 Hz, J ) 1.8 Hz), 7.47-7.60 (m, 3H), 7.37 (d, 2H, J ) 8.4
Hz), 4.61 (s, 2H, -CH2OH). 13C NMR (CD3OD, 50 MHz): δ
168.9, 139.0, 136.3, 132.9, 129.6, 128.6, 128.5, 122.2, 64.9. MS
(ESI+): m/z (intensity), 228.1 (MH+, 100%), 269.1 (MH+ + CH3-
CN, 10%).
4-Benzamidobenzyl Acetate (31). The product (94 mg) was
obtained after FCC (hexane/EtOAc, 2:1) in 70% isolated yield (97%
yield based on the recovery). 1H NMR (CDCl3, 200 MHz): δ 8.27
(brs, 1H, -NH), δ 7.86 (d, 2H, J ) 8.0 Hz), 7.67 (d, 2H, J ) 8.4
Hz), 7.39-7.54 (m, 3H), 7.34 (d, 2H, J ) 8.4 Hz), 5.08 (s, 2H),
2.10 (s, 3H). 13C NMR (CDCl3, 50 MHz): δ 171.1, 166.1, 138.2,
134.9, 132.1, 131.9, 129.3, 128.8, 127.2, 120.5, 66.0, 21.1. MS
(ESI+): m/z (intensity), 270.1 (MH+, 100%), 311.1 (MH+ + CH3-
CN, 30%).
4-Benzamidobenzoic Acid (24).50 The product (210 mg) was
obtained after FCC (hexane/EtOAc, 1:1) in 87% isolated yield. 1H
NMR (DMSO-d6, 200 MHz): δ 10.5 (s, -COOH), 7.95 (brs, 7H),
7.53-7.56 (m, 3H). 13C NMR (DMSO-d6, 50 MHz): δ 167.0,
166.0, 143.3, 134.7, 131.8, 130.2, 128.4, 127.8, 125.5, 119.5. MS
(ESI-): m/z (intensity), 240.1 (M - H-, 100%), 258.1 (M - H-
+ H2O, 10%).
Diphenyl N-Benzoylphosphoramidate (25). The product (337
mg) was obtained after FCC (hexane/EtOAc, 2:1) in 95% isolated
N-(4-Aminophenyl)benzamide (32).54 The product (57 mg) was
obtained after FCC (hexane/EtOAc, 2:1) in 54% isolated yield (96%
1
yield. H NMR (CDCl3, 200 MHz): δ 9.67 (brs, -NH), 7.98 (d,
1
yield based on the recovery). H NMR (CD3OD, 200 MHz): δ
2H, J ) 8.6 Hz), 7.56 (t, 1H, J ) 7.8 Hz), 7.39 (dd, 2H, J ) 8.0
Hz, J ) 7.6 Hz), 7.08-7.27 (m, 10H). 13C NMR (CDCl3, 50
MHz): δ 167.7, 150.2 (d, J ) 6.8 Hz), 133.0, 132.4 (d, J ) 11
Hz), 129.8, 128.6, 125.7, 120.7, 120.6. MS (ESI+): m/z (intensity),
354.1 (M + H+, 90%), 417.1 (M + Na+ + CH3CN, 85%).
N-(4-Toluenesulfonyl)benzamide (26).51 The product (270 mg)
was obtained after FCC (hexane/CH2Cl2/MeOH, 10:10:1) in 98%
7.89 (dd, 2H, J ) 8.0 Hz, J ) 1.8 Hz), 7.42-7.52 (m, 3H), 7.37
(dd, 2H, J ) 7.0 Hz, J ) 2.0 Hz), 6.73 (dd, 2H, J ) 8.0 Hz, J )
1.8 Hz). 13C NMR (CD3OD, 50 MHz): δ 168.6, 145.9, 136.3,
132.6, 130.4, 129.5, 128.4, 124.2, 116.6. MS (ESI+): m/z (intensity),
213.1 (MH+, 80%), 254.1 (MH+ + CH3CN, 100%).
N-{6-[(tert-Butyldiphenylsilyl)oxy]hexyl}benzamide (33). The
product (143 mg) was obtained after FCC (hexane/EtOAc, 4:1) in
62% isolated yield. 1H NMR (CDCl3, 200 MHz): δ 7.75 (dd, 2H,
J ) 7.2 Hz, J ) 1.4 Hz), δ 7.61-7.71 (m, 3H), 7.24-7.50 (m,
10H), 6.2 (brs, -NH), 3.65 (t, 2H, J ) 6.4 Hz), 3.41 (dd, 2H, J )
7.0 Hz, J ) 7.2 Hz), 1.51-1.61 (m, 4H), 1.35-1.47 (m, 4H),1.04
(s, 9H). 13C NMR (CDCl3, 50 MHz): δ 167.5, 135.2, 134.8, 134.0,
131.3, 129.5, 128.5, 127.5, 126.8, 63.7, 40.0, 32.4, 29.6, 26.8, 26.7,
25.5, 19.2. MS (ESI+): m/z (intensity), 482.3 (M + Na+, 100%),
523.3 (M + Na+ + CH3CN, 10%).
Stability Studies. Aliquots of freshly prepared benzeneseleno-
carboxylate in solvents were incubated at room temperature and at
55 °C under a nitrogen atmosphere. The amount of benzenesele-
nocarboxylate was measured at different time intervals by the
reaction with 2 equiv of 4-toluenesulfonyl azide for 1 h. The
reaction mixtures were separated on a Shimadzu 2010 LCMS
system using a Chromolith SpeedROD RP-18e column (50 × 4.6
mm) at 1 mL/min with a 10-min gradient of 10-90% acetonitrile
containing 0.1% formic acid. The N-4-toluenesulfonylbenzamide
was quantitated based on UV absorption at 220 nm, and its structure
was confirmed by the mass spectrometry detector. The relative peak
area of N-4-toluenesulfonylbenzamide represents the amount of
benzeneselenocarboxylate that remained in the solution at a given
time point and was plotted against the time of incubation prior to
the addition of 4-toluenesulfonyl azide. The data were fitted using
the single, two-parameter exponential decay equation in Sigma Plot
to obtain the pseudo first-order rate constant and calculate the t1/2
value.
1
isolated yield. H NMR (CDCl3, 200 MHz): δ 9.52 (brs, -NH),
8.05 (d, 2H, J ) 8.4 Hz), 7.83 (d, 2H, J ) 7.4 Hz), 7.32-7.58 (m,
5H), 2.43 (s, 3H). 13C NMR (CDCl3, 50 MHz): δ 164.4, 145.2,
135.4, 133.4, 131.1, 129.68, 128.8, 128.7, 127.9, 21.7. MS (ESI-):
m/z (intensity), 274.1 (M - H+, 100%).
N-Benzoylbenzamide (27).52 The product (91 mg) was obtained
1
after FCC (hexane/EtOAc, 1:1) in 81% isolated yield. H NMR
(CDCl3, 200 MHz): δ 9.26 (brs, -NH), δ 7.84 (dd, 4H, J ) 8.0
Hz, J ) 1.4 Hz), δ 7.40-7.60 (m, 6H). 13C NMR (CDCl3, 50
MHz): δ 166.7, 133.3, 133.0, 128.7, 127.9. MS (ESI+): m/z
(intensity), 226.1 (M + H+, 100%), 289.1 (M + Na+ + CH3CN,
85%).
N-(2,3,4,6-Tetra-O-acetyl-â-D-galactopyranosyl)benzamide (28).
The product (160 mg) was obtained after FCC (hexane/EtOAc, 4:1)
1
in 71% isolated yield. H NMR (CDCl3, 200 MHz): δ 7.77 (d,
2H, J ) 6.6 Hz), 7.40-7.54 (m, 3H), 5.44-5.49 (m, 2H), 5.22-
5.26 (m, 2H), 4.09-4.17 (m, 3H), 2.15 (s, 3H), 2.04 (s, 3H), 2.01
(s, 6H). 13C NMR (CDCl3, 50 MHz): δ 171.8, 170.5, 170.1, 169.9,
167.2, 133.0, 132.4, 128.8, 127.3, 79.3, 72.4, 70.9, 68.7, 67.4, 61.2,
20.9, 20.8, 20.7, 20.6. MS (ESI+): m/z (intensity), 452.2 (M +
H+, 45%), 474.2 (M + Na+, 100%), 515.2 (M + Na+ + CH3CN,
25%).
N-(4-Methoxyphenyl)benzamide (29).53 The product (72 mg)
was obtained after FCC (hexane/EtOAc, 4:1) in 63% isolated yield
(48) Lee, C. K.; Yu, J. S.; Ji, Y. R. J. Heterocycl. Chem. 2002, 39, 1219-
1227.
Acid Hydrolysis of Amides. Solutions (10 µL) of NR-benzyl-
oxycarbonyl-N′-(4-cyanophenyl)-L-phenylalaninamide 10 (0.03 mM),
NR-benzyloxycarbonyl-N′-benzyl-L-phenylalaninamide (0.03 mM),
and NR-benzyloxycarbonyl-phenylalanine (0.03 mM) were placed
separately in a 6 × 50 mm test tube. The samples were then dried
(49) Largerona, M.; Vuilhorgneb, M.; Potiera, I. L.; Auzeila, N.; Bacque´b,
E.; Parisa, J. M.; Fleury, M. B. Tetrahedron 1994, 50, 6307-6332.
(50) Lee, C. K.; Ahn, Y. M. J. Org. Chem. 1989, 54, 3744-3747.
(51) Singh, D. U.; Singh, P. R.; Samant, S. D. Tetrahedron Lett. 2004,
45, 4805-4807.
(52) Ette, M. C.; Reutzel, S. M. J. Am. Chem. Soc. 1991, 113, 2586-
2598.
(53) Nieuwendijk, A. M. C. H. v. d.; Pietra, D.; Heitman, L.; Go¨blyo¨s,
A.; IJzerman, A. P. J. Med. Chem. 2004, 47, 663-672.
(54) Mohmeyer, N.; Schmidt, H.-W. Chem.-Eur. J. 2005, 11, 863-
872.
J. Org. Chem, Vol. 72, No. 3, 2007 773

Wu and Hu
using a speed-vac at room temperature. The test tubes containing
amides were placed inside an acid hydrolysis vessel, and 200 µL
of 6 N HCl was added. The vessel was flushed three times
with nitrogen and then heated under a vacuum at 110 (
2 °C in a convection oven for 24 h. The test tubes were dried using
a speed-vac for 30 min prior to derivatization.
340 nm. The elution started with an isocratic step of 40%
acetonitrile for 5 min followed by a gradient step from 40-80%
acetonitrile in 25 min. The relative amount of D- and L-amino acids
was determined by comparing the relative peak areas of OPA/NBC-
derived isoindoles corresponding to the D- and L-amino acids.
OPA/NBC Derivatization and HPLC Analysis of Amino
Acids. The OPA/NBC reagent solutions were freshly prepared by
dissolving 11 mg of o-phthaldialdehyde (OPA) and 20 mg of NR-
tert-butyloxycarbonyl-L-cysteine (NBC) in 1 mL of methanol, each.
For derivatization, 50 µL of 0.1 M Na2B4O7 (pH 9.6), 50 µL of
deionized distilled water, 20 µL of OPA solution, and 20 µL of
NBC solution were added to each of the test tubes containing
hydrolyzed samples. The resulting solution was vortexed for 5 min
and then stored at -20 °C prior to HPLC analysis.
A 20 µL solution of each sample was injected into an HPLC
system using a C18 column (150 × 4.6 mm, 3 µm) and aqueous
acetonitrile containing 0.1% trifluoroacetic acid (TFA) as the mobile
phase at a flow rate of 1 mL/min and a detection wavelength of
Acknowledgment. We gratefully acknowledge the financial
support of a research grant from the State of New Jersey
Commission on Cancer Research and a research scholar grant
from the American Cancer Society. We thank Prof. Lawrence
Williams and Spencer Knapp for helpful discussions and
Prathima Surabhi for assistance with some of the experiments.
1
Supporting Information Available: General methods and H
NMR and 13C NMR spectra of new compounds. This material is
available free of charge via the Internet at http://pubs.acs.org.
JO061703N
774 J. Org. Chem., Vol. 72, No. 3, 2007