Ethyl 2-(tert-butoxycarbonyloxyimino)-2-cyanoacetate (Boc-Oxyma) as coupling reagent for racemization-free esterification, thioesterification, amidation and peptide synthesis

Article abstract of DOI:10.1002/adsc.201200645

Here we report the synthesis and utility of ethyl 2-(tert- butoxycarbonyloxyimino)-2-cyanoacetate (Boc-Oxyma) as an efficient coupling reagent for racemization-free esterification, thioesterification, amidation reactions and peptide synthesis that uses equimolar amounts of acids and alcohols, thiols, amines or amino acids, respectively. Its application to solid phase as well as solution phase peptide synthesis is also demonstrated and a mechanistic investigation is discussed. Boc-Oxyma is similar to the well known coupling agent COMU {1-[1-cyano-2-ethoxy-2-oxoethylideneaminooxy)- dimethylaminomorpholino] uronium hexafluorophosphate} in terms of its high reactivity and mechanism of action. However, it is not only much easier to prepare, but also to recover and reuse, thereby generating far less chemical waste.

Full text of DOI:10.1002/adsc.201200645

FULL PAPERS
DOI: 10.1002/adsc.201200645
Ethyl 2-(tert-Butoxycarbonyloxyimino)-2-cyanoacetate
(Boc-Oxyma) as Coupling Reagent for Racemization-Free
Esterification, Thioesterification, Amidation and Peptide
Synthesis
Kishore Thalluri,a Krishna Chaitanya Nadimpally,a
Maharishi Parasar Chakravarty,a Ashim Paul,a and Bhubaneswar Mandala,*
a
Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati, Assam – 781039, India
Fax : (+91)-358-258-2349; phone: (+91)-361-258-2319; e-mail: bmandal@iitg.ernet.in
Received: July 23, 2012; Revised: November 16, 2012; Published online: February 1, 2013
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201200645.
Abstract: Here we report the synthesis and utility of 2-oxoethylideneaminooxy)-dimethylaminomorpholi-
ethyl
2-(tert-butoxycarbonyloxyimino)-2-cyanoace- no]uronium hexafluorophosphate} in terms of its
tate (Boc-Oxyma) as an efficient coupling reagent high reactivity and mechanism of action. However, it
for racemization-free esterification, thioesterifica- is not only much easier to prepare, but also to recov-
tion, amidation reactions and peptide synthesis that er and reuse, thereby generating far less chemical
uses equimolar amounts of acids and alcohols, thiols, waste.
amines or amino acids, respectively. Its application to
solid phase as well as solution phase peptide synthe-
sis is also demonstrated and a mechanistic investiga- Keywords: amidation; Boc-Oxyma; coupling re-
tion is discussed. Boc-Oxyma is similar to the well agents; esterification; peptide synthesis; racemiza-
known coupling agent COMU {1-[1-cyano-2-ethoxy- tion; thioesterification
Introduction
sis and has gained popularity as a non-explosive alter-
native to HOBt and HOAt. However, a major disad-
Amidation, esterification and thioesterification reac- vantage of most coupling reagents invented to date is
tions of carboxylic acids and amino acids are key re- that they generate substantial amounts of undesired
actions in the organic synthesis[1] of many important by-products and chemical waste. Moreover, the prep-
molecules of biological interest.[2] To date, a variety of aration of these coupling reagents is often very diffi-
coupling reagents have been developed for peptide cult and involves multi-step syntheses that require
synthesis[3] which are also capable of effecting these harsh reagents.
same reactions. The most effective coupling agents
The use of Boc-protected reagents is also known to
are those that not only afford fast reaction rates and inhibit racemization and Boc2O has been shown to be
high yields, but also the suppression of any undesired quite effective in this regard.[7] However, Boc2O medi-
racemization.
ated coupling proceeds via mixed anhydride forma-
To prevent racemization in peptide synthesis, N-hy- tion which inevitably generates unwanted by-products
droxyamine reagents, for example, HOBt (hydroxy- (e.g., Boc-protected amines), thus reducing the yield
benzotriazole) and HOAt (hydroxyazabenzotriazole), of the desired product, increasing waste, and compli-
are frequently used as additives in conjunction with cating purification.
the coupling reagents such as carbodiimides, uronium
We wish to report herein the invention of the first
salts, phosphonium salts, phosphates, phosphinates member of a novel class of coupling reagent, Boc-
and immonium salts.[4] Recently, Oxyma (ethyl 2-hy- Oxyma. Similar to the recently developed COMU (1-
droxyimino 2-cyanoacetate) has also been used as {[1-(cyano-2-ethoxy-2-oxoethylideneaminooxy)dime-
a racemization suppressant in DIC (diisopropylcarbo- thylaminomorpholino]}uronium
hexafluorophos-
diimide)[5] and TFFH (tetramethylfluoroformamidini- phate), a coupling reagent leading to excellent yields
um hexafluorophosphate)[6] mediated peptide synthe- with virtually no racemization,[8] Oxyma is Boc-deriv-
448
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Ethyl 2-(tert-Butoxycarbonyloxyimino)-2-cyanoacetate (Boc-Oxyma) as Coupling Reagent
Table 1. Solvent screening.
atized to generate a transiently stable, yet highly acti-
vated species. Boc-Oxyma readily affords the Oxyma
ester of a carboxylic acid in basic milieu which in turn
generates an ester, thioester, amide or peptide bond
depending on the nucleophile that is added. Chiral in-
tegrity is also completely conserved.
Entry
Solvent
Yield [%][a]
1
2
3
4
5
6
dichloromethane
chloroform
acetonitrile
tetrahydrofuran
ethyl acetate
dimethylformamide
50
55
40
35
92
70
Results and Discussion
[a]
Isolated yield after column chromatography (200 mg
scale). Solvents were examined for the coupling of phe-
nylacetic acid (1 equiv.) and benzylamine (1 equiv.) using
2 (1 equiv.) at 0–58C for 2 h.
Herein, we describe the synthesis of a new coupling
reagent, Boc-Oxyma, and its application as a simple,
mild and racemization-free esterification, thioesterifi-
cation and amidation reagent that uses nearly equi-
molar amounts of acids and the corresponding nucleo-
philes. We prepared Boc-Oxyma by the reaction of to afford higher yields for both amidations and esteri-
Boc2O (1.2 equiv.) with Oxyma (1 equiv.) in the pres- fications (Table 1). Furthermore, the Oxyma that is
ence of DIPEA or triethylamine (1 equiv.) at 0–58C produced as a by-product could be readily recovered
for 1 h (Scheme 1).
by dilution of the reaction mixture with ethyl acetate,
washing with sodium bicarbonate solution, acidifying
the aqueous layer, and then extracting with ethyl ace-
tate. The Oxyma thus recovered can then be used to
regenerate Boc-Oxyma.
After optimization of the reaction procedure, the
esterification of various acids with different alcohols
was examined to explore the scope of the coupling re-
agent (Table 2). It was noted that the coupling of
a carboxylic acid with an equimolar amount of an al-
cohol was completed within 2 h with good to excellent
yields. The reaction works well with aromatic acids
(entries 1–3), aliphatic acids (entries 4–7), including
a long-chain aliphatic acid (entry 8) and amino acids
(entries 9–20), and no side reactions were observed.
The reaction is compatible with the common amino
Scheme 1. Prepation of the coupling reagent, Boc-Oxyma.
Once the Oxyma was completely consumed, the protecting groups (Cbz, Fmoc and Boc; entries 12–
solvent (chloroform) was evaporated to obtain Boc- 20). Various amino acids with bulky side chains also
Oxyma, which could be used for coupling reactions were tested and all of them resulted in excellent
without further purification. The reagent thus pre- yields (entries 12–20).
pared was added to the carboxylic acid or amino acid,
The esterification of secondary alcohols using ben-
which was predissolved in EtOAc with an equivalent zotriazole-derived coupling agents, for example,
amount of DIPEA, and the reaction mixture was TATU [(1H-7-azabenzotriazole-1-yl)-1,1,3,3-tetrame-
stirred for 1 h at 08C for preactivation. The final thyluronium tetrafluoroborate] and TBTU [2-(1H-
ester, thioester, amide or peptide product was ob- benzotriazole-1-yl)-1,1,3,3-tetramethyluronium tetra-
tained by further stirring for about 2 h after the addi- fluoroborate], requires a strong base, such as DBU,
tion of the corresponding nucleophile (Scheme 2).
and the reaction does not proceed with tertiary alco-
For optimization, we screened different solvents hols.[9] Esterifications with tert-butyl alcohol have
using the reaction between phenylacetic acid and ben- been achieved using COMU with good yield, but only
zylamine (200-mg scale) and ethyl acetate was found when the very strong base MTBD (7-methyl-1,5,7-
triazabicycloACTHNUTRGNEUNG
[4.4.0]dec-5-ene) is used.[9] In contrast,
Boc-Oxyma mediated esterification proceeds smooth-
ly with a wide variety of secondary alcohols, and even
with tert-butyl alcohol (entry 6) in good to excellent
yields, all with just DIPEA as the base. Thioesterifica-
tion using 2 also proceeds with excellent yields (en-
tries 21–23, Table 2).
The potential for racemization associated with the
use of Boc-Oxyma was investigated by use of the re-
Scheme 2. Esterification, amidation and thioesterification
using Boc-Oxyma.
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Table 2. Esterification using 2 as coupling reagent.
[a]
Isolated yield after column chromatography (200 mg scale).
dl-Boc-Phe-OH was used as substrate.
[b]
action of l-Boc-phenylalanine with (1R,2S,5R)-men- nine (entry 20) and the presence of the two diastereo-
thol (entry 19). It was clear from various characteriza- mers was demonstrated independently by HPLC
tion data that the product obtained in this reaction (Symmetry C8, 5 mm 3.0ꢁ150 mm analytical column,
contained a single diastereomer. As a control, the 70% CH3CN in H2O with 0.1% formic acid, isocratic
1
same reaction was carried out with dl-Boc-phenylala- gradient, 20 min), and H and 13C NMR (Figure 1).
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Ethyl 2-(tert-Butoxycarbonyloxyimino)-2-cyanoacetate (Boc-Oxyma) as Coupling Reagent
protected amino acids containing various side chains
(Table 3). All the reactions generated good to excel-
lent yields even on a small reaction scale. The current
coupling protocol is compatible with common amino
protecting groups for example, Cbz (entries 35–38),
Boc (entry 39) and Fmoc (entries 40–46). The reac-
tion works well for hindered (entry 29) and non-hin-
dered (entry 36) secondary amines. The dehydration
of N-hydroxyalkylamides affords a facile route to a va-
riety of heterocycles that are present in many biologi-
cally active natural products,[10] and several of these
were prepared by direct coupling of the carboxylic
acid and amino alcohol under mild conditions (en-
tries 26, 33 and 34).
Also of note is the ease with which aniline deriva-
tives undergo amidation with the use of 2. While
other reports of aniline amidation call for the use of
harsh conditions,[11] the reaction proceeds smoothly
with 2 at mild temperatures (0 to 58C) and with
better yields (entries 27, 31, 32 and 37).
The use of Boc-Oxyma was also successfully ap-
plied to solution phase peptide coupling (entries 40–
47). The potential for racemization during the course
of amidation as well as peptide coupling was tested
Figure 1. Examination of racemization by comparison of the
1
HPLC profiles, H NMR and 13C NMR of the product of en-
tries 19 and 20, respectively.
The peak with a retention time of 9 min corresponds by comparing the HPLC profiles (Symmetry C8 5 mm
to the pure stereoisomer (entry 19) whereas a second 3.0ꢁ150 mm analytical column, linear gradient of
peak with a retention time of 9.5 min (entry 20) ap- 18 min, 0 to 100% CH3CN in H2O with 0.1% formic
1
peared when both diastereomers were present. The acid was used), as well as the H and 13C NMR spec-
identity of the peaks in the HPLC profile was verified tra of Fmoc-l-Phe-l-Ala-OMe with those of the
using ESI-MS (see the Supporting Information). In Fmoc-dl-Phe-l-Ala-OMe (Figure 2). In the HPLC
1
the H and 13C NMR spectra of the product of the re- profile of the former (entry 43, Table 3), a single peak
action with l-Boc-phenylalanine (entry 19), the peaks with a retention time of 11.6 min was noted, which
at d=1.42 ppm and d=20.9 ppm, respectively, corre- corresponds to a single stereoisomeric product, Fmoc-
spond to the tertiary butyl proton and carbon, respec- l-Phe-l-Ala-OMe. In the case of the latter, two peaks
tively, of the Boc group of the ester product with retention times of 12.7 min and 14.2 min ap-
(entry 19) and confirm the presence of only one opti- peared in the HPLC profile, corresponding to the two
cal isomer. The appearance of twin peaks at d=1.42 diastereomers present in the product, Fmoc-dl-Phe-l-
and 1.40 ppm in 1H NMR and at d=21.0 and Ala-OMe (entry 44, Table 3). The identity of the
20.9 ppm in 13C NMR of the product of the reaction peaks in the HPLC profiles was verified using ESI-
with dl-Boc-Phe-OH (entry 20) indicates the pres- MS (see the Supporting Information). The doublet
1
ence of two diastereomers of the resulting ester. The centered at d=1.32 ppm in the H NMR and the sin-
comparison of the spectra of the single and mixed dia- glet at d=17.8 ppm in the 13C NMR correspond to
stereomers also allowed the assignment of several the protons and the carbon, respectively, of the
other peaks. For example, the peaks at d=16.3 and methyl group of the alanine residue of the single opti-
15.9 ppm in the 13C NMR were assigned to the methyl cal isomer of the dipeptide product (entry 43), where-
carbon of menthol ester (entry 20 and Figure 1). From as, the doublets centered at d=1.33 and 1.22 ppm in
1
the above comparative study, it can be concluded that the H NMR and the peaks at d=18.3 and 18.2 ppm
the Boc-Oxyma mediated esterification conserves the in the 13C NMR indicate the presence of two diaste-
chiral integrity of the substrate in the product. Simi- reomers for the same proton and carbon, respectively
larly, the HPLC profiles and NMR spectra of the (entry 44, Figure 2). From these comparative studies,
products of entries 13–15 and 17 also clearly indicate it can be concluded that there was no detectable race-
the absence of any racemization (see the Supporting mization in the amidation reactions mediated by 2 as
Information).
the coupling reagent.
We also evaluated the scope of using Boc-Oxyma
We have also synthesized a tripeptide, Z-Gly-Phe-
for amidation reactions under the optimized condi- Val-OMe (entry 47, Table 3), via the segment cou-
tions by reacting a variety of amines with a selection pling of Z-Gly-Phe-OH and H-Val-OMe in solution
of aromatic acids, long-chain aliphatic acids and N- using 2 and compared the results to those reported by
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Table 3. Amidation reactions using 2 as a coupling reagent.
[a]
Isolated yield after column chromatography (200-mg scale). All amino acids were l-amino acids except 44 which was the
dl-amino acid.
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Ethyl 2-(tert-Butoxycarbonyloxyimino)-2-cyanoacetate (Boc-Oxyma) as Coupling Reagent
Table 5. HPLC retention times and observed mass (ESI-MS)
of the peptides after each coupling step on solid phase.
The efficiency of the new coupling reagent was
verified for solid phase peptide synthesis as well. We
prepared the octapeptide H-Gly-Asn-Phe-Gly-Ala-
Ile-Leu-Gly-NH2, which contains the hexapeptide se-
quence NFGAIL, known to be the core sequence re-
sponsible for the initiation of aggregation of the
Amylin peptide that leads to type 2 diabetes. We syn-
Figure 2. HPLC profiles (18 min, linear gradient, 0 to 100%
1
CH3CN in water), H NMR and 13C NMR of the products of
the reactions entered as 43 and 44 in Table 3.
others with a variety of well known coupling reagents thesized the said octapeptide by stepwise coupling of
such as HATU {N-[(dimethylamino)-1H-1,2,3- the constituent amino acids on rink amide MBHA
triazolo
G
resin using Fmoc chemistry, and employing 2 as the
ACHTUNGTRENNUNGanaminium hexafluorophosphate N-oxide}, HDMA coupling reagent. Couplings proceeded smoothly at
{1-[(dimethylamino)(morpholino)methylene]-1H-
[1,2,3]triazolo
3-oxide}, HBTU {N-[(1H-benzotriazol-1-yl)(dimethyl- data for each fragment are reported in Table 5. The
amino)methylene]N-methylmethanaminium hexa- yield of the peptide after cleavage from the resin fol-
fluorophosphate N-oxide} and HDMB {1-[(dimethyl- lowed by ether precipitaion and semi-preparative
each step in less than 2 h and were confirmed by the
ACHTUNGTRENNUNG[4,5-b]pyridinium hexafluorophosphate Kaiser test. The HPLC retention times and ESI-MS
ACHTUNGTRENNUNG
A
HPLC was around 40% with respect to the resin load-
hexafluorophosphate 3-oxide}.[12] Unlike the other ing. The HPLC profile and ESI-MS of the purified
coupling reagents, no racemization was observed peptide are provided in Figure 3.
while using 2 by RP-HPLC and NMR techniques
(Table 4).
Table 4. Comparison of % yield and % racemization of a tri-
peptide synthesized using various coupling reagents.
Having demonstrated the applicability of 2 as a cou-
pling reagent for esterification, thioesterification, ami-
dation and peptide synthesis, we turned our attention
to the elucidation of the mechanism by which it
works. The reaction of a carboxylic acid with 2 can
lead to the generation of two possible intermediates,
the mixed anhydride, [X] (Figure 4) and the active
ester, [A]. The mixed anhydride [X] would evolve via
nucleophilic attack of the carboxylate anion at the
carbonyl carbon of 2 and expulsion of the Oxyma
anion (which is favored because of resonance stabili-
zation). Subsequently, the active ester [A] is likely
generated by further attack of the Oxyma anion at
the carbonyl carbon of the mixed anhydride, followed
by the release of CO2 and tert-butyl alcohol.
Entry Peptide
Coupling
reagent
Yield Racemization
[%]
[%]
1
Z-Gly-Phe-
Val-OMe
HATU
90
1.56
2
3
4
5
HDMA
HBTU
HDMB
2
90
89
90
90
0.65
6.90
2.90
0[a]
No racemization could be detected by HPLC, 1H and
13C NMR (see Supporting Information).
[a]
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sponding to the starting materials (a, b and c) had dis-
appeared, indicating complete conversion of the start-
ing carboxylic acid and the reagent 2 to the inter-
mediate Oxyma ester [A]. However, with the addi-
tion of benzylamine (the nucleophile), peaks d and
e shifted to new positions, f and g, respectively, as the
intermediate [A] was converted to the amide product
and the Oxyma was regenerated. When benzyl alco-
hol was added as the nucleophile, instead of the peak
corresponding to the amide carbonyl carbon f, the
benzyl ester carbonyl carbon, h, was observed. These
observations support the proposed mechanism, similar
to that of TBTU and COMU,[9] in which the coupling
first proceeds via the slow formation of the mixed an-
hydride [X], which then quickly converts to the
active Oxyma ester of the carboxylic acid [A]. Final
attack of the added nucleophile on [A] generates the
end product, as depicted in Figure 4.
The evolution of tert-butyl alcohol was confirmed
1
by both 13C NMR and H NMR. In the 13C NMR, the
peak which corresponds to the tert-butyl group of
Boc-Oxyma gradually shifted from 86.4 ppm (i) to
67.1 ppm (k, corresponding to the tert-butyl carbon of
the tert-butanol that is generated, right side of panel
Figure 3. (a) Sequence of the peptide synthesized by SPPS
using Boc-Oxyma. (b) HPLC profile and ESI-MS of the
final peptide after purification.
1
a in Figure 4). In the H NMR, the peak correspond-
ing to the tert-butyl proton of Boc-Oxyma gradually
shifted from 1.60 ppm to 1.20 ppm (corresponding to
This formation of the active ester [A] sets Boc- the tert-butyl alcohol protons) as shown in Figure 4
Oxyma apart from other butoxycarbonylation re- (panel b). Other peaks in 13C/1H NMR (Supporting
agents. For instance, Takahata et al. recently reported Information) and characteristic bands in IR spectra
coupling reactions using BBDI (1-tert-butoxy-2-tert- (CO stretching frequencies[14] are shown in Figure 5)
butoxycarbonyl-1,2-dihydroisoquinoline) and assumed also support the mechanistic pathway elucidated in
the reactions to proceed via the formation of the Figure 4. Furthermore, the evolution of CO2 was con-
mixed anhydride, similar to [X], which then under- firmed by passing the evolved gas in a saturated solu-
goes attack by a nucleophile to generate an ester or tion of Ca(OH)2 and observing the formation of an
amide. They found that the reaction did not proceed emulsion of CaCO3 (Figure 5, panel f, a video demon-
with the use of tert-butyl alcohol as the nucleophile strating the course of the experiment is provided in
owing to steric hindrance.[13] In contrast, the use of 2 the Supporting Information). Finally, when benzoic
with tert-butyl alcohol readily afforded the tert-butyl acid was used, the corresponding transient intermedi-
ester product (entry 6, Table 2), further supporting ate [A] could be isolated and its structure was re-
the proposed conversion of the mixed anhydride to solved crystallographically (panel g, Figure 5), thereby
the active ester.
proving the formation of [A]. All the relevant NMR
The lower steric congestion about the carbonyl and IR spectra, including that of the starting materi-
carbon of the intermediate [A] allows the formation als, are furnished in the Supporting Information.
of the tert-butyl ester.
To identify the intermediate [A], 2 was mixed with
equimolar amounts of phenylacetic acid in the pres- Conclusions
ence of DIPEA in CDCl3 in an NMR tube and
13C NMR spectra were recorded over time. Initially, The newly designed compound, ethyl 2-(tert-butoxy-
at five minutes, three CO peaks were present, one carbonyloxyimino)-2-cyanoacetate (2) is an efficient
corresponding to the phenyl acetic acid at 175.0 ppm coupling reagent for simple, mild and racemization-
(a, Figure 4) and the other two to Boc-Oxyma at free esterification, thioesterification, amidation reac-
155.9 and 147.5 ppm (b and c). At 30 min, two new tions and peptide synthesis that uses equimolar
peaks corresponding to the expected Oxyma ester of amounts of acids with equimolar amounts of alcohols,
phenylacetic acid [A] were clearly observed at 169.8 thiols and amines, respectively. This method can be
(d) and 163.0 ppm (e). At 60 min, only the two new used for both solid phase and solution phase peptide
peaks (d, e) could be observed, while the peaks corre- synthesis. Boc-Oxyma is much easier to prepare than
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Ethyl 2-(tert-Butoxycarbonyloxyimino)-2-cyanoacetate (Boc-Oxyma) as Coupling Reagent
Figure 4. Plausible mechanism of Boc-Oxyma mediated coupling reaction. (a) Time dependent 13C NMR experiments. d
Cppm a=175.0, b=155.9, c=147.5, d=169.8, e=163.0, h=171.5, f=165.9, g=156.4, h=171.5, i=86.4, j=63.4, k=67.1, l=
59.5, m=64.0, k=66.6. (b) Part of the proton NMR spectra, to demonstrate the release of tert-butyl alcohol.
other popular coupling reagents and produces only thylenamino]-N,N,N’,N’-tetramethyluronium
hexa-
tert-butyl alcohol, CO2 and Oxyma as by-products, the fluorophosphate}, COMU, etc.,[8] all of them are uro-
latter of which can be easily recovered and reused to nium salt-based coupling reagents. Boc-Oxyma is sim-
regenerate 2. Thus, use of this reagent reduces chemi- ilar to them with respect to efficiency and mechanism,
cal waste generation. Boc-Oxyma was successfully but completely different structurally, as it does not in-
used for the esterification of secondary and tertiary volve a uronium/aminium type salt. Thus, Boc-Oxyma
alcohols with DIPEA, whereas COMU, TBTU and is the first member of a new class of efficient coupling
TATU need stronger bases. Although there are a few reagents. We are continuing to develop other coupling
other Oxyma-based coupling reagents reported to reagents in this series and will be reporting on them
date, for example, TOTU {O-[(ethoxycarbonyl)cyano- in the near future.
methylenamino]-N,N,N’,N’-tetramethyluronium tetra-
fluoroborate}, HOTU {O-[(ethoxycarbonyl)cyanome-
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Experimental Section
Procedure for the Preparation of Ethyl 2-(tert-
Butoxycarbonyloxyimino)-2-cyanoacetate (Boc-
Oxyma)
Di-tert-butyl dicarbonate (1.2 equiv,) was added to a solution
of ethyl cyano(hydroxyimino)acetate (Oxyma) (1 equiv.)
and DIPEA (1 equiv) in 1 mL CHCl3 at 0–58C and the reac-
tion mixture was stirred for 1 h. (Caution!: Boc2O is
a known irritant and sensitizer and should be handled with
due care.) After complete disappearance of Oxyma, the sol-
vent CHCl3 and byproduct tert-butyl alcohol were evaporat-
ed with a rotary evaporator to obtain pure Boc-Oxyma;
1
yield: 96%; red colored liquid. H NMR (400 MHz, CDCl3):
d=4.44–4.39 (q, J=8 Hz, 2H), 1.54 (s, 9H) 1.38–1.34 (t, J=
8.4, 3H); 13C NMR (100 MHz, CDCl3): d=156.7, 148.3,
129.9, 106.6, 87.4, 64.3, 27.3, 13.8; LR-MS: m/z=265 [M+
Na]+,
HR-MS
(ESI):
m/z=265.0728,
calcd.
for
C10H14N2NaO5: 265.0800; IR (KBr): n=3444, 2925, 1764,
1742 cmÀ1; Rf product: 0.5 (10% EtOAc/hexane).
General Procedure for the Synthesis of Esters,
Amides and Thioesters
Boc-Oxyma was added to a solution of acid (1 equiv.) and
DIPEA (1 equiv.) in 1 mL of EtOAc. The reaction mixture
was stirred for 1 h at 0–58C followed by the addition of al-
cohol, amine or thiol (1 equiv.). The reaction mixture was
stirred at the same temperature for another 2 h. After com-
pletion of the reaction, the reaction mixture was diluted
with 50 mL of ethyl acetate; the organic phase was washed
with 5% citric acid (2ꢁ20 mL), saturated NaHCO3 (2ꢁ
20 mL) and dried over anhydrous Na2SO4. Finally, Na2SO4
was filtered off and the solvent was evaporated to obtain
the product which was purified by column chromatography.
General Procedure for the Synthesis of Dipeptides
To a solution of N-protected amino acid (1 equiv.) and
DIPEA (1 equiv.) in 1 mL of EtOAc, Boc-Oxyma (1 equiv.)
was added. The reaction mixture was stirred for 1 h at 0–
58C followed by the addition of methyl ester of the second
amino acid (1 equiv.) and DIPEA (1 equiv.) in 1 mL of
EtOAc. The reaction mixture was stirred at the same tem-
perature for another 2 h. After completion of the reaction,
the reaction mixture was diluted with 50 mL of ethyl ace-
tate, the organic phase was washed with 5% citric acid (2ꢁ
20 mL), saturated NaHCO3 (2ꢁ20 mL) and dried over
Figure 5. IR spectra of (I) phenyl acetic acid, (II) Boc-
Oxyma, (III) the reaction mixture after 1 h, Oxyma ester of
phenylacetic acid, intermediate [A], (IV) 10 min after addi-
tion of benzyl amine, (V) 10 min after addition of benzyl al-
cohol. (VI) formation of the CaCO3 emulsion when passed
the evolved gas through Ca(OH)2 solution, left side bottle
serves as control. (VII) ORTEP molecular diagram with el-
lipsoid at 50% probability of [A], when benzoic acid was
used. CCDC 903487 contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free
of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
456
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ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2013, 355, 448 – 462

Ethyl 2-(tert-Butoxycarbonyloxyimino)-2-cyanoacetate (Boc-Oxyma) as Coupling Reagent
Na2SO4 anhydrous. Finally, Na2SO4 was filtered and the sol-
vent was evaporated. The product was purified by silica gel
column chromatography.
Phenyl acetic acid tert-butyl ester (6): Yield: 87%; color-
1
less oil. H NMR (400 MHz, CDCl3): d=7.32–7.22 (m, 5H),
3.51 (s, 2H), 1.45 (s, 9H); 13C NMR (100 MHz, CDCl3): d=
170.9, 134.7, 129.2, 128.5, 126.8, 80.7, 42.6, 28.0; LR-MS: m/
z=215 [M+Na]+; HR-MS (ESI): m/z=215.0990, calcd. for
C12H16NaO2: 215.1048; IR (KBr): n=3230, 2989, 1754 cmÀ1
Rf product: 0.3 (5% EtOAc/hexane).
;
Solid Phase Synthesis of GNFGAILG-NH2
The octapeptide was manually assembled stepwise on Fmoc-
Rink Amide MBHA resin using Fmoc/t-Bu protection strat-
egy. The preactivation time of Fmoc-amino acids with Boc-
Oxyma (3 equiv.) and DIPEA (5 equiv.) was 1 h. The cou-
pling time was 2 h. Fmoc deprotection was carried out with
the use of piperidine/DMF (1:4, 3ꢁ7 min). The peptide was
cleaved from the resin with the use of TFA/DCM (1:1) mix-
ture for 2.5 h. Purification of the peptide was carried out by
semi-preparative HPLC and subsequent liophilization af-
forded the final peptide as a white powder.
Phenyl acetic acid naphthalen-1-yl ester (7): Yield: 95%;
white solid; mp 528C; 1H NMR (400 MHz, CDCl3): d=
7.82–7.80 (d, J=8 Hz, 1H), 7.70–7.68 (d, J=8.4, 1H), 7.61–
7.59 (d, J=8.4, 1H), 7.48–7.20 (m, 9H), 3.99 (s, 2H);
13C NMR (100 MHz, CDCl3): d=169.9, 146.5, 134.6, 133.5,
129.4, 128.8, 127.9, 127.4, 126.8, 126.4, 126.0, 125.3, 121.0,
118.0, 41.4; LR-MS: m/z=285 [M+Na]+; HR-MS (ESI):
m/z=285.0892, calcd. for C18H14NaO2: 285.0891; IR (KBr):
n=3444, 2925, 1748 cmÀ1; Rf product: 0.3 (10% EtOAc/
hexane).
1
Bz-Oxyma: Yield: 96%. H NMR (400 MHz, CDCl3): d=
Dodecanoic acid benzhydryl ester (8): Yield: 93%; white
8.18–7.51 (m, 5H), 4.52–4.46 (q, J=8 Hz, 2H), 1.44–1.40 (t,
J=8.4, 3H); 13C NMR (100 MHz, CDCl3): d=160.0, 157.0,
135.3, 131.7, 130.6, 129.2, 125.6, 107.0, 67.7, 13.9; LR-MS: m/
z=269 [M+Na]+; HR-MS (ESI): m/z=269.0007, calcd. for
C12H10N2NaO4: 269.0538.
Benzoic acid methyl ester (1): Yield: 92%; white solid;
mp 1128C. 1H NMR (400 MHz, CDCl3): d=8.02–7.38 (m,
5H), 3.88 (s, 3H); 13C NMR (100 MHz, CDCl3): d=167.3,
133.0, 130.3, 129.7, 128.5, 52.2; LR-MS: m/z=137 [M+H]+;
HR-MS (ESI): m/z=139.0649 [M+H]+, calcd. for C8H9O2:
137.0603; IR (KBr): n=3451, 2961, 1730 cmÀ1; Rf product:
0.5 (5% EtOAc/hexane).
Benzoic acid phenyl ester (2): Yield: 90%; white solid;
mp 708C. 1H NMR (400 MHz, CDCl3): d=8.21–8.19 (m,
2H), 7.64–7.48 (m, 3H), 7.50–7.15 (m, 5H); 13C NMR
(100 MHz, CDCl3): d=165.3, 151.2, 133.7, 130.3, 129.6,
129.5, 128.7, 126.0, 125.8; LR-MS: m/z=221 [M+Na]+; HR-
MS (ESI): m/z=221.0583 [M+Na]+, calcd. for C13H10NaO2:
221.0578; IR (KBr): n=3454, 2923, 1731 cmÀ1; Rf product:
0.15 (30% EtOAc/hexane).
Benzoic acid octyl ester (3): Yield: 94%; colorless oil.
1H NMR (400 MHz, CDCl3): d=8.05–7.41 (m, 5H), 4.33–
4.29 (t, J=6.8 Hz, 2H), 1.80–1.73 (m, 2H), 1.46–1.41 (m,
2H), 1.37–1.28 (m, 8H), 0.89–0.87 (t, J=4 Hz, 3H);
13C NMR (100 MHz, CDCl3): d=166.9, 132.9, 130.7, 129.7,
128.5, 65.3, 32.0, 29.4, 29.4, 28.9, 26.2, 22.8, 14.2; LR-MS: m/
z=235 [M+H]+; HR-MS (ESI): m/z=235.1692 [M+H]+,
calcd. for C15H23O2 is 235.1698; IR (KBr): n=3455, 2929,
1721 cmÀ1; Rf product: 0.3 (5% EtOAc/hexane).
1
solid; mp 558C. H NMR (400 MHz, CDCl3): d=7.37–7.22
(m, 10H), 5.79 (s, 1H), 2.31–2.27 (t, J=7.6 Hz, 2H), 1.59–
1.57 (m, 2H), 1.25 (s, 16H), 0.89–0.86 (t, J=6.4 Hz, 3H);
13C NMR (100 MHz, CDCl3): d=179.7, 158.8, 143.6, 128.4,
127.5, 126.6, 63.3, 34.1, 32.0, 29.7, 29.5, 29.4, 29.3, 29.1, 24.1,
22.7, 14.1; LR-MS: m/z=389 [M+Na]+, HR-MS (ESI):
m/z=389.2480 [M+Na]+, calcd. for C25H34NaO2: 389.2456;
IR (KBr): n3390, 2921, 1705; Rf product 0.25 (5% EtOAc/
Hexane)
Benzoylamino-acetic acid methyl ester (9): Yield: 93%;
white solid; mp: 828C. 1H NMR (400 MHz, CDCl3): d=
7.83–7.81 (d, J=7.2 Hz, 2H), 7.54–7.50 (t, J=7.2 Hz, 1H),
7.46–7.43 (t, J=7.2 Hz, 2H), 6.75 (br, 1H), 4.26–4.25 (d, J=
4.8 Hz, 2H), 3.80 (s, 3H); 13C NMR (100 MHz, CDCl3): d=
170.6, 167.7, 133.6, 131.7, 128.4, 127.1, 52.3, 41.7; LR-MS:
m/z=216 [M+Na]+; HR-MS (ESI): m/z=216.0631 [M+
Na]+, calcd. for C10H11NNaO3: 216.0637; IR (KBr): n=3433,
2925, 1736, 1627 cmÀ1; Rf product: 0.15 (30% EtOAc/
hexane).
Benzylamino acetic acid ethyl ester (10): Yield: 95%;
1
white solid; mp 618C. H NMR (400 MHz, CDCl3): d=7.82–
7.80 (d, J=8 Hz, 2H), 7.53–7.49 (t, J=6.8 Hz, 1H), 7.45–
7.42 (t, J=7.2 Hz, 2H), 6.79 (br, 1H), 4.28–4.22 (m, 4H),
1.32–1.29 (t, J=7 Hz, 3H); 13C NMR (100 MHz, CDCl3):
d=170.0, 167.8, 133.4, 131.5, 128.3, 127.0, 61.2, 41.6, 13.9;
LR-MS: m/z=208 [M+H]+; HRMS (ESI): m/z=208.0982
[M+Na]+, calcd. for C11H14NO3: 208.0974; IR (KBr): n=
3435, 3340, 1757, 1642 cmÀ1; Rf product: 0.15 (30% EtOAc/
hexane).
Phenyl acetic acid benzyl ester (4): Yield: 93%; white
1
Benzylamino acetic acid allyl ester (11): Yield: 93%;
solid; mp 528C. H NMR (400 MHz, CDCl3): d=7.34–7.25
1
(m, 10H), 5.11 (s, 2H), 3.64 (s,2H); 13C NMR (100 MHz,
CDCl3): d=171.3, 135.9, 133.9, 129.3, 128.5, 128.5, 128.2,
128.1, 127.1, 66.5, 41.3; LR-MS: m/z=249 [M+Na]+; HR-
MS (ESI): m/z=249.0893, calcd. for C15H14NaO2: 249.0891;
IR (KBr): n=3436, 2917, 1733 cmÀ1; Rf product: 0.3 (5%
EtOAc/hexane).
yellow oil. H NMR (400 MHz, CDCl3): d=7.8–7.78 (d, J=
8.4 Hz, 2H), 7.52–7.48 (t, J=8.2 Hz, 1H), 7.44–7.40 (t, J=
8 Hz, 2H), 6.68 (br, 1H), 5.96–5.86 (m, 1H), 5.36–5.31 (dd,
J=16 Hz, 1H), 5.27–5.25 (dd, J=10.4 Hz, 1H), 4.68–4.67 (d,
J=6 Hz, 2H), 4.26–4.25 (d, J=5.2 Hz, 2H); 13C NMR
(100 MHz, CDCl3): d=169.8, 167.8, 133.6, 131.7, 131.5,
128.5, 127.2, 118.8, 60.0, 41.8; LR-MS: m/z=242 [M+Na]+;
HR-MS (ESI): m/z=242.0791 [M+Na]+, calcd. for
C12H13NNaO3: 242.0793; IR (KBr): n=3427, 2928, 1751,
1650 cmÀ1; Rf product: 0.3 (30% EtOAc/hexane).
Phenyl acetic acid isopropyl ester (5): Yield: 90%; color-
1
less oil. H NMR (400 MHz, CDCl3): d=7.33–7.23 (m, 5H),
5.04–4.97 (m, 1H), 3.57 (s, 2H), 1.22 (d, J=0.8 Hz, 6H);
13C NMR (100 MHz, CDCl3): d=171.2, 134.4, 129.3, 128.6,
127.0, 68.2, 41.8, 21.8; LR-MS: m/z=179.1072 [M+H]+;
HR-MS (ESI): m/z=179.0885, calcd. for C11H15O2:
179.1072; IR (KBr): n=3440, 2981, 1732 cmÀ1; Rf product:
0.3 (5% EtOAc/hexane).
Benzyloxycarbonylamino-3-butyric acid benzyl ester (12):
1
Yield: 92%; colorless oil. H NMR (400 MHz, CDCl3): d=
7.33 (m, 10H), 5.3–5.27 (d, J=8.8 Hz, 1H), 5.19–5.11 (m,
2H), 5.09 (s, 2H), 4.36–4.32 (m, 1H), 2.19–2.14 (m, 1H),
Adv. Synth. Catal. 2013, 355, 448 – 462
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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457

Kishore Thalluri et al.
FULL PAPERS
0.94–0.92 (d, J=6.8 Hz, 3H), 0.84–0.82 (d, J=6.8 Hz, 3H);
13C NMR (100 MHz, CDCl3): d=171.8, 156.2, 136.3, 135.3,
128.4, 128.3, 128.3, 128.2, 127.9, 66.8, 59.0, 31.1, 18.8, 17.3;
ESI-MS: m/z=342 [M+H]+; HR-MS (ESI): m/z=342.1714
[M+H]+, calcd. for C20H24NO4: 342.1705; IR (KBr): n=
3342, 2963,1727, 1538 cmÀ1; Rf product: 0.35 (10% EtOAc/
hexane).
3-tert-Butoxy-2-(9H-fluoren-9-ylmethoxycarbonylamino)-
propionic acid 1-phenylethyl ester (17): Yield: 92%; white
solid; mp 1628C. H NMR (400 MHz, CDCl3): d=7.78–7.76
1
(d, J=7.6 Hz 2H), 7.64–7.61 (t, J=6.4 Hz, 2H), 7.42–7.25
(m, 9H), 5.99–5.97 (d, J=6.4 Hz, 1H) 4.92–4.87 (q, J=
6.4 Hz, 1H), 4.54–4.53 (d, J=2.4 Hz, 2H), 4.42–4.36 (m,
1H), 4.27–4.26 (t, J=5.2 Hz, 1H), 3.91–3.89 (dd, J=2 Hz,
1H), 3.85–3.82 (dd, J=2.4 Hz, 1H), 1.51- 1.49 (d, J=6.4 Hz,
3H), 1.81 (s, 9H); 13C NMR (100 MHz, CDCl3): d=170.0,
156.2, 144.0, 141.3, 141.0, 128.5, 128.4, 128.0, 127.7, 127.3,
127.1, 126.2, 126.0, 125.4, 73.5, 70.2, 67.2, 62.3, 54.8, 47.1,
27.3, 22.0; LR-MS: m/z=510 [M+Na]+; HR-MS (ESI):
m/z=510.2284 [M+H]+, calcd. for C30H33NNaO5: 510.2256;
IR (KBr): n=3443, 2975, 1727, 1700 cmÀ1; Rf product: 0.50
(15% EtOAc/hexane).
1-(4-Nitrophenyl)ethyl
2-(benzyloxycarbonylamino)-2-
phenylacetate (13): Yield: 93%; white solid; mp 1558C.
1H NMR (400 MHz, CDCl3): d=8.20–8.18 (d, J=8.4 Hz,
2H), 7.98–7.96 (d, J=8.4 Hz, 1H), 7.54–7.00 (m, 12H),
5.92–5.91 (d, J=6.4 Hz, 1H), 5.78–5.77 (q, J=7.6 Hz, 1H),
5.06 (s, 2H), 1.41–1.39 (d, J=6.8 Hz, 3H); 13C NMR
(100 MHz, CDCl3): d=170.1, 155.5, 147.9, 147.6, 136.1,
129.0, 128.8, 128.5, 128.2, 128.1, 127.4, 127.1, 126.7, 126.2,
72.9, 67.1, 58.2, 21.7; LR-MS: m/z=457 [M+Na]+; HR-MS
(ESI): m/z=457.1388, calcd. for C24H22N2NaO6: 457.1376;
IR (KBr): n=3404, 1726, 1716 cmÀ1; Rf product: 0.2 (5%
EtOAc/hexane).
2-Isopropyl-6-methylcyclohexyl 2-(benzyloxycarbonylami-
no)-2-phenylacetate (14): Yield: 93%; white solid; mp 608C.
1H NMR (400 MHz, CDCl3): d=7.33–7.12 (m, 10H), 5.98–
5.96 (d, J=6.8 Hz, 1H), 5.36–5.34 (d, J=7.2 Hz, 1H), 5.06 (s,
2H), 4.75–4.64 (m, 1H), 2.03–2.00 (m, 1H),1.95–1.86 (m,
1H), 1.73–1.57 (m, 1H), 1.37–1.28 (m, 1H), 1.05–1.95 (m,
4H), 0.89–0.87 (d, J=6.8 Hz, 3H), 0.81–0.74 (dd, J=6.4 Hz,
3H), 0.58–0.56 (d, J=6.8 Hz, 3H); 13C NMR (100 MHz,
CDCl3): d=170.2, 155.3, 137.0, 136.2, 128.6, 128.5, 128.3,
128.2, 128.2, 128.0, 127.2, 127.0, 76.0, 66.8, 58.0, 46.7, 40.6,
39.8, 34.0, 31.3, 26.1, 23.3, 21.9, 20.6; LR-MS: m/z=446
[M+Na]+; HR-MS (ESI): m/z=446.2356, calcd. for
C26H33NNaO4: 446.2307; IR (KBr): n=3368, 1735,
1708 cmÀ1; Rf product: 0.2 (5% EtOAc/hexane).
1-Phenylethyl 2-{[(9H-fluoren-9-yl)methoxy]carbonylami-
no}-4-methylpentanoate (15): Yield: 92%; white solid; mp
1508C. 1H NMR (400 MHz, CDCl3): d=7.76–7.74 (d, J=
7.6 Hz 2H), 7.58–7.25 (m, 11H), 5.92–5.90 (q, J=2.8 Hz,
1H), 5.22–5.20 (d, J=8.4 Hz, 1H) 4.91–4.86 (q, J=6.4 Hz,
1H), 4.39–4.38 (d, J=4 Hz, 2H), 4.21–4.18 (t, J=6.8 Hz,
1H), 1.69- 1.52 (m, 2H), 1.49–1.48 (d, J=6.4 Hz, 3H), 0.97–
0.95 (d, J=6 Hz, 3H), 0.92–0.87 (dd, J=6 Hz, 3H);
13C NMR (100 MHz, CDCl3): d=172.5, 156.0, 144.1, 143.9,
141.4, 128.7, 128.2, 127.8, 127.2, 126.2, 126.1, 125.5, 125.2,
73.6, 67.0, 52.8, 47.3, 42.0, 29.8, 25.3, 23.0, 22.3; LR-MS:
m/z=480 [M+Na]+; HR-MS (ESI): m/z=480.2764 [M+
H]+, calcd. for C29H31NNaO4: 480.2150; IR (KBr): n=3429,
1727, 1711 cmÀ1; Rf product: 0.35 (15% EtOAc/hexane).
2-(9H-Fluoren-9-ylmethoxycarbonylamino)-succinic acid
4-tert-butyl 1-phenyl ester (16): Yield: 90%; white solid; mp
122–1248C. 1H NMR (400 MHz, CDCl3): d=7.76–7.74 (d,
J=7.6 Hz, 2H), 7.59–7.58 (d, J=7.6 Hz, 2H), 7.40–7.36 (t,
J=7.2 Hz, 2H), 7.30–7.19 (m, 3H), 7.10–7.08 (d, J=7.6 Hz,
2H), 6.90–6.82 (m, 2H), 5.94–5.92 (d, J=8.4 Hz, 1H), 4.45–
4.43 (d, J=7.2 Hz, 2H), 4.38–4.34 (t, J=7.6 Hz, 1H), 4.27–
4.23 (t, J=6.8 Hz, 1H) 3.15–3.10 (dd, J=4.4 Hz, 1H), 2.92–
2.87 (dd, J=4.4 Hz, 1H), 1.41 (s, 9H); 13C NMR (100 MHz,
CDCl3): d=170.0, 169.7, 156.1, 150.5, 143.7, 141.2, 129.4,
128.7, 127.6, 127.0, 126.1, 125.0, 121.2, 82.0, 67.3, 50.7, 47.0,
37.8, 27.9; LR-MS: m/z=510 [M+Na]+; HR-MS (ESI):
m/z=510.2304 [M+H]+, calcd. for C30H33NNaO5: 510.1893;
IR (KBr): n=3361, 2935, 1750, 1720, 1690 cmÀ1; Rf product:
0.45 (15% EtOAc/hexane).
2-tert-Butoxycarbonylamino-4-methylpentanoic
acid
benzyl ester (18): Yield: 91%; white semi-solid. 1H NMR
(400 MHz, CDCl3): d=7.36–7.32 (m, 5H), 5.18–5.09 (m,
2H), 4.89–4.87 (d, J=8 Hz, 1H), 4.33–4.32 (m, 1H), 1.40 (s,
12H), 0.90–0.88 (m, 6H); 13C NMR (100 MHz, CDCl3): d=
173.3, 155.4, 135.5, 128.5, 128.2, 128.1, 79.7, 66.8, 52.2, 41.5,
28.3, 24.7, 22.8, 21.8; LR-MS: m/z=344 [M+Na]+; HR-MS
(ESI): m/z=344.1838, calcd. for C18H27NNaO4: 344.1838; IR
(KBr): n=3442, 1712, 1638 cmÀ1; Rf product: 0.3 (10%
EtOAc/hexane).
l-2-tert-Butoxycarbonylamino-3-phenylpropionic acid 5-
isopropyl-2-methylcyclohexyl ester (19): Yield: 92%; white
1
solid; mp 1628C. H NMR (400 MHz, CDCl3): d=7.29–7.14
(m, 5H), 5.01–4.99 (d, J=7.2 Hz, 1H), 4.73–4.68 (m,1H),
4.54–4.52 (d, J=6.4 Hz, 1H), 3.12–3.09 (dd, J=6 Hz, 1H),
3.05–3.00 (dd, J=6 Hz, 1H), 1.89–1.86 (m, 1H), 1.77–1.76
(m, 1H), 1.67–1.64 (m, 1H), 1.41 (s, 9H), 1.34–1.31 (m, 1H),
1.04–1.0 (m, 4H), 0.92–0.85 (dd, J=6.4 Hz, 6H), 0.72–0.71
(d, J=6.8 Hz, 3H); 13C NMR (100 MHz, CDCl3): d=171.6,
155.2, 136.3, 129.6, 128.5, 127.0, 79.8, 75.6, 54.6, 47.0, 40.8,
38.3, 34.3, 31.5, 28.4, 26.1, 23.4, 22.1, 20.9, 16.3; LR-MS:
m/z=426 [M+Na]+; HR-MS (ESI): m/z=426.2797, calcd.
for C27H37NNaO4: 426.2620; Rt: RP-HPLC [isocratic gradi-
ent, 20 min (70% CH3CN in water)]: Rt =9.1 min; IR (KBr):
n=3394, 2945, 1721, 1687 cmÀ1; Rf product: 0.3 (10%
EtOAc/hexane).
dl-2-tert-Butoxycarbonylamino-3-phenylpropionic acid 5-
isopropyl-2-methylcyclohexyl ester (20): Yield: 91%; white
1
solid; mp 1628C. H NMR (400 MHz, CDCl3): d=7.29–7.15
(m, 10H), 5.01–5.00 (d, J=7.2 Hz, 1H), 4.95–4.93 (d, J=
8 Hz, 1H) 4.72–4.67 (t, J=10.4 Hz, 2H), 4.54–4.52 (d, J=
6.4 Hz, 2H), 3.13–3.04 (dd, J=5.2, Hz, 2H), 3.02–2.97 (dd,
J=6 Hz, 2H), 2.03–1.86 (m, 2H), 1.77–1.72 (m, 2H), 1.67–
1.64 (m, 2H), 1.41 (s, 9H), 1.39 (s, 9H), 1.32–1.29 (m, 2H),
1.03–1.93 (m, 8H), 0.90–0.85 (dd, J=6.4 Hz, 12H), 0.72–0.70
(d, J=6.8 Hz, 6H); 13C NMR (100 MHz, CDCl3): d=171.7,
171.5, 155.1, 136.2, 129.6, 129.4, 128.5, 128.4, 126.9, 79.7,
75.7, 75.5, 54.7, 54.6, 47.0, 46.8, 40.7, 38.2, 34.2, 31.4, 29.7,
28.3, 26.0, 25.8, 23.3, 23.0, 22.0, 20.9, 20.8, 16.3, 15.9. Rf prod-
uct: 0.3 (10% EtOAc/hexane); RP-HPLC [isocratic gradi-
ent, 20 min (70% CH3CN in water)]: Rt =9 min and 9.5 min.
ESI-MS: m/z=426.27 and 426.28, calcd. for C27H37NNaO4:
426.26.
Thiobenzoic acid S-(4-nitrophenyl) ester (21): Yield: 87%;
yellow solid; mp 1258C. 1H NMR (400 MHz, CDCl3): d=
8.25–8.22 (dd, J=2 Hz, 2H), 7.96–7.94 (dd, J=1.2 Hz, 2H),
7.66–7.64 (dd, J=2 Hz, 2H), 7.59–7.44 (m, 3H); 13C NMR
458
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ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2013, 355, 448 – 462

Ethyl 2-(tert-Butoxycarbonyloxyimino)-2-cyanoacetate (Boc-Oxyma) as Coupling Reagent
(100 MHz, CDCl3): d=162.5, 137.4, 134.3, 133.3, 128.7,
126.3, 124.9, 124.2, 123.6; LR-MS: m/z=280 [M+Na]+; HR-
3285, 3256, 1657, 1601 cmÀ1; Rf product: 0.35 (20% EtOAc/
hexane).
MS
(ESI):
m/z=280.0200
[M+Na]+,
calcd.
for
N-Butyldodecanamide (28): Yield: 95%; white solid; mp:
1
C13H9NNaO3S: 280.0201; IR (KBr): =3390, 2927, 1750,
568C. H NMR (400 MHz, CDCl3): d=5.52 (br, 1H), 3.23–
1289 cmÀ1; Rf product: 0.5 (15% EtOAc/hexane).
3.18 (q, J=6.8 Hz, 2H), 2.14–2.10 (t, J=7.2 Hz, 2H), 1.60–
1.56 (m, 2H), 1.48–1.41 (m, 2H), 1.22 (s, 18H), 0.90–0.82
(m, 6H); 13C NMR (100 MHz, CDCl3): d=173.5, 39.2, 36.7,
31.9, 31.7, 29.6, 29.5, 29.4, 29.4, 25.9, 22.7, 20.1, 14.1, 13.7;
LR-MS: m/z=256 [M+H]+; HR-MS (ESI): m/z=256.2639
[M+H]+, calcd. for C16H34NO: 256.2640; IR (KBr): n=
3296, 2919, 1638, 1552 cmÀ1; Rf product: 0.25 (20% EtOAc/
hexane).
(9H-Fluoren-9-ylmethoxycarbonylamino)thioacetic acid
S-(4-methoxyphenyl) ester (22): Yield: 91%; white solid; mp
1688C. 1H NMR (400 MHz, CDCl3): d=7.77–7.75 (d, J=
7.6 Hz 2H), 7.61–7.60 (d, J=6.4 Hz, 2H), 7.41–7.25 (m,7H),
6.95–6.93(d, J=6.4 Hz, 1H), 5.45 (s, 1H), 4.46–4.44 (d, J=
2.4 Hz, 2H), 4.26–4.23 (t, J=6.8 Hz, 1H), 4.21–4.20 (d, J=
5.2 Hz, 2H), 3.82 (s, 3H); 13C NMR (100 MHz, CDCl3): d=
197.1, 161.0, 156.3, 143.8, 141.4, 136.4, 132.8, 127.9, 127.2,
125.2, 120.1, 116.9, 115.2, 67.5, 55.5, 47.3, 29.8; LR-MS:
m/z=442 [M+Na]+; HR-MS (ESI): m/z=442.1081 [M+
H]+, calcd. for C24H21NNaO4S: 442.1089; IR (KBr): n=3349,
1724, 1683, 1249 cmÀ1; Rf product: 0.5 (20% EtOAc/
hexane).
2-(9H-Fluoren-9-ylmethoxycarbonylamino)thiopropionic
acid S-benzyl ester (23): Yield: 92%; white solid; mp 1728C.
1H NMR (400 MHz, CDCl3): d=7.75–7.73 (d, J=7.6 Hz,
2H), 7.60–7.25 (m, 11H), 5.28–5.27 (d, J=2.4 Hz, 1H),
4.51–4.46 (q, J=6.8 Hz, 1H),), 4.38–4.36 (d, J=6.8 Hz, 2H)
4.22–4.19 (t, J=7.2 Hz, 1H), 4.10 (s, 2H), 1.41–1.39 (d, J=
7.6 Hz, 2H); 13C NMR (100 MHz, CDCl3): d=200.9, 155.7,
143.9, 143.8, 141.4, 137.0, 128.9, 128.7, 127.8, 127.4, 127.1,
125.1, 67.1, 56.7, 47.2, 33.3, 18.8; LR-MS: m/z=440 [M+
Na]+. HR-MS (ESI): m/z=440.1374 [M+Na]+, calcd. for
C25H23NNaO3S: 440.1296; IR (KBr): n3309, 1735, 1696,
1273 cmÀ1; Rf product: 0.3 (10% EtOAc/hexane).
3-Furan-2-yl-N,N-diisopropylacrylamide (29): Yield: 90%;
1
white solid; mp 1258C. H NMR (400 MHz, CDCl3): d=7.38
(d, J=1.6 Hz, 1H),7.36–7.32 (d, J=15.2 Hz, 1H), 6.74–6.71
(d, J=14.8 Hz, 1H), 6.46–6.45 (d, J=3.6 Hz, 1H), 6.40–6.39
(dd, J=2 Hz, 1H), 4.10–4.05 (q, J=7.4 Hz, 1H), 1.35–1.30
(d, J=11.6 Hz, 12H); 13C NMR (100 MHz, CDCl3): d=
166.0, 152.1, 143.6, 128.2, 118.2, 113.1, 112.2, 48.0, 46.0, 21.7,
20.8; LR-MS: m/z=222 [M+H]+; HR-MS (ESI): m/z=
222.1498 [M+H]+, calcd. for C13H20NO2: 222.1494; IR
(KBr): n=3458, 2970, 1646 cmÀ1; Rf product: 0.5 (20%
EtOAc/hexane).
N-Butylcarbamoylmethylbenzamide (30): Yield: 93%;
white solid; mp 1288C. 1H NMR (400 MHz, CDCl3): d=
7.82–7.38 (m, 5H), 6.78 (br, 1H), 4.12–4.10 (d, J=4.8 Hz,
2H), 3.27–3.22 (m, 2H), 1.51–1.43 (m, 2H), 1.36–1.28 (m,
2H), 0.89–0.85 (t, J=7.4 Hz, 3H); 13C NMR (100 MHz,
CDCl3): d=169.4, 168.2, 133.6, 132.1, 128.8, 127.4, 41.1, 39.7,
31.6, 20.2, 13.8; LR-MS: m/z=235 [M+H]+, HR-MS (ESI):
m/z=235.1458 [M+H]+, calcd. for C13H19N2O2: 235.1447;
IR (KBr): n=3320, 3298, 1652, 1635 cmÀ1; Rf product: 0.2
(40% EtOAc/hexane).
N-Benzylbenzamide (24): Yield: 91%; white solid; mp
1
1628C. H NMR (400 MHz, CDCl3): d=7.79–7.77 (m, 2H),
7.43–7.25 (m, 8H), 4.64–4.63 (d, J=5.6 Hz, 2H); 13C NMR
(100 MHz, CDCl3): d =167.6, 138.4, 134.4, 131.6, 128.8,
128.6, 127.9, 127.5, 127.1, 44.1; LR-MS: m/z 212 [M+H]+;
HR-MS (ESI): m/z=212.1073 [M+H]+, calcd. for
C14H14NO: 212.1075; IR (KBr): n=3290, 2924, 1637,
1551 cmÀ1; Rf product: 0.45 (20% EtOAc/hexane).
N-[(4-Chlorophenylcarbamoyl)methyl]benzamide
(31):
Yield: 90%; white solid; mp 2328C. 1H NMR (400 MHz,
CDCl3): d=7.93–7.33 (m, 10H), 4.25 (s, 2H); 13C NMR
(100 MHz, CDCl3, CD3OD for solubility): d=168.8, 167.8,
136.4, 133.2, 131.8, 129.1, 128.6, 128.4, 127.0, 121.1, 43.5;
LR-MS: m/z=311 [M+Na]+; HR-MS (ESI): m/z=311.0565
[M+Na]+, calcd. for C15H13ClN2Na O2: 311.0563; IR (KBr):
n=3306, 3192, 1680, 1638 cmÀ1; Rf product: 0.25 (40%
EtOAc/hexane).
N-Benzyl-2-phenylacetamide (25): Yield: 92%; white
1
solid; mp 1188C. H NMR (400 MHz, CDCl3): d=7.36–7.16
(m, 10H), 5.76 (br, 1H), 4.41–4.39 (d, J=6 Hz, 2H), 3.62 (s,
2H); 13C NMR (100 MHz, CDCl3): d=171.1, 138.3, 135.0,
129.4, 129.0, 128.7, 127.5, 127.4, 127.3, 43.7, 43.6; LR-MS:
m/z=226 [M+H]+; HR-MS (ESI): m/z=226.1231, [M+
H]+, calcd. for C15H16NO: 226.1232; IR (KBr): n=3289,
N-(p-Tolylcarbamoylmethyl)benzamide (32): Yield: 89%;
1
white solid; mp 2308C. H NMR (400 MHz, CDCl3): d=9.04
(br, 1H), 7.83–7.06 (m, 10H), 5.68 (br, 1H), 4.32–4.31 (d,
J=5.6 Hz, 2H), 2.27 (s, 3H); 13C NMR (100 MHz, CDCl3,
CD3OD for solubility): d=167.8, 167.4, 135.4, 133.4, 131.5,
130.8, 129.1, 128.3, 127.2, 119.8; LR-MS: m/z=269 [M+
H]+; HR-MS (ESI): m/z=269.1285 [M+H]+, calcd. for
C16H17N2O2: 269.1290; IR (KBr): n=3269, 2935, 1676,
1638 cmÀ1; Rf product: 0.2 (40% EtOAc/hexane).
2925, 1638, 1552 cmÀ1
hexane).
; Rf product: 0.3 (20% EtOAc/
N-[2-Hydroxy-1,1-bis(hydroxymethyl)ethyl]-2-phenylace-
1
tamide (26): Yield: 90%; white solid; mp 1258C. H NMR
(400 MHz, CD3OD): d=7.19–7.13 (m, 5H), 3.59 (s, 6H),
3.46 (s, 2H); 13C NMR (100 MHz, CDCl3): d=175.1, 136.7,
130.3, 129.7, 128.0, 63.6, 62.5, 44.3; LR-MS: m/z=240 [M+
H]+; HR-MS (ESI): m/z=240.1233 [M+H]+, calcd. for
C12H18NO4: 240.1236; IR (KBr): n=3283, 2934, 1644,
1542 cmÀ1; Rf product: 0.25 (80% EtOAc/hexane).
N-1-(2-Hydroxyethylcarbamoyl)ethylbenzamide
(33):
Yield: 91%; white solid; m.p 1278C. 1H NMR (400 MHz,
CD3OD): d=7.78–7.34 (m, 5H), 4.49–4.43 (m, 1H), 3.51–
3.49 (d, J=5.2 Hz, 2H), 3.23–3.16 (m, 2H), 1.84 (s, 1H),
1.37–1.35 (d, J=6.8 Hz, 3H); 13C NMR (100 MHz, CD3OD):
d=175.6, 170.1, 135.3, 133.0, 129.6, 128.7, 61.6, 51.3, 43.1,
18.4; LR-MS: m/z=237 [M+H]+; HR-MS (ESI): m/z=
237.1250 [M+H]+, calcd. for C12H17N2O3: 237.1239; IR
(KBr): n=3425, 3300, 1622 cmÀ1; Rf product: 0.2 (80%
EtOAc/hexane).
2,N-Diphenylacetamide (27): Yield: 85%; white solid;
1
mp: 1168C. H NMR (400 MHz, CDCl3): d=7.42–7.06 (m,
10H), 3.73 (s, 2H); 13C NMR (100 MHz, CDCl3): d=169.8,
137.9, 134.7, 129.5, 129.1, 128.9, 127.5, 124.5, 120.2, 44.6;
LR-MS: m/z=212 [M+H]+; HR-MS (ESI): m/z=212.1071
[M+H]+, calcd. for C14H14NO: 212.1075; IR (KBr): n=
Adv. Synth. Catal. 2013, 355, 448 – 462
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
459

Kishore Thalluri et al.
FULL PAPERS
N-{1-[2-Hydroxy-1,1-bis(hydroxymethyl)ethylcarbamoyl]-
ethyl}benzamide (34): Yield: 91%; semi solid. 1H NMR
(400 MHz, CD3OD): d=7.76–7.73 (m, 5H), 4.51–4.44 (q, J=
7.2 Hz, 1H), 3.62 (s, 6H), 1.37–1.36 (d, J=7.2 Hz, 3H);
13C NMR (100 MHz, CD3OD): d=176.0, 170.5, 135.1, 133.1,
129.7, 128.6, 63.5, 62.3, 51.8, 18.1; LR-MS: m/z=297 [M+
H]+; HR-MS (ESI): m/z=297.1455 [M+H]+, calcd. for
C14H21N2O5: 297.1450; IR (KBr): n=3436, 2922, 1671,
1640 cmÀ1; Rf product: 0.2 (80% EtOAc/hexane).
Cyclohexylcarbamoylmethylcarbamic acid benzyl ester
(35): Yield: 94%; brown solid; mp 1108C. 1H NMR
(400 MHz, CDCl3): d=7.31 (s, 5H), 6.1 (br, 1H), 5.6 (br,
1H), 5.0 (s, 2H), 3.79–3.78 (d, J=4 Hz, 2H), 3.74–3.70 (m,
1H), 1.84–1.09 (m, 10H), 13C NMR (100 MHz, CDCl3): d=
168.1, 156.8, 136.3, 128.7, 128.4, 128.2, 67.3, 48.5, 44.8, 33.0,
25.6, 24.9; LR-MS: m/z=291 [M+H]+; HR-MS (ESI):
m/z=291.1710 [M+H]+, calcd. for C16H23N2O3: 291.1709;
IR (KBr): n=3347, 2924, 1718, 1655 cmÀ1; Rf product: 0.1
(10% EtOAc/hexane).
2-Oxopiperidin-1-ylethylcarbamic acid benzyl ester (36):
Yield: 91%; white solid; mp 1148C 1H NMR (400 MHz,
CDCl3): d=7.33–7.29 (m, 5H), 5.84 (br, 1H), 5.09 (s, 2H),
3.98–3.97 (d, J=4 Hz, 2H), 3.54–3.52 (t, J=5.2 Hz, 2H),
3.29–3.27 (t, J=5.2 Hz, 2H), 1.70 (s, 2H), 1.63–1.61 (d, J=
4.8 Hz, 2H), 1.54–1.52 (d, J=5.6 Hz, 2H); 13C NMR
(100 MHz, CDCl3): d=166.0, 156.3, 136.6, 128.5, 128.1,
128.0, 66.8, 45.4, 43.1, 42.6, 34.0, 26.2, 25.7, 25.4, 25.0, 24.3;
LR-MS: m/z=299 [M+Na]+; HR-MS (ESI): m/z=299.1366
[M+Na]+, calcd. for C15H20N2NaO3: 299.1372; IR (KBr):
n=3293, 2936, 1707, 1641 cmÀ1; Rf product: 0.2 (30%
EtOAc/hexane).
(KBr): n=3332, 1684, 1661, 1650 cmÀ1; Rf product: 0.6 (50%
EtOAc/hexane).
2-[(9H-Fluoren-9-ylmethoxycarbonylamino)acetylamino]-
acetic acid methyl ester (40): Yield: 91%; white solid; mp
1628C. 1H NMR (400 MHz, CDCl3): d=7.76–7.74 (d, J=
7.6 Hz, 2H), 7.58–7.56 (d, J=7.6 Hz, 2H), 7.40–7.37 (t, J=
7.2 Hz, 2H), 7.31–7.27 (t, J=7.2 Hz, 2H), 6.80 (br, 1H), 5.73
(br, 1H), 4.43–4.41 (d, J=6.8 Hz, 2H), 4.22–4.19 (t, J=
7.2 Hz, 1H), 4.04–4.03 (d, J=6.8 Hz, 2H) 3.92–3.91 (d, J=
6.8 Hz, 2H), 3.73 (s, 3H); 13C NMR (100 MHz, CDCl3): d=
169.9, 169.7, 156.8, 143.7, 141.2, 128.6, 128.5, 128.3, 127.1,
67.1, 47.0, 44.2, 41.2, 33.8; LR-MS: m/z=391 [M+Na]+;
HR-MS (ESI): m/z=391.1366 [M+H]+, calcd. for
C20H20N2NaO5: 391.1270; IR (KBr): n=3316, 2931, 1734,
1692, 1654 cmÀ1; Rf product: 0.2 (40% EtOAc/hexane).
2-[2-(9H-Fluoren-9-ylmethoxycarbonylamino)-3-methyl-
butyrylamino]propionic acid methyl ester (41): Yield: 90%;
white solid; mp: 2058C. 1H NMR (400 MHz, CDCl3): d=
7.76–7.75 (d, J=7.6 Hz, 2H), 7.60–7.58 (d, J=6.8 Hz, 2H),
7.41–7.37 (t, J=7.2 Hz, 2H), 7.31–7.28 (t, J=7.2 Hz, 2H),
6.54–6.52 (d, J=6.8 Hz, 1H), 5.54–5.52 (d, J=8 Hz, 1H),
4.60–4.58 (d, J=7.2 Hz, 2H), 4.44–4.35 (m, 1H), 4.23–
4.21(d, J=6.4 Hz, 1H) 4.04–4.0 (t, J=7.4 Hz, 1H), 3.73 (s,
3H), 2.11–2.08 (m, 1H), 1.41–1.39 (d, J=6.8 Hz, 3H) 0.98–
0.96 (d, J=9.6 Hz, 6H); 13C NMR (100 MHz, CDCl3): d=
173.3, 171.2, 156.6, 144.0, 141.4, 127.8, 127.2, 125.2, 120.1,
67.2, 60.4, 52.6, 48.2, 47.3, 31.9, 29.8, 19.2, 18.2; LR-MS:
m/z=447 [M+Na]+; HR-MS (ESI): m/z=447.1908 [M+
Na]+, calcd. for C24H28N2NaO5: 447.1896; IR (KBr): n=
3296, 2953, 1745, 1692, 1650 cmÀ1; Rf product: 0.5 (30%
EtOAc/hexane).
Phenylcarbamoylmethyl-carbamic acid-benzyl ester (37):
Yield: 87%; light red solid; mp 1478C. H NMR (400 MHz,
Methyl 2-(2-{[(9H-fluoren-9 yl)methoxy]carbonylamino}-
4-methylpentanamido)propanoate (42): Yield: 88%; white
solid; mp 1628C. H NMR (400 MHz, CDCl3): d=7.76–7.75
1
1
CDCl3): d=7.45–7.10 (m, 10H), 5.57 (br, 1H), 5.13 (s, 2H),
3.99–3.98 (d, J=4 Hz, 2H); 13C NMR (100 MHz, CDCl3):
d=170.0, 156.5, 136.3, 135.3, 129.3, 128.8, 128.7, 128.5, 128.3,
128.2, 67.3, 43.0; LR-MS: m/z=307 [M+Na]+, HR-MS
(ESI): m/z=307.1062 [M+Na]+, calcd. for C16H16N2NaO3:
307.1059; IR (KBr): n=3340, 2929, 1696, 1675 cmÀ1; Rf
product: 0.2 (30% EtOAc/hexane).
1-Butylcarbomyl-2-methylpropylcarbamic acid benzyl
ester (38): Yield: 93%; white solid; m.p 1398C. 1H NMR
(400 MHz, CDCl3): d=7.34 (s, 5H), 5.95 ( br, 1H), 5.41–
5.39 (d, J=8 Hz, 1H), 5.1 (s, 2H), 3.92–3.82 (t, J=8.4 Hz,
(d, J=7.6 Hz, 2H), 7.59–7.57 (d, J=6.8 Hz, 2H), 7.41–7.37
(t, J=7.2 Hz, 2H), 7.32–7.28 (t, J=7.2 Hz, 2H), 6.62–6.60
(d, J=6.8 Hz, 1H), 5.36–5.34 (d, J=8 Hz, 1H), 4.58–4.52
(m, 1H), 4.41–4.35 (m, 3H), 4.22–4.19 (t, J=6.4 Hz, 1H),
3.73 (s, 3H), 1.81–1.74 (m, 1H), 1.65–1.63 (m, 2H), 1.40–
1.38 (d, J=6.8 Hz, 3H) 1.01–0.98 (d, J=9.6 Hz, 6H);
13C NMR (100 MHz, CDCl3): d=174.2, 163.1, 156.6, 143.8,
141.1, 129.3, 128.6, 127.5, 126.9, 124.9, 66.6, 57.4, 52.2, 46.9,
38.2, 38.3, 28.1, 18.0.; LR-MS: m/z=439 [M+H]+; HR-MS
(ESI): m/z=439.2047 [M+H]+, calcd. for C25H31N2O5:
439.2233; IR (KBr): n3307, 2925, 1754, 1694, 1654 cmÀ1; Rf
product: 0.5 (30% EtOAc/hexane).
1H), 3.27–3.18 (m, 2H), 2.11–2.10 (m, 1H) 1.47–1.45
ACHTUNGERTN(NUNG m,
2H), 1.35–1.29 (m, 2H), 0.96–0.83
AHCTUNGTRENNUNG
(100 MHz, CDCl3): d=171.4, 156.7, 136.4, 128.6, 128.2,
128.0, 67.0, 60.8, 39.3, 31.6, 31.2, 20.1, 19.3, 18.2, 13.8; LR-
MS: m/z=307 [M+H]+; HR-MS (ESI): m/z=307.2032
[M+H]+, calcd. for C17H27N2O3: 307.2022; IR (KBr): n=
2-[2-(9H-Fluoren-9-ylmethoxycarbonylamino)-3-phenyl-
propionylamino]propionic acid methyl ester (43): Yield:
89%; white solid; mp 1828C. H NMR (400 MHz, CDCl3):
1
d=7.75–7.73 (d, J=7.6 Hz, 2H), 7.53–7.50 (t, J=6.8 Hz,
2H), 7.40–7.36 (t, J=7.2 Hz, 2H), 7.30–7.18 (m, J=7.2 Hz,
7H), 6.63 (br, 1H), 5.37 (br, 1H), 4.51–4.39 (m, 3H), 4.32–
4.29 (m, 1H), 4.18–4.14(t, J=6.8 Hz, 1H) 3.09–3.03 (dd, J=
6 Hz, 2H), 3.68 (s, 3H), 1.33–1.31 (d, J=6.8 Hz, 3H);
13C NMR (100 MHz, CDCl3): d=172.8, 170.9, 156.0, 143.7,
141.1, 136.5, 129.4, 128.4, 127.6, 127.0, 126.8, 125.1, 119.8,
67.0, 56.0, 52.3, 48.2, 46.9, 38.4, 17.8; LR-MS: m/z=473
[M+H]+; HR-MS (ESI): m/z=473.2402 [M+H]+, calcd. for
C28H28N2NaO5: 473.2076; IR (KBr): n=3307, 2925, 1754,
1694, 1654 cmÀ1; Rf product: 0.5 (30% EtOAc/hexane).
2-[2-(9H-Fluoren-9-ylmethoxycarbonylamino)-3-phenyl-
propionylamino]propionic acid methyl ester (44): Yield:
3293, 1690, 1641 cmÀ1
hexane).
; Rf product: 0.1 (20% EtOAc/
2-Phenyl-1-pyridin-2-ylmethylcarbamoylethylcarbamic
acid tert-butyl ester (39): Yield: 91%; white solid; mp
1208C. 1H NMR (400 MHz, CDCl3): d=8.44–8.43 (d, J=
4 Hz, 1H), 7.65–7.62 (t, J=6.4 Hz, 1H), 7.22–7.14 (m, 8H),
5.16 (br, 1H), 4.49–4.48 (d, J=4 Hz, 2H), 4.45 (br, 1H),
3.07–3.05 (d, J=6.4 Hz, 2H), 1.36 (s, 9H); 13C NMR
(100 MHz, CDCl3): d=171.1, 156.4, 155.5, 148.9, 137.0,
136.8, 129.4, 128.7, 126.9, 122.5, 122.1, 80.2, 56.0, 44.5, 38.8,
28.4; LR-MS: m/z=356 [M+H]+; HR-MS (ESI): m/z=
356.1962 [M+H]+, calcd. for C20H26N3O3: 356.1974; IR
460
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ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2013, 355, 448 – 462

Ethyl 2-(tert-Butoxycarbonyloxyimino)-2-cyanoacetate (Boc-Oxyma) as Coupling Reagent
1
91%; white solid; mp 1828C. H NMR (400 MHz, CDCl3):
d=7.76–7.74(d, J=7.6 Hz, 4H), 7.55–7.12 (m, 20H), 6.41
(br, 1H), 6.29 (br, 1H), 5.52–5.50 (d, J=6.8 Hz, 1H), 5.45–
5.43 (d, J=6.8 Hz, 1H 4.53–4.37 (m, 6H), 4.35–4.31 (m,
2H), 4.19–4.16(t, J=6.8 Hz, 2H) 3.10–2.97 (m, 4H), 3.68 (s,
6H), 1.33–1.32 (d, J=7.2 Hz, 3H), 1.22–1.20 (d, J=6.8 Hz,
3H); 13C NMR (100 MHz, CDCl3): d=173.2, 173.1, 171.1,
170.8, 156.2,144.0, 141.5, 136.8, 129.6, 128.8, 128.3, 127.9,
127.6, 127.3, 127.2, 125.3, 120.2, 67.4, 56.3, 52.6, 48.3, 48.2,
47.3, 39.2, 38.9, 18.3, 18.1.
and IITG (start up grant) for financial assistance. The au-
thors also thank the reviewers for important suggestions and
Dr. R. Mimna for language correction.
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2-[2-(9H-Fluoren-9-ylmethoxycarbonylamino)-2-phenyl-
acetylamino]-4-methylpentanoic acid methyl ester (45):
Yield: 90%; white solid; mp 1948C. 1H NMR (400 MHz,
CDCl3): d=7.74–7.73 (d, J=7.6 Hz, 2H), 7.56–7.22 (m,
11H), 6.21–6.20 (d, J=4.8 Hz, 1H), 5.29ACTHNUTRGNE(NUG br, 1H), 4.60–4.55
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6.8 Hz, 2H), 3.61 (s, 3H), 1.63–1.60 (m, 2H), 1.54–1.47 (m,
1H), 0.92–0.91 (d, J=4.4 Hz, 3H) 0.90–0.89 (d, J=4.4 Hz,
6H); 13C NMR (100 MHz, CDCl3): d=172.7, 170.0, 155.9,
143.7, 143.7, 141.2, 137.6, 128.8, 128.2, 127.6, 127.1, 127.0,
125.1, 119.9, 67.2, 58.1, 52.0, 51.1, 47.0, 41.0, 24.7, 22.6, 21.8;
LR-MS: m/z=501 [M+H]+; HR-MS (ESI): m/z=501.2418
[M+H]+, calcd. for C30H33N2O5: 501.2389; IR (KBr): n=
3433, 1742, 1684, 1655; 1654 cmÀ1; Rf product: 0.5 (30%
EtOAc/hexane).
[2-(9H-Fluoren-9-ylmethoxycarbonylamino)-3-methylbu-
tyrylamino]phenylacetic acid methyl ester (46): Yield: 91%;
white solid; mp 2008C. 1H NMR (400 MHz, CDCl3): d=
7.75–7.74 (d, J=7.6 Hz, 2H), 7.56–7.55 (d, J=7.6 Hz, 2H),
(7.40–7.23 (m, 9H), 6.90–6.88 (d, J=6.8 Hz, 1H), 5.54–5.52
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(m, 1H), 4.19–4.18 (d, J=6.8, 1H) 4.10–4.07 (t, J=7.8 Hz,
2H), 3.7 (s, 3H), 2.16–2.13 (m, 1H), 1.01–0.99 (d, J=6.8 Hz,
3H), 0.96–0.95 (d, J=6.8 Hz, 6H); 13C NMR (100 MHz,
CDCl3): d=171.7, 169.1, 156.6, 144.0, 143.9, 141.4, 135.9,
129.1, 128.7, 127.8, 127.4, 127.2, 125.2, 120.0, 67.2, 60.2, 56.7,
52.9, 47.2, 31.6, 19.2, 18.1; LR-MS: m/z=487 [M+H]+; HR-
MS (ESI): m/z=487.2373 [M+H]+, calcd. for C29H31N2O5:
487.2233; IR (KBr): n=3445, 1739, 1681, 1650 cmÀ1; Rf
product: 0.5 (30% EtOAc/hexane).
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2-[2-(2-Benzyloxycarbonylaminoacetylamino)-3-phenyl-
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Yield: 90%; white solid; mp 1018C. 1H NMR (400 MHz,
CDCl3): d=7.32–7.13 (m, 10H), 6.99 (br, 1H), 6.2 (br, 1H),
5.8 (br, 1H), 5.08 (s, 2H), 4.58–4.54 (m, 1H), 4.43–4.38 (m,
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Acknowledgements
The authors TK, AP and NKC thank IIT Guwahati for fel-
lowships. We are thankful to DST for XRD facility, DST
(FAST TRACK scheme, sanction no/SR/FT/CS-011/2008)
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