Exploitation of the arndteistert homologation of N-methyl-α-amino acids for concomitant ester and N-mMethyl peptide formation

Article abstract of DOI:10.1071/CH07371

Results from studies of the Arndt?Eistert homologation of N-methyl-?-amino acids with concomitant ester and amide formation is discussed. The applicability of the use of ultrasound was investigated in the Wolff rearrangement of diazoketones for the produc

Full text of DOI:10.1071/CH07371

Full Paper
CSIRO PUBLISHING
www.publish.csiro.au/journals/ajc
Aust. J. Chem. 2008, 61, 131–137
Exploitation of the Arndt–Eistert Homologation
of N-Methyl-α-Amino Acids for Concomitant
Ester and N-Methyl Peptide Formation
Andrew B. HughesA,C and Brad E. SleebsA,B
ADepartment of Chemistry, La Trobe University, Vic. 3086, Australia.
BCurrent address: Medicinal Chemistry Group, Structural Biology Division, The Walter
and Eliza Hall Institute of Medical Research, Vic. 3086, Australia.
CCorresponding author. Email: a.hughes@latrobe.edu.au
Results from studies of the Arndt–Eistert homologation of N-methyl-α-amino acids with concomitant ester and amide
formation is discussed. The applicability of the use of ultrasound was investigated in the Wolff rearrangement of diazo-
ketones for the production of esters and amides. This methodology was then applied to a novel ‘N-methyl coupling’
that allows simultaneous β-amino acid formation. Conventional ‘PyBroP N-methyl couplings’ were also performed as a
comparison to establish the validity of the method.
Manuscript received: 23 October 2007.
Final version: 17 January 2008.
Introduction
intermediate derived from the Wolff rearrangement of amino
acid-derived diazoketones to generate dipeptides that have an α-
and β-amino acid residue.
The importance of β-amino acids has recently been realised,
and has resulted in a plethora of syntheses for these valu-
able intermediates.[1–8] However, the synthesis of β-amino acid
derivatives and development of methodology for their incorpo-
ration into peptides and peptidomimetics is ongoing. It is well
established that the Arndt–Eistert homologation is a method for
the synthesis of β-amino acids from α-amino acids.[9–13]
The Arndt–Eistert homologation involves the formation of a
diazoketone, by the activation of a N-protected α-amino acid
(either as an acid chloride or mixed anhydride) and subse-
quent reaction with diazomethane.The diazoketone formed then
undergoes reaction via an α-ketocarbene produced by thermo-
lysis, photolysis, or by metal catalysis[14] to form a ketene
intermediate. In most circumstances the ketene intermediate is
intercepted by hydroxide or water to give the homologated car-
boxylic acid. But the ketene intermediate formed is a reactive
electrophile that can react even with poor nucleophiles, such as
hindered alcohols and amines, to form homologated esters and
amides.[11,12,15–18]
Results and Discussion
Using amines as nucleophiles in theWolff rearrangement to form
amides is well known. Seebach et al.[11] used this methodology
with amino acid esters as nucleophiles to allow quick and easy
construction of peptides that contain β-amino acids. The pro-
cess is relatively high yielding; however, the major drawback is
the general requirement for four equivalents of the nucleophilic
amino ester.This method would be a poor one if used in the latter
stages of a peptidic sequence of any length, as a peptide with two
or more residues as a nucleophile would result in an inefficient
sequence.
A literature search revealed that neither N-methyl-α- nor β-
amino acid esters have been used as nucleophiles during a Wolff
rearrangement. Generally, N-methyl amide formation is more
difficult to achieve than ordinary amide bonds. In peptide chem-
istry, the use of exotic, sometimes toxic, and expensive coupling
reagents are required to form N-methyl amide bonds. Further-
more, addition of co-additives such as racemization suppressants
is another added cost. These reagents are generally used in
excess, and reaction yields are variable, but more often than
not, mediocre.
Muller and coworkers[19] found that ultrasound can cause
the diazoketone to decompose to the ketene, and, in the pres-
ence of water, to form a homologous acid product. Although this
method was created so N-Fmoc-protected amino acid residues
could be smoothly converted into their homologous acids, we
found this method was a reliable approach to other compounds
and reaction times were significantly reduced compared with
other methods.[9–12,15–18]
Recently, we used this methodology in the synthesis of a
series of novel N-methyl-β-amino derivatives from all 20 of the
common proteinogenic α-amino acids.[20–22]
We now report on the production of various synthetically
useful amino acid esters. In addition, we also investigated the
coupling of amino acids and N-methyl amino acids to the ketene
It was proposed that the exploitation of the ketene intermedi-
ate in the Wolff rearrangement might be a convenient method to
couple a β-amino acid and an α-amino acid or N-methyl-α-amino
acid residue. Although the procedure would require an excess of
the α-amino acid or N-methyl-α-amino acid residue, there is
no necessity for expensive coupling reagents or racemization
suppressants. The coupling reaction would be racemization-free
because no strong bases would be used. Also, the coupling
© CSIRO 2008
10.1071/CH07371
0004-9425/08/020131

132
A. B. Hughes and B. E. Sleebs
R, RЈ see Table 1
CO2RЈ
R
R
R
O
·
(ii)
Cbz
CHN2
Cbz
(i)
Cbz
N
N
N
O
H
5–17
1–4 (see refs 20, 21)
Scheme 1. Synthesis of N-methyl-β-amino acid esters employing ultrasound. R and Rꢀ refer to groups
described in Table 1. Reagents and conditions: (i) ultrasound, CF3COO−Ag+, 1,4-dioxane; (ii) RꢀOH.
Table 1. Yields of N-methyl-β-amino esters from Wolff rearrangements employing ultrasound
R and Rꢀ refer to groups in Scheme 1
Cpd
R
Rꢀ
Yield [%]
5
Ala
Orn
His
Ala
Ala
Orn
Ala
Val
Ala
Ala
Ala
Ala
Val
CH3
CH3
CH3
CH3CH2
(CH3)3C
(CH3)3C
PhCH2
PhCH2
PMB
Ph
Pfp
Allyl
Allyl
80
76
82
67
43
45
58
44
42
90
75
76
75
6[20]
7[20,21]
8
9
10[20]
11
12
13
14
15
16
17
O
CHN2
Cbz
Cbz
ϩ
(i)
Cbz
N
N
N
N
H
CO2CH3
CO2CH3
ClϪϩH3N
CO2CH3
18
O
O
1
19 66%
O
ϩ
(i)
CHN2
Cbz
N
N
H
ClϪϩH3N
CO2CH3
18
21
55%
20
Scheme 2. Synthesis of N-methyl-β-amino acid amides 19 and 21 employing ultrasound. Reagents
and conditions: (i) ultrasound, CF3COO−Ag+, 1,4-dioxane, Et3N.
time might be reduced significantly especially if ultrasound was
applied to accelerate the coupling reaction.
couplings using ultrasonic waves had not been reported. The
reaction yields were good compared with yields obtained by
Seebach et al.[11] who reported higher yields using the more
conventional method of silver catalysis and longer reaction
times.
The first N-methyl amino acid coupling was performed with
residues that induced the least amount of steric hindrance, an
alanine diazoketone 1[20,21] and the sarcosine ethyl ester 22
(Scheme 3). The reaction mixture, in dry 1,4-dioxane, with an
equivalent of triethylamine to remove the hydrochloride salt
of the amine, was placed in an ultrasound bath for 45 min.
The dipeptide 23 formed, albeit in only 38% yield (Scheme 3).
This initial 38% yield for compound 23 compared favourably
to that for compounds 19 (66%) and 21 (55%) using ultra-
sound (Scheme 2), and may potentially be improved using the
procedure of Seebach et al.[11]
Initially, certain alcohols were chosen as nucleophiles for
their applicability in peptide chemistry. Notable differences to
the procedure of Suresh Babu and coworkers[15–18] include the
use of the ultrasonic bath and the addition of only four equiva-
lents of the alcohol nucleophile. The transformation performed
in dry 1,4-dioxane, without addition of base, using ultrasound
and silver trifluoroacetate was generally complete in 30 min
(Scheme 1). Table 1 summarizes the novel N-methyl-β-amino
esters formed from diazoketones.[20,21]
The results for β-amino ester formation in Table 1 are gener-
ally good.The lowest yields are clearly associated with tert-butyl
and benzylic type esters. Good yields were achieved with the
simple alkyl, allyl, and phenolic esters. The results in Table 1
confirm the general reactivity of the ketene intermediate towards
the alcoholic nucleophiles.
A more challenging coupling was N-methyl-α-valine t-butyl
ester 24 with alanine diazoketone 1[20] (Scheme 3).As expected,
a lower yield (27%) of the dipeptide 25 was obtained, presumably
a result of steric hindrance. The formation of the same com-
pound was performed using the ‘traditional’ N-methyl coupling
In the next phase, initial coupling reactions of ketene inter-
mediates with amino acids as nucleophiles were conducted using
an L-valine ester 18 as a nucleophile (Scheme 2) to form amides
19and 21.Thesereactionswereundertakenbecausesuchpeptide

Exploitation of the Arndt–Eistert Homologation
133
O
(i)
(i)
ϪClϩH2N
CO2Et
Cbz
Cbz
Cbz
Cbz
CHN2
CHN2
ϩ
ϩ
N
N
CO2Et
N
N
O
O
1
22
23 38%
O
N
TosϪ ϩH2N
CO2tBu
O
N
CO2tBu
1
24
26
25 27%
ϩ
Cbz
CHN2
HN
N
H
CO2Me
Cbz
N
O
O
O
(i)
4
N
N
H
CO2Me
N
27 34%
Scheme 3. Synthesis of N-methyl-β-amino acid N-methyl amides 23, 25, and 27 using the ketene interme-
diate in theWolff rearrangement of theArndt–Eistert homologation. Reagents and conditions: (i) ultrasound,
CF3COO−Ag+, 1,4-dioxane, Et3N.
(i)
Cbz
CHN2
Cbz
CO2H
ϩ
N
TosϪ
CO2tBu
ϩH2N
N
O
1
28
24
(ii)
O
Cbz
N
N
CO2tBu
25 30% overall
O
(i)
Cbz
CHN2
ϩ
Cbz
CO2H
N
HN
(ii)
N
H
CO2Me
N
O
4
29
26
O
O
Cbz
N
N
H
CO2Me
N
27 44% overall
Scheme 4. Synthesis of N-methyl-β-amino acid N-methyl amides 25 and 27 using PyBrOP as the
coupling reagent. Reagents and conditions: (i) ultrasound, CF3COO−Ag+, 1,4-dioxane/H2O, Et3N;
(ii) iPr2NEt, CH2Cl2.
reagents and an overall yield ‘from the diazoketone 1’ of 30%
was obtained (Scheme 4). This compares with a 27% yield from
the ketene interception reaction (Scheme 3).
In another challenging coupling, the dipeptide 26 was used
as the nucleophile and coupled with the diazoketone 4.[20,21] The
reaction afforded the tripeptide 27 in a 34% yield (Scheme 3).
In comparison, a reaction using PyBrOP as a coupling reagent
gave an overall yield over two steps of 44% (Scheme 4).
α-amino acid and N-methyl-α-amino acid couplings. The possi-
bility to increase yields using the method of Seebach et al.[11]
also needs to be explored.Although the novel reaction is specific
to a coupling of a β-amino diazoketone to an N-methyl residue,
the method also demonstrates how quickly a peptide chain that
contains β-residues may be constructed.
Experimental
General Methods
Infrared spectra were recorded on a Brüker Vector 22 Fourier-
transform spectrometer or a Perkin Elmer 1720-X FT-IR spec-
trometer. Optical rotations were recorded on a Perkin–Elmer
141 polarimeter. NMR spectra were recorded in CDCl3 at 300 K
unless otherwise stated on a Brüker Avance DRX-300 spectro-
meter. Low-andhigh-resolutionmassspectra(LSIMS)werealso
measured at the University of Tasmania by Dr Noel Davies and
Conclusion
The results obtained from reaction of the ketene intermediates of
various compounds are promising: short reaction times and the
use of inexpensive reagents are major advantages of the reaction.
In future work, more results are needed to confirm that the
ketene interception method is a viable and general process for

134
A. B. Hughes and B. E. Sleebs
coworkers on a Kratos Concept mass spectrometer at 70 eV, all
using water/methanol/acetic acid (0/99/1 or 50/50/1) mixtures as
the mobile phase. Microanalyses were performed by E. Mocellin
and coworkers from Chemical and Microanalytical Services Pty
Ltd, Belmont, Victoria. ‘Flash’ chromatography was carried out
using silica gel 60 particle size 0.040–0.063 µm (230–400 mesh
ASTM) supplied by Merck Chemicals. Ethyl acetate and hexane
used for chromatography were distilled before use. All solvents
were purified by distillation. TLC was performed on Merck
kieselgel 60 F254 plates and visualized with a UV lamp. For
dry solvents, procedures from Perrin andArmarego[23] were fol-
lowed. All other reagents and solvents were purified or dried as
described by Perrin and Armarego.[23] All yields reported are
unoptimized.
(3S)-Ethyl-N-benzyloxycarbonyl-
3-aminomethylbutanoate 8
To the diazoketone 1[20] (0.7 mmol) dissolved in dry 1,4-
dioxane (3 mL) were added dry ethanol (2.8 mmol) and silver
trifluoroacetate (0.07 mmol). The solution was then sonicated
for 30 min. The mixture was concentrated under vacuum, the
residue was diluted with ether (20 mL), and the solution was
washed successively with 10% citric acid solution (20 mL), sat-
urated sodium bicarbonate solution (20 mL), and brine (20 mL).
The ethereal layer was dried (MgSO4) and concentrated under
reduced pressure. The resulting oil was purified using column
chromatography, with 15% ethyl acetate/hexane as eluent, to
afford the ester 8 as a clear colourless oil (67% yield). [α]2D0 0.0
(c 1.2, CH2Cl2). δH (300 MHz, CDCl3) (rotamers) 7.33–7.26
(5H, m), 5.11 (2H, s), 4.63–4.07 (1H, m), 4.06–4.04 (2H, m),
2.86–2.79 (3H, m), 2.54–2.41 (2H, m), 1.40–1.15 (6H, 2m). δC
(75 MHz, CDCl3) (rotamers) 170.5, 155.5, 136.6, 128.0, 127.5,
127.4, 127.3, 67.0, 66.6, 60.0, 48.8, 39.0, 28.4, 17.6, 13.7. νmax
(NaCl)/cm−1 2979, 2907 (CH), 1735, 1700 (CO), 1497, 1454,
1329, 1182, 1142, 768, 698. (Found: m/z 280.1543. Calc. for
C15H22NO4 (MH+): m/z 280.1549.)
β-Esters
(3S)-Methyl-N-benzyloxycarbonyl-
3-aminomethylbutanoate 5
To the diazoketone 1[20] (0.9 mmol) dissolved in dry 1,4-
dioxane (4 mL) were added dry methanol (3.2 mmol) and silver
trifluoroacetate (0.09 mmol). The solution was then sonicated
for 30 min. The mixture was concentrated under vacuum, the
residue was diluted with ether (20 mL), and the resulting organic
layer was washed successively with 10% citric acid solution
(20 mL), saturated sodium bicarbonate solution (20 mL), and
brine (20 mL). The ethereal layer was dried (MgSO4) and con-
centrated under reduced pressure. The resulting oil was purified
using column chromatography, with 15% ethyl acetate–hexane
as eluent, to furnish the ester 5 as a clear colourless oil (80%).
[α]2D0 0.0 (c 1.0, CH2Cl2). δH (300 MHz, CDCl3) (rotamers) 7.31
(5H, s), 5.10–5.07 (2H, m), 4.64–4.57 (1H, m), 3.58 (3H, s),
2.79–2.72 (3H, m), 2.55–2.37 (2H, m), 1.17 (3H, d, J 6.1). δC
(75 MHz, CDCl3) (rotamers) (323 K) 171.1, 155.6, 136.8, 128.2,
127.7, 127.6, 127.5, 67.1, 66.7, 66.2, 51.7, 51.2, 48.9, 38.8, 28.6,
27.3, 17.7. νmax (NaCl)/cm−1 3100–2900 (CH), 1737, 1696
(CO), 1454, 1331, 1254, 1139, 698. (Found: m/z 266.1386. Calc.
for C14H20NO4: m/z (LSIMS) 266.1392 (MH+).)
(3S)-tert-Butyl-N-benzyloxycarbonyl-
3-aminomethylbutanoate 9
To the diazoketone 1[20] (1.2 mmol) dissolved in dry 1,4-
dioxane (5 mL) were added dry tert-butanol (4.8 mmol) and
silver trifluoroacetate (0.12 mmol). The solution was then soni-
cated for 45 min. The mixture was concentrated under vacuum,
diluted with ether (25 mL), and the ethereal solution was washed
successively with 10% citric acid solution (25 mL), saturated
sodium bicarbonate solution (25 mL), and brine (25 mL). The
ethereal layer was dried (MgSO4) and concentrated under
reduced pressure. The resulting oil was purified using column
chromatography, with10%ethylacetate/hexaneaseluent, togive
the ester 9 as a clear colourless oil (43% yield). [α]2D0 +2.58
(c 1.5, CH2Cl2). δH (300 MHz, CDCl3) (rotamers) 7.30 (5H,
s), 5.08 (2H, s), 4.61–4.57 (1H, m), 2.85–2.76 (3H, m), 2.42–
2.30 (2H, m), 1.37 (9H, s), 1.14 (3H, d, J 6.6). δC (75 MHz,
CDCl3) (rotamers) (323 K) 170.0, 155.6, 136.7, 128.2, 127.6,
127.6, 80.4, 67.1, 66.7, 48.8, 40.3, 28.2, 27.7, 18.1, 17.6. νmax
(NaCl)/cm−1 2977–2935 (CH), 1728, 1702 (CO), 1455, 1329,
1302, 1162, 1140, 698. (Found: C 66.5, H 8.2, N 4.4. Calc. for
C17H25NO4: C 66.4, H 8.2, N 4.6%.)
(3S)-Methyl-N-benzyloxycarbonyl-3-aminomethyl-
6-phthalimidohexanoate 6
The diazoketone 2[20] (0.1 mmol) was dissolved in a solu-
tion of anhydrous 1,4-dioxane/methanol (9/1, v/v, 5 mL). On
addition of silver trifluoroacetate (0.01 mmol) the mixture was
sonicated in an ultrasound bath for 30 min or until no presence
of the diazoketone remained as indicated by TLC. The mixture
was concentrated under vacuum. The residue was dissolved in
ethyl acetate (15 mL), and the solution was washed successively
with 10% citric acid solution (15 mL), saturated sodium hydro-
gen carbonate solution (15 mL), and water (15 mL). The organic
layer was dried (MgSO4) and evaporated under vacuum. The
residue was subjected to column chromatography, with 40%
ethyl acetate–hexane as eluent, to afford the methyl ester 6 as a
clear colourless oil (76% yield). [α]2D0 −10.1 (c 1.7, CH2Cl2).
δH (300 MHz, CDCl3) 7.75–7.62 (4H, m), 7.28–7.12 (5H, m),
5.06–4.98 (2H, m), 4.45 (1H, br s), 3.60–3.47 (5H, 2m), 2.71
(3H, s), 2.55–2.29 (2H, m), 1.59–1.43 (4H, m). δC (75 MHz,
CDCl3) (rotamers) (325 K) 171.0, 167.9, 156.0, 136.8, 132.0,
136.8, 132.0, 128.2, 127.6, 122.9, 66.8, 53.4, 51.3, 37.6, 37.3,
29.4, 25.1. νmax (NaCl)/cm−1 3032, 2949 (CH), 1771, 1737,
1711 (CO), 1437, 1398, 1338, 1171, 1047, 721, 699. (Found:
C 65.7, H 6.0, N 6.3. Calc. for C24H26N2O6: C 65.7, H 6.0,
N 6.4%.)
(3S)-Benzyl-N-benzyloxycarbonyl-
3-aminomethylbutanoate 11
To the diazoketone 1[20] (0.9 mmol) dissolved in dry 1,4-
dioxane (5 mL) were added dry benzyl alcohol (3.2 mmol) and
silver trifluoroacetate (0.09 mmol). The solution was then soni-
cated for 30 min. The mixture was concentrated under vacuum,
the residue was diluted with ether (25 mL), and the ethereal
layer was washed successively with 10% citric acid solution
(25 mL), saturated sodium bicarbonate solution (25 mL), and
brine (25 mL). The ethereal layer was dried (MgSO4) and con-
centrated under reduced pressure. The resulting oil was purified
using column chromatography, with 20% ethyl acetate/hexane as
eluent, to yield the ester 11 as a clear colourless oil (58% yield).
[α]2D0 −3.16 (c 1.8, CH2Cl2). δH (300 MHz, CDCl3) (rotamers)
7.33–7.30 (10H, m), 5.14–5.00 (4H, m), 4.72–4.68 (1H, m),
2.90–2.77 (3H, m), 2.71–2.46 (2H, m), 1.16 (3H, d, J 4.7). δC
(75 MHz, CDCl3) (rotamers) (323 K) 170.4, 155.6, 136.8, 136.7,
128.2, 128.1, 127.9, 127.7, 127.6, 127.5, 67.1, 66.7, 66.0, 49.0,

Exploitation of the Arndt–Eistert Homologation
135
39.0, 28.6, 17.7. νmax (NaCl)/cm−1 3033, 2953(CH), 1736, 1699
(CO), 1455, 1329, 1174, 1142, 1017, 750, 698. (Found: C 70.4,
H 6.8, N 4.0. Calc. for C20H23NO4: C 70.4, H 6.8, N 4.1%.)
chromatography, with 10% ethyl acetate/hexane as eluent, to
furnish the ester 14 as a clear colourless oil (90% yield). [α]D20
+0.85 (c 1.7, CH2Cl2). δH (300 MHz, CDCl3) (rotamers) 7.32–
7.00 (10H, m), 5.13 (2H, s), 4.81–4.74 (1H, m), 2.87–2.65 (5H,
2m), 1.29 (3H, d, J 6.0). δC (75 MHz, CDCl3) (rotamers) (323 K)
169.2, 155.6, 150.3, 136.5, 129.1, 128.2, 127.6, 127.5, 125.5,
121.2, 67.0, 66.7, 48.9, 48.6, 39.2, 38.8, 28.4, 18.1, 17.7. νmax
(NaCl)/cm−1 3032, 2942 (CH), 1755, 1698 (CO), 1455, 1328,
1195, 1164, 1134, 696. (Found: C 69.8, H 6.4, N 4.2. Calc. for
C19H21NO4: C 69.7, H 6.5, N 4.3%.)
(3S)-Benzyl-N-benzyloxycarbonyl-3-aminomethyl-
4-methylpentanoate 12
To the diazoketone 4[20] (0.9 mmol) dissolved in dry 1,4-
dioxane (5 mL) were added benzyl alcohol (3.2 mmol) and silver
trifluoroacetate (0.09 mmol). The solution was then sonicated
for 30 min. The mixture was concentrated under vacuum, the
residue was diluted with ether (20 mL), and the ethereal layer was
washed successively with 10% citric acid solution (20 mL), sat-
urated sodium bicarbonate solution (20 mL), and brine (20 mL).
The ethereal layer was dried (MgSO4) and concentrated under
reduced pressure. The resulting oil was purified using column
chromatography, with 20% ethyl acetate/hexane as eluent, to
furnish the ester 12 as a clear colourless oil (44% yield). [α]D20
−4.40 (c 1.2, CH2Cl2). δH (300 MHz, CDCl3) (rotamers) 7.31
(10H, m), 5.18–4.93 (4H, m), 4.21–4.09 (1H, m), 2.78–2.47
(5H, 2m), 1.82–1.77 (1H, m), 0.94–0.81 (6H, m). δC (75 MHz,
CDCl3) (rotamers) 171.2, 171.0, 156.2, 156.0, 136.8, 136.6,
135.6, 135.4, 128.3, 128.2, 128.1, 127.9, 127.6, 127.6, 127.3,
66.5, 66.21, 66.16, 60.1, 59.4, 36.2, 35.7, 30.5, 30.1, 29.9,
29.4, 19.7, 19.3. νmax (NaCl)/cm−1 3030, 2964 (CH), 1737,
1698 (CO), 1470, 1305, 1173, 1141, 990, 751, 697. (Found:
m/z 370.2027. Calc. for C22H28NO4: m/z (LSIMS) 370.2018
(MH+).)
(3S)-Pentafluorophenyl-N-benzyloxycarbonyl-
3-aminomethylbutanoate 15
To the diazoketone 1[20] (12.7 mmol), dissolved in dry 1,4-
dioxane (10 mL), were added pentafluorophenol (50.8 mmol)
and silver trifluoroacetate (1.27 mmol). The solution was then
sonicated for 45 min. The mixture was concentrated under vac-
uum, the residue was diluted with ether (35 mL), and the ethereal
layer was washed successively with 10% citric acid solution
(35 mL), saturated sodium bicarbonate solution (35 mL), and
brine (35 mL). The ethereal layer was dried (MgSO4) and con-
centrated under reduced pressure. The resulting oil was purified
using column chromatography, with 15% ethyl acetate/hexane as
eluent, to afford the ester 15 as a clear colourless oil (75% yield).
[α]2D0 +0.57 (c 1.8, CH2Cl2). δH (300 MHz, CDCl3) (rotamers)
7.40–7.33 (5H, m), 5.15 (2H, s), 4.80–4.73 (1H, m), 2.95–2.83
(5H, 2m), 1.31 (3H, d, J 6.7). δC (75 MHz, CDCl3) (rotamers)
(323 K) 166.8, 156.4, 142.9, 141.4, 139.7, 138.0, 133.5, 131.6,
136.4, 128.5, 128.1, 127.9, 67.7, 49.4, 38.2, 29.2, 17.8. νmax
(NaCl)/cm−1 3153, 3036, 2975 (CH), 1789, 1679 (CO), 1520,
1337, 1229, 1160, 1108, 998, 698. (Found: C 54.7, H 3.8, N 3.4.
Calc. for C19H16F5NO4: C 54.7, H 3.9, N 3.4%.)
(3S)-(4-Methoxybenzyl)-N-benzyloxycarbonyl-
3-aminomethylbutanoate 13
To the diazoketone 1[20] (1.2 mmol) dissolved in dry 1,4-
dioxane (6 mL) were added dry p-methoxybenzyl alcohol
(4.8 mmol) and silver trifluoroacetate (0.12 mmol). The solution
was then sonicated for 30 min. The mixture was concentrated
under vacuum, the residue diluted with ether (25 mL), and the
etheral layer was washed successively with 10% citric acid solu-
tion (25 mL), saturated sodium bicarbonate solution (25 mL),
and brine (25 mL). The ethereal layer was dried (MgSO4)
and concentrated under reduced pressure. The resulting oil
was purified using column chromatography, with 20% ethyl
acetate/hexane as eluent, to afford the ester 13 as a clear colour-
less oil (42% yield). [α]2D0 −1.69 (c 2.3, CH2Cl2). δH (300 MHz,
CDCl3) (rotamers) 7.30–7.23 (7H, m), 6.82 (2H, d, J 8.4), 5.11–
4.95 (4H, 2m), 4.61–4.65 (1H, m), 3.71 (3H, s), 2.74–2.71 (3H,
m), 2.59–2.40 (2H, m), 1.13 (3H, d, J 6.0). δC (75 MHz, CDCl3)
(rotamers) (323 K) 170.5, 159.3, 155.5, 136.5, 129.8, 128.1,
127.6, 127.5, 127.4, 133.6, 67.0, 66.6, 66.1, 65.8, 54.8, 48.8,
48.4, 39.0, 38.7, 29.0, 27.1, 18.0, 17.4. νmax (NaCl)/cm−1 3088–
2837 (CH), 1731, 1699 (CO), 1516, 1330, 1300, 1249, 1138,
698. (Found: m/z 372.1805. Calc. for C21H26NO5: m/z (LSIMS)
372.1811 (MH+).)
(3S)-Allyl-N-benzyloxycarbonyl-
3-aminomethylbutanoate 16
To the diazoketone 1[20] (0.9 mmol) dissolved in dry 1,4-
dioxane (5 mL) were added allyl alcohol (3.2 mmol) and silver
trifluoroacetate (0.09 mmol). The solution was then sonicated
for 30 min. The mixture was concentrated under vacuum, the
residue diluted with ether (25 mL), and the ethereal layer was
washed successively with 10% citric acid solution (25 mL), sat-
urated sodium bicarbonate solution (25 mL), and brine (25 mL).
The ethereal layer was dried (MgSO4) and concentrated under
reduced pressure. The resulting oil was purified using column
chromatography, with 10% ethyl acetate/hexane as eluent, to
isolate the ester 16 as a clear colourless oil (76% yield). [α]D20
−0.83 (c 1.8, CH2Cl2). δH (300 MHz, CDCl3) (rotamers) 7.24,
(5H, m), 5.80–5.74 (1H, m), 5.22–5.05 (4H, 2m), 4.63–4.53 (1H,
m), 4.44 (2H, s), 2.84–2.73 (3H, m), 2.53–2.38 (2H, m), 1.12
(3H, d, J 7.3). δC (75 MHz, CDCl3) (rotamers) (323 K) 170.1,
155.5, 136.8, 131.9, 128.1, 127.9, 127.6, 127.5, 127.4, 117.8,
67.0, 66.6, 64.8, 48.9, 38.9, 28.6, 17.7. νmax (NaCl)/cm−1 3030,
2950 (CH), 1737, 1701 (CO), 1497, 1329, 1300, 1177, 1143,
769, 698. (Found: C 65.8, H 7.2, N 5.0. Calc. for C16H21NO4:
C 66.0, H 7.3, N 4.8%.)
(3S)-Phenyl-N-benzyloxycarbonyl-
3-aminomethylbutanoate 14
To the diazoketone 1[20] (1.2 mmol) dissolved in dry 1,4-
dioxane (6 mL) were added phenol (4.8 mmol) and silver tri-
fluoroacetate (0.12 mmol). The solution was then sonicated
for 30 min. The mixture was concentrated under vacuum, the
residue diluted with ether (25 mL), and the ethereal layer was
washed successively with 10% citric acid solution (25 mL), sat-
urated sodium bicarbonate solution (25 mL), and brine (25 mL).
The ethereal layer was dried (MgSO4) and concentrated under
reduced pressure. The resulting oil was purified using column
(3S)-Allyl-N-benzyloxycarbonyl-3-aminomethyl-
4-methylpentanoate 17
To the diazoketone 4[20] (0.9 mmol) dissolved in dry 1,4-
dioxane (5 mL) were added allyl alcohol (3.2 mmol) and silver
trifluoroacetate (0.09 mmol). The solution was then sonicated
for 30 min. The mixture was concentrated under vacuum, the
residue was diluted with ether (20 mL), and the ethereal layer was

136
A. B. Hughes and B. E. Sleebs
washed successively with 10% citric acid solution (20 mL), sat-
urated sodium bicarbonate solution (20 mL), and brine (20 mL).
The ethereal layer was dried (MgSO4) and concentrated under
reduced pressure. The resulting oil was purified using column
chromatography, with 10% ethyl acetate/hexane as eluent, to
yield the ester 17 as a clear colourless oil (75% yield). [α]D20
−20.0(c1.0, CH2Cl2). δH (300 MHz, CDCl3)7.32(5H, s), 5.88–
5.84 (1H, m), 5.20–5.05 (4H, 2m), 4.46 (2H, s), 4.08–4.02 (1H,
m), 2.80 (3H, s), 2.60 (2H, s), 1.78–1.70 (1H, m), 0.92–0.86
(6H, m). δC (75 MHz, CDCl3) (rotamers) 171.2, 156.5, 137.0,
132.2, 128.3, 127.7, 118.2, 67.1, 66.8, 65.2, 60.6, 59.9, 36.4,
35.9, 30.9, 30.6, 19.9, 19.5. νmax (NaCl)/cm−1 3030, 2965 (CH),
1737, 1699 (CO), 1470, 1404, 1335, 1305, 1176, 1142, 989, 768,
698. (Found: C 67.6, H 7.8, N 4.4. Calc. for C18H25NO4: C 67.7,
H 7.9, N 4.4%.)
1336, 1263, 1151, 767, 698. (Found: C 63.5, H 8.0, N 7.4. Calc.
for C20H30N2O5: C 63.5, H 8.0, N 7.4%.)
Ethyl-2-[(3S)-N-benzyloxycarbonyl-3-
aminomethylbutanamido]-2-methylaminoethanoate 23
To the diazoketone 1[20] (0.9 mmol) and sarcosine ethyl
ester hydrochloride 22 (3.6 mmol) dissolved in dry 1,4-dioxane
(5 mL) was added triethylamine (3.6 mmol) and silver triflu-
oroacetate (0.09 mmol). The solution was then sonicated for
45 min.The mixture was concentrated under vacuum, the residue
was diluted with ether (20 mL), and the ethereal layer was
washed successively with 10% citric acid solution (20 mL), sat-
urated sodium bicarbonate solution (20 mL), and brine (20 mL).
The ethereal layer was dried (MgSO4) and concentrated under
reduced pressure. The resulting oil was purified using column
chromatography, with 40% ethyl acetate/hexane as eluent, to
afford the dipeptide 23 as a clear colourless oil (38% yield). [α]D20
+6.50 (c 1.1, CH2Cl2). δH (300 MHz, CDCl3) 7.28 (5H, s), 5.08
(2H, s), 4.38–4.46 (1H, m), 4.12 (2H, q, J 7.1), 4.03–3.92 (2H,
m), 3.06–2.83 (6H, 2m), 2.59–2.20 (2H, m), 1.23–1.19 (6H, 2m).
δC (75 MHz, CDCl3) (323 K) 171.0, 169.0, 155.7, 136.9, 128.3,
127.7, 66.8, 60.8, 50.0, 49.4, 37.9, 36.4, 30.4, 17.7, 14.0. νmax
(NaCl)/cm−1 3320, 3032, 2938 (CH), 1746, 1697, 1651 (CO),
1456, 1402, 1330, 1202, 1150, 1024, 699. (Found: C 61.8, H 7.4,
N 7.9. Calc. for C18H26N2O5: C 61.7, H 7.5, N 8.0%.)
β-Peptides
(2S)-Methyl-2-[(3S)-N-benzyloxycarbonyl-3-
aminomethylbutanamido]-3-methylbutanoate 19
To the diazoketone 1[20] (0.9 mmol) and l-valine methyl ester
hydrochloride (3.6 mmol) dissolved in dry 1,4-dioxane (5 mL)
were added triethylamine (3.6 mmol) and silver trifluoroacetate
(0.09 mmol). The solution was then sonicated for 45 min. The
mixture was concentrated under vacuum, the residue was diluted
with ether (20 mL), and the ethereal layer was washed succes-
sively with 10% citric acid solution (20 mL), saturated sodium
bicarbonate solution (20 mL), and brine (20 mL). The ethe-
real layer was dried (MgSO4) and concentrated under reduced
pressure. The resulting oil was purified using column chro-
matography, with 40% ethyl acetate/hexane as eluent, to give the
dipeptide 19 as a clear colourless oil (66% yield). [α]2D0 +9.83
(c 1.7, CH2Cl2). δH (300 MHz, CDCl3) 7.20 (5H, s), 6.82–6.70
(1H, m), 4.99 (2H, s), 4.43–3.37 (2H, 2m), 3.55 (3H, s), 2.70
(3H, s), 2.49–2.31 (2H, m), 2.03–1.96 (1H, m), 1.06 (3H, s),
0.79–0.74 (6H, m). δC (75 MHz, CDCl3) (323 K) 171.9, 170.0,
155.6, 136.6, 128.0, 127.4, 127.3, 66.6, 57.0, 51.4, 49.7, 40.8,
30.6, 29.2, 18.4, 17.5. νmax (NaCl)/cm−1 3323 (NH), 3030,
2965 (CH), 1744, 1695, 1651 (CO), 1538, 1454, 1334, 1207,
1151, 1017, 769, 698. (Found: C 62.7, H 7.6, N 7.6. Calc. for
C19H28N2O5: C 62.6, H 7.7, N 7.7%.)
(2S)-tert-Butyl-2-[(3S)-N-benzyloxycarbonyl-
3-aminomethy-butanamido]-2-methylamino-
3-butanoate 25
Using the ketene intermediate. To the diazoketone 1[20]
(0.9 mmol) and N-methyl-α-valine tert-butyl ester tosylate
salt 24 (3.6 mmol) dissolved in dry 1,4-dioxane (5 mL) were
added triethylamine (3.8 mmol) and silver trifluoroacetate
(0.09 mmol). The solution was then sonicated for 45 min. The
mixture was concentrated under vacuum, the residue was diluted
with ether (20 mL), and the ethereal layer was washed succes-
sively with 10% citric acid solution (20 mL), saturated sodium
bicarbonate solution (20 mL), and brine (20 mL). The ethe-
real layer was dried (MgSO4) and concentrated under reduced
pressure. The resulting oil was purified using column chro-
matography, with 20% ethyl acetate/hexane as eluent, to fur-
nish the dipeptide 25 as a clear colourless oil (27% yield).
[α]2D0 −76.86 (c 1.1, CH2Cl2). δH (300 MHz, CDCl3) 7.27
(5H, s), 5.07 (2H, s), 4.77–4.44 (2H, 2m), 2.96–2.81 (6H,
2m), 2.60–2.48 (2H, m), 2.11–2.00 (1H, m), 1.41–1.39 (9H,
m), 1.21 (3H, s), 0.96 (3H, d, J 6.5), 0.78 (3H, d, J 6.7).
δC (75 MHz, CDCl3) (rotamers) (323 K) 170.9, 170.1, 169.0,
155.7, 136.9, 128.3, 127.7, 81.8, 81.1, 66.8, 62.3, 50.0, 38.4,
37.8, 31.4, 30.5, 27.9, 27.3, 19.7, 19.4, 18.9, 18.5, 17.7.
νmax (NaCl)/cm−1 2968, 2935 (CH), 1729, 1699, 1652 (CO),
1473, 1456, 1419, 1401, 1153, 698. (Found: C 65.6, H 8.6,
N 6.7. Calc. for C23H36N2O5: C 65.7, H 8.6, N 6.7%.)
Using PyBrOP as coupling reagent. To a stirred solution
of the N-methylated-β-amino acid 28[20] (0.45 mmol) and the
α-N-methylated residue 24 (0.54 mmol) in dichloromethane
(1 mL) were added PyBrOP (0.54 mmol) and diisopropylethyl-
amine (1.8 mmol) at 0◦C. The mixture was allowed to warm to
room temperature and it was stirred for 20 h. The solution was
diluted with dichloromethane (10 mL).The solution was washed
successively with 10% citric acid solution (10 mL), saturated
sodium bicarbonate solution (10 mL), and water (10 mL). The
organic layer was dried (MgSO4) and concentrated under vac-
uum. The residue was applied to a silica column, with 20% ethyl
(2S)-Methyl-2-[(3S)-N-benzyloxycarbonyl-3-
aminomethylpentanamido]-3-methylbutanoate 21
To the diazoketone 20[20] (0.9 mmol) and valine methyl ester
hydrochloride (3.6 mmol) dissolved in dry 1,4-dioxane (5 mL)
were added triethylamine (3.6 mmol) and silver trifluoroacetate
(0.09 mmol). The solution was then sonicated for 45 min. The
mixture was concentrated under vacuum, the residue was diluted
with ether (20 mL), and the ethereal layer was washed succes-
sively with 10% citric acid solution (20 mL), saturated sodium
bicarbonate solution (20 mL), and brine (20 mL). The ethe-
real layer was dried (MgSO4) and concentrated under reduced
pressure. The resulting oil was purified using column chro-
matography, with 40% ethyl acetate/hexane as eluent, to give
the dipeptide 21 as a clear colourless oil (55% yield). [α]D20
+0.68(c1.5, CH2Cl2). δH (300 MHz, CDCl3)7.18(5H, s), 6.81–
6.58 (1H, m), 5.11–4.89 (2H, m), 4.36–4.19 (2H, m), 3.53 (3H,
s), 2.66 (3H, s), 2.51–2.31 (2H, m), 1.99–1.92 (1H, m), 1.48–
1.36 (2H, m), 0.77–0.66 (9H, 2m). δC (75 MHz, CDCl3) (323 K)
171.8, 170.0, 156.1, 136.5, 128.0, 127.4, 127.2, 66.6, 57.0, 55.8,
51.4, 39.5, 30.5, 29.7, 25.0, 18.4, 17.5, 10.2. νmax (NaCl)/cm−1
3320 (NH), 2964, 2935 (CH), 1743, 1698 (CO), 1536, 1437,

Exploitation of the Arndt–Eistert Homologation
137
acetate/hexane as eluent, to furnish the dipeptide 25 (41% yield)
(two-step process from the diazoketone gave an overall yield of
30%), with data identical to that described above.
residue was purified by column chromatography, with 40% ethyl
acetate/hexane as eluent, to isolate the tripeptide 27 as a clear
colourless oil (52%) (two-step process from the diazoketone
gave an overall yield of 44%), with data identical to those
described above.
(2S)-Methyl-2-{(3S)-3-[(3S)-N-benzyloxycarbonyl-
3-aminomethylpentanamido]3-aminomethyl-4-methyl}-
3-methylbutanoate 27
Acknowledgement
The authors thank La Trobe University for scholarship funding of BES.
Using the ketene intermediate. The N-methyl carbamate
dipeptide 19 (1.2 mmol) was dissolved in methanol (5 mL), and
10% palladium on charcoal (0.01 mmol) was added. The solu-
tion was stirred for 20 h at room temperature, in an atmosphere
of hydrogen gas.The mixture was filtered through a bed of Celite
and concentrated under vacuum. To the residue 26 and the dia-
zoketone 4[20] (0.04 mmol) dissolved in dry 1,4-dioxane (5 mL)
were added triethylamine (3.8 mmol) and silver trifluoroacetate
(0.09 mmol). The solution was then sonicated. The mixture was
concentrated under vacuum, the residue was diluted with ether
(10 mL), and the ethereal layer was washed successively with
10% citric acid solution (10 mL), saturated sodium bicarbonate
solution (10 mL), and brine (10 mL).The ethereal layer was dried
(MgSO4) and concentrated under reduced pressure. The result-
ing oil was purified using column chromatography, with 40%
ethyl acetate/hexane as eluent, to furnish the tripeptide 27 as a
clear colourless oil (34% yield). [α]2D0 +39.5 (c 0.7, CH2Cl2).
δH (300 MHz, CDCl3) 7.29–7.23 (5H, m), 5.10–4.99 (2H, m),
4.41–4.15 (3H, 3m), 3.65 (3H, s), 2.86–2.64 (6H, 2m), 2.55–
2.08 (4H, 2m), 1.90–1.70 (2H, 2m), 1.20–0.78 (15H, 2m). δC
(75 MHz, CDCl3) (rotamers) (323 K) 172.2, 171.5, 171.0, 170.8,
170.0, 156.9, 156.7, 137.1, 128.3, 127.7, 127.3, 66.9, 66.8, 60.3,
59.9, 57.5, 51.7, 51.0, 48.6, 41.9, 40.8, 36.2, 35.3, 31.4, 31.1,
30.6, 30.5, 30.3, 26.3, 19.9, 19.5, 19.4, 19.2, 19.0, 18.8, 18.4,
17.8, 17.5. νmax (NaCl)/cm−1 3300 (NH), 3030, 2964 (CH),
1742, 1696, 1673, 1655, 1626 (CO), 1541, 1457, 1336, 1306,
1267, 1154, 844, 698. (Found: C 63.6, H 8.6, N 8.5. Calc. for
C26H41N3O6: C 63.5, H 8.4, N 8.6%.)
Using PyBrOP as coupling reagent. The dipeptide 19
(0.43 mmol) was dissolved in methanol (5 mL), and 10% pal-
ladium on charcoal (0.04 mmol) was added. The solution was
stirred for 20 h at room temperature, in an atmosphere of hydro-
gen. The mixture was filtered through a bed of Celite and
concentrated under vacuum. To the ensuing residue 26 dis-
solved in dichloromethane (1 mL) were added the β-amino
acid 29[20] (0.43 mmol), PyBroP (0.47 mmol), and diisopropyl-
amine (1.3 mmol) at 0◦C. The solution was allowed to warm
to room temperature where it was stirred for 20 h. The mixture
was diluted with dichloromethane (5 mL) and washed succes-
sively with 10% citric acid solution (10 mL), saturated sodium
bicarbonate solution (10 mL), and water (10 mL). The organic
layer was dried (MgSO4) and concentrated under vacuum. The
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