A biomimetic domino reaction for the concise synthesis of capreomycidine and epicapreomycidine

Article abstract of DOI:10.1007/s00726-012-1309-8

The non-proteinogenic amino acids capreomycidine and epicapreomycidine are constituents of antibiotically active natural products, but the synthesis of these unusual cyclic guanidine derivatives is challenging. The biosynthesis of capreomycidine has therefore beenemployed as a guideline to develop a concise biomimetic synthesis of both epimeric amino acids. The resulting domino-guanidinylation-Aza-Michael-Addition reaction provides the most convenient access to these amino acids in racemic form. Attempts to dissect the domino reaction into two separate transformations for a stereocontrolled version of this synthetic approach have also been made. The synthesized didehydro-Arginine derivatives with urethane-protected guanidine moieties did not undergo the aza-Michael-Addition anymore. These results may have wider implications for the 1,4-Addition of guanidines to a,b-unsaturated carbonyl compounds, particularly to didehydro amino acids. Springer-Verlag 2012.

Full text of DOI:10.1007/s00726-012-1309-8

Amino Acids (2012) 43:2313–2328
DOI 10.1007/s00726-012-1309-8
ORIGINAL ARTICLE
A biomimetic domino reaction for the concise synthesis
of capreomycidine and epicapreomycidine
•
•
Martin Bu¨schleb Markus Granitzka
•
Dietmar Stalke Christian Ducho
Received: 31 March 2012 / Accepted: 19 April 2012 / Published online: 23 May 2012
Ó Springer-Verlag 2012
Abstract The non-proteinogenic amino acids capreomyci-
dine and epicapreomycidine are constituents of antibiotically
active natural products, but the synthesis of these unusual
cyclic guanidine derivatives is challenging. The biosynthesis
ofcapreomycidinehastherefore beenemployedasa guideline
to develop a concise biomimetic synthesis of both epimeric
amino acids. The resulting domino-guanidinylation-aza-
Michael-addition reaction provides the most convenient
access to these amino acids in racemic form. Attempts to
dissect the domino reaction into two separate transformations
for a stereocontrolled version of this synthetic approach have
also been made. The synthesized didehydro-arginine deriva-
tives with urethane-protected guanidine moieties did not
undergo the aza-Michael-addition anymore. These results
may have wider implications for the 1,4-addition of guani-
dines to a,b-unsaturated carbonyl compounds, particularly to
didehydro amino acids.
Keywords Natural products Á Antibiotics Á Amino acids Á
Biomimetic synthesis Á Domino reactions Á Guanidines
Introduction
Bacterial infections continue to be a major threat to human
health, particularly with respect to emerging strains with
resistances to established antibiotics (Taubes 2008; Cooper
and Shlaes 2011). It is therefore highly desirable to develop
novel antibacterial agents which should ideally display new
or yet unexploited modes of action (Walsh 2003). Natural
products have been used as lead structures for many anti-
bacterial drugs (Mahady et al. 2008). However, their often
complex structures represent important challenges to
organic synthesis as a prerequisite for detailed structure–
activity relationship (SAR) studies. Among the many
bacterial secondary metabolites with antibiotic activity, a
significant number of compounds are of peptidic nature and
contain non-proteinogenic amino acid structures.
Electronic supplementary material The online version of this
article (doi:10.1007/s00726-012-1309-8) contains supplementary
material, which is available to authorized users.
The cyclic arginine-derived non-proteinogenic amino
acids (2S,3R)-capreomycidine 1 (‘capreomycidine’) and
(2S,3S)-capreomycidine 2 (‘epicapreomycidine’) are con-
stituents of several natural products with remarkable anti-
biotic potencies (Fig. 1). Capreomycidine 1 can be found
in tuberactinomycin peptide antibiotics (Nagata et al. 1968;
Wakamiya et al. 1970; Yoshioka et al. 1971; Ando et al.
1971; Izumi et al. 1972), e.g., the capreomycins (Herr et al.
1960; Bycroft et al. 1971a; Shiba et al. 1976; Nomoto
et al. 1977; Nomoto et al. 1978) and viomycin 3 (Finlay
et al. 1951; Bartz et al. 1951; Dyer et al. 1965; Bycroft et al.
1969; Noda et al. 1972), which have both found clinical use
as anti-tuberculosis drugs. The 3-epimer epicapreomyci-
dine 2 is a component of the naturally occurring protease
inhibitors chymostatin (Umezawa et al. 1970; Tatsuta et al.
M. Bu¨schleb Á C. Ducho
Department of Chemistry, Institute of Organic and Biomolecular
¨
Chemistry, Georg-August-University Gottingen, Tammannstr. 2,
¨
37077 Gottingen, Germany
M. Granitzka Á D. Stalke
Department of Chemistry, Institute of Inorganic Chemistry,
¨
Georg-August-University Gottingen, Tammannstr. 4,
¨
37077 Gottingen, Germany
C. Ducho (&)
Department of Chemistry, University of Paderborn,
Warburger Str. 100, 33098 Paderborn, Germany
e-mail: christian.ducho@uni-paderborn.de
URL: http://chemie.uni-paderborn.de/fachgebiete/oc/ak-ducho
123

2314
M. Bu¨schleb et al.
1973) and elastatinal (Umezawa et al. 1973; Okura et al.
1975) as well as of the Streptomyces-produced muray-
mycins, a subclass of nucleoside lipopeptide antibiotics
(e.g. muraymycin A1 4, McDonald et al. 2002). Further-
more, b-hydroxy-enduracididine, an amino acid unit with
structural similarities to 1 and 2, is a building block of the
mannopeptimycins, a novel subclass of glycopeptide anti-
biotics (He et al. 2002; Singh et al. 2003; Koehn 2008;
synthesis of 13C-labelled 1 starting from Garner’s aldehyde
has also been established (Jackson et al. 2002), but inves-
tigations in our own laboratory revealed that an application
of this approach to the synthesis of 2 is not possible
(unpublished data). The first practical synthesis of stereo-
chemically pure epicapreomycidine has been established in
recent years. The initial version of this synthetic route
involved an ornithine-derived sulfamate which was then
employed in a rhodium-catalyzed C–H insertion reaction to
construct the stereocenter in the C-3 position. However,
this elegant approach suffered from low yields and dia-
stereoselectivities of the C–H activation key step (Tanino
et al. 2008, 2010). A superior version of this C–H activa-
tion strategy starting from D-tyrosine and displaying perfect
stereocontrol has recently been described though (Tanino
et al. 2011).
¨
¨
Schworer and Oberthur 2009).
Due to their relevance in natural product chemistry,
several approaches for the synthesis of amino acids 1 and 2
have been developed. A first non-stereoselective synthesis
of 1 and 2 involved the preparation of an aromatic
2-aminopyrimidine precursor, which was then hydroge-
nated to provide a diastereomeric mixture of 1 and 2, each
in racemic form. The separation of the diastereomers was
carried out by crystallization of the respective picrates
(Bycroft et al. 1971b). A slightly improved version of this
synthesis has also been described (Yamashita et al. 2004).
The first syntheses of stereochemically pure 1 and 2
employed an aldol reaction for the synthesis of a protected
b-hydroxy-ornithine precursor, which then underwent
acylase-mediated enzymatic resolution. Further conversion
via an aziridine intermediate finally provided either 1 or 2,
but the synthesis was lengthy, and the overall yields were
rather low (Shiba et al. 1977; Wakamiya et al. 1978;
Teshima et al. 1980). The first stereoselective synthesis of
capreomycidine 1 involved a Mannich-type reaction with a
chiral glycine derivative as the key step. The subsequent
guanidinylation reaction only proceeded with one of the
two diastereomers obtained from the Mannich transfor-
mation, thus finally giving pure 1 after further conversions
(DeMong and Williams 2001, 2003). However, this also
implies that an according synthesis of 2 using the same
strategy is not feasible. An alternative ex-chiral pool
One of the highly useful strategies for the development
of efficient synthetic routes is to imitate the biosynthetic
pathways found in nature. Such biomimetic syntheses have
found widespread applications (Razzak and De Brabander
2011). Our goal was to investigate the feasibility of a
biomimetic synthesis of both capreomycidines 1 and 2. The
biosynthetic assembly of the cyclic guanidine structure of
capreomycidine 1 as part of viomycin biosynthesis in
Streptomyces vinaceus has been elucidated (Fig. 2, Yin and
Zabriskie 2004; Yin et al. 2004; Ju et al. 2004). Stereose-
lective 3-hydroxylation of L-arginine 5, catalyzed by the
non-heme 2-oxoglutarate (2-OG) dependent Fe(II)-oxy-
genase VioC, provides (3S)-3-hydroxy-L-arginine 6. The
subsequent ring closure reaction is mediated by the
pyridoxalphosphate-(PLP)-dependent enzyme VioD and most
likely proceeds via the a,b-unsaturated didehydro-arginine
intermediate 7, as demonstrated by labelling studies.
Michael-type conjugate 1,4-addition of the guanidine
moiety then stereoselectively provides capreomycidine 1
O
O
H2N
OH
NH
OH
O
OR1
HO
O
HN
HN
H
H
N
O
O
O
β-Lys
N
N
OR2
H
H
N
OH
N
H
N
N
NH
N
H
H
O
O
HO
N
NH
HN
O
H
O
1
H
N
O
O
NH
OH OH
HN
O
O
NH2
4
H2N
HN
O
HN
N
OH
HN
HN
H
O
NH
NH2
H2N
N
OH
H
O
OH
R1 =
R2 =
N
HN
N
H
11
3
OH
O
2
Fig. 1 Structures of non-proteinogenic amino acids capreomycidine 1 and epicapreomycidine 2 as well as structures of natural products
viomycin 3 and muraymycin A1 4 containing 1 and 2, respectively
123

A biomimetic domino reaction for the concise synthesis
2315
with overall formal inversion of the stereocenter at C-3.
The biosynthesis of 2 has not been elucidated yet, but the
recent analysis of the biosynthetic gene cluster of muray-
mycin nucleoside antibiotics suggests that it probably
occurs in a similar manner, just with a different stereo-
chemical course (Cheng et al. 2011; also see Lemke et al.
2010).
(Seiple et al. 2011). It was thus envisaged to investigate
different protecting group patterns both at the guanidine
group (to influence its nucleophilicity) and the didehydro
amino acid moiety (to tune its electrophilicity).
Materials and methods
Hence, the goal of this study was to synthesize dide-
hydro-arginine derivatives 8a–h as potential precursors for
biomimetic Aza-Michael-additions in order to obtain
amino acids 1 and 2 or protected derivatives thereof
(Table 1). There is very limited precendent for 1,4-addi-
tions of protected guanidines, with a striking example
provided by Baran, Seiple and coworkers as part of their
synthetic studies on palau’amine and related compounds
Chemicals were purchased from Sigma-Aldrich, Alfa Ae-
sar, ABCR and VWR. Amino acid phosphonates 9a–c and
3-azido-propionaldehyde 10 were synthesized as previ-
ously reported (Ducho et al. 2009; Schmidt et al. 1984;
Davies et al. 1967). Reactions involving oxygen and/or
moisture-sensitive reagents were carried out under an
atmosphere of argon using anhydrous solvents. Anhydrous
solvents were obtained in the following manner: THF was
dried over sodium/benzophenone and distilled, MeCN and
CH2Cl2 were dried over P2O5 and distilled and DMF was
O
O
H2N
H2N
HO
˚
dried over molecular sieves 4 A. All other solvents were of
OH
OH
NH
1
VioC
3
3
technical quality and distilled prior to their use, and dis-
tilled water was used throughout. Column chromatography
was carried out on silica gel 60 (0.040–0.063 mm,
230–400 mesh ASTM, VWR) except where indicated
under flash conditions. TLC was performed on aluminium
plates precoated with silica gel 60 F254 (VWR). Visual-
isation of the spots was carried out using UV light
(254 nm) where appropriate and/or KMnO4 staining under
heating (staining solution: 1 g KMnO4, 6 g K2CO3 and
1.5 mL 5 % NaOH(aq) (w/v), all dissolved in 100 mL
H2O). 300 MHz-1H as well as 75 MHz-, 76 MHz- and
126 MHz-13C NMR spectra were recorded on Varian
UNITY 300, MERCURY 300, INOVA 500 and INOVA
NH
Fe(II), 2-OG, O2
N
NH2
N
NH2
H
H
6
5
O
O
N
H2N
HN
PLP
OH
OH
VioD
VioD
3
H2N
HN
PLP
N
H
HN
N
H
7
1
Fig. 2 Biosynthesis of capreomycidine 1 in Streoptomyces vinaceus
Table 1 Target structures 8a–h envisaged as precursors for biomimetic cyclization reactions in this study
R2
O
N
R1
R4
O R3
NH
R5
N
N
R6
8a-h
R3
Compd.
R1
R2
R4
R5
R6
8a
8b
8c
8d
8e
8f
Boc
Boc
Ac
H
t-Bu
t-Bu
t-Bu
Me
H
H
H
Boc
H
H
H
H
H
H
H
Ac
H
H
H
H
Boc
Boc
Boc
Boc
H
t-Bu
t-Bu
t-Bu
t-Bu
Cbz
Cbz
Cbz
Cbz
Cbz
Cbz
Cbz
Cbz
H
H
Bn
H
8g
8h
Boc
Boc
Bn
123

2316
M. Bu¨schleb et al.
600 spectrometers. All 13C NMR spectra are H-decou-
pled. All spectra were recorded at room temperature
except of samples in DMSO-d6 and D2O (standard 35 °C)
and where indicated otherwise and were referenced
internally to solvent reference frequencies. Chemical
shifts (d) are quoted in ppm. Coupling constants (J) are
reported in Hz to the nearest 0.1 Hz. Assignment of sig-
nals was carried out using 1H,1H-COSY and HSQC
spectra obtained on the spectrometers mentioned above.
Low-resolution ESI mass spectrometry was performed on
a Varian MAT 311 A spectrometer operating in positive
ionization mode. High-resolution (HR) ESI mass spec-
trometry was carried out on a Bruker microTOF spec-
trometer or a Bruker 7 T FTICR APEX IV spectrometer.
Melting points (mp) were measured on a Bu¨chi instrument
and are not corrected. Infrared spectroscopy (IR) was
performed on a JASCO FT/IR-4100 spectrometer equip-
ped with an ATR unit. Wavenumbers (m) are quoted in
cm-1. UV spectroscopy was carried out on a JASCO
V-630 spectrometer. Wavelengths of maximum absorp-
tion (kmax) are reported in nm with the corresponding
logarithmic molar extinction coefficient (log e) given in
parenthesis.
ornithines, they were prepared freshly and directly used
for the subsequent guanidinylation reaction. Thus, the
crude didehydro-ornithines were dissolved in DMF, and
NEt3 and guanidinylation reagent 13 were added at rt.
The reaction mixture was stirred at rt or elevated tem-
perature (to drive the reaction to completion) for the
indicated time. The solvent was evaporated under
reduced pressure and the resultant crude product was
purified by column chromatography.
1
General procedure C (guanidinylation reactions
with reagent 31)
To a solution of the crude ornithine derivative in DMF,
guanidinylation reagent 31, NEt3 and AgOTf were added.
The reaction mixture was stirred at rt for 3 h, then fil-
tered through Celite and the Celite were washed with
EtOAc (3 9). The filtrate was washed with brine (3 9),
dried over Na2SO4 and evaporated under reduced pres-
sure. The resultant crude product was purified by column
chromatography.
Capreomycidine dihydrochloride ((rac)-1 Á 2HCl)
and epicapreomycidine dihydrochloride
((rac)-2 Á 2HCl)
General procedure A (synthesis of protected didehydro
amino acids by Wittig-Horner reactions)
From (rac)-14 and (rac)-15
To a solution of KOt-Bu in THF, a solution of the amino
acid phosphonate in THF was added at -78 °C. After
15 min, a solution of 3-azido-propionaldehyde 10 in THF
was added dropwise at -78 °C. The reaction mixture was
stirred overnight and slowly warmed to rt during this per-
iod. The reaction was quenched by addition of MeOH.
After the addition of EtOAc, the organic layer was washed
with water, dried over Na2SO4 and evaporated under
reduced pressure. The resultant crude product was purified
by column chromatography.
A solution of a mixture of protected capreomycidine
(rac)-14 and epicapreomycidine (rac)-15 (580 mg,
1.77 mmol) in aqueous HCl (6 M, 10 mL, 60 mmol) was
stirred at rt for 4 h. The solvent was evaporated under
reduced pressure to give a mixture of (rac)-1 Á 2HCl and
(rac)-2 Á 2HCl as a slightly brownish oil (430 mg, 99 %).
1H NMR (300 MHz, D2O): d = 1.90–2.23 (m, 2 9 2 H,
H-4), 3.36–3.55 (m, 2 9 2 H, H-5), 4.18–4.30 (m,
2 9 2 H, H-2, H-3). 13C NMR (76 MHz, D2O): d = 20.7
(1 9 C-4), 21.6 (1 9 C-4), 36.0 (1 9 C-5), 36.4 (1 9 C-
5), 48.4 (1 9 C-3), 48.7 (1 9 C-3), 55.3 (1 9 C-2), 55.4
(1 9 C-2), 154.5 (2 9 guanidine-C), 169.2 (1 9 C=O),
169.5 (1 9 C=O). MS (ESI?): m/z = 173.1 [M ? H]?.
HRMS (ESI?): calcd. for C6H12N4O2 ([M ? H]?)
173.1033, found 173.1034.
General procedure B (sequence of Staudinger reduction
and guanidinylation reactions with reagent 13)
To a solution of the protected d-azido-didehydro a-amino
acid in THF and water, PPh3 was added at rt. The reaction
mixture was stirred overnight at rt. After the addition of
EtOAc, the organic layer was washed with aqueous
AcOH (10 %, 3 9). The combined aqueous layers were
adjusted to pH 12 by the addition of diluted NaOH
solution and extracted with CH2Cl2 (3 9) and CH2Cl2/i-
PrOH (3:1, 1 9). The combined organics were dried over
Na2SO4 and evaporated under reduced pressure. The
resultant crude products were sufficiently pure ([95 % as
From (rac)-17 and (rac)-18
A solution of protected capreomycidine (rac)-17 and epi-
capreomycidine (rac)-18 (75 mg, 0.18 mmol) in aqueous
HCl (6 M, 1.8 mL, 11 mmol) was stirred at rt for 3 h. The
solvent was evaporated under reduced pressure to give a
mixture of (rac)-1 Á 2HCl and (rac)-2 Á 2HCl as a slightly
brownish oil (37 mg, 87 %). Analytical data were identical
to those given above.
1
judged by H NMR) without further purification. With
respect to the limited stability of the didehydro-
123

A biomimetic domino reaction for the concise synthesis
2317
N2-Acetyl-2,3-didehydro-arginine tert-butyl ester (8c)
8.26 (sbr, 1 H, 2-NH), 8.47 (t, J = 5.9 Hz, 1 H, guanidine-
NH), 11.59 (s, 1 H, guanidine-NH). 13C NMR (126 MHz,
DMSO-d6): d = 26.8 (C-4), 27.5 (t-Bu-CH3), 27.9 (t-Bu-
CH3), 39.0 (C-5), 66.3 (Cbz-CH2), 67.4 (Cbz-CH2), 78.5
(t-Bu-C), 79.9 (t-Bu-C), 127.5 (Cbz-aryl-C-2, Cbz-aryl-C-6),
127.7 (Cbz-aryl-C-2, Cbz-aryl-C-6), 128.1 (Cbz-aryl-C-4),
128.2 (Cbz-aryl-C-3, Cbz-aryl-C-5), 128.3 (Cbz-aryl-C-3,
Cbz-aryl-C-5), 130.3 (C-3), 134.9 (C-2), 136.6 (Cbz-aryl-
C-1), 152.3 (guanidine-C), 153.2 (Boc-C=O), 154.9 (Cbz-
C=O), 163.4 (C-1). MS (ESI?): m/z = 619.3 [M ? Na]?.
HRMS (ESI?): calcd. for C31H40N4O8 ([M ? Na]?)
619.2738, found 619.2742. IR (ATR): m = 3324, 2977,
1718, 1638, 1620, 1257, 1204, 1137, 1049, 695. UV
(MeCN) : kmax (log e) = 206 (4.54), 237 (4.41). Mp:
57 °C. TLC: Rf = 0.73 (petroleum ether–EtOAc 1:1).
General procedureBwith(Z)-tert-butylN2-acetyl-2-amino-5-
azido-pent-2-enoate (Z)-11b (334 mg, 1.32 mmol), PPh3
(689 mg, 2.63 mmol), water (1.3 mL) and THF (13 mL) for
the Staudinger reduction as well as guanidinylation reagent 13
(386 mg, 2.63 mmol), NEt3 (266 mg, 364 lL, 2.63 mmol)
and DMF (13 mL) for the guanidinylation reaction. The
reaction mixture was stirred at rt for 8 days and at 100 °C for
22 h. Evaporation of the solvent under reduced pressure gave
1
the crude product of 8c as yellow oil (417 mg). H NMR
(300 MHz, DMSO-d6): d = 1.39 (s, 9 H, t-Bu-CH3), 1.92 (s,
3 H, Ac-CH3), 2.21–2.34 (m, 2 H, H-4), 2.98–3.08 (m, 2 H,
H-5), 6.18 (t, J = 7.5 Hz, 1 H, H-3), 7.09 (sbr, 1 H, 2-NH),
9.39 (s, 1 H, guanidine-NH). MS (ESI?): m/z = 271.2
[M ? H]?. HRMS (ESI?): calcd. for C12H22N4O3
([M ? H]?) 271.1765, found 271.1769.
N2-Boc-N5-benzyl-N8,N9-bis-Cbz-2,3-didehydro-
arginine tert-butyl ester (8f)
N2-Acetyl-2,3-didehydro-arginine methyl ester (8d)
To a solution of crude N2-Boc-2,3-didehydro-ornithine
tert-butyl ester 12 (694 mg, obtained from (Z)-11a as
described for the synthesis of (rac)-14,15) in CH2Cl2
General procedure B with (Z)-methyl N2-acetyl-2-amino-5-
azido-pent-2-enoate (Z)-11c (204 mg, 0.962 mmol), PPh3
(504 mg, 1.92 mmol), water (0.4 mL) and THF (10 mL) for
the Staudinger reduction as well as guanidinylation reagent
13 (254 mg, 1.73 mmol), NEt3 (175 mg, 240 lL,
1.73 mmol) and DMF (9 mL) for the guanidinylation reac-
tion. The reaction mixture was stirred at rt for 2 days. Col-
umn chromatography (CH2Cl2–MeOH 10:1 ? EtOAc–
MeOH 1:1 ? MeOH) gave impure 8d as a yellow oil
(71 mg). 1H NMR (300 MHz, DMSO-d6): d = 1.93 (s, 3 H,
Ac-CH3), 2.26 (td, J = 7.4 Hz, 7.1 Hz, 2 H, H-4), 3.20 (dt,
J = 7.1 Hz, 7.0 Hz, 2 H, H-5), 6.18 (t, J = 7.4 Hz, 1 H,
H-3), 7.58 (s, 2 H, 2-NH), 8.14 (sbr, 3 H, guanidine-NH),
9.47 (s, 1 H, guanidine-NH). MS (ESI?): m/z = 229.2
[M ? H]?. HRMS (ESI?): calcd. for C9H16N4O3
([M ? H]?) 229.1295, found 229.1299.
˚
(20 mL), molecular sieves (4 A) were added at rt and the
mixture was stirred at rt for 30 min. Subsequently, benz-
aldehyde (259 mg, 247 lL, 2.45 mmol) was added, and the
reaction mixture was stirred at rt for 17 h. After filtration,
the filtrate was evaporated under reduced pressure. The
resultant crude product was dissolved in MeOH (20 mL)
and cooled to 0 °C. NaBH4 (229 mg, 6.07 mmol) was
added, and the reaction mixture was stirred at 0 °C for 3 h.
The reaction was quenched by addition of saturated NH4Cl
solution (5 mL). After addition of water (10 mL), the
aqueous layer was extracted with EtOAc (3 9). The
combined organics were washed with H2O (25 mL), dried
over Na2SO4 and evaporated under reduced pressure to
give 29 as a yellow oil (750 mg). Some of this crude
product (702 mg) was used without further purification for
the guanidinylation reaction according to general proce-
N2-Boc-N8,N9-bis-Cbz-2,3-didehydro-arginine tert-
butyl ester (8e)
dure
C with guanidinylation reagent 31 (869 mg,
2.43 mmol), NEt3 (566 mg, 775 lL, 5.60 mmol), AgOTf
(669 mg, 2.61 mmol) and DMF (9.5 mL). Purification by
General procedure C with crude N2-Boc-2,3-didehydro-
ornithine tert-butyl ester 12 (509 mg, obtained from
(Z)-11a as described for the synthesis of (rac)-14,15),
guanidinylation reagent 31 (828 mg, 2.31 mmol), NEt3
(539 mg, 739 lL, 5.34 mmol), AgOTf (638 mg,
2.49 mmol) and DMF (9 mL). Purification by column
chromatography (petroleum ether–EtOAc 6:1 ? 3:1) gave
8e as a yellow foam (761 mg, 71 % over 2 steps from
(Z)-11a). 1H NMR (300 MHz, DMSO-d6): d = 1.39 (s, 9 H,
t-Bu-CH3), 1.40 (s, 9 H, t-Bu-CH3), 2.36 (dt, J = 7.1 Hz,
6.8 Hz, 2 H, H-4), 3.42 (td, J = 6.8 Hz, 5.9 Hz, 2 H, H-5),
5.04 (s, 2 H, Cbz-CH2), 5.21 (s, 2 H, Cbz-CH2), 6.12 (t,
J = 7.1 Hz, 1 H, H-3), 7.27–7.44 (m, 10 H, Cbz-aryl-H),
column
chromatography
(petroleum
ether–EtOAc
4:1 ? 1:1) gave 8f as a white foam (820 mg, 55 % over 3
steps from (Z)-11a). 1H NMR (300 MHz, DMSO-d6):
d = 1.39 (s, 9 H, t-Bu-CH3), 1.42 (s, 9 H, t-Bu-CH3), 2.39
(dt, J = 7.4 Hz, 7.0 Hz, 2 H, H-4), 3.42 (t, J = 7.0 Hz,
2 H, H-5), 4.62 (s, 2 H, Bn-CH2), 4.95 (s, 2 H, Cbz-CH2),
5.08 (s, 2 H, Cbz-CH2), 6.04 (t, J = 7.4 Hz, 1 H, H-3),
7.19–7.42 (m, 15 H, aryl-H), 8.28 (sbr, 1 H, 2-NH), 10.07
(s, 1 H, guanidine-NH). 13C NMR (126 MHz, DMSO-d6):
d = 25.4 (C-4), 27.5 (t-Bu-CH3), 28.0 (t-Bu-CH3), 46.5
(C-5), 50.5 (Bn-CH2), 66.2 (Cbz-CH2), 78.6 (t-Bu-C), 80.0
(t-Bu-C), 126.9 (aryl-C-2, aryl-C-6), 127.3 (aryl-C-2, aryl-
123

2318
M. Bu¨schleb et al.
C-6), 127.6 (aryl-C-4), 127.8 (aryl-C-4), 128.0 (aryl-C-3,
aryl-C-5), 128.2 (aryl-C-3, aryl-C-5), 128.4 (C-2), 130.3
(C-3), 135.8 (aryl-C-1), 136.8 (aryl-C-1), 151.0 (Boc-
C=O), 152.3 (guanidine-C), 153.3 (Cbz-C=O), 163.3 (C-1).
MS (ESI?): m/z = 709.3 [M ? Na]?. HRMS (ESI?):
calcd. for C38H46N4O8 ([M ? Na]?) 709.3208, found
709.3206. TLC: Rf = 0.70 (petroleum ether–EtOAc 1:1).
NaBH4 (1.79 g, 47.6 mmol) was added in two portions (the
second after 4 h), and the reaction mixture was stirred at rt
for 22 h. The reaction was quenched by addition of satu-
rated NH4Cl solution (5 mL). After addition of water
(10 mL), the aqueous layer was extracted with EtOAc
(3 9). The combined organics were washed with H2O
(25 mL), dried over Na2SO4 and evaporated under reduced
pressure. The resultant crude product was separated by
column chromatography (petroleum ether–EtOAc 1:1) to
give impure 30 as a yellow oil (748 mg). This material was
used without further purification for the guanidinylation
reaction according to general procedure C with guanid-
inylation reagent 31 (758 mg, 2.12 mmol), NEt3 (493 mg,
676 lL, 4.88 mmol), AgOTf (584 mg, 2.28 mmol) and
DMF (8 mL). Purification by column chromatography
(petroleum ether–EtOAc 6:1 ? 3:1) gave 8 h as a yellow
foam (991 mg, 42 % over 3 steps from (Z)-11d). 1H NMR
(300 MHz, DMSO-d6): d = 1.32 (s, 18 H, t-Bu-CH3), 1.39
(s, 9 H, t-Bu-CH3), 2.27 (dt, J = 7.6 Hz, 7.1 Hz, 2 H,
H-4), 3.42 (t, J = 7.1 Hz, 2 H, H-5), 4.62 (s, 2 H,
Bn-CH2), 4.90 (s, 2 H, Cbz-CH2), 5.03 (s, 2 H, Cbz-CH2),
6.59 (t, J = 7.6 Hz, 1 H, H-3), 7.17–7.33 (m, 15 H, aryl-
H), 10.05 (s, 1 H, NH). 13C NMR (126 MHz, DMSO-d6):
d = 25.9 (C-4), 27.3 (t-Bu-CH3), 27.5 (t-Bu-CH3), 45.8
(C-5), 50.4 (Bn-CH2), 66.2 (Cbz-CH2), 80.7 (t-Bu-C), 81.8
(t-Bu-C), 126.2 (aryl-C-2, aryl-C-6), 126.9 (aryl-C-2, aryl-
C-6), 127.6 (aryl-C-4), 127.8 (aryl-C-4), 128.0 (aryl-C-3,
aryl-C-5), 128.3 (aryl-C-3, aryl-C-5), 130.9 (C-2), 136.0
(aryl-C-1), 136.6 (C-3) 136.8 (aryl-C-1), 149.5 (Boc-C=O),
152.3 (guanidine-C), 153.3 (Cbz-C=O), 161.8 (C-1). MS
(ESI?): m/z = 809.4 [M ? Na]?. HRMS (ESI?): calcd.
for C43H54N4O10 ([M ? Na]?) 809.3732, found 809.3736.
IR (ATR): m = 2977, 1718, 1602, 1454, 1366, 1250, 1150,
1090, 733, 696. UV (MeCN): kmax (log e) = 207 (4.61),
251 (4.26). TLC: Rf = 0.71 (petroleum ether–EtOAc 1:1).
N2,N2-bis-Boc-N8,N9-bis-Cbz-2,3-didehydro-arginine
tert-butyl ester (8 g)
General procedure C with crude N2,N2-bis-Boc-2,3-dide-
hydro-ornithine tert-butyl ester 16 (488 mg, obtained from
(Z)-11d as described for the synthesis of (rac)-17,18),
guanidinylation reagent 31 (588 mg, 1.64 mmol), NEt3
(383 mg, 525 lL, 3.79 mmol), AgOTf (173 mg,
0.675 mmol) and DMF (6.5 mL). Purification by column
chromatography (petroleum ether–EtOAc 10:1 ? 1:2)
gave 8 g as a white foam (642 mg, 50 % over 2 steps from
(Z)-11d). 1H NMR (300 MHz, DMSO-d6): d = 1.31 (s,
18 H, t-Bu-CH3), 1.38 (s, 9 H, t-Bu-CH3), 2.30 (dt,
J = 7.0 Hz, 6.6 Hz, 2 H, H-4), 3.46 (td, J = 6.6 Hz,
6.0 Hz, 2 H, H-5), 5.00 (s, 2 H, Cbz-CH2), 5.18 (s, 2 H,
Cbz-CH2), 6.70 (t, J = 7.0 Hz, 1 H, H-3), 7.24–7.41 (m,
10 H, Cbz-aryl-H), 8.53 (t, J = 6.0 Hz, 1 H, guanidine-
NH), 11.58 (s, 1 H, guanidine-NH). 13C NMR (76 MHz,
DMSO-d6): d = 27.2 (C-4, t-Bu-CH3), 27.5 (t-Bu-CH3),
39.5 (C-5), 66.3 (Cbz-CH2), 67.5 (Cbz-CH2), 80.6 (t-Bu-
C), 81.7 (t-Bu-C), 127.7 (Cbz-aryl-C-2, Cbz-aryl-C-6),
127.8 (Cbz-aryl-C-2, Cbz-aryl-C-6), 128.0 (Cbz-aryl-C-4),
128.2 (Cbz-aryl-C-4), 128.4 (Cbz-aryl-C-3, Cbz-aryl-C-5),
131.0 (C-2), 135.1 (Cbz-aryl-C-1), 136.8 (Cbz-aryl-C-1),
137.2 (C-3), 149.6 (Boc-C=O), 152.3 (guanidine-C), 155.1
(Cbz-C=O), 162.0 (Cbz-C=O), 162.9 (C-1). MS (ESI?):
m/z = 719.4 [M ? Na]?. HRMS (ESI?): calcd. for
C36H48N4O10 ([M ? Na]?) 719.3263, found 719.3274. IR
(ATR): m = 2978, 1719, 1638, 1366, 1249, 1151, 1094,
1047, 734, 696. UV (MeCN): kmax (log e) = 206 (4.57),
240 (4.45), 273 (4.48). TLC: Rf = 0.53 (petroleum ether–
EtOAc 1:1).
tert-Butyl N2-Boc-2-amino-5-azido-pent-2-enoate (11a)
General procedure A with phosphonate 9a (2.00 g,
5.45 mmol), crude 3-azido-propionaldehyde 10 (1.18 g),
KOt-Bu (641 mg, 5.72 mmol) and THF (11 mL (KOt-Bu),
16 mL (9a), 16 mL (10)). Purification by column chro-
matography (petroleum ether–EtOAc 6:1) gave (Z)-11a as
a colorless solid (1.31 g, 77 %) and (E)-11a as a colorless
N2,N2-Bis-Boc-N5-benzyl-N8,N9-bis-Cbz-2,3-
didehydro-arginine tert-butyl ester (8 h)
To a solution of crude N2,N2-bis-Boc-2,3-didehydro-orni-
thine tert-butyl ester 16 (1.10 g, obtained from (Z)-11d as
described for the synthesis of (rac)-17,18) in CH2Cl2
oil (62 mg, 4 %). (Z)-11a: H NMR (300 MHz, CDCl3):
1
d = 1.44 (s, 9 H, t-Bu-CH3), 1.47 (s, 9 H, t-Bu-CH3), 2.45
(dt, J = 7.4 Hz, 6.9 Hz, 2 H, H-4), 3.41 (t, J = 6.9 Hz,
2 H, H-5), 6.19 (sbr, 1 H, NH), 6.38 (t, J = 7.4 Hz, 1 H,
H-3). 13C NMR (76 MHz, CDCl3): d = 28.0 (t-Bu-CH3),
28.2 (t-Bu-CH3), 28.5 (C-4), 50.0 (C-5), 80.5 (t-Bu-C),
82.0 (t-Bu-C), 128.5 (C-3), 128.7 (C-2), 153.1 (Boc-C=O),
163.6 (C-1). MS (ESI?): m/z = 335 [M ? Na]?. HRMS
(ESI?): calcd. for C14H24N4O4 ([M ? Na]?) 335.1690,
˚
(25 mL), molecular sieves (4 A) were added at rt and the
mixture was stirred at rt for 30 min. Subsequently, benz-
aldehyde (504 mg, 478 lL, 4.75 mmol) was added, and the
reaction mixture was stirred at rt for 17 h. After filtration,
the filtrate was evaporated under reduced pressure. The
resultant crude product was dissolved in MeOH (25 mL).
123

A biomimetic domino reaction for the concise synthesis
2319
found 335.1690. IR: m = 3308, 2980, 2097, 1688, 1493,
1367, 1246, 1146, 847, 778. UV (MeCN): kmax
(log e) = 231 (3.85). Mp: 57 °C. TLC: Rf = 0.42 (petro-
leum ether–EtOAc 3:1). (E)-11a: 1H NMR (300 MHz,
CDCl3): d = 1.44 (s, 9 H, t-Bu-CH3), 1.52 (s, 9 H, t-Bu-
CH3), 2.79 (dt, J = 7.5 Hz, 7.3 Hz, 2 H, H-4), 3.35 (t,
J = 7.4 Hz, 2 H, H-5), 6.66 (t, J = 7.5 Hz, 1 H, H-3),
6.74 (sbr, 1 H, NH). 13C NMR (76 MHz, CDCl3): d = 28.0
(C-4), 28.1 (t-Bu-CH3), 28.3 (t-Bu-CH3), 51.0 (C-5), 80.3
(t-Bu-C), 83.3 (t-Bu-C), 121.3 (C-3), 128.0 (C-2), 153.0
(Boc-C=O), 163.1 (C-1). MS (ESI?): m/z = 335
[M ? Na]?. HRMS (ESI?): calcd. for C14H24N4O4
([M ? Na]?) 335.1690, found 335.1690. TLC: Rf = 0.42
(petroleum ether-EtOAc 3:1).
[2 M ? Na]?. HRMS (ESI?): calcd. for C8H12N4O3
([M ? Na]?) 235.0802, found 235.0803. IR (ATR):
m = 3261, 2962, 2086, 1742, 1676, 1372, 1210, 1021, 795,
714. UV (MeCN): kmax (log e) = 231 (3.80). Mp: 51 °C.
TLC: Rf = 0.16 (petroleum ether–EtOAc 1:1).
tert-Butyl N2,N2-bis-Boc-2-amino-5-azido-pent-2-
enoate (11d)
To a solution of (Z)-tert-butyl N2-Boc-2-amino-5-azido-
pent-2-enoate (Z)-11a (1.29 g, 4.14 mmol) in MeCN
12mL, DMAP (50 mg, 0.41 mmol) and Boc2O (1.98 g,
9.10 mmol) were added at rt. The reaction mixture was
stirred at rt for 20 h. The solvent was evaporated under
reduced pressure. After the addition of EtOAc, the solution
was washed with aqueous KHSO4 solution (1 M,
2 9 10 mL), saturated NaHCO3 solution (1 9 10 mL) and
brine (1 9 10 mL), dried over Na2SO4 and evaporated
under reduced pressure. The resultant crude product was
purified by column chromatography (petroleum ether–
EtOAc 10:1) to give (Z)-11d as a colorless oil (1.57 g,
92 %). 1H NMR (300 MHz, DMSO-d6): d = 1.37 (s,
18 H, t-Bu-CH3), 1.41 (s, 9 H, t-Bu-CH3), 2.29 (dt,
J = 7.3 Hz, 6.6 Hz, 2 H, H-4), 3.44 (t, J = 6.6 Hz, 2 H,
H-5), 6.65 (t, J = 7.3 Hz, 1 H, H-3). 13C NMR (76 MHz,
DMSO-d6): d = 27.0 (C-4), 27.3 (t-Bu-CH3), 27.5 (t-Bu-
CH3), 48.6 (C-5), 80.8 (t-Bu-C), 81.9 (t-Bu-C), 131.5
(C-2), 136.3 (C-3), 149.7 (Boc-C=O), 161.9 (C-1). MS
(ESI?): m/z = 847.6 [2 M ? Na]?. HRMS (ESI?): calcd.
for C19H32N4O6 ([M ? Na]?) 435.2214, found 435.2218.
IR (ATR): m = 2979, 2097, 1795, 1717, 1366, 1273, 1250,
1150, 1092, 849. UV (MeCN): kmax (log e) = 216 (4.01).
TLC: Rf = 0.23 (petroleum ether–EtOAc 10:1).
tert-Butyl N2-acetyl-2-amino-5-azido-pent-2-enoate
(11b)
General procedure A with phosphonate 9b (1.64 g,
5.83 mmol), crude 3-azido-propionaldehyde 10 (1.15 g),
KOt-Bu (686 mg, 6.12 mmol) and THF (11 mL (KOt-Bu),
16 mL (9b), 10 mL (10)). Purification by column chro-
matography (petroleum ether–EtOAc 3:2) gave (Z)-11b as
a yellow oil (910 mg, 61 %) while (E)-11b could not be
1
isolated. H NMR (300 MHz, CDCl3): d = 1.48 (s, 9 H,
t-Bu-CH3), 2.10 (s, 3 H, Ac-CH3), 2.40 (dt, J = 7.1 Hz,
J = 6.7 Hz, 2 H, H-4), 3.43 (t, J = 6.7 Hz, 2 H, H-5),
6.49 (t, J = 7.1 Hz, 1 H, H-3), 7.13 (sbr, 1 H, NH).
13C NMR (75 MHz, CDCl3): d = 23.5 (Ac-CH3), 27.9
(t-Bu-CH3), 29.0 (C-4), 49.9 (C-5), 82.3 (t-Bu-C), 127.6
(C-2), 130.8 (C-3), 163.4 (C-1), 168.2 (Ac-C=O). MS:
(ESI?): m/z = 531.3 [2 M ? Na]?. HRMS (ESI?): calcd.
for C11H18N4O3 ([M–H]-) 253.1306, found 253.1312. IR
(ATR): m = 3265, 2979, 2095, 1656, 1509, 1366, 1254,
1135, 848, 735. UV (MeCN): kmax (log e) = 230 (3.81).
TLC: Rf = 0.34 (petroleum ether-EtOAc 3:2).
N2-Boc-capreomycidine tert-butyl ester ((rac)-14)
and N2-Boc-epicapreomycidine tert-butyl ester
((rac)-15)
Methyl N2-acetyl-2-amino-5-azido-pent-2-enoate (11c)
General procedure B with (Z)-tert-butyl N2-Boc-2-amino-
5-azido-pent-2-enoate (Z)-11a (1.13 g, 3.61 mmol), PPh3
(2.28 g, 8.71 mmol), water (1.7 mL) and THF (44 mL) for
the Staudinger reduction as well as guanidinylation reagent
13 (964 mg, 6.58 mmol), NEt3 (1.66 g, 2.28 mL,
16.4 mmol) and DMF (17 mL) for the domino reaction.
The reaction mixture was stirred at rt for 20 h and at 70 °C
for 7 h. Purification by column chromatography (CH2Cl2–
MeOH gradient (3–10 %)) gave a mixture of (rac)-14 and
(rac)-15 (ratio 1.0 : 1.0) as a yellow foam (580 mg, 54 %
General procedure A with phosphonate 9c (1.62 g,
6.80 mmol), crude 3-azido-propionaldehyde 10 (1.50 g),
KOt-Bu (799 mg, 7.14 mmol) and THF (14 mL (KOt-Bu),
15 mL (9c), 10 mL (10)). Purification by column chro-
matography (petroleum ether–EtOAc 1:1 ? 1:3) gave
(Z)-11c as a yellow solid (751 mg, 52 %) while (E)-11c
could not be isolated. 1H-NMR (300 MHz, CDCl3):
d = 2.13 (s, 3 H, Ac-CH3), 2.43 (dt, J = 7.1 Hz, 6.7 Hz,
2 H, H-4), 3.46 (t, J = 6.7 Hz 2 H, H-5), 3.78 (s, 3 H,
OCH3), 6.64 (t, J = 7.1 Hz, 1 H, H-3), 7.09 (sbr, 1 H, NH).
13C NMR (76 MHz, CDCl3): d = 23.5 (Ac-CH3), 28.9
(C-4), 49.8 (C-5), 52.6 (OCH3), 126.5 (C-2), 132.7 (C-3),
164.8 (C-1), 168.3 (Ac-C=O). MS (ESI?): m/z = 447.2
1
over 2 steps from (Z)-11a). H NMR (300 MHz, CDCl3):
d = 1.40 (s, 1 9 9 H, t-Bu-CH3), 1.41 (s, 1 9 9 H, t-Bu-
CH3), 1.46 (s, 1 9 9 H, t-Bu-CH3), 1.47 (s, 1 9 9 H, t-Bu-
CH3), 1.79–1.93 (m, 1 9 2 H, H-4), 1.93–2.03 (m,
123

2320
M. Bu¨schleb et al.
tert-Butyl N2-Cbz-2-amino-5-azido-pent-2-enoate (21)
1 9 2 H, H-4), 3.21–3.33 (m, 1 9 2 H, H-5), 3.33–3.47
(m, 1 9 2 H, H-5), 3.76–3.83 (m, 1 9 1 H, H-3),
3.85–3.93 (m, 1 9 1 H, H-3), 4.25–4.35 (m, 2 9 1 H,
H-2), 5.71 (dbr, J = 8.2 Hz, 2 9 1 H, NH-2), 7.04 (sbr,
1 9 2 H, guanidine-NH), 7.15 (sbr, 1 9 2 H, guanidine-
NH), 7.82 (sbr, 2 9 1 H, guanidine-NH), 8.10 (sbr,
1 9 1 H, guanidine-NH), 8.18 (sbr, 1 9 1 H, guanidine-
NH). 13C NMR (76 MHz, CDCl3): d = 21.7 (1 9 C-4),
23.1 (1 9 C-4), 28.0 (2 9 t-Bu-CH3), 28.2 (1 9 t-Bu-
CH3), 28.3 (1 9 t-Bu-CH3), 36.6 (1 9 C-5), 36.7 (1 9
C-5), 50.1 (1 9 C-3), 51.6 (1 9 C-3), 56.4 (1 9 C-2), 56.9
(1 9 C-2), 80.4 (1 9 t-Bu-C), 80.7 (1 9 t-Bu-C), 83.6
(1 9 t-Bu-C), 83.7 (1 9 t-Bu-C), 154.9 (1 9 Boc-C=O),
155.1 (1 9 Boc-C=O), 155.8 (1 9 guanidine-C), 156.0
(1 9 guanidine-C), 168.0 (1 9 C-1), 168.7 (1 9 C-1). MS
(ESI?): m/z = 329.3 [M ? H]?. HRMS (ESI?) calcd. for
C15H28N4O4 ([M ? H]?) 329.2183, found 329.2183. TLC:
Rf = 0.33 (CH2Cl2–MeOH 9:1).
General procedure A with phosphonate 20 (3.46 g,
9.27 mmol), crude 3-azido-propionaldehyde 10 (1.84 g),
KOt-Bu (1.09 g, 9.73 mmol) and THF (19 mL (KOt-Bu),
28 mL (20), 28 mL (10)). Purification by column chro-
matography (petroleum ether–EtOAc 8:1) gave (Z)-21
(containing traces of aldehyde 10) as a colorless oil
1
(2.70 g, 81 % as calculated from the H NMR spectrum)
1
and (E)-21 as a colorless oil (118 mg, 3 %). (Z)-21: H
NMR (300 MHz, CDCl3): d = 1.49 (s, 9 H, t-Bu-CH3),
2.48 (dt, J = 7.2 Hz, 6.8 Hz, 2 H, H-4), 3.44 (t,
J = 6.8 Hz 2 H, H-5), 5.14 (s, 2 H, Cbz-CH2), 6.45 (tbr,
J = 7.2 Hz, 2 H, H-3, NH), 7.30–7.39 (m, 5 H, Cbz-aryl-
H). 13C NMR (76 MHz, CDCl3): d = 28.0 (t-Bu-CH3),
28.5 (C-4), 49.9 (C-5), 67.4 (Cbz-CH2), 82.3 (t-Bu-C),
128.2 (Cbz-aryl-C-2, Cbz-aryl-C-6), 128.3 (Cbz-aryl-C-4),
128.5 (C-2, Cbz-aryl-C-3, Cbz-aryl-C-5), 129.7 (C-3),
135.9 (Cbz-aryl-C-1), 153.9 (Cbz-C=O), 163.2 (C-1). MS
(ESI?): m/z = 715.3 [2 M ? Na]?. HRMS (ESI?): calcd.
for C17H22N4O4 ([M ? Na]?) 369.1533, found 369.1530.
IR (ATR): m = 2971, 2095, 1703, 1498, 1290, 1218, 1147,
1022, 750, 696. UV (MeCN): kma9 (log e) = 204 (4.13),
228 (3.83). TLC: Rf = 0.73 (petroleum ether–EtOAc 1:1).
N2,N2-Bis-Boc-capreomycidine tert-butyl ester
((rac)-17) and N2,N2-Bis-Boc-epicapreomycidine
tert-butyl ester ((rac)-18)
General procedure B with (Z)-tert-butyl N2,N2-bis-Boc-2-
amino-5-azido-pent-2-enoate (Z)-11d (671 mg, 1.63
mmol), PPh3 (1.32 g, 5.05 mmol), water (1.0 mL) and
THF (25 mL) for the Staudinger reduction as well as
guanidinylation reagent 13 (388 mg, 2.65 mmol), NEt3
(268 mg, 367 lL, 2.65 mmol) and DMF (13 mL) for the
domino reaction. The reaction mixture was stirred at rt for
4 d. Purification by column chromatography (CH2Cl2–
MeOH gradient (5–10 %)) gave a mixture of (rac)-17 and
(rac)-18 (ratio 1.8:1.0) as a yellow foam (340 mg, 49 %
1
(E)-21: H NMR (300 MHz, CDCl3): d = 1.52 (s, 9 H,
t-Bu-CH3), 2.82 (dt, J = 7.5 Hz, 7.2 Hz, 2 H, H-4), 3.36
(t, J = 7.2 Hz 2 H, H-5), 5.11 (s, 2 H, Cbz-CH2), 6.76 (t,
J = 7.5 Hz, 1 H, H-3), 6.98 (sbr, 1 H, NH), 7.27–7.39 (m,
5 H, Cbz-aryl-H). 13C NMR (76 MHz, CDCl3): d = 28.0
(C-4), 28.1 (t-Bu-CH3), 50.9 (C-5), 66.8 (Cbz-CH2), 83.6
(t-Bu-C), 122.2 (C-3), 127.6 (C-2), 128.2 (Cbz-aryl-C-2,
Cbz-aryl-C-6), 128.3 (Cbz-aryl-C-4), 128.6 (Cbz-aryl-C-3,
Cbz-aryl-C-5), 136.0 (Cbz-aryl-C-1), 153.5 (Cbz-C=O),
162.7 (C-1). MS (ESI?): m/z = 715.3 [2 M ? Na]?.
HRMS (ESI): calcd. for C17H22N4O4 ([M ? Na]?)
369.1533, found 369.1535. IR (ATR): m = 2972, 2095,
1697, 1509, 1363, 1215, 1151, 1046, 846, 696. UV
(MeCN): kmax (log e) = 238 (3.85). TLC: Rf = 0.78
(petroleum ether–EtOAc 1:1).
1
over 2 steps from (Z)-11d). H NMR (300 MHz, DMSO-
d6): d = 1.39 (s, 2 9 18 H, t-Bu-CH3), 1.40 (s, 1 9 9 H,
t-Bu-CH3), 1.45 (s, 1 9 9 H, t-Bu-CH3), 1.96–2.02 (m,
2 9 2 H, H-4), 3.21–3.28 (m, 2 9 2 H, H-5), 3.98–4.07
(m, 2 9 1 H, H-3), 4.61 (d, J = 8.9 Hz, 1 9 1 H, H-2),
4.70 (d, J = 8.3 Hz, 1 9 1 H, H-2), 7.01 (s, 1 9 2 H,
guanidine-NH), 7.09 (s, 1 9 2 H, guanidine-NH), 7.51 (sbr,
1 9 1 H, guanidine-NH), 7.56 (sbr, 1 9 1 H, guanidine-
NH), 8.13 (sbr, 1 9 1 H, guanidine-NH), 8.19 (sbr,
1 9 1 H, guanidine-NH). 13C NMR (76 MHz, DMSO-d6):
d = 22.8 (2 9 C-4), 27.4 (2 9 t-Bu-CH3), 27.5 (2 9
t-Bu-CH3), 27.5 (2 9 t-Bu-CH3), 34.2 (1 9 C-5), 35.0
(1 9 C-5), 46.8 (1 9 C-3), 47.3 (1 9 C-3), 60.0 (1 9
C-2), 60.3 (1 9 C-2), 81.6 (1 9 t-Bu-C), 82.0 (1 9 t-Bu-
C), 83.0 (1 9 t-Bu-C), 83.4 (1 9 t-Bu-C), 151.8
(1 9 Boc-C=O), 151.8 (1 9 Boc-C=O), 153.6 (2 9 gua-
nidine-C), 154.0 (2 9 Boc-C=O), 166.9 (2 9 C-1). MS
(ESI?): m/z = 429.3 [M ? H]?. HRMS (ESI?): calcd. for
C20H36N4O6 ([M ? H]?) 429.2708, found 429.2707. TLC:
Rf = 0.45 (CH2Cl2–MeOH 9:1).
N2-Cbz-capreomycidine tert-butyl ester ((rac)-23)
and N2-Cbz-epicapreomycidine tert-butyl ester
((rac)-24)
General procedure B with (Z)-tert-butyl N2-Cbz-2-amino-
5-azido-pent-2-enoate (Z)-21 (1.05 g, 3.04 mmol), PPh3
(2.41 g, 9.18 mmol), water (1.8 mL) and THF (46 mL) for
the Staudinger reduction as well as guanidinylation reagent
13 (433 mg, 2.96 mmol), NEt3 (326 mg, 446 lL,
3.23 mmol) and DMF (14 mL) for the domino reaction.
The reaction mixture was stirred at 80 °C for 19 h. Puri-
fication by column chromatography (CH2Cl2–MeOH gra-
dient (8–11 %)) gave a mixture of (rac)-23 and (rac)-24
123

A biomimetic domino reaction for the concise synthesis
2321
(ratio 1.0 : 1.0) as a brownish foam (683 mg, 62 % over 2
steps from (Z)-21). 1H NMR (300 MHz, CDCl3): d = 1.43
(s, 2 9 9 H, t-Bu-CH3), 1.65–1.99 (m, 2 9 2 H, H-4),
3.13–3.26 (m, 1 9 2 H, H-5), 3.26–3.42 (m, 1 9 2 H,
H-5), 3.70–3.80 (m, 1 9 1 H, H-3), 3.81–3.91 (m,
1 9 1 H, H-3), 4.29–4.40 (m, 2 9 1 H, H-2), 5.02–5.14
(m, 2 9 2 H, Cbz-CH2), 6.18 (dbr, J = 8.5 Hz, 2 9 1 H,
Cbz-NH), 7.03 (sbr, 1 9 2 H, guanidine-NH), 7.13 (sbr,
1 9 2 H, guanidine-NH), 7.22–7.35 (m, 2 9 5 H, aryl-H),
7.89 (sbr, 1 9 1 H, guanidine-NH), 8.02 (sbr, 1 9 1 H,
guanidine-NH), 8.10 (sbr, 1 9 1 H, guanidine-NH), 8.13
(sbr, 1 9 1 H, guanidine-NH). 13C NMR (76 MHz,
CDCl3): d = 21.8 (1 9 C-4), 23.0 (1 9 C-4), 27.9 (2 9
t-Bu-CH3), 36.6 (1 9 C-5), 36.7 (1 9 C-5), 50.2 (1 9
C-3), 51.4 (1 9 C-3), 56.9 (1 9 C-2), 57.4 (1 9 C-2), 67.3
(2 9 Cbz-CH2), 83.8 (1 9 t-Bu-C), 83.9 (1 9 t-Bu-C),
128.0 (2 9 Cbz-aryl-C-2, 2 9 Cbz-aryl-C-6), 128.1
(1 9 Cbz-aryl-C-4), 128.2 (1 9 Cbz-aryl-C-4), 128.5
(2 9 Cbz-aryl-C-3, 2 9 Cbz-aryl-C-5), 136.0 (1 9 Cbz-
aryl-C-1), 136.1 (1 9 Cbz-aryl-C-1), 154.9 (1 9 Cbz-
C=O), 155.0 (1 9 Cbz-C=O), 156.3 (1 9 guanidine-C),
156.6 (1 9 guanidine-C), 167.6 (1 9 C-1), 168.4 (1 9
C-1). MS (ESI?): m/z = 363.2 [M ? H]?. HRMS (ESI?):
calcd. for C18H26N4O4 ([M ? H]?) 363.2027, found
363.2029. TLC: Rf = 0.33 (CH2Cl2–MeOH 4:1).
172.4 (2 9 C-1), 174.6 (2 9 Ac-C=O). MS (ESI?):
m/z = 237.1 [M ? Na]?. HRMS (ESI?): calcd. for
C8H14N4O3 ([M ? Na]?) 237.0958, found 237.0958.
X-ray structure determination of (Z)-11a
Single crystals of (Z)-11a were selected and covered with
perfluorinated polyether oil on a microscope slide, which
was cooled with a nitrogen gas flow using the X-TEMP2
(Kottke and Stalke 1993; Kottke et al. 1996; Stalke 1998).
An appropriate crystal was selected using a polarize
microscope, mounted on the tip of a MITEGENÓ-Micro-
Mount, fixed to a goniometer head and shock-cooled by the
crystal cooling device.
The data for (Z)-11a were collected from shock-cooled
crystals at 100(2) K (Kottke and Stalke 1993; Kottke et al.
1996; Stalke 1998). The data of (Z)-11a were collected on
a INCOATEC Mo Microsource (Schulz et al. 2009) with
mirror optics and APEX II detector with a D8 goniometer.
The diffractometer was equipped with a low-temperature
device and used MoKa radiation, k = 71.073 pm. The
crystal of (Z)-11a used for data collection was a non-
merohedral twin with two twin domains. The two domains
were separated using RLATT. Two orientation matrices
were used for the integration with SAINT (Bruker AXS Inst.
Inc. 2011) and a semi-empirical absorption correction with
TWINABS (Sheldrick 2012) was applied. The structure was
solved by direct methods (SHELXS-97) (Sheldrick 1990)
using untwinned data (HKLF 4) and refined by full-matrix
least-squares methods against F2 (SHELXL-97) (Sheldrick
2008; Mu¨ller et al. 2006) using the twinned data (HKLF 5)
within the SHELXLE GUI (Hu¨bschle et al. 2011). The
fractional distribution of the second domain refines to
0.4669 (9). All non-hydrogen-atoms were refined with
anisotropic displacement parameters. The hydrogen atoms
were refined isotropically on calculated positions using a
riding model with their Uiso values constrained to equal to
1.5 times the Ueq of their pivot atoms for terminal sp3 carbon
atoms and 1.2 times for all other carbon atoms. Only the
hydrogenatomsofthenitrogenatoms(H1andH8)werefound
from difference density map and restrained to same distance
N2-Acetyl-capreomycidine hydrochloride ((rac)-27)
and N2-acetyl-epicapreomycidine hydrochloride
((rac)-28)
A mixture of N2-Cbz-capreomycidine tert-butyl ester
(rac)-23 and N2-Cbz-epicapreomycidine tert-butyl ester
(rac)-24 (174 mg, 0.481 mmol), acetic anhydride (123 mg,
113 lL, 1.20 mmol) and Pd/C (10 %, 13 mg, 12 lmol) in
anhydrous MeOH (4.8 mL) was stirred under a hydrogen
atmosphere (1 bar, balloon) at rt for 22 h. After filtration
through Celite and washing the Celite with MeOH (2 9),
the filtrate was evaporated under reduced pressure to give a
mixture of (rac)-25 and (rac)-26 containing acetic anhy-
dride as a yellow oil (170 mg). This mixture was then
dissolved in hydrochloric acid (6 M, 4.0 mL, 24 mmol),
and the solution was stirred at rt for 3 h. The solvent was
evaporated under reduced pressure to give a mixture of
(rac)-27 and (rac)-28 (ratio 1.2 : 1.0) as an orange foam
(93 mg, 77 % over 2 steps from (rac)-23 and (rac)-24). 1H
NMR (300 MHz, D2O): d = 1.88–2.20 (m, 2 9 2 H, H-4),
2.11 (s, 1 9 3 H, Ac-CH3), 2.13 (s, 1 9 3 H, Ac-CH3),
3.36–3.46 (m, 2 9 2 H, H-5), 4.02–4.11 (m, 2 9 1 H,
H-3), 4.65–4.82 (m, 2 9 1 H, H-2). 13C NMR (76 MHz,
D2O): d = 20.7 (1 9 C-4), 21.7 (1 9 Ac-CH3), 21.8
(1 9 Ac-CH3), 22.2 (1 9 C-4), 36.1 (1 9 C-5), 36.2
(1 9 C-5), 49.3 (2 9 C-3), 54.7 (1 9 C-2), 55.2 (1 9
C-2), 154.1 (1 9 guanidine-C), 154.4 (1 9 guanidine-C),
˚
within the esd (0.02 A). Disordered moieties were refined
using bond lengths and angles restraints and anisotropic dis-
placement parameter restraints.
Crystallographic data (excluding structure factors) for
the structures reported in this paper have been deposited
with the Cambridge Crystallographic Data Centre. The
CCDC number, crystal data and further details for the
X-ray measurement are listed in the Supplementary
Material. Copies of the data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif or from the corre-
sponding author.
123

2322
M. Bu¨schleb et al.
Results and discussion
O
H
N
R1
O R2
P(OR3)2
For the synthesis of potential cyclization precursors
8a–h (Table 1), a general synthetic approach was devel-
oped. In order to achieve diversity with respect to the
protecting group pattern of the guanidine moiety, it was
envisaged to introduce this functionality at a late stage.
Guanidinylation of primary or secondary amines is a
common way to prepare guanidines (for the terminology of
guanidine formation, see Jones 2002). However, since d-
amino-didehydro a-amino acid esters (i.e., didehydro-
ornithine derivatives) with an unprotected d-amino group
display limited stability, the corresponding d-azido ana-
logues were synthesized first. Reduction of the azide
moiety would then provide the amines suitable for gua-
nidinylation reactions. This approach has previously been
used in the synthesis of a protected didehydro-arginine
derivative (Yonezawa et al. 2000). However, to the best of
our knowledge, the literature precedent for the synthesis
and further transformation of a d-azido-didehydro a-amino
acid ester is limited to this one example.
O
9a: R1 = Boc, R2 = t-Bu, R3 = Et
9b: R1 = Ac, R2 = t-Bu, R3 = Me
9c: R1 = Ac, R2 = R3 = Me
O
KOt-Bu, THF
N3
10
O
H
O
H
R1
N
R1
N
O R2
O R2
+
N3
N3
R1 = Boc, R2 = t-Bu:
R1 = Ac, R2 = t-Bu:
R1 = Ac, R2 = Me:
(Z)-11a 77 %
(Z)-11b 61 %
(Z)-11c 52 %
(E)-11a
(E)-11b
(E)-11c
4 %
---
---
O
O
BocHN
Boc2N
Ot-Bu
Ot-Bu
(Boc)2O, DMAP
92 %
Thus, rapid access to protected d-azido-didehydro
a-amino acids was an essential requirement for the described
synthetic strategy. All d-azido-didehydro a-amino acids
used in this study were prepared by Wittig-Horner reac-
tions of amino acid phosphonates 9a–c (Ducho et al. 2009;
Schmidt et al. 1984) with 3-azido-propionaldehyde 10
(Davies et al. 1967) (Fig. 3). This approach proved to be
significantly more efficient than the previously used
lengthy transformation of protected didehydro-glutamate
into the respective d-azido compound (Yonezawa et al.
2000). Wittig-Horner reactions of amino acid phosphonates
with aldehydes are known to furnish the according
(Z)-didehydro amino acids with excellent diastereoselec-
tivities (Schmidt et al. 1992; for further examples, see
Ducho et al. 2009; Spork and Ducho 2010; Spork et al.
2011). Accordingly, the (Z)-configured products (Z)-11a–
c were obtained in isolated yields of 52–77 %. The syn-
thesis of cyclization precursors 8b and 8g,h required the
presence of a second Boc group at the a-amino moiety.
Thus, (Z)-11a was treated with di-tert-butyl dicarbonate
((Boc)2O) to afford (Z)-11d in 92 % yield. For the stereo-
chemical assignment of (Z)-11a, the according (E)-isomer
was needed as a reference compound. Following careful
column chromatography, the (E)-configured congener (E)-
11a was isolated as a byproduct from the Wittig-Horner
transformation of 9a with 10 in 4 % yield. Application of
N3
N3
(Z)-11d
(Z)-11a
Fig. 3 Synthesis of protected d-azido-didehydro a-amino acids 11
Fig. 4 Molecular structure of protected d-azido-didehydro a-amino
acid (Z)-11a. Ellipsoids are shown at 50 % probability level. Carbon
atoms are shown in black, nitrogen in blue, oxygen red and hydrogen
white. The other hydrogen atoms and the disorder of the azide-group
are omitted for clarity
1
the established H NMR criteria for the distinction of (Z)-
for X-ray crystallography could be obtained by slow
evaporation of a solution of the compound in diethyl ether.
The elucidated structure confirmed the (Z)-configuration of
the double bond (Fig. 4). The crystals of (Z)-11a grew as
non-merohedral twins in the space group P21/c with two
hydrogen-bonded (N–HÁÁÁO d = 206(2) pm) molecules
and (E)-didehydro amino acids (Mazurkiewicz et al. 2005)
on both isomers of 11a clearly revealed the major product to
display (Z)-configuration (for details see Supplementary
Material). For a completely unambiguous assignment of the
double-bond geometry, single crystals of (Z)-11a suitable
123

A biomimetic domino reaction for the concise synthesis
2323
within the asymmetric unit (see Supplementary Material).
The azide groups of both molecules are disordered on two
positions with an occupancy of 50 %.
that the (E)-didehydro-arginine underwent decomposition
under the reaction conditions.
The thus synthesized mixtures of protected cap-
reomycidine (rac)-14 and epicapreomycidine (rac)-15 as
well as (rac)-17 and (rac)-18, respectively, were depro-
tected under acidic conditions to furnish a mixture of target
compounds capreomycidine (rac)-1 and epicapreomyci-
dine (rac)-2 as their dihydrochlorides in yields of 99 and
87 %, respectively (Fig. 5). A procedure for the separation
of 1 and 2 by crystallization of the picrates is established
(Bycroft et al. 1971b). Our novel biomimetic synthesis can
therefore be considered a very convenient and rapid access
to pure 1 and/or 2 in racemic form.
The first attempt to achieve the desired biomimetic
cyclization was to synthesize precursor 8a without pro-
tecting groups at the guanidine moiety (Fig. 5). Azido
derivative (Z)-11a was therefore reduced under Staudinger
conditions to give amino intermediate 12, which was not
purified with respect to anticipated stability issues (see
Yonezawa et al. 2000). Guanidinylation of 12 was carried
out using commercially available reagent 13 in the pres-
ence of triethylamine to liberate the reactive agent from
the employed hydrochloride. However, instead of the
expected product (Z)-8a, a mixture of protected cap-
reomycidine (rac)-14 and epicapreomycidine (rac)-15
(diastereomeric ratio (d.r.) ca. 1 : 1) was isolated in 54 %
yield over two steps from (Z)-11a. Thus, after formation
of intermediate (Z)-8a, an immediate aza-Michael-addi-
tion occurred in a domino fashion (for the concept of
domino reactions, see Tietze and Beifuss 1993; Tietze
1996; Tietze et al. 2006). Similar results were obtained
with precursor (Z)-11d, which was reduced to ornithine
derivative 16. Intermediate 16 was then used in the
domino reaction to give a mixture of protected cap-
reomycidine (rac)-17 and epicapreomycidine (rac)-18
(d.r.*1.8 : 1.0) in 49 % yield over two steps from
(Z)-11d. Hence, the higher electrophilicity of the precursor
16 bearing two Boc groups did not result in higher isolated
yields of the domino product (Fig. 5). Interestingly, when
isomer (E)-11a was used in the sequence of Staudinger
reduction and subsequent domino-guanidinylation-aza-
Michael-addition, no product could be isolated. It was
therefore concluded that either the (E)-configured orni-
thine derivative was more instable than the (Z)-isomer or
It was envisaged that a stereocontrolled version of the
domino reaction would be difficult to achieve. First
attempts to employ chiral bases such as (-)-quinine or
(-)-sparteine instead of triethylamine resulted in low
conversions and side reactions. Hence, it was decided to
prepare N2-acetyl derivatives of 1 and 2 using the biomi-
metic route to enable acylase-mediated resolution of the
product mixtures for separation of the L- from the D-iso-
mers. Didehydro amino acids (Z)-11b and (Z)-11c were
therefore used in the newly established protocol of Stau-
dinger reduction (products 19a and 19b, not purified) and
subsequent biomimetic domino transformation (Fig. 6).
Surprisingly, the anticipated domino process stopped at the
didehydro-arginine stage after the reduction and guanid-
inylation steps for both precursors (Z)-11b and (Z)-11c.
Thus, compounds 8c and 8d were obtained in impure form
and did not undergo cyclization towards the capreomyci-
dine scaffold even at elevated temperatures up to 100 °C.
This indicates that the didehydro amino acid moiety is
required to display a certain electrophilicity to enable the
aza-Michael addition and that even slight changes, such as
O
O
O
O
BocHN
BocHN
BocHN
BocHN
Ot-Bu
Ot-Bu
Ot-Bu
Ot-Bu
13, NEt3, DMF
PPh3, THF
+
HN
HN
HN
HN
54 %
(over 2 steps
from (Z)-11a)
6 M aq.
HCl
99 %
N
N
N3
NH2
H
H
(Z)-11a
12
(rac)-14
(rac)-15
(rac)-1 2 HCl
+
NH2
Cl
N
H2N
N
(rac)-2 2 HCl
13
O
O
O
O
6 M aq.
HCl
Boc2N
HN
HN
Boc2N
Boc2N
Boc2N
87 %
Ot-Bu
Ot-Bu
Ot-Bu
Ot-Bu
13, NEt3, DMF
PPh3, THF
+
HN
HN
49 %
(over 2 steps
from (Z)-11d)
N
N
N3
NH2
H
H
(Z)-11d
16
(rac)-17
(rac)-18
Fig. 5 Synthesis of capreomycidine (rac)-1 and epicapreomycidine (rac)-2 using a biomimetic domino reaction
123

2324
M. Bu¨schleb et al.
the shift from carbamate to amide protection, can lead to a
complete loss of reactivity.
according N-Cbz-protected congeners first and then to
exchange the N-Cbz for an N-acetyl moiety after the bio-
mimetic cyclization process, followed by acidic cleavage
of the tert-butyl ester (Fig. 7). When Cbz-protected phos-
phonate 20 (Schmidt et al. 1982, 1984; Schmidt and Wild
1985; Hamzavi et al. 2003) was employed in the Wittig-
Horner reaction with aldehyde 10, Cbz-protected d-azido-
didehydro a-amino acid ester (Z)-21 was obtained in 81 %
yield (calculated as traces of aldehyde 10 could not com-
pletely be removed). Stereoisomer (E)-21 could also be
isolated in 3 % yield, and again, application of the estab-
lished 1H NMR criteria for the distinction of (Z)- and
(E)-didehydro amino acids (Mazurkiewicz et al. 2005)
confirmed the stereochemical assignment. Subsequent
Staudinger reduction of (Z)-21 furnished ornithine deriva-
tive 22, which was directly treated with guanidinylation
reagent 13 to initiate the domino reaction. Thus, formation
of the respective arginine derivative and domino-type ring
closure provided a mixture of protected capreomycidine
(rac)-23 and epicapreomycidine (rac)-24 (d.r.*1 : 1) in
62 % yield over two steps. The mixture of (rac)-23 and
(rac)-24 was then subjected to a sequence of hydrogenol-
ysis with in situ-acetylation (products (rac)-25 and
(rac)-26) and acidic cleavage of the tert-butyl ester to
afford a mixture of Na-acetylated products (rac)-27 and
(rac)-28 in 77 % yield over two steps from the mixture of
(rac)-23 and (rac)-24 (Fig. 7). However, attempts to
N-Acetylated capreomycidin derivatives for enzymatic
resolution therefore had to be prepared via a different
route. An attempted direct acetylation of target structures
(rac)-1 and (rac)-2 did not provide the desired Na-acety-
lated products. It was therefore envisaged to synthesize the
O
O
AcHN
AcHN
OR
OR
PPh3, THF
N3
NH2
R = t-Bu: (Z)-11b
R = Me: (Z)-11c
R = t-Bu: 19a
R = Me: 19b
O
O
AcHN
AcHN
HN
HN
OR
NH2
NH
OR
13, NEt3, DMF
N
N
H
H
R = t-Bu: 8c
R = Me: 8d
R = t-Bu
R = Me
Fig. 6 Synthesis of Na-acetylated capreomycidines for enzymatic
resolution: first attempt with the domino reaction stopping after the
guanidinylation step
O
N3
O
O
10
O
CbzHN
CbzHN
Ot-Bu
Ot-Bu
KOt-Bu, THF
CbzHN
O
Ot-Bu
+
P(OMe)2
N3
(Z)-21 81 %
N3
20
(E)-21 3 %
O
O
O
O
CbzHN
CbzHN
CbzHN
CbzHN
Ot-Bu
+
Ot-Bu
Ot-Bu
Ot-Bu
PPh3, THF
13, NEt3, DMF
HN
HN
HN
HN
62 %
(over 2 steps
from (Z)-21)
N
N
N3
NH2
H
H
(Z)-21
22
(rac)-23
(rac)-24
O
O
O
O
H2, Pd/C
Ac2O, MeOH
AcHN
HN
HN
AcHN
HN
HN
AcHN
HN
AcHN
HN
Ot-Bu
Ot-Bu
OH
OH
6 M aq. HCl
+
+
77 %
(over 2 steps)
N
N
H2N
Cl
N
H
H2N
Cl
N
H
H
H
(rac)-25
(rac)-26
(rac)-27
(rac)-28
Fig. 7 Synthesis of Na-acetylated capreomycidines for enzymatic resolution: successful route via Na-Cbz-protected derivatives
123

A biomimetic domino reaction for the concise synthesis
2325
subject this mixture of Na-acetylated capreomycidines to
acylase-mediated deacetylation to separate the L- from the
D-isomers failed as no conversion could be detected.
Apparently, Na-acetylated capreomycidines are no sub-
strates for standard acylases. This might result either from
steric hindrance due to the adjacent six-membered ring or
from the positively charged guanidine group being fixed in
a position close to the reaction site.
1) PhCHO
CH2Cl2
2) NaBH4
MeOH
R
O
R
O
BocN
BocN
Ot-Bu
Ot-Bu
NH2
NH
Bn
obtained by
reduction of
(Z)-11a,d
R = H: 12
R = Boc: 16
R = H: 29
R = Boc: 30
The synthesis of guanidine-protected cyclization pre-
cursors 8e–h (see Table 1) was then attempted. It was
anticipated that these compounds should not spontaneously
undergo the aza-Michael addition as a result from the
reduced nucleophilicity of the guanidine group. In contrast,
one should be able to isolate fully protected didehydro-
arginine derivatives 8e–h and to subject them to aza-
Michael cyclization in a separate step. This second reaction
would then require activation either with a Lewis acid (in
order to enhance the electrophilicity of the didehydro
amino acid moiety) or a base (to deprotonate the urethane-
protected guanidine and make it nucleophilic). Thus, the
domino reaction would be dissected into two separate
transformations, and a chiral activating agent for the
cyclization step might potentially provide stereoinduction
and enable a stereoselective biomimetic synthesis of
capreomycidines. In principle, the acidic properties of
urethane-protected guanidines are established (Feichtinger
et al. 1998). However, it was unclear if the guanidine
moiety had to be fully protected in order to allow for
sufficient deprotonation of the urethane moiety, and
therefore, the additional N-benzyl group was introduced
into potential cyclization precursors 8f and 8h.
SMe
NCbz
CbzHN
R
O
31
R
O
NEt3, AgOTf
DMF
BocN
BocN
Ot-Bu
Ot-Bu
NHCbz
NCbz
NH
R'
R = R' = H:
R = H, R' = Bn: 29
R = Boc, R' = H: 16
R = Boc, R' = Bn: 30
N
R'
12
R = R' = H:
8e 71 %*
R = H, R' = Bn: 8f 55 %**
R = Boc, R' = H: 8g 50 %*
R = Boc, R' = Bn: 8h 42 %**
*over 2 steps from (Z)-11a,d
**over 3 steps from (Z)-11a,d
Fig. 8 Synthesis of potential cyclization precursors 8e–h with pro-
tected guanidine moieties
anticipated to be more reactive, its possible cyclization in
the presence of DBU in THF was investigated next, but no
conversion was observed even at elevated temperatures. It
was therefore decided to skip cyclization studies on 8f and
to conduct more detailed investigations regarding the aza-
Michael cyclization of what should be the most reactive
precursor both in terms of guanidine-urethane acidity and
Michael acceptor activity, i.e. didehydro-arginine 8h.
Hence, compound 8h was treated with a series of bases
(K2CO3 in DMSO, basic Al2O3 in MeCN, KOt-Bu or NaH
in THF) as well as a series of Lewis acids (FeCl3, SnCl4,
Me2AlCl, EtAlCl2 or Yb(OTf)3 in dichloromethane, reac-
tions not displayed). Furthermore, Takemoto’s bifunctional
chiral organocatalyst (Inokuma et al. 2006) was added to a
solution of 8h in toluene to study if the combination of
thiourea activation and basic moiety might be able to
trigger the cyclization reaction. However, no conversion
towards the cyclic capreomycidine system was detected in
any of the listed cases. It was therefore concluded that the
reactivity of the didehydro amino acid moiety was just not
sufficient for aza-Michael additions if the nucleophilicity
of the guanidine group is too low resulting from its
derivatization. This again demonstrated that the biomi-
metic cyclization reaction could only work within a certain
frame of reactivity of both moieties involved (vide supra).
Thus, a dissection of the domino reaction into two separate
transformations was impossible, preventing further
For an efficient synthesis of 8e–h, d-azido-didehydro
a-amino acids (Z)-11a and (Z)-11d were used and again
reduced to moderately stable ornithine derivatives 12 and
16 (vide supra, Fig. 8). For the introduction of the benzyl
group, 12 and 16 were subjected to reductive amination
reactions with benzaldehyde to furnish Nd-benzylated or-
nithines 29 and 30, which could not be obtained in pure
form. All didehydro-ornithines 12, 16, 29 and 30 were
transformed into the respective arginine derivatives by
guanidinylation with reagent 31 (Tian et al. 1992) in the
presence of silver(I) triflate. Thus, target structures 8e–
h were obtained in yields of 42–71 % (over two or three
steps from the respective d-azido-didehydro a-amino acid).
Guanidine-protected cyclization precursors 8e–h were
then subjected to numerous attempts to activate them for
the aza-Michael addition. Initially, 8e was treated with a
series of bases (K2CO3 in MeCN, NaH, KHMDS or DBU
in THF) for urethane deprotonation or with iron(III) chlo-
ride as a Lewis acid in dichloromethane for activation of
the Michael acceptor system (reactions not displayed). No
conversion towards the cyclic capreomycidine system was
detected. As the bis-Boc-protected congener 8g was
123

2326
M. Bu¨schleb et al.
Baldwin JE, Merritt KD, Schofield CJ (1993a) Synthesis of
L-ornithines stereospecifically deuterated at C-3. Tetrahedron
Lett 34:3919–3920
attempts to conduct the biomimetic cyclization in a ste-
reocontrolled fashion.
Baldwin JE, Merritt KD, Schofield CJ, Elson SW, Baggaley KH
(1993b) Studies on the stereospecificity of the Clavaminic acid
synthase catalysed hydroxylation reaction. J Chem Soc Chem
Commun 1993:1301–1302
Conclusion
Bartz QR, Ehrlich J, Mold JD, Penner MA, Smith RM (1951)
Viomycin, a new tuberculostatic antibiotic. Amer Rev Tubercu-
losis 63:4–6
In summary, we have developed an unprecedented biomi-
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teinogenic amino acids capreomycidine
1
and
epicapreomycidine 2 in racemic form. The two key steps of
this efficient synthetic route were (1) the preparation of
protected d-azido-didehydro a-amino acid precursors using
Wittig-Horner reactions and (2) a sequence of Staudinger
reduction and a novel domino reaction for the construction
of the capreomycidine scaffold. This domino-guanidiny-
lation-aza-Michael-addition sequence mimics the biosyn-
thetic ring closure of didehydro-arginine towards
capreomycidine. Attempts to subject the furnished racemic
mixtures of L- and D-amino acids to acylase-mediated
enzymatic resolution were unsuccessful as Na-acetylated
capreomycidines were apparently no substrates for acy-
lases. Investigations on didehydro-arginine cyclization
precursors with different protecting group patterns revealed
that the domino reaction would only work within a certain
frame of reactivity, i.e. both sufficient electrophilicity of
the Michael acceptor and nucleophilicity of the guanidine
moiety were essential. Fully protected didehydro-arginine
derivatives 8e–h therefore did not undergo the aza-Michael
addition anymore, even in the presence of highly reactive
activating agents. However, both compounds 8e–h and
their respective azide precursors might be useful synthetic
intermediates for the preparation of isotope-labelled ornithine
or arginine derivatives (also see Baldwin et al. 1993a, b).
Overall, the obtained results demonstrate that a biomimetic
1,4-addition of a guanidine moiety to a didehydro amino acid
unit is synthetically feasible, but that a fine-tuning of the
reactivity is essential.
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Acknowledgments This work was supported by the Deutsche
Forschungsgemeinschaft (DFG, SFB 803 ‘‘Functionality controlled
by organization in and between membranes’’) and the Fonds der
Chemischen Industrie (FCI, Sachkostenzuschuss). MG and DS thank
the DFG Priority Programme 1178 and the Danish National Research
Foundation (DNRF) funded Center for Materials Crystallography
(CMC).
Conflict of interest The authors declare that they have no conflict
of interest.
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