Asymmetric syntheses of (+)-negamycin, (+)-3-epi-negamycin and sperabillin C via lithium amide conjugate addition

Article abstract of DOI:10.1016/j.tet.2010.10.067

The chemo- and enantioselective reduction of ethyl 4-chloroacetoacetate and the diastereoselective conjugate addition of enantiopure lithium N-benzyl-N-(α-methylbenzyl)amide to an α,β-unsaturated ester have been used as the key steps in the total asymmetric syntheses of (+)-negamycin (in 13 steps and 24% overall yield), (+)-3-epi-negamycin (in 13 steps and 10% overall yield) and sperabillin C (in 17 steps and 13% overall yield) from commercially available starting materials.

Full text of DOI:10.1016/j.tet.2010.10.067

Tetrahedron 67 (2011) 216e227
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Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Asymmetric syntheses of (þ)-negamycin, (þ)-3-epi-negamycin and sperabillin C
via lithium amide conjugate addition
*
Stephen G. Davies , Osamu Ichihara, Paul M. Roberts, James E. Thomson
Department of Chemistry, Chemistry Research Laboratory, University of Oxford, Mansfield Road, Oxford, OX1 3TA, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 16 August 2010
Received in revised form 6 October 2010
Accepted 25 October 2010
Available online 29 October 2010
The chemo- and enantioselective reduction of ethyl 4-chloroacetoacetate and the diastereoselective
conjugate addition of enantiopure lithium N-benzyl-N-(a-methylbenzyl)amide to an a,b-unsaturated
ester have been used as the key steps in the total asymmetric syntheses of (þ)-negamycin (in 13 steps
and 24% overall yield), (þ)-3-epi-negamycin (in 13 steps and 10% overall yield) and sperabillin C (in 17
steps and 13% overall yield) from commercially available starting materials.
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Asymmetric synthesis
Lithium amide
(þ)-Negamycin
(þ)-3-epi-Negamycin
Sperabillin C
1. Introduction
core fragment of sperabillins B 5 and D 7, and subsequently the total
asymmetric syntheses of both sperabillins B 5 and D 7 (Fig. 1).11 Our
(þ)-Negamycin 1 is a pseudopeptide antibiotic, which displays
strong inhibitory activity against Gram-positive and Gram-negative
bacteria, and exhibits low toxicity.1 (þ)-Negamycin 1 also exhibits
genetic miscoding activity on bacterial ribosome systems2 and is
a specific inhibitor of protein synthesis in Escherichia coli K12.3 Since
Umezawa et al. first isolated (þ)-negamycin 1 in 1970 from the
culture filtrate of three strains related to Streptomyces purpeofuscus,1
it has received a great deal of attention from the synthetic com-
munity.4,5 The structure of (þ)-negamycin 1 was initially elucidated
via degradation studies6 and was subsequently confirmed in 1972 by
strategy for the synthesis of these natural products employed the
conjugate addition of an enantiopure lithium amide12 to an
a,b-
unsaturated ester as one of the key steps. We have used this reaction
in a series of natural product syntheses,13 kinetic and parallel kinetic
resolutions,14 and for the synthesis of a range of
b-amino acids and
their derivatives.15 It was also envisaged that this reaction could be
employed for the total asymmetric syntheses of (þ)-negamycin 1,
(þ)-3-epi-negamycin 3 and sperabillin C 6, and we report herein our
full investigations within this area. Part of this work has been
communicated previously.4h
a total enantiospecific synthesis from
D
-galacturonic acid.7 The
sperabillin family of antibiotics 4e7, which were isolated from the
culture filtrates of Pseudomonas fluoresens YK-437,8 are structurally
related to (þ)-negamycin 1 and are also active against Gram-positive
and Gram-negative bacteria, including antibiotic resisitant strains.8b
The structures of sperabillins A-D 4e7, including their absolute
configurations, were elucidiated by degradation studies9 and by the
total enantiospecific synthesis of sperabillin D 7.5a Sperabillins A 4
and C 6 have the same core amino acid unit as (þ)-negamycin 1,
whilst sperabillins B 5 and D 7 bear an additional C(6)-methyl sub-
stituent and consequently an additional stereogenic centre at C(6).
We have previously reported the asymmetric synthesis of (R,R,R)-
3,6-diamino-5-hydroxyheptanoic acid 3,10 the highly functionalised
2. Results and discussion
Retrosynthetic analysis of (þ)-negamycin 1 proceeded via dis-
connection of the hydrazinoacetate component to give b-amino acid
9. It was anticipated that
intermediacy of a -amino ester, such as 10, which in turn could be
synthesised via the conjugate addition of lithium (R)-N-benzyl-N-
-methylbenzyl)amide (R)-11 to enantiopure N-Boc-N,O-acetonide
protected -stereogenic
-unsaturated ester 12.12 Given that the
b-amino acid 9 could be accessed via the
b
(a
a,b
d
centre within 12 is fairly remote from the site of reaction it was en-
visaged that the powerful stereocontrol exerted by the lithium amide
reagent 11 would overwhelm any inherent stereofacial bias of the
a,b
-unsaturated ester 12 during the conjugate addition reaction16,17
and thus allow the highly stereoselective preparation of the (3R)-
stereocentre required for (þ)-negamycin 1. We have previously
* Corresponding author. E-mail address: steve.davies@chem.ox.ac.uk (S.G. Davies).
0040-4020/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2010.10.067

S.G. Davies et al. / Tetrahedron 67 (2011) 216e227
217
NH2
Me
N
OH
OH
O
O
O
H2N
H2N
CO2H
CO2H
Boc
N
Boc
N
N
H
CO2CHEt2
CHO
(+)-negamycin 1
12
13
NH2
Me
N
O
N
H
OH
(+)-3-epi-negamycin 2
O
N3
CO2Et
Boc
N
CO2Et
NH2
OH
15
14
H2N
CO2H
Me
3
OH
O
OH
NH2
O
NH
Cl
CO2Et
Cl
CO2Et
H
R2
N
(R)-16
17
N
NH2
H
R3
O
R1
Fig. 3. Retrosynthetic analysis of enantiopure N-Boc-N,O-acetonide protected
saturated ester 12.
a
,
b-un-
sperabillin A 4, R1 = H, R2 = Me, R3 = H
sperabillin B 5, R1 = Me, R2 = Me, R3 = H
sperabillin C 6, R1 = H, R2 = H, R3 = Me
sperabillin D 7, R1 = Me, R2 = H, R3 = Me
We also anticipated that this strategy would be equally appli-
cable to the total asymmetric synthesis of (þ)-3-epi-negamycin 2
Fig. 1. The structures of (þ)-negamycin 1, (þ)-3-epi-negamycin 2, (R,R,R)-3,6-diamino-
5-hydroxyheptanoic acid 3 and sperabillins AeD 4e7.
since the very high stereocontrol of lithium (S)-N-benzyl-N-(a-
methylbenzyl)amide (S)-11 was expected to dominate16 any in-
herent stereocontrol of -unsaturated ester 12 and therefore the
a,b
conjugate addition of lithium amide (S)-11 to 12 could be exploited
as the key step for the generation of the (3S)-stereogenic centre
within (þ)-3-epi-negamycin 2 (Fig. 4).
shown that the use of a bulky ester moiety (usually a tert-butyl ester)
effectively eliminates competing 1,2-addition in these systems;12
however, it was anticipated that a tert-butyl ester would not be
compatible with the proposed N-Boc-N,O-acetonide protecting group
strategy as an orthogonal deprotection of
b-amino ester 10 to give
b-
2.1. Asymmetric synthesis of (D)-negamycin
amino acid 9 would be required. 3-Pentyl
a,b-unsaturated ester 12
was therefore selected in this case as it was anticipated that its steric
bulk would suppress the potential for 1,2-addition of the lithium
amide whilst also being susceptible to selective base-catalysed
ester hydrolysis in the presence of the acid-labile protecting
groups18 (Fig. 2).
The synthesis of
a,b-unsaturated ester 12 began with the
enantioselective reduction of commercially available ethyl
4-chloroacetoacetate 17, which was achieved under 5 atmospheres
of hydrogen in the presence of an [(S)-BINAP]Ru(II) complex to give
g
-chloro-
b
-hydroxy ester (R)-16 in 91% yield and 98:2 er.20 Treat-
Ph
Ph
N
O
Me
N
OH
5
NH2
3
O
Me
N
CO2Bn
N
H
H2N
CO2H
N
H
O
(+)-negamycin 1
N
Boc
8
Ph
Ph
Ph
Ph
N
(R)-11
Li
+
Ph
N
Ph
N
CO2CHEt2
CO2CHEt2
CO2H
O
O
O
N
N
N
Boc
Boc
Boc
12
10
9
Fig. 2. Retrosynthetic analysis of (þ)-negamycin 1.
We proposed that
a
,
b
-unsaturated ester 12 could be obtained
ment of (R)-16 with a large excess of NaI in acetone gave essentially
quantitative conversion to -iodo- -hydroxy ester 20, which was
via Wittig reaction of 3-pentyl (triphenylphosphoranylidene)ace-
tate with aldehyde 13. In turn, 13 could be accessed via manipu-
lation of alcohol (R)-16, which is known to be formed in high
enantiomeric purity following catalytic hydrogenation of com-
mercially available ethyl 4-chloroacetoacetate 17 in the presence of
an [(S)-BINAP]Ru(II) complex19 (Fig. 3).
g
b
treated with NaN3 to give 15 in 95% isolated yield (two steps).
Hydrogenolytic reduction of 15 in the presence of di-tert-butyl
dicarbonate (Boc2O) followed by acetonide formation gave 14 in
81% yield over the two steps. Attempted conversion of ester 14 into
aldehyde 13 directly by treatment with diisobutylaluminum

218
S.G. Davies et al. / Tetrahedron 67 (2011) 216e227
diastereoisomeric products of the conjugate addition reaction dif-
fered in their configurations at C(5) and not at C(3): the conjugate
addition of lithium amide (R)-11 to -unsaturated ester 12 is
a,b
therefore completely diastereoselective. Purification of the crude
reaction mixture gave 10 in 85% isolated yield and >99:1 dr
(Scheme 2). The configuration of the newly formed C(3) stereogenic
centre within the major diastereoisomer 10 was assigned as (R) by
analogy to the transition state mnemonic developed by us for this
class of lithium amide conjugate addition reaction,21 and this was
subsequently confirmed by the elaboration of 10 into (þ)-neg-
amycin 1 (vide infra).
Fig. 4. Retrosynthetic analysis of (þ)-3-epi-negamycin 2.
hydride (DIBAL-H) at ꢀ78 ꢁC was unsuccessful and therefore a re-
duction and re-oxidation protocol was employed. Thus, ester 14
was reduced with sodium bis(2-methoxyethoxy)aluminium hy-
dride (Red-AlÒ) and the resultant alcohol was oxidised under
Swern conditions to give aldehyde 13 in 94% yield (two steps).
Wittig reaction of 13 with 3-pentyl (triphenylphosphoranylidene)
acetate in toluene at 70 ꢁC gave quantitative conversion to a 91:9
mixture of (E)-12 and (Z)-21, respectively. Subsequent chromato-
graphic purification gave (E)-12 in 86% yield and (Z)-21 in 9% yield
and >99:1 dr and 98:2 er20 in each case (Scheme 1).
Scheme 2. Reagents and conditions: (i) lithium (R)-N-benzyl-N-(
amide (R)-11, THF, ꢀ78 ꢁC, 1 h then NH4Cl (satd, aq).
a-methylbenzyl)
LiOH-Mediated hydrolysis of the ester functionality within 10
proceeded to give the corresponding carboxylic acid 9. Subsequent
coupling of 9 with benzyl [N(1)-methylhydrazino]acetate 22 was
achieved upon treatment with DCC or via conversion to the cor-
responding mixed anhydride, giving 8 in 79% yield (two steps) and
57% yield (two steps), respectively. Deprotection of 8 was achieved
via treatment with TFA in THF/H2O (to remove both the N-Boc and
acetonide protecting groups) followed by catalytic hydrogenolysis,
which gave (þ)-negamycin 1 in quantitative conversion. Purifica-
tion via ion-exchange chromatography (Amberlite CG-50) enabled
isolation of (þ)-negamycin 1 in 71% yield from 8 (Scheme 3). The
spectroscopic data of our sample of (þ)-negamycin 1 were in ex-
cellent agreement with those reported for the natural product by
Umezawa et al.1 {[
a
]
20 þ2.7 (c in H2O); lit.1 [
a]
29 þ2.5 (c 2.0 in H2O)}
D
D
and with those reported for other synthetic samples4 {e.g., lit.4a
20
20
[a
]
þ2.4 (c 1.5 in H2O); lit.4f
[
a
]
þ2.5 (c 1.4 in H2O)}. The over-
D
D
all yield of (þ)-negamycin 1 produced using this strategy was 24%
(in 13 steps) from commercially available starting materials,
Scheme 1. Reagents and conditions: (i) H2 (5 atm), Ru[(S)-BINAP]Cl2, EtOH, 100 ꢁC,
6 h; (ii) NaI, acetone, reflux, 2 days; (iii) NaN3, MeCN/H2O (5:1), 80 ꢁC, 7 h; (iv) H2
(1 atm), Pd/C, Boc2O, EtOAc, 16 h; (v) (ꢀ)-CSA, Me2C(OMe)2/acetone (1:1), 70 ꢁC, 2 h;
(vi) Red-AlÒ, PhMe, 0 ꢁC, 1 h; (vii) DMSO, (COCl)2, iPr2NEt, CH2Cl2, ꢀ63 ꢁC, 25 min; (viii)
3-pentyl (triphenylphosphoranylidene)acetate, PhMe, 70 ꢁC, 4 h.
The conjugate addition of lithium (R)-N-benzyl-N-(
benzyl)amide (R)-11 to -unsaturated ester (E)-12 gave quanti-
tative conversion to
-amino ester 10. The rt 1H NMR spectrum of
a-methyl-
a,b
b
the crude reaction mixture in CDCl3 displayed extremely broad
peaks (presumably due to the rotameric nature of the N-Boc group)
and so an accurate determination of the reaction diaster-
eoselectivity by peak integration of this spectrum was not possible.
1H NMR spectroscopic analysis of this sample at 363 K in DMSO-d6,
however, gave sufficient resolution of peaks and it was found that
10 was produced in 98:2 dr. Considering the enantiomeric purity of
(E)-12 (98:2 er) it was reasoned that the major and minor
Scheme 3. Reagents and conditions: (i) LiOH, MeOH/THF/H2O (3:1:1), reflux, 24 h; (ii)
benzyl [N(1)-methylhydrazino]acetate 22, DCC, HOBt, Et3N, THF, 0 ꢁC to rt, 5 h; (iii)
Et3N, ClCO2Et, CH2Cl2, ꢀ15 ꢁC, 5 min then 0 ꢁC, 30 min; (iv) TFA, THF/H2O (1:1), rt, 16 h;
(v) H2 (5 atm), Pd(OH)2/C, AcOH (five drops), MeOH, rt, 22 h.

S.G. Davies et al. / Tetrahedron 67 (2011) 216e227
219
representing one of the most efficient syntheses of (þ)-negamycin
1 reported to date.
in agreement with that reported for 3-epi-negamycin
2
by
20
Kibayashi et al.4e {[
appears anomalous.24
a]
ꢀ3.2 (c 4.4 in H2O)}, which therefore
D
2.2. Asymmetric synthesis of (D)-3-epi-negamycin
2.3. Asymmetric synthesis of sperabillin C
The synthesis of (þ)-3-epi-negamycin 222 via the conjugate
addition of lithium (S)-N-benzyl-N-(a-methylbenzyl)amide (S)-11
to
Given the similar structures of (þ)-negamycin 1 and sper-
a
,b
-unsaturated ester 12 was next investigated. Treatment of 12
-amino
abillin C 6 it was envisaged that b-amino ester 10 could be easily
with (S)-11 gave quantitative conversion to a mixture of
b
elaborated into sperabillin C 6: this would require only protecting
esters. Peak integration of the 1H NMR spectrum of the crude re-
action mixture (obtained at 363 K in DMSO-d6) revealed that
a major diastereoisomeric product 19 accounted for w95% of the
product distribution.23 Unfortunately, attempts to enrich the di-
astereoisomeric purity of 19 by column chromatography were un-
successful and 19 was isolated in 61% yield and w95%
diastereoisomeric purity.23 The configuration of the C(3) stereo-
genic centre within 19 was initially assigned as (S) on the as-
sumption that the conjugate addition reaction proceeds under the
dominant stereocontrol of the lithium amide reagent (S)-11,21 and
this was subsequently confirmed by the conversion of 19 into
(þ)-3-epi-negamycin 2. Hydrolysis of 19 under basic conditions
gave the corresponding carboxylic acid, which was coupled with
benzyl [N(1)-methylhydrazino]acetate 22 to give 18 in 38% yield
(two steps) and w95% diastereoisomeric purity.23 Removal of the
N-Boc and acetonide protecting groups within 18, followed by
hydrogenolysis completed the synthesis of (þ)-3-epi-negamycin 2
in 96:4 dr and 75% yield over the final two steps (10% overall yield
in 13 steps from 17) (Scheme 4).
group manipulation and two amide bond forming reactions. Re-
moval of the N-benzyl and N-
within 10 was achieved by catalytic hydrogenolysis with
Pearlman0s catalyst under 6 atm of hydrogen to give primary
a-methylbenzyl protecting groups
b
-
amino ester 23 in 96% yield. Subsequent treatment of 23 with
benzyl chloroformate (CbzCl) gave 24 in 82% yield. Treatment of
24 with a solution of HCl in MeOH resulted in cleavage of the N-
Boc and acetonide protecting groups as well as promoting in situ
transesterification to give the corresponding methyl ester, which
was isolated as its hydrochloride salt 25$HCl in 93% yield
(Scheme 5).
Ph
Ph
N
O
CO2CHEt2
CO2CHEt2
(i)
O
N
N
Boc
Boc
12, >99:1 dr, 98:2 er
19,a 61%
(ii), (iii)
Scheme 5. Reagents and conditions: (i) H2 (6 atm), Pd(OH)2/C, AcOH (five drops),
Ph
O
MeOH, rt, 20 h; (ii) CbzCl, NaHCO3, CH2Cl2, rt, 45 min; (iii) HCl, MeOH, 0 ꢁC, 30 min.
Ph
N
O
Me
OH
NH2
O
(iv), (v)
Using an analogous procedure to that described by Natsugari
et al.5a for the transformation of the 6-methyl substituted congener
N
CO2Bn
Me
N
N
H
H2N
CO2H
N
H
of N-Cbz protected b-amino ester 25 into sperabillin D 7, the hy-
drochloride salt 25$HCl was treated with sorbyl chloride in the
presence of Et3N to give 26 in 80% yield as a single diastereoisomer.
Protection of both the C(5)-hydroxyl and C(6)-amide groups within
26 was achieved by treatment with dimethoxypropane in acetone
giving 27 in 80% yield, which was found to be prone to polymeri-
sation when left to stand at rt and therefore was used immediately
in the next step. Thus, ester 27 was hydrolysed by treatment with
NaOH in THF/MeOH to give carboxylic acid 28 in 88% yield
(Scheme 6).
(+)-3-epi-negamycin 2
18,a 38% (2 steps)
N
75% (2 steps), 96:4 dr
Boc
Scheme 4. Reagents and conditions: (i) lithium (S)-N-benzyl-N-(a-methylbenzyl)am-
ide (S)-11, THF, ꢀ78 ꢁC, 1 h then NH4Cl (satd, aq); (ii) LiOH, MeOH/THF/H2O (3:1:1),
reflux, 24 h; (iii) benzyl [N(1)-methylhydrazino]acetate 22, DCC, HOBt, Et3N, THF, 0 ꢁ
C
to rt, 5 h; (iv) TFA, THF/H2O (1:1), rt, 16 h; (v) H2 (5 atm), Pd(OH)2/C, AcOH (five drops),
MeOH, rt, 20 h. [acompounds 18 and 19 were isolated in w95% diastereoisomeric
purity].
By analogy to the method reported by Natsugari et al.,5a,25 the
coupling of carboxylic acid 28 with 2-amidinoethylamine 2926
was achieved upon treatment with DCC. This was immediately
followed by treatment of 30 with TMSI in MeCN for 3.5 h, which
gave sperabillin C 6 in 50% isolated yield (two steps) after se-
quential purification on Amberlite XAD-II resin then Amberlite
402 resin (Scheme 7). The spectroscopic data of our synthetic
The 1H and 13C NMR spectroscopic data obtained for (þ)-3-epi-
negamycin 2 were in good agreement with those reported for other
samples of this compound.4e,g,5e However, there are some dis-
crepancies between the specific rotation data for epi-negamycin 2
reported in the literature. The specific rotation for our sample of
22
(þ)-3-epi-negamycin 2 {[
a
]
D
þ8.5 (c 0.7 in H2O)} was consistent
with the values for its enantiomer (ꢀ)-5-epi-negamycin 2 reported
sample of 6 were in excellent agreement with those reported by
25
by Hegedus et al.4g {[
a
]
25 ꢀ9.4 (c 0.5 in H2O)} and the group of I.C.I.
Hida et al.9 for a sample isolated from the natural source {[
a]
D
D
25
Pharma5e {[
a
]
23 ꢀ9.9 (c 3.5 in H2O)} although none of these date are
ꢀ10.2 (c 0.3 in H2O); lit.9
[
a
]
ꢀ11.0 (c 0.7 in H2O)} and other
D
D

220
S.G. Davies et al. / Tetrahedron 67 (2011) 216e227
MeCN and DMSO were distilled from CaH2 before use. All other re-
agents were used as supplied (analyticalor HPLC grade) without prior
purification. Reactions performed at ꢀ63 ꢁC were cooled by means of
a chloroform/dry ice bath. Reactions performed at ꢀ15 ꢁC were
cooled by means of a ethylene glycol/dry ice bath. Organic layers were
dried over MgSO4. Thin layer chromatography was performed on al-
uminium plates coated with 60 F254 silica. Plates were visualised us-
ing UV light (254 nm), iodine, 1% aq KMnO4, or 10% ethanolic
phosphomolybdic acid. Flash column chromatography was per-
formed on Kieselgel 60 silica.
Elemental analyses were recorded by the microanalysis service
of the Inorganic Chemistry Laboratory, University of Oxford, U.K.
Melting points were recorded on a Gallenkamp Hot Stage apparatus
and are uncorrected. Optical rotations were recorded on a Perkin-
Elmer 241 polarimeter with a water-jacketed 10 cm cell. Specific
rotations are reported in 10ꢀ1 deg cm2 gꢀ1 and concentrations in
g/100 mL. IR spectra were recorded on a Bruker Tensor 27 FT-IR
Scheme 6. Reagents and conditions: (i) sorbyl chloride, Et3N, CH2Cl2, 0 ꢁC, 1 h; (ii)
spectrometer. Selected characteristic peaks are reported in cmꢀ1
.
TsOH, Me2C(OMe)2/acetone (1:1), rt, 26 h; (iii) NaOH, MeOH/THF (2:1), 0 ꢁC to rt, 4 h.
NMR spectra were recorded on Bruker Avance spectrometers in the
deuterated solvent stated. The field was locked by external refer-
encing to the relevant deuteron resonance. The 13C NMR spectra of
many N-Boc protected compounds contained peaks that were
doubled due to the presence of rotamers; the chemical shifts of
these peaks are reported in italics. Low-resolution mass spectra
were recorded on either a VG MassLab 20e250 or a Micromass
Platform 1 spectrometer. Accurate mass measurements were run
on either a Bruker MicroTOF, which was internally calibrated with
polyalanine, or a Micromass GCT instrument fitted with a Scientific
Glass Instruments BPX5 column (15 mꢂ0.25 mm) using amyl ace-
tate as a lock mass.
synthetic samples {lit.25
(c 0.4 in H2O)}. The overall yield of sperabillin C 6 produced using
this strategy was 13% (in 17 steps) from commercially available
starting materials.
[
a
]
25 ꢀ10.1 (c 0.5 in H2O); lit.5d
[
a
]D ꢀ10.2
D
4.1.1. Ethyl (R)-3-hydroxy-4-chlorobutanoate 16.
28
Ru[(S)-BINAP]Cl2 (270 mg, 145 mmol) was suspended in
1,2-dichlorobenzene (3 mL) and the resultant mixture was
thoroughly degassed and heated at 160 ꢁC for 10 min, during
which time the initially purple suspension became dark brown.
The reaction mixture was then allowed to cool to rt and con-
centrated in vacuo. The brown solid residue was dried under high
vacuum to give RuCl2[(S)-BINAP]. A solution of ethyl 4-chlor-
oacetoacetate (6.23 g, 37.9 mmol) in EtOH (7 mL) was then
treated with RuCl2[(S)-BINAP] (180 mg) and the resultant mixture
was stirred under hydrogen (5 atm) at 100 ꢁC for 6 h. Purification
Scheme 7. Reagents and conditions: (i) 29$2HCl, DCC, HOBt, THF, rt, 2 h; (ii) TMSI,
MeCN, rt, 3.5 h.
3. Conclusion
In conclusion, the chemo- and enantio-selective reduction of
commercially available ethyl 4-chloroacetoacetate and the diaster-
eoselective conjugate addition of enantiopure lithium N-benzyl-N-
via reduced pressure distillation gave 16 as a colourless oil
21
(5.74 g, 91%, 98:2 er20);19 bp 85e87 ꢁC (3 mmHg); [
a
]
þ20.7 (c
D
(a-methylbenzyl)amide to a chiral a,b-unsaturated ester have been
7.3 in CHCl3); {lit.19 for 97% ee [
a
]
þ20.9 (c 7.7 in CHCl3)}; dH
21
D
used as the key steps for the introduction of stereochemistry at the C
(3) and C(5) positions in the total asymmetric syntheses of (þ)-neg-
amycin, (þ)-3-epi-negamycin and sperabillin C. The overall yields
from ethyl 4-chloroacetoacetate were: (þ)-negamycin, 24% over 13
steps; (þ)-3-epi-negamycin, 10% over 13 steps; and sperabillin C,13%
over 17 steps.
(200 MHz, CDCl3) 1.28 (3H, t, J 7.2, OCH2CH3), 2.60e2.66 (2H, m, C
(2)H2), 3.21 (1H, br s, OH), 3.61 (2H, d, J 4.9, C(4)H2), 4.12e4.32
(1H, m, C(3)H), 4.19 (2H, q, J 7.2, OCH2CH3).
4.1.2. Ethyl (R)-3-hydroxy-4-iodobutanoate 20.
4. Experimental
4.1. General experimental
NaI (27.0 g, 180 mmol) was added to a stirred solution of 16
(15.0 g, 90.0 mmol) in acetone (100 mL) and the resultant mixture
was heated at reflux for 2 days. The reaction mixture was then
allowed to cool to rt, diluted with Et2O (150 mL) and filtered. The
filtrate was then passed through a short column of silica gel (eluent
All reactions involving organometallic or other moisture-sensitive
reagents were carried out under a nitrogen or argon atmosphere
using standard vacuum line techniques and glassware that was flame
dried and cooled under nitrogen before use. Solvents were dried
according to the procedure outlined by Grubbs et al.27 Water was
purified by a Millipore ElixÒ UV-10 system. 1,2-Dichlorobenzene,
Et2O) and concentrated in vacuo to give 20 as a pale yellow oil
20
(22.9 g, 99%);29
[
a
]
þ10.0 (c 3.0 in EtOH); {lit.29 for enantiomer
D

S.G. Davies et al. / Tetrahedron 67 (2011) 216e227
221
20
[
a]
ꢀ10.9 (c 3.0 in EtOH)}; dH (200 MHz, CDCl3) 1.30 (3H, t, J 7.1,
q, J 7.1, OCH2CH3), 4.41e4.45 (1H, m, C(50)H); dC (50 MHz, CDCl3)
14.0 (OCH2CH3), 24.0, 25.2, 26.2, 27.4 (C(20)Me2), 28.3 (CMe3), 38.4 (C
(2)), 50.6 (C(40)), 60.7 (OCH2CH3), 70.0 (C(50)), 79.6, 80.1 (CMe3),
93.2, 93.6 (C(20)), 152.2 (NCO), 170.6 (C(1)); m/z (CIþ) 288 ([MþH]þ,
28%), 188 (100%).
D
OCH2CH3), 2.65e2.69 (2H, m, C(2)H2), 3.20 (1H, d, J 5.0, OH),
3.31e3.36 (2H, m, C(4)H2), 3.99e4.05 (1H, m, C(3)H), 4.21 (2H, q,
J 7.0, OCH2CH3).
4.1.3. Ethyl (R)-3-hydroxy-4-azidobutanoate 15.
4.1.6. (R)-2-[20,20-Dimethyl-N(30)-tert-butoxycarbonyl-10,30-
oxazolidin-50-yl]ethanol 32.
NaN3 (20.2 g, 310 mmol) was added to a stirred solution of 20
(20.0 g, 77.5 mmol) in MeCN/H2O (5:1, 200 mL) and the resultant
mixture was heated at 80 ꢁC for 7 h. The reaction mixture was then
concentrated in vacuo and the residue was diluted with brine
(500 mL). The resultant mixture was extracted with Et2O (3ꢂ100 mL)
and the combined organic extracts were dried and concentrated in
Red-AlÒ (3.4 M in PhMe, 3.2 mL,11 mmol) was added to a stirred
solution of 14 (790 mg, 2.75 mmol) in PhMe (30 mL) and the re-
sultant mixture was stirred at 0 ꢁC for 1 h. H2O (2 mL) was added,
the reaction mixture was allowed to warm to rt and the resultant
mixture was diluted with H2O (25 mL) and stirred for 30 min. The
mixture was then filtered through Celite (eluent Et2O) and the
aqueous layer was extracted with Et2O (3ꢂ30 mL). The combined
organic extracts were dried and concentrated in vacuo to give 32 as
a viscous, colourless oil (591 mg, 86%); C12H23NO4 requires C, 58.75;
vacuo to give 15 as a pale yellow oil (12.9 g, 96%);29
[
a
]
20 þ7.1 (c 4.1 in
D
MeOH); {lit.29
[a]
20 þ7.4 (c 4.1 in MeOH)}; dH (200 MHz, CDCl3) 1.28
D
(3H, t, J 7.0, OCH2CH3), 2.42e2.66 (2H, m, C(2)H2), 3.23 (1H, d, J 5.0,
OH), 3.33e3.38(2H, m, C(4)H2), 4.20(2H, q, J 7.0, OCH2CH3), 4.06e4.28
(1H, m, C(3)H).
H, 9.45; N, 5.7%; found C, 58.85; H, 9.5; N, 5.7%; [
a
]
D
25 ꢀ22.4 (c 2.1 in
4.1.4. Ethyl (R)-3-hydroxy-4-(N-tert-butoxycarbonylamino)buta-
noate 31.
CHCl3); nmax (film) 3600, 3100, 2980, 2940, 2880, 1700 (C]O), 1480,
1260, 1175, 1150, 1100,1060; dH (200 MHz, CDCl3) 1.46 (9H, s, CMe3),
1.49 (3H, s, C(20)MeA), 1.57 (3H, s, C(20)MeB), 1.82e1.87 (2H, m, C(2)
H2), 2.30 (1H, br s, OH), 3.13 (1H, dd, J 9.5, 9.5, C(40)HA), 3.40e3.82
(1H, m, C(40)HB), 3.77 (2H, t, J 6.3, C(1)H2), 4.20e4.26 (1H, m, C(50)
H); dC (50 MHz, CDCl3) 24.1, 25.0, 26.1, 27.1 (C(20)Me2), 28.3 (CMe3),
35.3 (C(2)), 50.8 (C(40)), 59.6 (C(1)), 72.2 (C(50)), 79.6, 80.2 (CMe3),
93.1, 93.5 (C(20)), 152.1, 152.5 (NCO); m/z (CIþ) 246 ([MþH]þ, 15%),
146 (100%).
Boc2O (13.5 g, 61.9 mmol) and 15 (8.88 g, 51.3 mmol) were
added to a suspension of Pd/C (10% wt, 2.40 g) in EtOAc (100 mL)
and the resultant mixture was stirred vigorously under hydrogen
(1 atm) for 16 h. The reaction mixture was then filtered through
Celite (eluent EtOAc) and concentrated in vacuo. Purification via
flash column chromatography (eluent 30e40 ꢁC petrol/CH2Cl2/
Et2O, 1:1:1) gave 31 as a colourless oil (11.2 g, 88%); C11H21NO5
4.1.7. (R)-[20,20-Dimethyl-N(30)-tert-butoxycarbonyl-10,30-oxazolidin-
50-yl]ethanal 13.
requires C, 53.4; H, 8.6; N, 5.7%; found C, 53.2; H, 8.8; N, 5.7%;
20
[
a
]
þ6.6 (c 1.1 in CHCl3); nmax (film) 3650, 3100, 2480, 2440,
D
1735 (C]O), 1700 (C]O), 1520, 1370, 1170; dH (200 MHz, CDCl3)
1.27 (3H, t, J 7.2, OCH2CH3), 1.45 (9H, s, CMe3), 2.60 (2H, d, J 6.3, C
(2)H2), 3.05e3.18 (1H, m, C(4)HA), 3.28e3.33 (1H, m, C(4)HB), 3.53
(1H, br s, NH), 4.07e4.23 (1H, m, C(3)H), 4.17 (2H, q, J 7.2,
OCH2CH3), 5.00 (1H, br s, OH); dC (50 MHz, CDCl3) 14.0
(OCH2CH3), 28.2 (CMe3), 38.6 (C(2)), 45.4 (C(4)), 60.8 (OCH2CH3),
67.7 (C(3)), 79.6 (CMe3), 156.8 (NCO), 172.8 (C(1)); m/z (CIþ) 248
([MþH]þ, 65%), 192 (100%).
DMSO (6.47 g, 76.9 mmol) was added to a stirred solution of
(COCl)2 (4.47 g, 35.2 mmol) in CH2Cl2 (100 mL) at ꢀ63 ꢁC. The re-
action mixture was stirred for 10 min then a solution of 32 (7.85 g,
32.0 mmol) in CH2Cl2 (50 mL) was added and the resultant mixture
i
was stirred at ꢀ63 ꢁC for 15 min. A solution of Pr2NEt (10.3 g,
79.8 mmol) in CH2Cl2 (10 mL) was then added and the reaction
mixture was allowed to warm to rt before H2O (100 mL) was added.
The resultant mixture was extracted with CH2Cl2 (2ꢂ50 mL) and
the combined organic extracts were dried and concentrated in
vacuo. The residue was dissolved in CH2Cl2 and the resultant so-
lution was filtered through a short plug of silica gel (eluent CH2Cl2).
The filtrate was concentrated in vacuo to give 13 as a pale yellow oil,
which was used immediately without further purification (7.48 g,
96%); dH (200 MHz, CDCl3) 1.47 (9H, s, CMe3), 1.51 (3H, s, C(20)MeA),
1.55 (3H, s, C(20)MeB), 2.70 (1H, dd, J 15.8, 6.0, C(2)HA), 2.80e2.95
(1H, m, C(2)HB), 2.90e3.20 (1H, m, C(40)HA), 3.70e3.90 (1H, m, C(40)
HB), 4.47e5.52 (1H, m, C(50)H), 9.82 (1H, s, C(1)H); dC (50 MHz,
CDCl3) 24.2, 25.1, 26.1, 27.1 (C(20)Me2), 28.3 (CMe3), 47.0 (C(2)), 50.6
(C(40)), 68.5 (C(50)), 80.0 (CMe3), 93.3 (C(20)), 151.7 (NCO), 199.3 (C
(1)); m/z (CIþ) 244 ([MþH]þ, 10%), 144 (100%).
4.1.5. Ethyl
(R)-[20,20-dimethyl-N(30)-tert-butoxycarbonyl-10,30-
oxazolidin-50-yl]ethanoate 14.
(ꢀ)-CSA (500 mg, 2.15 mmol) was added to a stirred solution of
31 (13.9 g, 56.3 mmol) in 2,2-dimethoxypropane/acetone (1:1,
200 mL) and the resultant mixture was heated at 70 ꢁC for 2 h. The
reaction mixture was then allowed to cool to rt, Et3N (1.0 mL) was
added and the resultant mixture was concentrated in vacuo. Puri-
fication via flash column chromatography (eluent CH2Cl2/Et2O, 2:1)
gave 14 as a pale yellow oil (14.9 g, 92%); C14H25NO5 requires C,
58.5; H, 8.8; N, 4.9%; found C, 58.45; H, 8.8; N, 4.8%; [
a
]
20 ꢀ26.8 (c
D
4.1.8. 3-Pentyl (triphenylphosphoranylidene)ethanoate 33.
2.7 in CHCl3); nmax (film) 2480, 2440, 1730 (C]O), 1695 (C]O),
1395, 1260, 1195, 1055, 870, 770; dH (200 MHz, CDCl3) 1.27 (3H, t, J
7.1, OCH2CH3), 1.47 (9H, s, CMe3), 1.50 (3H, s, C(20)MeA), 1.55 (3H, s, C
(20)MeB), 2.56 (1H, dd, J 15.9, 7.0, C(2)HA), 2.68e2.72 (1H, m, C(2)
HB), 3.11e3.16 (1H, m, C(40)HA), 3.78e3.82 (1H, m, C(40)HB), 4.17 (2H,
Step 1: Concd aq H2SO4 (0.2 mL) was added to a solution of
bromoacetic acid (10.8 g, 77 mmol) and 3-pentanol (8.8 g,

222
S.G. Davies et al. / Tetrahedron 67 (2011) 216e227
100 mmol) in C6H6 (180 mL) and the resultant mixture was heated
at reflux in a DeaneStark apparatus until the evolution of H2O
ceased (w5 h). The reaction mixture was then allowed to cool to rt
and concentrated in vacuo. H2O (50 mL) was added to the residue
and the resultant mixture was extracted with Et2O (2ꢂ20 mL). The
combined organic extracts were washed with 1% aq NaHCO3
(30 mL), then dried. Distillation of the resultant solution gave 3-
pentyl bromoacetate 34 as a colourless oil (13.6 g, 84%); C7H13BrO2
requires C, 40.2; H, 6.3%; found C, 40.2; H, 6.45%; bp 65e67 ꢁC
(3 mmHg); nmax (film) 2970, 2880, 1730 (C]O), 1460, 1280, 1170,
1105; dH (200 MHz, CDCl3) 0.92 (6H, t, J 7.3, OCH(CH2CH3)2), 1.62
(4H, dq, J 7.3, 6.3, OCH(CH2CH3)2), 3.82 (2H, s, CH2Br), 4.83 (1H,
quintet, J 6.3, OCHEt2); m/z (CIþ) 228 ([M(81Br)þNH4]þ, 100%), 226
([M(79Br)þNH4]þ, 100%).
76.3 (OCHEt2), 79.3, 79.9 (CMe3), 93.1, 93.4 (C(20)), 122.3 (C(2)),
143.7 (C(3)), 152.0 (NCO), 165.9 (C(1)); m/z (CIþ) 356 ([MþH]þ,
25%), 256 (100%). Further elution gave (E)-12 as a colourless oil
(8.11 g, 86%, >99:1 dr); C19H33NO5 requires C, 64.2; H, 9.4; N, 3.9%;
20
found C, 64.3; H, 9.2; N, 3.85%; [
a
]
ꢀ17.9 (c 2.1 in CHCl3); nmax
D
(film) 2970, 2930, 2880, 1705 (C]O), 1660, 1390, 1265, 1170, 1105,
1145; dH (200 MHz, CDCl3) 0.88 (6H, t, J 6.7, OCH(CH2CH3)2),
1.40e1.70 (10H, m, C(20)Me2, OCH(CH2CH3)2), 1.47 (9H, s, CMe3),
2.4e2.53 (2H, m, C(4)H2), 3.10 (1H, br s, C(40)HA), 3.70 (1H, br s, C
(40)HB), 4.15e4.21 (1H, m, C(50)H), 4.82 (1H, quintet, J 6.0, OCHEt2),
5.93 (1H, d, J 15.7, C(2)H), 6.91 (1H, dt, J 15.7, 5.7, C(3)H); dC
(50 MHz, CDCl3) 9.5 (OCH(CH2CH3)2), 24.2, 25.2, 26.2, 27.1 (C(20)
Me2), 26.4 (OCH(CH2CH3)2), 28.3 (CMe3), 35.6 (C(4)), 50.3 (C(40)),
71.9 (C(50)), 76.6 (OCHEt2), 80.1 (CMe3), 93.2 (C(20)), 124.3 (C(2)),
142.8 (C(3)), 152.1 (NCO), 165.9 (C(1)); m/z (CIþ) 373 ([MþNH4]þ,
5%), 356 ([MþH]þ, 4%), 256 (100%).
Step 2: PPh3 (10.4 g, 39.8 mmol) was added to a stirred solution
of 34 (9.36 g, 39.8 mmol) in PhMe (40 mL) and the resultant mix-
ture was stirred at rt for 16 h. Fine crystals precipitated from the
mixture and were subsequently washed with PhMe then dried
under high vacuum to give 2-[2-oxo-2-(30-pentyl)ethyl]triphenyl-
4.1.10. 300-Pentyl (R,R,R)-3-[N-benzyl-N-(
a-methylbenzyl)amino]-4-
[20,20-dimethyl-N(30)-tert-butoxycarbonyl-10,30-oxazolidin-50-yl]bu-
phosphonium bromide 35 as
a
colourless oil (17.3 g, 92%);
tanoate 10.
C25H28BrO2P requires C, 63.7; H, 6.0%; found C, 63.4; H, 6.0%; nmax
(KBr) 3560, 3410, 2970, 2760, 1725 (C]O),1440, 1260, 1150, 1110; dH
(200 MHz, CDCl3) 0.71 (6H, t, J 7.0, OCH(CH2CH3)2), 1.41 (4H, dq,
J 7.0, 6.3, OCH(CH2CH3)2), 4.62 (1H, quintet, J 6.3, OCHEt2),
5.58e5.63 (2H, m, C(2)H2), 7.27e7.97 (15H, m, Ph); m/z (CIþ) 391
([MꢀBr]þ, 7%), 279 (100%).
Step 3: Phenolphthalein (1.0 M in EtOH, two drops) was added
to a suspension of 35 (1.56 g, 33 mmol) in H2O/PhMe (1:1, 50 mL)
in a separating funnel. NaOH (2.0 M, aq) was added in small por-
tions to the suspension with vigorous shaking until the aqueous
layer turned pink. The aqueous layer was then extracted with
PhMe (2ꢂ25 mL) and the combined organic extracts were dried
and concentrated in vacuo. The residue was dried at 50 ꢁC under
high vacuum for 1 h to give 33 as a viscous, colourless oil (1.30 g,
quant); C25H27O2P requires C, 76.9; H, 7.0%; found C, 76.7; H, 6.8%;
nmax (film) 3060, 2960, 2940, 2880, 1625, 1435, 1380, 1320, 1110; dH
(200 MHz, CDCl3) 0.72 (6H, br s, OCH(CH2CH3)2), 1.34 (4H, br s,
OCH(CH2CH3)2), 2.87 (1H, br s, C(2)H), 4.64 (1H, quintet, J 6.3,
OCHEt2), 7.21e7.73 (15H, m, Ph); m/z (CIþ) 391 ([MþH]þ, 100%),
303 (65%).
BuLi (1.6 M in hexanes, 9.9 mL, 15.8 mmol) was added to a stir-
red solution of (R)-N-benzyl-N-(a-methylbenzyl)amine (3.36 g,
15.9 mmol) in THF (25 mL) at ꢀ78 ꢁC and the resulting pink solu-
tion was stirred at ꢀ78 ꢁC for 30 min. A solution of 12 (3.77 g,
10.6 mmol) in THF (5 mL) was then added dropwise and the re-
sultant mixture was stirred at ꢀ78 ꢁC for 1 h. Satd aq NH4Cl (4 mL)
was then added and the mixture was allowed to warm to rt. The
reaction mixture was poured into brine (150 mL) and extracted
with Et2O (3ꢂ50 mL). The combined organic extracts were dried
and concentrated in vacuo to give 10 in 98:2 dr. Purification via flash
chromatography (eluent 30e40 ꢁC petrol/Et2O, 10:3) gave 10 as
a viscous, colourless oil (5.11 g, 85%, >99:1 dr); C34H50N2O5 requires
4.1.9. 300-Pentyl (R,E)-4-[20,20-dimethyl-N(30)-tert-butoxycarbonyl-
10,30-oxazolidin-50-yl]but-2-enoate 12.
C, 72.05; H, 8.9; N, 4.9%; found C, 72.25; H, 8.5; N, 5.0%; [
(c 2.0 in CHCl3); nmax (film) 2970, 2930, 1730 (C]O), 1700 (C]O),
1600, 1495, 1455, 1245; dH (500 MHz, DMSO-d6, 363 K) 0.81 (6H, t, J
a
]
21 ꢀ23.9
D
7.4, OCH(CH2CH3)2), 1.34 (3H, d, J 6.9, C(a)Me), 1.44 (15H, s, CMe3, C
(20)Me2), 1.46e1.57 (4H, m, OCH(CH2CH3)2), 1.76 (1H, dd, J 9.4, 3.7, C
(4)HA),1.79 (1H, dd, J 9.4, 4.2, C(4)HB), 2.11 (2H, m, C(2)H2), 2.89 (1H,
dd, J 9.5, 9.5, C(40)HA), 3.50e3.55 (1H, m, C(3)H), 3.54 (1H, dd, J 9.8,
5.8, C(40)HB), 3.57 (1H, d, J 15.1, NCHAHBPh), 3.85 (1H, d, J 15.1,
a
)H), 4.23e4.28 (1H, m, C(50)H),
A solution of 13 (6.43 g, 26.5 mmol) and 33 (16.5 g, 42.3 mmol)
in PhMe (300 mL) was stirred at 70 ꢁC for 4 h then concentrated in
vacuo. 30e40 ꢁC Petrol was added to the residue and the resultant
mixture was filtered. The filtrate was concentrated in vacuo to give
a 91:9 mixture of (E)-12 and (Z)-21. Purification via flash column
chromatography (eluent 30e40 ꢁC petrol/Et2O, 3:2) gave (Z)-21 as
NCHAHBPh), 3.91 (1H, q, J 6.9, C(
4.56e4.61 (1H, m, OCHEt2), 7.41e7.21 (10H, m, Ph); dC (50 MHz,
CDCl3) 9.6 (OCH(CH2CH3)2), 19.7 (C( )Me), 24.3, 25.2, 26.3, 27.3 (C
(20)Me2), 26.3 (OCH(CH2CH3)2), 28.5 (CMe3), 37.0 (C(2)), 37.0 (C(4)),
a
37.1, 49.8 (C(3), NCH2Ph), 50.9, 51.2 (C(40)), 57.5 (C(
a)), 70.3, 70.7 (C
(50)), 76.5 (OCHEt2), 79.0, 79.9 (CMe3), 92.9 (C(20)), 126.8, 127.3,
128.4 (o,m,p-Ph), 140.9 (i-Ph), 151.8, 152.5 (NCO), 172.1 (C(1)); m/z
(CIþ) 567 ([MþH]þ, 100%).
a colourless oil (840 mg, 9%, >99:1 dr); C19H33NO5 requires C,
20
64.2; H, 9.4; N, 3.9%; found C, 64.4; H, 9.3; N, 4.2%; [
a
]
ꢀ12.7 (c
D
2.2 in CHCl3); nmax (film) 2970, 2940, 2880, 1700 (C]O), 1645,
1390, 1180; dH (200 MHz, CDCl3) 0.89 (6H, t, J 7.4, OCH(CH2CH3)2),
1.01e1.70 (10H, m, C(20)Me2, OCH(CH2CH3)2), 1.47 (9H, s, CMe3),
3.00 (2H, m, C(4)H2), 3.06e3.13 (1H, m, C(40)HA), 3.64e3.69 (1H,
m, C(40)HB), 4.18e4.25 (1H, m, C(50)H), 4.77e4.81 (1H, m, OCHEt2),
5.92 (1H, d, J 11.5, C(2)H), 6.25e6.34 (1H, m, C(3)H); dC (50 MHz,
CDCl3) 9.5 (OCH(CH2CH3)2), 24.2, 25.2, 26.3, 27.1 (C(20)Me2), 28.3
(CMe3), 28.3 (OCH(CH2CH3)2), 32.0 (C(4)), 50.1 (C(40)), 72.7 (C(50)),
4.1.11. Benzyl N(1)-methylhydrazinylethanoate$TsOH 22$TsOH.
Step 1: Chloroacetic acid (4.5 g, 47.6 mmol) was added to a solu-
tion of methylhydrazine (12.0 g, 260 mmol) in H2O (80 mL) and the

S.G. Davies et al. / Tetrahedron 67 (2011) 216e227
223
resultant mixture was allowed to stand at rt for 5 days. Excess
methylhydrazine was removed by reduced pressure distillation. The
residue was dissolved in H2O (80 mL) and purified by ion-exchange
chromatography (Dowex 50WX8-200, eluent 1.0 M NH4OH). EtOH
(50 mL) was added to the residue and the resultant mixture was
allowed to stand at ꢀ20 ꢁC for 16 h. The crystals that formed were
filtered off and dried under high vacuum to give N(1)-methylhy-
drazinylacetic acid 36 as a white solid (2.57 g, 52%);30 mp
151e152 ꢁC; {lit.30 mp 153e154 ꢁC}; dH (200 MHz, D2O) 2.90 (3H, s,
NMe), 3.70 (2H, s, C(2)H2).
Step 2: TsOH (8.36 g, 44.2 mmol) was added to a stirred solution
of 36 (2.00 g, 19.2 mmol) and BnOH (20 mL) in C6H6 (80 mL) and
the resultant mixture was heated at reflux for 6 h using a Deane
Stark apparatus. The reaction mixture was then allowed to cool to
rt, poured into Et2O (500 mL), and the white precipitate that
formed was collected by filtration. Purification via recrystallisation
(MeOH/Et2O) gave 22$TsOH as a white powder (3.80 g, 54%);31 mp
93e94 ꢁC; {lit.31 mp 96e99 ꢁC}; dH (200 MHz, D2O) 2.10 (3H, s,
ArMe), 2.60 (3H, s, NMe), 3.67 (2H, s, C(2)H2), 4.98 (2H, s, OCH2Ph),
7.06 (2H, d, J 8.2, Ar), 7.41 (2H, d, J 8.2, Ar), 7.12e7.25 (5H, m, Ph).
dr);32 C39H52N4O6 requires C, 69.6; H, 7.8; N, 8.3%; found C, 69.7; H,
7.9; N, 8.4%; [
a]
25 ꢀ9.8 (c 0.8 in CHCl3); dC (50 MHz, CDCl3) 19.6 (C
D
(a
)Me), 24.2, 25.2, 26.3, 27.3 (C(2000)Me2), 28.5 (CMe3), 34.2, 36.2, 37.1
(C(200), C(400)), 43.8, 44.8 (NMe), 49.9, 50.1 (C(300), NCH2Ph), 51.1 (C
(4000)), 57.3, 57.6 (C( )), 58.0, 58.7 (C(2)), 66.5 (OCH2Ph), 70.7 (C(5000)),
a
79.3, 79.8 (CMe3), 92.9, 93.2 (C(2000)), 126.6, 126.8, 127.1, 127.3, 128.2,
128.7 (o,m,p-Ph),135.1,140.7 (i-Ph), 151.9, 152.3 (NCO), 170.0, 170.5 (C
(100)), 175.1 (C(1)); m/z (FABþ) 673 ([MþH]þ, 37%), 105 (100%).
Step 2B: Ethyl chloroformate (0.06 mL, 0.60 mmol) was added to
a stirred solution of 9 (300 mg, 0.60 mmol) and Et3N (61 mg,
0.60 mmol) in CH2Cl2 (40 mL) at ꢀ15 ꢁC and the resultant solution
was stirred at ꢀ15 ꢁC for 10 min. A solution of 22$TsOH (220 mg,
0.60 mmol) and Et3N (61 mg, 0.60 mmol) in CH2Cl2 (5 mL) was then
added and the resultant mixture was stirred at ꢀ15 ꢁC for 5 min,
then allowed to warm to 0 ꢁC and stirred for a further 30 min. The
reaction mixture was then concentrated in vacuo and H2O (50 mL)
was added to the residue. The resultant mixture was extracted with
Et2O (3ꢂ30 mL) and the combined organic extracts were washed
with satd aq NaHCO3 (50 mL), dried and concentrated in vacuo.
Purification via flash column chromatography (eluent Et2O) gave 8
as a white solid (286 mg, 57% from 10, >99:1 dr).
4.1.12. Benzyl (R,R,R)-N(10)-methyl-N(20)-{300-[N-benzyl-N-(
a-meth-
ylbenzyl)amino]-400-[2000,2000-dimethyl-N(3000)-tert-butoxycarbonyl-
4.1.13. (R,R)-N(10)-Methyl-N(20)-(300,600-diamino-500-hydroxyhex-
anoyl)hydrazinylethanoic acid [(þ)-negamycin] 1.
1000,3000-oxazolidin-5000-yl]butanoyl}hydrazinylethanoate 8.
A solution of 8 (1.29 g, 1.92 mmol) in TFA (3 mL) was stirred at
0 ꢁC for 15 min, then allowed to warm to rt and stirred for a further
15 min. A mixture of THF/H2O (1:1, 50 mL) was then added and
stirring was continued at rt for 16 h. The reaction mixture was then
poured into H2O (100 mL) and satd aq NaHCO3 was added until pH
9 was achieved. The aqueous layer was extracted with EtOAc
(3ꢂ30 mL) and the combined organic extracts were dried and
concentrated in vacuo. The residue was dissolved in MeOH (5 mL),
then AcOH (five drops) and Pd(OH)2/C (10% wt, 630 mg) were
added and the resultant mixture was stirred under hydrogen
(5 atm) at rt for 22 h. The reaction mixture was then filtered
through a short plug of Celite (eluent MeOH) and the filtrate was
concentrated in vacuo. Purification via ion-exchange chromatog-
raphy (Amberlite CG-50 resin, eluent satd aq NH4OH) gave
Step 1: LiOH$H2O (2.27 g, 543 mmol) was added to a stirred
solution of 10 (3.06 g, 5.43 mmol) in MeOH/THF/H2O (3:1:1,
180 mL) and the resultant mixture was heated at reflux for 24 h.
The reaction mixture was then allowed to cool to rt and diluted
with H2O (250 mL). HCl (1.0 M, aq) was then added until pH 5 was
reached. The resultant mixture was extracted with Et2O (2ꢂ50 mL),
and the combined organic extracts were dried and concentrated in
vacuo to give 9 as a white solid (>99:1 dr); C29H40N2O5 requires C,
70.1; H, 8.1; N, 5.6%; found C, 70.4; H, 8.1; N, 5.6%; [
a
]
D
21 ꢀ25.4 (c 0.8
(þ)-negamycin 1 as a white solid (331 mg, 71%, >99:1 dr);4 mp
20
in CHCl3); nmax (KBr) 2980, 2935, 1700 (C]O), 1395, 1255, 1170,
1150; dH (200 MHz, CDCl3) 1.49 (CMe3), 1.84e1.91 (1H, m, C(4)HA),
104e110 ꢁC (dec); {lit.1 mp 110e120 ꢁC (dec)}; [
a
]
þ2.7 (c 1.6 in
D
H2O); {lit.1 [
a
]
D
29 þ2.5 (c 2.0 in H2O)}; dH (200 MHz, D2O) 1.46e1.70
1.33e1.77 (10H, m, C(4)HB, C(20)Me2, C(
a
)Me), 2.00e2.52 (2H, m, C
(2)H2), 3.02 (1H, dd, J 9.7, 9.7, C(40)HA), 3.50e3.55 (2H, m, C(3)H, C
(40)HB), 3.68 (2H, s, NCH2Ph), 4.03 (1H, q, J 6.9, C(
)H), 4.07e4.14
(1H, m, C(50)H), 7.27e7.37 (10H, m, Ph); dC (50 MHz, CDCl3) 18.6 (C
)Me), 24.3, 26.4, 27.4, 28.5 (C(20)Me2), 28.7 (CMe3), 35.9 (C(4)), 50.0
(2H, m, C(400)H2), 2.33 (2H, d, J 7.2, C(200)H2), 2.58 (3H, s, NMe), 2.82
(1H, dd, J 13.2, 3.6, C(600)HA), 2.96 (1H, dd, J 13.2, 8.9, C(600)HB), 3.33
(2H, s, C(2)H2), 3.37e3.42 (1H, m, C(300)H), 3.92e3.97 (1H, m, C(500)
H); dC (50 MHz, D2O) 40.3 (C(400)), 41.8 (C(200)), 44.6 (NMe), 45.7 (C
(300)), 45.9 (C(600)), 61.7 (C(2)), 66.4 (C(500)), 171.6 (C(100)), 177.9 (C(1)).
a
(a
(NCH2Ph), 51.3 (C(3)), 57.7 (C(a
)), 52.0, 52.4 (C(40)), 71.2 (C(50)), 79.7,
80.3 (CMe3), 93.6 (C(20)), 127.6, 128.4, 128.8 (o,m,p-Ph), 138.8 (i-Ph),
141.2, 152.0 (NCO), 175.6 (C(1)); m/z (FABþ) 497 ([MþH]þ, 26%), 105
(100%).
4.1.14. 300-Pentyl (3S,5R,
aS)-3-[N-benzyl-N-(a-methylbenzyl)amino]-
4-[20,2’-dimethyl-N(30)-tert-butoxycarbonyl-10,3’-oxazolidin-50-yl]
butanoate 19.
Step 2A: Et3N (120 mg, 1.20 mmol) was added to a stirred so-
lution of 9 (500 mg, 1.00 mmol), 22$TsOH (366 mg, 1.00 mmol) and
HOBt (135 mg, 1.00 mmol) in THF (50 mL) and the resultant mix-
ture was cooled to 0 ꢁC. A solution of DCC (227 mg, 1.00 mmol) in
THF (10 mL) was then added and the resultant mixture was stirred
at 0 ꢁC for 15 min, then allowed to warm to rt and stirred at rt for
another 5 h. The reaction mixture was then concentrated in vacuo.
CH2Cl2 (30 mL) was added to the residue (which caused the for-
mation of white precipitate) and the resultant solution was filtered.
The filtrate was washed with satd aq NaHCO3, dried and concen-
trated in vacuo. Purification via flash column chromatography (el-
uent Et2O) gave 8 as a white solid (558 mg, 79% from 10, >99:1
BuLi (1.3 M in hexanes, 1.20 mL, 1.56 mmol) was added to
stirred solution of (S)-N-benzyl-N-(a-methylbenzyl)amine
a

224
S.G. Davies et al. / Tetrahedron 67 (2011) 216e227
(600 mg, 2.84 mmol) in THF (20 mL) at ꢀ78 ꢁC and the resultant
pink solution was stirred for 15 min. A solution of 12 (474 mg,
1.41 mmol) in THF (5 mL) at ꢀ78 ꢁC was then added dropwise and
the resultant mixture was stirred for 1 h at ꢀ78 ꢁC. Satd aq NH4Cl
(2 mL) was then added and the reaction mixture was poured into
brine (100 mL). The resultant mixture was extracted with Et2O
(3ꢂ30 mL), dried and concentrated in vacuo to give 19 in w95%
diastereoisomeric purity. Purification via flash column chroma-
tography (eluent 30e40 ꢁC petrol/Et2O, 9:1; increased to 30e40 ꢁC
petrol/Et2O, 4:1) gave 19 as a colourless oil (490 mg, 61%, w95%
diastereoisomeric purity); C34H50N2O5 requires C, 72.05; H, 8.9; N,
filtrate was washed with satd aq NaHCO3, then dried and concen-
trated in vacuo. Purification via flash column chromatography (el-
uent Et2O/EtOAc, 9:1) gave 18 as a viscous, colourless oil (233 mg,
45%, w95% diastereoisomeric purity);32 C39H52N4O6 requires C,
69.6; H, 7.8; N, 8.3%; found: C, 69.7; H, 7.9; N, 8.4%; [
a]
25 ꢀ19.5 (c 0.8
D
in CHCl3); nmax (KBr) 2980, 2935, 1745 (C]O), 1695 (C]O), 1395,
1180; dC (50 MHz, CDCl3) 20.7 (C(
27.3 (C(200)MeB), 28.5 (CMe3), 36.8, 37.7 (C(20), C(40)), 43.8, 44.8
(NMe), 50.1, 50.9, 51.0 (C(30)), C(400), NCH2Ph), 57.6 (C(
)), 58.2 (C
a
)Me), 24.2, 25.2 (C(200)MeA), 26.2,
a
(2)), 66.5 (OCH2Ph), 71.7, 72.0 (C(500)), 79.2, 80.0 (CMe3), 93.2 (C(200),
126.7, 126.8, 126.9, 127.4, 128.2, 128.7 (o,m,p-Ph), 135.1, 141.2þ(i-Ph),
143.1, 151.9 (NCO), 169.7, 170.5 (C(10)), 174.8 (C(1)); m/z (CI ) 673
([MþH]þ, 73%), 178 (100%).
4.9%; found C, 71.8; H, 9.0; N, 5.2%; [
a]
25 ꢀ13.5 (c 1.7 in CHCl3); nmax
D
(film) 2980, 1730 (C]O), 1700 (C]O), 1600, 1495, 1455; dH
(500 MHz, DMSO-d6, 363 K) 0.80 (6H, t, J 7.5, OCH(CH2CH3)2), 1.28
(3H, d, J 6.9, C(
a
)Me), 1.40e1.60 (4H, m, OCH(CH2CH3)2), 1.42 (3H, s,
4.1.16. (300S,500R)-N(10)-Methyl-N(20)-(300,600-diamino-500-hydroxyhex-
anoyl)hydrazinylethanoic acid [(þ)-3-epi-negamycin] 2.
C(20)MeA), 1.44 (9H, s, CMe3), 1.45 (9H, s, C(20)MeB), 1.91e1.96 (2H,
m, C(4)H2), 2.18 (2H, dd, J 14.9, 3.3, C(2)HA), 2.53 (2H, dd, J 14.9, 9.1, C
(2)HB), 2.79 (1H, dd, J 9.4, 9.4, C(40)HA), 3.17e3.24 (1H, m, C(3)H),
3.28e3.35 (1H, m, C(40)HB), 3.58 (2H, d, J 15.4, NCHAHBPh), 3.92 (2H,
d, J 15.4, NCHAHBPh), 3.91.3.98 (1H, m, C(
a)H), 4.20e4.26 (1H, m, C
(50)H), 4.60e4.63 (1H, m, OCHEt2), 7.21e7.41 (10H, m, Ph); dC
A solution of 18 (230 mg, 0.340 mmol, w95% diastereoisomeric
purity) in TFA (2 mL) was stirred at 0 ꢁC for 15 min, then allowed to
warm to rt and stirred for a further 15 min. A mixture of THF/H2O
(1:1, 5 mL) was then added and stirring was continued at rt for
16 h. The reaction mixture was then poured into H2O (40 mL) and
satd aq NaHCO3 was added until pH 9 was achieved. The aqueous
layer was extracted with EtOAc (3ꢂ20 mL) and the combined or-
ganic extracts were dried and concentrated in vacuo. The residue
was dissolved in MeOH (4 mL), then AcOH (five drops) and Pd
(OH)2/C (10% wt, 170 mg) were added and the resultant mixture
was stirred under hydrogen (5 atm) at rt for 20 h. The reaction
mixture was then filtered through a short plug of Celite (eluent
MeOH) and the filtrate was concentrated in vacuo. Purification via
ion-exchange chromatography (Amberlite CG-50 resin, eluent satd
(50 MHz, CDCl3) 9.6 (OCH(CH2CH3)2), 20.4, 21.3 (C(a)Me), 24.2, 25.2,
26.3, 27.3 (C(20)Me2), 26.3 (OCH(CH2CH3)2), 28.5 (CMe3), 35.7, 36.5,
37.2 (C(2), C(4)), 50.1, 50.8 (C(40), NCH2Ph), 51.8 (C(3)), 57.9, 58.9 (C
(a
)), 71.8 (C(50)), 76.6 (OCHEt2), 79.2, 79.9 (CMe3), 92.9 (C(20)), 126.7,
126.9, 127.3, 127.4, 128.1, 128.3 (o,m,p-Ph), 141.5, 143.2 (i-Ph), 152.2
(NCO), 172.1 (C(1)); m/z (CIþ) 567 ([MþH]þ, 100%).
4.1.15. Benzyl
(a
(3S,5R,00
a
S)-N(10)-methyl-N(20)-{300-[N-benzyl-N-
-methylbenzyl)amino]-4 -[2000,2000-dimethyl-N(3000)-tert-butox-
ycarbonyl-1000,3000-oxazolidin-5000-yl]butanoyl}hydrazinylethanoate 18.
aq NH4OH) gave (þ)-3-epi-negamycin 2 as a white solid (63 mg.
22
75%, 96:4 dr);4g mp 145e168 ꢁC (dec); [
a
]
þ8.5 (c 0.7 in H2O);
D
25
{lit.4g for enantiomer [
a
]
ꢀ9.4 (c 0.5, H2O); lit.5e for enantiomer
D
[
a
]
23 ꢀ9.9 (c 3.5 in H2O)}; nmax (KBr) 3700, 2000, 1650, 1580, 1400,
D
1315, 1130, 1045; dH (200 MHz, D2O) 1.53e1.77 (2H, m, C(4)H2),
2.31 (1H, dd, J 14.9, 5.6, C(200)HA), 2.42 (1H, dd, J 14.9, 7.5, C(200)HB),
2.58 (3H, s, NMe), 2.82 (1H, dd, J 13.2, 8.4, C(600)HA), 2.97 (1H, dd, J
13.2, 3.2, C(600)HB), 3.35 (2H, s, C(2)H2), 3.46 (1H, m, C(600)HA),
3.91.3.96 (1H, m, C(400)H); dC (50 MHz, D2O) 39.4 (C(500)), 40.2 (C
(200)), 44.6 (NMe), 45.7 (C(600)), 46.5 (C(3)), 63.9 (C(2)), 67.5 (C(5)),
171.2 (C(100)), 177.8 (C(1)).
Step 1: LiOH$H2O (380 mg, 9.10 mmol) was added to a solution of
19 (510 mg, 0.90 mmol, w95% diastereoisomeric purity) in MeOH/
THF/H2O (3:1:1, 40 mL) and the resultant mixture was heated at
reflux for 24 h. The reaction mixture was then allowed to cool to rt
and poured into H2O (75 mL). aq HCl (1.0 M) was added until pH 6
was achieved then the mixture was extracted with Et2O (3ꢂ30 mL).
The combined organic extracts were dried and concentrated in vacuo
4.1.17. 300-Pentyl (R,R)-3-amino-4-[20,2’-dimethyl-N(30)-tert-butox-
ycarbonyl-10,3’-oxazolidin-50-yl]butanoate 23.
to give 37 as a colourless oil (379 mg, 85%, w95% diastereoisomeric
32
purity);
C H40N2O5 requires C, 70.1; H, 8.1; N, 5.6%; found: C, 70.3;
29
25
H, 8.0; N, 5.3%; [
2980, 1700 (C]O), 1455, 1395, 1260, 1170, 1150; dC (50 MHz, CDCl3)
18.1, 18.7 (C(
)Me), 24.2, 25.1 (C(20)MeA), 26.3, 27.3 (C(20)MeB), 28.4
(CMe3), 35.1 (C(2), C(4)), 49.8, 51.1 (C(40), NCH2Ph), 52.1 (C(3)), 59.6 (C
)), 70.6, 71.0 (C(40)), 79.5, 80.2 (CMe3), 93.2 (C(20)), 127.2, 127.7,
a]
D
þ13.4 (c 0.9 in CHCl3); nmax (KBr) 3700, 2200,
a
(a
128.4, 128.5, 128.7, 129.0 (o,m,p-Ph), 137.2, 137.9, 140.2, 140.8 (i-Ph),
151.8 (NCO), 174.4 (C(1)); m/z (CI) 497 ([MþH]þ, 78%), 212 (100%).
Step 2: Et3N (78 mg, 0.78 mmol) was added to a stirred solution
of 37 (382 mg, 0.77 mmol), HOBt (104 mg, 0.77 mL) and 22$TsOH
(282 mg, 0.77 mmol) in THF (30 mL) and the resultant mixture was
cooled to 0 ꢁC. A solution of DCC (174 mg, 0.84 mmol) in THF (5 mL)
was then added. The reaction mixture was stirred at 0 ꢁC for 15 min,
allowed to warm to rt and stirred for a further 6 h then concen-
trated in vacuo. The residue was dissolved in CH2Cl2 (40 mL), the
white precipitate that formed was removed by filtration, and the
AcOH (five drops) and Pd(OH)2/C (10% wt, 1.70 g) were added to
a stirred solution of 10 (5.15 g, 9.70 mmol) in MeOH (10 mL). The
resultant mixture was stirred under hydrogen (6 atm) for 20 h.
Solid NaHCO3 (4.00 g) was then added and the reaction mixture
was filtered through Celite (eluent MeOH). The filtrate was con-
centrated in vacuo and the residue was purified by flash column
chromatography (eluent EtOAc) to give 23 as a colourless oil
(3.31 g, 96%, >99:1 dr); C19H36N2O5 requires C, 61.3; H, 9.7; N, 7.5%;
found C, 61.1; H, 9.8; N, 7.8%; [
a
]
25 ꢀ15.7 (c 0.6 in CHCl3); nmax (film)
D

S.G. Davies et al. / Tetrahedron 67 (2011) 216e227
225
2970, 2940, 2880, 1730 (C]O), 1700 (C]O), 1460, 1395, 1315, 1260,
1175, 1110; dH (200 MHz, CDCl3) 0.88 (6H, t, J 7.4, OCH(CH2CH3)2),
1.47 (9H, s, CMe3), 1.50e1.80 (14H, m, C(4)H2, C(20)Me2, OCH
(CH2CH3)2, NH2), 2.34 (1H, dd, J 15.7, 4.2, C(2)HA), 2.51 (1H, dd, J
15.7, 8.6, C(2)HB), 3.07 (1H, t, J 8.1, C(40)HA), 3.39e3.44 (1H, m, C(3)
H), 3.69 (1H, m, C(40)HB), 4.22 (1H, m, C(50)H), 4.79 (1H, quintet, J
6.0, OCHEt2); dC (50 MHz, CDCl3) 9.5 (OCH(CH2CH3)2) 51.0 (C(4)),
24.2, 25.1, 26.3, 27.2 (C(20)Me2),26.3 (OCH(CH2CH3)2), 28.3 (CMe3),
40.5 (C(40)), 43.2 (C(3)), 70.8, 71.1 (C(50)), 76.7 (OCHEt2), 79.3, 79.9
(CMe3), 92.8, 93.3 (C(20)), 151.7, 152.1 (NCO), 171.9 (C(1)); m/z (CIþ)
373 ([MþH]þ, 100%).
solution was washed sequentially with brine (50 mL) and satd aq
NaHCO3 (50 mL), then dried and concentrated in vacuo. Purification
via recrystallisation (30e40 ꢁC petrol/Et2O) gave 26 as a white solid
(2.00 g, 80%, >99:1 dr); C21H28N2O6 requires C, 62.4; H, 6.9; N, 6.9%;
25
found C, 62.4; H, 6.9; N, 6.75%; mp 100e109 ꢁC; [
a
]
þ6.67 (c
Hg365
0.68 in CHCl3); nmax (KBr) 3320, 3073, 3030, 2960, 1735 (C]O),
1715, 1700 (C]O), 1660 (C]O), 1540, 1260, 965; dH (200 MHz,
CDCl3) 1.45e1.66 (2H, m, C(4)H2),1.67 (1H, s, OH),1.85 (3H, d, J 5.3, C
(60)H3), 2.55 (1H, dd, J 16.2, 5.0, C(2)HA), 2.64 (1H, dd, J 16.2, 5.4, C(2)
HB), 3.13 (1H, dd, J 8.3, 4.5, C(6)HA), 3.69 (3H, s, OMe), 3.60e3.80
(2H, m, C(3)H, C(6)HB), 4.23 (1H, m, C(5)H), 4.31 (1H, d, J 3.0, NH),
5.07e5.14 (2H, m, OCH2Ph), 5.76 (1H, s, NH), 5.81 (1H, d, J 15.2, C(20)
H), 5.88e6.25 (2H, m, C(40)H), 7.14e7.30 (1H, m, C(30)H), 7.28e7.38
(5H, m, Ph); dC (50 MHz, CDCl3) 18.5 (C(60)), 39.8, 39.6 (C(2), C(4)),
44.9, 44.8 (C(3), C(5)), 51.8 (C(6)), 67.3, 67.1 (OMe, OCH2Ph), 121.4 (C
(20)), 128.5, 128.2, 128.0 (o,m,p-Ph), 129.6 (C(50)), 136.1 (i-Ph), 137.9
(C(40)),141.3 (C(30)),157.3 (NCO), 166.8 (C(10)),171.9 (C(1)); m/z (CIþ)
405 ([MþH]þ, 93%), 222 (100%).
4.1.18. 300-Pentyl (R,R)-3-(N-benzyloxycarbonylamino)-4-[N(30)-tert-
butoxycarbonyl-20,2’-dimethyl-10,3’-oxazolidin-50-yl]butanoate 24.
4.1.20. Methyl
(R,R,E,E)-3-(N-benzyloxycarbonylamino)-4-[20,2’-di-
methyl-N(30)-(200,400-hexadienoyl)- 10,3’-oxazolidin-50-yl]butanoate 27.
A solution of NaHCO3 (790 mg, 9.40 mol) and NaCl (4.00 g) in
H2O (30 mL) was added to a solution of 23 (3.20 g, 8.60 mmol) in
CHCl3 (50 mL).
A solution of benzyl chloroformate (1.54 g,
9.00 mmol) in CH2Cl2 (3 mL) was then added and the resultant
mixture was stirred at rt for 45 min. The aqueous layer was
extracted with Et2O (2ꢂ30 mL) and the combined organic extracts
were dried and concentrated in vacuo. Purification via flash column
chromatography (eluent Et2O) gave 24 as a colourless oil (3.55 g,
82%, >99:1 dr); C27H42N2O7 requires C, 64.0; H, 8.4; N, 5.5%; found
TsOH (10 mg) was added to a stirred solution of 26 (1.67 mg,
4.13 mmol) in acetone and 2,2-dimethoxypropane (1:1, 50 mL)
and the resultant mixture was stirred at rt for 26 h. Et3N (five
drops) was added and the reaction mixture was concentrated in
vacuo to give a pale yellow oil. Purification via filtration through
a short plug of silica gel (eluent EtOAc) gave 27 as pale yellow oil,
which was used immediately in the next step (1.47 g, 80%, >99:1
dr); dH (200 MHz, CDCl3) 1.53 (3H, s, C(20)MeA), 1.64 (3H, s, C(20)
MeB), 1.80e2.12 (2H, m, C(4)H2), 1.83 (3H, d, J 5.3, C(60)H3), 2.69
(2H, m, C(2)H2), 3.25 (1H, dd, J 9.5, 9.5, C(40)HA), 3.67 (3H, s, OMe),
3.52e3.82 (1H, m, C(3)H), 4.22 (2H, m, C(40)HB, C(50)H), 5.00 (2H,
m, OCH2Ph), 5.56 (1H, d, J 9.5, NH), 5.95 (1H, d, J 14.2, C(200)H),
6.00e6.26 (2H, m, C(400)H, C(500)H), 7.20e7.33 (1H, m, C(300)H),
7.25e7.35 (5H, m, Ph).
25
C, 64.1; H, 8.7; N, 5.5; [
a
]
ꢀ1.90 (c 0.6 in CHCl3);
nmax (film)
Hg365
3320, 2980, 2940, 2880, 1730 (C]O), 1700 (C]O), 1530, 1390, 1175,
1105, 1055; dH (200 MHz, CDCl3) 0.86 (6H, t, J 7.2, OCH(CH2CH3)2),
1.40e1.65 (19H, m, C(20)Me2, OCH(CH2CH3)2, CMe3), 1.68e1.97 (2H,
m, C(4)H2), 2.58e2.69 (2H, m, C(2)H2), 3.04 (1H, app t, J 9.6, C(40)
HA), 3.60e3.69 (1H, m, C(40)HB), 4.08e4.21 (2H, m, C(3)H, C(50)H),
4.77 (1H, quintet, J 6.1, OCHEt2), 5.09 (2H, s, OCH2Ph), 5.58 (1H, br s,
NH), 7.27e7.36 (5H, m, Ph); dC (50 MHz, CDCl3) 9.6 (OCH(CH2CH3)2),
24.2, 25.1, 26.3, 27.2 (C(20)Me2), 26.3 (OCH(CH2CH3)2), 28.4 (CMe3),
37.0 (C(2), C(4)), 38.8, 46.1 (C(3)), 50.9 (C(40)), 66.5 (OCH2Ph), 70.9,
71.2 (C(50)), 77.1 (OCHEt2), 79.4, 80.0 (CMe3), 93.1, 93.5 (C(20)), 127.9,
128.1, 128.4 (o,m,p-Ph), 136.5 (i-Ph), 151.8, 152.1, 155.6 (2ꢂNCO),
171.3 (C(1)); m/z (CIþ) 507 ([MþH]þ, 7%), 299 (100%).
4.1.21. (R,R,E,E)-3-(N-Benzyloxycarbonylamino)-4-[N(30)-(200,400-hex-
adienoyl)-20,2’-dimethyl-10,3’-oxazolidin-50-yl]butanoic acid 28.
4.1.19. Methyl (R,R,E,E)-3-(N-benzyloxycarbonylamino)-5-hydroxy-
6-[N-(20,4’-hexadienoyl)amino]hexanoate 26.
HCl gas was bubbled through
a solution of 24 (3.29 g,
6.50 mmol) in MeOH (150 mL) at 0 ꢁC for 1 min and the resultant
mixture was stirred at 0 ꢁC for 30 min then concentrated in vacuo to
give 25$HCl as a white solid (2.10 g, 93%, >99:1 dr). This residue
was dissolved in CH2Cl2 (100 mL), treated with Et3N (1.97 g,
19.5 mmol) and cooled to 0 ꢁC. A solution of freshly distilled sorbyl
chloride (1.19 g, 9.10 mmol) in CH2Cl2 (2 mL) was then added
dropwise. After 1 h the reaction mixture was concentrated in vacuo.
The residue was dissolved in EtOAc (100 mL) and the resultant
A solution of NaOH (460 mg, 11.5 mol) in H2O (2 mL) was added
to a solution of 27 (1.30 g, 2.87 mmol) in MeOH/THF (2:1, 40 mL) at
0 ꢁC. The reaction mixture was then allowed to warm to rt and
stirred for 4 h before being concentrated in vacuo. The residue was
dissolved in H2O (40 mL) and the resultant solution was acidified to
pH 4 by the addition of aq KHSO4 then extracted with EtOAc
(3ꢂ30 mL). The combined organic extracts were dried and con-
centrated in vacuo. Purification via flash column chromatography

226
S.G. Davies et al. / Tetrahedron 67 (2011) 216e227
6. Kondo, S.; Shibahara, S.; Talraheshi, S.; Maeda, K.; Umezawa, H.; Ohno, M. J. Am.
Chem. Soc. 1971, 93, 6305.
7. Shibahara, S.; Kondo, S.; Maeda, K.; Umezawa, H.; Ohno, M. J. Am. Chem. Soc.
(eluent CH2Cl2/MeOH, 4:1) gave 28 as a white solid (1.11 g, 88%,
>99:1 dr); C23H30N2O6 requires C, 64.2; H, 7.0; N, 6.5%; found C,
64.5; H, 6.85; N, 6.2%; [
a]
25 þ17.2 (c 1.1 in CHCl3); nmax (KBr) 3700,
D
1972, 94, 4353.
8. (a) Harada, S.; Ono, H. Eur. Patent Appl., 1986, EP 206068. (b) Katayama, N.;
Nozaki, Y.; Tsubotani, S.; Kondo, M.; Harada, S.; Ono, H. J. Antibiot. 1992, 45, 10.
9. Hida, T.; Tsubotani, S.; Funabashi, Y.; Ono, H.; Harada, S. Bull. Chem. Soc. Jpn.
1993, 66, 863.
3200, 2980, 2940, 1740 (C]O), 1705 (C]O), 1650, 1625, 1595, 1425,
1245, 1150, 1060, 1000; dH (200 MHz, CDCl3) 1.55 (3H, s, C(20)MeA),
1.63 (3H, s, C(20)MeB), 1.70e2.07 (2H, m, C(4)H2), 1.84 (3H, d, J 4.9, C
(600)H3), 2.62e2.69 (2H, m, C(2)H2), 3.23 (1H, dd, J 8.8, 8.8, C(40)HA),
3.72 (1H, m, C(3)H), 4.19 (2H, m, C(40)HB, C(50)H), 5.10 (2H, m,
OCH2Ph), 5.72 (1H, br s, NH), 5.93 (1H, d, J 14.8, C(200)H), 6.00e6.23
(2H, m, C(400)H, C(500)H), 7.18e7.33 (1H, m, C(300)H), 7.26e7.37 (5H,
m, Ph); m/z (CIþ) 431 ([MþH]þ, 36%), 373 (100%).
10. Davies, S. G.; Icihara, O. Tetrahedron Lett. 1999, 40, 9313.
11. (a) Davies, S. G.; Kelly, R. J.; Price Mortimer, A. J. Chem. Commun. 2003, 2132; (b)
Davies, S. G.; Haggitt, J. R.; Ichihara, O.; Kelly, R. J.; Leech, M. A.; Price Mortimer,
A. J.; Roberts, P. M.; Smith, A. D. Org. Biomol. Chem. 2004, 2, 2630.
12. For a review of the synthetic applications of lithium amides, see: Davies, S. G.;
Smith, A. D.; Price, P. D. Tetrahedron: Asymmetry 2005, 16, 2833.
13. (a) Bunnage, M. E.; Davies, S. G.; Goodwin, C. J.; Ichihara, O. Tetrahedron 1994,
50, 3975; (b) Chippindale, A. M.; Davies, S. G.; Iwamoto, K.; Parkin, R. M.;
Smethurst, C. A. P.; Smith, A. D.; Rodriguez-Solla, H. Tetrahedron 2003, 59, 3253;
(c) Abraham, E.; Candela-Lena, J. I.; Davies, S. G.; Georgiou, M.; Nicholson, R. L.;
4.1.22. 20-Amidinoethyl
(R,R,E,E)-3-amino-5-hydroxy-6-[N-(200,400-
hexadienoyl)amino]butanamide [sperabillin C] 6.
ꢀ
ꢀ
Roberts, P. M.; Russell, A. J.; Sanchez-Fernandez, E. M.; Smith, A. D.; Thomson, J.
E. Tetrahedron: Asymmetry 2007, 18, 2510; (d) Abraham, E.; Davies, S. G.; Mil-
lican, N. L.; Nicholson, R. L.; Roberts, P. M.; Smith, A. D. Org. Biomol. Chem. 2008,
6, 1655; (e) Abraham, E.; Brock, E. A.; Candela-Lena, J. I.; Davies, S. G.; Georgiou,
OH
NH2
O
NH
H
N
Me
N
H
NH
ꢀ
M.; Nicholson, R. L.; Perkins, J. H.; Roberts, P. M.; Russell, A. J.; Sanchez-
O
ꢀ
Fernandez, E. M.; Scott, P. M.; Smith, A. D.; Thomson, J. E. Org. Biomol. Chem.
2008, 6, 1665; (f) Davies, S. G.; Fletcher, A. M.; Roberts, P. M.; Smith, A. D.
Tetrahedron 2009, 65, 10192; (g) Davies, S. G.; Hughes, D. G.; Price, P. D.; Roberts,
P. M.; Russell, A. J.; Smith, A. D.; Thomson, J. E.; Williams, O. M. H. Synlett 2010,
567. See also Refs. 4h and 11.
Step 1: HOBt (320 mg, 2.37 mmol) and DCC (530 mg, 2.57 mmol)
were added to a solution of 29$2HCl11b (850 mg, 1.98 mmol) in THF
(15 mL) and the resultant mixture was stirred at rt for 2 h. The
reaction mixture was filtered then 28 (350 mg, 2.19 mmol) and satd
aq NaHCO3 (4.36 mmol, 2 mL) were added. The resultant mixture
was stirred at rt for 31 h then concentrated in vacuo. The residue
was dissolved in CHCl3/EtOH (3:1, 50 mL), dried and concentrated
in vacuo to give 30.
14. For instance, see: (a) Bailey, S.; Davies, S. G.; Smith, A. D.; Withey, J. M. Chem.
Commun. 2002, 2910; (b)Aye, Y.; Davies, S. G.;Garner, A. C.;Roberts, P. M.; Smith, A.
D.; Thomson, J. E. Org. Biomol. Chem. 2008, 6, 2195; (c) Abraham, E.; Davies, S. G.;
Docherty, A. J.; Ling, K. B.; Roberts, P. M.; Russell, A. J.; Thomson, J. E.; Toms, S. M.
Tetrahedron: Asymmetry 2008, 19, 1356 and references cited therein.
15. (a) Davies, S. G.; Sheppard, R. L.; Smith, A. D.; Thomson, J. E. Chem. Commun.
2005, 3802; (b) Davies, S. G.; Russell, A. J.; Sheppard, R. L.; Smith, A. D.;
Thomson, J. E. Org. Biomol. Chem. 2007, 5, 3190; (c) Davies, S. G.; Garner, A. C.;
Goddard, E. C.; Kruchinin, D.; Roberts, P. M.; Smith, A. D.; Rodriguez-Solla, H.;
Thomson, J. E.; Toms, S. M. Org. Biomol. Chem. 2007, 5, 1961; (d) Cailleau, T.;
Cooke, J. W. B.; Davies, S. G.; Ling, K. B.; Naylor, A.; Nicholson, R. L.; Price, P. D.;
Roberts, P. M.; Russell, A. J.; Smith, A. D.; Thomson, J. E. Org. Biomol. Chem.
2007, 5, 3922; (e) Davies, S. G.; Mujtaba, N.; Roberts, P. M.; Smith, A. D.;
Thomson, J. E. Org. Lett. 2009, 11, 1959; (f) Davies, S. G.; Nicholson, R. L.; Price,
P. D.; Roberts, P. M.; Savory, E. D.; Smith, A. D.; Thomson, J. E. Tetrahedron:
Asymmetry 2009, 20, 756; (g) Davies, S. G.; Garner, A. C.; Nicholson, R. L.;
Osborne, J.; Roberts, P. M.; Savory, E. D.; Smith, A. D.; Thomson, J. E. Org. Bi-
omol. Chem. 2009, 7, 2604; (h) Bentley, S. A.; Davies, S. G.; Lee, J. A.; Roberts, P.
M.; Russell, A. J.; Thomson, J. E.; Toms, S. M. Tetrahedron 2010, 66, 4604; (i)
Abraham, E.; Bailey, C. W.; Claridge, T. D. W.; Davies, S. G.; Ling, K. B.; Odell, B.;
Rees, T. L.; Roberts, P. M.; Russell, A. J.; Smith, A. D.; Smith, L. J.; Storr, H. R.;
Sweet, M. J.; Thompson, A. L.; Thomson, J. E.; Tranter, G. E.; Watkin, D. J. Tet-
rahedron: Asymmetry 2010, 21, 1797.
Step 2: TMSI (1.1 mL, 7.9 mmol) was added to a suspension of 30
in MeCN (40 mL) at rt. The resultant solution was stirred for 3.5 h
then concentrated in vacuo. The residue was dissolved in H2O
(20 mL), washed with Et2O (10 mL) and concentrated in vacuo. Pu-
rification via ion-exchange chromatography (Amberlite XAD-II
resin, eluent H2O) gave sperabillin C 6 as a pale yellowsolid (420 mg,
51% from 28, >99:1 dr);9 [
0.7 in H2O)};
a
]
25 ꢀ10.2 (c 0.3 in H2O); {lit.9 [
a
]
25 ꢀ11 (c
D
D
n
max (KBr) 3700, 2650, 2650, 2050, 1655, 1635, 1540,
1435, 1160, 1090; dH (200 MHz, D2O) 1.46e1.74 (2H, m, C(4)H), 1.57
(3H, d, J 5.3, C(600)H3), 2.46e2.53 (4H, m, C(2)H2, C(20)H2), 2.97e3.34
(4H, m, C(6)H2, C(10)H2), 3.71e3.75 (1H, m, C(5)H), 3.82e3.88 (1H, m,
C(3)H), 5.72 (1H, d, J 15.9, C(200)H), 6.00e6.04 (2H, m, C(400)H, C(500)H),
6.86 (1H, dd, J 9.5,14.8, C(300)H); dC (50 MHz, D2O) 18.5 (C(600)), 33.1 (C
(20)), 36.0 (C(4)), 37.0 (C(10)), 37.6 (C(2)), 45.7 (C(6)), 47.0 (C(3)), 67.0
(C(5)), 121.0 (C(200)), 130.1 (C(400)), 141.2 (þC(500)), 143.4 (C(300)), 170.6,
172.8, 175.4 (C(1), C(30), C(100)); m/z (FAB ) 326 ([MþH]þ, 24%), 242
(100%).
16. We have previously shown that the doubly diastereoselective conjugate addi-
tion of lithium N-benzyl-N-(
a-methylbenzyl)amide to a range of chiral
a,b
-unsaturated carbonyl compounds proceeds under the dominant stereo-
control of the lithium amide, for instance see: (a) Davies, S. G.; Hermann, G. J.;
Sweet, M. J.; Smith, A. D. Chem. Commun. 2004, 1128; (b) Davies, S. G.; Fletcher,
A. M.; Hermann, G. J.; Poce, G.; Roberts, P. M.; Smith, A. D.; Sweet, M. J.;
Thomson, J. E. Tetrahedron: Asymmetry 2010, 21, 1635. See also Ref. 15d.
17. For reviews concerning the phenomenon of double asymmetric induction, see:
(a) Masamune, S.; Choy, W.; Petersen, J. S.; Sita, L. R. Angew. Chem., Int. Ed. Engl.
1985, 24, 1; (b) Kolodiazhnyi, O. I. Tetrahedron 2003, 59, 5953.
References and notes
18. For the utility of 3-pentyl esters in synthesis see, for example: (a) Davies, S. G.;
ꢀ
Dõez, D.; Dominguez, S. H.; Garrido, N. M.; Kruchinin, D.; Price, P. D.; Smith, A.
1. Hamada, M.; Takeuchi, T.; Kondo, S.; Ikeda, Y.; Naganawa, H.; Maeda, K.; Okami,
Y.; Umezawa, H. J. Antibiot. 1970, 23, 170.
2. (a) Mizuno, S.; Nitta, K.; Umezawa, H. J. Antibiot. 1970, 23, 589; (b) Uehara, Y.;
Kondo, S.; Umezawa, H.; Suzukalre, K.; Hori, M. J. Antibiot. 1972, 25, 685.
3. Mizuno, S.; Nitta, K.; Umezawa, H. J. Antibiot. 1970, 23, 581.
D. Org. Biomol. Chem. 2005, 3, 1284; (b) Karlstroem, A.; Unden, A. Int. J. Pept.
Protein Res. 1996, 48, 305.
19. Kitamura, M.; Ohkuma, T.; Takaya, H.; Noyori, R. Tetrahedron Lett. 1988, 29, 1555.
20. The enantiomeric purity of (R)-16 was calculated by reference to literature data
21
21
f[a
]
þ20.7 (c 7.3 in CHCl3); lit.19 for 97% ee [
a
]
þ20.9 (c 7.7 in CHCl3)g. The
D
D
4. (a) Wang, Y.-F.; Izawa, T.; Kibayashi, S.; Ohno, M. J. Am. Chem. Soc. 1982, 104,
6465; (b) Iida, H.; Kasahara, K.; Kibayashi, C. J. Am. Chem. Soc. 1986, 108, 4647;
(c) Tanner, D.; Somfai, P. Tetrahedron Lett. 1988, 29, 2373; (d) De Bernado, S.;
Tengi, J. P.; Sasso, G.; Weigele, M. Tetrahedron Lett. 1988, 29, 4077; (e) Kasahara,
enantiomeric purities of compounds 12 and 21 were inferred from that of (R)-
16.
21. Costello, J. F.; Davies, S. G.; Ichihara, O. Tetrahedron: Asymmetry 1994, 5, 1999.
22. For previous syntheses of epi-negamycin 2, see Refs. 4e,g,5e. For previous
syntheses of epi-negamycin trifluoroacetic acid salt 2$TFA, see Ref. 4l.
23. Line broadening in the 1H NMR spectra of the crude reaction mixtures and the
isolated products precluded a more accurate determination of the reaction
diastereoselectivity and/or product diastereoisomeric ratio.
24. In the case of the Hegedus synthesis a common intermediate was also derivatised
to (þ)-negamycin 1, and in the case of the I.C.I. Pharma synthesis the C(3)-ster-
eogenic centrewithin (ꢀ)-5-epi-negamycin 2 was derived from (R)-aspartic acid;
their stereochemical assignments are therefore secure.
€
K.; Iida, H.; Kibayashi, C. J. Org. Chem. 1989, 54, 2225; (f) Schmidt, U.; Stabler, F.;
Lieberkneht, A. Synthesis 1992, 482; (g) Masters, J. J.; Hegedus, L. S. J. Org. Chem.
1993, 58, 4547; (h) Davies, S. G.; Ichihara, O. Tetrahedron: Asymmetry 1996, 7,
1919; (i) Jain, R. P.; Williams, R. M. J. Org. Chem. 2002, 67, 6361; (j) Naidu, S. V.;
Kumar, P. Tetrahedron Lett. 2007, 48, 3793; (k) Hayashi, Y.; Regnier, T.; Nishi-
guchi, S.; Sydnes, M. O.; Hashimoto, D.; Hasegawa, J.; Katoh, T.; Kajimoto, T.;
Shiozuka, M.; Matsuda, R.; Nodec, M.; Kiso, Y. Chem. Commun. 2008, 2379; (l)
Nishiguchi, S.; Sydnes, M. O.; Taguchi, A.; Regnier, T.; Kajimoto, T.; Node, M.;
Yamazaki, Y.; Yakushiji, F.; Kiso, Y.; Hayashi, Y. Tetrahedron 2010, 66, 314.
5. For the syntheses of related structures, see: (a) Hashiguchi, S.; Kawada, A.;
Natsugari, H. J. Chem. Soc., Perkin Trans. 1 1991, 2435; (b) Socha, D.; Jurczak, M.;
Chmlelewski, M. Tetrahedron Lett. 1995, 36, 135; (c) Shimizu, M.; Morita, A.;
Fujisawa, T. Chem. Lett. 1998, 467; (d) Allmendinger, L.; Bauschke, G.; Paintner,
F. F. Synlett 2005, 2615; (e) I.C.I. Pharma, French Patent 2433508 A1 19800314,
1980.
25. Hashiguchi, S.; Kawada, A.; Natsugari, H. Synthesis 1992, 403.
26. 3-Aminopropanamidine dihydrochloride 29$2HCl was prepared in 25% overall
yield from commercially available 3-aminopropionitrile fumarate via N-tosyl
protection, ethanolysis, treatment with ammonia and deprotection. For a sim-
ꢀ
ꢀ
ilar procedure, see: Pierdet, A.; Nedelec, L.; Delaroff, V.; Allais, A. Tetrahedron
1980, 36, 1763.

S.G. Davies et al. / Tetrahedron 67 (2011) 216e227
227
27. Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.; Timmers, F. J.
Organometallics 1996, 15, 1518.
30. Carmi, A.; Pollak, G.; Yellin, H. J. Org. Chem. 1960, 25, 44.
31. Streicher, W.; Reinshagen, H. Chem. Ber. 1975, 108, 813.
28. Stephensˇ on, T. A.; Wilkinson, G. J. Inorg. Nucl. Chem. 1966, 28, 945.
32. The signals in the 1H NMR spectrum of this compound were too broad to be
assigned.
29. Larcheveque, M.; Henrot, S. Tetrahedron 1990, 46, 4277.