Synthesis of the 'northern-hemisphere' fragments of the thiopeptide antibiotic nosiheptide

Article abstract of DOI:10.1055/s-2006-951502

The northern-hemisphere fragments 4 and 5 of the thiopeptide antibiotic nosiheptide have been synthesized from Boc-Glu-OBn 7 and Boc-Thr 12 in 21.8% and 16.8% overall yields, respectively. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2006-951502

LETTER
3033
Synthesis of the ‘Northern-Hemisphere’ Fragments of the Thiopeptide
Antibiotic Nosiheptide
Thiopeptide
An
a
tibiotic No
h
a
Department of Chemistry, University of Exeter, Stocker Road, Exeter EX4 4QD, UK
b
School of Chemistry, University of Nottingham, University Park, Nottingham NG7 2RD, UK
Fax +44(115)9513564; E-mail: c.j.moody@nottingham.ac.uk
Received 23 March 2006
Dedicated to Professor Richard Heck in recognition of his outstanding contributions to organic synthesis
Nosiheptide has traditionally been regarded as comprising
six fragments (Figure 1) – dehydroalanine and fragments
A (2,3,5,6-tetrasubstituted pyridine), B (threonine), C
(threonine–cysteine-derived propenylthiazole), D (modi-
fied glutamate) and E (2,3,4-trisubstituted indole). Al-
though nosiheptide has yet to succumb to total synthesis,
routes to various fragments have been described, includ-
ing the indole fragment E,10–12 the modified glutamate
fragment D,13–15 and the pyridine fragment A.16,17 The
synthesis of a potential precursor to the B-C-D-fragment
has also been described,14 although in many of these ex-
amples, the use of non-orthogonal protecting groups
would appear to preclude their use in any total-synthesis
campaign. In continuation of our interest in the synthesis
of the thiopeptide antibiotics,18–21 including nosiheptide,12
we now report concise routes to the ‘northern-hemi-
sphere’ fragments of the antibiotic.
Abstract: The northern-hemisphere fragments 4 and 5 of the thio-
peptide antibiotic nosiheptide have been synthesized from Boc-Glu-
OBn 7 and Boc-Thr 12 in 21.8% and 16.8% overall yields, respec-
tively.
Key words: thiazole, amino acids, heterocycles, antibiotics
The antibiotic nosiheptide 1 (RP9671), originally isolated
from Streptomyces actuous 40037 in the early 1960s by
French workers,1,2 with its structure determined by chem-
ical degradation3 and X-ray crystallography,4,5 is charac-
terized by the presence of seven heterocyclic rings (five
thiazoles, one indole, one pyridine) in a double macrocy-
clic array. Nosiheptide is a member of the thiopeptide an-
tibiotics, a growing class of sulfur-rich modified cyclic
peptides,6 and has been subject of detailed biosynthetic
studies which establish the origin of the heterocyclic rings
from modification of the amino acid side-chains with cy-
clization.7 Although none of the thiopeptides are used
clinically as yet, nosiheptide is in commercial use as a
feed additive to increase weight gain in poultry and
pigs.8,9
Our overall plan for the synthesis of nosiheptide (1) is
shown in Scheme 1, and involves formation of the macro-
lactone/thiolactone from a suitable indole 2, previously
synthesized in our laboratory,12 and macrocyclic pyridine
fragment 3, which in turn could be derived from the
‘northern-hemisphere’ fragments 4 and 5 and the pyridine
6 by two amide couplings and one Pd-catalyzed biaryl
formation (Scheme 1).
Me
fragment C
H
N
O
H
O
N
The modified glutamate 4, corresponding to fragment D
in the original analysis, has been synthesized on four pre-
vious occasions: from D-glucose (ca. 12 steps),13 from
(S)-2,2-dimethyl-1,3-dioxolane-4-acetaldehyde (10 steps,
overall yield ca. 11%),15 from (S)-pyroglutamic acid
(11 steps, overall yield ca. 2%),14 or from D-glyceralde-
hyde (11 steps, overall yield ca. 8%).14 Our route that
starts from commercially available N-Boc-glutamic acid
benzyl ester 7 delivers the required fragment in just seven
steps in an overall yield of 22%, and importantly, with
orthogonal protecting groups on the two carboxyl,
hydroxyl and amino groups (Scheme 2).
fragment B
OH
fragment D
OH HN
S
HN
H
O
Me
O
N
S
O
O
S
Me
NH
N
fragment A
OH
N
S
H
S
N
O
N
N
H
O
fragment E
S
O
HN
nosiheptide
NH2
(1)
dehydroAla
CH2
Thus glutamate 7 was esterified using methyl chlorofor-
mate,22 and then subjected to the excellent Hanessian pro-
tocol for the stereocontrolled hydroxylation of the
glutamate dianion, formed upon treatment with LiHMDS
(2 equiv), with the Davis oxaziridine.23,24 The resulting 4-
hydroxy compound was not purified, but immediately
converted into its TBS-ether 9, which after purification
was obtained as a single diastereomer in 60% yield over
Figure 1
SYNLETT 2006, No. 18, pp 3033–3036
Advanced online publication: 25.10.2006
DOI: 10.1055/s-2006-951502; Art ID: S03706ST
© Georg Thieme Verlag Stuttgart · New York
0
3
.1
1
.2
0
0
6

3034
T. Belhadj et al.
LETTER
NHBoc
OBn
Me
NHBoc
OBn
MeO2CCl, Et3N
O
O
H
N
DMAP, CH2Cl2
78%
O
H
O
N
ester
OH
O
OMe
O
formation
7
8
S
OH HN
OH
Me
‘northern
hemisphere’
HN
H
O
O
O
N
i. LiHMDS
THF, –78 °C
S
O
S
Me
Davis oxaziridine
ii. TBSOTf
2,6-lut, CH2Cl2
60% (2 steps)
NH
N
N
S
H
OH
attach dehydro
amino acid
S
N
O
N
O
N
H
O
O
thioester
formation
i. TFA, CH2Cl2
ii. basify
NH
H
TBSO
OMe
NHBoc
OBn
S
OBn
O
HN
TBSO
ca. 100%
NH2
H
nosiheptide
O
O
(1)
CH2
9
10
i. H2, cat Pd/C
MeOH, 96%
ii. EtO2CCl, Et3N
NH4OH, 74%
iii. LR, THF
Me
H
N
O
amide
formation
O
N
S
H
77%
OR HN
OR
HN
H
O
Me
O
N
S
O
NH
TBSO
O
NHBoc
NH2
OMe
S
Pd(0)
coupling
N
RO
N
amide
formation
Me
OMe
S
O
H
OR
CO2tBu
SR
S
11
N
Br
CO2R
i. KHCO3
DME, –15 °C
ii. TFAA
2,6-lut, DME
85%
N
3
N
O
S
H
TBSO
OMe
NHBoc
2
OR
Me
O
N
H
t
CO2 Bu
TBSO
O
NHBoc
N
O
H
S
N
O
N
4
MeO2C
S
OTBS
Me
OtBu
HN
OMe
S
H
Scheme 2 LR = Lawesson’s reagent
4
S
O
NHR
N
X
The synthesis of fragment 5 started with N-Boc-threonine
12, readily converted into the known carboxamide 13 in
good yield (Scheme 3). Protection of the hydroxyl group
as its 4-methoxybenzyl (PMB) ether 14 using 2-tert-bu-
tylimino-2-diethylamino-1,3-dimethylperhydro-1,3,2-di-
azaphosphorine (BEMP) as base, was followed by
conversion into the thioamide 15 and Hantzsch reaction
with methyl bromopyruvate to give the thiazole 16.30
Treatment of 16 with TFA in dichloromethane cleaved
both Boc and PMB protecting groups, and the resulting
amine trifluoroacetate salt was coupled to N-Boc-O-tert-
butyldimethylsilylthreonine (17)31 to give the dipeptide
18. The unprotected threonine side-chain underwent de-
hydration upon treatment with methanesulfonyl chloride
and triethylamine, followed by DBU32 to give the desired
Z-alkene 19 in good yield, the stereochemistry of the
double bond being confirmed by NOE studies. Finally,
selective removal of the N-Boc-group in TFA–dichloro-
methane was followed by coupling to 2-bromothiazole-4-
carboxylic acid33 to give the complete fragment 534
(Scheme 3) in an overall yield of 16.8% over the nine
steps.
5
Br
N
H
S
OR
N
SR
N
O
S
6
OR
Scheme 1
the two steps.25 That the hydroxyl group had indeed been
introduced with the desired S-stereochemistry was con-
firmed by removal of the Boc protecting group under
acidic conditions, basification and cyclization to the pyro-
glutamate derivative 10. NMR studies on compound 10
established that the hydrogens at C-2 and C-4 are syn to
each other. In order to complete the synthesis, the benzyl
ester in glutamate 9 was removed by hydrogenolysis, and
the resulting acid converted into the thioamide 11 by treat-
ment of the corresponding carboxamide with Lawesson’s
reagent (LR). Finally, Hantzsch reaction with tert-butyl
bromopyruvate,26 under the modified conditions to pre-
vent racemization,27 gave the orthogonally protected
glutamate fragment 4 (Scheme 2).28,29
Synlett 2006, No. 18, 3033–3036 © Thieme Stuttgart · New York

LETTER
Thiopeptide Antibiotic Nosiheptide
3035
HO
Me
i-BuO2CCl, NMM
References and Notes
RO
Me
NH4OH
(1) Rhone-Poulenc, S. A. FR 1392453, 1961.
HO2C
H
NHBoc
98%
H2NOC
NHBoc
(2) Benazet, F.; Cartier, M.; Florent, J.; Godard, C.; Jung, G.;
Lunel, J.; Mancy, D.; Pascal, C.; Renaut, J.; Tarridec, P.;
Theilleux, J.; Tissier, R.; Dubost, M.; Ninet, L. Experientia
1980, 36.
(3) Depaire, H.; Thomas, J.-P.; Brun, A.; Lukacs, G.
Tetrahedron Lett. 1977, 1365.
(4) Prange, T.; Ducruix, S.; Pascard, C.; Lunel, J. Nature
(London) 1977, 265, 189.
(5) Pascard, C.; Ducroix, A.; Lunel, J.; Prange, T. J. Am. Chem.
Soc. 1977, 99, 6418.
(6) Bagley, M. C.; Dale, J. W.; Merritt, E. A.; Xiong, X. Chem.
Rev. 2005, 105, 685.
(7) Mocek, U.; Knaggs, A. R.; Tsuchiya, R.; Nguyen, T.; Beale,
J. M.; Floss, H. G. J. Am. Chem. Soc. 1993, 115, 7557; and
references therein.
H
12
PMBBr
13 R = H
BEMP
MeCN
60%
14 R = PMB
LR, THF
77%
PMBO
Me
O
CO2Me
PMBO
Me
Br
N
S
NHBoc
H
i. KHCO3
DME, –15 °C
ii. TFAA
2,6-lut, DME
74%
MeO2C
H2NSC
NHBoc
H
16
15
(8) Casteels, M.; Bekaert, H.; Buysse, F. X. Rev. Agric. 1980,
33, 1069.
HO2C
i. TFA, CH2Cl2
ii. PyBOP, CH2Cl2
i-Pr2NEt, 71%
H
(9) Horii, S.; Oku, N. J. AOAC Int. 2000, 83, 17.
(10) Koerber-Plé, K.; Massiot, G. Synlett 1994, 759.
(11) Shin, C.; Yamada, Y.; Hayashi, K.; Yonezawa, Y.;
Umemura, K.; Tanji, T.; Yoshimura, J. Heterocycles 1996,
43, 891.
(12) Bentley, D. J.; Fairhurst, J.; Gallagher, P. T.; Manteuffel, A.
K.; Moody, C. J.; Pinder, J. L. Org. Biomol. Chem. 2004, 2,
701.
(13) Iwakawa, M.; Kobayashi, Y.; Ikuta, S.; Yoshimura, J. Chem.
Lett. 1982, 1975.
(14) Shin, C.; Nakamura, Y.; Yamada, Y.; Yonezawa, Y.;
Umemura, K.; Yoshimura, J. Bull. Chem. Soc. Jpn. 1995, 68,
3151.
(15) Umemura, K.; Tate, T.; Yamaura, M.; Yoshimura, J.;
Yonezawa, Y.; Shin, C. Synthesis 1995, 1423.
(16) Umemura, K.; Noda, H.; Yoshimura, J.; Konn, A.;
Yonezawa, Y.; Shin, C. G. Tetrahedron Lett. 1997, 38, 3539.
(17) Umemura, K.; Noda, H.; Yoshimura, J.; Konn, A.;
Yonezawa, Y.; Shin, C. G. Bull. Chem. Soc. Jpn. 1998, 71,
1391.
OTBS
BocHN
H
Me
17
Me
H
HO
N
N
O
H
MsCl, Et3N, CH2Cl2
H
MeO2C
then DBU
81%
S
OTBS
Me
BocHN
H
Me
18
H
N
O
H
N
S
MeO2C
OTBS
Me
i. TFA, CH2Cl2
ii. PyBOP, CH2Cl2
i-Pr2NEt, 87%
BocHN
HO2C
H
S
N
Br
19
Me
H
N
(18) Moody, C. J.; Bagley, M. C. Chem. Commun. 1998, 2049.
(19) Bagley, M. C.; Bashford, K. E.; Hesketh, C. L.; Moody, C.
J. J. Am. Chem. Soc. 2000, 122, 3301.
(20) Hughes, R. A.; Thompson, S. P.; Alcaraz, L.; Moody, C. J.
Chem. Commun. 2004, 946.
(21) Hughes, R. A.; Thompson, S. P.; Alcaraz, L.; Moody, C. J.
J. Am. Chem. Soc. 2005, 127, 15644.
(22) Adamczyk, M.; Johnson, D. D.; Reddy, R. E. Tetrahedron:
Asymmetry 1999, 10, 775.
O
H
N
MeO2C
S
OTBS
Me
HN
H
S
O
N
5
Br
Scheme
agent
3
NMM = N-methylmorpholine; LR = Lawesson’s re-
(23) Hanessian, S.; Vanasse, B. Can. J. Chem. 1993, 71, 1401.
(24) Hanessian, S.; Margarita, R. Tetrahedron Lett. 1998, 39,
5887.
(25) Analytical Data of (S,S)-1-Benzyl-5-methyl 2-tert-
butoxycarbonylamino-4-tert-butyldimethylsiloxy
Pentanedioate (9).
In summary, we have developed concise routes to thiaz-
oles 4 and 5 that together comprise the complete northern
hemisphere of nosiheptide, with appropriate orthogonal
protecting groups for further elaboration to the natural
product.
1H NMR (300 MHz, CDCl3): d = 7.40–7.26 (5 H, br s, ArH),
5.30 (1 H, d, J = 8.3 Hz, NH), 5.22 (1 H, d, J = 12.4 Hz,
CHHPh), 5.11 (1 H, d, J = 12.4 Hz, CHHPh), 4.47 (1 H, m,
CHNH), 4.28 (1 H, m, CHOSi), 3.70 (3 H, s, OMe), 2.28–
2.09 (2 H, m, 3-CH2), 1.42 (9 H, s, CMe3), 0.91 (9 H, s,
CMe3), 0.04 (3 H, s, Me), 0.03 (3 H, s, Me). 13C NMR (75
MHz, CDCl3): d = 173.8 (C), 172.5 (C), 155.9 (C), 135.9
(C), 129.3 (CH), 128.8 (CH), 128.6 (CH), 80.2 (C), 69.9
(CH), 67.5 (CH2), 52.5 (Me), 51.5 (CH), 36.7 (CH2), 28.7
(Me), 26.1 (Me), 18.6 (C), –3.18 (Me).
Acknowledgment
We the EPSRC for financial support, and the EPSRC Mass Spectro-
metry Service at Swansea for mass spectra.
(26) Singh, Y.; Stoermer, M. J.; Lucke, A. J.; Guthrie, T.; Fairlie,
D. P. J. Am. Chem. Soc. 2005, 127, 6563.
Synlett 2006, No. 18, 3033–3036 © Thieme Stuttgart · New York

3036
T. Belhadj et al.
LETTER
(27) Bredenkamp, M. W.; Holzapfel, C. W.; van Zyl, W. J. Synth.
Commun. 1990, 20, 2235.
(28) It is established that Boc-groups can be deprotected in the
presence of tert-butyl esters (for an example, see ref. 21) and
of tert-butyldimethylsilyl ethers (cf. deprotection of
compound 19).
Hz, CHNH), 4.39 (1 H, d, J = 11.3 Hz, CHHPh), 4.31 (1 H,
m, CHHMe), 4.14 (1 H, d, J = 11.1 Hz, CHPh), 3.93 (3 H, s,
OMe), 3.76 (3 H, s, OMe), 1.46 (9 H, s, CMe3), 1.27 (3 H, d,
J = 6.2 Hz, Me). 13C NMR (75 MHz, CDCl3): d = 175.0 (C),
162.3 (C), 159.6 (C), 156.1 (C), 147.3 (C), 130.1 (C), 129.8
(CH), 127.8 (CH), 114.0 (CH), 80.8 (C), 76.0 (CH), 71.5
(CH2), 58.1 (CH), 55.6 (Me), 52.8 (Me), 28.7 (Me), 16.9
(Me).
(29) Synthesis of (S,S)-tert-Butyl 2-[(1-tert-butoxycarbonyl-
amino-3-tert-butyldimethylsiloxy-3-methoxy-
carbonyl)propyl]thiazole-4-carboxylate (4).
(31) Muir, J. C.; Pattenden, G.; Thomas, R. M. Synthesis 1998,
613.
To a solution of (S,S)-2-tert-butoxycarbonylamino-4-tert-
butyldimethylsiloxy-4-methoxycarbonyl-thiobutanamide
(11, 1.20 g, 2.95 mmol) and tert-butyl bromopyruvate (2.30
g, 10.33 mmol) in 1,2-dimethoxyethane (42 mL) at –30 °C
was added KHCO3 (1.18 g, 11.80 mmol). The mixture was
stirred at –10 °C for 6 h. The colorless solid was filtered off
and washed with 1,2-dimethoxyethane. The filtrate was
concentrated in vacuo. The mixture was diluted in 1,2-
dimethoxyethane (42 mL) and cooled at –30 °C before
addition of TFAA (1.24 mL, 8.85 mmol) and 2,6-lutidine
(2.06 mL, 17.70 mmol). The mixture was stirred overnight
at –20 °C. The mixture was partitioned between EtOAc and
brine, the organic layer separated and concentrated in vacuo.
The crude product was purified by flash chromatography
(EtOAc–light PE, 1:5) to give the title compound (1.33 g,
85%) as a colorless sticky solid. 1H NMR (300 MHz,
CDCl3): d = 7.93 (1 H, s, H-4), 5.77 (1 H, d, J = 8.1 Hz,
NHBoc), 5.15 (1 H, m, CHOSi), 4.42 (1 H, m, CHNH), 3.70
(3 H, s, OMe), 2.51–2.40 (2 H, m, CH2), 1.57 (9 H, s, CMe3),
1.42 (9 H, s, CMe3), 0.91 (9 H, s, CMe3), 0.05 (3 H, s, Me),
0.01 (3 H, s, Me). 13C NMR (75 MHz, CDCl3): d = 173.8 (2
× C), 160.8 (C), 155.6 (C), 149.0 (C), 126.9 (CH), 82.3 (C),
80.5 (C), 69.9 (CH), 52.5 (CH), 50.5 (Me), 39.1 (CH2), 28.7
(Me), 28.6 (Me), 26.1 (Me), 18.5 (C), –4.6 (Me), –5.1 (Me).
MS (FI): m/z = 531 (14) [MH+], 473 (100), 418 (8), 417 (32),
132 (38), 57 (84).
(32) Bower, J.; Drysdale, M.; Hebdon, R.; Jordan, A.; Lentzen,
G.; Matassova, N.; Murchie, A.; Powles, J.; Roughley, S.
Bioorg. Med. Chem. Lett. 2003, 13, 2455.
(33) Silberg, A.; Ursu, A. Rev. Roum. Chim. 1965, 10, 897.
(34) Synthesis of Methyl 2-{(S)-1-[2-(2-Bromothiazole-4-
carbonylamino)-(R)-3-(tert-butyldimethyl-
siloxy)butanoylamino]-(Z)-propenyl}thiazole-4-
carboxylate (5).
To a stirred solution of methyl 2-{(S)-1-[(S)-2-tert-butoxy-
carbonylamino-(R)-3-(tert-butyldimethylsiloxy)butanoyl-
amino]-(Z)-propenyl}thiazole-4-carboxylate (19, 482 mg,
0.94 mmol) in CH2Cl2 (13 mL) was added TFA (1.74 mL,
23.46 mmol). The mixture was stirred 30 min at 20 °C. The
solvent was removed in vacuo and the residue was
azeotroped with toluene. To the crude thiazole amine
trifluoroacetate and 2-bromo-4-thiazolecarboxylic acid (254
mg, 1.22 mmol) in CH2Cl2 (12 mL) at 0 °C was added
PyBOP (587 mg, 1.13 mmol) and N,N-diisopropylethyl-
amine (0.80 mL, 4.70 mmol). The mixture was stirred 15
min at 0 °C and then overnight at r.t. The solvent was
removed in vacuo. The mixture was partitioned between
EtOAc and sat. solution of NaHCO3. The organic layer was
washed with brine, dried over Na2SO4 and concentrated. The
crude product was purified by flash chromatography (light
PE–EtOAc, 1:1 to 1:2) to give the title compound (496 mg,
87%) as a colorless solid, mp 69–73 °C; [a]D25 +24.0 (c 0.5,
CHCl3). HRMS: m/z calcd for C22H31BrN4O5S2Si + H:
603.0767; found: 603.0768. IR (CH2Cl2): nmax = 3387, 3294,
2954, 2930, 2856, 1727, 1670, 1536, 1473, 1432, 1244,
1215, 1094, 1014, 838, 779 cm–1. 1H NMR (300 MHz,
CDCl3): d = 8.41 (1 H, br s, NH), 8.22 (1 H, d, J = 6.4 Hz,
NH), 8.07 (2 H, s, 2 × H-4), 6.66 (1 H, q, J = 7.1 Hz,
=C=CH), 4.71 (1 H, dd, J = 6.4, 3.8 Hz, CHNH), 4.57 (1 H,
m, CHOSi), 3.92 (3 H, s, OMe), 1.85 (3 H, d, J = 7.1 Hz,
=CMe), 1.31 (3 H, d, J = 6.4 Hz, CHMe), 0.92 (9 H, s,
CMe3), 0.22 (3 H, s, Me), 0.18 (3 H, s, Me). 13C NMR (75
MHz, CDCl3): d = 168.3 (C), 167.8 (C), 162.0 (C), 160.5
(C), 149.6 (C), 147.2 (C), 136.6 (C), 128.1 (CH), 127.7
(CH), 127.4 (CH), 68.1 (CH), 58.8 (CH), 52.8 (Me), 26.2
(Me), 19.1 (Me), 18.3 (C), 14.7 (Me), –4.3 (Me), –4.6 (Me);
one quaternary C unobserved. MS (CI): m/z = 605/603 (95)
[MH+], 587 (15), 525 (13), 199 (33), 133 (21), 115 (14).
(30) Synthesis of Methyl 2-[(S)-1-(tert-Butoxycarbonyl-
amino)-(R)-2-(4-methoxybenzyloxy)propyl]thiazole-4-
carboxylate (16).
To a solution of N-tert-butoxycarbonyl-O-(4-methoxy-
benzyl)thiothreoninamide (15, 1.60 g, 4.51 mmol) in 1,2-
dimethoxyethane (31 mL) was added at 0 °C methyl
bromopyruvate (1.68 mL, 15.80 mmol) and KHCO3 (1.80 g,
18.04 mmol). The mixture was stirred 1 h at 0 °C before
addition of TFAA (1.89 mL, 13.53 mmol) and 2,6-lutidine
(3.15 mL, 27.06 mmol) at the same temperature. The
mixture was stirred 2 h at 0 °C. The mixture was diluted in
EtOAc and washed with brine. The organic layer was dried
over Na2SO4 and concentrated in vacuo. The crude product
was purified by flash chromatography (light PE–EtOAc, 2:1
to 1:2) to give the title compound (1.46 g, 74%) as a yellow-
brown oil. 1H NMR (300 MHz, CDCl3): d = 8.09 (1 H, s, H-
4), 6.95 (2 H, d, J = 8.6 Hz, ArH), 6.75 (2 H, d, J = 8.6 Hz,
ArH), 5.70 (1 H, d, J = 8.7 Hz, NHBoc), 5.00 (1 H, d, J = 8.5
Synlett 2006, No. 18, 3033–3036 © Thieme Stuttgart · New York