Total Synthesis of Mannopeptimycins α and β

Article abstract of DOI:10.1021/jacs.6b01384

The mannopeptimycins are a class of glycopeptide natural products with unusual structures and potent antibiotic activity against a range of Gram-positive multidrug-resistant bacteria. Their cyclic hexapeptide core features a pair of unprecedented β-hydroxyenduracididines (l- and d-βhEnd), an O-glycosylated d-Tyr carrying an α-linked dimannose, and a β-methylated Phe residue. The d-βhEnd unit also carries an α-linked mannopyranose at the most hindered N of its cyclic guanidine ring. Herein, we report the first total synthesis of mannopeptimycin α and β with fully elaborated N- and O-linked sugars. Critically, a gold-catalyzed N-glycosylation of a d-βhEnd substrate with a mannosyl ortho-alkynylbenzoate donor enabled the synthesis of the most challenging N-Man-d-βhEnd unit with excellent efficiency and stereoselectivity. The l-βMePhe unit was prepared using a Pd-catalyzed C-H arylation method. The l-βhEnd, d-Tyr(di-Man), and l-βMePhe units were prepared in gram quantities. A convergent assembly of the cyclic peptide scaffold and a single global hydrogenolysis deprotection operation provided mannopeptimycin α and β.

Full text of DOI:10.1021/jacs.6b01384

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Article
Total Synthesis of Mannopeptimycins # and #
Bo Wang, Yunpeng Liu, Rui Jiao, Yiqing Feng, Qiong Li, Chen Chen, Long Liu, Gang He, and Gong Chen
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Journal of the American Chemical Society
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Total Synthesis of Mannopeptimycins α and β
Bo Wang,3 Yunpeng Liu,3 Rui Jiao,3 Yiqing Feng,3 Qiong Li,3 Chen Chen,3 Long Liu,1 Gang He,1,2 and Gong
Chen1,2,3
*
1State Key Laboratory and Institute of Elemento-Organic Chemistry, Nankai University, Tianjin 300071, China
2Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300071, China
3Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802, United States
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ABSTRACT: The mannopeptimycins are a class of glycopeptide natural products with unusual structures and potent antibiotic activity
against a range of Gram-positive multidrug-resistant bacteria. Their cyclic hexapeptide core features a pair of unprecedented β-
hydroxyenduracididines (L- and D-βhEnd), an O-glycosylated D-Tyr carrying an α-linked di-mannose, and a β-methylated Phe residue. The
D-βhEnd unit also carries an α-linked mannopyranose at the most hindered N of its cyclic guanidine ring. Herein, we report the first total
synthesis of mannopeptimycin α and β with fully elaborated N- and O-linked sugars. Critically, a gold-catalyzed N-glycosylation of a D-βhEnd
substrate with a mannosyl ortho-alkynylbenzoate donor enabled the synthesis of the most challenging N-Man-D-βhEnd unit with excellent
efficiency and stereoselectivity. The L-βMePhe unit was prepared using a Pd-catalyzed C−H arylation method. The L-βhEnd, D-Tyr(di-
Man) and L-βMePhe units were prepared in gram quantities. A convergent assembly of the cyclic peptide scaffold and a single global hydro-
genolysis deprotection operation provided mannopeptimycin α and β.
Iadonisi have reported syntheses of the O-linked di-mannose resi-
INTRODUCTION
due.10 The laboratories of Oberthür and Van Nieuwenhze have
reported syntheses of unglycosylated L- and D-βhEnd units.11 In
The mannopeptimycins (MPP) are a class of glycopeptide natu-
ral products produced by Streptomyces hygroscopicus LL-AC98.1
They have shown potent antibiotic activity against a range of
Gram-positive multidrug-resistant pathogens including methicillin-
resistant S. aureus (MRSA) and vancomycin-resistant enterococci
(VRE) and have demonstrated compelling potential as clinically
useful antibacterials.2 The MPPs were originally isolated in 1950s,
but their structures were first elucidated in 2002 by researchers at
Wyeth Pharmaceuticals based on NMR and chemical degradation
studies.3 The MPPs contain a cyclic hexapeptide core comprised of
alternating L- and D-α-amino acids (αAAs). Among the αAA units
are a pair of unprecedented β-hydroxyenduracididine (L- and D-
βhEnd)4,5, a L-β-methylated Phe (βMePhe), and a O-glycosylated
2014, Fuse and Doi reported the first total synthesis of mannopep-
timycin aglycone and revised the Cβ stereochemistry of the L-
βMePhe unit.12a However, the synthesis of N-man-D-βhEnd re-
mains elusive, posing a formidable obstacle to the total synthesis of
the mannopeptimycins.
OH
OR3
OR2
OR1
OH
HO
NH
OH
HN
OH
OHO
O
O
HO
NH
O
L-βhEnd
O
O
HN
OH
O
N
HO
D-Tyr
OH
HN
NH HN
D-βhEnd
O
NH
HN
O
D-Tyr carrying an α-(1,4-linked)-bis-manno-pyranosyl pyranoside
(Scheme 1). More strikingly, it was proposed that the D-βhEnd
mannopeptimycins α−ε
R1 = R2 = R3 = H,
R2, R3 = H, R1 = i-valeryl,
R1, R3 = H, R2 = i-valeryl,
R1, R2 = H, R3 = i-valeryl,
with unglycosylated D-Tyr,
HO
L-Ser
NH HN
O
α
γ
O
β
unit bears a α-mannopyranose in 1C4 conformation6 at the most
hindered N atom on the cyclic guanidine ring. An N-glycosylated
guanidine motif has not been found in any other natural product.
Biological studies have indicated that the MPPs interfere with the
late stages of bacterial cell wall synthesis by binding cell wall pre-
cursor lipid II7 in a manner unlike other lipid II binders such as
ramoplanin and vancomycin.8 Bio- and semisynthetic studies of
MPPs suggest that both N- and O-linked sugars are necessary for
antibiotic activity.9
Gly
δ
ε
L-βMePhe
β
HO
α-Man 1C4
H1' : δ ~5.0, d
J1',2' > 8.0 Hz
OH
O
HO
H2'
H1'
OH
H
OH
HO
H
a
NH2
NH2
N
N
c
NH2
N
b
β
HN
HN
HN
α
γ
CO2H
CO2H
HN
CO2H
HN
δ
HN
(2R,3S,4S)-L-βhEnd
N-α−Man-(2S,3S,4S)-D-βhEnd
L-enduricididine (End)
The highly unusual structures, novel mode of action, and prom-
ising antibiotic activity of the MPPs have generated great interest in
their chemical synthesis and structural modification over the past
decade.9-12 Modifications on the O-linked sugar residues and
βMePhe unit have provided significantly improved lead com-
pounds for preclinical trials.9c The laboratories of O’Doherty and
Scheme 1. Structure of mannopeptimycins
Herein, we report the first total synthesis of mannopeptimycin α
and β with fully elaborated N- and O-linked sugars. Key features
include a highly efficient gold-catalyzed N-mannosylation for the
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Page 2 of 8
synthesis of the N-man-D-βhEnd unit, a stereoselective synthesis of To address this issue, we first investigated the N-mannosylation
1
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3
4
5
6
7
8
the L-βMePhe unit via Pd-catalyzed directed C−H arylation, a
gram-scale preparation of the L-βhEnd and O-di-mannosyl-D-Tyr
units, and a convergent assembly of the cyclic peptide backbone
followed by a global hydrogenolysis deprotection operation to give
the final product.
of simpler di-Cbz-protected cyclic guanidine model substrate 1
(Table 1). Compound 1 can be quickly prepared from serine me-
thyl ester 3 via guanylation with Goodman reagent 414 followed by
MsCl-mediated C−N cyclization.15 N-mannosylation of 1 with
mannosyl trichloroacetimidate donor 5 and ethyl sulfide donor 6
under various Lewis acid-promoted conditions (e.g. with TMSOTf,
BF3OEt2, NIS) failed to give any N-mannosylated product. N-
mannosylation of 1 with bromide donor 7 promoted by weakly
RESULTS AND DISCUSSION
N-mannosylation of a model cyclic guanidine
13e
basic Ag2CO3 in toluene at 80 °C gave product 216 in 12% yield.
9
Intrigued by the extraordinary structure and highly promising
antibiotic activity of the MPPs, we began our attempt at the total
synthesis of this class of glycopeptide natural products seven years
ago. The most difficult challenge in the synthesis of MPP is the
preparation of a suitable N-α-mannosyl-D-βhEnd unit. Compared
with the array of established methods for O-glycosylation, methods
for N-glycosylation are much less developed, and primarily limited
to the synthesis of N-glycosides of nucleotides and heteroarenes.13
Moreover, poor compatibility with Lewis acid-promoted condi-
tions, steric hindrance about the N-glycosylation site on the cyclic
guanidine ring, and the delicate structure of the D-βhEnd substrate
However, the Koenigs-Knorr-type N-mannosylation of more com-
plex substrates (e.g. D-βhEnd 16 in Scheme 2) with 6 only gave
trace amount of product (<5%). The failure of these conventional
glycosylation methods prompted us to test a gold(I)-catalyzed
glycosylation method, recently reported by Yu, using ortho-
alkynylbenzoate donors.17 Encouraged by a successful application
in nucleoside synthesis,17b we expected that the unique π-acid acti-
vation mode of Yu’s method, orthogonal to the Lewis basic guani-
dine NH, might provide an efficient N-mannosylation method for
cyclic guanidines. To our delight, the Ph3PAuNTf2-catalyzed N-
mannosylation of 1 with mannosyl ortho-alkynylbenzoate 10 pro-
ceeded in excellent yield and with exclusive α-stereoselectivity at
room temperature (entries 4 and 5). The reaction time can be
shortened at elevated temperature (entry 6). As in donor 7, the 2-
OAc group of 10 is required to control the α stereoselectivity via
neighboring group participation. Donor 9 carrying a 2-OBz group
gave slightly lower yield (entry 8). Disarmed tetra-OAc substituted
donor 8 gave considerably lower mannosylation yield under the
same reaction conditions (entries 2 and 3, see Supporting Infor-
mation for preparation of 8-10).
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
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further complicate of the synthesis of N-α-mannosyl-D-βhEnd.
BnO
H1'
CO2Me
OBn
O
H
donor
N
OAc
N
CO2Me
CbzN
CbzN
OBn
NCbz
H2'
N
1
2
Cbz
entry
donor
(equiv)
reagents (equiv), conditions
yield
(%)a
1
2
3
7 (3)
Ag2CO3 (2), 4A MS, toluene, 80 oC, 12h
Ph3PAuNTf2 (0.2), DCM, 4A MS, rt, 18h
Ph3PAuNTf2 (0.2), DCM, 4A MS, 45 oC, 24h
12
15
54
8 (1.5)
8 (1.5)
Preparation of the βhEnd units
With a gold-catalyzed N-mannosylation method in hand, we
proceeded to investigate the synthesis of N-Man-D-βhEnd and L-
βhEnd units.11 As shown in Scheme 2A, our initial synthetic route
for the βhEnd units began from a common precursor 12, which can
be prepared from Garner aldehyde 11 in large quantity in 2 steps.18,
4
10 (1.5)
Ph3PAuNTf2 (0.2), DCM, 4A MS, rt, 18h
Ph3PAuNTf2 (0.2), toluene, 4A MS, rt, 18h
Ph3PAuNTf2 (0.2), toluene, 4A MS, 65 oC,
85
5
6
10 (1.5)
10 (1.5)
87
83
4h
7
8
10 (1.5)
9 (1.5)
Ph3PAuNTf2 (0.1), toluene, 4A MS, 65 oC,
55
80
18h
19
OsO4-catalyzed dihydroxylation of 12 and TBDPS protection of
Ph3PAuNTf2 (0.2), toluene, 4A MS, 65 oC,
4h
the terminal OH group gave a separable diastereomeric mixture of
13 and 14 with 1:8 selectivity.20 Mitsunobu reaction of 14 gave an
azido compound. The removal of N,O-acetonide and Boc groups,
followed by guanylation with Goodman reagent 4, and
PPh3/DIAD-mediated C−N cyclization provided 18. Removal of
the TBDPS group of 18 with TBAF, TEMPO oxidation, and esteri-
fication with MeI gave α-azido methyl ester 16. A diastereomeric
mixture of 13 and 14 can be subjected to the same reaction se-
quence (from 14 to 16) without separating the diastereomeric
intermediates until the final azido ester products 15 and 16, which
are easily separable by silica gel column chromatography. Starting
from 11, both βhEnd compounds 15 and 16 were obtained in 14%
combined yield via a single sequence of 11 steps and 7 column
purifications.
Preparation of 1
i.
CbzHN
NTf
NHCbz
TEA, toluene, 45 o
4
HCl.H2N
HO
CO2Me
H
N
CO2Me
C
CbzN
CbzN
3
ii. MsCl, DIPEA, DCM, 0 o
31% over 2 steps
C
1
donors:
OAc
O
BnO
BnO
OAc
O
BnO
BnO
BnO
O
NH
CCl3
BnO
SEt
5
6
nBu
OAc
O
BnO
BnO
BnO
OAc
O
AcO
AcO
AcO
Br
O
7
8
O
As shown in Scheme 2B, azido ester 16 can be converted to 19
via reduction with PPh3 followed by Boc protection. Disappoint-
ingly, N-mannosylation of 19 using various methods failed to give
any of the desired product 20 possibly due to steric hindrance or
interference from the Boc-protected NH group.21 On the other
hand, N-mannosylation of azido ester 16 with 10 proceeded suc-
nBu
nBu
OBz
O
OAc
O
BnO
BnO
BnO
BnO
BnO
BnO
O
O
O
O
9
10
Table 1. N-mannosylation of model cyclic guanidine 1. a) isolated
yield on a 0.2 mmol scale.
o
cessfully under the gold-catalyzed conditions at 65 C to give prod-
uct 22 in 65% yield and complete α-stereoselectivity.22 However,
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Scheme 2. Our initial synthetic route for L-βhEnd and N-Man-D-βhEnd
1
2
3
4
5
6
7
8
9
A) Synthesis of βhEnd 15 and 16
OPMB
CO2Me
PMBO
OTBDPS
H
N
CbzN
CbzN
O
O
N3
i.
NBoc OH
OPMB
MgBr
THF, 0 oC, 70%
O
iii. OsO4, NMO,
acetone/H2O, rt
mixture of 13 & 14
used as SM
13 (9%)
L-βhEnd 15
H
O
+
O
NBoc
12
11
NBoc
iv. TBDPSCl,
Im, DCM, rt
7 steps
4 column purifications
ii. PMBBr, THF
/DMF, 0 oC, 92%
PMBO
OTBDPS
OPMB
H
α
N
CO2Me
CbzN
CbzN
β
NBocOH
N3
D-βhEnd 16
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
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39
40
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42
43
44
45
46
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48
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14 (72%)
i. HN3, PPh3, DIAD,
toluene, rt, 93%
i. TBAF, THF, rt
ii. TEMPO, NaClO2,
NaClO, 40 o
ii. HOAc/H2O/dioxane,
C
80 o
C
iii. MeI, KHCO3,
DMF, rt
iii. 4, TEA, THF, rt,
54% over 2 steps
56% over 3 steps
PMBO
OTBDPS
PMBO
OTBDPS
H
HO
CbzHN
N
PPh3, DIAD, THF, rt, 86%
CbzN
CbzN
NH N3
17
N3
18
NCbz
B) Attempted synthesis of N-α-Man-D-βhEnd 21
CO2Me
i. PPh3, THF/H2O, 55 o
ii. Boc2O, TEA, THF, rt, 48%
C
OPMB
H
BnO
PMBO
NHBoc
N
CO2Me
OBn
O
CbzN
CbzN
N
N-mannosylation
OAc
NHBoc
NCbz
BnO
19
20
CbzN
CO2H
(HO)
PMBO
BnO
OPMB
CO2Me
NHBoc
H
OBn
O
N
CbzN
CbzN
D-βhEnd 16
N
OH
N3
NCbz
BnO
21
CbzN
CO2Me
N3
10 (1.5 equiv)
CO2Me
BnO
PMBO
Ph3PAuNTf2 (0.2 equiv)
toluene, 4AMS, 65 o
BnO
PMBO
NHAc
OBn
O
C
OBn
N
O
OAc
N
PPh3
N-mannosylation
OH
NCbz
BnO
THF/H2O
55 oC, 92%
NCbz
CbzN
BnO
23
CbzN
22 (65%, α only)
attempted reduction of the azido group of 22 under various con-
ditions failed to give the desired amine product, predominately
forming acetamide byproduct 23 through an intramolecular O to N
acyl transfer process. The attempted removal of the OAc group of
22 under acidic or basic conditions failed due to serious side reac-
tions and decomposition of 22.23
strate 16, little undesired side product was formed, possibly be-
cause the guanidine NH group of the acetonide protected substrate
is less hindered. Finally, treatment of 31 with LiOH successfully
removed the 2-OAc group on mannose, the methyl ester group,
and one Cbz group on the cyclic guanidine moiety to give N-man-
D-βhEnd 32 in good yield.
The success of the gold-catalyzed N-mannosylation with the
complex D-βhEnd substrate 16 followed by the failed reduction of
the azido group to amine prompted us to investigate other βhEnd
substrates bearing a more properly protected N-terminus. Encour-
aged by the report of MPP aglycone synthesis by Fuse and Doi,12a
we wondered whether their D-βhEnd unit 30 protected by N,O-
acetonide and Boc at N-terminus might be useful for the synthesis
of N-man-D-βhEnd (Scheme 3A). Following the reported proce-
dure, a separable mixture of 26 and 27 was obtained via a tandem
aldol/cyclization reaction between tribenzyl protected 2-
aminopropanol 24 and N-(diphenylmethylene) glycine t-butyl
ester 25. Compound 27 was then converted to compound 30 in 7
steps.24 To our delight, the gold-catalyzed N-mannosylation of D-
Although intermediate 26 can be converted to L-βhEnd 28 via a
similar sequence in Fuse and Doi’s report, the overall yield of this
route was very low in our hands. As shown in Scheme 3C, a more
scalable synthesis of L-βhEnd 39 was achieved based on modifica-
tion of a method recently reported by Oberthür.11c The synthesis of
39 began with Wittig reaction of compound 33 and
Ph3P=CHCO2Bn followed by diastereoselective dihydroxylation to
form 34. The Cα OH group was then converted to BocNH. The
acetonide, Bn and Cbz groups of 36 were then removed to give 37.
The use of the free carboxylic acid form of 37 was necessary to
avoid a competing intramolecular γ-lactamization observed when
ester derivatives of 37 were subjected to basic conditions. For-
mation of the cyclic guanidine group followed by saponification of
o
βhEnd 30 with ortho-alkynylbenzoate donor 10 in toluene at 65 C
the Cα ester gave L-βhEnd 39. Starting from 33, 39 was obtained
in 7% yield over 12 steps and 6 column purifications.
proceeded very cleanly to give desired product 31 in 86% isolated
yield and with complete α-stereoselectivity on a gram scale
(Scheme 3B). Compared to the N-mannosylation reaction of sub-
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Journal of the American Chemical Society
Scheme 3. Synthesis of N-Man-D-βhEnd 32 and L-βhEnd 39
Page 4 of 8
1
2
A) Fuse-Doi synthesis of D-βhEnd 30
3
4
5
6
7
8
9
OH
Ph
Ph
O
Bn2N
BnO
CHO
24
H
N
Bn2N
BnO
N
N
Boc
CbzN
CbzN
CO2tBu
CO2Me
i. LiN(TMS)2
THF, -78 o
ii. SiO2
28
26 (30%)
prepared from
H-Ser(Bn)-OH in 4 steps
C
i. Pd/C, H2 (50 atm)
MeOH, 45 o
+
C
+
Aldol /
isomeric
resolution
ii. 4, TEA, dioxane
/H2O, 50 oC,
Ph
i. TFA, Et3SiH, H2O, rt
ii. Boc2O, K2CO3, THF
/H2O, rt
Ph
Ph
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
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60
O
tBuO2C
N
Ph
O
O
N
54% over 2 steps
H
N
N
Bn2N
BnO
NH
Bn2N
BnO
25
Boc
CbzN
CbzN
Boc
iii. DIAD, PPh3,
THF, rt, 86%
iii. MeI, K2CO3, DMF
rt, 66% over 3 steps
iv. 2-methoxypropene,
PPTS, DCM, rt, 86%
CO2Me
CO2tBu
27 (33%)
CO2Me
29
30
B) Synthesis of N-α-Man-D-βhEnd 32
Boc
N
Boc
10 (1.5 equiv)
N
LiOH (7.5 equiv)
THF/MeOH/H2O,
55 oC, 86%
BnO
BnO
O
Ph3PAuNTf2 (0.2 equiv)
toluene, 4AMS, 65 o
O
CO2H
CO2Me
C
OBn
OBn
O
30
O
N
N
OH
OAc
N-mannosylation
NH
NCbz
BnO
BnO
CbzN
CbzN
31 (86%, α only)
N-α−Man-D-βhEnd 32
~6% yield from 24
10 steps, 6 colum purifications
C) Synthesis of L-βhEnd 39
OH
OH
OH
i. NosCl, TEA,
DCM, 0 o
C
i. PPh3=CHCO2Bn
CHO
i. PPh3, THF/H2O, 70 o
C
α
CO2Bn
CO2Bn
CO2Bn
O
THF, reflux, 81%
O
O
O
NCbz
33
NCbz
NCbz
OH
NCbz
NHBoc
N3
ii. NaN3, DMF, 70 o
61% over 2 steps
C
ii. AD-mix-α
tBuOH/H2O, rt
60%, dr~ 10/1
ii. Boc2O, TEA
85% over 2 steps
34
35
36
i. TsOH, MeOH, 45 oC
ii. H2 (1 atm), Pd/C,
MeOH, rt
i. 4, TEA, dioxane/H2O,
50 oC,
OH
OH
H
N
i. NosCl, Py,
rt, 65%
CO2H
OH
CO2Me
NH NHBoc
NCbz
CbzN
CbzN
HO
CbzHN
ii. MeI, K2CO3, DMF, rt
CO2H
NHBoc
NHBoc
L-βhEnd 39
HO
NH2
ii. LiOH (4 equiv)
THF/H2O, 0 oC,
87%
48% over 4 steps
37
38
~7% yield from 33
12 steps, 6 colum purifications
As shown in Scheme 4B, O-di-mannosyl-D-Tyr unit 51 was pre-
pared via glycosylation of Boc-D-Tyr-OMe 50 with dimannosyl
trichloroacetimidate donor 49.10 Mannose 44 with a PMP group26
at the anomeric position was first protected as a 4,6-O-benzylidene
intermediate, and then treated with BzCl to give 45. The benzyli-
dene group of 45 was selectively opened via treatment with Et3SiH
and BF3OEt2 to give 46. BF3OEt-promoted glycosylation of 46
with 2-O-benzoyl-3,4,6-tri-O-benzyl-D-mannsoyl trichloroacetimi-
date 47 gave 1,4-linked dimannose 48 in excellent yield and α-
selectivity. Treatment of 48 with cerium ammonium nitrate
(CAN) removed the anomeric PMP group and reaction with
CCl3CN and DBU gave corresponding trichloroacetimidate donor
49. The BF3OEt2-promoted O-glycosylation of 50 with 49 gave
Preparation of the other αAA units and the final
assembly of mannopeptimycins
With the two βhEnd units in hand, we next turned our attention
to the preparation of the remaining αAA units and the final assem-
bly of the cyclic hexapeptide. Following Fuse and Doi’s peptide
coupling strategy in the synthesis of MPP aglycone,12a we planed to
join the two tripeptide fragments at the D-Tyr/L-βhEnd site and
then cyclize at the Ser/Gly site.12b To simplify the final deprotec-
tion operation, Bn was used as the protecting group for Ser and D-
Tyr units. As shown in Scheme 4A, the (2S,3R)-threo-βMePhe unit
43 was prepared using our previously developed Pd-catalyzed ami-
noquinoline (AQ)-directed C−H arylation chemistry.25 Pd-
catalyzed β-C(sp3)−H arylation of phthaloyl-L-2-aminobutyramide
40 with PhI gave 42 in excellent yield and diastereoselectivity. The
stereochemistry of C−H arylation was controlled by the α,β-trans-
configuration of 5-membered palladacycle intermediate 41. The
AQ auxiliary was cleaved with LiO2H following Boc activation. The
Phth group was then replaced with Boc to give Boc-βMePhe-OH
43 in good yield and with excellent stereoretention at Cα.
Boc-D-Tyr(di-Man)-OMe 51 in excellent yield and α-selectivity.
Boc deprotection and HATU-mediated amide couplings of the
αAA units 51, 43 and Boc-Gly-OH followed by saponification with
LiOH gave the tripeptide Boc-Gly-βMePhe-D-Tyr(di-Man)-OH
52. Similarly, peptide coupling of Boc-D-Tyr(Bn)-OMe 53, Boc-
βMePhe-OH 43 and Boc-Gly-OH gave the un-glycosylated tripep-
tide 54.
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Scheme 4. Synthesis of tripeptides 52 and 54
1
A) Synthesis of (2S,3R)-βMePhe 43
2
O
3
4
5
6
7
8
9
PhthN
N
i. Boc2O, DMAP, CH3CN, rt, 89%
ii. LiOH, H2O2, THF/H2O, rt;
then reflux in toluene
(AQ)
O
PdII
O
N
2
BocHN
CO2H
Ph
H
PhthN
AQ
PhthN
41
N
3
N
H
H
PhI (2.5 equiv)
iii. NH2NH2 (1.3 equiv)
N
Ph
H
MeOH, 70 o
C
Pd(OAc)2 (10 mol%)
Ag2CO3, DCE, 70 o
(2S,3R)-βMePhe 43
H
42
Phth-Abu-AQ 40
C
iv. Boc2O, TEA, rt,
66% over 3 steps
78%, dr > 10:1
C−H arylation
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
B) Synthesis of tripeptides 52 and 54
BnO
BnO
BnO
OBz
O
OBz
O
BnO
BnO
OBn
OBz
OBn
OBz
OH
OH
BnO
Et3SiH, BF3OEt2
DCM, 0 oC, 75%
i. PhCH(OMe)2
CSA, DMF
OBz
O
Ph
OC(=NH)CCl3
BF3OEt2, DCM, rt, 87%
i. CAN, CH3CN/H2O, 0 o
O
47
O
BzO
O
O
O
HO
O
HO
HO
BzO
BzO
ii. BzCl, Py, rt
68% over 2 steps
OR
OPMP
OPMP
OPMP
44
45
46
48 R= PMP
C
ii. CCl3CN, DBU, DCM,
rt, 61% (2 steps)
49 R= C(=NH)CCl3
BnO
OBn
BnO
49 (0.5 equiv),
O
O
O
BnO
OBn
OH
OH
BF3OEt2, DCM, rt,
93% (based on 49)
α only
i. TFA, DCM, rt
OBn
OBn
O
O
OH
ii. 43 (1.2 equiv), HATU,
O
DIPEA, DCM, rt, 94%
O
HO2C
OBn
OBz
O
OH
OBz
iii. TFA, DCM, rt
iv. Boc-Gly-OH, HATU,
DIPEA, DCM, rt, 89%
v. LiOH, THF/MeOH/H2O,
rt, >90%
BocHN
CO2Me
HN
O
OBz
BocHN
CO2Me
Boc-D-Tyr-OMe 50
HN
Boc-D-Tyr(di-Man)-OMe 51
(25% yield starting from 44)
BocHN
O
52
(~75% yield starting from 51)
OBn
OBn
i. 43 (0.75 equiv), HATU, DIPEA,
DCM, rt, 91% (based on 43)
ii. TFA, DCM, rt
HO2C
HN
MeO2C
H2N
O
iii. Boc-Gly-OH, HATU,
DIPEA, DCM, rt, 76%
iv. LiOH, THF/H2O, rt, >90%
HN
D-Tyr(Bn)-OMe 53
BocHN
O
54
(~62% yield starting from 53)
As shown in Scheme 5, the HATU-mediated amide coupling of
N-man-D-βhEnd 32 with H-Ser(Bn)-OAll 55 gave 56 in good
yield. To our delight, the N-linked mannose residue of 56 remained
intact during the deprotection of acetonide and Boc groups under
acidic conditions. HATU-mediated amide coupling between the
two sterically hindered βhEnd sites also proceeded smoothly to
give tripeptide 57 in excellent yield. The Boc group of 57 was re-
moved and HATU-mediated amide coupling with tripeptide 54
gave linear hexapeptide 58. Removal of the C-terminus allyl group,
removal of the N-terminus Boc group, and HATU-mediated mac-
rolactamization provided the cyclized hexapeptide in ~44% yield
over 3 steps. Finally, a global deprotection of the Cbz and Bn
groups provided mannopeptimycin β, following reverse phase
HPLC purification. Following the same sequence, the tripeptide
fragments 57 and 52 were coupled, cyclized and deprotected to
The unique peptide scaffold and unprecedented glycosylation
pattern of the mannopeptimycins has until now prevented their
total synthesis. The structural complexity of the MPPs requires
judicious choices at the level of individual αAA building block syn-
thesis as well as peptide assembly. Our early studies revealed that
Yu’s gold-catalyzed N-glycosylation with
a mannosyl ortho-
alkynylbenzoate donor offered a uniquely powerful means to install
N-linked mannose moiety on cyclic guanidine substrates with high
efficiency and stereoselectivity. Our subsequent investigation re-
vealed that the choice of protecting groups for the N-terminus and
β-OH group of the D-βhEnd substrate is critical to successfully
access the N-Man-D-βhEnd building block. Building upon the
earlier reports by Fuse-Doi and Oberthür, we developed efficient
and scalable syntheses for both N-Man-D-βhEnd and L-βhEnd
units. Boc-βMePhe-OH was prepared via Pd-catalyzed C−H aryla-
tion chemistry. O-di-mannosyl-D-tyrosine was prepared via glyco-
sylation of Boc-D-Tyr-OMe with a dimannosyl trichloroacetimi-
give mannopeptimycin α in similar yield. The 1H and 13C NMR
spectra of both synthetic products was fully consistent with data of
isolated samples from the literature (see SI for details).27
date donor. Each of these αAA building blocks can be prepared in
gram quantities. Finally, a convergent assembly of the cyclic pep-
tide backbone and a single global hydrogenolysis deprotection
Conclusion
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Scheme 5. Total synthesis of mannopeptimycins α and β
Page 6 of 8
1
2
3
4
5
6
7
8
OBn
OBn
OBn
BnO
NCbz
NH
OBn
O
BnO
O
Boc
i. TFA, DCM, H2O, rt
II. 39 (1.5 equiv)
HATU, DIPEA,
CbzN
N
55 H-Ser(Bn)-OAll
HATU, DIPEA,
DCM, rt, 76%
HO
BnO
O
N
HO
CbzN
HN
CO2H
N
O
O
OH
NHBoc
57
OH
OBn
O
N
DCM, rt, 82%
CbzN
OH
CbzN
NBoc
N
HN
NH
H
BnO
O
O
32
NH
NH
56
9
BnO
CO2All
BnO
CO2All
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
OH
HO
NH
NH
i. Pd(PPh3)4, DMBA,
dioxane/H2O, rt
ii. TFA, DCM, rt
iii. HATU, DIPEA, DCM
(0.5 mM), rt, 44% over
3 steps
OH
OBn
BnO
O
i. TFA/DCM, rt
ii. 54 (2 equiv),
HATU, DIPEA,
DCM, rt, 71%
over 2 teps
NCbz
HN
OBn
O
HO
N
OH
HN
HN
NH
CbzN
OH O
OH
O
HO
OBn
CbzN
HN
N
OH O
OH
O
N
H
HN
O
3mer + 3mer
iv. H2 (50 atm), Pd(OH)2
THF/MeOH/HCO2H/H2O, rt
C18 HPLC seperation, ~42%
N
H
HN
NH
HN
O
O
NH
HN
HN
O
HO
NH HN
O
O
BnO
CO2All
BocHN
O
mannopeptimycin β
~8% yield starting from 32
9 steps, 5 column purifications
58
57
HO
BnO
OH
OH
OH
OBn
i. Pd(PPh3)4, DMBA,
OBn
OH
OBn
HO
BnO
dioxane/H2O, rt
HO
O
BnO
O
O
O
NH
NCbz
NH
OH
O
OBn
O
O
ii. TFA, DCM, rt
O
OH
NH
HN
CbzN
iii. HATU, DIPEA, DCM
(0.5 mM), rt, 47% over
3 steps
OH
OH
OH
OH
HO
HO
N
O
O
HN
HN
CbzN
N
OH O
OH
OH O
OH
O
3mer + 3mer
O
HN
N
H
HN
N
H
HN
iv. H2 (50 atm), Pd(OH)2
THF/MeOH/HCO2H/H2O,
rt, C18 HPLC seperation,
~45%
i. TFA/DCM, rt
ii. 52 (2 equiv),
HATU, DIPEA,
DCM, rt, 76%
over 2 teps
O
O
NH
NH
HN
NH HN
HN
O
O
HO
BnO
CO2All
BocHN
HN
O
59
O
O
mannopeptimycin α
~10% yield starting from 32
9 steps, 5 column purifications
Groton, CT 06340, US.; Dr. Jiao, R: Lianyungang Teachers College,
Jiangsu, 22066, China; Dr. Li, Q.: School of Chemistry and Molecular
Engineering, East China University of Science and Technology,
Shanghai, 200237, China.
operation provided mannopeptimycins α and β. Our synthesis
provides conclusive evidence for the structural determination of
these highly complex glycopeptide natural products. We hope that
this work will enable exploration of previously inaccessible manno-
peptimycin derivatives, provide mechanistic understanding of their
mode of action, and promote the development of new analogues
with enhanced antibacterial activity.
ACKNOWLEDGMENT
We gratefully thank The Pennsylvania State University and the State
Key Laboratory of Elemento-Organic Chemistry at Nankai University
for financial support of this work. We dedicate this work to Prof. Samu-
el J. Danishefsky on the occasion of his eightieth birthday.
ASSOCIATED CONTENT
Additional experimental procedures and spectroscopic data for all new
compounds are supplied. This material is available free of charge via
the Internet at http://pubs.acs.org.
REFERENCES
1. De Voe, S. E.; Kunstmann, M. P. Antibiotic AC98 and production. US
Patent 3495004, 1970.
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try 1977, 16, 1957.
AUTHOR INFORMATION
Corresponding Author
gongchen@nankai.edu.cn, guc11@psu.edu
Current addresses
Dr. Liu, Y: Department of Chemistry, Georgia State University, Atlan-
ta, GA 30302, US.; Dr. Feng, Y.: Worldwide MedChem, Pfizer Inc.,
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5. For biosynthesis study of β-hydroxyenduracididines: Haltli, B.; Tan, Y.;
20. A 1.1:1 ratio of 13 and 14 was obtained in 91% combined yield when
the AD-mix-β catalyst was used in the dihydroxylation step.
21. Protection of NH2 with Phth gave very low yield (<15%).
22. An unidentified side product with the same molecular weight of 22 was
also formed in 20% yield. However, its NMR spectra do not fully agree
with an ortho ester structure (see SI). Gold-catalyzed N-
mannosylation of compound 18 with 10 also worked well. However,
the attempted conversion of the resulting N-mannosylated intermedi-
ate to 21 was unsuccessful.
Magarvey, N. A.; Wagenaar, M.; Yin, X. H.; Greenstein, M.; Hucul, J.
1
2
3
4
5
6
7
8
A.; Zabriskie, T. M. Chem. Biol. 2005, 12, 1163.
1
6. For references on C4 conformation of mannose: (a) Lemieux, R. U.;
Morgan, A. R. Can. J. Chem. 1965, 43, 2205. (b) Onodera, K.; Hirano,
S.; Masuda, F.; Kashimura, N. J. Org. Chem. 1966, 31, 2403.
7. Breukink, E.; de Kruijff, B. Nat. Rev. Drug Discovery 2006, 5, 321.
8. (a) Ruzin, A.; Singh, G.; Severin, A.; Yang, Y.; Dushin, R. G.; Suther-
land, A. G.; Minnick, A.; Greenstein, M.; May, M. K.; Shlaes, D. M.;
Bradford, P. A. Antimicrob. Agents Chemother. 2004, 48, 728. (b)
Magarvey, N. A.; Haltli, B.; Greenstein, M.; Hucul, J. A. Antimicrob.
Agents Chemother. 2006, 50, 2167.
23. The conformation of 22 might affect the accessibility of the OAc group.
24. The procedures for converting 29 to 30 have been modified to obtain
more reliable yields. See SI for details.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
9. For previous SAR studies based on bio- and semisynthesis: (a) Sum, P.
E.; How, D.; Torres, N.; Petersen, P. J.; Lenoy, E. B.; Weiss, W. J.;
Mansour, T. S. Bioorg. Med. Chem. Lett. 2003, 13, 1151. (b) Dushin, R.
J.; Wang, T.-Z.; Sum, P.-E.; He, H.; Sutherland, A. G.; Ashcroft, J. S.;
Graziani, E. I.; E., K. F.; Bradford, P. A.; Petersen, P. J.; Wheless, K. L.;
How, D.; Torres, N.; Lenoy, E. B.; Weiss, W. J.; Lang, S. A.; Projan, S. J.;
Shlaes, D. M.; Mansour, T. S. J. Med. Chem. 2004, 47, 3487. (c) Pe-
tersen, P. J.; Wang, T. Z.; Dushin, R. G.; Bradford, P. A. Antimicrob.
Agents Chemother. 2004, 48, 739. (d) He, H.; Shen, B.; Petersen, P. J.;
Weiss, W. J.; Yang, H. Y.; Wang, T.-Z.; Dushin, R. G.; Koehn, F. E.;
Carter, G. T. Bioorg. Med. Chem. Lett. 2004, 14, 279.
10. For previous syntheses of the O-linked di-mannose residue: (a) Babu,
R. S.; Guppi, S. R.; O’Doherty, G. A. Org. Lett. 2006, 8, 1605. (b)
Adinolfi, M.; Giacomini, D.; Iadonisi, A.; Quintavalla, A.; Valerio, S.
Eur. J. Org. Chem. 2008, 2895.
11. For previous syntheses of unglycosylated βhEnd units: (a) Schwörer, C.
J.; Oberthür, M. Eur. J. Org. Chem. 2009, 6129. (b) Olivier, K. S.; Van
Nieuwenhze, M. S. Org. Lett. 2010, 12, 1680. (c) Fischer, S. N.;
Schwörer, C. J.; Oberthür, M. Synthesis 2014, 46, 2234.
12. (a) Fuse, S.; Koinuma, H.; Kimbara, A.; Izumikawa, M.; Mifune, Y.; He,
H.; Shin-ya, K.; Takahashi, T.; Doi, T. J. Am. Chem. Soc. 2014, 136,
12011. (b) For a solid phase synthesis of a simplified MPP aglycone
substituted with L-, D-Arg and L-Phe residues: Wang, T.-Z.; Wheless,
K. L.; Sutherland, A. G.; Dushin, R. G. Heterocycles 2004, 62, 131.
13. For selected syntheses of N-glycosides: (a) Vorbrüggen, H. Acc. Chem.
Res. 1995, 28, 509. (b) Gallant, M.; Link, J. T.; Danishefsky, S. J. J. Org.
Chem. 1993, 58, 343. (c) Keynes, M. N.; Earle, M. A.; Sudharshan, M.;
Hultin, P. G. Tetrahedron 1996, 52, 8685.(d) Lin, P.; Lee, C. L.; Sim,
M. M. J. Org. Chem. 2001, 66, 8243. (e) Sugimura, H.; Natsui, Y. Tet-
rahedron Lett. 2003, 44, 4729. (f) Schnabel, M.; Rompp,
B.; Ruckdeschel, D.; Unverzagt, C. Tetrahedron Lett. 2004, 45, 295. (g)
Tanaka, H.; Iwata, Y.; Takahashi, D.; Adachi, M.; Takahashi, T. J. Am.
Chem. Soc. 2005, 127, 1630. (h) Ohno, H.; Terui, T.; Kitawaki, T.;
Chida, N. Tetrahedron Lett. 2006, 47, 5747. (i) Hager, D.; Mayer, P.;
Paulitz, C.; Tiebes, J.; Trauner, D. Angew. Chem., Int. Ed. 2012, 51,
6525.
25. Zhang, S.-Y.; Li, Q.; He, G. Nack, W. A.; Chen, G. J. Am. Chem. Soc.
2013, 135, 12135.
26. Han, Z.; Pinkner, J.; Ford, B.; Obermann, R.; Nolan, W.; Wildman, S.;
Hobbs, D.; Ellenberger, T.; Cusumano, C.; Hultgren, S.; Janetka, J. J.
Med. Chem. 2010, 53, 4779.
27. No NMR spectra of isolated MPPs were provided in the original struc-
1
tural determination paper (ref 3). Our H and 13C-NMR spectra of
both MPP α and β fully agree with the listed NMR data within an error
of 0.1 ppm for 1H-NMR and 0.2 ppm for 13C-NMR.
14. Feichtinger, K.; Sings, H. L.; Baker, T. J.; Matthews, K.; Goodman, M. J.
Org. Chem. 1998, 63, 8432.
15. The low cyclization yield is caused by the competing elimination reac-
tion. For selected syntheses of cyclic guanidines: (a) DeMong, D. E.;
Williams, R. M. J. Am. Chem. Soc. 2003, 125, 8561. (b) Shimokawa, J.;
Shirai, K.; Tanatani, A.; Hashimoto, Y.; Nagasawa, K. Angew. Chem., Int.
Ed. 2004, 43, 1559. (c) Ref 11.
16. The unique large coupling constant of H1’ at the anomeric position of
mannose in 2 (δ 5.86 ppm, d, J1’,2’ = 8.9 Hz, in CDCl3) agrees with the
proposed 1C4 conformation of N-linked α-mannose in MPPs.
17. (a) Li, Y.; Yang, Y.; Yu, B. Tetrahedron Lett. 2008, 49, 3604. (b) Zhu, Y.;
Yu, B. Angew. Chem., Int. Ed. 2011, 50, 8329. (c) Tang, Y.; Li, J.; Zhu,
Y.; Li, Y.; Yu, B. J. Am. Chem. Soc. 2013, 135, 18396.
18. (a) Liang, X.; Andersch, J.; Bols, M. J. Chem. Soc., Perkin Trans. 2001,
2136. (b) Ojima, I.; Vidal, E. J. Org. Chem. 1998, 63, 7999. (c) 11 was
also used as SM in ref 11b and 11c .
19. The corresponding Bn-protected analogues of 16, 19, and 22 (Scheme
2B) were also prepared using the same strategy for precursor 12. How-
ever, we found that a Bn protecting group at Cβ OH of these analogues
cannot be cleanly removed under catalytic hydrogenolysis conditions.
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Page 8 of 8
1
2
3
4
5
6
HO
OH
7
8
9
OH
HO
OH
OH
HO
O
O
NH
NH
OH
O
Gold-catalyzed
N-mannosylation
O
L-βhEnd
HN
OH
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
HO
O
HN
trichloroacetimidate
O-glycosylation
N
OH O
OH
O
N-Man-D-βhEnd
OH
HN
D-Tyr(di-Man)
N
H
HN
O
NH
HN
NH HN
Gly
L-Ser
O
HO
Pd-catalyzed
C−H arylation
O
O
L-βMePhe
mannopeptimycin α
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