Efficient synthesis of lactate-containing depsipeptides by the mitsunobu reaction of lactates

Article abstract of DOI:10.1002/adsc.200404252

The Mitsunobu reaction has been used as an efficient tool for the synthesis of orthogonally-protected and unprotected depsipeptides such as Boc/Fmoc-L-Lys(Alloc)-D-Ala-D-Lac-OAllyl or L-Lys-D-Ala-D-Lac that are bacterial cell wall precursor analogues foun

Full text of DOI:10.1002/adsc.200404252

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DOI: 10.1002/adsc.200404252
Efficient Synthesis of Lactate-Containing Depsipeptides by the
Mitsunobu Reaction of Lactates
Tobias Grab, Stefan Bräse*
Institut für Organische Chemie, Universität Karlsruhe (TH), Fritz-Haber-Weg 6, 76131 Karlsruhe, Germany
Fax: þ497216088581, e-mail: braese@ioc.uka.de
Received: August 8, 2004; Revised: May 21, 2005; Accepted: June 9, 2005
Abstract: The Mitsunobu reaction has been used as
an efficient tool for the synthesis of orthogonally-
protected and unprotected depsipeptides such as
Boc/Fmoc-l-Lys(Alloc)-d-Ala-d-Lac-OAllyl or l-Lys-
d-Ala-d-Lac that are bacterial cell wall precursor
analogues found in vancomycin-resistant entero-
cocci.
Keywords: allyl lactate; antibiotic resistance; depsi-
peptides; Mitsunobu reaction; vancomycin
The synthesis of the small depsipeptide l-Lys-d-Ala-d-
Lac (1) is relevant to problems associated with the resist-
ance of bacteria to the glycopeptide antibiotic vancomy-
cin (2), which is considered to be the last resort for the
treatment of infections caused by Gram-positive bacte-
[1]
ria. The latter is well known to inhibit cell wall biosyn-
thesis of pathogenic bacteria and effects bacterial cell ly-
sis by binding to the C-terminal l-Lys-d-Ala-d-Ala mo- Figure 1. l-Lys-d-Ala-d-Lac (1) and vancomycin (2).
3
tif present in cell-wall precursors (Figure 1). A 10 -fold
loss of binding affinity to the resistant cell-wall sequence
is due to the bacterial formation of l-Lys-d-Ala-d-Lac.
The simple replacement of an amide NHgroup for an es-
ter oxygen results in the replacement of a stabilizing hy-
drogen bond with a destabilizing lone pair-lone pair re-
[
2]
pulsion within the vancomycin-ligand complex.
In the course of an antimicrobial discovery project, we
needed a larger amount of the orthogonally protected
depsipeptides 3, 4 or their derivatives (Figure 2). In con- Figure 2. Depsipeptides Boc-l-Lys(Alloc)-d-Ala-d-Lac-OAll-
[
3]
trast to the published syntheses by Williams and Bo- yl (3) and Fmoc-l-Lys(Alloc)-d-Ala-d-Lac-OAllyl (4).
[4]
ger, we proposed to generate the ester bond via the
Mitsunobu reaction, the advantages being economical
starting material l-lactate and potentially high yields. (DEAD) and triphenylphosphine (PPh ) proceeds
3
While Li-d-Lac costs EUR 195.52 per g, l-lactic acid is with complete Walden inversion of the alcohol compo-
[
5]
[7]
obtainable for EUR 0.04 per g.
The Mitsunobu reaction, first reported by Oyo Mitsu-
nent.
However, Mitsunobu reactions at sterically and/or
[6]
nobu in 1967, is today one of the most important organ- electronically disfavoured hydroxy compounds can
ic transformations. The generation of the esters in high proceed in part via acyloxyphosphonium ions and,
yields from the reaction of alcohols and carboxylic thus, they might occur with retention of the configura-
[
8]
acid in the presence of diethyl azodicarboxylate tion.
Adv. Synth. Catal. 2005, 347, 1765 – 1768
ꢀ 2005 Wiley-VCHVerlag GmbH& Co. KGaA, Weinheim
1765

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Tobias Grab, Stefan Bräse
[9]
Boger reported the first total synthesis of a cyclic hep- group already took place at a temperature of 08C. The
tadepsipeptide via a Mitsunobu reaction in 1999. Howev- Fmoc protective group is labile under the conditions
er, the full exploration of this useful method using a range used. The basic hydrazide anion, formed during the Mit-
of aliphatic carboxylic acids has not yet been devised. sunobu reaction, induces the deprotection. The nucleo-
[
10]
This reaction is known both in liquids and on solid sup- philes benzoic acid (5c), 2-naphthoic acid (5d) and 2-
[11]
ports. Therefore, we decided to choose this etherifica- chloro-4-nitrobenzoic acid (5e) reacted in the same
tion of the depsipeptides with lactates. way with l-Lac-OAllyl (6). In addition to DEAD, the
The protecting group strategy is based on the demands Mitsunobu reaction was also tested with di-tert-butyl
of former studies. Thus, the carbonyl function of l-lac- azodicarboxylate, and dibenzyl azodicarboxylate with-
tate was protected as an allyl ester. For the Mitsunobu out significant differences. To prove the stereochemical
reaction, d-alanine was protected by the common amino integrity of this reaction, both enantiomers of alanine,
protective groups Boc and Fmoc. In a first trial, the Mit- Boc-d-Ala-OH( 5) and Boc-l-Ala-OH( 5a) were tested.
sunobu reaction was tested with the nucleophiles Boc-d- The optical rotation showed that the stereochemical in-
Ala-OH( 5), Boc-l-Ala-OH( 5a), Fmoc-d-Ala-OH tegrity remained throughout the reaction.
(
5b), benzoic acid (5c), 2-naphthoic acid (5d), and 2-
chloro-4-nitrobenzoic acid (5e) with different results derivatives, the N-terminus of the protected depsipepti-
Scheme 1, Table 1). des PG-d/l-Ala-d-Lac-OAllyl 7, 7a, 7b were deprotect-
The starting materials and PPh were dissolved in ed under classical conditions. The controlled deprotec-
In preparation for the following coupling with l-Lys
(
3
THF, DEAD was added dropwise at 08C and the reac- tion of Boc and Fmoc shows an interesting feature.
tion mixture was warmed to room temperature within The deprotection of Boc (5% TFA/CH Cl ) under clas-
2
2
4
h while stirring. To work up the reaction mixtures sical conditions without any scavenger was successful
the solvent was removed, the residue was dissolved in (90, 92%) and left the labile ester bond intact to give
ethyl acetate and washed with saturated aqueous NaH- the amine 8 (Scheme 2), removing the Fmoc group
CO solution. The crude products were purified by col- (20% piperidine/DMF) in 7b lead to a diketomorpho-
3
umn chromatography (cyclohexane/ethyl acetate).
line 9 (79%). Obviously, basic conditions induce a nucle-
While Boc-d-Ala-OH( 5) reacted with allyl l-lactate ophilic attack of the deprotected amine to the lactate
6) in quite good yields to Boc-d-Ala-d-Lac-OAllyl (7) carboxylic carbon (Figure 3).
(
(
89% with 2 equivalents of DEAD/PPh , 84% with 4
The Mitsunobu products d-Ala-d-Lac-OAllyl (8) and
3
equivalents of DEAD/PPh , 5a to 7a in 83%), the Mitsu- l-Ala-d-Lac-OAllyl (8a) were coupled to Fmoc-l-Lys
3
[
12]
nobu reaction with Fmoc-d-Ala-OH( 5b) resulted in (Alloc)-OH(DMF, HBTU
, 1 h, room temperature)
only 14% of the desired product 6b (2 equivalents of and the depsipeptides Fmoc-l-Lys(Alloc)-d-Ala-d-
DEAD/PPh ). Even increasing the amount of DEAD/ Lac-OAllyl (4) and Fmoc-l-Lys(Alloc)-l-Ala-d-Lac-
3
PPh up to four equivalents did not result in higher OAllyl (4a) were generated in 89% (4) and 91% (4a)
3
31
1
13
yields. By means of P, Hand C NMR, it became ob- yield (Scheme 2).
vious that the particular cleavage of the Fmoc protecting
Scheme 1. Mitsunobu reactions of acids with l-Lac-OAllyl
6).
Figure 3. Diketomorpholine 9 after deprotection of Fmoc
from 7b.
(
Table 1.
[a]
Carboxylic acid
Conditions
Product
Yield [%]
Boc-d-Ala-OH( 5)
Boc-d-Ala-OH( 5)
Boc-l-Ala-OH( 5a)
Fmoc-d-Ala-OH( 5b)
benzoic acid (5c)
2 equivs. DEAD, PPh3
4 equivs. DEAD, PPh3
2 equivs. DEAD, PPh3
2 equivs. DEAD, PPh3
2 equivs. DEAD, PPh3
2 equivs. DEAD, PPh3
2 equivs. DEAD, PPh3
4 equivs. DEAD, PPh3
7
7
89
84
83
14
51
43
64
40
7a
7b
7c
7d
7e
3
2-naphthoic acid (5d)
2-chloro-4-nitrobenzoic acid (5e)
Boc-l-Lys(Alloc)-d-Ala-OH( 10)
[
a]
After column chromatography.
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Adv. Synth. Catal. 2005, 347, 1765 – 1768

Synthesis of Lactate-Containing Depsipeptides by the Mitsunobu Reaction
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Scheme 4. Synthesis of the depsipeptide 3.
Scheme 2. Synthesis of the depsipeptide 4 (analogue 7a via 8a
to 4a in 91% yield).
al way to this and other important depsipeptides involv-
ing the lactate moiety.
For former studies l-Lys was orthogonally protected
in a different manner. Using CuCO Cu(OH) the e-po-
3
2
[
13]
Experimental Section
sition of l-Lys was protected with Alloc (96%). After
removing the a-complexing Cu via thioacetate, the a-
[
14]
position was protected with Boc (79%). Final coupling Boc-d-Ala-d-Lac-OAllyl (7) via Mitsunobu Reaction
with the Mitsunobu product d-Ala-d-Lac-OAllyl (8)
A solution of l-Lac-OAllyl (1.30 g, 10 mmol), Boc-d-Ala
also succeeded to the depsipeptide Boc-l-Lys(Alloc)-
(
1.89 g, 10 mmol) and triphenylphosphine (5.24 g, 20 mmol)
d-Ala-d-Lac-OAllyl (3), a starting material for further
investigations (Scheme 3).
To verify and compare the efficiency of the Mitsunobu
in 80 mL absolute THF was cooled to 08C under an argon at-
mosphere. DEAD (3.14 mL, 20 mmol) was added dropwise
and the reaction mixture was stirred at room temperature for
reaction for the synthesis of larger depsipeptides, the es- 4 h. The solvent was removed under reduced pressure. The res-
ter bond was also created as the final assembly step. d- idue was dissolved in 50 mL EtOAc and washed three times
[
15]
with 50 mL of a saturated solution of NaHSO . The organic lay-
Ala-OBn
(
was coupled with Boc-l-Lys(Alloc)-OH
4
er was dried over Na SO . The solvent was removed under re-
DMF, HBTU, 1 h, 97%). The benzyl ester was cleaved
during the work-up, a controlled deprotection was not
necessary. The Mitsunobu coupling with allyl l-lactate
6) gave the orthogonally protected depsipeptide 3
2
4
duced pressure. The crude product was purified by flash chro-
matography (ethyl acetate/cyclohexane, 3:1) to give the de-
sired product; yield: 2.68 g (89%); R ¼0.28 (ethyl acetate/cy-
f
(
(
1
clohexane 3:1); HNMR (400 MHz, CDCl ): d¼1.44 (d, 3H,
3
Scheme 4).
3
J¼7.15 Hz, CH -Lac), 1.46 (s, 12H, CH -Boc), 1.51 (d, 3H,
3
3
These results revealed that an initial Mitsunobu
reaction and subsequent peptide coupling led to a
3
3
J¼7.10, CH -Ala), 4.15 (q, 1H, J¼7.10 Hz, CH-Ala), 4.45
3
3
3
4
(
q, 1H, J¼7.10 Hz, CH-Lac), 4,66 (ddd, 2H, J¼5.70, J¼
3
much better overall yield (72%) than an inverted strat- 2.40, 1.26 Hz, methylene-CH ), 5.24 (ddt, 1H, J¼10.20, 2.40,
2
egy (39%).
1.26 Hz, allyl-CH ), 5.32 (m, 1H, allyl-CH ), 5.91 (m, 1H, all-
2 2
13
yl-CH); C NMR (100 MHz, CDCl ): d¼17.1 (CH -Ala),
In comparison to the published syntheses of the cell
3
3
[
3,4]
1
7.5 (CH -Lac), 28.7 (CH -Boc), 50.3 (CH-Ala), 66.8 (CH -
3 3 2
wall precursor l-Lys-d-Ala-d-Lac, this new strategy
starting with a Mitsunobu reaction of allyl l-lactate fol-
lowed by peptide coupling opens an efficient and ration-
methylene), 70.4 (CH-Lac), 80.5 (Cquart-Boc), 118.8 (CH -all-
2
yl), 133.0 (CH-Allyl), 156.7 (carbamate-Boc), 170.6 (car-
bonyl-Ala), 174.6 (carbonyl-Lac); IR (KBr): n¼3382 (s),
2
1
1
7
980 (s), 2939 (s), 1747 (m), 1716 (w), 1649 (s), 1513 (m),
454 (m), 1367 (m), 1297 (m), 1251 (m), 1166 (w), 1132 (s),
097 (m), 1067 (m), 979 (s), 937 (s), 880 (s), 856 (s), 781 (s),
ꢀ
1
þ
58 (s) cm ; FAB-MS: m/z (%)¼603 (4) [2 M þH], 302
þ
þ
þ
(
ꢀ
16) [M þH], 246 (48), 202 (100) [M ꢀBoc], 189 (8) [M
2
0
LacꢀOAllyl]; [a] :þ5.92 (c 0.10 g, THF). [Boc-l-Ala-d-
D
2
0
Lac-OAllyl (7a): [a] : ꢀ0.90 (c 0.12 g, THF)].
D
Fmoc-l-Lys(Alloc)-d-Ala-d-Lac-OAllyl (4) via
Peptide Coupling
A solution of d-Ala-d-Lac-OAllyl [(generated via Mitsunobu
reaction, 89% yield, and Boc-deprotection, 90% yield);
1.00 g, 5.0 mmol] and DIPEA (646 mg, 5.0 mmol) was pre-
pared in 4 mL of absolute DMF. Fmoc-l-Lys(Alloc)-OH
Scheme 3. Synthesis of the orthogonally protected depsipep-
tide 3.
Adv. Synth. Catal. 2005, 347, 1765 – 1768
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Tobias Grab, Stefan Bräse
(
2.34 g, 5 mmol) and HBTU (2.09 g, 5.5 mmol) were also dis-
Vanderheiden and Sabrina Lumpp for dedicated experimental
help.
solved in 4 mL of DMF. Under argon atmosphere both mix-
tures were added in parallel and dropwise into a vial. The reac-
tion mixture was stirred at room temperature for 1 h and then
poured on 250 mL of cold water. The crude product was fil-
tered, washed with water and pentane and purified by flash
chromatography (ethyl acetate, methanol) to give a white sol-
References and Notes
[
1] K. C. Nicolaou, C. N. C. Boddy, S. Bräse, N. Winssinger,
Angew. Chem. 1999, 111, 2230–2287.
id; yield: 2.34 g (91%); R ¼0.75 (dichloromethane/methanol);
f
1
HNMR (500 MHz, DMSO): d¼1.25–1.67 (m, 14H, CH -Lys,
2
[
2] C. T. Walsh, S. L. Fisher, I. S. Park, M. Prahalad, Z. Wu,
Chem. Biol. 1996, 3, 21–28.
CH -Lac, CH -Ala), 4.14 (m, 1H, CH-Fmoc), 4.20 (q, 1H,
3
3
3
3
J¼7.10 Hz, CH-Ala), 4.59 (q, 1H, J¼7.10 Hz, CH-Lac),
3
[3] Y. R. Cho, R. M. H. Entress, D. H. Williams, Tetrahedron
Lett. 1997, 38, 5229–5232.
[4] C. C. McComas, B. M. Crowley, D. L. Boger, J. Am.
Chem. Soc. 2003, 125, 9314–9315.
4
.80 (m, 4H, O-CH ), 5.21 (d, 2H, J¼2.21 Hz, O-CH -
2
2
Fmoc), 5.89 (m, 6H, allyl/Alloc), 7.36–7.74 (m, 8H, Fmoc);
13
C NMR (125 MHz, DMSO): d¼17.1, 17.4, 23.4, 29.5, 30.8,
1.1, 31.2, 32.3, 36.2, 39.5, 39.6, 40.2, 40.4, 47.1, 54.2, 54.7,
4.5, 65.6, 66.0, 117.3, 118.6, 120.5, 125.8, 127.5, 128.1, 132.4,
34.3, 141.1, 144.2, 144.3, 156.3, 162.7, 174.4; IR (KBr): n¼
302 (s), 3068 (w), 2968 (m), 1956 (w), 1916 (w), 1693 (s),
651 (m), 1536 (s), 1451 (m), 1410 (w), 1335 (w), 1249 (m),
150 (w), 1105 (w), 1045 (m) 992 (m), 938 (m), 859 (m), 777
3
6
1
3
1
1
[
5] Acros Organics, Li-d-Lac: EUR 97.76 (500 mg); l-lactic
acid: EUR 11.13 (250 mL).
[
6] a) O. Mitsunobu, M. Yamada, T. Mukaiyama, Bull.
Chem. Soc. Jpn. 1967, 40, 435–439; b) O. Mitsunobu,
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1992, 42, 335.
ꢀ
1
(
w), 759 (m), 740 (s), 660 (m), 621 (w) cm ; FAB-MS: m/z
þ
(
%)¼622 (12) [M þH], 506 (8), 414 (15), 356 (10), 202 (20)
[7] T. Onozawa, Org. Synth. 1995, 72, 273.
[8] a) H. Kunz, P. Schmidt, Chem. Ber. 1979, 112, 3886–
3894; b) H. Kunz, P. Schmidt, Liebigs Ann. Chem. 1982,
þ
20
[
M ꢀFmocꢀLys(Alloc)], 179 (100), 136 (15); [a] : þ11.6 (c
D
0
.10, THF).
1
Fmoc-l-Lys(Alloc)-l-Ala-d-Lac-OAllyl (4a): HNMR
1
245–1260; c) J. McNulty, A. Capretta, V. Laritchev, J.
(
500 MHz, DMSO): d¼1.28–1.69 (m, 14H, CH -Lys, CH -
2
3
Dyck, A. J. Robertson, Angew. Chem. 2003, 115, 4185–
4188.
3
Lac, CH -Ala), 4.14 (m, 1H, CH-Fmoc), 4.23 (q, 1H, J¼
3
3
7
(
5
(
3
6
1
.10 Hz, CH-Ala), 4.61 (q, 1H, J¼7.10 Hz, CH-Lac), 4.82
[
9] D. L. Boger, H. Keim, B. Oberhauser, E. P. Schreiner,
C. A. Foster, J. Am. Chem. Soc., 1999, 121, 6197–6205.
3
m, 4H, O-CH ), 5.21 (d, 2H, J¼2.21 Hz, OꢀCH ꢀFmoc),
2
2
1
3
.88 (m, 6H, allyl/Alloc), 7.36–7.74 (m, 8H, Fmoc); C-NMR
[
[
[
10] J. McNulty, A. Capretta, V. Laritchev, J. Dyck, A. J. Rob-
ertson, Angew. Chem. 2003, 115, 4185–4188.
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2001, 66, 2178–2180.
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um hexafluorophosphate; CAS No. 94790-37-1.
13] A. Crivici, G. Lajoie, Synth. Commun. 1993, 1, 23, 49–53.
14] R. Schwyzer, D. K. Quang, A. N. Eberle, J. L Fauchere,
Hel. Chim. Acta 1981, 64, 2078–2083.
125 MHz, DMSO): d¼17.2, 17.3, 23.3, 29.4, 29.6, 30.8, 31.2,
2.1, 36.2, 39.4, 39.6, 40.3, 40.4, 47.1, 54.2, 54.6, 64.5, 65.6,
6.0, 117.3, 118.5, 120.5, 125.7, 127.5, 128.1, 132.5, 134.3,
2
0
41.1, 144.2, 144.3, 156.6, 162.7, 174.4; [a] : ꢀ0.20 (c 0.15,
D
THF).
[
[
Acknowledgements
[
15] S. D. Bull, S. G. Davies, G. Fenton, A. W. Mulvaney, R. S.
Prasad, A. D. Smith, J. Chem. Soc. Perkin Trans. 1 2000,
Financial support from the DFG (SFB 624) and the Friedrich-
Naumann-Stiftung is gratefully acknowledged. We thank Sylvia
3
765–3774.
1768
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Adv. Synth. Catal. 2005, 347, 1765 – 1768