Synthesis of the AviMeCys-containing D-ring of mersacidin

Article abstract of DOI:10.1021/ol2034806

A chemical synthesis of the D-ring of mersacidin is reported. The synthetic route relied upon development of a method for late-stage introduction of an unusual S-[(Z)-2-aminovinyl]-(3S)-3-methyl-d-cysteine (AviMeCys) functional group via an oxidative decarbonylation/decarboxylation reaction.

Full text of DOI:10.1021/ol2034806

ORGANIC
LETTERS
2012
Vol. 14, No. 4
1034–1037
Synthesis of the AviMeCys-Containing
D-Ring of Mersacidin
Angela K. Carrillo and Michael S. VanNieuwenhze*
Department of Chemistry, Indiana University, 800 East Kirkwood Avenue, Bloomington,
Indiana 47405-7102, United States
mvannieu@indiana.edu
Received December 29, 2011
ABSTRACT
A chemical synthesis of the D-ring of mersacidin is reported. The synthetic route relied upon development of a method for late-stage introduction
of an unusual S-[(Z)-2-aminovinyl]-(3S)-3-methyl-D-cysteine (AviMeCys) functional group via an oxidative decarbonylation/decarboxylation
reaction.
Mersacidin (1) is a 20-residue polycyclic lantibiotic
peptide that possesses promising antibacterial activity
against Gram-positive organisms. In addition, it has dis-
played good activity against problematic Gram-positive
pathogens, in particular, methicillin-resistant Staphylococ-
cus aureus (MRSA) and vancomycin-resistant entercocci
(VRE), which have shown significant levels of resistance to
commonly used members of the currently available anti-
biotic arsenal.1,2
Mersacidin is believed to inhibit the transglycosylation
reaction in the bacterial cell wall biosynthetic pathway by
forming a stoichiometric complex with lipid II, the final
monomeric intermediate utilized by the bacterial cell for
assembly of its cell wall. Sequestration of lipid II from the
transglycosylases, enzymes that polymerize lipid II into the
glycan strands that typify bacterial cell wall architecture,
ultimately compromises the integrity of the bacterial cell
wall and results in lysis and subsequent cell death.
Interestingly, mersacidin does not interact with the Lys-
D-Ala-D-Ala portion of the stem peptide present in the lipid
II intermediate and thus appears to target a site on lipid II
that differs from the site utilized by vancomycin.3 Con-
sidering the serious public health threat posed by bacterial
resistance to glycopeptide antibiotics, mersacidin may rep-
resent a promising avenue for development of new anti-
bacterial therapeutics with activity versus problematic re-
sistant organisms.
The CD bicyclic fragment is believed to have a
major role in the binding of mersacidin with lipid II.4
(3) (a) Brotz, H.; Bierbaum, G.; Markus, A.; Molitor, E.; Sahl, H. G.
Antimicrob. Agents Chemother. 1995, 39, 714.3. (b) Brotz, H.; Bierbaum,
G.; Reynolds, P. E.; Sahl, H.-G. Eur. J. Biochem. 1997, 246, 193.3. (c)
Brotz, H.; Bierbaum, G.; Leopold, K.; Reynolds, P. E.; Sahl, H.-G.
Antimicrob. Agents Chemother. 1998, 42, 154.
(1) Chatterjee, S.; Chatterjee, D. K.; Rajendra, H. J.; Blumbach, J.;
Ganguli, B. N.; Klesel, N.; Limbert, M.; Seibert, G. J. Antibiot. 1992, 45,
839.
(2) (a) Chatterjee, C.; Paul, M.; Xie, L.; van der Donk, W. A. Chem.
Rev. 2005, 105, 633. (b) Willey, J. M.; van der Donk, W. A. Annu. Rev.
Microbiol. 2007, 61, 477–501.
(4) Hsu, S-T.D.; Breukink, E.; Bierbaum, G.; Sahl, H.-G.; de Kruijff,
B.; Kaptein, R.; van Nuland, N. A. J.; Bonvin, A. M. J. Biol. Chem. 2003,
278, 13110.
r
10.1021/ol2034806
Published on Web 02/01/2012
2012 American Chemical Society

However, the exact nature of this interaction is unknown.
StructureÀactivity relationship studies (SAR) of mersaci-
din and its derivatives could reveal important insights into
its mechanism of action and could contribute to the
rational design of new lead compounds.5
As part of our effort directed toward the total synthesis
of mersacidin, we initially focused our attention on the
development of an efficient methodology that would in-
troduce the synthetically challenging S-[(Z)-2-aminovinyl]-
(3S)-3-methyl-D-cysteine (AviMeCys) into the polypeptide
structure of a D-ring derivative. Thus, we report herein the
synthesis of the D-ring derivative (2) containing the Avi-
MeCys unit.
our knowledge, however, these methods have not been
tested on cysteine containing peptides as would be required
for the AviMeCys subunit of mersacidin.10 In order to
evaluatethe potential utility of these methods for late-stage
introduction of the AviMeCys subunit on a fully function-
alized (C)D-ring precursor, a model study was carried out
on a simplified β-methyllanthionine (MeLan) derivative 8.
The synthesis of protected 8 relied on the alkylation of
β-MeCys 5 with bromoalanine 611 following methodol-
ogy previously developed in our laboratory12 (Scheme 1).
Cleavage of the Fmoc group under standard condi-
tions (20% piperidine, DMF), followed by reprotection
of the free amine with Cbz (BnOCOCl, NaHCO3,
EtOAc, H2O), provided diester 7. Finally, cleavage of
the tert-butyl ester (TFA, CH2Cl2) provided the target
MeLan derivative 8.
Scheme 1
Figure 1. Biosynthetic pathway for introduction of AviMeCys.
Our initial results are summarized in Scheme 2. Expo-
sure of 8 to the oxidative decarbonylation conditions (2.2
equiv of diphenylphosphoryl azide (DPPA), 2.2 equiv of
TEA, toluene) resulted in the formation of protected
AviMeCys 9 as a mixture of stereoisomers in good
yields. The same occurred when subjecting 8 to 1 equiv
of Pb(AcO)4 in the presence of 1 equiv of Cu(OAc)2 under
oxidative decarboxylation conditions. Although, the Avi-
MeCys derivative was obtained as a mixture of Z and
E isomers, we were encouraged by these results and were
hoping that conformational constraints imposed by the
cyclic peptide substrate would facilitate preferential for-
mation of the desired (Z)-AviMeCys moiety upon subject-
ing a suitably protected D-ring precursor to the optimized
oxidative decarboxylation conditions.
The (Z)-aminovinyl methyl sulfide bond found in mersa-
cidin is the result of posttranslational modification mediated
by the enzyme MrsD.6 The proposed mechanism is believed
to involve the formation of a thioaldehyde intermediate,
which spontaneously decarboxylates to an enethiolate.
This enethiolate intermediate subsequently carries out a
1,4-conjugate addition reaction to an adjacent dehydroa-
mino acid to install the vinyl sulfide bond (Figure 1).7
Inspired by this mechanism, we set out to test different
routes that would leverage a carboxyl handle for late-stage
introduction of the enamide linkage from a readily avail-
able peptide precursor. Previous reports in the literature
have described similar strategies for the synthesis of en-
amides. These include the oxidative decarbonylation of
activated carboxylic acids8 and the oxidative decarboxyla-
tion of amino acids using lead tetraacetate.9 To the best of
Scheme 2
(5) (a) Szekat, C.; Jack, R. W.; Skutlarek, D.; Farber, H.; Bierbaum,
G. Appl. Environ. Microbiol. 2003, 69 (7), 3777–3783. (b) Appleyard,
A. N.; Choi, S.; Read, D. M.; Lightfoot, A.; Boakes, S.; Hoffmann, A.;
Chopra, I.; Bierbaum, G.; Rudd, B. A. M.; Dawson, M. J.; Cortes, J.
Chem. Biol. 2009, 16, 490–498. (c) Levengood, M. R.; Knerr, P. J.;
Oman, T. J.; van der Donk, W. A. J. Am. Chem. Soc. 2009, 131, 12024–
12025.
(6) Majer, F.; Schmid, D. G.; Altena, K.; Bierbaum, G.; Kupke, T.
J. Bacteriol. 2002, 184, 1234.
(7) For a recent review of AviCys-containing lantibiotics, see: Sit,
C. S.; Yoganathan, S.; Vederas, J. C. Acc. Chem. Res. 2011, 44, 261–268.
(8) (a) Martin-Lopez, M. J.; Bermejo-Gonzalez, F. Tetrahedron Lett.
1994, 4235. (b) Martin-Lopez, M. J.; Bermejo-Gonzalez, F. Tetrahedron
Lett. 1994, 35, 8843.
(10) For a review on recent developments in lantibiotic synthesis, see:
Tabor, A. B. Org. Biomol. Chem. 2011, 9, 7606–7628.
(9) Wang, X.; Porco, J. A. J. Org. Chem. 2001, 66, 8215.
Org. Lett., Vol. 14, No. 4, 2012
1035

Cyclic peptide 3 was selected as the precursor for intro-
duction of the AviMeCys subunit. Since the AviMeCys
unit is acid-sensitive, the side-chain protecting groups were
selected in order to achieve the dual goals of mutual
orthogonality while maintaining nonacidic cleavage con-
ditions. Additionally, orthogonal protection for each of
the serine residues is required since subsequent conversion
into a CD-ring system requires that one must be converted
into a didehydroalanine, while the other will be incorpo-
rated into the C-ring lanthionine bridge.
Our retrosynthetic analysis for 2 is shown in Scheme 3.
Our macrocyclization precursor was selected with Ile at the
C-terminus. This site was selected for amide bond forma-
tion since the Ile is less prone to epimerization than Ser or
Glu. In addition, this site is proximate to the site at which
MrsD catalyzes the cyclization reaction in mersacidin
(Figure 1), thus presenting the possibility that, although
in protected form, the peptide might exist in a conforma-
tion to facilitate macrocyclization.
TBDPS ether (TBDPS-Cl, imidazole, THF) followed by
cleavage of the allyl ester (Pd(OAc)2, PS-PPh3, PhSiH3,
CH2Cl2) afforded the acid in 65% overall yield. Finally,
serine derivative 15 was prepared by protection of alcohol
19 as a p-nitrobenzyl ether (Ag2O, p-nitrobenzyl bromide,
toluene) followed by cleavage of the methyl ester under
standard conditions (LiOH, THF, H2O).
Scheme 4
Scheme 3
Our synthesis of 3 began with the coupling of 16 and 15,
under standard peptide coupling conditions (DEPBT,
DIEA, THF), affording 21 in 75% yield (Scheme 5).
DEPBT (3-(diethoxyphosphoryloxy)-1,2,3-benzotriazin-
4(3H)-one) was the coupling reagent of choice since its
ability to suppress racemization at the adjacent stereocen-
ter of activated carboxylates has been demonstrated.15
The N-terminal Boc protecting group found in 21 was
cleanly removed (4 N HCl, dioxane), and the resulting
free amine was coupled with Fmoc-Glu(OMe)-OH 14
(DEPBT, DIEA, THF) to provide 22 in 85% yield.
Cleavage of the N-terminal Fmoc protecting group of 22
(20% piperidine, DMF) followed by coupling with 13
(DEPBT, DIEA, THF) provided tetrapeptide 23 in 75%
yield. Fmoc deprotection (20% piperidine, DMF) of 23
and coupling with MeLan 12 (DEPBT, DIEA, THF)
provided 11 in 75% yield. Cleavage of the C-terminal allyl
ester (Pd(OAc)2, PS-PPh3, PhSiH3, CH2Cl2) provided the
free carboxylic acid in 98% yield. Deprotection of the
N-terminal Fmoc protecting group (20% piperidine, DMF),
followed by cyclization of the peptide (PyBOP, DMAP,
DIEA, DMF, CH2Cl2), afforded D-ring precursor 10 in
65% overall yield.
Ile 16 and Glu 14 derivatives were commercially avail-
able, but the serine residues (13 and 15) and MeLan 12 had
to be synthesized (Scheme 4). MeLan 12 was prepared by
selective deprotection of the methyl ester of 4 using Nico-
laou conditions ((CH3)3SnOH, 1,2-dichloroethane).13 For
the synthesis of 13, Fmoc-Ser-OH 17 was converted to its
allyl ester in excellent yields (allyl-Br, CsCO3, DMF).14
Protection of the side chain hydroxyl group of 18 as a
In order to set the stage for investigation of reaction
conditions for oxidative decarboxylation, the C-terminal
tert-butyl ester of 10 was cleaved under standard condi-
tions (50% TFA in CH2Cl2, 0 °C), affording carboxylic
acid 3 in good yield. In addition, carboxylic acid 3 was
easily converted to the corresponding thioester 24 under
standard conditions (PhSH, PyBOP, DIEA, CH2Cl2, 0 °C
to rt, 78%).
(11) Bromoalanine 6 was prepared following reported procedures.
(a) Ginisty, M.; Gravier-Pelletier, C.; Le Merrier, Y. Tetrahedron:
Asymmetry 2006, 17, 142. (b) Zhu, X.; Schmidt, R. R. Chem.;Eur. J.
2004, 10, 875.
(12) Narayan, R. S.; VanNieuwenhze, M. S. Org. Lett. 2005, 7, 2655.
(13) Nicolaou, K. C.; Estrada, A. A.; Zak, M.; Lee, S. H.; Safina,
B. S. Angew. Chem., Int. Ed. 2005, 44, 1378.
Our initial attempt to introduce the (Z)-AviMeCys
subunit sought to take advantage of Ni(0)-mediated
(14) Ficht, S.; Payne, R. J.; Guy, R. T.; Wong, C. H. Chem.;Eur. J.
2008, 14, 3620.
(15) Li, H.; Jiang, X.; Ye, Y-h; Fan, C.; Romoff, T.; Goodman, M.
Org. Lett. 1999, 1, 91.
1036
Org. Lett., Vol. 14, No. 4, 2012

Scheme 5
Scheme 6
DABCO, and 1,4-dioxane gave the best results, providing
D-ring derivative 2 containing the desired (Z)-AviMeCys
in 25À30% yield.
Acid 3 was also exposed to the oxidative decarboxyla-
tion conditions utilizing lead tetraacetate. However, when
using 1 equiv of both Pb(OAc)4 and Cu(OAc)2, the reac-
tion produced none ofthe desiredproduct. We feltthat this
result may be due tothe greater number of Lewis basic sites
present in 3 relative to those in MeLan 8, our test substrate.
Thus, when 5 equiv of each reagent were added under the
same conditions, the desired product 2 was formed in
25À30% yield.
In summary, we were able to synthesize the D-ring of
mersacidin that contains the unusual amino acid S-[(Z)-2-
aminovinyl]-(3S)-3-methyl-D-cysteine (AviMeCys). Our
strategy took advantage of a C-terminal carboxyl group
that enabled late-stage introduction of the enamide sub-
unit via an oxidative decarbonylation reaction promoted
by DPPA or an oxidative decarboxylation promoted by
Pb(OAc)4. These reactions produced the desired (Z)-en-
amide predominantly; only minor amounts of the (E)-
enamide were detected. The methodology reported above
is currently being applied toward the synthesis of the CD-
ring system of mersacidin, with the goal of gaining a better
understanding of how this subunit interacts with lipid II.
The results of these studies will be presented in due course.
decarbonylation reactions as described in the preceding
manuscript. Unfortunately, all efforts to facilitate oxida-
tive decarbonylation of 24 under the optimized reaction
conditions (1.2 equiv of Ni(PPh3)4, copper(I) 3-methylsa-
licylate (CuMeSal), dioxane) were unsuccessful as only
complex product mixtures were obtained.16 In response to
this disappointing result, we then turned our attention to
oxidative decarbonylation/decarboxylation of carboxylic
acid 3 via the reaction conditions described in Scheme 2.
As shown in Scheme 6 when 3 was exposed to the
oxidative decarbonylation conditions (DPPA, TEA,
toluene), the desired (major) product possessing the (Z)-
olefin geometry was obtained, albeit in poor yield. The
vicinal coupling constant found for the olefinic hydrogens
was consistent with coupling constant values for 1,2-dis-
ubstituted double bonds of (Z)-geometry (J = 8 Hz). A
2D-NOESY experiment (see Supporting Information)
also provided support for the cis geometry of the double
bond.
Acknowledgment. Financial support for this work was
provided by the NIH (AI 059327). Helpful discussions
with William Turner (Indiana University) and Pablo
Garcia-Reynaga (Indiana University) are gratefully
acknowledged.
Various bases and solvents were screened in an effort to
improve the reaction yield. The combination of DPPA,
Supporting Information Available. Experimental pro-
cedures describing the synthesis of all new compounds as
well as the characterization data are provided. This
material is available free of charge via the Internet at
http://pubs.acs.org.
(16) Spectroscopic examination of the reaction product mixture
appeared to show evidence of thiol elimination (described in the
preceding manuscript) as well as a product(s) arising from loss of the
p-nitrobenzyl protecting group. See also the preceding manuscript:
Garcıa-Reynaga, P.; Carrillo, A. K.; VanNieuwenhze, M. S. Org. Lett.
´
2012, 14, DOI: 10.1021/ol203399x.
The authors declare no competing financial interest.
Org. Lett., Vol. 14, No. 4, 2012
1037