Versatile and stereoselective syntheses of orthogonally protected β-methylcysteine and β-methyllanthionine

Article abstract of DOI:10.1021/ol0507930

(Chemical Equation Presented) Lantibiotics are a class of lanthionine (and/or β-methyllanthionine)-containing peptides with antibioitic activity against Gram-positive bacteria. As part of our research effort directed toward the synthesis and mechanistic s

Full text of DOI:10.1021/ol0507930

ORGANIC
LETTERS
2005
Vol. 7, No. 13
2655-2658
Versatile and Stereoselective Syntheses
of Orthogonally Protected
â
â
-Methylcysteine and
-Methyllanthionine
Radha S. Narayan and Michael S. VanNieuwenhze*
Department of Chemistry and Biochemistry, UniVersity of California, San Diego,
9500 Gilman DriVe, MC 0358, La Jolla, California 92093
msV@ucsd.edu
Received April 12, 2005
ABSTRACT
Lantibiotics are a class of lanthionine (and/or
â-methyllanthionine)-containing peptides with antibioitic activity against Gram-positive bacteria.
As part of our research effort directed toward the synthesis and mechanistic study of the lantibiotic peptide mersacidin (1), we report
stereoselective syntheses of orthogonally protected
acid components of the mersacidin architecture.
â-methylcysteine (â-MeCys) and â-methyllanthionine (â-MeLan), two key nonnatural amino
The onset of vancomycin resistance, particularly among
enterococcal and multidrug-resistant staphylococcal patho-
gens, has underscored the urgent need for an upgrade of the
antibiotic armamentarium for the treatment of infections
arising from Gram-positive pathogens. In this connection,
we have initiated a broad research program that entails
chemical synthesis and mechanistic studies of several peptide
antibiotics that inhibit bacterial peptidoglycan (PG) biosyn-
thesis via mechanisms distinct from that utilized by vanco-
mycin. Specifically, we are interested in lantibiotic mersa-
cidin (1), which has shown great promise in this area.
Mersacidin is active against several bacterial strains including
streptococci, bacilli, and methicillin-resistant Staphylococcus
aureus with mimimum inhibitory concentration (MIC) values
comparable to that of vancomycin.1 Even more interesting
is the observation that mersacidin is active against vanco-
mycin-resistant Enterococcus faecium, a strain that synthe-
sizes PG precursors terminating in D-Ala-D-Lac.2
Mersacidin is believed to exert its antibiotic activity by
inhibition of transglycosylation reaction in PG biosynthesis
via complexation with lipid II at the disaccharide-pyro-
phosphate domain, a site that is not believed to be targeted
by other known antibiotics.2,3 Its unique binding site as well
as inhibition of the transglycosylation event make it an
attractive target for development of novel therapeutics
because (i) transglycosylation occurs on the cell surface so
cellular penetration of the antibiotic is not a requirement for
activity and (ii) the transglycosylation reaction utilized in
PG biosynthesis is unique to bacterial cells. With the ultimate
(2) Bro¨tz, H.; Bierbaum, G.; Leopold, K.; Reynolds, P. E.; Sahl, H.-G.
Antimicrob. Agents Chemother. 1998, 42, 154.
(1) Chatterjee, S.; Chatterjee, D. K.; Jani, R. H.; Blumbach, J.; Ganguli,
B. N.; Klesel, N.; Limbert, M.; Siebert, G. J. Antibiot. 1992, 45, 839.
(3) Bro¨tz, H.; Bierbaum, G.; Markus, A.; Molitor, E.; Sahl, H.-G.
Antimicrob. Agents Chemother. 1995, 39, 714.
10.1021/ol0507930 CCC: $30.25
© 2005 American Chemical Society
Published on Web 05/20/2005

Scheme 1
Scheme 2
goal of delineating the exact mode of action of mersacidin
and its variants with lipid substrates, as well as developing
novel analogues with increased efficacy, we have embarked
on the total synthesis of 1.
The presence of the sensitive (Z)-amidovinyl sulfide
linkage in the CD ring system, as well as the unnatural amino
acid (2S,3S)-â-methylcysteine (â-MeCys) and (2S,3S,6R)-
â-methyllanthionine (â-MeLan) fragments in the A, B, and
CD ring systems render mersacidin a challenging synthetic
target. We initially focused our attention on development of
an efficient and enantioselective synthesis of these two
nonnatural amino acids since both are embedded in all four
rings of mersacidin. Moreover, an orthogonal protective
group scheme would be necessary for the regioselective
manipulation of these core residues. To the best of our
knowledge, despite the apparent simplicity in their structures,
efficient and stereoselective methods for preparation of
orthogonally protected versions of these two amino acids
(2, 5, and 6; Scheme 1) are not currently available.4,5 Herein,
we report versatile routes to â-MeCys and â-MeLan that
fulfill all the aforementioned requirements.
Our synthesis of protected â-MeLan (5 and 6) relies on
the stereoselective alkylation of â-MeCys derivative 2 with
serine-derived electrophiles of general representation 3 and
4 (Scheme 1). â-MeCys 2 would in turn be obtained by the
regio- and stereoselective ring opening of D-threonine derived
aziridine such as 1 by a thiol nucleophile.
Initially, we explored the use of benzyloxycarbonyl
aziridine 10 (Scheme 2) as a precursor for synthesis of
protected â-MeCys derivatives.6 Wakamiya has reported the
acid-promoted ring opening of 10 by thiobenzoic acid.
However, competitive ring-opening by oxygen led to forma-
tion of a thionoester byproduct along with the desired
thioester.6 Hydrogen sulfide gas has also been used as the
nucleophile, but its high toxicity makes its use inconvenient,
especially on preparative scale.7 Thus, it became clear to us
that the success of this strategy will depend on the regio-
and stereoselective opening of the aziridine at C3 with an
appropriate S-nucleophile as well as the ability to cleave the
thiol protective group under mild conditions so as to avoid
â-elimination and C2 epimerization.
Aziridine 10 was prepared from D-threonine (7) by
modification of reported procedures as outlined in Scheme
2.6,8 In each example, BF3‚OEt2-mediated ring-opening
reactions of 10 with various thiol nucleophiles smoothly
afforded the corresponding â-MeCys derivatives (11a-c) as
a single diastereomer.9 Subsequent thiol deprotection, how-
ever, proved to be more difficult. In case of 11a, both
reductive as well as oxidative cleavage of the S-Bn group
was incompatible with Cbz and methyl ester functionalities.10
Deblocking of S-PMB ether utilizing reported protocols
resulted either in incomplete cleavage or decomposition of
the starting material.11,12 After some experimentation, we
found that by using 2 equiv each of Hg(OAc)2 and DTT in
TFA, the PMB group could be removed cleanly to afford
thiol 12 (75% yield).
The mild conditions for cleavage of S-trityl ethers prompted
us to explore the use of triphenylmethyl thiol as a nucleophile
in the aziridine ring-opening reactions. We were pleased to
find that under optimized conditions, S-trityl protected
â-MeCys (11c) was obtained in 40% yield along with an
almost equal amount of free thiol 12. In addition, detritylation
(4) For recent efforts directed toward synthesis of orthogonally protected
lanthionines, see: (a) Bregant, S.; Tabor, A. B. J. Org. Chem. 2005, 70,
2430. (b) Mustapa, M. F. M.; Harris, R.; Bulic-Subanovic, N.; Chubb, N.
A. L.; Gaffney, P. R. J.; Driscoll, P. C.; Tabor, A. B. J. Org. Chem. 2003,
68, 8185. (c) Matteucci, M.; Bhalay, G.; Bradley, M. Tetrahedron Lett.
2004, 45, 1399. (d) Swali, V.; Matteucci, M.; Elliot, R.; Bradley, M.
Tetrahedron 2002, 58, 9101.
(5) For a recent review on chemical and enzymatic synthesis of
lanthionines, see: Paul, M.; van der Donk, W. A. Mini-ReV. Org. Chem.
2005, 2, 23.
(7) Nakajima, K.; Okawa, K. Bull. Chem. Soc. Jpn. 1983, 56, 1565.
(8) McKeever, B.; Pattenden, G. Tetrahedron 2003, 59, 2713.
(9) Xiong, C. Y.; Wang, W.; Hruby, V. J. J. Org. Chem. 2002, 67, 3514.
(10) (a) Koide, T.; Otaka, A.; Suzuki, H.; Fujii, N. Synlett 1991, 345.
(b) Akaji, K.; Tatsumi, T.; Yoshida, M.; Kimura, T.; Fujiwara, Y.; Kiso,
Y. J. Chem. Soc., Chem. Commun. 1991, 167.
(11) Nishimura, O.; Kitada, C.; Fujino, M. Chem. Pharm. Bull. 1978,
26, 1576.
(12) Macmillan: D.; Anderson, D. W. Org. Lett. 2004, 6, 4659.
(6) Wakamiya, T.; Shimbo, K.; Shiba, T.; Nakajima, K.; Neya, M.;
Okawa, K. Bull. Chem. Soc. Jpn. 1982, 55, 3878.
2656
Org. Lett., Vol. 7, No. 13, 2005

Scheme 3
Scheme 4
of 11c could be carried out separately with TFA/Et3SiH in
excellent yields.13 Taken together, thiol 12 was obtained in
73% yield starting from aziridine 10 via S-Trt derivative 11c.
Although nucleophilic opening of N-alkoxycarbonyl aziri-
dines allows efficient access to â-MeCys derivatives, it is
restricted to the use of Cbz-protected aziridines rendering it
unsuitable for Fmoc- or Boc-based peptide synthesis in
solution or on the solid phase. To get around this limitation,
we have utilized p-nitrophenylsulfonyl (Ns) aziridine (14,
Scheme 3) as the electrophile, which has provided a more
concise and versatile route to orthogonally protected â-MeCys
derivatives. As shown in Scheme 3, 14 was prepared in only
three steps starting from D-threonine (compared to five steps
for Cbz aziridine 10). D-Threonine allyl ester was transformed
to the p-nitrophenylsulfonamide 13 by treatment with NsCl/
Et3N in 75% yield. Ring closure of 14 under Mitsunobu
conditions afforded the desired aziridine in 84% yield as a
single diastereomer.14 The ring-opening reaction of 14 by
thiol nucleophiles under our optimized conditions provided
the desired N-nitrosulfonyl â-MeCys derivatives (15a and
15b) with high regio- and stereoselectivity.
For the Ns-aziridine route to construct â-MeCys deriva-
tives to be synthetically useful, it was critical to establish
conditions for cleavage of the Ns group while avoiding any
undesired side reactions. Maligres and co-workers found that
deprotection of primary nitrophenylsulfonamides was in-
complete under Fukuyama conditions and prolonged heating
in the presence of PhSH and K2CO3 was needed.15 This was
of particular concern to us since nosyl-protected â-MeCys
derivatives (15a,b) could be prone to epimerization under
the Maligres protocol. Fortunately, we were able to ac-
complish Ns cleavage of 15a and 15b in 3 h at room
temperature simply by using p-methoxyphenyl thiol instead
of PhSH, to afford the primary amines in 80-85% yields
(16a,b, Scheme 4). In addition, primary amine 16a could
be quantitatively reprotected as its Boc derivative (16c).
Furthermore, a highly efficient, one-pot, Ns to Boc switch
could also be carried out by simply adding (Boc)2O to the
reaction mixture after Ns cleavage (15a to 16c, Scheme 4).
In a similar manner, Fmoc or any other carbamate function-
ality can easily be incorporated in protected â-MeCys
derivatives providing flexibility in choice of the amine
protecting group. Thus, the use of N-nitrosulfonyl aziridine
14 has enabled a much more direct access to orthogonally
protected â-MeCys derivatives such as 16c (5 steps, 45%
overall yield starting from D-threonine). In contrast, prepara-
tion of â-methylcysteine derivatives via ring-opening of Cbz-
aziridine intermediates requires additional amino protecting
group manipulations for target structures in which Cbz
protection is not desirable.16
Having accessed â-MeCys with the desired protective
group scheme, we next explored the synthesis of differently
protected â-MeLan. Van der Donk has employed a biomi-
metic route involving intramolecular Michael addition of
MeCys to dehydrobutyrine (Dhb).16 This strategy is suitable
only for cyclic â-methyllanthionines where the stereoselec-
tivity of Michael addition originates from the intrinsic
preference of the acyclic peptide precursor to produce a
particular diastereomer upon cyclization. Another approach
involving coupling of alanyl â-cation equivalents with
â-MeCys has remained problematic due to the formation of
norlanthionines.17,18 Finally, BF3‚OEt2-mediated ring opening
of aziridines such as 10 by cysteine-derived nucleophiles
provided the corresponding â-MeLan derivative in only 12%
yield.19
In light of these issues, we set out to investigate the
methods reported by Goodman and Schmidt for synthesis
of our target â-methyllanthionines (Schemes 5 and 6).
Goodman utilized the ring-opening reactions of serine
â-lactone derivatives such as 17 (Scheme 5) by cesium
thiolates of penicillamine (â,â-dimethylcysteine) to access
the corresponding â,â-dimethyllanthionine derivatives in
high yields (78-92%).20 When the same protocol was
applied to coupling of â-MeCys 12 with lactone 17, 52%
of orthogonally protected â-MeLan 18 was obtained as a
1
single diastereomer (as determined by H and 13C NMR
(16) Zhou, H.; van der Donk, W. A. Org. Lett. 2002, 4, 1335.
(17) Dugave, C.; Me´nez, A. Tetrahedron: Asymmetry 1997, 8, 1453.
(18) Mustapa, M. F. M.; Harris, R.; Mould, J.; Chubb, N. A. L.; Schultz,
D.; Driscoll, P. C.; Tabor, A. B. Tetrahedron Lett. 2002, 43, 8359.
(19) Nakajima, K.; Oda, H.; Okawa, K. Bull. Chem. Soc. Jpn. 1983, 56,
520.
(13) Moreau, X.; Campagne, J.-M. J. Org. Chem. 2003, 68, 5346.
(14) Ibuka, T.; Mimura, N.; Aoyama, H.; Akaji, M.; Ohno, H.; Miwa,
Y.; Taga, T.; Nakai, K.; Tamamura, H.; Fujii, N.; Yamamoto, Y. J. Org.
Chem. 1997, 62, 999.
(15) Maligres, P. E.; See, M. M.; Askin, D.; Reider, P. J. Tetrahedron
Lett. 1997, 38, 5253.
(20) Shao, H.; Wang, S. H. H.; Lee, C. W.; Osapay, G.; Goodman, M.
J. Org. Chem. 1995, 60, 2956.
Org. Lett., Vol. 7, No. 13, 2005
2657

Scheme 5
Scheme 6
analysis) along with 20% of dehydrobutyrine 19. Out of the
two reaction pathways (a and b, Scheme 5) available for
nucleophilic ring opening of 17, path a leads to formation
of the desired product 18. On the other hand, path b generates
the thioester intermediate, which likely undergoes â-elimina-
tion under the reaction conditions to produce 19. This
strategy, though reasonably efficient, suffers from the limita-
tion that the coupling conditions are not compatible with the
Fmoc protecting group.
We therefore explored conditions reported by Schmidt and
co-workers for the synthesis of unsubstituted, differently
protected lanthionines.21 This method involved S-alkylation
of protected cysteine derivatives by protected â-bromoala-
nines under phase transfer conditions (Bu4N‚HSO4, EtOAc,
aq NaHCO3). Under Schmidt conditions, displacement of
bromide 2121 by â-MeCys (12) afforded fully protected
â-MeLan 22 in 60% yield. Although no dehydroalanine by-
products were reported in reactions utilizing unsubstituted
cysteine nucleophiles, we found the â-elimination pathway
to be competitive, resulting in the isolation of dehydroalanine
23 in 21% yield. This is likely a consequence of the
diminished reactivity of â-MeCys derivative 12 as compared
to its unsubstituted cysteine counterpart. Use of aqueous
Cs2CO3 (pH 12) as base, in place of saturated NaHCO3,
improved the yield of 22 to 72% while minimizing the
formation of 23. The isomeric purity of 22 was established
by 1H, 13C, and LCMS analysis (see the Supporting Informa-
tion).
via D-threonine-derived aziridine derivatives. Use of as yet
unexplored Ns-aziridine derivative has allowed a more
concise and versatile access to â-MeCys analogues. Synthesis
of linear, orthogonally protected â-methyllanthionines has
been accomplished for the first time by extension of
previously reported methods for the synthesis of lanthionines.
Modified Schmidt conditions (use of aqueous Cs2CO3 instead
of NaHCO3) were found to provide the fully protected
â-methyllanthionine derivative in the highest yield. With the
two unnatural amino acids available in multigram quantities,
we have initiated an effort toward the total synthesis of
mersacidin. Our progress toward this end will be reported
in due course.
Acknowledgment. This paper is dedicated to the memory
of Professor Murray Goodman. Financial support provided
by the National Institutes of Health (AI 059327), The
Hellman Foundation, and the Regents of the University of
California is gratefully acknowledged. M.S.V. also thanks
Eli Lilly and Company for support in the form of an Eli
Lilly and Company New Faculty Award.
Supporting Information Available: Experimental details
and tabulated spectroscopic data for all new compounds. This
material is available free of charge via the Internet at
http://pubs.acs.org.
In conclusion, we have prepared orthogonally protected
â-methylcysteines enantioselectively in an efficient manner
(21) Zhu, X. M.; Schmidt, R. R. Eur. J. Org. Chem. 2003, 4069.
OL0507930
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Org. Lett., Vol. 7, No. 13, 2005