Synthesis of orthogonally protected lanthionines

Article abstract of DOI:10.1021/jo0346398

Synthetic approaches to the lantibiotics, a family of thioether-bridged antimicrobial peptides, require flexible synthetic routes to a variety of orthogonally protected derivatives of lanthionine 1. The most direct approaches to lanthionine involve the re

Full text of DOI:10.1021/jo0346398

Syn th esis of Or th ogon a lly P r otected La n th ion in es
M. Firouz Mohd Mustapa,† Richard Harris,‡,§ Nives Bulic-Subanovic,† Susan L. Elliott,†
Sarah Bregant,† Marianne F. A. Groussier,† J essica Mould,| Darren Schultz,|
,#
Nathan A. L. Chubb,| Piers R. J . Gaffney,†, Paul C. Driscoll,‡,§,# and Alethea B. Tabor*,†
Department of Chemistry, University College London, Christopher Ingold Laboratories, 20 Gordon Street,
London, WC1H OAJ U.K., Department of Biochemistry and Molecular Biology, University College
London, Darwin Building, Gower Street, London WC1E 6BT, U.K., Bloomsbury Centre for Structural
Biology, Department of Biochemistry and Molecular Biology, University College London, Gower Street,
London WC1E 6BT, U.K., Pfizer Limited, Sandwich, Kent CT13 9NJ , U.K., Department of Chemistry,
Imperial College London, Exhibition Road, London SW7 2AZ, U.K., and Ludwig Institute for Cancer
Research, 91 Riding House Street, London W1W 7BS, U.K.
a.b.tabor@ucl.ac.uk
Received May 14, 2003
Synthetic approaches to the lantibiotics, a family of thioether-bridged antimicrobial peptides, require
flexible synthetic routes to a variety of orthogonally protected derivatives of lanthionine 1. The
most direct approaches to lanthionine involve the reaction of cysteine with an alanyl â-cation
equivalent. Several possibilities exist for the alanyl â-cation equivalent, including direct activation
of serine under Mitsunobu conditions: however, the low reactivity of sulfur nucleophiles in the
Mitsunobu reaction has previously precluded its use in the synthesis of the lantibiotics. We report
here a new approach to the synthesis of protected lanthionine, using a novel variant of the Mitsunobu
reaction in which catalytic zinc tartrate is used to enhance the nucleophilicity of the thiol. In the
course of these studies, we have also demonstrated that the synthesis of lanthionine from trityl-
protected â-iodoalanines is prone to rearrangement, via an aziridine, to give predominantly trityl-
protected R-iodo-â-alanines, and hence norlanthionines, as the major products.
In tr od u ction
The unusual amino acid lanthionine (1) is the key
component of the lantibiotics, a family of antimicrobial
peptides such as nisin, duramycin, and subtilin produced
by Gram-positive bacteria.1 Many of these peptides show
potent antibiotic properties; others are believed to act as
enzyme inhibitors. Peptides-containing lanthionine resi-
dues are polycyclic structures, with multiple thioether
bridges between side-chains resulting from the lanthion-
ine residues. As part of an ongoing program to develop a
new approach for the solid-phase synthesis of cyclic
peptides with unnatural side-chain linkages,2 we are
currently investigating the synthesis of mono-and poly-
cyclic lanthionine-bridged peptides.
sulfur extrusion, from cystine and from suitably protected
cyclic analogues,3 afforded protected lanthionines, in low
to moderate yields. Ring opening of serine â-lactone with
protected cysteine derivatives afforded lanthionines in
good yields;4 however, competing formation of the un-
desired thioester analogue was also observed, and the
procedure was not compatible with the Fmoc-protecting
group. Approaches based on Michael additions of cysteine
to dehydroalanine have also been reported5 but these of
necessity afford a mixture of diastereoisomers. Recently,
a strategy based on the use of iodoalanine as an alanyl
â-cation equivalent was reported by Dugave and Me´nez.6
This route had many apparent advantages; it was direct,
high-yielding, and amenable to large-scale synthesis, and
it was compatible with a very wide range of protecting
For our approach to the solid-phase synthesis of
lantibiotics, we required the orthogonally protected
lanthionine 2. Various approaches to the synthesis of
lanthionine have previously been published. The use of
† Department of Chemistry, UCL.
‡ Department of Biochemistry, UCL.
§ Bloomsbury Centre for Structural Biology.
| Pfizer Limited.
Present address: Department of Chemistry, Imperial College
London.
#
Ludwig Institute for Cancer Research.
(3) Harpp, D. N.; Gleason, J . G. J . Org. Chem. 1971, 36, 73; Cavelier-
Frontin, F.; Daunis, J .; J acquier, R. Tetrahedron: Asymmetry 1992, 3,
85.
(4) Shao, H.; Wang, S. H. H.; Lee, C.-W.; O¨ sapay, G.; Goodman, M.
J . Org. Chem. 1995, 60, 2956.
(5) Probert, J . M.; Rennex, D.; Bradley, M. Tetrahedron Lett. 1996,
37, 1101.
(6) Dugave, C.; Me´nez, A. Tetrahedron: Asymmetry 1997, 8, 1453.
(1) For recent reviews, see: (a) Guder, A.; Wiedemann, I.; Sahl, H.-
G. Biopolymers 2000, 55, 62. (b) van Kraaij, C.; de Vos, W. M.; Siezen,
R. J .; Kuipers, O. P. Nat. Prod. Rep. 1999, 16, 575. (c) Breukink, E.;
de Kruijff, B. Biochim. Biophys. Acta 1999, 1462, 223. (d) Sahl, H.-G.;
Bierbaum, G. Annu. Rev. Microbiol. 1998, 52, 41.
(2) Alexander McNamara, L. M.; Andrews, M. J . I.; Mitzel, F.;
Siligardi, G.; Tabor, A. B. J . Org. Chem. 2001, 66, 4585.
10.1021/jo0346398 CCC: $25.00 © 2003 American Chemical Society
Published on Web 09/24/2003
J . Org. Chem. 2003, 68, 8185-8192
8185

Mohd Mustapa et al.
SCHEME 1
lanthionine derivatives formed from iodoalanines similar
to 5, and indeed, we again also observed two clearly
distinct sets of signals, a major component and a minor
component, in the 1H NMR spectrum of 7. However, even
when the bulky trityl group had been replaced by Aloc
1
in 8, several sets of peaks were still observed in the H
NMR spectrum, although the differences in chemical
shifts between the isomers were less pronounced. This
caused us to question whether the isomeric forms ob-
served in the 1H NMR spectra of 5, 7, and 8 were, in fact,
due to the presence of two rotameric forms of each
compound and raised doubts concerning the identity of
5, 7, and 8. We therefore undertook a careful investiga-
tion of the iodoalanine and lanthionine derivatives.
Rota m er s, Dia ster eoisom er s, or Regioisom er s? To
eliminate the possibility of the additional 1H NMR signals
arising from impurities, the purity and homogeneity of
the samples were confirmed. Although 7 and 8 were
purified by flash column chromatography, and 5 by
recrystallization, and appeared homogeneous by TLC,
HPLC analysis revealed the presence of two distinct
peaks. Although these could not be detected or resolved
by flash column chromatography, a partial separation of
the two components observed in the NMR spectrum of 5
was achieved by normal-phase preparative HPLC, en-
abling the NMR spectrum to be analyzed. In the case of
8, the two isomers could be separated and isolated by
normal-phase preparative HPLC, which allowed us to
access sufficient material for the characterization of each
of these compounds.
We also sought to establish whether the major and
minor isomers were rotamers by VT NMR experiments.
Predictably, the thermal instability of 5 precluded a
successful outcome. Extensive decomposition of the sample,
giving aziridine 6 and other unidentified byproducts, was
observed at temperatures in excess of 50 °C. VT experi-
ments with lanthionine derivative 7 in DMSO-d6 were
also performed, and even at temperatures up to 100 °C,
no coalescence of the signals was observed.8
groups. We therefore elected to use this route to synthe-
size the required orthogonally protected lanthionine 2.
However, during the course of our synthetic studies,
problems became evident, and we therefore carried out
a full investigation of the regio- and stereochemical
outcome of the key reaction between the iodoalanine and
cysteine derivatives. This led us to reassess this reaction,
and to the conclusion that, in fact, the regioisomeric
norlanthionine 3 is predominantly produced via this
route. In this paper, we report full details of this
investigation7 and propose an alternative approach to
lanthionine synthesis which circumvents these problems.
Resu lts a n d Discu ssion
Attem p ted Syn th esis of La n th ion in e 2 fr om Iod o-
a la n in e. We undertook the synthesis of lanthionine 2
(Scheme 1) following the route reported by Dugave and
Me´nez6 and adjusting the protecting groups to suit the
requirements of our strategy for solid-phase synthesis of
side-chain linked peptides.2 Thus, (R)-serine was con-
verted by standard methods to the mesylate 4. Reaction
with sodium iodide afforded iodoalanine 5, with up to
10% of aziridine 6 as a byproduct that could be removed
by recrystallization. As previously described,6 two distinct
These results suggested that the two isomers formed
in the conversion of 4 to 7 via 5 were unlikely to be
rotamers. It was therefore necessary to consider the
1
possibility that the two isomers observed in the H NMR
1
spectrum of 7 were either diastereoisomers or regio-
isomers. The most likely route for diastereoisomer forma-
tion would be as a result of competing elimination of HI
from 5 to form the dehydroalanine; subsequent Michael
addition5 would result in a mixture of isomers R to the
-NHTrt group. However, Dugave and Me´nez had un-
ambiguously previously demonstrated, via desulfuriza-
tion, derivatization and chiral HPLC measurements of
the resulting alanines, that racemization of this chiral
center does not occur during the coupling of iodoalanines
to cysteine.6
We confirmed these observations by the independent
synthesis of the diastereoisomer 10. L-Serine was again
protected and converted via the mesylate to 9, and two
sets of peaks were again observed in the 1H NMR
spectrum. Using the same reaction sequence (Scheme 2),
sets of peaks were observed in the H NMR spectrum of
the purified product 5. The peaks were in an uneven
ratio, with a major component and a minor component
initially in a 2:1 ratio; after recrystallization, the ratio
of major isomer to minor isomer was 5:1. In the previous
1
work,6 the same doubling of the peaks in the H NMR
spectra of similar protected iodoalanines, formed via this
route, was observed, and this was attributed to the
presence of two rotameric forms of these iodoalanines.
Compound 5 was then reacted with Fmoc-Cys-OtBu in
the presence of Cs2CO3 to give lanthionine 7; again,
aziridine 6 was observed as a byproduct. Removal of the
trityl group and replacement with Aloc afforded 8.
Finally, removal of the tert-butyl ester gave the desired
lanthionine 2, with the requisite orthogonal protecting
groups for SPPS. Dugave and Me´nez had also reported
the presence of two rotameric forms for other protected
(8) We have also prepared a range of other lanthionine derivatives
following this method: in all cases two isomers were observed in the
1H NMR, and VT experiments failed to show coalescence of the peaks.
Details are given in the Supporting Information.
(7) For preliminary results reassessing this synthesis, see: Mohd
Mustapa, M. F.; Harris, R.; Mould, J .; Chubb, N. A. L.; Schultz, D.;
Driscoll, P. C.; Tabor, A. B. Tetrahedron Lett. 2002, 43, 8359.
8186 J . Org. Chem., Vol. 68, No. 21, 2003

Synthesis of Orthogonally Protected Lanthionines
TABLE 1. Key Ch em ica l Sh ifts for r- a n d â-H a n d -C for th e Ma jor a n d Min or Iod oa la n in e a n d La n th ion in e Isom er s
a
b
The signal was too weak to be unambiguously determined. Two signals were observed, due to the presence of two diastereoisomers.
SCHEME 2
SCHEME 3
9 was then converted to 10.9 Two sets of peaks were also
observed in the NMR spectra of 10. However, the chemi-
cal shifts of the signals corresponding to the minor isomer
were different from those observed for 7,10 indicating that
the two sets of signals in 7 do not arise from the presence
of the undesired diastereoisomer 10.
the trityl-protected side, and also between the R-CH on
the Fmoc-protected side of the lanthionine and the â-C
on the trityl-protected side, were observed for the major
isomer. This is only compatible with the formation of the
norlanthionine regioisomer 12 from attack of Fmoc-Cys-
OtBu on the major regioisomer R-iodo-â-alanine 11.
Similar HSQC and HMBC experiments were carried out
on the mixtures of isomers for 8 and 2, with the same
pattern of chemical shifts and connectivities observed for
the major components in each case. To verify that the
unexpected synthesis of 11 and 12 was not an effect
arising from the different protecting groups used in our
work, L-serine was protected and converted to the
mesylate 13.6 Treatment of this material with sodium
iodide again led to a mixture of iodoalanine isomers,
identified by 1H NMR as N-trityl-â-iodo-R-alanine benzyl
ester 14 (minor isomer) and N-trityl-R-iodo-â-alanine
benzyl ester 15 (major isomer: Scheme 3).
Inspection of the 13C NMR spectrum of the mixture of
7 and 12 revealed a total of three peaks for many of the
carbon signals. A sample of pure 12 (synthesized from
the partially purified R-iodo-â-alanine 11) showed dou-
bling of the carbon signals, indicating that a diastereo-
meric mixture of norlanthionines had been formed.
F or m a t ion of t h e Ma jor R egioisom er , r-Iod o-â-
a la n in e 12, via Azir id in e Rin g Op en in g. It is likely
that the aziridine 6, rather than being a minor byproduct
of the reaction, plays a key role as an intermediate in
the formation of the two iodoalanine regioisomers 5 and
11. We hypothesized that during the course of the
reaction mesylate 4 is in fact converted to aziridine 6.
R-Attack would then lead to the formation of the major
regioisomer R-iodo-â-alanine 11, whereas the minor
The only possibility remaining was that the two sets
of signals arose from two structural isomers of the
iodoalanine 5, and likewise from structural isomers of
the lanthionines 7, 8 and 2. As it was not possible to
obtain crystals suitable for X-ray diffraction, the struc-
tures of 5 and 7 were examined in detail by means of
1
1H-13C NMR correlation experiments. A H-13C HSQC
spectrum was acquired on iodoalanine 5, which revealed
that the chemical shift of the R-carbon in the major
component was ∼30 ppm upfield of that in the minor
component and that of the â-carbon was ∼40 ppm
downfield (Table 1). These chemical shifts are only
compatible with attachment of iodine at the R-carbon in
the major component, implying that the major component
is the regioisomer R-iodo-â-alanine 11. Conversely, the
iodine atom must be attached at the â-carbon in the
minor component, implying that this is the desired
iodoalanine 5. To confirm these observations, we carried
out an HMBC experiment on the mixture of lanthionine
isomers 7. 3J correlations between the â-CH2 on the
Fmoc-protected side of the lanthionine and the R-C on
(9) Satisfactory microanalytical data were produced for both prod-
ucts.
(10) Previous work in which two lanthionine diastereoisomers (with
different protecting groups) were unambiguously prepared have shown
the chemical shifts to be similar in value, but distinguishable: O¨ sapay,
G.; Zhu, Q.; Shao, H.; Chadha, R. K.; Goodman, M. Int. J . Pept. Prot.
Res. 1995, 46, 290.
J . Org. Chem, Vol. 68, No. 21, 2003 8187

Mohd Mustapa et al.
SCHEME 4
SCHEME 6
giving exclusively â-attack13-15 or exclusively R-attack,16
depending on the substitution of the aziridines and the
presence of Lewis acids. Hata and Watanabe17 utilized
the ring opening of N-unprotected aziridines with un-
protected cysteine to afford mixtures of lanthionine (via
â-attack) and norlanthionine (via R-attack). We therefore
treated the pure aziridine 6 with Fmoc-Cys-OtBu in the
presence of Cs2CO3, under the conditions of the original
synthesis (Scheme 5); however, no reaction was observed.
This must indicate that norlanthionine 12 must derive
from the major regioisomer R-iodo-â-alanine 11, and
lanthionine 7 from the minor regioisomer â-iodo-R-
alanine 5.
SCHEME 5
Both our work and that of Dugave and Me´nez show
that the minor desired lanthionine isomer 7 is a single
diastereoisomer. In light of the preceding discussion, this
must indicate that the minor regioisomer â-iodo-R-
alanine 5 is formed without racemization. However, the
major norlanthionine regioisomer 12 is formed as a
mixture of two diastereoisomers, indicating that the
major regioisomer R-iodo-â-alanine 11 is in turn a
mixture of enantiomers. The cyclization of 5 to 6, ring-
opening of 6 to afford 11, and the ring-closure of 11 to
give 6 (Scheme 4) should all occur via SN2 mechanisms,
preserving the chirality of the intermediate aziridine 6
and initially affording a single enantiomer, (R)-11. How-
ever, once (R)-11 has been formed, it is in turn susceptible
to nucleophilic attack, and we believe that under the
conditions used for the reaction (10 equiv of NaI) it
undergoes Walden inversion, via further attack of the
excess I-, leading to partial racemization at this center.
Both the nucleophilicity of I- relative to other halides18
and the facile racemization of optically active alkyl
iodides by excess iodide19 are precedented (Scheme 6).
Righi et al. report that enantiopure R-iodo-â-amino acid
derivatives were formed during the reaction of enan-
tiopure derivatives of aziridine-2-carboxylates with a
single equivalent of NaI in the presence of Amberlyst
15.11 We treated 6 under the same conditions and
obtained 5 and 11 in good yield as an inseparable mixture
(1:9 ratio) of regioisomers.
regioisomer, â-iodo-R-alanine 5, could be formed either
by â-attack or by direct reaction on mesylate 4 (Scheme
4).
Literature precedent supported this rationale. Righi
et al. demonstrated that C3-substituted (2R,3R)-1-ethoxy-
carbonylaziridine-2-carboxylic acid methyl and ethyl
esters undergo nucleophilic ring opening with sodium
iodide and Amberlyst 15 in acetone at -40 °C to give a
mixture of R- and â-amino acids, with the â-amino acid
isomers predominating.11 We confirmed this by synthe-
sizing the aziridine 6, treating mesylate 4 with Et3N at
reflux for 18 h (Scheme 5).12 The pure aziridine was then
treated with 10 equiv of sodium iodide, under the
conditions used for the iodination of 4, affording a
mixture of the major regioisomer R-iodo-â-alanine 11 and
the minor regioisomer, â-iodo-R-alanine 5 in a 3:1 ratio,
together with 75% of unreacted aziridine. This confirms
that both 5 and 11 may arise from an aziridine inter-
mediate. The ratio of the minor isomer 5 to the major
isomer 11 is, however, greater with the original synthetic
route (Scheme 1, reaction of mesylate 4 with sodium
iodide), indicating that some of the minor isomer 5 must
be formed by direct SN2 attack on the mesylate (direct
reaction pathway, Scheme 4). Clearly the slow reaction
of the aziridine, with incomplete conversion even with
10 equiv of NaI over 72 h (attributable to the electron-
withdrawing nature of the aziridine protecting group)
also indicates that a direct reaction pathway must
operate and that re-conversion of the iodoalanines to
aziridine must occur.
Having shown that attempted preparation of iodoala-
nine 5 from mesylate 4 leads to an inseparable mixture
of iodoalanine regioisomers 5 and 11, and thence to a
mixture of lanthionine 7 and norlanthionine 12 (Scheme
7), we then turned our attention to developing methodol-
ogy that would unambiguously produce only the desired
lanthionine.
La n th ion in e Syn th esis via th e Mitsu n obu Rea c-
tion . In light of the above observations, we sought an
The aziridine 6 is also formed during the reaction with
Fmoc-Cys-OtBu, and it was possible that it might also
be an intermediate in this reaction. Several groups have
reported the ring opening of aziridines with thiols, either
(13) Nakajima, K.; Oda, H.; Okawa, K. Bull. Chem. Soc. J pn. 1983,
56, 520.
(14) Parry, R. J .; Naidu, M. V. Tetrahedron Lett. 1983, 24, 1133.
(15) Baldwin, J . E.; Adlington, R. M.; Moss, N.; Robinson, N. G. J .
Chem. Soc., Chem. Commun. 1987, 1664.
(16) Crousse, B.; Narizuka, S.; Bonnet-Delpon, D.; Be´gue´, J .-P.
Synlett 2001, 679.
(11) (a) Righi, G.; D’Achille, R.; Bonini, C. Tetrahedron Lett. 1996,
37, 6893. (b) Righi, G.; Chionne, A.; D’Achille, R.; Bonini, C. Tetrahe-
dron: Asymmetry 1997, 8, 903.
(12) (a) Olofsson, B.; Wijtmans, R.; Somfai, P. Tetrahedron 2002,
58, 5979. (b) Willems, J . G. H.; Hersmis, M. C.; de Gelder, R.; Smits,
J . M. M.; Hammink, J . B.; Dommerholt, F. J .; Thjis, L.; Zwanenburg,
B. J . Chem. Soc., Perkin Trans. 1 1997, 963.
(17) Hata, Y.; Watanabe, M. Tetrahedron 1987, 43, 3881.
(18) Edwards, J . O.; Pearson, R. G. J . Am. Chem. Soc. 1962, 84, 16.
(19) Hughes, E. D.; J uliusburger, F.; Masterman, S.; Topley, B.;
Weiss, J . J . Chem. Soc. 1935, 1525.
8188 J . Org. Chem., Vol. 68, No. 21, 2003

Synthesis of Orthogonally Protected Lanthionines
SCHEME 7
SCHEME 8
alternative pathway for the synthesis of the desired
protected lanthionines that would be both stereo- and
regioselective. In addition to providing a route to the
necessary amino acid building blocks, it was also envis-
aged that this would provide further proof of the struc-
tures of the lanthionines and norlanthionines synthesized
above.
The Mitsunobu reaction20 is well-known as a mild
method for conversion of alcohols into a range of different
functionalities. Cherney and Wang21 have demonstrated
that Mitsunobu reactions under standard conditions with
N-trityl-protected serine are extremely efficient for pro-
ducing amino acids functionalized at the â-position. For
the formation of carbon-sulfur bonds, however, masked
reagents such as thio acids or thioamides, or aryl thiols,
are generally used,20b as aliphatic thiols generally lack
sufficient acidity to react under the standard Mitsunobu
conditions.22,23 However, recently aliphatic sulfides and
thioglycosides have been successfully prepared using 1,1′-
(azodicarbonyl)dipiperidine (ADDP) and Me3P in the
presence24 or absence25,26 of imidazole. We therefore
studied the Mitsunobu reaction as a potential methodol-
ogy for the synthesis of protected lanthionines.
Reaction of N-trityl-(R)-serine allyl ester (16) with
ADDP, Me3P, and Fmoc-Cys-OtBu, in the presence or
absence of imidazole, using the range of conditions
previously described,24-26 failed to produce the desired
lanthionine 7. Recovery of unreacted starting materials,
with some dimerization of the cysteine, resulted. Zinc
chloride is known27 to enhance sluggish Mitsunobu
reactions, but under the conditions described (between
0.2 and 4 equiv of ZnCl2), extensive detritylation of 16
was observed, presumably as a result of the generation
of HCl. Catalytic zinc tartrate has been used to increase
the nucleophilicity of thiols in epoxide ring-opening
reactions,28 and as a less acidic byproduct would thereby
be generated, this seemed a viable alternative. Accord-
ingly, 16 was added to the preformed ADDP/Me3P
adduct, followed by addition of a preformed mixture of
zinc tartrate (0.2 equiv) and Fmoc-Cys-OtBu (Scheme 8).
After the mixture was stirred at room temperature for 7
days, the desired lanthionine 7 was produced, as a single
isomer, in 50% yield. The reaction was then repeated
using the corresponding (S)-serine derivative; this also
yielded only one isomer, the desired lanthionine 10.
HSQC experiments carried out as before on both di-
astereoisomers, 7 and 10, synthesized via this Mitsunobu
route, confirmed the correct thioether bridge connectivity
in each case, with similar 13C chemical shift values
observed between the R and â-Cs on both sides of the
thioether bridge. HMBC experiments confirmed this with
the presence of 3J couplings between â-Hs and â-Cs
1
across the thioether bridge. Comparison of the H NMR
1
spectrum of 7 with the H NMR spectrum of the mixture
of isomers produced via the iodoalanine route (Scheme
1) confirmed that the minor isomer produced from this
latter route is, in fact, the desired lanthionine. Similarly,
1
comparison of the H NMR spectrum of 10 synthesized
1
via the Mitsunobu route with the H NMR spectrum of
the mixture of isomers produced via the iodoalanine route
(Scheme 2) also confirmed that the minor isomer pro-
duced from this latter route is the diastereomeric lanthion-
ine. As expected,10 small differences were observed in the
1H NMR chemical shift values between the two diaste-
reoisomers 7 and 10.
(20) (a) Mitsunobu, O. Synthesis 1981, 1. (b) Hughes, D. L. Org.
React. 1992, 42, 335.
(21) Cherney, R. J .; Wang, L. J . Org. Chem. 1996, 61, 2544.
(22) Tanigawa, Y.; Kanamaru, H.; Murahashi, S.-I. Tetrahedron Lett.
1975, 4655.
(23) For an exception, see: Alpegiani, M.; Perrone, E.; Franceschi,
G. Heterocycles 1988, 27, 49.
(24) Falck, J . R.; Lai, J .-Y.; Cho, S.-D.; Yu, J . Tetrahedron Lett. 1999,
40, 2903.
(25) Falconer, R. A.; J ablonkai, I.; Toth, I. Tetrahedron Lett. 1999,
40, 8663.
(26) Ohnishi, Y.; Ichikawa, M.; Ichikawa, Y. Bioorg. Med. Chem.
Lett. 2000, 10, 1289.
(27) Kang, S. B.; Ahn, E. J .; Kim, Y.; Kim, Y. H. Tetrahedron Lett.
1996, 37, 9317.
Con clu sion s
In this paper, we have shown that the reaction of
N-trityl-O-methanesulfonyl-(R)-serine allyl ester 4 with
sodium iodide leads to a mixture of two iodoalanine
regioisomers. The major product from this reaction is
R-iodo-â-alanine 11 and not, as previously reported,6
â-iodo-R-alanine 5, which is present as a minor compo-
nent. We have demonstrated that the complexity of the
1H NMR spectra observed with these reactions arises
from the inseparable mixture of regioisomers produced,
(28) Yamashita, H.; Mukaiyama, T. Chem. Lett. 1985, 1643.
J . Org. Chem, Vol. 68, No. 21, 2003 8189

Mohd Mustapa et al.
5 and 11 in a ratio of 1:5 (19.9 g, 40 mmol, 87% overall yield
for two regioisomers). A partial separation of the two regio-
isomers was possible using preparative normal-phase HPLC,
with a gradient of 2-3% ethyl acetate in hexane over 10 min,
15 mL/min: the retention time of 5 was12.1 min and that of
11 12.84 min: 1H NMR (CDCl3, 600 MHz) for 5 δ 7.51 (6H, m,
Trt), 7.29 (6H, m, Trt), 7.20 (3H, m, Trt), 5.73 (1H, m,
CHdCH2), 5.22 (1H, dd, J ) 17.3, 1.4 Hz, CHdCH2), 5.19 (1H,
dd, J ) 10.2, 1.1 Hz, CHdCH2), 4.22 (1H, dd, J ) 13.1, 6.1
Hz, OCH2CH), 4.09 (1H, dd, J ) 13.1, 6.1 Hz, OCH2CH), 3.50
(1H, m, HNCHCO2), 3.30 (1H, dd, J ) 9.8, 3.4 Hz, CHCH2I),
3.23 (1H, dd, J ) 9.8, 6.9 Hz, CHCH2I), 2.88 (1H, d, J ) 9.8
Hz, TrtNHCH); for 11 δ 7.52 (6H, m, Trt), 7.30 (6H, m, Trt),
7.21 (3H, m, Trt), 5.94 (1H, m, CHdCH2), 5.40 (1H, dd, J )
17.2, 1.4 Hz, CHdCH2), 5.30 (1H, dd, J ) 10.5, 1.1 Hz,
CHdCH2), 4.67 (2H, m, OCH2CH), 4.42 (1H, m, CHICO2), 2.73
(1H, dd, J ) 12.9, 8.5 Hz, HNCH2CHI), 2.57 (1H, dd, J ) 12.9,
5.9 Hz, HNCH2CHI), 2.27 (TrtNHCH2); 13C NMR (CDCl3, 150
MHz) for 5 δ 171.9, 145.6, 131.3, 128.4, 127.9, 126.4, 118.6,
70.9, 65.8, 55.8, 9.55; for 11 δ 170.4, 145.4, 131.1, 128.3, 128.0,
and not from the presence of two rotameric forms of 5.
We have presented evidence that the reaction proceeds
via an intermediate aziridine, such as 6, with concomi-
tant partial racemization of the chiral center of the
predominant R-iodo-â-alanine regioisomer 11. The mix-
ture of regioisomers, 5 and 11, in turn leads to a mixture
of lanthionine 7 and norlanthionine 12, on treatment
with Fmoc-Cys-OtBu in the presence of Cs2CO3. These
mixtures of regioisomers are difficult to separate; how-
ever, in our hands purification at each subsequent
synthetic stage resulted in the isolation of norlanthionine
3, suitable for a three-dimensional orthogonal protecting
group strategy for the synthesis of cyclic peptides. In the
following paper, we have demonstrated the use of this
approach to synthesize norlanthionine bridged cyclic
peptides.29 Finally, we have developed a new approach
to the synthesis of protected lanthionine, using a variant
of the Mitsunobu reaction, which gives exclusively the
correct lanthionine regioisomer. Further studies to op-
timize this reaction and to synthesize lanthionine-bridged
peptides are in hand.
126.5, 118.8, 70.8, 66.1, 48.2, 20.1; IR νmax 1730, 1489 cm-1
;
mass spectrum m/z (APCI+) 498 ([M + H]+, 2), 243 (Trt+, 100),
256 ([M - Trt + 2]+, 2); HRMS (FAB) calcd for C25H24NO2INa
([M + Na]+) 520.0732, found 520.0750.
Mixtu r e of 3-[(R)-2-ter t-Bu toxyca r bon yl-2-(flu or en -9-
y lm e t h o x y c a r b o n y la m in o )e t h y ls u lfa n y l]-(S )-2-(t r i-
p h en ylm eth yla m in o)p r op ion ic Acid Allyl Ester (7) a n d
(R/S)-2-[(R)-2-ter t-Bu t oxyca r bon yl-2-(flu or en -9-ylm et h -
oxyca r b on yla m in o)et h ylsu lfa n yl]-3-(t r ip h en ylm et h yl-
a m in o)p r op ion ic Acid Allyl Ester (12). N-9-Fluorenyl-
methoxycarbonyl-(R)-cysteine tert-butyl ester (5.6 g, 14 mmol)
was dissolved in dry DMF (75 mL) under inert conditions. A
solution of the regioisomeric mixture of N-triphenylmethyl-â-
iodo-(S)-alanine allyl ester (5) and (R/ S)-2-iodo-3-(triphenyl-
methylamino)propionic acid allyl ester (11) (7.7 g, 16 mmol,
1.1 equiv) (1:5 ratio) in DMF (75 mL) was added. Cesium
carbonate (4.6 g, 14 mmol) was then added, and the mixture
was stirred for 4 h at 25 °C. The solvent was removed in vacuo,
and the residue was dissolved in a mixture of ethyl acetate
(250 mL) and citric acid (5% aq w/v, 100 mL). The organic layer
was separated, washed with water (8 × 100 mL), and dried
over anhydrous sodium sulfate. Removal of the solvent in
vacuo yielded a pale yellow oil. This was purified via flash
column chromatography (hexane/ethyl acetate, 4:1, Rf 0.28)
to give a 1:5 mixture of the regioisomers 7 and 12 as a very
pale yellow foam (9.7 g, 12.6 mmol, 90%): 1H NMR (CDCl3,
500 MHz) for 7 δ 7.74 (2H, m, Fmoc (Ar)), 7.60 (2H, m, Fmoc
(Ar)), 7.49 (6H, m, Trt), 7.38 (2H, m, Fmoc (Ar)), 7.20-7.31
(2H, m, Fmoc (Ar) + 6H, m, Trt), 7.16 (3H, m, Fmoc (Ar)),
5.69 (1H, m, CHdCH2), 5.63 (1H, bd, FmocNH), 5.18 (1H, m,
CHdCH2), 5.13 (1H, m, CHdCH2), 4.49 (1H, m, FmocNHCH),
4.34 (2H, m, CH2OCONH), 4.18 (1H, m, CHCH2OCONH), 4.04
(2H, m, OCH2CHdCH2), 3.56 (1H, m, TrtNHCH), 2.89-3.03
(2H, m, CHCH2S), 2.70-2.89 (2H, m, TrtNHCHCH2), 2.20 (1H,
bs, TrtNH), 1.50 (9H, s, C(CH3)3); for 12 (diastereoisomeric
mixture) δ 7.73 (2H, m, Fmoc (Ar)), 7.56 (2H, m, Fmoc (Ar)),
7.43 (6H, m, Trt), 7.36 (2H, m, Fmoc (Ar)), 7.20-7.31 (2H, m,
Fmoc (Ar) + 6H, m, Trt), 7.16 (3H, m, Fmoc (Ar)), 5.91 (1H,
m, CHdCH2), 5.58 + 5.69 (1H, bd, FmocNH), 5.33 (1H, m,
CHdCH2), 5.23 (1H, m, CHdCH2), 4.66 (2H, m, OCH2-
CHdCH2), 4.50 (1H, m, FmocNHCH), 4.29-4.41 (2H, m, CH2-
OCONH), 4.18-4.25 (1H, m, CHCH2OCONH), 3.49 + 3.56
(1H, t, J ) 6.4 + 6.7, TrtNHCH2CH), 2.81-3.12 (2H, m,
CHCH2S), 2.61 (1H, m, TrtNHCH2), 2.45 (1H, m, TrtNHCH2),
1.48-1.49 (9H, s, C(CH3)3); 13C NMR (CDCl3, 126 MHz) for 7
δ 171.9, 169.7, 156.1, 146.0, 144.2, 141.7, 132.1, 129.2, 128.4,
128.1, 127.0, 127.5, 125.6, 120.4, 119.2, 83.4, 71.2, 67.6, 66.3,
54.3, 48.9, 47.5, 44.9, 34.3, 28.4; for 12 δ 171.8, 169, 156.1,
146, 144.1, 141.6, 132, 129, 128, 128.1, 127.5, 127, 125.5, 120.3,
119, 83, 71, 67.6, 66, 54, (48.8 + 48.0), 47.5, (44.9 + 44.6), (34.8
+ 34.3), 28.3; IR νmax1724, 1504, 1450, 1215, 1153 cm-1; mass
spectrum m/z (FAB) 527 ([M - Trt + 1]+, 0.4), 471 ([M - Trt
Exp er im en ta l Section
N-Tr ip h en ylm eth yl-(R)-ser in e(O-m eth a n esu lfon yl) Al-
lyl Ester (4). N-Triphenylmethyl-(R)-serine allyl ester 16 (19
g, 49 mmol) was dissolved in dry THF (200 mL) under Ar, and
the solution was cooled to 0 °C. Triethylamine (7.0 mL, 49
mmol) and methanesulfonyl chloride (8.3 mL, 98 mmol, 2
equiv) were added, the ice bath removed, and the mixture
stirred for 4 h. The reaction mixture was diluted with ether
(400 mL) and washed with ice-cold water (6 × 100 mL) and
then brine (4 × 100 mL). The organic layer was dried over
anhydrous sodium sulfate, and removal of the solvent yielded
a pale yellow liquid. Recrystallization of the crude product was
carried out in DCM and MeOH (1:4) yielding white crystals
(Rf 0.3 (hexane/ethyl acetate, 2:1)) (21 g, 44 mmol, 91%): mp
116-118 °C dec; [R]20 -32.7 (c 0.46, CHCl3); H NMR (400
1
D
MHz, CDCl3) δ 7.55 (6H, m, Trt), 7.31 (6H, m, Trt), 7.23 (3H,
m, Trt), 5.74 (1H, m, CHdCH2), 5.24 (1H, dd, J ) 15.3, 1.4
Hz, CHdCH2), 5.20 (1H, dd, J ) 8.3, 1.1 Hz, CHdCH2), 4.47
(1H, dd, J ) 4.3 Hz, 10.1 Hz, CH2CHdCH2), 4.27 (1H, dd, J )
6.1, 10.1 Hz, CH2-CHdCH2), 4.24 (1H, dd, J ) 4.4, 12 Hz,
CH2SO2), 4.01 (1H, dd, J ) 5.8, 12 Hz, CH2SO2), 3.74 (1H, dd,
J ) 5.8, 4.4 Hz, HNCHCO2), 2.95 (3H, s, CH3SO2); 13C NMR
(CDCl3, 100.61 MHz) δ 171.1, 145.2, 131.4, 129.5, 128.0, 126.7,
118.7, 71.0, 70.6, 66.0, 55.3, 37.4; IR νmax (CHCl3) 1736, 1489,
1346 + 1177 cm-1; mass spectrum m/z (FAB) 488 ([M + Na]+,
1), 243 (Trt+, 100); HRMS (FAB) calcd for C26H27NO5SNa ([M
+ Na]+) 488.1520, found 488.1508.
Mixtu r e of N-Tr ip h en ylm eth yl-â-iod o-(S)-a la n in e Allyl
Ester (5) a n d (R/S)-2-Iod o-3-(tr ip h en ylm eth yla m in o)-
p r op ion ic Acid Allyl Ester (11). A solution of N-triphenyl-
methyl-(R)-serine(O-methanesulfonyl) allyl ester 4 (22 g, 46
mmol) in acetone (80 mL) was added to a solution of sodium
iodide (70 g, 0.46 mol, 10 equiv) in dry acetone (100 mL) under
argon. The mixture was stirred for 72 h at 25 °C, and the
resulting yellow-brown slurry was then concentrated in vacuo.
Ether (400 mL) was added, and sodium thiosulfate (10% aq
w/v, approximately 25 mL) was then slowly added, dissolving
the solids and substantially decolorizing the organic layer to
afford a pale yellow solution. The organic layer was separated
and dried over anhydrous sodium sulfate, and the solvent was
removed in vacuo to give a yellow oil. This was purified by
flash column chromatography (hexane/ethyl acetate, 2:1, Rf
0.60) followed by precipitation from CH2Cl2/MeOH (1:5) to give
(29) Mohd Mustapa, M. F.; Harris, R.; Esposito, D.; Mould, J .;
Chubb, N. A. L.; Schultz, D.; Driscoll, P. C.; Tabor, A. B. J . Org. Chem.
2003, 68, 8193.
8190 J . Org. Chem., Vol. 68, No. 21, 2003

Synthesis of Orthogonally Protected Lanthionines
t
- Bu + 2]+, 0.3), 243 (Trt+, 100); HRMS (FAB) calcd for
with a solution of TFA (5 mL) in CHCl3 (5 mL), and the
mixture was stirred for 4 h. The mixture was concentrated in
vacuo and redissolved in CHCl3 (100 mL) and MeOH (100 mL).
The solvents were again removed in vacuo. The crude material
was purified via reversed-phase flash column chromatography
(0-35% MeCN, 20% saturated NaHCO3, H2O, Rf 0.31) yielding
only 3 as a colorless foam (1.4 g, 2.6 mmol, 76%): 1H NMR
(CD3OD, 500 MHz) δ 7.76 (2H, m), 7.62 (2H, m), 7.35 (2H, m),
7.27 (2H, m), 5.80-5.96 (2H, m, 2 × CHdCH2), 5.10-5.35 (4H,
m, 2 × CHdCH2), 4.60 (2H, m, OCH2CHdCH2), 4.48 (2H, m,
OCH2CHdCH2), 4.38 (1H, m, FmocNHCH), 4.28-4.41 (2H, m,
CH2OCONH), 4.22 (1H, m), 3.64 (1H, m, AlocNHCH2CH), 3.44
(2H, m, AlocNHCH2), 2.88-3.24 (2H, m, CHCH2S); 13C NMR
(CD3OD, 125.8 MHz) δ (172.5 + 172.4), 172.0, 158, 145.2,
142.5, (134.2 + 134.1), (133.2 + 133.0), 129.2, (128.7 + 128.6),
126.3, 120.6, 118.8, (117.7 + 117.6), (68.23 + 68.18), (66.99 +
66.96), (66.6 + 66.5), 55.6, 48.3, 47.2, (43.1 + 42.8), (34.7 +
C
47H48N2O6SCs ([M + Cs]+) 901.2264, found 901.2287.
Mixtu r e of 3-[(R)-2-ter t-Bu toxyca r bon yl-2-(flu or en -9-
ylm eth oxyca r bon yla m in o)eth ylsu lfa n yl]-(S)-2-(a llyloxy-
ca r bon yla m in o)p r op ion ic Acid Allyl Ester (8) a n d th e
Regioisom er (R/S)-2-[(R)-2-ter t-Bu toxyca r bon yl-2-(flu o-
r en -9-ylm et h oxyca r b on yla m in o)et h ylsu lfa n yl]-3-(a lly-
loxyca r b on yla m in o)p r op ion ic Acid Allyl E st er . The
mixture (1:5) of 3-[(R)-2-tert-butoxycarbonyl-2-(fluoren-9-yl-
methoxycarbonylamino)ethylsulfanyl]-(S)-2-(triphenylmeth-
ylamino)propionic acid allyl ester 7 and (R/S)-2-[(R)-2-tert-
butoxycarbonyl-2-(fluoren-9-ylmethoxycarbonylamino)ethyl-
sulfanyl]-3-(triphenylmethylamino)propionic acid allyl ester 12
(3.5 g, 4.6 mmol) was treated with a 5% solution of trifluoro-
acetic acid (1.4 mL, 18 mmol, 4 equiv) in CHCl3 (27 mL) under
inert conditions for 4 h. The resulting solution was diluted with
CHCl3 (200 mL) and washed with sodium hydrogen carbonate
(5% aq w/v, 2 × 75 mL) and water (2 × 50 mL). The solvent
was removed in vacuo. The material was then redissolved in
CHCl3 (20 mL) and MeOH (20 mL), and the solvents were
again removed in vacuo to yield a pale yellow liquid. This was
then treated with sodium hydrogen carbonate (1.5 g, 18 mmol,
4 equiv) in water (30 mL). The mixture was then cooled to 0
°C, and a solution of allyl chloroformate (0.80 mL, 9.2 mmol,
2 equiv) in dioxane (30 mL) was added. The resulting mixture
was stirred at below 5 °C for 18 h. The solvents were then
removed in vacuo, and the residue was redissolved in ethyl
acetate (250 mL). The organic layer was washed with water
(8 × 50 mL), dried over anhydrous sodium sulfate, and
concentrated yielding a yellow liquid. This was purified via
flash column chromatography (hexane/ethyl acetate, 4:1 then
2:1, Rf 0.29) yielding a mixture of 8 and the regioisomer in a
1:6 ratio as a viscous yellow oil (2.4 g, 4.0 mmol, 87%).
Separation of the regioisomers was carried out via preparative
normal-phase HPLC (20-23% ethyl acetate in hexane over 45
min, 15 mL/min) yielding both products as viscous, pale yellow
liquids. The retention time of 8 was 16.6 min and that of the
regioisomer (R/S)-2-[(R)-2-tert-butoxycarbonyl-2-(fluoren-9-yl-
methoxycarbonylamino)ethylsulfanyl]-3-(allyloxycarbonylami-
no)propionic acid allyl ester 20.1 min: 1H NMR (CDCl3, 400
MHz) for 8 δ 7.78 (2H, m), 7.63 (2H, m), 7.42 (2H, m), 7.33
(2H, m), 5.80-5.96 (2H, m, CHdCH2), 5.75 (1H, m, FmocNH),
5.66 (1H, m, AlocNH), 5.17-5.35 (4H, m, CHdCH2), 4.56-
4.65 (4H, m, OCH2CHdCH2), 4.61 (1H, m, AlocNHCH), 4.50
(1H, m, FmocNHCH), 4.39 (2H, m, CH2OCONH), 4.25 (1H,
m), 2.94-3.10 (4H, m, CH2SCH2), 1.50 (9H, s, C(CH3)3); for
the regioisomer δ 7.76 (2H, m), 7.62 (2H, m), 7.40 (2H, m),
7.32 (2H, m), 5.90 (2H, m, CHdCH2), 5.76 (1H, br m,
FmocNH), 5.14-5.36 (4H, m, CHdCH2), 5.31 (1H, m, AlocNH),
4.50-4.70 (4H, m, OCH2CHdCH2), 4.54 (1H, m, FmocNHCH),
4.40 (2H, m, CH2OCONH), 4.25 (1H, m, CHCH2OCONH),
3.45-3.66 (2H, m, SCHCH2), 3.52 (1H, m, AlocNHCH), 2.97-
3.25 (2H, m, SCH2), 1.50 (9H, s, C(CH3)3); 13C NMR (CDCl3,
126 MHz) for 8 δ 170.2, 169.2, 156.1, 155.7, 143.8, 141.3, 132.5,
131.3, 127.7, 127.1, 125.1, 120.0, 119.1, 117.7, 83.1, 67.2, 66.4,
66.1, 54.0, 47.1, 35.5, 28.0; for the regioisomer δ 170.7, (169.2
+ 169.1), 156, 157, 143.7, 141.2, 132.6, (131.4 + 131.3), 127.7,
127.0, 125.1, 119.9, (119.0 + 118.9), (117.72 + 117.65), 83.1,
(67.24 + 67.16), (66.1 + 66.0), 65.7, (54.3 + 53.9), 47.0, (46.7
+ 46.0), (41.5 + 41.4), (34.34 + 34.26), 27.9; IR νmax 3429, 1724,
1512, 1450, 1226 + 1153 cm-1; mass spectrum m/z 633 ([M +
34.2); IR νmax 3433, 3337, 1720, 1512, 1450, 1223 + 1150 cm-1
mass spectrum m/z (ES+) 577 ([M + Na]+, 100), 555 ([M +
H]+, 4), 333 ([M 2]+, 4). Anal. Calcd for
Fmoc
44H48N2O8S2: C, 60.64; H, 5.45; N, 5.05; S, 5.78. Found: C,
61.06; H, 6.02; N, 4.68; S, 5.22.
;
-
+
C
(R)-1-Tr ip h en ylm eth yla zir id in e-2-ca r boxylic Acid Al-
lyl Ester (6). Triethylamine (1.8 mL, 12.9 mmol, 3 equiv) was
added at room temperature to a solution of N-triphenylmethyl-
(R)-serine(O-methanesulfonyl) allyl ester 4 (2 g, 4.3 mmol) in
THF (20 mL). The solution was heated at reflux for 12 h. The
mixture was then diluted with EtOAc (20 mL), washed with
citric acid (5% aq, 2 × 10 mL) and brine (2 × 10 mL), dried
over MgSO4, and concentrated to afford a viscous oil. Silica
chromatography purification (hexane/EtOAc, 12:1) afforded the
pure aziridine 6 (1.4 g, 88%) as a colorless oil. Starting material
was recovered in 8% yield: Rf 0.60 (hexane/ethyl acetate, 2:1);
1H NMR (CDCl3 500 MHz) δ 7.60 (6H, m, Trt), 7.34 (6H, m,
Trt), 7.26 (3H, m, Trt), 5.95 (1H, m, CHdCH2), 5.36 (1H, dd,
J ) 17.2, 1.5 Hz, CHdCH2), 5.27 (1H, dd, J ) 10.4 Hz, 1.3
Hz, CHdCH2), 4.69 (m, 2H, CH2CHdCH2), 2.29 (1H, dd, J )
2.7, 1.6 Hz, NCH2), 1.94 (1H, dd, J ) 2.7, 6.15 Hz, NCHCO2),
1.44 (1H, dd, J ) 6.15, 1.6 Hz, NCH2);13C NMR (CDCl3, 126
MHz) δ 171.1, 143.6, 131.9, 129.0, 129.8, 126.9, 118.5, 74.3,
65.5, 31.7, 28.7; mass spectrum m/z (ES+) 370 ([M + H]+, 9),
243 (Trt+, 100); (FAB) 393 ([M + Na + H]+, 13), 243 (Trt+,
100); HRMS (FAB) calcd for
392.1645, found 392.1626.
C
25H23NO2Na ([M + Na]+)
Rea ction of Azir id in e 6 w ith 10 equ iv of Na I To Give
5 a n d 11. (R)-N-Triphenylmethylaziridine-2-carboxylic acid
allyl ester 6 (0.012 g, 33 µmol) was treated with sodium iodide
(0.049 g, 0.33 mmol, 10 equiv) in acetone (0.5 mL) under Ar,
and the mixture was stirred for 72 h at 25 °C. The solvent
was then removed in vacuo. Ether (10 mL) was added, and
sodium thiosulfate (10% aq w/v, 1 mL approximately) was then
slowly added, decolorizing the organic layer to a pale yellow
solution. The organic layer was separated and dried over
anhydrous sodium sulfate, and the solvent was removed in
vacuo. The crude product was purified by preparative normal-
phase HPLC (2-3% ethyl acetate in hexane over 60 min, 15
mL/min). A mixture of 5 (46.0 min) and 12 (47.1 min) was
isolated and partially resolved (combined mass 0.0044 g, 8.8
µmol, 27%) in a 1:3 ratio, identical by NMR to the mixture
analyzed previously. The starting material was also recovered
at 53.6 min as the predominant species.
Na]+, 31), 611 ([M + H]+, 38), 555 ([M - Bu + 2]+, 56), 333
t
([M - tBu - Fmoc + 3]+, 100); HRMS (FAB) calcd for
C
32H38N2O8SNa ([M + Na]+) 633.2265, found 633.2247.
3-[(R/S)-1-Allyloxycar bon yl-2-(allyloxycar bon ylam in o)-
Rea ction of Azir id in e 6 w ith 1 equ iv of Na I To Give 5
a n d 11. A solution of 6 (235 mg, 6.38 mmol), NaI (96 mg, 6.38
mmol, 1 equiv), and Amberlyst 15 (80 mg, 6.38 mmol, 1 equiv)
in distilled acetone (7 mL) was stirred for 12 h at -40 °C. After
filtration of the beads, the solution was diluted with EtOAc,
washed with brine, dried over MgSO4, and concentrated. The
purification by silica chromatography of the resulting residue
(hexane/EtOAc: 12/1) afforded 5 (10%) and 11 (90%) as an
inseparable mixture (2.79 g, 88%).
eth ylsu lfan yl]-(R)-2-(flu or en -9-ylm eth oxycar bon ylam in o)-
p r op ion ic Acid (3). A 1:6 mixture of 3-[(R)-2-tert-butoxy-
carbonyl-2-(fluoren-9-ylmethoxycarbonylamino)ethylsulfan-
yl-(S)-2-(allyloxycarbonylamino)propionic acid allyl ester 8 and
the regioisomer (R/S)-2-[(R)-2-tert-butoxycarbonyl-2-(fluoren-
9-ylmethoxycarbonylamino)ethylsulfanyl]-3-(allyloxycarbonyl-
amino)propionic acid allyl ester (2.1 g, 3.4 mmol) was treated
J . Org. Chem, Vol. 68, No. 21, 2003 8191

Mohd Mustapa et al.
Syn th esis of 3-[(R)-2-ter t-Bu toxyca r bon yl-2-(flu or en -
9-ylm eth oxycar bon ylam in o)eth ylsu lfan yl]-(S)-2(Tr iph en -
ylm eth yla m in o)p r op ion ic Acid Allyl Ester (7) via th e
Mitsu n obu Rea ction . To a precooled solution of ADDP (0.12
g, 0.5 mmol, 2 equiv) in anhydrous CHCl3 (3.0 mL) under N2
was added PMe3 (1 M solution in toluene, 0.5 mL, 0.5 mmol,
2 equiv). The reaction was stirred at 0 °C for 1 h, during which
time the solution changed its color from yellow to almost
transparent. At the same time, a solution of N-9-fluorenyl-
methoxycarbonyl-(R)-cysteine tert-butyl ester (0.10 g, 0.25
mmol, 1 equiv) and zinc tartrate (0.01 g, 0.05 mmol, 0.2 equiv)
in dry CHCl3 (2 mL) under N2 were stirred at room temper-
ature for 1 h. N-Triphenylmethyl-(R)-serine allyl ester 16 (0.09
g, 0.25 mmol, 1 equiv) was then added to the PMe3-ADDP
mixture. After being stirred for a further 5 min, this mixture
was transferred to the zinc tartrate-cysteine mixture. The
reaction was then stirred for 7 days at room temperature under
inert conditions. The solvents were removed in vacuo, and the
crude material was redissolved in ethyl acetate (10 mL).
Hexane (40 mL) was added, causing precipitation of the
hydrazide, which was removed by filtration. The solvents were
then removed in vacuo to afford the crude product, which was
purified by flash chromatography (hexane/ethyl acetate, 4:1)
to give pure 7 as a yellow oil (0.100 g, 0.13 mmol, 52%),
spectroscopically identical to the minor isomer described above.
Graduate School Research Scholarship (to M.F.M.M.),
and Pfizer Ltd. for a CASE award (to M.F.M.M.). We
also thank the Leverhulme Trust for a Research Project
Grant (to S.B.), UCL for the award of a Provost’s
Studentship (to M.F.A.G.), and the Ludwig Institute for
Cancer Research for a fellowship (to P.R.J .G.). This is
a publication from the Bloomsbury Centre for Structural
Biology, funded by the BBSRC.
Su p p or tin g In for m a tion Ava ila ble: Full experimental
details and characterization for the preparation of 16 from (R)-
serine, the preparation of Fmoc-Cys-OtBu, and the preparation
of veratryl-protected lanthionines, together with the complete
experimental details and characterization for the pathways,
based on (S)-serine, described in Schemes 2 and 3. 1H NMR
spectra of all compounds described in the Experimental Section
and in the Supporting Information (both mixtures of regioi-
somers, pure 8 and the regioisomer after HPLC separation,
and pure lanthionines 7 and 10 prepared via the Mitsunobu
route) and the benzyl ester 14/15; VT NMR of the regioisomer
1
mixtures 5/11, 7/12, and veratryl-protected lanthionine; H-
13C HSQC spectra of the regioisomer mixtures 5/11, 7/12, and
pure lanthionines 7 and 10 prepared via the Mitsunobu route;
HMBC spectra of the regioisomer mixtures 7/12 and pure
lanthionines 7 and 10 prepared via the Mitsunobu route. This
material is available free of charge via the Internet at
http://pubs.acs.org.
Ack n ow led gm en t. We thank the Overseas Re-
search Students’ Award Scheme for the award of a
Scholarship (to M.F.M.M.), UCL for the award of a
J O0346398
8192 J . Org. Chem., Vol. 68, No. 21, 2003