The stereospecific synthesis of 'orthogonally' protected lanthionines

Article abstract of DOI:10.1016/S0040-4020(02)01091-8

Lanthionine is an attractive monomer for the design and synthesis of novel conformationally constrained peptidomimetics, since unlike cystine, the monosulfur bridge of lanthionine is chemically far more robust and also displays a greater degree of conformational rigidity. The synthesis of lanthionine residues for use in peptide synthesis is non-trivial due to the protectional requirements necessary for this tetra-functional amino acid. In this paper an efficient stereo-specific route to orthogonally protected lanthionine is described.

Full text of DOI:10.1016/S0040-4020(02)01091-8

Tetrahedron 58 (2002) 9101–9109
The stereospecific synthesis of ‘orthogonally’
protected lanthioninesq
Vinay Swali,a Mizio Matteucci,a Richard Elliotb and Mark Bradleya
,
*
aDepartment of Chemistry, University of Southampton, Southampton SO17 1BJ, UK
bGSK, Medicines Research Centre, Gunnels Wood Road, Stevenage SG1 2NY, UK
Received 3 May 2002; revised 2 August 2002; accepted 29 August 2002
Abstract—Lanthionine is an attractive monomer for the design and synthesis of novel conformationally constrained peptidomimetics, since
unlike cystine, the monosulfur bridge of lanthionine is chemically far more robust and also displays a greater degree of conformational
rigidity. The synthesis of lanthionine residues for use in peptide synthesis is non-trivial due to the protectional requirements necessary for this
tetra-functional amino acid. In this paper an efficient stereo-specific route to orthogonally protected lanthionine is described. q 2002 Elsevier
Science Ltd. All rights reserved.
1. Introduction
contrast to the labile disulfide bond of cystine, the
monosulfur bridge of lanthionine is chemically far more
robust, while cyclic structures based on lanthionines also
display a greater degree of conformational rigidity. For
these reasons, lanthionine is an attractive monomer for the
design of novel conformationally constrained peptido-
mimetics. However, the synthesis of lanthionine residues
for use in peptide synthesis is non-trivial for two main
reasons. Often, the reactive synthons employed in the
construction of the lanthionine skeleton can undergo
eliminative side reactions, thus the maintainance of
stereochemical integrity throughout the reaction course is
frequently a problem. A second issue is the discrimination
of the two amine and two acid functions of the lanthionine
residue. The selection of orthogonal protecting groups can
be pivotal to the success of lanthionine forming reactions.
Lanthionine is a non-proteinogenic amino acid comprised of
two alanyl residues bridged by a thioether linkage. This
unusual amino acid was first detected in 1941 in alkaline
hydrolysates of wool,1 and was subsequently found in
human hair,2 feathers,3 chick embryos4 and epidermal
keratin.5 Lanthionine has also been found in cataractous
human lenses, and it is believed that the photo-oxidative
degradation of cystine residues results in the formation of
lanthionine; the subsequent increase in the extent of protein
cross-bridging leading to increased tissue rigidity and
hardening of the eye lens.6 Other bridged amino acids
such as lysinoalanine have also been implicated in the
ageing process.7 Biosynthetically, lanthionine is derived
from the Michael addition of cysteine onto dehydroalanine.8
The processing of many foods involves treatment with heat
or alkali at some stage, treatments which are known to give
rise to novel amino acids such as histidinoalanine,
lysinoalanine and lanthionine,9 which in turn have conse-
quences for nutrition, food safety and health.7 Lanthionine
is also of interest in the silk10 and leather11 industries. In
The first synthesis of lanthionine was performed by du
Vigneaud and Brown12,13 in 1940 (Scheme 1). Their
strategy involved the S-alkylation of L-cysteine with L-b-
chloroalanine. However, the strongly basic conditions
required resulted in the formation of dehydroalanine from
Scheme 1. The first synthesis of lanthionine.
q Swali, V. PhD Thesis, University of Southampton, 2000.
Keywords: lanthionine; peptidomimetics; peptides.
*
Corresponding author. Fax: þ44-1703-596766; e-mail: mb14@soton.ac.uk
0040–4020/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4020(02)01091-8

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V. Swali et al. / Tetrahedron 58 (2002) 9101–9109
Scheme 2. Mono-desulfurisation of disulfides.
the chloroalanine and the subsequent 1,4-addition of
the thiolate anion afforded a diastereomeric mixture of
lanthionine 1 in poor yield.
peptides, relatively little has been reported concerning the
solid phase incorporation of lanthionine into synthetic
peptides. The reasons for this as discussed earlier being the
formation of an optically homogeneous lanthionine skeleton
and the chemo-differentiation of the two acid and two amino
functions. Goodman has reported methodology which
allows for the solid phase synthesis of cyclolanthionines:
the technique utilised lanthionyl monomers derived from
serine b-lactones which were subsequently cyclised and
cleaved using an oxime linker.21 Mayer and co-workers
have detailed a procedure for the in situ synthesis of
lanthionines on the solid phase.22 Their approach involved
the conversion of a serine unit within the peptide
sequence into bromoalanine. This residue was then used
to alkylate a cysteinyl monomer, also contained within the
peptide. However, the stereochemical integrity of these
conversions is in doubt, it being probable that the reactions
proceed via a dehydroalanine residue. It is thus evident that
despite much activity in this area, there is still a requirement
for a high yielding synthesis of a lanthionine derivative,
An attractive approach to the synthesis of lanthionines
involves the extrusion of sulfur from cystine residues, a
methodology which was reported by Harpp and Gleason for
a range of alkyl thioethers14 (Scheme 2).
However the application of this method to lanthionine
synthesis has proven to be problematic for several groups,
including our own, with large amounts of the dehydroamino
acid being formed.15 Despite these problems, Shiba et al.
accomplished the total synthesis of the lantibiotic nisin
using the sulfur extrusion method to generate lanthionine
ring fragments which were duly condensed.16 Another route
to lanthionines involves the opening of a serine b-lactone17
with a protected cysteine residue. This method was utilised
by Goodman to prepare a series of tri-protected lanthionine
residues (Scheme 3).18
Scheme 3. Synthesis of lanthionine by ring opening of serine b-lactones.
This methodology was used to generate lanthionine
derivatives for the solid phase synthesis of peptidomimetics,
although only moderate yields were obtained due to
competing O-acyl fission.19 However, the use of the Cbz
protecting groups limits application. Bradley utilised a
biomimetic synthesis to allow the preparation of a fully
protected lanthionine derivative suitable for solid phase
synthesis. Thus the Michael reaction between Boc-Dha-
OMe and Fmoc-Cys-OAll afforded the desired lanthionine
in 70% yield.15 The two diastereoisomers were separable by
HPLC, however drawbacks to this approach included the
low overall yield of the desired meso isomer. Recently,
Dugave and Menez detailed a solution phase synthesis of
lanthionines20 involving the alkylation of cysteinyl deriva-
tives with N-triphenylmethyl (trityl) protected iodoalanines
(Scheme 4). It was found that steric buttressing by the trityl
group protected the iodoalanine residue from b-elimination,
though small amounts of the aziridine were isolated.
Analysis of the lanthionines showed the stereochemistry
of the iodoalanine chiral centre to be retained during
substitution. However, the scope of this thioalkylation
methodology was limited due to the protecting groups
utilised.
which is appropriately protected for solid phase initiatives.
Furthermore, it is essential that the reaction proceeds
without epimerisation of either chiral centre and that the
desired lanthionine product can be isolated without
difficulty.
2. Results and discussion
2.1. Lanthionine synthesis
Our first attempts to prepare lanthionine derivatives used
Mitsunobu chemistry. A major consideration in Mitsunobu
reactions performed on serine is the fate of the activated
alcohol. In the case of N-protected serines the activated
intermediate decomposes intramolecularly, yielding lac-
tones. Bis-protected derivatives however are highly prone to
elimination, forming dehydroalanines. In order to circum-
vent this general problem the amino protecting group,
triphenylmethyl (Trt) has been used. The steric bulk of this
functionality shields the a-carbon centre of amino acids,
thus protecting them from base promoted racemisation.23
N-Trityl amino acid esters have also been shown to
withstand saponification, again due to the spatial properties
of the trityl group. In light of these results, we attempted the
Despite the interest surrounding the lantibiotic family of
Scheme 4. The Dugave–Menez approach to lanthionines utilising b-iodoalanines.

V. Swali et al. / Tetrahedron 58 (2002) 9101–9109
9103
Scheme 5. Successful synthesis of lanthionine by thioalkylation.
synthesis of lanthionine derivatives via the Mitsunobu
reaction of N-trityl-serine methyl ester24 using Fmoc-Cys-
OMe as the nucleophile, with DIAD and PPh3. However,
this reaction failed to generate the desired lanthionine. The
reaction was repeated with a variety of redox carriers
(DEAD, DIAD, TMAD, PPh3, PBu3) without success.
It was believed that following the initial activation of the
triphenylphosphine by DIAD, the nucleophilic thiol com-
ponent reacted at the phosphorus centre. At this stage, a
report on the synthesis of thioethers using modified
Mitsunobu chemistry caused us to change our strategy.
Falck demonstrated that a variety of aliphatic thiols could
undergo Mitsunobu reactions with primary alcohols using
ADDP/PMe3 in the presence of imidazole.25 However the
only material isolated when these conditons were applied
was, surprisingly, the trityl protected dehydroamino acid.
Other N-trityl protected alanine b-cation equivalents were
therefore utilised in subsequent attempts to synthesise
lanthionines. Sulfonate esters of serine in thioalkylations
were fruitless and resulted in formation of the aziridine.
Dugave and Menez20 reported similar findings during this
work, however, they showed that the iodide, generated from
the serine mesylate was much more pliable. It was
envisaged therefore that iodoalanine could be a potentially
useful intermediate in the synthesis of lanthionines. The
literature route to the iodide via the mesylate was found to
be capricious, often giving significant amounts of aziridine
or poor conversion. A simple one-step phosphine mediated
iodination26 of N-Trt-Ser-OMe (2) was therefore developed
to afford the desired iodide (Scheme 5). The iodide (3) was
found to be highly photosensitive and thermally unstable,
but could be stored for up to a month at 2208C without
appreciable degradation. NMR analysis indicated the
presence of two rotamers arising from restricted rotation
about the Ca–Cb bond, but variable temperature NMR
experiments showed no coalescence of rotameric signals
below 323 K, while above this temperature the sample
decomposed to the aziridine.
The synthesis of lanthionines utilising the iodide (3) via
solution phase thioalkylation was attempted. It was hoped
that dehydroalanine formation would be suppressed by the
steric protection of the trityl group, while aziridine
formation would be minimised by the selection of reaction
conditions. Thus cystine was suitably protected to give (4)
prior to conversion into cysteine (5) by reduction with zinc
in acetic acid. The coupling of the iodide (3) with cysteine
(5) was performed in DMF with 1 equiv. of Cs2CO3
(Scheme 5), which after purification by column chroma-
tography afforded the desired lanthionine (6) in 71% yield.
A small quantity (12%) of aziridine (7) was also isolated.
2.2. Stereochemical course of lanthionine formation
The other lanthionine diastereomer was prepared in similar
fashion. Optical rotations correlated well with those of the
corresponding compounds from the L-series and with
literature data. Analysis was also carried out by NMR
and, to this end, the two diastereomeric lanthionines were
converted into their Mosher amides by detritylation and
amidation with (2R)-2-methoxy-2-phenyl-3,3,3-trifluoro-
acetyl chloride.27 Analysis confirmed that lanthionine
synthesis had transpired stereoselectively, in particular,
the resonance of the methoxy group was found to vary
significantly according to the lanthionine stereochemistry.
By recording the 1H NMR of a mixed sample it was possible
to demonstrate the optical purity of the individual samples
was .95% and hence the stereo-integrity of the thio-
alkylation step.
2.3. An orthogonally protected lanthione
Following successful stereospecific synthesis of lanthio-
nines it was necessary to extend the methodology. In
particular, we envisaged a Trt/Allyl/Fmoc strategy would
facilitate entry into the cyclo-lanthionyl systems via
immobilisation onto a solid support via esterification or
Figure 1. Protecting group strategy for synthesis of cyclic lanthionine fragments.

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V. Swali et al. / Tetrahedron 58 (2002) 9101–9109
Scheme 6. Orthogonally protected lanthionine synthesis.
amidation of the unprotected C-terminus (Fig. 1), followed
by Fmoc peptide chemistry, Pd(0) removal of the Allyl
ester, cyclisation and chain extension following Trityl
removal.
regioselective tranformations. To this end the enzymatic
monohydrolysis of lanthionine (6a) with PLE and PPL was
attempted. Despite using a variety of reaction conditions, it
was not possible to isolate the requisite acid, the lack of
hydrolysis being ascribed to either the poor solubility in
acetone/water mixtures or the bulky nature of the substrate.
The problems associated with the regioselective mono-
hydrolysis of lanthionyl diesters led to a reassessment of the
route to the lanthionine acid. Differential protection of the
acid moieties in (6) would afford a tetra-functionalised
lanthionine unit (8) (Scheme 6). Acidolysis would furnish
(9), which could be re-protected to give (10).
Initial attempts at generating tri-protected lanthionines
utilising Fmoc-Cys-OH as the nucleophilic synthon part-
nered with an iodoalanine furnished poor yields of the
required lanthionine with the mass balance being accounted
for by aziridine formation. Another strategy was therefore
adopted subjecting the fully protected lanthionine residue
(6) to regioselective saponification in an attempt to provide
a suitable monomer for solid phase attachment and
synthesis. Although lanthionine (6) contains three poten-
tially base labile functionalities, two esters and the Fmoc
group, Zervas8 demonstrated that the steric properties of
the N-Trt group will block a-methyl ester hydrolysis. Two
groups have reported the selective removal of a methyl ester
in the presence of an Fmoc group. Burke for example
saponified a tyrosine derivative without significant loss of
the Fmoc group using ice-cold 0.2 M LiOH in dioxane,28
while Pascal and Sola29 observed that the addition of high
concentrations of calcium chloride to the reaction media
dramatically increased the lifetime of the Fmoc group in
basic solution. The regioselective saponification of (6) in
alkaline solution containing calcium chloride was therefore
attempted. Unfortunately, work-up of the reaction furnished
only starting material, due to the limited solubility of the
substrate under the reaction conditions (iPrOH/H2O, 7:3),
while other mixed solvent systems were also unsuccessful.
Attempts at performing selective hydrolysis with lithium
hydroxide in dioxane were equally fruitless, a single
equivalent of base failed to afford the tri-protected monomer
while an excess led to deprotection of the Fmoc group.
A chemoenzymatic approach to the lanthionyl acids was
also considered. The use of hydrolytic enzymes, especially
esterases and lipases, in organic synthesis has been well
documented,30 in particular, porcine liver esterase (PLE)
has found considerable use in a variety of stereo and
Synthesis of iodoalanine (13) began with esterification of
D-serine under Dean–Stark conditions with allyl alcohol,31
followed by N-tritylation of the ester affording di-protected
serine, which was transformed into the desired, photo-
sensitive, iodide (13) as previously described. As expected,
the iodide exhibited rotational isomerism by NMR, and
degraded to the corresponding aziridine at elevated
temperatures. The thiol component was realised from
cystine (11) by protection of both acid functions with tert-
butyl acetate in perchloric acid,32 double N-protection
under standard conditions (Fmoc-OSu) to furnish the
symmetrical cystine derivative, which was reduced with
zinc/acetic acid33 to give the desired thiol (12). The cesium
salt of the thiol (12) was alkylated with the b-iodoalanine
(13) in DMF, to afford the desired tetra-functional
lanthionine (8) as a white foam (74%). The aziridine,
formed by the intra-molecular degradation, was also
obtained (16%). Acidolytic deprotection of the trityl group
was effected with TFA in DCM (our fears that the TIS/TFA
system could reduce the allyl olefin34 were assuaged
by ES–MS and NMR analysis) and N-protection with
Boc2O in dioxane proved facile, affording the orthogonally
protected monomer in 68% yield. Following the suc-
cessful synthesis of the lanthionine monomer its utility in
solid phase peptide synthesis was tested. Thus the
lanthionine monomer (10) was coupled onto a solid support

V. Swali et al. / Tetrahedron 58 (2002) 9101–9109
9105
(Fmoc-Val-Rink linker-PS). TFA cleavage gave the
expected lanthionine derivative (2S,6R)-N 6-(9H-fluorenyl-
methoxycarbonyl)-lanthionyl-(O 1-allyl)-valinamide (14).
Data agreed with the literature.20 Rf (ether/hexane, 1:9 v/v):
0.41; LRMS (ESIþ) m/z: 243 (Trt)þ; 472 (MþH)þ; dH
(300 MHz; CDCl3; p denotes minor rotamer): 2.20 (bs, 1H,
NH); 2.51p (dd, J¼6, 13 Hz)þ3.14 (dd, J¼7, 10 Hz) (1H,
CHb); 2.68p (dd, J¼9, 13 Hz)þ3.27 (dd, J¼4, 10 Hz) (1H,
p
p
0
CHb ); 3.23 (s)þ3.69 (s) (3H, CO2CH3); 3.39 (m)þ4.39
3. Conclusion
(m) (1H, CHa); 7.09–7.28 (m, 9H, Trt ArH); 7.41–7.56
(m, 6H, Trt ArH). dC (75 MHz; CDCl3; p denotes minor
rotamer): 9.8pþ48.5 (CHb); 20.3þ56.4p (CHa); 52.1pþ53.0
(CO2CH3); 71.2pþ77.4 (Ph3CNH); 126.7–129.4 (complex,
Ar CH); 145.6þ145.8p (ipso Ar-C); 171.3þ172.9p
(CO2CH3); IR (neat) n (cm21): 3100–2800 (br, w); 1733
(m; CvO); 1595 (w); 1489 (w); 1447 (w); 1208 (m); 1158
(m); 1028 (w); 900 (w); 743 (s); 701 (s); [a]2D0¼þ23 (c¼1,
CHCl3), lit. [a]2D5¼þ21 (c¼1, CHCl3).
Despite the simplicity of its structure, the synthesis of
lanthionine derivatives, which have utility as peptido-
mimetics, is by no means trivial. We have demonstrated
the utility of iodoalanines in the generation of synthetically
valuable lanthionines, useful for solid phase synthesis. Such
iodide synthons may be rapidly accessed from the chiral
pool, though their instability should not be forgotten. It has
been shown by chemical correlation and the method of
Mosher that the formation of lanthionines by thioalkylation
of iodides is a stereospecific process. These lanthionines are
now available for the synthesis of a range of constrained
peptido-mimetics and other synthetic targets, including the
lantibiotics.
4.1.2. (2S)-N-Triphenylmethyl-3-iodoalanine methyl
ester (3b). Repetition of the above methodology afforded
the title compound in 85% yield. Data as above except:
[a]2D0¼221.0 (c¼1, CHCl3).
4.1.3. (2R)-N-(9H-Fluorenylmethoxycarbonyl) cysteine
methyl ester (5). The title compound was prepared using a
modification of the Kihlberg procedure.33 To a solution of
(2R,7R)-N,N-bis-(9H-fluorenylmethoxycarbonyl) cystine
dimethyl ester (5.00 g, 7.0 mmol) in acetic acid (50 mL)
and DCM (25 mL) was added acid washed zinc dust (4.56 g,
70.2 mmol, 10 equiv.) portionwise. The mixture was stirred
under nitrogen overnight and the solvent removed in vacuo.
2 M HCl (250 mL) was added to the residue and the mixture
extracted with DCM (2£250 mL). The combined organics
were washed with water, dried over MgSO4 and concen-
trated in vacuo to afford the title compound as a white solid
(4.71 g, 94%) which was used immediately.
4. Experimental
4.1. General
NMR spectra were obtained at 298 K using a Bruker
AC-300 spectrometer (300 MHz for 1H NMR, 75 MHz
for 13C NMR and 282 MHz for 19F NMR) and a Bruker
DPX-400 spectrometer (400 MHz for 1H NMR and
100 MHz for 13C NMR). Chemical shifts (d) were
referenced to the residual proton signals of deuterated
solvents. ES–MS was performed on a VG Platform
quadrupole electrospray ionisation mass spectrometer.
FAB mass spectra were obtained on a VG analytical
70-250-SE normal geometry double focusing mass spectro-
meter, using argon as a bombarding gas. High resolution
mass measurements were recorded at 10,000 resolution
using mixtures of polyethylene glycol and/or polyethylene
glycol methyl ethers as mass calibrants for FAB. Infra-red
spectra were recorded on a Bio-Rad Golden Gate FTS 135
spectrophotometer with neat solids or solutions. Optical
rotations were measured using an AA-100 polarimeter from
Optical Activity Ltd using a path length of 5 cm. Melting
points were determined using open capillaries on a
Gallenkamp apparatus and remain uncorrected. TLC was
performed using Alugramw silica gel 60 F254 (0.25 mm)
plates and visualised using UV, phosphomolybdic acid,
ninhydrin or bromocresol green.
4.1.4. (2R,6R)-N 2-Triphenylmethyl-N 6-(9H-fluorenyl-
methoxycarbonyl)-lanthionine dimethyl ester (6a). The
title compound was prepared using a modification of the
Dugave and Menez procedure.20 Thus, (2R)-N-triphenyl-
methyl-3-iodoalanine methyl ester (1.70 g, 3.61 mmol) and
(2R)-N-(9H-fluorenylmethoxycarbonyl) cysteine methyl
ester (1.29 g, 3.61 mmol, 1 equiv.) were dissolved in dry
DMF (20 mL). Cesium carbonate (1.18 g, 3.61 mmol,
1 equiv.) was added and the reaction mixture stirred under
nitrogen in the dark for 4 h. 10% citric acid solution
(100 mL) was added and the solution extracted with ether
(2£50 mL). The combined organics were washed with
brine, dried over MgSO4 and the solvent removed in vacuo.
The residue was purified by column chromatography to
afford the title compound as a white foam (1.80 g, 71%); Rf
(ethyl acetate/hexane, 3:17 v/v): 0.12; LRMS (ESIþ) m/z:
243 (Trt)þ; 701 (MþH)þ; 723 (MþNa)þ; 739 (MþK)þ;
HRMS (FT-MS) calcd for C42H41O6N2S 701.2680. Found
701.2682; dH (400 MHz; CDCl3): 2.27 (br s, 1H, Trt
NH); 2.40–3.01 (m, 4H, CH2SCH2); 3.14 (s, 3H,
TrtNHCH(R)CO2CH3); 3.39–3.51 (m, 1H, CHa); 3.81 (s,
3H, Fmoc NHCH(R)CO2CH3); 4.15 (t, J¼7 Hz, 1H, Fmoc
4.1.1. (2R)-N-Triphenylmethyl-3-iodoalanine methyl
ester (3). The title compound was prepared by a
modification to the Garegg procedure.26 Thus (2S)-N-
triphenylmethyl serine methyl ester (2.00 g, 5.54 mmol),
triphenylphosphine (1.45 g, 5.54 mmol, 1 equiv.) and
imidazole (0.38 g, 5.54 mmol, 1 equiv.) were dissolved in
dry DCM (50 mL) with stirring. The solution was cooled on
ice to 08C and stirring continued for 5 min. Iodine (1.41 g,
5.54 mmol, 1 equiv.) was added portionwise over 2 min and
stirring continued for 2.5 h in the dark. Solvent was
removed in vacuo and the residue was purified by column
chromatography to afford the title compound as a photo-
sensitive amorphous off white solid solid (2.19 g, 84%).
0
H-9); 4.21–4.43 (m, 2H, Fmoc CH2O); 4.52 (m, 1H, CHa );
5.70 (d, J¼7 Hz, 1H, Fmoc NH); 7.24 (t, J¼7 Hz, 3H, Trt
Ar p-H); 6.95–7.51 (m, 16H, Trt ArHþFmoc H); 7.68 (br d,
J¼7 Hz, 2H, Fmoc H-1þH-8); 7.83 (d, J¼7 Hz, 2H, Fmoc
H-4þH-5); dC (100 MHz; CDCl3): 34.5 (CH2SCH2); 35.8
(CH2SCH2); 47.5 (Fmoc CH); 52.3 (CO2CH3); 53.2

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V. Swali et al. / Tetrahedron 58 (2002) 9101–9109
0
(CO2CH3); 54.2 (CHa); 56.9 (CHa ); 67.6 (Fmoc CH2); 71.6
(Ph3C); 120.4, 125.6, 127.0, 127.5, 128.2, 128.4, 129.2
(Fmoc Ar CHþTrt Ar CH); 141.7, 144.2 (Fmoc Ar-C);
146.0 (Trt ipso Ar-C); 156.2 (Fmoc OCONH); 171.4
(CO2CH3); 174.3 (CO2CH3); IR (neat) n (cm21): 1722
(br; s); 1493 (m); 1446 (m); 1205 (br; s); 1028 (br; m); 739
(s); 704 (s); [a]2D0¼þ48.8 (c¼0.5, ethyl acetate).
1448 (m); 1440 (m); 1342 (m); 1317 (w); 1264 (m); 1213
(s); 1164 (s); 1105 (m); 911 (m); 734 (s); 697 (m).
4.1.7. (2S,6R)-N 2-[(2R)-(2-Methoxy-2-phenyl-3,3,3-tri-
fluoropropionoyl]-N 6-(9H-fluorenylmethoxycarbonyl)-
lanthionine dimethyl ester. The title compound was pre-
pared as above from (6b) (selected data): (33 mg, 45%);
white foam; HRMS (FAB) calcd for C33H34F3O8N2S
675.1988. Found 675.1988; dF (CDCl3): 8.38 (s, 3F, CF3).
4.1.5. (2S,6R)-N 2-Triphenylmethyl-N 6-(9H-fluorenyl-
methoxycarbonyl)-lanthionine dimethyl ester (6b). White
foam, LRMS (ESIþ) m/z: 243 (Trt)þ; 701 (MþH)þ; 723
(MþNa)þ; 739 (MþK)þ; dH (400 MHz; CDCl3): 1.95 (br
4.1.8. (2R)-N-Triphenylmethyl serine allyl ester. The title
compound was prepared by using a modification of the
Baldwin procedure.24 (2R)-Serine allyl ester toluenesulfo-
nate salt (5.60 g, 16.7 mmol) was dissolved in dry DCM
(100 mL) with stirring and cooled to 08C. Triethylamine
(3.38 g, 4.66 mL, 2 equiv.) was added dropwise, followed
by triphenylmethyl chloride (5.12 g, 18.4 mmol, 1.1 equiv.)
in DCM (50 mL). The solution was stirred at 08C overnight
and filtered. The filtrate was washed with 1 M citric acid
(2£100 mL), water (2£100 mL), dried over MgSO4 and
solvent removed in vacuo. The residue was purified by
column chromatography to afford the title compound as
a clear oil (4.39 g, 68%); LRMS (ESIþ) m/z: 243 (Trtþ);
388 (MþH)þ; 410 (MþNa)þ; HRMS (FAB) calcd for
C25H26O3N 388.1913. Found 388.1907; [a]2D0¼þ28 (c¼1,
MeOH); dH (400 MHz; CDCl3): 2.42 (br s, 1H, OH or NH);
3.07 (br s, 1H, OH or NH); 3.62 (m, 2H, CH2OH); 3.77 (dd,
J¼7, 13 Hz, 1H, CHa); 4.14 (dd, J¼6, 13 Hz, 1H, allyl
CHH0CHvCH2); 4.26 (dd, J¼6, 13 Hz, 1H, allyl
CHH0CHvCH2); 5.23 (dd, J¼1, 11 Hz, 1H, allyl CH2-
CHvCHH0); 5.24 (dd, J¼2, 17 Hz, 1H, allyl CH2-
CHvCHH0); 5.76 (ddt, J¼6, 12, 17 Hz, 1H, allyl
CH2CHvCHH0); 7.21–7.43 (m, 9H, Trt ArH); 7.45–7.62
(m, 6H, Trt ArH); dC (100 MHz; CDCl3): 58.2 (CHa);
65.4 (CH2OH or allyl CH2CHvCH2); 66.1 (CH2OH
or allyl CH2CHvCH2); 71.4 (Ph3C); 119.0 (allyl
CO2CH2CHvCH2); 127.1, 128.4, 129.1 (ArCH); 132.0
(allyl CO2CH2CH¼CH2); 146.0 (ipso Ar-C); 173.6
(CO2CH2); IR (neat) n (cm21): 3056 (br, w); 1731 (s);
1489 (w); 1447 (w); 1169 (s); 1154 (s); 984 (m); 933 (m);
745 (s); 697 (s).
0
s, 1H, Trt NH); 2.80 (dd, J¼8, 13 Hz, 1H, CHbHb S); 2.94–
0
0
3.03 (m, 2H, CHbHb SþSCHbHb ); 3.09 (dd, J¼5, 14 Hz,
0
1H, SCHbHb ); 3.30 (s, 3H, TrtNHCH(R)CO2CH3);
3.59 (dd, J¼5, 7 Hz, 1H, CHa); 3.81 (s, 3H, Fmoc
NHCH(R)CO2CH3); 4.30 (t, J¼7 Hz, 1H, Fmoc H-9);
0
4.48 (d, J¼7 Hz, 2H, Fmoc CH2O); 4.69 (m, 1H, CHa );
5.74 (d, J¼8 Hz, 1H, Fmoc NH); 7.24 (t, J¼7 Hz, 3H, Trt
Ar p-H); 7.32 (dd, J¼7, 7 Hz, 6H, Trt Ar m-H); 7.35–7.40
(m, 2H, Fmoc H-2þH-7); 7.46 (dd, J¼7, 7 Hz, 2H, Fmoc
H-3þH-6); 7.56 (d, J¼7 Hz, 6H, Trt Ar o-H); 7.68 (br d,
J¼7 Hz, 2H, Fmoc H-1þH-8); 7.83 (d, J¼7 Hz, 2H, Fmoc
H-4þH-5); dC (100 MHz; CDCl3): 34.2 (CH2SCH2); 35.0
(CH2SCH2); 47.2 (Fmoc CH); 52.0 (CO2CH3); 52.6
0
(CO2CH3); 53.7 (CHa); 56.3 (CHa ); 67.4 (Fmoc CH2);
71.3 (Ph3C); 120.1, 125.3, 126.7, 127.2, 127.9, 128.1, 128.9
(Fmoc Ar CHþTrt Ar CH); 141.4, 143.9 (Fmoc Ar-C);
145.7 (Trt ipso Ar-C); 155.9 (Fmoc OCONH); 171.2
(CO2CH3); 174.1 (CO2CH3); IR (DCM) n (cm21): 1721
(br; s); 1495 (m); 1444 (m); 1205 (br; s); 1027 (br; m); 738
(s); 699 (s); [a]2D0¼251.2 (c¼0.5, ethyl acetate).
4.1.6. (2R,6R)-N 2-[(2R)-(2-Methoxy-2-phenyl-3,3,3-tri-
fluoropropionoyl]-N 6-(9H-fluorenylmethoxycarbonyl)-
lanthionine dimethyl ester. The title compound was
prepared by using a modification of the Mosher procedure.27
Thus, (2R,6R)-N 6-(9H-fluorenylmethoxycarbonyl)-O 1,O 7-
dimethyl lanthionine (6a), (50 mg, 109 mmol) in DMF
(5 mL) was treated with (2R)-(2-methoxy-2-phenyl-3,3,3-
trifluoroacetyl chloride 36 mg, 141 mmol) and DIPEA
(57 mL, 328 mmol). The reaction mixture was stirred
overnight and ethyl acetate (15 mL) added. The solution
was extracted with 10% citric acid (3£15mL), 10%
NaHCO3 (3£15 mL), brine (2£15 mL) dried over MgSO4
and the solvent removed in vacuo to afford a white foam
(41 mg, 57%); LRMS (ESIþ) m/z: 675 (MþH)þ; 697
(MþNa)þ; 713 (MþK)þ; 1371 (2MþH)þ; HRMS (FT-MS)
calcd for C33H34F3O8N2S 675.1988. Found 675.1998; dH
(300 MHz; CDCl3): 2.79–3.21 (m, 4H, CH2SþSCH2); 3.52
(br s, 3H, OCH3); 3.75 (s, 3H, Fmoc NHCH(R)CO2CH3);
3.77 (s, 3H, CONHCH(R)CO2-CH3); 4.24 (t, J¼7 Hz, 1H,
Fmoc H-9); 4.46 (d, J¼7 Hz, 2H, Fmoc CH2O); 4.51 (m,
4.1.9. (2S)-N-Triphenylmethyl-3-iodoalanine allyl ester
(13). The title compound was prepared by using a
modification of the Garegg procedure.26 (2S)-N-Triphenyl-
methyl serine allyl ester (1.19 g, 3.07 mmol), triphenyl-
phosphine (0.80 g, 3.07 mmol, 1 equiv.) and imidazole
(0.29 g, 3.07 mmol, 1 equiv.) were dissolved in dry DCM
(50 mL) with stirring. The solution was cooled to 08C and
stirring continued for 5 min. Iodine (0.78 g, 3.07 mmol,
1 equiv.) was added portionwise over 2 min and stirring
continued for 2.5 h in the dark. The solvent was removed in
vacuo and the residue purified by column chromatography
(SiO2, ether/hexane, 1:15 v/v) to afford the title compound
as a clear photosensitive gum (1.28 g, 84%); LRMS (ESIþ)
m/z: 243 (Trt)þ; 498 (MþH)þ; HRMS (FAB) calcd for
C25H25O2NI 498.0930. Found 498.0925; dH (300 MHz;
CDCl3; rotamers 2:1; p denotes minor rotamer): 2.25–2.49p
(br s)þ2.93 (d, J¼10 Hz) (1H, TrtNH); 2.54–2.65p
(m)þ3.25 (dd, J¼7, 10 Hz) (1H, Hb); 2.70–2.82p
0
1H, CHa); 4.89 (m, 1H, CHa ); 5.60 (d, J¼8 Hz, 1H, Fmoc
NH); 7.28–7.70 (m, 11H, C6H5þFmoc ArH); 7.78 (d,
J¼7 Hz, 2H, Fmoc H-4þH-5); dC (75 MHz; CDCl3): 34.9
(CH2SCH2); 35.0 (CH2SCH2); 47.2 (Fmoc CH); 52.0
0
(CHa); 53.0 (CO2CH3); 53.1 (CO2CH3); 53.8 (CHa ); 55.4
(OCH3); 67.3 (Fmoc CH2); 77.4 (CF3); 120.2, 125.2, 127.3,
127.8, 127.9, 128.7, 129.8, 132.6 (Ph CHþFmoc Ar CH);
155.8 (Fmoc OCONH); 166.7 (CONH); 170.5 (CO2CH3);
171.0 (CO2CH3); dF (CDCl3): 8.65 (s, 3F, CF3); IR (neat) n
(cm21): 3346 (br, w); 1742 (s); 1721 (s); 1692 (s); 1511 (s);
0
(m)þ3.36 (dd, J¼4, 10 Hz) (1H, Hb ); 3.50–3.58
(m)þ4.45p (dd, J¼7, 9 Hz) (1H, Ha); 4.12 (dddd, J¼1, 1,
6, 13 Hz)þ4.27 (dddd, J¼1, 1, 6, 13 Hz)þ4.68p (ddd, J¼1,

V. Swali et al. / Tetrahedron 58 (2002) 9101–9109
9107
1, 6 Hz) (2H, allyl CH2CHvCH2); 5.20 (dd, J¼1,
10 Hz)þ5.32p (dd, J¼1, 10 Hz) (1H, allyl CH2CHvCHH);
5.24 (dd, J¼1, 018 Hz)þ5.42p (dd, J¼1, 18 Hz) (1H, allyl
CH2CHvCHH ); 5.77 (dddd, J¼6, 6, 10, 18 Hz)þ5.97p
(dddd, J¼6, 6, 10, 18 Hz) (1H, allyl CH2CHvCH2); 7.18–
7.25 (m, 3H, Trt ArH); 7.26–7.36 (m, 6H, Trt ArH); 7.45–
7.57 (m, 6H, Trt ArH); dC (75 MHz; CDCl3; p denotes
minor rotamer): 10.1þ48.6p (CHb); 20.6pþ56.3 (CHa);
66.2þ66.5p (allyl CO2CH2CHvCH2); 71.0pþ71.3
(Ph3CNH); 118.9þ119.1p (allyl CO2CH2CHvCH2);
126.7, 126.8, 128.2, 128.6, 128.8 (Ar CH); 131.5pþ131.8
(allyl CO2CH2CHvCH2); 145.7pþ145.8 (ipso Ar-C);
170.7pþ172.1 (CO2CH2); IR (neat) n (cm21): 3039 (br,
w); 1730 (s); 1596 (w); 1490 (m); 1448 (m); 1415 (w); 1364
(w); 1323 (w); 1272 (w); 1206 (m); 1171 (s); 1115 (w); 1031
(w); 985 (w); 935 (w); 901 (w); 774 (w); 747 (m); 705 (s).
125.3, 127.2, 127.9 (Fmoc Ar CH); 141.4, 143.9 (Fmoc
Ar-C); 155.9 (Fmoc OCONH); 169.5 (COt2Bu); IR (neat) n
(cm21): 3341 (br, w); 2979 (br, w); 1708 (s); 1503 (m); 1449
(m); 1394 (w); 1368 (m); 1341 (m); 1221 (s); 1149 (s); 1045
(m); 842 (m); 756 (m); 736 (s); [a]2D0¼25.1 (c¼2, CHCl3),
lit. [a]2D3¼26.4 (c¼0.56, CHCl3).
The title compound was prepared using a modification of
the Kihlberg procedure.33 Thus to a solution of (2R,7R)-
N,N-bis-(9H-fluorenylmethoxycarbonyl) cystine di-tert-
butyl ester (3.00 g, 3.9 mmol) in acetic acid (30 mL) was
added zinc dust (2.53 g, 39.0 mmol, 10 equiv.) portionwise.
The mixture was stirred under nitrogen overnight and the
solvent removed in vacuo. 2 M KHSO4 (100 mL) was added
to the residue and the mixture extracted with DCM
(2£50 mL). The combined organics were washed with
water (2£50 mL), dried over MgSO4 and concentrated in
vacuo. Purification by flash column chromatography (SiO2,
ether/hexane, 1:4 v/v) afforded the title compound as a
white foam (2.64 g, 88%); LRMS (ESIþ) m/z: 417
(MþNH4)þ; 438 (MþK)þ; 799 (2MþH)þ; 816 (2Mþ
NH4)þ; 821 (2MþNa)þ; 837 (2MþK)þ; HRMS (FAB)
calcd for C22H25O4NS 400.1583. Found 400.1584; dH
(400 MHz; CDCl3): 1.26 (dd, J¼9, 9 Hz, 1H, CH2SH); 1.42
4.1.10. (2R)-N-(9H-Fluorenylmethoxycarbonyl) cysteine
tert-butyl ester (12). Cystine (11) was protected according
to the method of Amaral.32 Thus (2R,7R)-cystine (1.92 g,
8.0 mmol) was dissolved in 60% perchloric acid (5.88 g,
35.2 mmol, 4.4 equiv.) with stirring. To the solution was
added tert-butyl acetate (50 mL) and the mixture stirred for
2 h to give a homogeneous solution. The solution was
cooled to 08C for 24 h and filtered. The white solid was
dissolved in ether (50 mL) and 1 M NaHCO3 (25 mL)
added, the organic layer was washed with 1 M NaHCO3
(2£25 mL) and saturated sodium chloride solution
(3£25 mL). The organic phase was dried over MgSO4 and
reduced in vacuo to furnish the desired compound as a clear
oil (1.81 g, 64%); LRMS (ESIþ) m/z: 353 (MþH)þ; 375
(MþNa)þ; 705 (2MþH)þ; dH (300 MHz; CDCl3): 1.50 (s,
18H, C(CH3)3); 1.71 (s, 4H, NH2); 2.85 (dd, J¼7, 14 Hz,
0
(s, 9H, C(CH3)3); 2.86–2.98 (m, 2H, CHbHb ); 4.15 (t,
J¼7 Hz, 1H, Fmoc H-9); 4.28–4.40 (m, 2H, Fmoc CH2O);
4.43–4.49 (m, 1H, CHa); 5.61 (d, J¼7 Hz, 1H, Fmoc NH);
7.24 (ddd, J¼8, 8, 2 Hz, 2H, Fmoc H-2þH-7); 7.33 (dd,
J¼8, 8 Hz, 2H, Fmoc H-3þH-6); 7.53 (d, J¼8 Hz, 2H,
Fmoc H-1þH-8); 7.68 (d, J¼8 Hz, 2H, Fmoc H-4þH-5); dC
0
(100 MHz; CDCl3): 27.2 (CHbHb ); 28.4 (C(CH3)3); 47.6
(Fmoc CH); 55.9 (CHa); 67.5 (Fmoc CH2); 83.5 (C(CH3)3);
120.4, 125.5, 127.5, 128.2 (Fmoc Ar CH); 141.8, 144.1,
144.3 (Fmoc Ar-C); 156.1 (Fmoc OCONH); 169.4
(COt2Bu); IR (neat) n (cm21): 2978 (br, w); 2360 (br, w);
1721 (br, s); 1511 (m); 1345 (m); 1249 (w); 1220 (w); 1155
(s); 1060 (w); 913 (w); 844 (w); 740 (m).
0
2H, Hb); 3.12 (dd, J¼4, 14 Hz, 2H, Hb ); 3.67 (dd, J¼4,
7 Hz, 2H, Ha); dC (75 MHz; CDCl3): 28.2 (C(CH3)3); 44.2
0
t
(CHbHb ); 54.4 (CHa); 81.2 (C(CH3)3); 173.0 (CO2Bu); IR
(neat) n (cm21): 2977 (w); 1725 (s); 1478 (w); 1459 (w);
1393 (w); 1368 (m); 1251 (m); 1152 (s); 991 (w); 845 (w);
752 (w); [a]2D0¼27.8 (c¼2, CH3OH). (2R,7R)-cystine di-
tert-butyl ester was protected according to the method of
Jung.32 N-(9H-Fluorenylmethoxycarbonyloxy) succinimide
(5.40 g, 16.0 mmol, 0.8 equiv.) and (2R,7R)-cystine di-tert-
butyl ester (3.52 g, 10.0 mmol) were dissolved in THF
(10 mL). A solution of N-methylmorpholine (2.02 g,
2.20 mL, 20.0 mmol, 1.0 equiv.) in THF (5 mL) was
added dropwise and the solution stirred for 3 h. Solvent
was removed in vacuo and the residue taken up in ethyl
acetate (100 mL). The solution was washed with 2 M
KHSO4 (3£100 mL) and water (2£100 mL), dried over
MgSO4 and reduced in vacuo. Further purification by flash
column chromatography (SiO2, chloroform) afforded the
requisite compound as a white foam (6.11 g, 77%); mp:
150–1528C (lit. 151.5–1528C); LRMS (ESIþ) m/z: 797
(MþH)þ; 814 (MþNH4)þ; 819 (MþNa)þ; 835 (MþK)þ;
dH (300 MHz; CDCl3): 1.50 (s, 18H, C(CH3)3); 3.18 (dd,
4.1.11. (2S,6R)-N 2-Triphenylmethyl-N 6-(9H-fluorenyl-
methoxycarbonyl)-lanthionine O 1-allyl-O 7-tert-butyl
esters (8). The title compound was prepared using a
modification of the Dugave and Menez procedure.20 Thus,
(2S)-N-triphenylmethyl-3-iodoalanine allyl ester (497 mg,
1 mmol, 1eq) and (2R)-N-(9H-fluorenylmethoxycarbonyl)
cysteine tert-butyl ester (399 mg, 1 mmol, 1 equiv.) were
dissolved in dry DMF (10 mL). Cesium carbonate (326 mg,
1 mmol, 1 equiv.) was added and the reaction mixture
stirred under nitrogen in the dark for 4 h. 10% Citric acid
solution (100 mL) was added and the solution extracted
with ether (2£50 mL). The combined organics were washed
with brine, dried over MgSO4 and solvent removed in
vacuo. The residue was purified by column chromatography
(SiO2, ethyl acetate/hexane, 3:17 v:v) to afford the title
compound as a white foam (568 mg, 74%) and the aziridine
(59 mg, 16%); LRMS (ESIþ) m/z: 769 (MþH)þ; 791
(MþNa)þ; 807 (MþK)þ; 1537 (2MþH)þ; HRMS (FAB)
calcd for C47H48O6N2S 769.3311. Found 769.3293; dH
(300 MHz; CDCl3): 1.49 (s)þ1.50 (s) (9H, C(CH3)3); 2.25
(br s, 1H, NH); 2.42–2.73 (m, 2H, CH2SCH2); 2.83–3.19
(m, 2H, CH2SCH2); 3.53 and 3.61 (2t, J¼9, 9 Hz, 1H, CHa);
4.24 (dd, J¼7, 15 Hz, 1H, Fmoc H-9); 4.32–4.44 (m, 2H,
0
J¼6, 14 Hz, 2H, Hb); 3.26 (dd, J¼5, 14 Hz, 2H, Hb ); 4.22
(t, J¼7 Hz, 2H, Fmoc H-9); 4.46 (d, J¼7 Hz, 4H, Fmoc
CH2O); 4.60 (m, 2H); 5.78 (d, J¼7 Hz, 2H, Fmoc NH); 7.30
(dd, J¼7, 7 Hz, 4H, Fmoc H-2þH-7); 7.40 (dd, J¼7, 7 Hz,
4H, Fmoc H-3þH-6); 7.60 (d, J¼7 Hz, 4H, Fmoc H-1þ
H-8); 7.76 (d, J¼7 Hz, 4H, Fmoc H-4þH-5); dC (75 MHz;
0
0
CDCl3): 28.2 (C(CH3)3); 42.0 (CHbHb ); 47.2 (Fmoc CH);
Fmoc CH2O); 4.48–4.60 (m, 1H, CHa ); 4.68 (dd, J¼4,
54.3 (CHa); 67.3 (Fmoc CH2); 83.3 (C(CH3)3); 120.1,
6 Hz, 2H, allyl CO2CH2CHvCH2); 5.27 (dd, J¼1, 10 Hz,

9108
V. Swali et al. / Tetrahedron 58 (2002) 9101–9109
1H, allyl CO2CH2CHvCHH0); 5.36 (ddd, J¼1, 1, 18 Hz,
1H, allyl CO2CH2CHvCHH0); 5.63 (d, J¼8 Hz)þ5.74
(d, J¼8 Hz) (1H, conformers, NH); 5.95 (m, allyl
CO2CH2CHvCHH0); 7.14–7.83 (m, 23H, Fmoc ArCHþ
Trt CH); dC (75 MHz; CDCl3): 28.2 (C(CH3)3); 34.1þ34.6
(CH2SCH2); 44.4þ44.7 (CH2SCH2); 47.3 (Fmoc CH-9);
(75 MHz; CDCl3): 28.3 (C(CH3)3); 35.1 (CH2SCH2); 41.2
0
(CH2SCH2); 47.1 (Fmoc CH-9); 53.7 (CHa); 56.3 (CHa );
66.3þ67.3 (allyl CO2CH2CHvCH2þFmoc CH2O); 80.4
(C(CH3)3); 119.1 (allyl CO2CH2CHvCH2); 119.9 (allyl
CO2CH2CHvCH2); 125.3, 127.2, 127.7, 131.5 (Fmoc Ar CH);
141.2, 144.1 (Fmoc Ar C); 156.1 (Fmoc OCONH); 157.5 (Boc
OCONH); 171.3 (allyl CO2CH2CHvCH2); 177.2 (CO2H).
0
47.8þ48.6 (CHa); 54.1þ54.4 (CHa ); 66.1þ66.2 (allyl
CO2CH2CHvCH2); 67.4 (Fmoc CH2O); 80.0 (C(CH3)3);
83.1 (Ph3C); 119.0þ119.1 (allyl CO2CH2CHvCH2); 120.1,
125.3, 126.6, 127.3, 128.12, 128.7 (Fmoc ArCHþTrt CH);
131.8þ131.9 (allyl CO2CH2CH¼CH2); 141.4, 144.0, 145.8
(Fmoc ArCHþTrt CH); 155.9 (Fmoc OCONH);169.6,
171.6 (CO2R); IR (neat) n (cm21): 3308 (w); 2974 (w);
1720 (s); 1492 (w); 1448 (w); 1216 (m); 1148 (s); 1032 (m);
739 (s); 704 (s).
4.1.14. (2S,6R)-N 6-(9H-Fluorenylmethoxycarbonyl)-
lanthionyl-(O 1-allyl)-valinamide (14). Fmoc-Val-Rink
Linker-Polystyrene Resin (1.20 g, 0.99 mmol) was shaken
with 20% piperidine in DMF (20 mL) for 5 min. The resin
was filtered and again shaken with 20% piperidine in
DMF for 15 min. The resin was filtered and washed with
DMF (2£10 mL£1 min), DCM (2£10 mL£1 min) and
ether (2£10 mL£1 min). (2S,6R)-N 2-tert-Butoxycarbonyl-
N 6-(9H-fluorenylmethoxycarbonyl)-O 1-allyl lanthionine
(1.13 g, 1.98 mmol) and HOBt (0.30 g, 1.98 mmol) were
dissolved in DCM (9 mL) and DMF (1 mL) with stirring for
5 min. DIC (0.31 mL, 1.98 mmol) was added and stirring
continued for a further 5 min. The coupling mixture was
added to the resin and the reaction mixture shaken for 2 h.
The resin was filtered and washed as above. A small aliquot
of resin gave a negative ninhydrin test. A sample of the resin
(200 mg) was shaken with TFA (1.9 mL) and TIS (0.1 mL)
for 3 h the mixture filtered and the resin was washed with
TFA (0.5 mL). The organic fractions were pooled and
reduced in vacuo. The residue was dissolved in TFA
(0.2 mL) and cold ether (5 mL) added dropwise.
The supernatant was decanted and the procedure repeated.
The resultant white solid was purified by column chroma-
tography (SiO2, methanol/chloroform, 1:19 v:v) to give the
title compound as a white foam (15 mg, 16%); LRMS
(ESIþ) m/z: 569 (MþH)þ; 591 (MþNa)þ; HRMS (FAB)
calcd for C29H37O6N4S 569.2434. Found 569.2447; dH
(400 MHz; CD3OD): 0.87 (d, J¼6 Hz, 3H, Val CH3); 0.89
(d, J¼6 Hz, 3H, Val CH3); 1.91–2.10 (m, 1H, Val CHb);
2.71–2.91 (m, 4H, CH2SCH2); 3.60–3.71 (m, 1H, Lan
CHa); 4.09–4.17 (m, 2H, Val CHaþFmoc CH-9);
4.1.12. (2S,6R)-N 6-(9H-Fluorenylmethoxycarbonyl)-
lanthionine O 1-allyl ester (9). (2S,6R)-N 2-Triphenyl-
methyl-N 6-(9H-fluorenylmethoxycarbonyl)-lanthionine
O 1-allyl-O 7-tert-butyl esters (2.00 g, 2.71 mmol) was
stirred with TFA (4.9 mL), DCM (4.9 mL) and TIS
(0.2 mL) for 1 h. Solvent was removed in vacuo and the
residue purified by column chromatography (SiO2,
MeOH/CHCl3, 1:9 v:v) to afford the title compound as a
white solid (1.16 g, 91%); LRMS (ESIþ) m/z: 471
(MþH)þ; 493 (MþNa)þ; HRMS (FAB) calcd for
C24H27O6N2S 471.1590. Found 471.1571; dH (400 MHz;
CDCl3): 2.71–3.40 (br m, 4H, CH2SCH2); 4.01–4.47 (br m,
0
5H, Fmoc CH-9þFmoc OCH2þCHaþCHa ); 4.50–4.71 (br
m, 2H, allyl CO2CH2CHvCH2); 5.00–5.47 (br m, 2H, allyl
CO2CH2CHvCHH0þallyl CO2CH2CHvCHH0); 5.61–
5.91 (br m, 1H, allyl CO2CH2CHvCH2); 6.13–6.35 (br s,
1H, NH); 7.10–7.84 (m, 8H, Fmoc ArH); dC (100 MHz;
CDCl3): 33.1 (CH2SCH2); 36.2 (CH2SCH2); 47.3
0
(Fmoc CH-9); 53.4 (CHa); 54.7 (CHa ); 67.1þ67.9
(allyl CO2CH2CHvCH2þFmoc CH2O); 120.3 (allyl
CO2CH2CHvCH2); 120.6 (allyl CO2CH2CHvCH2);
125.5, 127.5, 128.2, 130.6 (Fmoc ArCH); 141.6,
143.9 (Fmoc ArC); 157.1 (Fmoc OCONH); 167.9 (allyl
CO2CH2CHvCH2); 174.2 (CO2H); IR (neat) n (cm21): 1673
(br; s); 1512 (br; m); 1181 (br; s); 1133 (br; s); 721 (br; m);
739 (br; m).
0
4.21–4.38 (m, 3H, Lan CHa þFmoc CH2O); 4.59 (d, J¼
5 Hz, 2H, allyl CO2CH2CHvCH2); 5.10–5.32 (m, 2H, allyl
CO2CH2CHvCH2); 5.78–5.92 (m, 1H, allyl CO2CH2-
CHvCH2); 7.14–7.73 (m, 8H, Fmoc ArH); dC (100 MHz;
CD3OD): 18.7þ20.2 (2£Val CH3); 32.3 (Val CHb); 36.0þ
37.5 (CH2SCH2); 48.7 (Fmoc CH-9); 55.2þ56.5þ60.1
(2£Lan CHaþVal CHa); 67.6 (allyl CO2CH2CHvCH2);
68.6 (Fmoc CH2O); 119.6 (allyl CO2CH2CHvCH2); 121.4
allyl CO2CH2CHvCH2); 126.6, 128.6, 129.2, 133.5 (Fmoc
Ar CH); 142.9, 145.5 (Fmoc Ar C); 158.9 (Fmoc OCONH);
163.9, 173.3, 176.3 (2£CONHþCO2R); IR (neat) n (cm21):
3352 (br, w); 1732 (br s); 1727 (s); 1687 (s); 1521 (s); 1432
(m); 1324 (m); 1326 (w); 1254 (m); 1161 (s); 1105 (w); 914
(m); 704 (m); 697 (m).
4.1.13. (2S,6R)-N 2-tert-Butoxycarbonyl-N 6-(9H-fluorenyl-
methoxycarbonyl)-lanthionine O 1-allyl ester (10). The
title compound was prepared using a modification of
the procedure of Yamamoto et al.35 To a solution of
(2S,6R)-N 6-(9H-fluorenylmethoxycarbonyl)-lanthionine
O 1-allyl ester (1.35 g, 2.87 mmol, 1 equiv.) in dioxane at
08C was added di-tert-butyl dicarbonate (626 mg, 1 equiv.)
and DIPEA (0.50 mL, 372 mg, 1 equiv.). The solution was
stirred for 3 h and the solvent removed in vacuo. The residue
was purified by column chromatography (SiO2, methanol/
chloroform, 1:19 v:v) to yield the title compound as a white
solid (1.12 g, 68%); LRMS (ESIþ) m/z: 571 (MþH)þ; 593
(MþNa)þ; 609 (MþNa)þ; 1163 (2MþNa)þ; HRMS (FAB)
calcd for C29H35O8N2S 571.2114. Found 571.2090; dH
(300 MHz; CDCl3): 1.38 (br s, 9H, C(CH3)3); 2.61–3.20
(br m, 4H, CH2SCH2); 3.81–4.63 (br m, 7H, Fmoc
Acknowledgements
We thank the EPSRC for a CASE studentship.
0
CH-9þFmoc OCH2þCHaþCHa þallyl CO2CH2CHvCH2);
References
4.89–5.31 (br m, 2H, allyl CO2CH2CHvCHH0þallyl
CO2CH2CHvCHH0); 5.40 – 5.91 (br m, 1H, allyl
CO2CH2CHvCH2); 7.00–7.71 (m, 8H, Fmoc ArH); dC
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