Derivatized Amino Acids Relevant to Native Peptide Synthesis by Chemical Ligation and Acyl Transfer

Article abstract of DOI:10.1021/jo030192r

Three amino acids were converted into the derivatives 5.2 (from glycine), 6.4a and 6.4b (from alanine), and 8.3a and 8.3b (from O-benzyl serine). These N-alkylated amino acids, which can be deprotected after conversion of the carboxyl into an amide, correspond to the general structure 2.1, a compound class of use in the study of peptide segment coupling by the ligation-acyl transfer method.

Full text of DOI:10.1021/jo030192r

Der iva tized Am in o Acid s Releva n t to Na tive P ep tid e Syn th esis by
Ch em ica l Liga tion a n d Acyl Tr a n sfer
Derrick L. J . Clive,* Soleiman Hisaindee, and Don M. Coltart
Chemistry Department, University of Alberta, Edmonton, Alberta, Canada T6G 2G2
derrick.clive@ualberta.ca
Received J une 13, 2003
Three amino acids were converted into the derivatives 5.2 (from glycine), 6.4a and 6.4b (from
alanine), and 8.3a and 8.3b (from O-benzyl serine). These N-alkylated amino acids, which can be
deprotected after conversion of the carboxyl into an amide, correspond to the general structure
2.1, a compound class of use in the study of peptide segment coupling by the ligation-acyl transfer
method.
SCHEME 1
In tr od u ction
The synthesis of large peptides and proteins by native
chemical ligation1 has attracted considerable interest,
and many impressive applications have been reported.1,2
Although the method constitutes a dramatic advance in
the area of peptide and protein synthesis, the require-
ment for a cysteine residue at the ligation site can be an
awkward restriction because cysteine is not a common
amino acid, making up only 1.7%3,4 of the residues in
proteins. Accordingly, much effort has been devoted to
modifying the original native chemical ligation method
so that it can afford peptides with other amino acids at
the ligation site.5,6 The most ambitious development has
been pioneered by Kent et al.6 and is based on the design
(1) (a) Dawson, P. E.; Muir, T. W.; Clark-Lewis, I.; Kent, S. B. H.
Science 1994, 266, 776-779. (b) Dawson, P. E.; Kent, S. B. H. Annu.
Rev. Biochem. 2000, 69, 923-960. (c) Coltart, D. M. Tetrahedron 2000,
56, 3449-3491.
(2) Wilken, J .; Kent, S. B. H. Curr. Opin. Biotechnol. 1998, 9, 412-
426.
(3) (a) McCaldon, P.; Argos, P. Proteins 1988, 4, 99-122. (b) Low,
D. W.; Hill, M. G.; Carrasco, M. R.; Kent, S. B. H.; Botti, P. Proc. Natl.
Acad. Sci. U. S. A. 2001, 98, 6554-6559.
(4) Voet, D.; Voet, J . G. Biochemistry, 2nd ed.; Wiley: New York,
1995; p 58.
(5) For leading references, see: (a) Liu, C.-F.; Rao, C.; Tam, J . P.
Tetrahedron Lett. 1996, 37, 933-936. (b) Huang, H.; Carey, R. I. J .
Peptide Res. 1998, 51, 290-296. (c) Gieselman, M. D.; Xie, L.; van der
Donk, W. A. Org. Lett. 2001, 3, 1331-1334. (d) Quaderer, R.; Sewing,
A.; Hilvert, D. Helv. Chim. Acta 2001, 84, 1197-1206. (e) Hondal, R.
J .; Nilsson, B. L.; Raines, R. T. J . Am. Chem. Soc. 2001, 123, 5140-
5141. (f) Yan, L. Z.; Dawson, P. E. J . Am. Chem. Soc. 2001, 123, 526-
533. (g) Tam, J . P.; Yu, Q. Biopolymers 1998, 46, 319-327. (h) Miao,
Z.; Tam, J . P. Org. Lett. 2000, 2, 3711-3713. (i) Zhang, L.; Tam, J . P.
Tetrahedron Lett. 1997, 38, 3-6. (j) Muir, T. W.; Dawson, P. E.; Kent,
S. B. H. Methods Enzymol. 1997, 289, 266-298. (k) Nilsson, B. L.;
Kiessling, L. L.; Raines, R. T. Org. Lett. 2001, 3, 9-12. (l) Nilsson, B.
L.; Kiesling, L. L.; Raines, R. T. Org. Lett. 2000, 2, 1939-1941. (m)
Soellner, M. B.; Nilsson, B. L.; Raines, R. T. J . Org. Chem. 2002, 67,
4993-4996. (n) Nilsson, B. L.; Hondal, R. J .; Soellner, M. B.; Raines,
R. T. J . Am. Chem. Soc. 2003, 125, 5268-5269. (o) Canne, L. E.; Bark,
S. J .; Kent, S. B. H. J . Am. Chem. Soc. 1996, 118, 5891-5896. (p) Shao,
Y.; Lu, W.; Kent, S. B. H. Tetrahedron Lett. 1998, 39, 3911-3914. (q)
Offer, J .; Boddy, C. N. C.; Dawson, P. E. J Am. Chem. Soc. 2002, 124,
4642-4646. (r) Offer, J .; Dawson, P. E. Org. Lett. 2000, 2, 23-26. (s)
Hackeng, T. M.; Griffin, J . H.; Dawson, P. E. Proc. Natl. Acad. Sci. U.
S. A. 1999, 96, 10068-10073. (t) Kawakami, T.; Akaji, K.; Aimoto, S.
Org. Lett. 2001, 3, 1403-1405. (u) Marinzi, C.; Bark, S. J .; Offer, J .;
Dawson, P. E. Bioorg. Med. Chem. 2001, 9, 2323-2328.
of an auxiliary that is initially attached to nitrogen.5q,r,t,6
The auxiliary must allow ligation and acyl transfer and
should then be removable. In principle, the auxiliary
could be attached to the eventual nitrogen terminus of
one complete peptide segment; alternatively, it could be
attached to each of the common amino acids, and then
the appropriate derivatized amino acid, already carrying
the auxiliary, would be used in the last step of assembling
the peptide segment. The first of these methods has been
investigated in much greater detail than the second.
Kent’s method to allow ligation by means of an
auxiliary at a noncysteinyl residue is based on the amines
1.1 (X ) H or OMe) (Scheme 1).6 The auxiliaries have
(6) Botti, P.; Carrasco, M. R.; Kent, S. B. H. Tetrahedron Lett. 2001,
42, 1831-1833.
10.1021/jo030192r CCC: $25.00 © 2003 American Chemical Society
Published on Web 10/28/2003
J . Org. Chem. 2003, 68, 9247-9254
9247

Clive et al.
SCHEME 2
SCHEME 3
the essential property of resisting cleavage under the
acidic conditions used in peptide synthesis as long as they
are linked to an amine nitrogen; however, after ligation
and acyl transfer, the attachment is to an amide nitrogen
(see 1.7) and each auxiliary is now easily cleaved by acid.
The auxiliaries are attached to a resin-bound peptide via
bromide displacement (Scheme 1, 1.1 + 1.2 f 1.3).6 After
deprotection and resin cleavage (1.3 f 1.4), reaction with
a thioester segment (1.5) results in ligation and acyl
transfer (1.4 f 1.6 f 1.7). Finally, the auxiliary is
removed under acidic conditions, giving the native pep-
tide 1.8.3b,6
synthesis of 2.1 and can be removed easily in order to
prime the auxiliary for ligation.
We examined a number of potential methods for
nitrogen protection and, after considerable exploratory
work, found that the nitrogen was best protected as its
Troc carbamate (2.1, Pg′ ) C(O)OCH2CCl3).
Discu ssion
Our aim was to prepare specially derivatized amino
acids that could be incorporated as the N-terminus of a
peptide, the nature of the derivatization being such that
sequential reaction with a thioester segment, acyl trans-
fer, and detachment of the auxiliary could be achieved.
The compounds we have made for this purpose cor-
respond to structure 2.1 (Pg, Pg′ ) protecting groups),7
in which the role of the methoxy group is to ensure acid
lability of the auxiliary after acyl transfer. If an amino
acid corresponding to 2.1 is incorporated as the N-
terminus of a peptide segment, then removal of the
nitrogen and sulfur protecting groups and reaction with
another peptide segment having a C-terminal thioester
should result in thioester exchange, followed by S f N
acyl transfer. Removal of the auxiliary would then give
a native peptide. The availability of a range of derivatized
amino acids 2.1, preferably with values of R correspond-
ing to all the common amino acids, would allow a detailed
study to be made of the permissible values of R for each
of the steps ligation, acyl transfer, and deprotection to
work efficiently.Of a number of potential routes to 2.1,
our preference7 was to make the compounds from pro-
tected amino acids and so we sought methods in which
the C-N bond linking the amino acid unit to the
auxiliary is the key step. We generally protected the
sulfur as a tert-butyl thioether (2.1, Pg ) Bu-t) but have
also used a 4-methylbenzyl thioether (2.1, Pg ) CH2C6H4-
Me-4); these protecting groups behaved well in the
Our initial studies7 were based on the use of ketone
2.2 for reductive amination with H2NCH2CO2Me. These
experiments were unsuccessful,8 and after a great deal
of further effort, we were led to consider the possibility
of using quinone methides9 as precursors to the auxiliary
segment.8 To this end, phenolic alcohol 3.3 was prepared
from the known10 bromide 3.1, as shown in Scheme 3,
and treated with 2 equiv of Me3SiBr and 3 equiv of
freshly distilled (()-alanine ethyl ester. The expected
product, 3.6, was obtained as a 1:1 mixture of two
diastereoisomers in 58% yield. Optimization of the condi-
tions showed that use of 1 equiv of Me3SiBr and 2 equiv
of (()-alanine ethyl ester was just as effective. Although
formation of a quinone methide (3.5) does account for the
observed products, we have no evidence to exclude the
possibility of direct nucleophilic displacement from bro-
mide 3.4 (X ) H or SiMe3). At this point, our plan to make
compounds of type 2.1 would require that the phenolic
hydroxyl of 3.6 be methylated selectively, but instead,
we examined a corresponding series of reactions in which
a methoxy substituent was actually in place from the
beginning.
In these experiments, bromide 4.111 was converted into
the sulfide 2.2 (Scheme 4) and reduction then gave
alcohol 4.2. When this compound was treated with Me3-
SiBr and glycine ethyl ester, using conditions optimized
for the phenolic series of Scheme 3, we obtained a good
yield of the desired N-alkylated product 4.3. We had
appreciated the possibility that a reactive entity gener-
ated at C(R) (see 4.2) might be captured by neighboring
(7) Our preliminary studies (see: Coltart, D. M., Ph.D. Thesis,
University of Alberta, 2000) were based on thiophenolic and 1-phenyl-
2-mercaptoethyl systems exemplified by i and ii. Work on the former
was stopped when similar studies by Dawson and Offer (ref 5r) were
published. A noteworthy feature of ii (and 2.1) is that the mercapto
group ultimately released is an alkyl thiol and, as such, is more
nucleophilic than an aryl thiol (cf. i).
(8) The possibility of preparing compounds of type 2.1 by reductive
amination or by halide displacement (cf. the present method) is shown
in a generic scheme in two patents: (a) Botti, P.; Bradburne, J . A.;
Kent, S. B. H.; Low, D. W. PCT Int. Appl. WO 2002/20557. (b) Kent,
S. B. H.; Botti, P.; Low, D. W.; Bradburne, J . A.; Hunter, C. L., Chen,
S.-Y.; Cressman, S.; Kochendoerfer, G. PCT Int. Appl. WO 2002/20034.
(9) Shevchenko, S. M.; Apushkinskii, A. G.; Gindin, V. A.; Zarubin,
M. Ya. Russ. J . Org. Chem. 1990, 26, 921-925.
(10) Park, C.-H.; Givens, R. S. J . Am. Chem. Soc. 1997, 119, 2453-
2463.
(11) Nieuwenhuis, S. A. M.; Vertegaal, L. B. J .; de Zoete, M. C.; van
der Gen, A. Tetrahedron 1994, 50, 13207-13230.
9248 J . Org. Chem., Vol. 68, No. 24, 2003

Derivatized Amino Acids
SCHEME 4
5.3, and the Troc group was removed (5.3 f 5.4) by the
action of cadmium in DMF-AcOH, according to a known
procedure.14 The sulfur protecting group was removed by
treatment with Hg(OAc)215 in the presence of anisole (as
a cation scavenger) and CF3CO2H. The required thiol was
then released by treatment with H2S, and oxidation,
which occurred during exposure to air, afforded disulfide
5.5. An attempt to effect oxidation with I2-MeOH was
unsuccessful and did not afford the expected disulfide.
It should be noted that a disulfide is suitable for ligation
chemistry when that is done in the presence of a reducing
agent.16
The yields in the sequence of Scheme 5 were accept-
able, and the route represents our optimized version of
making a specially derivatized glycine corresponding to
the general structure 2.1. Coupling of 5.2 with H2NCH2-
CO2Et was, of course, done to demonstrate in a simple
way that deprotection of both nitrogen and sulfur could
be accomplished in a situation similar to the one that
would be present when the derivatized amino acid 5.2 is
the N-terminus of a peptide.
SCHEME 5
Der iva tized Ala n in e a n d Ser in e. At this point we
examined two other amino acids, L-alanine and L-serine,
and for both of these it was convenient to protect the
carboxyl group as its tert-butyl ester; the ethyl ester is
suitable only for glycine, where the absence of an asym-
metric center allows base hydrolysis to be used. We hoped
that nitrogen protection as a Troc carbamate would again
prove suitable, and in the event, this was the case.
Alcohol 4.2 was coupled with L-alanine tert-butyl ester
(6.1)17 under our optimized conditions to afford a sepa-
rable mixture of the less polar (6.2a , 31%) and more polar
(6.2b, 30%) adducts, respectively. The former was con-
verted into the protected acid 6.4a , and again we carried
out a model sequence: coupling with H2NCH2CONHBn25
(6.4a f 6.5a ), removal of the Troc group (Cd, DMF,
AcOH, 74%; 6.5a f 6.6a ), and deprotection of the sulfur
(Hg(OAc)2, CF3CO2H, anisole, H2S) to obtain 6.7a , largely
group participation of the sulfur, followed by loss of the
tert-butyl group; evidently, this is not a significant side
reaction, if it occurs at all.
(14) Di Giorgio, C.; Pairot, S.; Schwergold, C.; Patino, N.; Condom,
R.; Farese-Di Giorgio, A.; Guedj, R. Tetrahedron 1999, 55, 1937-1958.
(15) Nishimura, O.; Kitada, C.; Fujino, M. Chem. Pharm. Bull. 1978,
26, 1576-1585.
(16) (a) Burns, J . A.; Butler, J . C.; Moran, J .; Whitesides, G. M. J .
Org. Chem. 1991, 56, 2648-2650. (b) Tam, J . P.; Lu, Y.-A.; Liu, C.-F.;
Shao, J . Proc. Natl. Acad. Sci. U. S. A. 1995, 92, 12485-12489.
(17) L-Alanine was protected as its Cbz carbamate (CbzCl, aqueous
NaOH, 93%) (ref 18) and the tert-butyl ester was prepared using tert-
butyl trichloroacetimidate [Cl3C(dNH)OBu-t, BF3‚Et2O, 87%] (refs 19,
20). Hydrogenolysis then gave L-alanine tert-butyl ester (6.1) (ref 24).
The Mosher amide was prepared and found to give a single peak in
the 19F NMR spectrum; the Mosher amides of racemic material give
separated signals (19F δ -69.6 and -69.4 ppm).
(18) (a) Bergmann, M.; Zervas, L. Ber. Dtsch. Chem. Ges. 1932, 65,
1192-1205. (b) Cf. Bodanszky, M.; Bodanszky, A. The Practice of
Peptide Synthesis, 2nd ed.; Springer: Berlin, 1944; pp 11-13.
(19) Armstrong, A.; Brackenridge, I.; J ackson, R. F. W.; Kirk, J . M.
Tetrahedron Lett. 1988, 29, 2483-2486.
(20) (a) Other reagents, such as DCC-DMAP-t-BuOH (ref 21); Boc2O-
DMAP-t-BuOH (ref 22) gave partially racemized material. (b) Trans-
esterification with tert-butyl acetate (ref 23) was very slow and
proceeded in poor yield. (c) For other preparations of tert-butyl esters,
see: Anderson, G. W.; Callahan, F. M. J . Am. Chem. Soc. 1960, 82,
3359-3363.
When alcohol 4.2 was treated with Me3SiBr, the
expected bromide was formed, but could not be purified,
and so the procedure summarized in Scheme 4, in which
the bromide is generated in situ, was followed.12
With a method for making 4.3 now available, the next
tasks were to deprotect the carboxyl and protect the
nitrogen to arrive at a structure that corresponds to 2.1.
In the present case (4.3), hydrolysis of the ethyl ester
was easily effected (Scheme 5) with aqueous NaOH or
LiOH and we used the resulting amino acid (5.1) to
evaluate a number of protecting groups (Fmoc, Boc, Troc)
for nitrogen, but only Troc proved satisfactory as it alone
could be removed in good yield.
Der iva t ized Glycin e. The nitrogen of 5.1 could be
protected as its Troc carbamate under standard condi-
tions (Scheme 5).13 Coupling with glycine ethyl ester gave
(21) Wiener, H.; Gilon, C. J . Mol. Catal. 1986, 37, 45-52.
(22) Takeda, K.; Akiyama, A.; Nakamura, H.; Takizawa, S.; Mizuno,
Y.; Takayanagi, H.; Harigaya, Y. Synthesis 1994, 1063-1066.
(23) Taschner, E.; Chimiak, A.; Bator, B.; Sokolowska, T. Liebigs
Ann. Chem. 1961, 646, 134-136.
(24) (a) Strazzolini, P.; Melloni, T.; Giumanini, A. G. Tetrahedron
2001, 57, 9033-9044. (b) L-Alanine tert-butyl ester is commercially
available.
(12) We also investigated briefly the conversion of 4.2 into the
corresponding chloride (Cl instead of OH in 4.2). This was achieved
by treatment with SOCl2 (see Supporting Information). The chloride
could not be purified by chromatography and was used crude; reaction
with glycine ethyl ester was slow (CH2Cl2, rt, 6 h) and less efficient
(64% of 4.3) than the route of Scheme 4.
(13) Carson, J . F. Synthesis 1981, 268-270.
J . Org. Chem, Vol. 68, No. 24, 2003 9249

Clive et al.
SCHEME 6 a ,b
a
Yields beside the arrows refer to the “a ” series, those in parentheses to the “b” series. bThe product 6.7b was isolated as the disulfide.
SCHEME 7
in the form of the thiol. The intermediate 6.6a had an ee
of 99.5%.The same reactions were repeated with the more
polar isomer 6.2b to give a final product (6.7b) that
differs stereochemically from 6.7a at the starred atom.
The intermediate 6.6b had an ee of 99.5%.
For comparison purposes, racemic amides correspond-
ing to 6.6a and 6.6b were prepared from racemic alanine.
Again a more polar and a less polar isomer were obtained,
differing in stereochemistry at the benzylic carbon of the
auxiliary.28
Finally, we decided to study an example of an amino
acid with a functionalized side chain and chose L-serine
as a suitable representative of this class. The required
starting material was O-benzyl L-serine tert-butyl ester
(7.4). Although this is a known compound,24a several of
the literature methods we examined for making it caused
extensive epimerization in the step in which the carboxyl
group was converted20 into its tert-butyl ester. We
eventually found that the route shown in Scheme 7 is
satisfactory.
N-Boc O-benzyl L-serine (Aldrich) was converted into
its tert-butyl ester 7.229 by the action of tert-butyl
trichloroacetimidate.19 Removal of the N-Boc group with
HCl in EtOAc30,31 took place without disturbing the ester,
and the free amine (7.4) was obtained from the hydro-
chloride salt by treatment with NaOH. However, the acid
treatment for the N-deprotection should not be extended
beyond 14 h; hydrochloride obtained at that time (34%
yield) has an ee of g94%, as judged by 19F NMR
measurements on the derived Mosher amides and was
used in further work. An additional crop, obtained after
36 h (23%), had an ee of 57%.
The serine derivative 7.4 was subjected to the series
of reactions shown in Scheme 8; the reactions are
identical to those used for L-alanine, except for the fact
that the tert-butyl ester unit was deprotected with Me3-
SiOSO2CF3, since CF3CO2H caused cleavage of the ben-
zylic C-N bond. The final products were obtained as
mixtures of the disulfides shown (8.6a and 8.6b) and the
corresponding thiols. We did not examine 8.5a or 8.5b
by chiral HPLC; no epimerization was found in the
corresponding alanine series, as mentioned above.
(25) The compound was prepared as follows: CF3CO2H (5 mL) was
added slowly to a stirred and cooled (0 °C) solution of BocHNCH2-
CONHBn (ref 26) (2.65 g, 10.0 mmol) in dry CH2Cl2 (5 mL). Stirring
was continued at 0 °C for 3.5 h, and the solvent was evaporated. The
residue was mixed with Et2O (50 mL), and the resulting precipitate
was filtered, washed thoroughly with Et2O, and left under oil pump
vacuum to give CF3CO2H. H2NCH2CONHBn (ref 27) (2.79 g, 100%)
as a white solid: mp 154-157 °C; FTIR (microscope) 1672 cm-1 1H
;
Der iva tized Glycin e w ith p-Meth ylben zyl P r otec-
tion of Su lfu r . To show additional generality of our
method, we decided to make compound 10.5 (see Scheme
10), which has the same sulfur protecting group that was
used by the Kent6 and Dawson5u groups in their bromide
displacement method.6 p-Methylbenzyl chloride (9.1) was
converted into thioacetate 9.2, and treatment with BuLi
gave the thiolate 9.3. This was alkylated with bromide
4.1,6,11 and finally, reduction gave alcohol 9.5. The alcohol
was treated with H2NCH2CO2Et and Me3SiBr, according
to our general procedure, and the product 10.1 (Scheme
10) was hydrolyzed and protected on nitrogen with
TrocCl. Coupling in the usual way with H2NCH2-
CONHBn and removal of the Troc group gave 10.5. The
NMR (CD3OD, 300 MHz) δ 3.70 (s, 2 H), 4.42 (s, 2 H), 7.24-7.32 (m,
5 H) (three protons not observed); 13C NMR (CD3OD, 100 MHz) δ 41.5
(t′), 44.3 (t′), 128.5 (d′), 128.7 (d′), 129.6 (d′), 139.4 (s′), 167.1 (s′).
(26) Dinsmore, C. J .; Bergman, J . M. J . Org. Chem. 1998, 63, 4131-
4134.
(27) Fournie´-Zaluski, M.-C.; Coulaud, A.; Bouboutou, R.; Chaillet,
P.; Devin, J .; Waksman, G.; Costentin, J .; Roques, B. P. J . Med. Chem.
1985, 28, 1158-1169.
(28) In this sequence, the nitrogen was not protected as a carbamate
before coupling with H2NCH2CONHBn, i.e., compounds corresponding
to 6.3a and 6.3b were coupled directly with H2NCH2CONHBn (EDCI,
i-Pr2NEt).
(29) Winterfeld, G. A.; Ito, Y.; Ogawa, T.; Schmidt, R. R. Eur. J .
Org. Chem. 1999, 1167-1171.
(30) Footnote 10 in: Cavelier, F.; Enjalbal, C. Tetrahedron Lett.
1996, 37, 5131-5134.
(31) Gibson, F. S.; Bergmeier, S. C.; Rapoport, H. J . Org. Chem.
1994, 59, 3216-3218.
9250 J . Org. Chem., Vol. 68, No. 24, 2003

Derivatized Amino Acids
SCHEME 8 a
a
Yields beside the arrows refer to the “a ” series, those in parentheses to the “b” series.
SCHEME 9
having a functionalized side chain. The availability of
such derivatized amino acids should be helpful in studies
on conformational or steric factors in the derivatizing unit
that control the rate of the ligation and acyl transfer steps
for different amino acids.
Exp er im en ta l Section
2-ter t-Bu tylsu lfan yl-1-(4-m eth oxyph en yl)eth an on e (2.2).
Br2 (300 µL, 5.77 mmol) was added dropwise over 10 min to a
stirred and warmed (40 °C) solution of 4-methoxyacetophenone
(867 mg, 5.77 mmol) in bench CHCl3 (10 mL). At the end of
the addition, the mixture was diluted with Et2O (100 mL),
washed with saturated aqueous NaHCO3, dried (MgSO4), and
evaporated to give bromide 4.1 (1.24 g, 93%) as a white solid,
which was used for the next step without purification.
A solution of the above bromide (8.82 g, 38.5 mmol) in dry
THF (40 mL) was added dropwise over 10 min to a stirred
and cooled (0 °C) solution of t-BuSLi made by slow addition of
BuLi (2.5 M in hexanes, 17.7 mL, 44.3 mmol) to a stirred and
cooled (0 °C) solution of t-BuSH (5.21 mL, 46.2 mmol) in dry
THF (100 mL), the solution being stirred for 2.5 h before use.
When addition was complete, the cold bath was removed and
stirring was continued overnight. The mixture was diluted
with Et2O (200 mL), washed thoroughly with water (3 × 100
mL), dried (MgSO4), and evaporated. Flash chromatography
of the residue over silica gel, using 1:6 EtOAc-hexanes, gave
2.2 (9.05 g, 98%) as a colorless oil: FTIR (CDCl3, cast) 1671
cm-1; 1H NMR (CDCl3, 500 MHz) δ 1.34 (s, 9 H), 3.81 (s, 2 H),
3.85 (s, 3 H), 6.88-6.90 (m, 2 H), 7.89-7.91 (m, 2 H); 13C NMR
(CDCl3, 125 MHz) δ 30.7 (q′), 35.5 (t′), 43.6 (s′), 55.4 (q′), 113.8
(d′), 128.7 (s′), 131.1 (d′), 163.6 (s′), 194.9 (s′); exact mass m/z
calcd for C13H18O2S 238.1027, found 238.1027.
SCHEME 10
[2-ter t-Bu tylsu lfa n yl-1-(4-m eth oxyp h en yl)eth yla m in o]-
a cetic Acid Eth yl Ester (4.3). Me3SiBr (162 µL, 1.23 mmol)
was added dropwise to a stirred and cooled (0 °C) solution of
alcohol 4.2 (295 mg, 1.23 mmol) in dry CH2Cl2 (4.0 mL). After
30 min, freshly distilled (distilled under water pump vacuum)
H2NCH2CO2Et (253 mg, 2.46 mmol) was added in one portion.
The cold bath was removed, and stirring was continued for 1
h. Evaporation of the solvent and flash chromatography of the
residue over silica gel (2 × 15 cm), using 1:6 EtOAc-hexanes,
gave 4.3 (323 mg, 80%) as a colorless oil: FTIR (CDCl3 cast)
protecting group in compounds of this type has been
removed with HF,6 and so our procedure clearly tolerates
at least minor changes in the sulfur protecting group.
1
1739 cm-1; H NMR (CDCl3, 400 MHz) δ 1.22 (t, J ) 7.1 Hz,
Con clu sion
3 H), 1.31 (s, 9 H), 2.60 (br s, 1 H), 2.73 (dd, J AB ) 12.3 Hz,
J AX ) 9.1 Hz, 1 H), 2.81 (dd, J AB ) 12.3 Hz, J BX ) 4.7 Hz, 1
H), 3.21 (ABq, ∆νAB ) 48.5 Hz, J AB ) 17 Hz, 2 H), 3.73 (dd,
J AX ) 9.1 Hz, J BX ) 4.6 Hz, 1 H), 3.78 (s, 3 H), 4.14 (q, J ) 7.1
Hz, 2 H), 6.86 (d, J ) 8.7 Hz, 2 H), 7.24 (d, J ) 8.6 Hz, 2 H);
13C NMR (CDCl3, 125 MHz) δ 14.3 (q′), 31.1 (q′), 37.0 (t′), 42.5
Our method for making derivatized amino acids of type
2.1 (Pg ) Bu-t or p-MeC6H4CH2) provides an alternative
to the bromide displacement route developed by Kent et
al.6 and has been shown to work with an amino acid
J . Org. Chem, Vol. 68, No. 24, 2003 9251

Clive et al.
yellow oil: FTIR (CDCl3 cast) 3307 (br), 1748, 1672, 1610 cm-1
(s′), 48.8 (t′), 55.3 (q′), 60.7 (t′), 61.6 (d′), 113.9 (d′), 128.2 (d′),
;
134.2 (s′), 159.0 (s′), 172.1 (s′); exact mass m/z calcd for C17H27
NNaO3S (M + Na) 348.1609, found 348.1610.
-
1H NMR (CDCl3, 400 MHz) δ 1.27 (t, J ) 7.2, Hz, 3 H); 1.30
(s, 9 H), 1.60 (br s, 1 H), 2.73 (dd, J AB ) 12.8 Hz, J AX ) 9.5 Hz,
1 H), 2.79 (dd, J AB )12.8 Hz, J BX ) 4.4 Hz, 1 H), 3.17 (ABq,
∆νAB ) 38.2 Hz, J AB ) 17.1 Hz, 2 H), 3.68 (dd, J ) 9.5, 4.4 Hz,
1 H), 3.77 (s, 3 H), 3.98 (dd, J ) 5.6, 3.1 Hz, 2 H), 4.19 (q, J )
7.2 Hz, 2 H), 6.81-6.83 (m, 2 H), 7.17-7.19 (m, 2 H), 7.70-
7.72 (m, 1 H); 13C NMR (CDCl3, 125 MHz) δ 14.3 (q′), 31.1
(q′), 36.5 (t′), 40.9 (t′), 42.7 (s′), 49.9 (t′), 55.3 (q′), 61.4 (t′), 62.4
(d′), 114.1 (d′), 128.0 (d′), 133.6 (s′), 159.2 (s′), 169.6 (s′), 171.6
(s′); exact mass m/z calcd for C19H31N2O4S (M + H) 383.2004,
found 383.2002.
[2-[2-[2-[[(Eth oxyca r bon ylm eth ylca r ba m oyl)m eth yl]-
a m in o]-2-(4-m eth oxyp h en yl)eth yld isu lfa n yl]-1-(4-m eth -
oxyp h en yl)eth yla m in o]a cetyla m in o]a cetic Acid Eth yl
Ester (5.5). In this experiment the initial thiol product was
not protected from air.
[[2-ter t-Bu tylsu lfa n yl-1-(4-m eth oxyp h en yl)eth yl](2,2,2-
tr ich lor oeth oxyca r bon yl)a m in o]a cetic Acid (5.2). Neat
Cl3CCH2OCOCl (400 µL, 2.90 mmol) and 1 N NaOH (380 µL)
were added alternately in 10 equal portions by syringe over
1.5 h to a stirred and cooled (0 °C) suspension of 5.1 (430 mg,
1.29 mmol) in 1 N NaOH (1.70 mL). When addition was
complete, the cold bath was removed and stirring was contin-
ued for 11 h, by which time all of 5.1 had reacted. The mixture
was cooled (0 °C), acidified to pH 2 with concentrated hydro-
chloric acid, and extracted with EtOAc (3 × 20 mL). The
combined organic extracts were dried (MgSO4) and evaporated.
Flash chromatography of the residue over silica gel (2 × 20
cm), using 2:100 and then 4:96 MeOH-CH2Cl2, gave 5.2 (510
mg, 83%) as a thick oil: FTIR (CDCl3 cast) 1717, 1612 cm-1
;
1H NMR (CDCl3, 400 MHz) (mixture of rotamers) δ 1.31 (s, 4
H), 1.33 (s, 5 H), 3.01-3.15 (m, 2 H), 3.67-3.83 (m containing
a singlet at δ 3.77, 5 H in all), 4.73-4.81 (m, 1 H), 4.87 (ABq,
∆νAB ) 92.0 Hz, J AB ) 11.9 Hz, 2 H), 5.43-5.50 (m, 1 H), 6.84-
6.87 (m, 2 H), 7.21-7.24 (m, 2 H) (one proton not observed in
this spectrum); 13C NMR (CDCl3, 100 MHz) (mixture of
rotamers) δ 29.1 (s′), 29.6 (s′), 30.9 (q′), 43.0 (t′), 43.2 (t′), 44.9
(t′), 45.2 (t′), 55.3 (q′), 59.6 (d′), 60.0 (d′), 75.4 (t′), 75.6 (t′),
95.1 (s′), 95.2 (s′), 114.2 (d′), 114.3 (d′), 128.4 (s′), 128.5 (d′),
129.4 (d′), 129.6 (d′), 129.7 (d′), 129.8 (d′), 154.2 (s′), 154.4 (s′),
CF3CO2H (2 mL) was added to thioether 5.4 (118 mg, 0.311
mmol) contained in a flask immersed in an ice bath. The
mixture was stirred, and PhOMe (50 µL), followed by Hg(OAc)2
(99.3 mg, 0.311 mmol), was added. Stirring was continued for
25 min, and the solvent was evaporated. The residue was
dissolved in MeCN (15 mL), and H2S gas was bubbled through
the solution for 2 min. The resulting black suspension was
filtered through a tightly packed Celite column (2 × 4 cm),
and the solid was washed with several portions of MeCN.
Evaporation of the combined filtrate and washings and flash
chromatography of the residue over silica gel (2 × 18 cm), using
4:100 MeOH-CH2Cl2, gave disulfide 5.5 (92.4 mg, 92%) as a
159.6 (s′), 173.5 (s′), 174.1 (s′); exact mass m/z calcd. for C18H24
Cl3NNaO5S (M + Na) 494.0338, found 494.0338.
-
colorless oil: FTIR (CDCl3 cast) 1745, 1669 cm-1 1H NMR
;
[2-[[2-ter t-Bu t ylsu lfa n yl-1-(4-m et h oxyp h en yl)et h yl]-
(2,2,2-t r ich lor oe t h oxyca r b on yl)a m in o]a ce t yla m in o]-
a cetic Acid Eth yl Ester (5.3). Ethyl glycinate hydrochloride
(5.2 g, 38.6 mmol) was mixed with solid K2CO3 (10 g, 72.4
mmol) and a few drops of saturated aqueous NaCl, and the
mixture was ground with a pestle and mortar to form a thick
paste. This was extracted with Et2O, and the combined
extracts were dried (MgSO4) and evaporated. The resulting
crude H2NCH2CO2Et was distilled (60 °C, water pump) to
afford pure (1H NMR) H2NCH2CO2Et (2.3 g, 62%).
N-(3-Dimethylamino)propyl-N-ethylcarbodiimide (207 mg,
1.08 mmol) was added to a stirred and cooled (0 °C) solution
of acid 5.2 (510 mg, 1.08 mmol) and H2NCH2CO2Et (110 µL,
1.08 mmol) in dry CH2Cl2 (12 mL). After 15 min the cold bath
was removed and stirring was continued for 2 h. The mixture
was diluted with Et2O (100 mL), washed with water (2 × 25
mL), dried (MgSO4), and evaporated. Flash chromatography
of the residue over silica gel (2 × 20 cm), using 1:99, 2:98, and
then 4:96 MeOH-CH2Cl2, gave 5.3 (374 mg, 62%) as a gum:
FTIR (CDCl3 cast) 3323 (br), 1748, 1716, 1611 cm-1; 1H NMR
(CDCl3, 300 MHz) (mixture of rotamers) δ 1.24 (t, J ) 7.1 Hz,
3 H), 1.32 (br s, 9 H), 3.04-3.29 (br m, 2 H), 3.64-3.76 (br, 2
H), 3.76 (s, 3 H), 3.81-3.96 (m, 2 H), 4.17 (q, J ) 7.1 Hz, 2 H),
4.74-4.83 (br m, 1.6 H), 5.00-5.10 (br m, 0.4 H), 5.32-5.56
(br m, 1 H), 6.70-6.80 (br s, 1 H), 6.80-6.94 (m, 2 H), 7.21-
7.30 (m, 2 H); 13C NMR (CDCl3, 100 MHz) (mixture of
rotamers) δ 14.1 (q′), 29.8 (t′), 30.9 (q′), 41.2 (t′), 42.9 (t′), 47.7
(t′), 55.2 (q′), 59.8 (d′), 60.4 (d′), 61.4 (t′), 75.2 (s′), 75.5 (s′),
95.2 (s′), 114.2 (d′), 129.1 (d′), 129.6 (s′), 154.6 (s′), 159.6 (s′),
168.8 (s′), 169.2 (s′) (one carbon not observed in this spectrum);
exact mass m/z calcd for C22H31Cl3N2NaO6S (M + Na) 579.0866,
found 579.0868.
(CDCl3, 500 MHz) δ 1.26 (t, J ) 7.1 Hz, 6 H), 2.74 (dd, J AB
13.7 Hz, J AX ) 8.0 Hz, 2 H), 3.23 (ABq, ∆νAB ) 15.4 Hz, J AB
)
)
17.0 Hz, 4 H), 2.85 (dd, J AB ) 13.7 Hz, J BX ) 5.3 Hz, 2 H),
3.71 (dd, J AX ) 7.8 Hz, J BX ) 5.4 Hz, 2 H), 3.76 (s, 6 H), 3.96
(dd, J AB ) 18.3 Hz, J AX ) 5.4 Hz, 2 H), 4.02 (dd, J AB ) 18.3
Hz, J BX ) 5.6 Hz, 2 H), 4.19 (q, J ) 7.1 Hz, 4 H), 6.82-6.85
(m, 4 H), 7.14-7.17 (m, 4 H), 7.65 (br t, J ) 4.5 Hz, 2 H) (two
protons not observed in this spectrum); 13C NMR (CDCl3, 125
MHz) δ 14.2 (q′), 31.6 (t′), 41.0 (t′), 49.5 (t′), 55.3 (q′), 61.5 (t′),
64.3 (d′), 114.2 (d′), 128.3 (d′), 131.8 (s′), 159.4 (s′), 169.7 (s′),
170.9 (s′); exact mass m/z calcd for C30H43N4O8S (M + H)
651.2522, found 651.2524.
(2S)-2-[2-ter t-Bu t ylsu lfa n yl-1-(4-m et h oxyp h en yl)et h -
yla m in o]p r op ion ic Acid ter t-Bu tyl Ester (Less P ola r
Isom er ) (6.2a ) a n d (Mor e P ola r Isom er ) (6.2b). Me3SiBr
(295 µL, 2.23 mmol) was added dropwise to a stirred and cooled
(0 °C) solution of alcohol 4.2 (537 mg, 2.23 mmol) in dry CH2-
Cl2 (11 mL). After 20 min, freshly prepared amine 6.1 (see
preparation of 6.1 in Supporting Information) (653 mg, 4.47
mmol) was added in one portion. The cold bath was left in place
but was not recharged, stirring was continued for 2 days.
Evaporation of the solvent and flash chromatography of the
residue over silica gel (3 × 20 cm), using 1:7 EtOAc-hexanes,
gave a fast-eluting diastereoisomer 6.2a (262 mg, 31%) as a
pale-yellow oil and a slow-eluting diastereoisomer 6.2b (251
mg, 30%) as a colorless oil. Isomer 6.2a : [R]20 -89.8° (c 1.0,
D
CHCl3); FTIR (CHCl3 cast) 1728 cm-1; H NMR (CDCl3, 400
1
MHz) δ 1.17 (d, J ) 7.0 Hz, 3 H), 1.30 (s, 9 H), 1.45 (s, 9 H),
2.46-2.54 (br s, 1 H), 2.66 (dd, J AB ) 11.7 Hz, J AX ) 9.5 Hz,
1 H), 2.76 (dd, J AB ) 12.1 Hz, J BX ) 4.2 Hz, 1 H), 2.93 (q, J )
7.1 Hz, 1 H), 3.67 (dd, J AX ) 9.5 Hz, J BX ) 4.2 Hz, 1 H), 3.78
(s, 3 H), 6.83-6.86 (m, 2 H), 7.22-7.25 (m, 2 H); 13C NMR
(CDCl3, 125 MHz) δ 19.7 (q′), 28.2 (q′), 31.0 (q′), 37.4 (t′), 42.4
(s′), 54.7 (d′), 55.3 (q′), 60.2 (d′), 80.6 (s′), 113.9 (d′), 128.3 (d′),
[2-[2-ter t-Bu tylsu lfa n yl-1-(4-m eth oxyp h en yl)eth yla m i-
n o]a cetyla m in o]a cetic Acid Eth yl Ester (5.4). Cd powder
(1.20 g, 10.7 mmol) was added in one portion to a stirred
solution of 5.3 (179 mg, 0.322 mmol) in 1:1 DMF-AcOH (8
mL). Stirring was continued for 45 min at room temperature,
and the mixture was filtered through a Celite pad (2 × 4 cm),
using EtOAc (3 × 20 mL). The combined filtrates and washings
were washed with saturated aqueous NaHCO3 (2 × 15 mL)
and water (15 mL), dried (MgSO4), and evaporated. The
residue was kept under oil pump vacuum to remove residual
DMF, and amine 5.4 (118 mg, 97%) was obtained as a pale-
134.9 (s′), 158.9 (s′), 174.9 (s′); exact mass m/z calcd for C20H33
-
NNaO3S (M + Na) 390.2079, found 390.2074. Isomer 6.2b:
1
[R]20 + 6.3° (c 1.0, CHCl3); FTIR (CHCl3 cast) 1729 cm-1; H
D
NMR (CDCl3, 500 MHz) δ 1.22 (d, J ) 6.9 Hz, 3 H), 1.27 (s, 9
H), 1.37 (s, 9 H), 1.9-2.5 (very br, 1 H), 2.80 (br d, J ) 5.5 Hz,
2 H), 3.20 (q, J ) 6.8 Hz, 1 H), 3.72-3.76 (overlapping signals
containing a multiplet and a singlet at δ 3.74, 4 H in all), 6.82
(d, J ) 8.6 Hz, 2 H), 7.21 (d, J ) 8.6 Hz, 2 H); 13C NMR (CDCl3,
9252 J . Org. Chem., Vol. 68, No. 24, 2003

Derivatized Amino Acids
125 MHz) δ 18.4 (q′), 28.1 (q′), 31.1 (q′), 35.9 (t′), 42.4 (s′), 54.7
(d′), 55.3 (q′), 60.1 (d′), 80.9 (s′), 113.9 (d′) 128.4 (d′), 158.9 (s′)
(two carbons not observed in this spectrum s′); exact mass m/z
calcd for C20H33NNaO3S 390.2079 (M + Na), found 390.2080.
signals, 7 H in all) (one proton not observed in this spectrum);
13C NMR (CDCl3, 125 MHz) δ 15.1 (q′), 30.2 (s′), 31.0 (q′), 43.3
(t′), 54.8 (t′), 55.4 (t′), 61.9 (q′), 74.9 (t′), 95.0 (s′), 114.5 (d′),
127.3 (d′), 127.8 (d′), 128.6 (d′), 128.9 (d′), 129.4 (s′), 138.0 (s′),
153.0 (s′), 159.9 (s′), 168.5 (s′), 170.5 (s′); exact mass m/z calcd
for C28H36Cl3N3NaO5S (M + Na) 654.1339, found 654.1337.
(2S)-2-[2-ter t-Bu t ylsu lfa n yl-1-(4-m et h oxyp h en yl)et h -
yla m in o]p r op ion ic Acid Tr iflu or oa ceta te (Less P ola r
Isom er ) (6.3a ). CF3CO2H (2 mL) was added to a stirred and
cooled (0 °C) mixture of 6.2a (315 mg, 0.856 mmol) and PhOMe
(150 µL), and stirring was continued for 5 h at 0 °C. Evapora-
tion of the solvents and flash chromatography of the residue
over silica gel (2 × 15 cm), using 8:92 and then 1:1 MeOH-
CH2Cl2, gave 6.3a (264 mg, 72%) as a white solid: [R]20D -18.1°
(2S)-N-(Ben zylca r b a m oylm et h yl)-2-[2-ter t-b u t ylsu lf-
an yl-1-(4-m eth oxyph en yl)eth ylam in o]pr opion am ide (6.6a).
Cd powder (880 mg, 7.87 mmol) was added in one portion to a
stirred solution of 6.5a (173 mg, 0.275 mmol) in 1:1 DMF-
AcOH (6 mL). Stirring was continued for 3 h at room
temperature, and the mixture was filtered through a Celite
pad (2 × 4 cm), using EtOAc (50 mL). The combined filtrates
and washings were evaporated, and flash chromatography of
the residue over silica gel (2 × 20 cm), using 5:100 MeOH-
CH2Cl2, gave 6.6a (114 mg, 74%) as a white solid: [R]20D -49.8°
1
(c 1.0, MeOH); FTIR (MeOH cast) 1678 cm-1; H NMR (CD3-
OD, 500 MHz) δ 1.25-1.28 (overlapping signals containing a
singlet at δ 1.26, 12 H in all), 1.90 (br s, 1 H), 3.01 (dd, J AB
)
13.1 Hz, J AX ) 8.2 Hz, 1 H), 3.09-3.16 (m, 2 H), 3.73 (s, 3 H),
4.25-4.31 (m, 1 H), 4.80 (br s, 1 H), 6.89 (d, J ) 8.8 Hz, 2 H),
7.21 (d, J ) 8.8 Hz, 2 H); 13C NMR (CD3OD, 100 MHz) δ 17.0
(q′), 31.3 (q′), 32.7 (s′), 44.2 (t′), 55.9 (q′), 57.1 (d′), 63.2 (d′),
115.8 (d′), 126.7 (s′), 131.0 (d′), 162.3 (s′), 173.9 (s′); 19F NMR
1
(c 1.0, CHCl3); FTIR (CHCl3 cast) 3304, 1656 cm-1; H NMR
(CDCl3, 300 MHz) δ 1.13 (d, J ) 7.0 Hz, 3 H), 1.26 (s, 9 H),
2.62-2.75 (m, 2 H), 3.00 (q, J ) 7.0 Hz, 1 H), 3.49 (dd, J )
9.2, 5.1 Hz, 1 H), 3.76 (s, 3 H), 3.88-4.04 (m, 2 H), 4.35-4.48
(m, 2 H), 4.59 (br s, 1 H), 6.81-6.85 (m, 2 H), 6.91 (br t, J )
5.0 Hz, 1 H), 7.03-7.07 (m, 2 H), 7.19-7.30 (m, 5 H), 8.19 (br
t, J ) 5.9 Hz, 1 H); 13C NMR (CDCl3, 125 MHz) δ 20.0 (q′),
30.9 (q′), 36.5 (t′), 42.6 (s′), 43.4 (t′), 43.5 (t′), 55.2 (q′), 61.8
(d′), 114.1 (d′), 127.3 (d′), 127.4 (d′), 127.5 (d′), 128.5 (d′), 133.9
(s′), 137.9 (s′), 159.0 (s′), 169.2 (s′), 176.2 (s′); exact mass m/z
calcd for C25H36N3O3S (M + H) 458.2477, found 458.2481.
The compound had ee ) 99.5% (HPLC, Chirex 3020 (S-Leu
R-NEA) column, 7.5% EtOH-hexane).
(2S)-N-(Ben zylca r b a m oylm et h yl)-2-[2-m er ca p t o-1-(4-
m eth oxyp h en yl)eth yla m in o]p r op ion a m id e (6.7a ). In this
experiment, the initial thiol product was not protected from
air.
CF3CO2H (1.0 mL) was added to thioether 6.6a (94.4 mg,
0.207 mmol) contained in a flask immersed in an ice bath. The
mixture was stirred, and anisole (40 µL) followed by Hg(OAc)2
(66.0 mg, 0.207 mmol) was added. Stirring was continued for
25 min, and the solvent was evaporated. The residue was
dissolved in MeCN (15 mL), and H2S gas was bubbled through
the solution for 2 min. The resulting black suspension was
filtered through a tightly packed Celite column (2 × 4 cm),
and the solid was washed with several portions of MeCN.
Evaporation of the combined filtrate and washings and flash
chromatography of the residue over silica gel (2 × 18 cm), using
4:100 MeOH-CH2Cl2, gave 6.7a and the corresponding dis-
ulfide (80.8 mg, 97%) as a pale-brown oil: 1H NMR (CDCl3,
500 MHz) δ 1.17 (d, J ) 7.0 Hz, 3 H), 1.43 (br s, 1 H), 1.80 (br
s, 1 H), 2.59-2.73 (m, 2 H), 2.97-3.05 (m, 1 H), 3.49 (dd, J )
5.7 Hz, 1 H), 3.76 (s, 3 H), 3.91 (dd, J AB ) 16.1 Hz, J AX ) 5.5
Hz, 1 H), 3.98 (dd, J AB ) 16.1 Hz, J BX ) 6.1 Hz, 1 H), 4.42 (d,
J ) 5.7 Hz, 2 H), 6.70 (br s, 1 H), 6.78-6.83 (m, 2 H), 7.01-
7.03 (m, 2 H), 7.18-7.28 (m, 5 H), 7.92 (t, J ) 5.6 Hz, 1 H);
13C NMR (CDCl3, 125 MHz) δ 20.2 (q′), 32.3 (t′), 43.3 (t′), 43.6
(t′), 55.3 (q′), 55.4 (q′), 63.2 (d′), 114.1 (d′), 127.5 (d′), 127.7
(d′), 127.8 (d′), 128.7 (d′), 133.0 (s′), 137.8 (s′), 159.1 (s′), 168.8
(s′), 175.8 (s′); exact mass m/z calcd for disulfide C42H53N6O6S2
(M + H) 801.3462, found 801.3467. We did not establish if the
compound is the trifluoroacetate salt or the free base; the
percent yield quoted is based on the assumption that the
compound is the free base.
(CD3OD, 376.5 MHz) δ -77.5; exact mass m/z calcd for C16H25
-
NNaO3S (M + Na) 334.1453, found 334.1450. Anal. Calcd for
C18H26F3NO5S: C, 50.81; H, 6.16; N, 3.29; S, 7.54. Found: C,
50.55; H, 6.26; N, 3.36; S, 7.89.
(2S)-2-[[2-ter t-Bu tylsu lfan yl-1-(4-m eth oxyph en yl)eth yl]-
(2,2,2-t r ich lor oet h oxyca r b on yl)a m in o]p r op ion ic Acid
(6.4a ). Neat Cl3CCH2OCOCl (280 µL, 2.03 mmol) and 1 N
NaOH (263 µL) were added simultaneously by syringe over 4
h to a stirred and cooled (0 °C) suspension of 6.3a (316 mg,
0.74 mmol) in 1 N NaOH (1.18 mL). When addition was
complete, the cold bath was removed and stirring was contin-
ued for 11 h, by which time all 6.3a had reacted. The mixture
was cooled (0 °C), acidified to pH 2 with concentrated hydro-
chloric acid, and extracted with EtOAc (3 × 20 mL). The
combined organic extracts were dried (MgSO4) and evaporated.
Flash chromatography of the residue over silica gel (2 × 15
cm), using 1:99, 2:99, and then 4:96 MeOH-CH2Cl2, gave 6.4a
(349 mg, 96%) as a white foam: [R]20 -61.3° (c 1.0, MeOH);
D
mp 58-61 °C; FTIR (MeOH cast) 1714 cm-1; H NMR (CD3-
1
OD, 500 MHz) (mixture of rotamers) δ 1.34 (s, 9 H), 1.56 (d, J
) 6.6 Hz, 1 H), 1.63 (d, J ) 6.9 Hz, 1 H), 3.13-3.19 (m, 2 H),
3.76 (s, 3 H), 3.82-3.88 (m, 1 H), 4.61 (d, J ) 11.8 Hz, 0.35
H), 4.77 (d, J ) 12.9 Hz, 0.33 H), 4.91-4.98 (m, 1 H), 5.45-
5.49 (m, 1 H), 6.82-6.86 (m, 2 H), 7.30-7.36 (m, 2 H) (one
proton not observed in this spectrum); 13C NMR (CDCl3, 100
MHz) δ 15.9 (q′), 17.2 (q′), 29.8 (t′), 30.1 (t′), 30.9 (q′), 42.9 (s’),
43.1 (s’), 51.6 (d′), 52.2 (d′), 55.2 (q′), 60.0 (d′), 60.8 (d′), 75.1
(t′), 75.5 (t′), 94.7 (s’), 95.3 (s’), 113.8 (d′), 114.1 (d′), 128.4 (s′),
129.9 (d′), 130.0 (d′), 152.9 (s′), 153.9 (s′), 159.4 (s′), 175.9 (s′),
176.4 (s′); exact mass m/z calcd for C19H26Cl3NNaO5S (M +
Na) 508.0495, found 508.0496.
[(1S)-1-[(Ben zylca r ba m oylm eth yl)ca r ba m oyl]eth yl][2-
ter t-b u t ylsu lfa n yl-1-(4-m et h oxyp h en yl)et h yl]ca r b a m ic
Acid 2,2,2-Tr ich lor oeth yl Ester (6.5a ). O-Benzotriazol-1-
yl-N,N,N′,N′-tetramethyluronium hexafluorophosphate (230
mg, 0.713 mmol) was added to a stirred mixture of acid 6.4a
(334 mg, 0.686 mmol), Et3N (288 µL, 2.06 mmol), and the
amine salt CF3CO2H‚H2NCH2CONHB25 (198 mg, 0.713 mmol)
in MeCN (3 mL). The mixture was stirred for 4 h, diluted with
EtOAc (25 mL), and washed successively with 1 N hydrochloric
acid (2 × 15 mL) and saturated aqueous NaHCO3 (2 × 15 mL).
The organic phase was dried (MgSO4) and evaporated. Flash
chromatography of the residue over silica gel (2 × 15 cm), using
4:96 MeOH-CH2Cl2, gave 6.5a (345 mg, 79%) as a white
Th ioa cetic Acid S-(4-Meth ylben zyl) Ester (9.2). NaI (50
mg) was added to a stirred mixture of 4-methylbenzyl chloride
(9.1) (1.27 mL, 9.59 mmol) and AcSK (1.21 g, 10.6 mmol) in
dry DME (30 mL) (N2 atmosphere). Stirring was continued
for 11 h, and the mixture was diluted with Et2O (200 mL),
washed with water (3 × 75 mL), dried (MgSO4), and evapo-
rated. Flash chromatography of the residue over silica gel (2
× 20 cm), using 1:10 EtOAc-hexanes, gave 9.2 (1.56 g, 90%)
crystalline solid: [R]20 -51.4° (c 1.0, CHCl3); mp 61-63 °C;
D
FTIR (CHCl3 cast) 1695, 1666 cm-1
;
1H NMR (CDCl3, 300
MHz) δ 1.34 (s, 9 H), 1.55 (d, J ) 6.9 Hz, 3 H), 3.01-3.08 (m,
1 H), 3.15-3.32 (m, 2 H), 3.66-3.79 (overlapping signals
containing a singlet at δ 3.78, 4 H in all), 4.05 (dd, J ) 17.7,
6.8 Hz, 1 H), 4.25 (dd, J ) 15.3, 4.5 Hz, 1 H), 4.36-4.55 (m, 2
H), 5.24-540 (m, 1 H), 6.03 (br s, 0.7 H), 6.4 (br s, 0.3 H), 6.90
(d, J ) 7.6 Hz, 2 H), 7.00 (br s, 1 H), 7.22-7.32 (overlapping
1
as a pale-yellow oil: FTIR (CDCl3 cast) 1691 cm-1; H NMR
(CDCl3, 300 MHz) δ 2.29 (s, 3 H), 2.32 (s, 3 H), 4.10 (s, 2 H),
7.11 (ABq, ∆νAB ) 21.5, J AB ) 8.0 Hz, 4 H); 13C NMR (CDCl3,
125 MHz) δ 21.0 (q′), 30.3 (q′), 33.2 (t′), 128.7 (d′), 129.3 (d′),
J . Org. Chem, Vol. 68, No. 24, 2003 9253

Clive et al.
134.5 (s′), 137.0 (s′), 195.2 (s′); exact mass m/z calcd for C10H12
OS 180.0609, found 180.0610.
-
N-Cbz-alanine tert-butyl ester, 6.1, 6.2a , 6.2b, 6.3b, 6.4a , 6.4b,
6.5a , 6.5b, 6.6a , 6.6b, 6.7a , 6.7b, 7.2, 7.3, Mosher amide of
7.3, 8.1a , 8.1b, 8.2a , 8.2b, 8.3a , 8.3b, 8.4a , 8.4b, 8.5a , 8.5b,
8.6a , 8.6b, 9.2, 9.5, 10.1-10.5, and experimental general
techniques and procedures for chloride derived from 4.2, 4.3
(from chloride derived from 4.2), 5.1, N-Cbz-alanine tert-butyl
ester, 6.1, 6.3b, 6.4b, 6.5b, 6.6b, 6.7b, 7.2, 7.3, Mosher amide
of 7.3, 8.1a , 8.1b, 8.2a , 8.2b, 8.3a , 8.3b, 8.4a , 8.4b, 8.5a , 8.5b,
8.6a , 8.6b, 9.4, 9.5, 10.1-10.5. 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 Natural Sciences
and Engineering Research Council of Canada and the
Merck Frosst Therapeutic Research Centre for financial
support. D.M.C. held an Alberta Heritage Foundation
for Medical Research Fellowship. We also thank C.
Boucher (Boehringer Ingelheim Canada) for ee mea-
surements.
Su p p or tin g In for m a tion Ava ila ble: NMR spectra of
compounds 2.2, 4.2, 4.3, chloride derived from 4.2, 5.1-5.5,
J O030192R
9254 J . Org. Chem., Vol. 68, No. 24, 2003