Synthesis of the side chain cross-linked tyrosine oligomers dityrosine, trityrosine, and pulcherosine

Article abstract of DOI:10.1021/jo051076m

An efficient synthesis of dityrosine and the first syntheses of the tyrosine trimers trityrosine and pulcherosine have been achieved. Protected 3-iodotyrosine underwent tandem Miyaura borylation-Suzuki coupling to give protected dityrosine. The choice of

Full text of DOI:10.1021/jo051076m

Synthesis of the Side Chain Cross-Linked Tyrosine Oligomers
Dityrosine, Trityrosine, and Pulcherosine
Ojia Skaff,† Katrina A. Jolliffe,† and Craig A. Hutton*,†,‡,§
School of Chemistry, The University of Sydney, NSW 2006, Australia, School of Chemistry, The
University of Melbourne, VIC 3010, Australia, and Bio21 Molecular Science and Biotechnology Institute,
The University of Melbourne, VIC 3010, Australia
chutton@unimelb.edu.au
Received May 30, 2005
An efficient synthesis of dityrosine and the first syntheses of the tyrosine trimers trityrosine and
pulcherosine have been achieved. Protected 3-iodotyrosine underwent tandem Miyaura borylation-
Suzuki coupling to give protected dityrosine. The choice of benzyl carbamate, ester, and ether
protecting groups enabled a one-step global deprotection to give dityrosine. Suzuki coupling of
protected 3,5-diiodotyrosine and tyrosine-3-boronic acid derivatives gave the corresponding trity-
rosine, but in low yield. However, use of a potassium tyrosine-3-trifluoroborate derivative in place
of the corresponding pinacol boronate ester, in combination with protecting group variation, gave
protected trityrosine in good yield. Access to pulcherosine was achieved through copper-catalyzed
coupling of phenylalanine-4-boronic acid and 4-O-protected dopa derivatives to give an isodityrosine
derivative. Selective halogenation followed by Suzuki coupling with the potassium tyrosine-3-
trifluoroborate gave protected pulcherosine. Global deprotection of the protected trityrosine and
pulcherosine derivatives completed the first syntheses of the corresponding tris-R-amino acids.
Introduction
The unusual chemical structure and biological activity
of these peptides and proteins engenders great interest
in the development of methods for the preparation of
these pivotal functionalized tyrosine derivatives and
determination of their physiological roles.
In recent years the presence of a number of unheralded
structures in a variety of peptides and proteins has been
unveiled. Many of these peptides and proteins incorpo-
rate cross-linked tyrosine residues in their structures.
These cross-links are believed to impart a wide range of
properties to the modified peptides and proteins, ranging
from stabilization against degradation and denaturation1
to modification of electronic properties of enzyme active
sites2 and conformational restriction of cyclic peptides.3
Dityrosine 1,4 a tyrosine dimer formed by 3,3′-biaryl
bond formation, occurs naturally in fungal cell wall
proteins,5 invertebrate egg/oocyte envelopes,6 and verte-
(2) Okeley, N. M.; van der Donk, W. A. Chem. Biol. 2000, 7, R159.
(3) Boger, D. L.; Yohannes, D.; Zhou, J.; Patane, M. A. J. Am. Chem.
Soc. 1993, 115, 3420.
† The University of Sydney.
(4) (a) Lygo, B. Tetrahedron Lett. 1999, 40, 1389. (b) Malencik, D.
A.; Anderson, S. R. Amino Acids 2003, 25, 233. (c) Malencik, D. A.;
Sprouse, J. F.; Swanson, C. A.; Anderson, S. R. Anal. Biochem. 1996,
242, 202.
‡ School of Chemistry, The University of Melbourne.
§ Bio21 Molecular Science and Biotechnology Institute, The Uni-
versity of Melbourne.
(1) Souza, J. M.; Giasson, B. I.; Chen, Q.; Lee, V. M.-Y.; Ischiro-
poulos, H. J. Biol. Chem. 2000, 275, 18344.
(5) Smail, E. H.; Briza, P.; Panagos, A.; Berenfeld, L. Infect. Immun.
1995, 63, 4078.
10.1021/jo051076m CCC: $30.25 © 2005 American Chemical Society
Published on Web 08/10/2005
J. Org. Chem. 2005, 70, 7353-7363
7353

Skaff et al.
FIGURE 1. Cross-linked tyrosine residues in proteins.
stress and plays a critical role in the signaling of protein
degradation and cellular damage.15
Cross-linked tyrosine residues are also common in
biologically active cyclic peptides, including the cyclo-
isodityrosine-containing bouvardins,17 RA-series com-
pounds,1,18 and the neurotensin antagonist RP-66453,19
which contains a pulcherosine moiety. The functionalized
tyrosine derivatives in these complex peptides are be-
lieved to be of critical importance to their three-
dimensional structure and biological activity.
FIGURE 2. Cross-linked tyrosine residues in cyclic peptides.
Recently, we communicated the synthesis of dityrosine
1 through a tandem Miyaura borylation-Suzuki coupling
of 3-iodotyrosine derivatives.20 Shortly thereafter Yoburn
and Van Vranken21 independently reported a closely
related synthesis of dityrosine-containing peptides. We
hereby present a full account of the expedient synthesis
of dityrosine 1 and the elaboration of this approach to
the first syntheses of the tyrosine trimers trityrosine 3
and pulcherosine 5.
brate proteins such as elastin and collagen.7 In these
examples the formation of dityrosine is believed to accord
a strengthening and/or defensive role to the proteins.8
Dimerization of proteins through dityrosine linkages is
also necessary for the proper functioning of proteins such
as thyroglobulin, the precursor to the thyroid hormone
thyroxine.9 Isodityrosine 2, a tyrosine dimer in which the
tyrosine units are linked through a biaryl ether moiety,
occurs in plant cell wall proteins and presumably conveys
a similar structural/defensive role.10 Oxidatively coupled
trimers of tyrosine-trityrosine 3, isotrityrosine 4 and
pulcherosine 5 (Figure 1)-have also been isolated from
bacteria,11 plants,10 yeast,12 and metazoans.6
Although many of these cross-links are formed specif-
ically by the organisms for strength and protective
purposes, the formation of tyrosine cross-links in proteins
has also been associated with a variety of diseases and
disorders, including the neurodegenerative Alzheimer’s
and Parkinson’s diseases,1,13 cystic fibrosis,14 atheroscler-
osis,15 and cataract formation.16 In these conditions
nonspecific formation of cross-linked tyrosine residues
has been shown to be a biological marker of oxidative
Results and Discussion
Tyrosine-3-boronic acid derivatives were chosen as
common precursors to a variety of cross-linked tyrosines,
as these functionalized tyrosine derivatives are suitable
for the preparation of biaryl-linked tyrosines, through
(13) (a) Reynolds, M. R.; Berry, R. W.; Binder, L. I. Biochemistry
2005, 44, 1690. (b) Barnham, K. J.; Haeffner, F.; Ciccotosto, G. D.;
Curtain, C. C.; Tew, D.; Mavros, C.; Beyreuther, K.; Carrington, D.;
Masters, C. L.; Cherny, R. A.; Cappai, R.; Bush, A. I. FASEB J. 2004,
18, 1427. (c) Hensley, K.; Maidt, M. L.; Yu, Z.; Sang, H.; Markesbury,
W. R.; Floyd, R. A. J. Neurosci. 1998, 18, 8126.
(14) van der Vliet, A.; Nguyen, M. N.; Shigenaga, M. K.; Eiserich,
J. P.; Marelich, G. P.; Cross, C. E. Am. J. Physiol., Lung Cell. Mol.
Physiol. 2000, 279, L537.
(6) (a) Belli, S. I.; Wallach, M. G.; Luxford, C.; Davies, M. J.; Smith,
N. C. Eukaryotic Cell 2003, 2, 456. (b) Li, J.; Hodgeman, B. A.;
Christensen, B. A. Insect Biochem. Mol. Biol. 1996, 26, 309. (c) Nomura,
K.; Suzuki, N. Arch. Biochem. Biophys. 1995, 319, 525.
(7) LaBella, F.; Waykole, P.; Queen, G. Biochem. Biophys. Res.
Commun. 1968, 30, 333.
(15) Jacob, J. S.; Cistola, D. P.; Hsu, F. F.; Muzaffar, S.; Mueller,
D. M.; Hazen, S. L.; Heinecke, J. W. J. Biol. Chem. 1996, 271, 19950.
(16) Wells-Knecht, M. C.; Huggins, T. G.; Dyer, S. R.; Thorpe, S.
R.; Baynes, J. W. J. Biol. Chem. 1993, 268, 12348.
(17) Boger, D. L.; Patane, M. A.; Zhou, J. C. J. Am. Chem. Soc. 1994,
116, 8544.
(8) Kanwar, R.; Balasubramanian, D. Biochemistry 2000, 39, 14976.
(9) Baudry, N.; Lejeune, P.-J.; Niccoli, P.; Vinet, L.; Carayon, P.;
Mallet, B. FEBS Lett. 1996, 396, 223.
(18) Bigot, A.; Dau, M. E. T. H.; Zhu, J. J. Org. Chem. 1999, 64,
6283.
(19) (a) Bios-Choussy, M.; Cristau, P.; Zhu, J. Angew. Chem., Int.
Ed. 2003, 42, 4238. (b) Krenitsky, P. J.; Boger, D. L. Tetrahedron Lett.
2003, 44, 4019. (c) Helynck, G.; Dubertret, C.; Frechet, D.; Leboul, J.
J. Antibiot. 1998, 51, 512.
(10) Brady, J. D.; Sadler, I. H.; Fry, S. C. Phytochemistry 1998, 47,
349.
(11) Pandey, N. K.; Aronson, A. I. J. Bacteriol. 1979, 137, 1208.
(12) Briza, P.; Winkler, G.; Kalchhauser, H.; Breitenbach, M. J. Biol.
Chem. 1986, 261, 4288.
(20) Hutton, C. A.; Skaff, O. Tetrahedron Lett. 2003, 44, 4895.
(21) Yoburn, J. C.; Van Vranken, D. L. Org. Lett. 2003, 5, 2817.
7354 J. Org. Chem., Vol. 70, No. 18, 2005

Synthesis of Dityrosine, Trityrosine, and Pulcherosine
SCHEME 1
SCHEME 2
Suzuki coupling, and biaryl ether-linked tyrosines, through
copper(II)-catalyzed coupling with tyrosine derivatives.
Additionally, the tyrosine-3-boronic acid derivatives are
available through borylation of the corresponding 3-io-
dotyrosine derivatives, which constitute the other coup-
ling partner in the formation of biaryl-linked tyrosines,
thereby providing a highly convergent overall process.
A. Synthesis of Dityrosine. Following protection of
3-iodo-L-tyrosine 6 under standard conditions, Miyaura
borylation22 with bis(pinacolatodiboron) gave the ty-
rosine-3-boronate derivative 8. Suzuki coupling23 of the
tyrosine-3-boronate 8 and 3-iodo-L-tyrosine derivative 7
then gave the protected dityrosine 9 in reasonable yield
(Scheme 1). This procedure was not optimized as a more
efficient route to the symmetrical biaryl compound 9 was
developed through a one-pot, tandem Miyaura boryla-
tion-Suzuki coupling from the 3-iodo-L-tyrosine deriva-
tive 7. In theory, treatment of an aryl iodide with 0.5
equiv of bis(pinacolato)diboron under standard Suzuki
coupling conditions should result in 50% of the iodide
being transformed to the corresponding boronate, which
then couples with the remaining iodide to give the
symmetrical biaryl product. In practice, optimization of
the amount of the diboron reagent is required for differ-
ent iodide substrates due to the differing relative rates
of conversion of the iodide to the boronate intermediate,
protodeborylation, and Suzuki coupling. The optimum
conditions for the one-pot conversion of iodotyrosine
derivative 7 to dityrosine derivative 9 were found to be
the use of 0.95 equiv of bis(pinacolato)diboron, giving
dityrosine derivative 9 in 70% yield (Scheme 2).
The use of benzyl carbamate, ester, and ether protect-
ing groups was designed to enable a one-step global
deprotection of the protected dityrosine derivative 9.
Accordingly, dityrosine 1 was prepared in quantitative
yield by treatment of 9 with palladium-on-charcoal under
an atmosphere of hydrogen. The overall procedure en-
ables preparation of dityrosine 1 in just four steps and
53% yield from 3-iodo-L-tyrosine 6.
B. Synthesis of Trityrosine. It was envisaged that
an analogous Suzuki coupling of 3,5-diiodo-L-tyrosine
derivative 12 (in place of 3-iodo-L-tyrosine derivative 7)
with excess tyrosine-3-boronate 8 would provide the
corresponding protected trityrosine derivative 17. Ac-
cordingly, several methods were investigated toward the
preparation of protected 3,5-diiodo-L-tyrosine derivative
12. Protection of commercially available 3,5-diiodo-L-
tyrosine 10 was initially attempted, with Cbz-protection
under standard conditions proceeding to give Cbz-3,5-
diiodo-L-tyrosine in 99% crude yield. However, the ben-
zylation of Cbz-3,5-diiodo-L-tyrosine by treatment with
excess benzyl bromide in acetone in the presence of
potassium carbonate (Scheme 3) gave the fully protected
compound 12 in low yield (19%), along with 28% of the
intermediate phenol 11. Use of cesium carbonate as base
in DMF improved the yield of the fully protected di-
iodotyrosine derivative 12 to a moderate level (32%). The
difficulty in forcing this reaction to completion is presum-
ably due to the steric hindrance around the phenolic
(22) Ishiyama, T.; Murate, M.; Miyaura, N. J. Org. Chem. 1995, 60,
7508.
(23) Miyaura, N.; Suzuki, A. Chem. Rev. 1995, 95, 2457.
J. Org. Chem, Vol. 70, No. 18, 2005 7355

Skaff et al.
SCHEME 3
SCHEME 5
SCHEME 4
group by the two ortho iodine atoms. Evidence for this
presumption was that the benzylation of 11 to give 12
was very slow, with mixtures of 11 and 12 being isolated
despite the use of excess benzyl bromide and long reaction
times. In contrast, the methylation of 11 with io-
domethane proceeded to give 13 in good yield (86%) under
similar conditions (Scheme 3).
SCHEME 6
An alternative approach to the fully protected diiodoty-
rosine derivative 12 through the iodination of tyrosine
derivative 14 was also investigated, but again the reac-
tion could not be driven to completion and mixtures of
iodotyrosine 7 and diiodotyrosine 12 were obtained
(Scheme 4).
Despite the problems associated with the preparation
of 3,5-diiodo-L-tyrosine derivative 12, Suzuki coupling
with tyrosine-3-boronate 8 was investigated. Treatment
of 12 with excess tyrosine boronate 8 under standard
Suzuki coupling conditions gave none of the doubly
coupled product 17, with the monocoupled iodo-dityrosine
derivative 16 isolated in 21% yield (Scheme 5). Use of
more forcing conditions (prolonged heating with excess
boronate 8) gave no improvement. Interestingly, coupling
of the corresponding methyl ether compounds 13 and 15
gave the trityrosine derivative 19 in 22% yield, in
addition to the iodo-dityrosine derivative 18 (31%). The
successful preparation of the methyl ether protected
trityrosine derivative 19 provides further evidence that
the high degree of steric hindrance associated with the
all-benzyl protected 3,5-diiodo-L-tyrosine derivative 12
gives rise to both the difficulty in its preparation and its
subsequent lack of reactivity.
Given the problems associated with both the prepara-
tion and subsequent Suzuki coupling of 12, attention was
turned to the discovery of more reactive coupling part-
ners. It was reasoned that the high degree of steric
hindrance encountered through the presence of three
adjacent, bulky aromatic substituents may be somewhat
alleviated through the use of the 3,5-diiodo-L-tyrosine
derivative containing a free phenolic group. That is,
diiodotyrosine 11 should be both easier to access and
more reactive in the double-Suzuki coupling reaction
than the corresponding benzyl-ether 12, while eliminat-
ing the need for a separate deprotection of the aryl-
methyl ether as would be required from 13. While the
phenol 11 had already been prepared in low yield through
mono-benzylation of N-Cbz-3,5-diiodo-L-tyrosine (Scheme
3), a more efficient procedure was found to be the
diiodination of tyrosine derivative 20 (Scheme 6). Fur-
thermore, recent reports by Molander24 and Batey,25
indicating that organotrifluoroborates are more reactive
7356 J. Org. Chem., Vol. 70, No. 18, 2005

Synthesis of Dityrosine, Trityrosine, and Pulcherosine
SCHEME 7
coupling partners than the corresponding organobor-
onates, led us to investigate the reactivity of the potas-
sium tyrosine-3-trifluoroborate salt 21. The tyrosine-3-
trifluoroborate 21 was synthesized in quantitative yield
from the corresponding pinacol boronate derivative 8 by
treatment with potassium hydrogen fluoride, according
to the procedure of Vedejs et al.26 Molander24 found that
optimal conditions for Suzuki couplings of organotrifluo-
roborates employed a solvent mixture of water and water-
miscible polar solvents such as THF, 2-propanol, or
methanol. Accordingly, the 3,5-diiodo-L-tyrosine deriva-
tive 11 was treated with 2 equiv of the tyrosine trifluo-
roborate 21 in THF/water to afford the trityrosine
derivative 23 in 67% yield (Scheme 6). The monocoupled
product 22 was also isolated in 22% yield. Protodeboryl-
ated product 14 and tyrosine boronic acid derivative 24
(arising from hydrolysis of the trifluoroborate salt) were
isolated as minor byproducts. Resubjecting the iodo-
dityrosine 22 to the reaction conditions gave more of the
trityrosine derivative, increasing the overall yield of 23
from 11 to 74%.
With an efficient procedure for the preparation of
protected trityrosine derivative 23 in hand, efforts were
focused on a global deprotection to furnish trityrosine 3.
Initial attempts at the hydrogenolysis of trityrosine
derivative 23 in methanol, as employed in the prepara-
tion of dityrosine 1, were plagued by solubility problems
resulting in the recovery of starting material and several
partially deprotected compounds. Various solvent mix-
tures, including 5% acetic acid in methanol and THF/
water were also unsuccessful due to solubility problems.
Eventually, hydrogenolysis of 23 in a mixture of THF,
methanol and water (5:5:1) was found to afford the fully
deprotected product, trityrosine 3, in quantitative yield
(Scheme 6).
tyrosine moiety is a 3,5-difunctionalized tyrosine, con-
nected through a 3-3′-biaryl linkage to a second tyrosine
moiety and through a biaryl ether linkage to the 4-oxygen
of the third tyrosine moiety. It was envisaged that the
biaryl linkage could be generated by Suzuki coupling of
iodotyrosine and tyrosine boronic acid derivatives in a
manner analogous to that developed for the syntheses
of di- and trityrosine, while the biaryl ether could be
generated by a copper(II)-catalyzed coupling27 of a ty-
rosine-3-boronic acid with a tyrosine phenolic group. This
retrosynthesis leads back to two tyrosine-3-boronic acid
units (e.g., 24) and a protected tyrosine unit (20) as the
three constituent tyrosine moieties (Scheme 7, path a).
Although the tyrosine-3-pinacolyl boronate 8 had been
prepared, the corresponding boronic acid 24 was required
for the Cu(II)-catalyzed biaryl ether formation. The
boronic acid 24 was prepared by transesterification of the
pinacolyl boronate 8 with solid-supported boronic acid28
or alternatively by borylation of the iodotyrosine deriva-
tive 7 with bis(neopentylglycolato)boron, followed by
facile hydrolysis of the neopentylglycolyl boronate ester
28 (Scheme 8). Treatment of the tyrosine-3-boronic acid
24 with protected tyrosine 20 in the presence of Cu(OAc)2,
DMAP, and molecular sieves gave the isodityrosine
derivative 29 in only 33% yield. Protected tyrosine 14,
generated by protodeborylation of 24, was isolated in 15%
yield, along with trace amounts of the dopa derivative
26 (X ) H) (formed by oxidation of 24). The formation of
protodeborylated and oxidized byproducts is a known
complication of the Cu(II)-catalyzed coupling of phenols
and arylboronic acids when the arylboronic acid possesses
an ortho heteroatom.27c
It was reasoned that switching the phenol and boronic
acid groups would enable a more efficient coupling
reaction as the arylboronic acid would no longer contain
C. Synthesis of Pulcherosine. Pulcherosine 5 is a
structural isomer of trityrosine in which the central
(27) (a) Chan, D. M. T.; Monaco, K. L.; Wang, R.-P.; Winters, M. P.
Tetrahedron Lett. 1998, 39, 2933. (b) Lam, P. Y. S.; Clark, C. G.;
Saubern, S.; Adams, J.; Winters, M. P.; Chan, D. M. T.; Combs, A.
Tetrahedron Lett. 1998, 39, 2941. (c) Evans, D. A.; Katz, J. L.; West,
T. R. Tetrahedron Lett. 1998, 39, 2937. (d) Cundy, D. J.; Forsyth, S. A.
Tetrahedron Lett. 1998, 39, 7979. (e) Collman, J. P.; Zhong, M. Org.
Lett. 2000, 2, 1233.
(24) (a) Molander, G. A.; Biolatto, B. J. Org. Chem. 2003, 68, 4302.
(b) Molander, G. A.; Bernardi, C. R. J. Org. Chem. 2002, 67, 8424.
(25) Batey, R. A.; Thadani, A. N.; Smil, D. V. Org. Lett. 1999, 1,
1683.
(26) Vedejs, E.; Chapman, R. W.; Fields, S. C.; Lin, S.; Schrimpf,
M. R. J. Org. Chem. 1995, 60, 3020.
(28) Pennington, T. E.; Kardiman, C.; Hutton, C. A. Tetrahedron
Lett. 2004, 45, 6657.
J. Org. Chem, Vol. 70, No. 18, 2005 7357

Skaff et al.
SCHEME 8
acetate gave the neopentylglycolyl boronate 32, which
was immediately hydrolyzed to afford the tyrosine-3-
boronic acid derivative 33 in good yield (Scheme 9).
Treatment of 33 with 1 equiv of hydrogen peroxide for
30 min gave the 4-O-PMB protected dopa derivative 34
in 81% yield.30
The phenylalanine-4-boronic acid component 27 was
prepared using a similar procedure to that employed for
the preparation of tyrosine-3-boronic acids 24 and 33.
Protection of 4-iodophenylalanine (35) under standard
conditions gave 36 in 86% yield. Borylation of iodide 36
with bis(neopentylglycolato)diboron gave the boronate
ester 37. Hydrolysis of the phenylalanine neopentylgly-
colyl boronate 37 was much slower than that of the
corresponding tyrosine derivatives 28 and 32, taking
several days rather than several hours, but did afford
the boronic acid 27 in 79% yield (based on recovered
starting material).
Treatment of phenylalanine-4-boronic acid 27 with
dopa derivative 34 in the presence of copper(II) acetate,
pyridine, and 4 Å molecular sieves afforded the biaryl
ether 38 in good yield (80%). Selective removal of the
PMB group with DDQ then provided isodityrosine de-
rivative 39 in 78% yield. Note that selective removal of
the 4-O-protecting group was required as 3,4-O,O-dialkyl-
protected dopa derivatives selectively halogenate at the
6-position,31 whereas those only protected at the 3-posi-
tion halogenate at the 5-position,32 presumably as a result
of the greater directing effect of the 4-phenolic group.
an ortho donating group (Scheme 7, path b). The biaryl
ether linkage would therefore be generated by the
coupling of a phenylalanine-4-boronic acid derivative (27)
and a functionalized dopa derivative (26). Such a route
to isodityrosine derivatives has previously been reported
by Jung and Lazarova.29 Furthermore, it was reasoned
that the requisite differentially protected dopa derivative
26 would be easily accessible through oxidation of the
corresponding tyrosine-3-boronic acid derivative 24.30
Disconnection of pulcherosine according to path b (Scheme
7) therefore leads back yet again to two tyrosine-3-boronic
acid units, with a phenylalanine-4-boronic acid unit
utilized as the third amino acid constituent. When
employing this reworked strategy, the p-methoxybenzyl
(PMB) group was chosen to protect the 4-oxygen of the
central tyrosine in order that this group could specifically
be removed in the presence of all the other benzyl
protecting groups at a later stage in the synthesis (vide
infra). Accordingly, PMB-protected iodotyrosine deriva-
tive 31 was prepared from 30 under standard conditions.
Subsequent treatment with bis(neopentylglycolato)dibo-
ron in the presence of Pd(dppf)Cl2‚CH2Cl2 and potassium
With the isodityrosine derivative 39 in hand, selective
halogenation was investigated. However, all efforts to-
ward iodination of the isodityrosine derivative 39 by
treatment with iodine and silver(I) salts were unsuccess-
ful. No explanation for the lack of success of this reaction
in light of the successful iodinations of other tyrosine
derivatives is currently forthcoming. Attention was there-
fore turned toward bromination of isodityrosine deriva-
tive 39, and it was found that treatment of 39 with 1.5
SCHEME 9
7358 J. Org. Chem., Vol. 70, No. 18, 2005

Synthesis of Dityrosine, Trityrosine, and Pulcherosine
SCHEME 10
ficient through Suzuki coupling of a tyrosine-3-trifluo-
roborate with a 3,5-diiodotyrosine derivative. Access to
pulcherosine was achieved through copper-catalyzed
coupling of phenylalanine-4-boronic acid and 4-O-pro-
tected dopa derivatives to give an isodityrosine deriva-
tive. Selective halogenation followed by Suzuki coupling
with the potassium tyrosine-3-trifluoroborate gave pro-
tected pulcherosine. One-step global deprotection of the
protected trityrosine and pulcherosine derivatives com-
pleted the first syntheses of the corresponding tris-R-
amino acids.
Tyrosine-3-boronic acid derivatives have been shown
to be highly useful precursors to cross-linked tyrosines
as these organoboron compounds are suitable for the
preparation of both biaryl-linked tyrosines, through
Suzuki coupling, and biaryl ether-linked tyrosines, through
copper(II)-catalyzed coupling with tyrosine derivatives.
Additionally, tyrosine-3-boronic acid derivatives can be
converted directly to differentially protected dopa deriva-
tives, which are alternative precursors to biaryl ether-
linked tyrosine oligomers. These general procedures
should allow for the preparation of a wide range of cross-
linked tyrosine oligomers present in naturally occurring
peptides and proteins, as well as unnatural oligomers.
Experimental Section
equiv of N-bromosuccinimide afforded the aryl bromide
40 selectively in 70% yield.
General procedures are available in Supporting Information.
N-Cbz-4-O-Benzyl-3-iodo-l-tyrosine Benzyl Ester 7. To
an ice-cold solution of potassium carbonate (6.75 g, 48.8 mmol)
in water (60 mL) was added 3-iodo-L-tyrosine 6 (5.00 g, 16.3
mmol). A solution of 50% benzylchloroformate in toluene (8.35
mL, 25.0 mmol) was added over 30 min at 0 °C. The mixture
was stirred at room temperature for 21 h. The resulting
mixture was acidified to pH 4 with 3 M HCl and extracted
with EtOAc (3 × 100 mL). The combined organic extracts were
washed with saturated sodium chloride solution (2 × 100 mL),
dried (MgSO4), and concentrated in vacuo to give Cbz-3-iodo-
L-tyrosine (7.2 g) as a white powder. The crude product was
added to a solution of potassium carbonate (6.80 g, 49.2 mmol)
and tetrabutylammonium iodide (40 mg) in dry acetone (200
mL). Nitrogen was bubbled through the mixture for 15 min.
Benzyl bromide (4.27 mL, 35.9 mmol) was added and the
mixture was heated at reflux in the dark for 20 h. The
resulting mixture was cooled and concentrated in vacuo. To
the residue was added water (100 mL) and the mixture was
extracted with DCM (2 × 100 mL). The combined organic
extracts were dried (MgSO4) and concentrated in vacuo to give
a pale yellow oil. The oil was chromatographed on silica eluting
with EtOAc/hexane (1:2-1:1 gradient) to give the dibenzylated
product 7 (7.63 g, 76%) as a white solid: mp 88.0-88.5 °C;
[R]D +11.3 (c 2.0, CHCl3); IR (Nujol) υmax (cm-1) 2922, 2853,
1735, 1460, 1377, 721; 1H NMR (400 MHz, CDCl3) δ 7.53 (1H,
d, J ) 2.1 Hz), 7.49-7.29 (15H, m), 6.91 (1H, dd, J ) 2.1, 8.4
Hz), 6.65 (1H, d, J ) 8.4 Hz), 5.25 (1H, br d, J ) 8.1 Hz), 5.13-
5.07 (6H, m), 4.64 (1H, m), 3.02-3.00 (2H, m); 13C NMR (100
MHz, CDCl3) δ 171.1 (s), 156.4 (s), 155.5 (s), 140.2 (d), 136.4
(s), 134.9 (s), 130.3 (d), 130.2 (d), 130.1 (s), 128.6 (d), 128.6
(d), 128.5 (s), 128.5 (d), 128.4 (d), 128.1 (d), 128.0 (d), 127.9
(d), 127.4 (d), 112.5 (d), 86.8 (s), 70.8 (t), 67.3 (t), 67.0 (t), 54.9
(d), 36.9 (t); MS (ESI, +ve) m/z 644 (M + Na+, 100%); HRMS
(ESI, +ve) m/z calcd for (M + Na)+ 644.0905, found 644.0906.
It was envisaged that the Suzuki coupling procedure
previously utilized in the synthesis of trityrosine deriva-
tive 23 could be adopted for the preparation of protected
pulcherosine 41. Treatment of bromo-isodityrosine de-
rivative 40 with excess tyrosine trifluoroborate 21, potas-
sium carbonate, and Pd(dppf)Cl2‚CH2Cl2 in a THF/water
solvent mixture afforded the coupled product 41 in only
7% yield. Unreacted starting material 40 and protode-
borylated product 14 were isolated in 67% and 26% yield,
respectively. Following Molander’s investigations,24 the
use of a 2-propanol/water solvent mixture and 6 equiv of
triethylamine were then examined and found to give the
optimum yield of the coupled product to date, affording
the pulcherosine derivative 41 in 32% yield (Scheme 10).
With the protected pulcherosine derivative 41 in hand,
a global deprotection was required to furnish pulche-
rosine 5. Hydrogenolysis of pulcherosine derivative 41
employing the conditions previously optimized for depro-
tection of the trityrosine derivative 23 (H2, Pd/C, THF/
MeOH/water) afforded the deprotected pulcherosine 5 in
quantitative yield (Scheme 10).
Conclusion
An efficient synthesis of dityrosine and the first
syntheses of the tyrosine trimers trityrosine and pulche-
rosine have been achieved. Dityrosine was prepared in
53% overall yield in four steps from 3-iodo-L-tyrosine,
employing a tandem Miyaura borylation-Suzuki coup-
ling strategy. Preparation of trityrosine was most ef-
N-Cbz-4-O-Benzyl-3-pinacolatoborono-l-tyrosine Ben-
zyl Ester 8. To a solution of bis(pinacolato)diboron (2.46 g,
9.66 mmol), Pd(dppf)Cl2‚CH2Cl2 (0.24 g), and potassium ac-
etate (1.90 g, 19.3 mmol) in DMSO (30 mL) was added 3-iodo-
tyrosine derivative 7 (3.00 g, 4.83 mmol). The mixture was
stirred under nitrogen at 80 °C for 24 h. A 50% saturated
(29) Jung, M. E.; Lazarova, T. I. J. Org. Chem. 1999, 64, 2976.
(30) Hunter, L.; Hutton, C. A. Aust. J. Chem. 2003, 56, 1095.
(31) Adam, M. J.; Ponce, Y. Z.; Berry, J. M.; Hoy, K. J. Labelled
Compd. Radiopharm. 1990, 28, 155.
(32) Boothe, T. E.; Finn, R. D.; Vora, M. M.; Emran, A.; Kothari, P.
J.; Kabalka, G. W. J. Labelled Compd. Radiopharm. 1985, 22, 1109.
J. Org. Chem, Vol. 70, No. 18, 2005 7359

Skaff et al.
sodium chloride solution (50 mL) was added, and the resulting
mixture was extracted with EtOAc (2 × 100 mL). The
combined organic extracts were washed with 50% saturated
sodium chloride solution (3 × 100 mL), dried (MgSO4), and
concentrated in vacuo to give a dark brown oil. The oil was
chromatographed on alumina eluting with EtOAc/hexane (1:
3-1:2 gradient) to give the boronate derivative 8 (2.55 g, 85%)
as a pale brown oil: [R]D +5.31 (c 1.08, CHCl3); IR (Nujol) υmax
(cm-1) 2897, 1735, 1456, 1377, 1366, 1151, 721; 1H NMR (300
MHz, CDCl3) δ 7.60 (2H, dd, J ) 1.4, 8.0 Hz), 7.48 (1H, d, J )
2.4 Hz), 7.41-7.28 (13H, m), 7.07 (1H, dd, J ) 2.4, 8.4 Hz),
6.77 (1H, d, J ) 8.4 Hz), 5.24 (1H, br d, J ) 8.2 Hz), 5.12-
5.07 (6H, m), 4.66 (1H, m), 3.07-3.04 (2H, m), 1.34 (12H, s);
13C NMR (100 MHz, CDCl3) δ 171.6 (s), 162.6 (s), 155.7 (s),
137.6 (d), 136.3 (s), 135.1 (s), 133.3 (d), 128.6 (d), 128.5(3) (d),
128.4(8) (d), 128.1(9) (d), 128.1(5) (s), 128.0(6) (d), 127.6 (d),
127.4(4) (s), 127.3(6) (d), 127.0 (d), 126,7 (d), 112.2 (d), 83.5
(s), 70.0 (t), 67.3 (t), 67.0 (t), 55.1 (d), 37.3 (t), 25.0 (q), 24.9 (q)
(C-B not observed); MS (ESI, +ve) m/z 644 (M + Na+, 100%);
HRMS (ESI, +ve) m/z calcd for (M + Na)+ 644.2798, found
644.2790.
Protected Dityrosine 9. Method 1. To a solution of iodo-
tyrosine derivative 7 (52.9 mg, 0.09 mmol), Pd(dppf)Cl2‚CH2-
Cl2 (2.00 mg), and potassium carbonate (44.7 mg, 0.36 mmol)
in DMSO (1.5 mL) was added bis(pinacolato)diboron (20.5 mg,
0.086 mmol). The mixture was stirred under nitrogen at 80
°C for 48 h. The resulting mixture was extracted with EtOAc
(2 × 30 mL) and the combined organic extracts were washed
with 50% saturated sodium chloride solution (2 × 30 mL),
dried (MgSO4), and concentrated in vacuo to give a dark brown
oil. The oil was chromatographed on silica eluting with ether/
DCM/hexane (1:2:3-1:2:2 gradient) to give dityrosine deriva-
tive 9 (29.3 mg, 70%) as a pale yellow oil: [R]D +5.70 (c 3.14,
CHCl3); IR (Nujol) υmax (cm-1) 3339, 2926, 1713, 1499, 1454,
1344, 1217, 1053, 696; 1H NMR (300 MHz, CDCl3) δ 7.36-
7.11 (30H, m), 6.98 (2H, m), 6.92 (2H, dd, J ) 1.9, 8.3 Hz),
6.78 (2H, d, J ) 8.3 Hz), 5.23 (2H, br d, J ) 7.9 Hz), 5.14-
4.99 (8H, m), 4.90 (4H, s), 4.65 (2H, m), 3.05-3.03 (4H, m);
13C NMR (100 MHz, CDCl3) δ 171.5 (s), 155.7 (s), 155.4 (s),
137.4 (s), 136.3 (s), 135.2 (s), 132.6 (d), 129.3 (d), 128.6 (d),
128.5 (d), 128.4 (d), 128.3 (d), 128.1 (d), 128.0 (d), 127.6 (s),
127.4 (d), 126.6 (d), 113.2 (d), 70.3 (t), 67.1 (t), 66.9 (t), 55.0
(d), 37.8 (t) (2 coincident C-H); MS (ESI, +ve) m/z 1011 (M +
Na+, 100%), 921 (32); HRMS (ESI, +ve) m/z calcd for (M +
Na)+ 1011.3828, found 1011.3865.
Method 2. To a solution of iodo-tyrosine derivative 7 (23.4
mg, 0.04 mmol), Pd(dppf)Cl2‚CH2Cl2 (1.00 mg), and potassium
carbonate (10.4 mg, 0.08 mmol) in DMSO (3 mL) was added
boronate derivative 8 (46.8 mg, 0.08 mmol). The mixture was
stirred under nitrogen at 80 °C for 24 h. The resulting mixture
was extracted with EtOAc (2 × 30 mL) and the combined
organic extracts were washed with saturated sodium chloride
solution (2 × 30 mL), dried (MgSO4), and concentrated in vacuo
to give a dark brown oil. The oil was chromatographed on
silica, eluting with ether/DCM/hexane (1:2:3-1:1:1 gradient)
to give dityrosine derivative 9 (21.3 mg, 56%) as a pale yellow
oil.
Dityrosine 1.4a To a solution of protected dityrosine deriva-
tive 9 (0.29 g, 0.30 mmol) in MeOH (12 mL) was added 10%
palladium on charcoal (0.03 g, 0.03 mmol). The reaction
mixture was stirred under hydrogen at room temperature for
21 h. The resulting mixture was filtered through Celite,
washed with HCl (1 M, 30 mL), and concentrated in vacuo to
give dityrosine 1 in quantitative yield. For analytical purposes,
the crude product was chromatographed on reverse-phase
HPLC eluting with H2O/CH3CN/TFA (95:5:0.05) to give dity-
rosine 1 as an off-white solid: mp 136-140 °C; [R]D -2.6 (c
0.08, 1 M HCl), (lit.4a [R]D -7 (c 1.00, 1 M HCl)); 1H NMR (400
MHz, 1 M DCl‚D2O) δ 7.12 (2H, d, J ) 8.3 Hz), 7.02 (2H, s),
6.89 (2H, d, J ) 8.3 Hz), 4.23 (2H, m), 3.21(2H, dd, J ) 5.4,
14.5 Hz), 3.09 (2H, dd, J ) 7.3, 14.5 Hz); 13C NMR (100 MHz,
1 M DCl‚D2O) δ 171.1 (s), 152.6 (s), 132.2 (d), 130.4 (d), 125.8
(s), 120.0 (s), 116.4 (d), 54.0 (d), 38.7 (t); MS (ESI, +ve) m/z
361 (M + Na+, 100%).
N-Cbz-3,5-Diiodo-L-tyrosine Benzyl Ester 11. To a solu-
tion of silver sulfate (0.16 g, 0.50 mmol) and iodine (0.13 g,
0.50 mmol) in MeOH (5 mL) was added N-Cbz-L-tyrosine
benzyl ester 20 (0.10 g, 0.25 mmol). The mixture was stirred
under nitrogen in the dark at room temperature for 5 h. The
yellow precipitate was removed by filtration, a 1:1 mixture of
saturated sodium hydrogen carbonate solution and saturated
sodium chloride solution (50 mL) was added to the filtrate,
and the mixture was extracted with EtOAc (2 × 50 mL). The
combined organic extracts were washed with a 1:1 mixture of
saturated sodium hydrogen carbonate solution and saturated
sodium chloride solution (50 mL), saturated sodium thiosulfate
solution (50 mL), dried (MgSO4), and concentrated in vacuo
to give a yellow oil. The oil was chromatographed on silica,
eluting with ether/DCM/hexane (1:2:3) to give the 3,5-diiodo-
L-tyrosine derivative 11 (85.3 mg, 52%) as a yellow solid: mp
93.0-94.5 °C; [R]D +17.3 (c 2.02, CHCl3); IR (Nujol) υmax (cm-1
)
1
2927, 2852, 1724, 1456, 1376, 1157, 724; H NMR (400 MHz,
CDCl3) δ 7.41-7.29 (12H, m), 5.74 (1H, br s), 5.37 (1H, br d,
J ) 7.8 Hz), 5.18-5.08 (4H, m), 4.62 (1H, br m), 3.00-2.91
(2H, br m); 13C NMR (100 MHz, CDCl3) δ 171.6 (s), 156.2 (s),
153.4 (s), 140.6 (d), 136.8 (s), 135.4 (s), 132.5 (s), 129.4 (d),
129.3 (d), 129.3 (d), 129.1 (d), 128.9 (d), 128.7 (d), 82.9 (s), 68.2
(t), 67.8 (t), 55.5 (d), 37.0 (t); MS (EI) m/z 522 (M+• - CO2Bn,
1%), 507 (M+• - CbzNH, 18), 460 (5), 380 (25), 359 (8), 333
(4), 252 (10), 232 (3), 91 (100); HRMS (ESI, +ve) m/z calc. for
(M + Na)+ 679.9409, found 679.9415.
N-Cbz-4-O-Benzyl-3,5-diiodo-L-tyrosine Benzyl Ester
12. To an ice-cold solution of potassium carbonate (2.06 g, 15.1
mmol) in water (20 mL) was added 3,5-diiodo-L-tyrosine 10
(2.00 g, 4.3 mmol). A solution of 50% benzyl chloroformate in
toluene (3.04 mL, 9.09 mmol) was added over 30 min at 0 °C.
The mixture was stirred at room temperature for 20 h. The
resulting mixture was acidified to pH 4 with 3 M HCl and
extracted with EtOAc (3 × 100 mL). The combined organic
extracts were washed with saturated sodium chloride solution
(50 mL), dried (MgSO4), and concentrated in vacuo to give the
crude Cbz-3,5-diiodo-L-tyrosine (2.46 g, 99%) as a pale yellow
solid. To a solution of cesium carbonate (46.1 mg, 0.14 mmol)
in MeOH (1 mL) was added N-Cbz-3,5-diiodo-tyrosine (80.2
mg, 0.14 mmol) and the mixture was concentrated in vacuo.
The residue was dissolved in DMF (2 mL) then benzyl bromide
(33.6 µL, 0.28 mmol) was added and the mixture stirred under
nitrogen at room temperature for 20 h. The resulting mixture
was cooled and concentrated in vacuo. To the residue was
added water (30 mL) and the mixture was extracted with DCM
(2 × 30 mL). The combined organic extracts were dried
(MgSO4) and concentrated in vacuo to give a pale yellow oil.
The oil was chromatographed on silica, eluting with ether/
DCM/hexane (1:3:7-1:3:5 gradient) to give the dibenzylated
product 12 (33.8 mg, 32%) as a white solid: mp 124-125 °C;
[R]D +8.00 (c 2.07, CHCl3); IR (Nujol) υmax (cm-1) 2930, 1725,
1454, 1377, 1366, 1306, 1155, 721; 1H NMR (400 MHz, CDCl3)
δ 7.64 (2H, d, J ) 7.0 Hz), 7.54 (2H, s), 7.44-7.30 (13H, m),
5.34 (1H, br d, J ) 8.2 Hz), 5.20-5.08 (4H, m), 4.95 (2H, s),
4.64 (1H, m), 3.05-2.95 (2H, m); 13C NMR (100 MHz, CDCl3)
δ 170.8 (s), 156.5 (s), 155.6 (s), 140.7 (d), 136.2 (s), 136.0 (s),
134.7 (s), 128.8 (d), 128.8 (d), 128.7 (d), 128.5 (d), 128.5 (d),
128.5 (d), 128.4 (d), 128.3 (d), 128.1 (d), 91.1 (s), 74.4 (t), 67.6
(t), 67.2 (t), 54.8 (d), 36.5 (t) (1 coincident C-C); MS (ESI, +ve)
m/z 770 (M + Na+, 60%); HRMS (ESI, +ve) m/z calcd for (M
+ Na)+ 769.9870, found 769.9887.
N-Cbz-4-O-Benzyl-3-(N-Cbz-4′-O-benzyl-L-3′-tyrosyl ben-
zyl ester)-5-iodo-L-tyrosine Benzyl Ester 16. To a solution
of 3,5-diiodo-L-tyrosine derivative 12 (24.0 mg, 0.03 mmol), Pd-
(dppf)Cl2‚CH2Cl2 (2.36 mg), and potassium carbonate (22.2 mg,
0.15 mmol) in DMSO (1 mL) was added boronate derivative 8
(40.0 mg, 0.06 mmol). The mixture was stirred under nitrogen
at 80 °C for 18 h. A further portion of boronate derivative 8
(40.0 mg, 0.06 mmol) was then added and the reaction mixture
7360 J. Org. Chem., Vol. 70, No. 18, 2005

Synthesis of Dityrosine, Trityrosine, and Pulcherosine
stirred at 80 °C for 21 h. The resulting mixture was extracted
with EtOAc (2 × 30 mL) and the combined organic extracts
were washed with saturated sodium chloride solution (2 × 30
mL), dried (MgSO4), and concentrated in vacuo to give a dark
brown oil. The oil was chromatographed on silica, eluting with
EtOAc/hexane (1:2-1:1 gradient) to give iodo-dityrosine de-
rivative 16 (7.4 mg, 21%) as a pale yellow oil: [R]D +15.6 (c
0.54, CHCl3); IR (neat) υmax (cm-1) 3373, 2089, 1636, 1454,
1342, 1215, 1057, 694; 1H NMR (400 MHz, CDCl3) δ 7.54 (1H,
d, J ) 1.2 Hz), 7.28-7.12 (30H, m), 6.99-6.94 (3H, m), 6.78
(1H, dd, J ) 1.2, 5.8 Hz), 5.28 (1H, br d, J ) 7.9 Hz), 5.23
(1H, br d, J ) 9.7 Hz), 5.14-5.00 (8H, m), 4.90-4.89 (4H, m),
4.64 (1H, m), 4.37 (1H, m), 3.03-2.96 (4H, m); 13C NMR (100
MHz, CDCl3) δ 171.4 (s), 171.2 (s), 155.6 (s), 155.3 (s), 155.0
(s), 139.1 (d), 137.4 (s), 137.0 (s), 136.5 (s), 136.2 (s), 134.9 (s),
133.2(8) (d), 133.1(6) (s), 132.6 (d), 132.3 (d), 130.0 (d), 129.2
(d), 128.6(6) (d), 128.5(8) (s), 128.5(4) (d), 128.4(8) (s), 128.4(1)
(d), 128.3 (d), 128.1(4) (s), 128.1(0) (d), 127.9 (d), 127.6 (d), 127.4
(d), 126.9 (d), 126.6 (d), 113.0 (d), 92.9 (s), 74.9 (t), 70.3 (t),
67.3(7) (t), 67.2(6) (t), 67.1 (t), 67.0 (t), 54.8(7) (d), 54.8(0) (d),
37.3 (t), 37.0 (t) (2 coincident C-C and 6 C-H); MS (ESI, +ve)
m/z 1137 (M + Na+, 57%), 1011 (M - I + Na+, 100%).
6.69 (5H, m), 6.43 (1H, s), 5.24-5.19 (2H, br m), 5.04-4.81
(10H, m), 4.59-4.57 (2H, m), 3.01-2.85 (4H, m); 13C NMR (100
MHz, CDCl3) δ 171.3 (s), 171.2 (s), 155.6 (s), 154.8 (s), 154.3
(s), 151.8 (s), 139.2 (d), 136.2 (s), 135.0 (s), 132.9 (d), 132.7 (d),
130.5 (d), 129.5 (d), 129.4 (s), 128.6 (d), 128.5 (d), 128.4 (d),
128.3 (d), 128.1 (d), 128.0 (d), 127.1 (d), 127.0 (d), 126.9 (s),
125.6 (s), 114.1 (d), 86.1 (s), 71.1 (t), 67.3 (t), 67.2 (t), 67.0 (t),
66.4 (t), 55.8 (d), 55.0 (d), 37.7 (t), 36.9 (t) (4 coincident C-C
and 6 C-H); MS (ESI, +ve) m/z 1048 (M + Na+, 44%); HRMS
(ESI, +ve) m/z calcd for (M + Na)+ 1047.2325, found 1047.2314.
Trityrosine 3. To a solution of protected trityrosine deriva-
tive 23 (90.3 mg, 0.06 mmol) in H2O/MeOH/THF (1:5:5, 10 mL)
was added 10% palladium on charcoal (7.0 mg). The reaction
mixture was stirred under hydrogen at room temperature for
7 d. The resulting mixture was filtered through Celite, washed
with NH3(aq)/MeOH/CHCl3 (2:18:80, 3 × 20 mL), and concen-
trated in vacuo to give the deprotected trityrosine in quantita-
tive yield. For analytical purposes, the crude product was
chromatographed on reverse-phase HPLC using H2O/CH3CN/
TFA (95:5:0.05) as eluent to give pure trityrosine 3 as an off-
white solid: mp 152.5-154 °C; [R]D -15.7 (c 0.30, 1 M HCl);
1H NMR (400 MHz, D2O) δ 7.17 (2H, dd, J ) 2.2, 8.3 Hz), 7.13
(2H, d, J ) 2.2 Hz), 7.12 (2H, s), 6.95 (2H, d, J ) 8.3 Hz),
4.09-4.03 (3H, m, RH’s), 3.24-3.08 (6H, m, âH’s);33 13C NMR
(100 MHz, D2O) δ 173.1 (s), 173.0 (s), 152.6 (s), 149.9 (s), 132.5
(d), 132.0 (d), 130.7 (d), 127.1 (s), 126.8 (s), 126.5 (s), 125.3 (s),
117.8 (d), 55.4(0) (d), 55.3(9) (d), 35.1(2) (t), 35.0(6) (t); MS (ESI,
+ve) m/z 540 (M + H+, 90%).
N-Cbz-3-Iodo-L-tyrosine Benzyl Ester 30. To a solution
of silver sulfate (0.77 g, 2.47 mmol) and iodine (0.63 g, 2.47
mmol) in MeOH (60 mL) was added N-Cbz-L-tyrosine benzyl
ester 20 (1.00 g, 2.47 mmol). The mixture was stirred under
nitrogen in the dark at room temperature for 5 h. The yellow
precipitate was removed by filtration, a 1:1 mixture of satu-
rated sodium hydrogen carbonate solution and saturated
sodium chloride solution (100 mL) were added to the filtrate,
and the mixture was extracted with EtOAc (2 × 100 mL). The
combined organic extracts were washed with 50% saturated
sodium chloride solution (100 mL) and saturated sodium
thiosulfate solution (100 mL), dried (MgSO4), and concentrated
in vacuo to give a dark brown oil. The oil was chromatographed
on silica, eluting with ether/DCM/hexane (1:3:4) to give the
diiodo-tyrosine derivative 11 (0.21 g, 13%) and the iodo-
tyrosine derivative 30 (0.57 g, 43%) as a yellow solid: mp
131.5-133.0 °C; [R]D +11.9 (c 2.06, CHCl3); IR (Nujol) υmax
(cm-1) 3354, 1765, 1725, 1498, 1223, 1027, 697; 1H NMR (400
MHz, CDCl3) δ 7.39-7.27 (11H, m), 6.85 (1H, dd, J ) 2.0, 8.3
Hz), 6.76 (1H, d, J ) 8.3 Hz), 5.76 (1H, br s, OH), 5.35 (1H, br
d, J ) 8.0 Hz), 5.19-5.07 (4H, m), 4.65 (1H, m, RH), 3.04-
2.94 (2H, m, âH’s);34 13C NMR (100 MHz, CDCl3) δ 171.2 (s),
155.6 (s), 154.1 (s), 138.9 (d), 136.0 (s), 134.8 (s), 131.0 (d),
129.5 (s), 128.6 (d), 128.6 (d), 128.5 (d), 128.5 (d), 128.2 (d),
128.0 (d), 115.0 (d), 85.4 (q), 67.4 (t), 67.1 (t), 54.9 (d), 36.8 (t);
MS (EI) m/z 380 (M+• - CbzNH, 59%), 335 (13), 334 (21), 253
(53), 233 (M+• - CbzNHCHCO2Bn, 29), 207 (19), 91 (100).
N-Cbz-4-O-p-Methoxybenzyl-3-iodo-L-tyrosine Benzyl
Ester 31. To a solution of potassium carbonate (2.65 g, 19.2
mmol) and tetrabutylammonium iodide (0.18 g) in dry acetone
(100 mL) was added iodotyrosine derivative 30 (2.55 g, 4.80
mmol). The mixture was stirred under nitrogen at room
temperature for 30 min. p-Methoxybenzyl chloride (2.61 mL,
19.2 mmol) was added and the mixture was heated at reflux
in the dark for 24 h. The resulting mixture was cooled and
concentrated in vacuo. To the residue was added water (100
mL) and the mixture was extracted with DCM (3 × 200 mL).
The combined organic extracts were dried (Na2SO4) and
concentrated in vacuo to give a brown/maroon oil. The oil was
Potassium N-Cbz-4-O-Benzyl-L-tyrosyl-3-trifluorobo-
rate Benzyl Ester 21. To a solution of tyrosine boronate
derivative 8 (50.0 mg, 0.08 mmol) in MeOH (1 mL) was added
potassium hydrogen fluoride (4.5 M, 0.13 mL). The mixture
was stirred at room temperature for 2 h. The resulting mixture
was filtered and the solid washed with DCM and hot acetone.
The filtrate was concentrated in vacuo to give the potassium
trifluoroborate derivative 21 (48.0 mg, 100%) as white crystals;
[R]D +10.4 (c 0.95, CHCl3); IR (Neat) υmax (cm-1) 3416, 2359,
1
1717, 1645, 1452, 1211, 1018, 737, 696; H NMR (400 MHz,
CD3CN) δ 7.52-7.50 (3H, m), 7.38-7.28 (13H, m), 6.86 (1H,
dd, J ) 2.5, 8.3 Hz), 6.68 (1H, d, J ) 8.3 Hz), 6.05 (1H, br d,
J ) 8.1 Hz), 5.15-4.98 (6H, m), 4.45 (1H, m), 3.05 (1H, dd, J
) 5.5, 13.8 Hz), 2.86 (1H, dd, J ) 8.5, 13.8 Hz); 13C NMR (100
MHz, CD3CN) δ 173.1 (s), 161.5 (s), 157.1 (s), 139.7 (s), 138.0
(s), 136.9 (s), 135.4 (d), 129.5(2) (d), 129.4(7) (d), 129.4(1) (d),
129.2 (d), 129.0 (d), 128.9 (d), 128.7 (d), 112.8 (d), 75.4 (d), 70.4
(t), 67.6 (t), 67.2 (t), 57.0 (d), 41.0 (d), 37.8 (t), 25.2 (d) (1
coincident C-C) (C-B not observed); MS (ESI, -ve) m/z 563
([M - K]-, 100%), 1163 ([M2 - K]-, 20%); HRMS (ESI, -ve)
m/z calcd for (M - K)- 562.2010, found 562.1970.
Protected Trityrosine 23. To a solution of diiodo-tyrosine
derivative 11 (50.1 mg, 0.08 mmol) and tyrosyl-3-trifluorobo-
rate derivative 21 (91.7 mg, 0.15 mmol) in H2O:THF (1:10, 3
mL) was added potassium carbonate (31.6 mg, 0.23 mmol) and
Pd(dppf)Cl2‚CH2Cl2 (5.50 mg). The reaction mixture was
stirred under nitrogen at reflux for 26 h. The resulting mixture
was filtered through Celite, washed with DCM (3 × 20 mL),
and concentrated in vacuo to give a brown oil. The oil was
chromatographed on silica eluting with EtOAc/hexane (1:2-
1:1 gradient) to give protected trityrosine derivative 23 (70.8
mg, 67%) as an off-white solid: mp 55.5-58 °C; [R]D +5.78 (c
1.98, CHCl3); IR (Nujol) υmax (cm-1) 3346, 3032, 1720, 1499,
1456, 1340, 1213, 1057, 1026, 696; 1H NMR (400 MHz, CDCl3)
δ 7.35-7.16 (40H, m), 6.97-6.93 (6H, m), 6.82-6.79 (2H, m),
6.23 (1H, br s), 5.29-5.25 (3H, br m), 5.14-4.89 (16H, m),
4.68-4.65 (3H, m), 3.07-2.97 (6H, m); 13C NMR (100 MHz,
CDCl3) δ 171.4 (s), 162.9 (s), 155.6 (s), 154.7 (s), 150.2 (s), 136.7
(s), 136.2 (s), 135.9 (s), 135.1 (s), 133.5 (d), 133.1 (d), 132.1 (d),
131.8 (d), 129.5 (d), 128.9 (d), 128.7 (d), 128.5 (d), 128.4(2) (s),
128.3(8) (d), 128.2 (d), 128.1 (d), 128.0(3) (d), 127.9(6) (d), 127.8
(d), 127.7 (d), 126.9 (d), 126.6 (s), 120.8 (d), 114.0 (d), 111.4
(d), 71.0 (t), 67.3 (t), 67.2 (t), 67.0 (t), 66.9 (t), 55.7 (d), 55.0
(d), 37.9 (t), 37.4 (t) (4 coincident C-C); MS (ESI, +ve) m/z
1415 (M + Na+, 100%); HRMS (ESI, +ve) m/z calcd for (M +
Na)+, 1414.525, found 1414.526. Further elution gave the
iododityrosine derivative 22 (17.2 mg, 22%) as an off-white
solid: mp 59-61 °C; [R]D +7.04 (c 1.95, CHCl3); IR (Nujol) υmax
(cm-1) 3346, 3032, 1720, 1499, 1456, 1340, 1213, 1059, 1026,
696; 1H NMR (400 MHz, CDCl3) δ 7.46-7.10 (25H, m), 6.97-
(33) Nomura, K.; Suzuki, N.; Matsumoto, S. Biochemistry 1990, 29,
4525.
(34) Burke, T. R. Jr; Yao, Z.-J.; Zhao, H.; Milne, G. W. A.; Wu, L.;
Zhang, Z.-Y.; Voigt, J. H. Tetrahedron 1998, 54, 9981.
J. Org. Chem, Vol. 70, No. 18, 2005 7361

Skaff et al.
chromatographed on silica, eluting with EtOAc/hexane (1:4-
1:2 gradient) to give the protected iodo-tyrosine derivative 31
(2.39 g, 76%) as a pale yellow oil: [R]D +11.7 (c 0.78, CHCl3);
IR (Neat) υmax (cm-1) 3317, 2955, 2359, 1734, 1489, 1456, 1240,
N-Cbz-4-Iodo-L-phenylalanine Benzyl Ester 36. To an
ice-cold solution of potassium carbonate (3.33 g, 24.1 mmol)
in water (30 mL) was added 4-iodo-L-phenylalanine 35 (2.00
g, 6.88 mmol). A solution of 50% benzyl chloroformate in
toluene (2.36 mL, 7.06 mmol) was added over 30 min. The
mixture was stirred at room temperature for 6 h. The resulting
mixture was acidified to pH 4 with 3 M HCl, extracted with
i-PrOH/CHCl3 (1:3, 3 × 300 mL), dried (Na2SO4), and concen-
trated in vacuo to give Cbz-4-iodophenylalanine (2.77 g, 95%)
as an off-white solid. To a solution of potassium carbonate (1.31
g, 9.45 mmol) and tetrabutylammonium iodide (0.11 g) in dry
acetone (100 mL) was added Cbz-iodo-phenylalanine (2.68 g,
6.30 mmol). The mixture was stirred under nitrogen in the
dark at room temperature for 30 min. Benzyl bromide (1.50
mL, 12.6 mmol) was added and the mixture was heated at
reflux for 2 h. The resulting mixture was cooled and concen-
trated in vacuo. To the residue was added water (100 mL) and
the mixture was extracted with DCM (3 × 200 mL). The
combined organic extracts were dried (Na2SO4) and concen-
trated in vacuo to give a pale yellow oil. The oil was chro-
matographed on silica, eluting with EtOAc/hexane (1:4-1:3
gradient) to give protected iodophenylalanine derivative 36
(2.86 g, 86%) as a white solid: mp 89.3-90.1 °C; [R]D +4.00 (c
1.20, CHCl3); IR (Neat) υmax (cm-1) 3408, 2359, 1718, 1498,
1340, 1256, 1211, 1059, 1007, 696; 1H NMR (400 MHz, CDCl3)
δ 7.48 (2H, d, J ) 8.2 Hz), 7.38-7.24 (10H, m), 6.70 (2H, d, J
) 8.2 Hz), 5.28 (1H, br d, J ) 8.0 Hz), 5.17-5.04 (4H, m), 4.66
(1H, m, RH), 3.07-2.97 (2H, m, âH’s);35 13C NMR (100 MHz,
CDCl3) δ 171.1 (s), 155.5 (s), 137.6 (d), 136.2 (s), 135.3 (s), 134.9
(s), 131.3 (d), 128.7(1) (d), 128.7(0) (d), 128.6 (d), 128.3 (d), 128.1
(d), 92.6 (s), 67.4 (t), 67.1 (t), 54.6 (d), 37.8 (t) (1 coincident
C-H); MS (ESI, +ve) m/z 538 (M + Na+, 100%).
1
1173, 1026, 694; H NMR (400 MHz, CDCl3) δ 7.43 (1H, d, J
) 1.8 Hz), 7.29 (2H, d, J ) 8.5 Hz), 7.26-7.14 (10H, m), 6.82
(2H, d, J ) 8.5 Hz), 6.78 (1H, br d, J ) 8.4 Hz), 6.55 (1H, d, J
) 8.4 Hz), 5.24 (1H, br d, J ) 7.9 Hz), 5.25-4.99 (4H, m), 4.89
(2H, s), 4.54 (1H, m), 3.70 (3H, s), 2.94-2.84 (2H, m); 13C NMR
(100 MHz, CDCl3) δ 171.2 (s), 159.4 (s), 156.5 (s), 155.6 (s),
136.3 (s), 135.0 (s), 130.3 (d), 130.1 (s), 128.7 (d), 128.6 (d),
128.2 (d), 128.1 (d), 127.6 (s), 127.0 (d), 114.0 (d), 112.7 (d),
87.0 (s), 70.8 (t), 67.4 (t), 67.1 (t), 55.3 (q), 55.0 (d), 36.9 (t) (3
coincident C-H); MS (ESI, +ve) m/z 674 (M + Na+, 100%);
HRMS (ESI, +ve) m/z calcd for (M + Na)+ 674.1010, found
674.1017.
N-Cbz-4-O-p-Methoxybenzyl-L-tyrosyl-3-boronic Acid
Benzyl Ester 33. To a solution of bis(neopentylglycolato)-
diboron (1.55 g, 6.85 mmol), Pd(dppf)Cl2‚CH2Cl2 (84.0 mg), and
potassium acetate (1.35 g, 13.7 mmol) in DMSO (100 mL) was
added 3-iodotyrosine derivative 31 (2.23 g, 3.43 mmol). The
mixture was stirred under nitrogen at 80 °C for 6 h. Water
(100 mL) was added and the resulting mixture was extracted
with EtOAc (3 × 100 mL). The combined organic extracts were
washed with saturated sodium chloride solution (3 × 100 mL),
dried (Na2SO4), and concentrated in vacuo to give a dark brown
oil. The oil was chromatographed on silica, eluting with EtOAc/
hexane (1:2-1:1 gradient) to give a mixture of neopentylgly-
colyl boronate 32 and boronic acid 33 (1.71 g) as a pale yellow
oil. To a solution of the above mixture (1.71 g) in EtOAc (100
mL) was added water (250 mL). The biphasic reaction was
stirred at room temperature for 20 h. The organic extract was
washed with water (4 × 100 mL), dried (Na2SO4), and
concentrated in vacuo to give tyrosine boronic acid derivative
N-Cbz-4-Neopentylglycolylborono-L-phenylalanine Ben-
zyl Ester 37. To a solution of bis(neopentylglycolato)diboron
(2.18 g, 9.64 mmol), Pd(dppf)Cl2‚CH2Cl2 (0.12 g) and potassium
acetate (1.89 g, 19.3 mmol) in DMSO (100 mL) was added
N-Cbz-4-iodo-L-phenylalanine benzyl ester 36 (2.48 g, 4.82
mmol). The mixture was stirred under nitrogen at 80 °C for
4.5 h. Water (100 mL) was added and the resulting mixture
extracted with EtOAc (3 × 100 mL). The combined organic
extracts were washed with saturated sodium chloride solution
(3 × 100 mL), dried (Na2SO4), and concentrated in vacuo to
give a dark brown oil. The oil was chromatographed on silica,
eluting with EtOAc/hexane (1:3-1:1 gradient) to give neopen-
tylglycolyl boronate derivative 37 (2.21 g, 92%) as a white
solid: mp 118-119 °C; [R]D +8.22 (c 1.11, CHCl3); IR (Neat)
υmax (cm-1) 3343, 2961, 1720, 1610, 1498, 1420, 1317, 1248,
33 (1.73 g, 89%) as a pale yellow oil: IR (Neat) υmax (cm-1
)
3408, 2955, 1713, 1610, 1516, 1339, 1251, 1034, 698; 1H NMR
(400 MHz, CDCl3) δ 7.67 (1H, d, J ) 2.2 Hz), 7.37-7.28 (12H,
m), 7.12 (1H, dd, J ) 2.2, 8.4 Hz), 6.95 (2H, d, J ) 8.5 Hz),
6.85 (1H, d, J ) 8.4 Hz), 6.18 (2H, br s), 5.38 (1H, br d, J )
8.2 Hz), 5.18 (2H, m), 5.12 (2H, s), 5.02 (2H, s), 4.72 (1H, m),
3.84 (3H, s), 3.16-3.06 (2H, m); 13C NMR (100 MHz, CDCl3) δ
171.5 (s), 163.1 (s), 159.9 (s), 155.7 (s), 137.8 (s), 136.3 (d), 135.1
(d), 133.5 (s), 129.6 (d), 129.5 (d), 128.6(0) (d), 128.5(8) (s),
128.5(1) (d), 128.3 (s), 128.1(4) (d), 128.0(7) (d), 127.9(4) (d),
114.4 (d), 111.5 (d), 70.5 (t), 67.3 (t), 67.0 (t), 55.3 (d), 55.2 (q),
37.4 (t) (C-B not observed); MS (ESI, +ve) m/z 592 (M + Na+,
100%); HRMS (ESI, +ve) m/z calcd for (M + Na)+ 592.2131,
found 592.2134.
1
1132, 1057, 698; H NMR (400 MHz, CDCl3) δ 7.67 (2H, d, J
) 7.7 Hz), 7.31-7.23 (10H, m), 7.01 (2H, d, J ) 7.7 Hz), 5.35
(1H, br d, J ) 8.2 Hz), 5.19-5.02 (4H, m), 4.69 (1H, m), 3.72
(4H, s) 3.13-3.03 (2H, m), 0.97 (6H, s); 13C NMR (100 MHz,
CDCl3) δ 171.4 (s), 155.7 (s), 138.2 (s), 136.4 (s), 135.2 (s), 134.3
(d), 128.7 (d), 128.6 (d), 128.2 (d), 128.1 (d), 72.3 (t), 67.3 (t),
67.0 (t), 55.1 (d), 38.3 (t), 31.9 (s), 22.0 (q) (2 coincident C-H)
(C-B not observed); MS (ESI, +ve) m/z 524 (M + Na+, 59%),
457 (100%); HRMS (ESI, +ve) m/z calcd for (M + Na)+
524.2220, found 524.2210.
N-Cbz-4-O-p-Methoxybenzyl-L-dopa Benzyl Ester 34.
To a solution of tyrosine boronic acid derivative 33 (1.66 g,
2.91 mmol) in MeOH (35 mL) was added a solution of 30%
hydrogen peroxide in water (33 mL). The mixture was stirred
at room temperature for 30 min. The resulting mixture was
concentrated in vacuo. To the residue was added water (50
mL) and the mixture was extracted with DCM (3 × 50 mL).
The combined organic extracts were dried (Na2SO4) and
concentrated in vacuo to give a pale yellow oil. The oil was
chromatographed on silica, eluting with EtOAc/hexane (2:3)
to give dopa derivative 34 (1.28 g, 81%) as a pale yellow oil:
[R]D +4.88 (c 2.85, CHCl3); IR (Neat) υmax (cm-1) 3416, 1705,
1636, 1514, 1250, 1028, 696; 1H NMR (400 MHz, CDCl3) δ
7.34-7.28 (12H, m), 6.91 (2H, d, J ) 8.5 Hz), 6.74 (1H, d, J )
8.2 Hz), 6.65 (1H, d, J ) 2.1 Hz), 6.47 (1H, dd, J ) 2.1, 8.2
Hz), 5.61 (1H, br s), 5.24 (1H, br d, J ) 8.6 Hz), 5.13-5.08
(4H, m), 4.96 (2H, s), 4.65 (1H, m), 3.81 (3H, s), 3.02-2.99 (2H,
m); 13C NMR (100 MHz, CDCl3) δ 171.7 (s), 159.8 (s), 155.9
(s), 146.2 (s), 145.3 (s), 136.5 (s), 135.4 (s), 129.6 (d), 129.3 (s),
128.6(9) (d), 128.6(1) (d), 128.5(3) (d), 128.1(9) (d), 128.1(5) (d),
127.5 (d), 120.8 (d), 116.1 (d), 114.2 (d), 112.7 (d), 71.0 (t), 67.2
(t), 67.0 (t), 55.3 (q), 55.2 (d), 37.6 (t) (1 coincident C-C); MS
(ESI, +ve) m/z 564 (M + Na+, 100%); HRMS (ESI, +ve) m/z
calcd for (M + Na)+ 564.1993, found 564.1995.
N-Cbz-L-Phenylalanyl-4-boronic Acid Benzyl Ester 27.
To a solution of neopentylglycolyl boronate derivative 37 (2.00
g, 3.99 mmol) in EtOAc (60 mL) was added water (120 mL).
The biphasic reaction was stirred at room temperature for 3
d. The organic extract was washed with water (4 × 100 mL),
dried (Na2SO4), and concentrated in vacuo to give phenylala-
nine boronic acid derivative 27 (accompanied by varying
amounts of boroxine) as a pale yellow oil (0.87 g, 50%): IR
(Neat) υmax (cm-1) 3337, 1723, 1609, 1551, 1344, 1213, 1057,
1
696; H NMR (400 MHz, CDCl3) δ 7.58 (2H, d, J ) 7.9 Hz),
7.38-7.24 (10H, m), 7.02 (2H, d, J ) 7.9 Hz), 5.33 (1H, br d,
J ) 8.0 Hz), 5.16-5.07 (4H, m), 4.72 (1H, m), 3.29-3.09 (2H,
(35) Nakamura H.; Fujiwara M.; Yamamoto Y. Bull. Chem. Soc. Jpn.
2000, 73, 231. (b) Yao, Z.-J.; Gao, Y.; Burke, T. R., Jr. Tetrahedron:
Asymmetry 1999, 10, 3727.
7362 J. Org. Chem., Vol. 70, No. 18, 2005

Synthesis of Dityrosine, Trityrosine, and Pulcherosine
m); MS (ESI, +ve) m/z 456 (M + Na+, 15%), 872 (2M - OH +
Na+, 100%); HRMS (ESI, +ve) m/z calcd for (M + Na)+
456.1605, found 456.1598. Starting material 37 (0.53 g, 27%)
was also recovered.
(1H, br s), 5.30-5.25 (2H, m), 5.14-4.98 (8H, m), 4.66-4.56
(2H, m), 3.06-2.87 (4H, m); 13C NMR (100 MHz, CDCl3) δ
171.2 (s), 171.0 (s), 155.7 (s), 144.3 (s), 143.9 (s), 136.2 (s), 135.1
(s), 134.8 (s), 131.1 (s), 130.9 (d), 129.0 (s), 128.7 (s), 128.6(9)
(d), 128.6(5) (d), 128.6(0) (d), 128.5(6) (d), 128.4 (d), 128.2 (d),
128.1 (d), 128.0 (d), 119.4 (d), 117.9 (d), 109.9 (s), 67.4 (t), 67.3
(t), 67.0(7) (t), 67.0(1) (t), 55.7 (d), 54.9 (d), 37.3 (t), 37.1 (t) (5
coincident C-H and 2 C-C); MS (ESI, +ve) m/z 909 (79Br M
+ Na+, 85%), 911 (81BrM + Na+, 100%); HRMS (ESI, +ve) m/z
calcd for (M + Na)+ 909.1993, found 909.1988.
PMB-Protected Isodityrosine 38. To a solution of copper-
(II) acetate (0.32 g, 1.77 mmol), pyridine (0.71 mL, 8.85 mmol),
and 4 Å molecular sieves (5.00 g) in DCM (35 mL) was added
dopa derivative 34 (0.96 g, 1.77 mmol) and phenylalanine
boronic acid derivative 27 (1.09 g, 2.48 mmol). The mixture
was stirred under nitrogen at room temperature for 48 h. The
resulting mixture was filtered and the solid washed with DCM
(2 × 30 mL). The filtrate was concentrated in vacuo to give a
green oil. The oil was chromatographed on silica, eluting with
EtOAc/hexane (1:2-1:1 gradient) to give the protected isodi-
tyrosine derivative 38 (1.31 g, 80%) as a yellow oil: [R]D +7.48
(c 1.23, CHCl3); IR (Neat) υmax (cm-1) 3354, 1720, 1506, 1250,
Protected Pulcherosine 41. To a solution of tyrosine
trifluoroborate derivative 21 (0.13 g, 0.21 mmol), triethylamine
(52.0 µL, 0.36 mmol), and Pd(dppf)Cl2‚CH2Cl2 (5.00 mg) was
added bromoisodityrosine derivative 40 (55.2 mg, 0.06 mmol)
in H2O/i-PrOH (1:2, 4 mL). The mixture was stirred at reflux
for 48 h. The resulting mixture was filtered through Celite
and the solid washed with DCM (3 × 30 mL). The filtrate was
concentrated in vacuo to give a dark brown oil. The oil was
chromatographed on silica, eluting with EtOAc/hexane (1:2-
2:3 gradient) to give pulcherosine derivative 41 (26.1 mg, 32%)
as a pale yellow oil: [R]D +8.25 (c 1.63, CHCl3); IR (Neat) υmax
(cm-1) 3356, 2922, 1734, 1498, 1215, 1057, 696; 1H NMR (400
MHz, CDCl3) δ 7.24-7.07 (35H, m), 6.98 (1H, s), 6.90 (1H, d,
J ) 8.0 Hz), 6.80-6.78 (3H, m), 6.72-6.67 (3H, m), 6.54 (1H,
s), 5.82 (1H, br s), 5.24 (1H, br d, J ) 7.9 Hz), 5.15-5.13 (2H,
m), 5.05-4.90 (14H, m), 4.58-4.54 (3H, m), 3.00-2.80 (6H,
m); 13C NMR (100 MHz, CDCl3) δ 171.4 (s), 171.3 (s), 156.5
(s), 155.6 (s), 154.7 (s), 144.7 (s), 143.9 (s), 136.7 (s), 136.2 (s),
135.1 (s), 132.8 (d), 130.6 (d), 130.1 (d), 130.0 (d), 128.7 (d),
128.6 (d), 128.5(3) (d), 128.4(9) (d), 128.4 (s), 128.3 (d), 128.2
(d), 128.1(3) (d), 128.0(5) (d), 127.9(7) (d), 127.8(5) (d), 126.9
(d), 119.9 (d), 117.7 (d), 113.8 (d), 71.0 (t), 67.3 (t), 67.2 (t),
67.0 (t), 54.9 (d), 37.5 (t), 37.3 (t) (27 coincident carbons; 1 Cd
O, 20 aryl, 6 aliphatic); MS (ESI, +ve) m/z 1325 (M + Na+,
100%); HRMS (ESI, +ve) m/z calcd for (M + Na)+ 1324.4778,
found 1324.4824.
Pulcherosine 5. To a solution of protected pulcherosine
derivative 41 (51.8 mg, 0.04 mmol) in H2O/MeOH/THF (1:5:5,
5 mL) was added 10% palladium on charcoal (5.0 mg). The
reaction mixture was stirred under hydrogen at room tem-
perature for 7 d. The resulting mixture was filtered through
Celite, washed with NH3(aq)/MeOH/CHCl3 (2:18:80, 3 × 20 mL),
and concentrated in vacuo to give pulcherosine 5 in quantita-
tive yield. For analytical purposes, the crude product was
chromatographed on reverse-phase HPLC using H2O/CH3CN/
TFA (95:5:0.05) as eluent to give pure pulcherosine 5 as an
off-white solid: mp 170-172 °C; [R]D +19.9 (c 0.33, 1 M HCl);
1H NMR (400 MHz, D2O) δ 7.17 (2H, d, J ) 8.6 Hz), 7.11 (1H,
dd, J ) 2.2, 8.3 Hz), 7.05 (1H, d, J ) 2.2 Hz), 6.92 (2H, d, J )
8.6 Hz), 6.88 (1H, d, J ) 8.3 Hz), 6.86 (1H, d, J ) 2.1 Hz),
6.84 (1H, d, J ) 2.1 Hz), 3.95-3.87 (3H, m), 3.16-2.93 (6H,
m);33 13C NMR (100 MHz, D2O) δ 173.2 (s), 173.0 (s), 156.6 (s),
152.5 (s), 144.5 (s), 144.1 (s), 132.2 (d), 130.9 (d), 130.7 (d),
129.6 (d), 127.8 (s), 127.6 (s), 127.4 (s), 126.8 (s), 124.9 (s), 121.0
(d), 117.8 (d), 116.3 (d), 55.7 (d), 55.6 (d), 35.4 (t), 35.3 (t), 35.2
(t); MS (ESI, +ve) m/z 562 (M + Na+, 100%).
1
1028, 696; H NMR (400 MHz, CDCl3) δ 7.31-7.21 (20H, m),
7.10 (2H, d, J ) 8.5 Hz), 6.89-6.78 (5H, m), 6.75-6.70 (4H,
m), 5.26-5.21 (2H, m), 5.16-5.00 (8H, m), 4.93 (2H, s), 4.69-
4.61 (2H, m), 3.74 (3H, s), 3.04-2.93 (4H, m); 13C NMR (100
MHz, CDCl3) δ 171.3(4) (s), 171.2(6) (s), 159.4 (s), 157.4 (s),
155.6(8) (s), 155.6(3) (s), 149.7(8) (s), 149.7(5) (s), 145.2 (s),
136.3 (s), 135.1 (s), 135.0 (s), 130.4 (d), 129.3 (s), 129.1 (s),
128.8(6) (s), 128.8(0) (d), 128.6(7) (d), 128.6(4) (d), 128.6(1) (d),
128.5(7) (d), 128.5(5) (d), 128.5(0) (d), 128.4(0) (d), 128.2(2) (d),
128.2(0) (d), 128.1(1) (d), 125.8 (d), 122.8 (d), 116.9 (d), 115.5
(d), 113.9 (d), 70.8 (t), 67.3(6) (t), 67.2(9) (t), 67.2(6) (t), 67.0
(t), 55.3 (q), 54.9(5) (d), 54.9(2) (d), 37.4(8) (t), 37.4(0) (t) (2
coincident C-H); MS (ESI, +ve) m/z 951 (M + Na+, 100%);
HRMS (ESI, +ve) m/z calcd for (M + Na)+ 951.3464, found
951.3467.
Protected Isodityrosine 39. To a solution of PMB-
protected isodityrosine derivative 38 (43.2 mg, 0.05 mmol) in
H2O/DCM (1:10, 2 mL) was added DDQ (12.7 mg, 0.06 mmol).
The mixture was stirred at reflux for 24 h. Saturated sodium
hydrogen carbonate solution (30 mL) was added and the
resulting mixture extracted with DCM (3 × 30 mL). The
combined organic extracts were washed with saturated sodium
hydrogen carbonate solution (30 mL) and saturated sodium
chloride solution (30 mL), dried (Na2SO4), and concentrated
in vacuo to give a yellow oil. The oil was chromatographed on
silica, eluting with EtOAc/hexane (1:2-2:3 gradient) to give
the deprotected product 39 (29.5 mg, 78%) as an off-white
solid: mp 50-51 °C; [R]D +6.02 (c 1.05, CHCl3); IR (Neat) υmax
(cm-1) 3346, 1718, 1506, 1437, 1217, 1057, 696; 1H NMR (400
MHz, CDCl3) δ 7.25-7.10 (20H, m), 6.81-6.78 (3H, m), 6.66
(2H, d, J ) 8.5 Hz), 6.60 (1H, dd, J ) 1.8, 8.2 Hz), 6.47 (1H,
d, J ) 1.8 Hz), 5.54 (1H, br s), 5.18-5.16 (2H, br m), 5.09-
4.90 (8H, m), 4.57-4.48 (2H, m), 2.96-2.82 (4H, m); 13C NMR
(100 MHz, CDCl3) δ 171.2 (s), 171.0 (s), 155.9 (s), 155.6(4) (s),
155.5(9) (s), 146.8 (s), 143.1 (s), 136.2 (s), 135.1 (s), 135.0 (s),
130.8 (d), 128.7(7) (d), 128.6(7) (d), 128.6(4) (d), 128.5(6) (d),
128.4(6) (d), 128.2(5) (d), 128.2(1) (d), 128.1(4) (d), 128.0(6) (d),
128.0(0) (s), 125.9 (d), 120.1 (d), 117.7 (d), 116.4 (d), 113.9 (d),
67.3(3) (t), 67.2(6) (t), 67.0(3) (t), 67.0(0) (t), 55.2 (d), 54.9 (d),
37.5 (t), 37.3 (t) (2 coincident C-H and 2 C-C); MS (ESI, +ve)
m/z 831 (M + Na+, 100%); HRMS (ESI, +ve) m/z calcd for (M
+ Na)+ 831.2889, found 831.2915.
Bromoisodityrosine Derivative 40. To a solution of
N-bromosuccinimide (54.0 mg, 0.30 mmol) in DMF (4 mL) was
added isodityrosine derivative 39 (0.16 g, 0.20 mmol). The
mixture was stirred under nitrogen at 80 °C for 19 h. Ether
(30 mL) was added and the organic extract washed with
saturated sodium thiosulfate solution (30 mL), water (30 mL),
dried (Na2SO4), and concentrated in vacuo to give a yellow oil.
The oil was chromatographed on silica, eluting with EtOAc/
hexane (1:2-2:3 gradient) to give the brominated product 40
(0.13 g, 70%) as a pale yellow oil: [R]D +11.3 (c 0.87, CHCl3);
IR (Neat) υmax (cm-1) 3354, 2916, 2849, 1718, 1497, 1213, 1059,
696; 1H NMR (400 MHz, CDCl3) δ 7.36-7.26 (18H, m), 7.21-
7.18 (2H, m), 7.00 (1H, d, J ) 1.6 Hz), 6.90 (2H, d, J ) 8.4
Hz), 6.73 (2H, d, J ) 8.4 Hz), 6.51 (1H, d, J ) 1.6 Hz), 5.77
Acknowledgment. This work was supported by the
Australian Research Council (DP0208190) and a Henry
Bertie and Florence Mabel Gritton Research Scholar-
ship to O.S. The authors thank Woan Mei Kok for the
preparation of starting materials.
Supporting Information Available: General procedures
and experimental methods for compounds 13, 15, 18, 19, 24
1
and 29; H and 13C NMR spectra for compounds 1, 3, 5, 7-9,
11, 12, 16, 21, 23, 27, 31, 33, 34, and 36-41; 13C DEPT NMR
spectra for compounds 9, 23, 41. This material is available
free of charge via the Internet at http://pubs.acs.org.
JO051076M
J. Org. Chem, Vol. 70, No. 18, 2005 7363