Oxidation of threonine residues with IBX reagents

Article abstract of DOI:10.1016/j.tetlet.2009.03.107

The treatment of protected Thr peptides with excess polymer-bound IBX effects sequential oxidation of the Thr secondary alcohol to the ketone, followed by hydroxylation of the alpha carbon of the Thr residues with loss of stereochemistry at this position.

Full text of DOI:10.1016/j.tetlet.2009.03.107

Tetrahedron Letters 50 (2009) 2601–2604
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Oxidation of threonine residues with IBX reagents
*
P. Manohari Abeysinghe, Yu Han, Margaret M. Harding
School of Chemistry, The University of New South Wales, Sydney, NSW 2052, Australia
a r t i c l e i n f o
a b s t r a c t
Article history:
The treatment of protected Thr peptides with excess polymer-bound IBX effects sequential oxidation of
the Thr secondary alcohol to the ketone, followed by hydroxylation of the alpha carbon of the Thr resi-
dues with loss of stereochemistry at this position.
Received 29 January 2009
Revised 3 March 2009
Accepted 16 March 2009
Available online 20 March 2009
Ó 2009 Elsevier Ltd. All rights reserved.
Keywords:
o-Iodoxybenzoic acid (IBX)
Threonine oxidation
Antifreeze proteins
O
O
The hypervalent iodine agent o-iodoxybenzoic acid 1 (IBX) is a
mild oxidising agent that has been applied widely in organic chem-
istry1 since it was reported to oxidise alcohols in 1994.2 The scope
of the reagent has been significantly extended with the develop-
ment of both water-soluble derivatives3 and several polymer-
bound IBX reagents.4,5 The reaction mechanism is highly depen-
dent on the specific IBX reagent employed and solvent effects are
important in the selectivity of the oxidations and the type of oxida-
tion products that are formed.1,3,6,7
While IBX and its derivatives have been used to oxidise a wide
variety of substrates including alcohols, silyl enol ethers, amines
and amides,1–7 there are limited applications of these reagents in
peptide chemistry,5 and there are no examples of the use of these
reagents to oxidise the side chains in Ser or Thr in either amino
acids or peptides. The incorporation of an aldehyde or ketone into
a peptide chain is an important reaction as it allows selective func-
tionalisation at a single site by the formation of Schiff base deriva-
tives. Bertozzi and co-workers have elegantly demonstrated this
approach with site-specific introduction of a carbonyl group into
peptides and proteins via solid-phase peptide synthesis or enzy-
matic oxidation of glycosidated residues.8 For example, the unnat-
ural amino acid 29 was introduced into peptides via solid-phase
synthesis followed by reaction with the amino-oxy-sugar deriva-
tive of GalNAc 38,10 to give stable oxime derivatives that were bio-
logically active and hence acted as excellent mimics of natural
glycosidated peptides.8
OH
O
I
OH
HO
HO
Me
OH
O
O
Fmoc
N
H
AcHN
ONH
2
O
1
2
3
As part of our studies on the design and synthesis of mimics of
naturally occurring fish antifreeze glycoproteins (AFGPs) (Fig. 1a),
we were interested in the application of IBX reagents to the oxida-
tion of the side-chain of Thr residues in small peptides and build-
ing blocks suitable for the assembly of polypeptides via chemical
ligation.11 The direct introduction of the ketone functional group
would allow easy addition of the GalNAc sugar via reaction with
the amino-oxo sugar 3 to give AFGP mimics (Fig. 1b). The design
of AFGP mimics is only possible now due to recent structure–activ-
ity studies of AFGPs that have identified the critical requirement
for the Thr-
c-methyl group and the presence of the GalNAc sugar
as an
a
-glycoside.12 In addition, oxidation of Thr side chains would
allow direct entry to a range of derivatives of peptides by the addi-
tion of different nucleophiles, which is of interest for both the
introduction of labels into peptides and proteins as well as in com-
binatorial chemistry for the introduction of diversity into peptide
libraries.
In this Letter we report the application of polymer-bound IBX
(pIBX) to the oxidation of protected Thr and Thr side chains in
polypeptides. While the desired side chain oxidation occurred,
unexpected oxidation at the Thr
a-C was also observed to provide
a new class of peptide derivatives.
pIBX is an attractive reagent for applications with peptides as
removal of the resin following the reaction should allow access
to pure product requiring minimal purification.4 While oxidation
* Corresponding author. Tel.: +61 293855638; fax: +61 2 93856545.
E-mail address: harding@unsw.edu.au (M.M. Harding).
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.03.107

2602
P. M. Abeysinghe et al. / Tetrahedron Letters 50 (2009) 2601–2604
a
OH
OH
b
OH
HO
HO
HO
O
HO
HO
O
O
n = 4 - 50
OH
O
OH
AcHN
AcHN
O
O
O
H
N
N
H N
2
N
H
O
H
N
O
O
n
N
H
N
n
Ala
Ala
Thr
H
O
O
Figure 1. (a) Structure of naturally occurring AFGPs; (b) proposed AFGP mimic.
of the side chain of Thr has been reported using a variety of condi-
tions13 including Dess–Martin periodinane, CrO3, PCC and dimethyl
dioxirane, these reagents have specific solvent requirements that
are generally not suitable for peptides or for the very hydrophobic
peptide backbone present in our target AFGP mimics (Fig. 1b).
The optimal reaction conditions for oxidation of the secondary
alcohol side chain in Thr to the corresponding ketone with pIBX
were determined using a simple NMR assay with Z-Thr as the sub-
strate and MeCN-d3 as solvent. The progress of the reaction was
monitored by the disappearance of the Me doublet in Z-Thr at
1.17 ppm and by the appearance of the ketone Me singlet at
2.37 ppm. In the presence of 2 equiv of pIBX at room temperature,
no oxidation occurred after 24 h. Increased equivalents of pIBX
(4 equiv) resulted in formation of the ketone but significant
amounts of unreacted alcohol remained (3:1). Quantitative conver-
sion of Z-Thr to the ketone required 10 equiv of pIBX and heating at
45 °C for 2.5 days. The reaction tolerated the presence of cosolvents
DME or DMF (25%), but required significantly longer reaction times
of up to 3–4 days. However, the addition of water (30–50%)
resulted in no reaction due to inhibition of polymer swelling.5
The optimised conditions (10 equiv pIBX, MeCN, 45 °C, 2 d)
were applied to the methyl ester 4 and amide 7 protected Thr
derivatives and resulted in complete oxidation of the Thr side
chain in both cases (Scheme 1). However, the elevated temperature
resulted in some minor leaching of the polymer resin into the solu-
tion, as evidenced by the appearance of aliphatic multiplets in the
NMR spectrum of the crude products, requiring purification by
chromatography.
In the case of the methyl ester 4, analysis of the product showed
none of the desired material 5, but instead the oxidised product 6
was formed (Scheme 1). 1H NMR analysis showed the absence of
the characteristic Thr H-a proton expected in 5. In addition, a sig-
nal that integrated for 1H and exchanged with D2O was present at
5.31 ppm. Keto-enol tautomerism has been reported in a number
of related peptides and amino acid derivatives and could explain
these properties.14 However, the 13C NMR spectrum of 6 showed
clear evidence for the new ketone carbonyl at 197.9 ppm and no
evidence for a possible enol tautomer of 5. Changing the solvent
from MeCN to MeOH did not change the appearance of the spec-
trum consistent with the absence of tautomerism. The high-resolu-
tion mass spectrum of the product identified that an additional
oxygen atom was present, and hence all data were consistent with
the formation of the
a
-substituted product 6.15 This assignment
was confirmed by DEPT and HSQC spectra.
On one occasion where the reaction had not gone to completion,
analysis of the Me signals in the crude product was consistent with
formation of the desired ketone 5 as a very minor product (<10%)
along with 6. Both compounds had very similar Rf values and the
low yield of 5 made isolation difficult.16
Similar oxidation products were obtained with the amide deriv-
ative 7. Thus, treatment of the amide 7 with 10 equiv of pIBX in
MeCN resulted in formation of the corresponding
a-hydroxy-
substituted ketone 9. However, compared to the reaction with es-
ter 4, a significantly higher amount of the desired ketone 8 was
formed (25%) and while both 8 and 9 had similar polarities, both
products were isolated and fully characterised.17 The pure ketone
polymer IBX
or
Me
O
Me
OH
Me
O
IBX
O
O
O
OH
COR
MeCN
BnO
N
H
BnO
N
H
BnO
N
H
COR
COR
6 R = OMe (87% from 4)
9 R = NHBn (51% from 7)
4 R = OMe
7 R = NHBn
5 R = OMe
8
R = NHBn
PhCH2NH2
CH2Cl2
Ph
H
Me
N
Ph
NaCNBH3
EtOH
Me
N
O
O
BnO
11
N
H
BnO
N
H
COR
COR
10 R = NHBn
R = NHBn
Scheme 1.

P. M. Abeysinghe et al. / Tetrahedron Letters 50 (2009) 2601–2604
2603
Me Me
Me Me
N
polymer IBX
O
O
H
H
N
COOMe
N
COOMe
OH
BnO
N
H
BnO
R
H
O
MeCN
O
Me
O
Me
13 R = OH 14 R = H
ONH
12
2
, HCl, DMSO
COOMe
Me Me
N
Me Me
N
O
O
H
N
H
N
COOMe
OH
COOMe
+
BnO
BnO
H
O
H
O
Me N O
COOMe
Me N O
COOMe
15 (38%)
16 (10%)
Scheme 2.
8 was converted to the imine 10 (d CH3C@N 2.16 ppm) which was
reduced to the amine 11 with sodium cyanoborohydride.
The results of treatment of both 4 and 7 with pIBX were consis-
tent with initial oxidation of the Thr side chain, followed by oxida-
There was no evidence for oxidation at the C-
Val residue, confirming that the
C- of Thr.
Oxidation of Z-Ala-Ala-Thr-OBn 17 (Scheme 3), the tripeptide
a
position of the
a
-hydroxylation occurs only at
a
tion at the C-
a
position to give 6 and 9, respectively, due to the
building block present in AFGPs (Fig. 1) was performed in DMF/
CH2Cl2 (4:1), due to the poor solubility of 17 in all organic solvents
except DMF and DMSO. While 20 equiv of pIBX was required,20
clear evidence for the formation of 18 as a mixture of diastereo-
mers was obtained by NMR spectroscopy and high-resolution mass
spectrometric analysis of the product.21
Hypervalent iodine reagents, including IBX and Dess–Martin
periodinane, have been reported to oxidise a range of substrates
including amides into imides and secondary amides to imines.7
The mechanisms of these reactions are dependent on the exact
nature of the oxidising agent and reaction conditions, and include
single-electron transfer processes as well as ionic pathways.6,7
Scheme 4 shows a proposed mechanism for the products observed
excess pIBX used in the reaction. The two stepwise oxidation reac-
tions (Scheme 1) were confirmed by monitoring the reaction by 1H
NMR spectroscopy with a gradual increase in the number of equiv-
alents of pIBX. All attempts to optimise the reaction conditions
to produce the ketones 5 and 8 as the major products were
unsuccessful.
For comparison, the oxidations were also performed with IBX.
As expected, similar products were obtained but additional purifi-
cation was required compared with pIBX to remove the excess re-
agent and by-products.
In order to establish whether the oxidation at the C-a positions
of 5 and 8 was specific for Thr and to investigate the stereochem-
istry of the reaction, the oxidation conditions were applied to Z-
Val-Thr-OMe 12 (Scheme 2). The initial oxidation product(s), pre-
sumably 13 and 14 were not isolated, but the mixture was directly
treated with methyl p-aminooxybenzoate18 to form the corre-
sponding oximes 15 and 16, which were purified by chromatogra-
in this study. The oxidation only at the
a-C of Thr residues is con-
sistent with the presence of a highly acidic C-
to two carbonyl groups once the side chain is oxidised to the
a
Àproton, which is
a
ketone. A recent study has reported a similar IBX-mediated
a
-hydroxylation of a
-alkynyl carbonyl systems.22 The alkynyl
phy.19 As expected, the Thr H-
in 16, while doubling of one of the methyl ester signals and the Val
Me signals was consistent with loss of stereochemistry at the
carbon and the formation of 15 as a mixture of diastereomers.
a
signal appeared as a clean doublet
group was essential for the rapid and clean oxygen transfer from
IBX at room temperature.
In summary, oxidation of Thr-containing protected peptides
and polypeptides with excess pIBX provides entry to the corre-
a
-
Me
HO
Me
O
polymer IBX
O
Me
O
O
Me
O
H
N
H
N
OH
BnO
N
H
N
H
BnO
N
H
N
H
COOBn
DMF/CH Cl
COOBn
O
Me
O
Me
2
2
(4:1)
17
18 (95%)
Scheme 3.

2604
P. M. Abeysinghe et al. / Tetrahedron Letters 50 (2009) 2601–2604
P. S.; Zhong, Y. L. J. Am. Chem. Soc. 2002, 124, 2245–2258; (f) Nicolaou, K. C.;
O
O
Gray, D. L. F.; Montagnon, T.; Harrison, S. T. Angew. Chem., Int. Ed. 2002, 41, 996–
1000.
7. (a) Nicolaou, K. C.; Mathison, C. J. N. Angew. Chem., Int. Ed. 2005, 44, 5992–5997;
(b) Nicolaou, K. C.; Mathison, C. J. N.; Montagnon, T. J. Am. Chem. Soc. 2004, 126,
5192–5201.
8. (a) Grogan, M. J.; Pratt, M. R.; Marcaurelle, L. A.; Bertozzi, C. R. Ann. Rev.
Biochem. 2002, 71, 593–634; (b) Hang, H. C.; Bertozzi, C. R. Acc. Chem. Res. 2001,
34, 727–736.
9. (a) Marcaurelle, L. A.; Bertozzi, C. R. Tetrahedron Lett. 1998, 39, 7279–7282; (b)
Marcaurelle, L. A.; Rodriguez, E. C.; Bertozzi, C. R. Tetrahedron Lett. 1998, 39,
8417–8420.
10. Marcaurelle, L. A.; Shin, Y.; Goon, S.; Bertozzi, C. R. Org. Lett. 2001, 3, 3691–
3694.
11. (a) Haase, C.; Seitz, O. Angew. Chem., Int. Ed. 2008, 47, 1553–1556; (b) Besret, S.;
Ollivier, N.; Blanpain, A.; Melnyk, O. J. Peptide Sci. 2008, 14, 1244–1250; (c)
Pentelute, B. L.; Gates, Z. P.; Dashnau, J. L.; Vanderkooi, J. M.; Kent, S. B. H. J. Am.
Chem. Soc. 2008, 130, 9702–9707; (d) Blanco-Canosa, J. B.; Dawson, P. E. Angew.
Chem., Int. Ed. 2008, 47, 6851–6855.
O
HO
O
O
O
I
I
OH
H
HO
O
O
O
H
O
BnO
N
O
BnO
N
H
R
O
R
Me
Me
O
-H O
2
12. Tachibana, Y.; Fletcher, G. L.; Fujitani, N.; Tsuda, S.; Monde, K.; Nishimura, S.-I.
Angew. Chem., Int. Ed. 2004, 116, 856–862.
Me
Me
O
O
R
O
O
13. (a) Benovsky, P.; Stephenson, G. A.; Stille, J. R. J. Am. Chem. Soc. 1998, 120, 2493–
2500; (b) Wipf, P.; Cunningham, A.; Rice, R. L.; Lazo, J. S. Bioorg. Med. Chem.
1997, 5, 165–177; (c) Crescenza, A.; Botta, M.; Corelli, F.; Santini, A.; Tafi, A. J.
Org. Chem. 1999, 64, 3019–3025; (d) Burtscher, P.; Kohler, B.; Metz, Y.; Voges,
R.; Wenger, R. J. Labelled Compd. Radiopharm. 2000, 43, 201–216; (e) Saladino,
R.; Mezzetti, M.; Mincione, E.; Torrini, I.; Paradisi, M. P.; Mastropietro, G. J. Org.
Chem. 1999, 64, 8468–8474.
O
O
BnO
N
H
BnO
N
O
OH
R
H
14. (a) Wipf, P.; Miller, C. P. J. Org. Chem. 1993, 58, 3604–3606; (b) Christensen, C.;
Tornoe, C. W.; Meldal, M. QSAR Comb. Sci. 2004, 23, 109–116; (c) Stachulski, A.
V. Tetrahedron Lett. 1982, 23, 3789–3790; (d) Fehrentz, J. A.; Paris, M.; Heitz, A.;
Velek, J.; Winternitz, F.; Martinez, J. J. Org. Chem. 1997, 62, 6792–6796; (e)
Ganneau, C.; Moulin, A.; Demange, L.; Martinez, J.; Fehrentz, J. A. J. Peptide Sci.
2006, 12, 497–501.
H
+
O
O
= polymer
I
15. Selected data for 6: Purified by flash column chromatography on silica using a
gradient of EtOAc in hexane (10:90)?(60:40); yield of ketone 6 (45 mg, 87%);
mp 66–68 °C; 1H NMR (300 MHz, CDCl3): d 7.50–7.20 (5H, m, ArH), 6.71 (1H, br
s, NH), 5.31(1H, br s, OH, D2O exchangeable), 5.10 (2H, s, ArCH2), 3.82 (3H, s,
OCH3), 2.24 (3H, s, CH3C(O)). 13C NMR (75 MHz, CDCl3): d 197.9 (CH3C(O)),
167.8 (C(O)OCH3), 154.5 (C(O)NH), 135.4, 128.5, 128.3, 128.1 (CAr), 83.9
(NC(OH)C(O)), 67.4 (OCH2Ar), 54.1 (OCH3), 23.0 (CH3). MS (ESI) m/z 304
(M+Na+, 100%). HRMS: calcd for C13H15NO6Na 304.0792 found 304.0797.
16. Four cycles of column chromatography on silica using Et2O:EtOAc:hexane
(6:3:1) permitted isolation of a small amount of 5 for characterisation. 1H NMR
(300 MHz, CDCl3): d 7.50–7.20 (5H, m, ArH), 5.96 (1H, d, J 5.7 Hz, NH), 5.20–
5.00 (3H, m, ArCH2, CH), 3.81 (3H, s, OCH3), 2.37 (3H, s, CH3C(O)). (ESI) m/z 288
(M+Na+, 100%). HRMS: calcd for C13H15NO5Na 288.0848, found 288.0844.
17. Selected data for 8 and 9: ketone 9 (47 mg, 51%); mp 95–97 °C; 1H NMR
(300 MHz, CDCl3): d 7.50–7.20 (10H, m, ArH), 6.92 (1H, br s, NHCH2Ar), 6.68
(1H, br s, NHC(OH)C(O)), 5.42 (1H, br s, OH), 5.11 (2H, s, ArCH2), 4.42 (2H, d, J
5.6 Hz, NHCH2Ar), 2.28 (3H, s, CH3C(O)). 13C NMR (75 MHz, CDCl3): d 201.5
(CH3C(O)), 165.7 (C(O)NH), 136.6, 135.5, 128.7, 128.5, 128.3, 128.1, 127.8,
127.4 (CAr), 85.8 (NHC(OH)), 67.4 (OCH2Ar), 44.1 (NHCH2Ar), 22.8 (CH3). (ESI)
m/z 379 (M+Na+, 100%). HRMS: calcd for C19H20N2O5Na 379.1270, Found
O
HO
Scheme 4.
sponding keto-peptides, which are hydroxylated at the Thr-a C po-
sition. Structure–activity studies of AFGPs have indicated that
modification of the peptide backbone is tolerated,12 and hence
testing of AFGP mimics that are hydroxylated at the a-C of the gly-
cosidated Thr residues may lead to novel new antifreeze com-
pounds. Oxidation of Thr residues in the first generation library
of Thr-containing peptides with pIBX may also be useful in combi-
natorial chemistry. The reaction provides a mechanism to intro-
duce another level of diversity into the peptide library via
a-hydroxylation. In addition, the ke-
tones can be further derivatised by reactions with Schiff bases to
introduce additional diversity into the library.
reaction of the ketone and
379.1271. Ketone
8 isolated as a
colourless oil (22 mg, 25%); 1H NMR
(300 MHz, CDCl3) d 7.50–7.20 (10H, m, ArH), 6.70 (1H, br s, NH), 6.26 (1H, br
s, NH), 5.12 (2H, s, ArCH2), 4.85 (1H, d, J 5.3 Hz, CH), 4.42 (2H, d, J 5.6 Hz,
NHCH2Ar), 2.35 (3H, s, CH3). 13C NMR (75 MHz, CDCl3): d 203.4 (CH3C(O)),
165.2 (C(O)NH), 156.7 (C(O)NH), 137.5, 136.2, 129.2, 129.0, 128.8, 128.6, 128.1,
128.0 (CAr), 68.2 (OCH2Ar), 65.8 (CH), 44.3 (NHCH2Ar), 27.8 (CH3). (ESI) m/z 363
(M+Na+, 100%). HRMS: calcd for C19H20N2O4Na 363.1321, found 363.1314.
18. (a) Petrassi, H. M.; Sharpless, K. B.; Kelly, J. W. Org. Lett. 2001, 3, 139–142; (b)
Johnson, S. M.; Petrassi, H. M.; Palaninathan, S. K.; Mohamedmohaideen, N. N.;
Purkey, H. E.; Nichols, C.; Chiang, K. P.; Walkup, T.; Sacchettini, J. C.; Sharpless,
K. B.; Kelly, J. W. J. Med. Chem. 2005, 48, 1576–1587.
Acknowledgement
Financial support from the Australian Research Council Discov-
ery Grant Scheme is gratefully acknowledged.
19. Selected data for 15 and 16: Less polar oxime 16 (7 mg, 10%); 1H NMR
(300 MHz, CDCl3): d 8.00 (2H, d, J 8.0 Hz, ArH), 7.40–7.30 (5H, m, ArH), 7.17
(2H, d, J 9.0 Hz, ArH), 7.02 (1H, br d, J 7.1 Hz, NH), 6.92 (1H, br d, J 8.2 Hz, NH),
5.32 (1H, d, J 7.5 Hz, CH), 5.10 (2H, s, CH2), 4.20–4.00 (1H, m, NHCHCH(CH3)2),
3.89 (3H, s, OCH3), 3.81 (3H, s, OCH3), 2.30–2.15 (1H, m, CH(CH3)(CH3)), 2.12
(3H, s, CH3C@N), 1.01–0.91 (6H, m, CH(CH3)(CH3)). (ESI) m/z 536 (M+Na+,
100%). HRMS: calcd for C26H31O8N3Na 536.2003; found, 536.2004. More polar
oxime 15 isolated as a colourless oil (27 mg, 38%); 1H NMR (300 MHz, CDCl3): d
8.17 (1H, br s, NH), 8.08 (1H, br s, NH), 8.01 (2H, dd, J 9.0, 2.2 Hz, ArH), 7.50–
7.30 (5H, m, ArH), 7.21 (2H, d, J 8.3 Hz, ArH), 5.35–5.20 (1H, m, OH), 5.13 (2H, s,
CH2), 4.20–4.10 (1H, m, NHCHCH(CH3)2), 3.90 (3H, s, OCH3), 3.83 (3H, s, OCH3),
2.40–2.20 (1H, m, CH(CH3)(CH3)), 2.08 (3H, s, CH3C@N), 1.10–0.90 (6H, m,
CH(CH3)(CH3)). (ESI) m/z 552 (M+Na+, 85%). HRMS: calcd for C26H31O9N3Na
552.1958; found 552.1964.
References and notes
1. For recent reviews see: (a) Tohma, H.; Kita, Y. Adv. Synth. Catal. 2004, 346, 111–
124; (b) Wirth, T. Angew. Chem., Int. Ed. 2005, 44, 3656–3665; (c) Wirth, T.
Angew. Chem., Int. Ed. 2001, 40, 2812–2814.
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Frigerio, M.; Santagostino, M.; Sputore, S.; Palmisano, G. J. Org. Chem. 1995, 60,
7272–7276.
3. (a) Thottumkara, A. P.; Vinod, T. K. Tetrahedron Lett. 2002, 43, 569–572; (b)
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4. Sorg, G.; Mengel, A.; Jung, G.; Rademann, J. Angew. Chem., Int. Ed. 2001, 40,
4395–4397.
5. pIBX has been used to oxidise C-terminal peptide alcohols: Sorg, G. T. B.;
Mader, O.; Rademann, J.; Jung, G. J. Peptide Sci. 2005, 11, 142–152.
6. For recent examples of mechanistic studies see: (a) Dess, D. B.; Martin, J. C. J.
Am. Chem. Soc. 1991, 113, 7277–7287; (b) Nicolaou, K. C.; Mathison, C. J. N.;
Montagnon, T. Angew. Chem., Int. Ed. 2003, 42, 4077–4082; (c) Nicolaou, K. C.;
Baran, P. S.; Zhong, Y. L.; Barluenga, S.; Hunt, K. W.; Kranich, R.; Vega, J. A. J. Am.
Chem. Soc. 2002, 124, 2233–2244; (d) Nicolaou, K. C.; Baran, P. S.; Zhong, Y. L. J.
Am. Chem. Soc. 2001, 123, 3183–3185; (e) Nicolaou, K. C.; Montagnon, T.; Baran,
20. pIBX was purchased from Merck and was used as provided. A different batch of
pIBX, supplied with the same resin loading, showed less reactivity and hence
more equivalents of pIBX and longer reaction times were required for the
reaction to go to completion.
21. Mass spectroscopy data for 18 (ESI) m/z 522 (M+Na+, 23%). HRMS: calcd for
C25H29N3O8Na 522.1852, found 522.1854.
22. Kirsch, S. F. J. Org. Chem. 2005, 70, 10210–10212.