Solid-phase synthesis of tetrahydro-β-carbolines and tetrahydroisoquinolines by stereoselective intramolecular N-carbamyliminium pictet-spengler reactions

Article abstract of DOI:10.1002/chem.200600138

A solid-phase method for the synthesis of tri-, tetra-, and pentacyclic compounds containing tetrahydro-β-carboline, tetrahydroisoquinoline or analogous scaffolds is presented. The reaction proceeds with high stereoselectivity through an intermediate N-ca

Full text of DOI:10.1002/chem.200600138

DOI: 10.1002/chem.200600138
Solid-Phase Synthesis of Tetrahydro-b-carbolines and
Tetrahydroisoquinolines by Stereoselective Intramolecular
N-Carbamyliminium Pictet–Spengler Reactions
Frederik Diness, Jürgen Beyer, and Morten Meldal*[a]
Abstract: A solid-phase method for the
synthesis of tri-, tetra-, and pentacyclic
compounds containing tetrahydro-b-
carboline, tetrahydroisoquinoline or
analogous scaffolds is presented. The
reaction proceeds with high stereose-
lectivity through an intermediate N-
carbamyliminium ion that exclusively
converts into Pictet–Spengler-type
products with a variety of C-nucleo-
philes. Amino aldehydes masked with
3-Boc-(1,3)-oxazinane (Box) have been
synthesized from amino acids, amino
alcohols, or 2-nitro benzaldehydes. The
amino moiety of these masked alde-
hydes has been converted into penta-
fluorophenyl carbamate to serve as a
urea precursor. The building blocks
were incorporated at the N-terminal of
lines with a de (de=diastereomeric
excess) above 95% and purity in the
range of 90–99%. This reaction has
been extended to a range of other aro-
matic C-nucleophiles, including substi-
tuted indoles, benzenes, pyrene, furan,
thiophenes, and benzothiophene with
a
resin-supported dipeptide through
urea formation. Subsequent treatment
with acid liberated the aldehyde quan-
titatively.
A penultimate tryptophan
residue gave rise, under the acetic con-
ditions, to a spontaneous intramolecu-
lar Pictet–Spengler reaction with the li-
berated aldehyde. The reaction pro-
ceeded with a high degree of stereose-
lectivity affording tetrahydro-b-carbo-
comparable
purity. Prolonged exposure of the ben-
zaldehyde-derived Pictet–Spengler
products to strong acid and air lead to
quantitative auto-oxidation which
stereoselectivity
and
yielded compounds with a 3,4-dihydro-
b-carboline, a 3,4-dihydroisoquinoline,
or a similar core structure.
Keywords: C–C coupling · cycliza-
tion · intramolecular cascade reac-
tion · solid-phase synthesis
Introduction
These compounds are known for their antibiotic[3–6] and
anticancer[7–10] properties and the most common method re-
ported for generating these structures is through an intermo-
lecular Pictet–Spengler reaction[11] between aldehydes and
hetrocyclic b-ethyl amine derivatives.[8,12–16] The reported
syntheses have generally afforded the desired products in
acceptable yields, but with few exceptions, as mixtures of di-
astereomers. Development of a general method for diaster-
eoselective synthesis of these core structures is therefore
still a challenge. As a part of ongoing research within solid-
phase synthesis, the chemistry of solid-supported aldehydes/
ketones has been investigated[17–20] and versions of the
Pictet–Spengler reaction in which both the formation of the
intermediate N-acyliminium ion and the nucleophillic attack
are intramolecular have recently been developed.[21–23] The
number of reports on solid-phase intramolecular Pictet–
Spengler reactions are limited, particularly within solid-
phase synthesis. In contrast, the intermolecular Pictet–Spen-
gler reaction in which the iminium ion is formed between
two separate molecules has been extensively investigated
and is known as one of natureꢀs favored reactions prevalent
Compounds containing tetrahydro-b-carboline and tetrahy-
droisoquinoline core structures are of great pharmaceutical
interest because of the inherent biological activity of a wide
range of both alkaloids and synthetic mimics containing
these scaffolds.[1] Among the alkaloids are woodinine,[2] eu-
distomins,[3–6] eudistomidins,[7] ecteinascidins, and saframy-
cins[8] (Figure 1). Common for most of these alkaloids is an
a-aminomethyl substituent at the tetrahydro-b-carboline or
tetrahydroisoquinoline core structure.
[a] Dr. F. Diness, Dr. J. Beyer, Prof. M. Meldal
Carlsberg Laboratory
Department of Chemistry
Gamle Carlsberg Vej 10, 2500 Valby (Denmark)
Fax : (+45)3327-4708
E-mail: mpm@crc.dk
1
Supporting information for this article contains H NMR spectra for
all compounds reported and HPLC chromatograms for all com-
pounds synthesized by solid-phase synthesis and is available on the
WWW under http://www.chemeurj.org/ or from the author.
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FULL PAPER
TFA
acid)
(TFA=trifluoroacetic
in acetonitrile/H2O.
Almost instantaneously, the
acid removed the Boc-protect-
ing group and hydrolyzed the
1,3-oxazinane, which gave the
free aldehyde. This was fol-
lowed by intramolecular for-
mation of the N,O-acetal and
acid-promoted elimination of
water afforded the highly reac-
tive N-carbamyliminium ion,
which upon nucleophilic attack
by the indole formed the
Pictet–Spengler product 3 con-
taining an a-amino methyl tet-
rahydro-b-carboline skeleton.
Figure 1. Natural products containing 1-aminomethyl tetrahydro-b-carboline (A) and tetrahydroisoquinoline
(B) substructures.
in the biosynthesis of alkaloids.[24,25] The application of the
reaction in the solid-phase synthesis of tetrahydro-b-carbo-
lines and tetrahydroisoquinolines has also been reported as
recently reviewed by the present authors.[26] These scaffolds
are closely related to other small-constrained peptide mim-
etics, such as aryl-piperidines and indoles, known as privi-
leged structures for G-protein coupled receptors (GPCR).[27]
In our search for novel G-protein coupled receptor agonists
that are peptide-derived small-molecules, the intramolecular
Pictet–Spengler reaction is interesting, as it generates two
new heterocyclic rings in one reaction step. This leads to tet-
rahydro-b-carbolines and tetrahydroisoquinolines with an
additional heterocyclic ring and thereby may result in en-
tropically favored constrained structures with higher activity
and selectivity towards a given GPCR target. In the present
report, we describe a versatile intramolecular Pictet–Spen-
gler reaction. This reaction provides a facile and fast route
from commercially available amino acids, amino alcohols, or
nitrobenzaldehydes, to complex tri-, tetra-, and pentacyclic
scaffolds by an N-carbamyliminium ion intermediate. The
reaction proceeds with various sources of C-nucleophiles,
thus providing a diverse range of pharmacologically interest-
ing products in excellent stereoselectivity and purity.
Results and Discussion
The present methodology was initially developed for solu-
tion-phase reactions by coupling of tryptophan ethyl ester
hydrochloride with a urea precursor building block contain-
ing
a 3-Boc-1,3-oxazinane (Box) masked aldehyde 1a
(Scheme 1).[28] The reaction was performed in DMF by
using 4-ethylmorpholine (NEM) for neutralization of the hy-
drochloride. This cleanly afforded the unsymmetrical urea, 2
in an 85% isolated yield and no major byproduct was de-
tected. The masked aldehyde was then liberated by 10%
Scheme 1. Solid-phase intramolecular Pictet–Spengler reaction with tryp-
tophan: a) 1a–f; b) TFA (10%, aq); c) NaOH (0.1m, aq).
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M. Meldal et al.
Table 1. Acid concentrations and reaction times for the Pictet–Spengler
reaction of 5b.
The Pictet–Spengler product was obtained in a 92% yield
with exclusive formation of the R diastereomer.
Entry Concentration of
TFA [%]
Reaction
time [h]
Products 5a/hemiacetal/
The mild, quantitative, and stereoselective nature of this
reaction prompted for further investigation into the use in
solid-phase combinatorial chemistry. Here the reaction
could be exploited in combination with highly refined tech-
niques for solid-phase peptide synthesis to generate valuable
and diverse small molecule compound collections and split-
mix libraries. Libraries of such natural product like com-
pounds have in recent years gained interest as a source of
novel drug candidates.[29,30]
other [%][a]
1
0.1
289:11:0
:2 36:62
12
21
3
2
1
0:
0:
>95:<5
>95:<5
0:>95:<5
4
5
10
95
2
5
[a] The major byproducts were the S diastereomer of 7b and 1,4-substi-
tuted imidazolone.
Polyethylene glycol acryl amide copolymer (PEGA)
resin[31] was selected as the solid-support due to excellent
swelling in polar solvents, such as dilute aqueous acid, which
was employed during the synthesis. The amino groups of the
resin were functionalized with a base-labile HMBA linker[32]
(HBMA=hydroxymethylbenzamide) by using TBTU[33]
(TBTU=N-[(1H-benzotriazol-1-yl)-(dimethylamino)methy-
lene]-N-methylmethanaminium tetrafluoroborate N-oxide)
and NEM. The first Fmoc-protected amino acid was esteri-
fied to the linker by using MSNT and N-methylimidazole.[34]
A second amino acid was attached by standard Fmoc-pep-
tide coupling by using TBTU and NEM providing a solid-
supported dipeptide.
found to be stable to prolonged exposure to high concentra-
tions of TFA.
Aqueous TFA (10%) for 1 h was selected as a standard
condition for the deprotection and Pictet–Spengler reaction.
The six different resin-bound compounds 4a–f all yielded
the expected products 5a–f with 90–95% purity (Scheme 5
and Table 3). Products containing a side-chain-protecting
group 5d–e were subsequently treated twice with 95%
aqueous TFA for 15 min for complete side-chain deprotec-
tion before cleavage from the resin. The stereoselectivity is
controlled by the transition state of the reaction and was
found to be more than 20:1, independent of the selection of
building block. The stereochemistry of the products has
been determined by NOE studies and by using both l- and
d-amino acids in the reactions (Figure 2).
The pentafluorophenyl carbamate[35–37] building blocks
1a–f[28] were coupled to the N-terminal of a resin bound
Trp-Ile dipeptide by adding three equivalents in DMF. Stud-
ies of the coupling reaction showed considerable variation
in the rate of reaction; howev-
er, after 12h the reactions
were all complete. Addition of
DIPEA (DIPEA=diisopropy-
lethylamine)
or
DMAP
(DMAP=4-dimethylamino-
pyridine) did not catalyze the
reaction significantly. The puri-
ties of the crude products 4a–f
were ꢀ95% as analyzed by
HPLC after release from the
support. For 4b the purity was
Figure 2. Elucidation of the stereochemistry by NOE.
confirmed by 1H NMR spec-
troscopy.
The deprotection and Pictet–Spengler reaction were ini-
tially successfully performed by using 10% aqueous TFA.
Various acids, acid concentrations, and solvents as well as re-
action times were studied. TFA, aqueous HCl (1n), and
aqueous H2SO4 (1n) all performed well in this reaction
yielding products with more than 95% purity. Lewis acids,
such as BF3·OEt2 or TiCl4 in dichloromethane also yielded
the desired product; however, with a slightly lower purity of
approximately 85%. The reaction only showed little de-
pendence of the solvent and 10% TFA in MeOH, DMF,
THF, acetone, EtOAc, acetonitrile, dichloromethane, tol-
uene, or hexane all gave more than 90% pure product. Ap-
plication of different concentrations of TFA (Table 1) dem-
onstrated that as little as 1% aqueous TFA could complete
the reaction in 12h. Furthermore, the formed products were
In analogy with intramolecular reactions involving N-acy-
liminium ions,[21] the new stereocenter gained the trans con-
figuration relative to the configuration of the tryptophan
giving rise to either (a-S, g-R)- or (a-R, g-S)-tetrahydro-b-
carboline derivatives.
Imidazolidin-2-one fused Pictet–Spengler products has
only been reported for the intramolecular reaction of a,a-
disubstituted a-amino aldehydes because the intermediate
iminium ions are prone to elimination of an a proton and
double bond rearrangement to yield the enamine.[38] In the
present investigation, this conversion was also observed
when tryptophan was replaced by less reactive phenylala-
nine as the C-nucleophile 7 (Scheme 2). In preference to re-
action with less reactive nucleophiles, the iminium ion C re-
arranges with a-proton abstraction and quantitatively af-
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Novel N-carbamyliminium Pictet–Spengler Reactions
FULL PAPER
Scheme 2. Formation of imidazolones as a competing reaction in the
presence of poor C-nucleophiles and labile a-protons: a) 10% TFA (aq).
fords the uncharged 1,4-substituted imidazolone 8. Detailed
analysis of the crude products of the Pictet–Spengler reac-
tion with more reactive tryptophan 5a–f showed that the
corresponding 1,4-substituted imidazolone was formed as a
byproduct in minute amounts by this competing reaction
pathway (0–5%). The 1,4-substituted imidazolone is in itself
an interesting heterocycle and further investigations on the
rearrangement reaction will be reported subsequently.
To extend the scaffold diversity, it was attempted to form
two fused six-membered rings (Schemes 4 and 5) in the in-
tramolecular Pictet–Spengler reaction, instead of the five/
six-membered bicyclic ring system. Only a few examples of
such products have previously been reported.[39–41] The reac-
tion could be accomplished with both aliphatic and aromatic
urea building blocks containing Box-protected aldehyde.
Thus 3-aminopropanal building block (ApaBB-OPfp) 11
(Scheme 3) was synthesized from N-Cbz-3-amino-1-propa-
nol (9)[42] by oxidation with Dess–Martin periodane (DMP)
and masking the formed aldehyde by reacting the crude
product with 3-amino-1-propanol and sodium sulfate fol-
lowed by reaction with Boc-anhydride to give 74% overall
yield of 10. The Cbz group was then removed with Pd/C and
H2 and the crude product was reacted with bispentafluoro-
phenylcarbonate affording ApaBB-OPfp 11 in an isolated
yield of 82% with no formation of the symmetric urea prod-
uct.
In addition to compound 11, two 3-Boc-(1,3)-oxazinane
(Box) masked 2-amino-benzaldehyde building blocks were
synthesized (Scheme 3). The synthesis of the nonsubstituted
building block (ApaBB-OPfp) 15 has previously been de-
scribed[28] and the 4,5-dimethoxy-2-nitro-benzaldehyde de-
rived building block (DMApaBB-OPfp) 14 was synthesized
in three steps in a similar manner (Scheme 3). The starting
material 12 was protected with the Box-protecting group by
reaction with 3-amino-1-propanol and sodium sulfate fol-
lowed by reaction with Boc-anhydride. This provided 40–
70% of compound 13 depending on reaction temperature
Scheme 3. Synthesis of 3-aminopropanal and 2-amino-3,4-dimethoxy-
benzaldehyde building blocks: a) DMP; b) 3-aminopropanol and Na2SO4
then Boc2O; c) 10% Pd/C, H2 (1 atm); d) bispentafluorophenyl carbo-
nate; e) 10% Pd/C, H2 (1 atm), DMAP; f) bispentafluorophenyl carbo-
nate, DMAP.
and timing of reagent addition. The nitro group was hydro-
genated in the presence of DMAP as basic conditions were
crucial for the stability of the Box-group. The crude product
containing DMAP required for catalysis was converted into
the pentafluorophenyl carbamate by reaction with bispenta-
fluorophenyl carbonate in 79% overall yield.
The Pictet–Spengler reaction is known to proceed with a
range of aromatic heterocycles.[25] To investigate the reactivi-
ty of compounds 11, 14, and 15, less prone to elimination of
the a proton in the Pictet–Spengler reactions, a series of di-
peptides with a terminal b-aryl alanine residue were synthe-
sized on solid support as described above. Compound 11
(ApaBB-OPfp) was coupled to the N-terminus of these pep-
tides under the same conditions as described in Scheme 1.
These reactions provided the unsymmetrical ureas 16a–k
(Scheme 4) with purities >95% after cleavage from the
resin. Application of 10% aqueous TFA for 1 h liberated
the aldehyde, which subsequently underwent Pictet–Spen-
gler reaction with various intramolecular nucleophiles in-
cluding indole, benzothiophene, and 3-thiophene (18a–e)
(Schemes 4, 5, and Table 3). In these cases, the deprotection
of the aldehyde was the rate-limiting step. With weaker nu-
cleophiles, such as 2-thiophene, furan, and phenyl deriva-
tives the N,O-acetal intermediate could be isolated (17 f–k,
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M. Meldal et al.
acetal intermediate could be cleaved off the resin and isolat-
ed. This observation was also supported by the reaction with
the nonsubstituted phenyl group in which only the cyclic
acetal (20h) could be isolated, even after treatment with
100% TFA for 24 h (Table 3).
Interestingly, prolonged treatment of Pictet–Spengler
products 21a–g derived from the AbaBB and DMAbaBB
with 95% TFA (aq) in the presence of oxygen (air) lead to
oxidation of the newly formed stereocenter, which yielded
22a–g as the only products. These products all had an in-
tense color varying from yellow to deep red and were stable
to further oxidation. The oxidation of, for example, 21a to
22a results in loss of two mass units in high resolution ESI-
MS and the loss of the benzylic proton at 6.12ppm. The
spectrophotometric properties of these oxidized products
are currently being investigated and are subject to future
publication. It was found that the electronic nature of the C-
nucleophile, the access to oxygen, and the acidity influenced
the oxidation reaction. The indole-derived Pictet–Spengler
products 21a–c were oxidized in less than 5 h, whereas the
dimethoxyphenyl product 21 f needed 7 d for completion of
the reaction under the same conditions. The reaction rate
was greatly enhanced by adding just 1% H2O2 to the acid
and in this case only 50% TFA was needed to convert 21a
within 1 h. However, prolonged reaction times under these
conditions led to decomposition. The reversible reaction was
easily accomplished by adding excess of sodium borohydride
in DMF with 5% AcOH which gave rise to 21a from 22a in
about 80% de within 5 min.
Scheme 4. Pictet–Spengler reaction with 2-amino-benzaldehyde building
blocks and oxidation. a) 10–100% TFA (aq); b) Air, TFA (100%)
(Table ).
To the best of our knowledge, the acid-promoted air oxi-
dation of this type of Pictet–Spengler products has not been
previously reported as a synthetic reaction; however, a
recent report described an oxidized Pictet–Spengler product
formed by degradation during storage of an NMR sample.[43]
A mechanism for the oxidation may be suggested in which
electrophilic oxygen reacts with p-electrons at the 3-position
of the indole, assisted by the indole nitrogen followed by
formation and elimination of an intermediate oxirane and
concurrent elimination of a proton from the benzylic stereo-
center formed in the Pictet–Spengler reaction (Scheme 6).
The lone pair of the b-nitrogen in the tetrahydro-b-carboline
then displaces the formed double bond, which rearranges to
form the indole while expelling the oxygen at the 3-position.
Stabilization of intermediates similar to that by indole can
occur in all the compounds prone to oxidation. However, it
should be noted that in the non-oxidizing compounds syn-
thesized from the aliphatic building block 11 that do not
contain the benzylic proton no trapping of expected oxirane
intermediates could be observed.
Schemes 4 and 5). However, these intermediates could be
fully converted to the Pictet–Spengler products by applying
more acidic conditions (Table 3). By using optimized TFA
concentrations all Pictet–Spengler products were formed
within 1 h in 90–99% purity and with a de of more than
90% (Table 3).
The
2-amino-4,5-dimethoxy-benzaldehyde
(DMAba-
OPfp) building block 14 and 2-amino-benzaldehyde
(AbaBB-OPfp) 15 were successfully coupled to the N-termi-
nal of the solid-supported dipetide (Scheme 2) The reaction
afforded the Pictet–Spengler precursor 19a–h with the same
high purity and similar coupling rates as for the aliphatic
amino aldehydes.
During deprotection with various acid concentrations
(Table 2) it was demonstrated that the 3-Boc-1,3-oxazinane-
masked benzaldehydes were indeed highly acid labile and as
little as 0.1% TFA (aq) liberated the aldehyde completely
within 1 h.
Various C-nucleophiles were tested in this reaction and
the indole, thiophene, and 3,4-dimethoxy-phenyl all worked
well in this reaction to form only one diastereomer
(Scheme 5 and Table 3). The conjugated carbamyliminium
ions formed from 20a–h were found to be less reactive com-
pared to the nonconjugated carbamyliminium ions (from
17a–k, Scheme 5) and in case of 20e, 20 f, and 20h the
Conclusion
We have demonstrated the versatile chemistry of using
amino aldehyde building blocks in the N-carbamyliminium-
ion-mediated intramolecular Pictet–Spengler reaction. This
chemistry has led to imidazolidin-2-one fused tetrahydro-b-
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Novel N-carbamyliminium Pictet–Spengler Reactions
FULL PAPER
Scheme 5. Pictet–Spengler products obtained through acid-catalyzed intramolecular cascade reactions involving intramolecular formation of peptidic N-
carbamyliminium ions.
carbolines with formation of a five and a six-membered ring
in a single reaction. New amino aldehyde building blocks
containing 3-Boc-1,3-oxazinane (Box) masking of the alde-
hyde, and a pentaflourophenyl carbamate moiety could be
synthesized from N-Cbz-3-aminopropanol and of 3,4-di-me-
thoxy-2-nitro-benzaldehyde in overall yields of 61 and 55%
respectively. These two new amino aldehyde building blocks
have expanded the scope of the intramolecular version of
the Pictet–Spengler reaction and given easy access to tetra-
hydro-b-carbolines, tetrahydroisoquinolines, and analogues
fused with tetrahydropyrimidin-2-one or 1,6-dihydropyrimi-
din-2-one.
colored/fluorescent compounds with a 3,4-dihydro-b-carbo-
line, 3,4-dihydro-isoquinoline, or analogue core structures.
Experimental Section
General aspects: All purchased chemicals were used without further puri-
fication. All solvents were HPLC grade. PEGA800 resin was from Versa-
Matrix A/S. All washing steps were performed for a period of 2min
unless stated differently. Flash chromatography was carried out on Merck
silica gel 60 (0.040–0.063 mm) and analytical TLC was performed by
using Merck silica gel 60 F254 aluminum plates. NMR spectra were re-
corded on
a Bruker AVANCE DPX 250 MHz instrument. CDCl3,
With only one exception, all solid-phase reactions afford-
ed products with 90–99% purity and with a stereoselectivity
above 95% de. Furthermore, we have demonstrated that
electron-rich Pictet–Spengler products are prone to oxida-
tion by a novel acid promoted electrophilic oxygenation.
The oxidation proceeds in a quantitative manner and yields
CD3OD, and [D6]DMSO were used as internal standards (7.26, 3.31, and
1
2.50 ppm for H and 77.0, 49.00, and 39.52ppm for 13C NMR). High-reso-
lution MS was determined with a Micromass QTOF Global Ultima in-
strument by using an appropriate internal reference. All reported oxi-
nanes were mixtures of the R and S isomers at the chiral center of the
oxinane ring. Purities of crude materials were accessed by a combination
of HPLC and ESIMS to identify all byproducts. The integration of peaks
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M. Meldal et al.
Table 2. Products formed from 23a in the presence of air under various
conditions.
Entry Concentration of TFA (aq)
[%]
Reaction time
[h]
19a/21a/22a
[%]
1
3
3
4
5
6
7
0.1
10
10
0.16
0.25
24
20:71:2
5
42:58:0
0:100:0
0:90:10
95
95
9
0:0:100
0:0:100
50 (+1% H2O2)
10 (then 10% H2O2)
1
0.25 (then 0.5 h) 0:100:0
Table 3. Products, conditions, reaction times, and purities/yields for com-
pounds in Scheme 5.
Product
Building
block
Conditions
(% aq TFA)
Reaction
time [h]
Yield[a]
/
A
purity[b] [%]
[a]
3
5a
5b1b
5c
5d
5e
1a
1a
10
10
10
10
10
10
10
10
10
10
10
95
50
95
95
95
95
TFA
10
10
10
10
95
95
10
95/air
95/air
95/air
95/air
95/air
95/air
95/air
292
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
24
2
2
2
2
0.5
295
1
5
5
92[b]
90[b]
1c
1d
1e
1 f
14
90[b]
92[b]
95[b]
5 f
95[b]
18a
18b14
18c
18d
18e
18 f
18g
18i
94[a]
Scheme 6. Proposed mechanism for oxidation reaction. Similar stabiliza-
tion can be realized to various degrees for the intermediates obtained
during oxidation to form 26b–26g.
>95[b]
>95[b]
>95[b]
>95[b]
>95[b]
>95[b]
>95[b]
>95[b]
95[b]
14
14
14
14
14
14
14
14
21
21
obtained as colorless glassy foam. 1H NMR (250 MHz, CDCl3): d=8.50
(s, 0.5H), 8.3 (s, 0.5H), 7.53 (d, J=7.0 Hz, 1H), 7.41–7.01 (m, 8H), 6.94
(d, J=7.9 Hz, 1H), 5.27–5.06 (m, 1H), 4.91–4.62 (m, 2H), 4.27–3.51 (m,
6H), 3.39–2.59 (m, 6H), 2.01–1.51 (m, 2H), 1.49 (s, 4.5H), 1.34 (s, 4.5H),
1.26–1.09 ppm (m, 3H); 13C NMR (62.5 MHz, CDCl3): d=136.8, 129.9,
129.8, 129.5, 129.3, 128.3, 127.6, 126.3, 124.7, 123.4, 122.2, 121.9, 119.6,
119.3, 118.5, 118.3, 111.2, 110.7, 61.4, 59.6, 59.3, 49.9, 37.5, 37.0, 36.5, 32.9,
28.4, 28.2, 27.9, 24.9, 23.6, 14.1, 14.0 ppm; HRMS: m/z: calcd for
C31H41N4O6: 565.3021 [M+H]+; found: 565.3027.
18j
18k
20h
21a
21b21
21c
21d
21e
21 f
21g
22a
22b21
22c
22d
22e
22 f
22g
95[b]
>95[b]
>95[b]
>95[b]
>95[b]
21
21
21
21
20
21
Pictet–Spengler
reaction
of
PheBB-Trp-OEt
(1-benzyl-3-oxo-
>95[b]
2,3,4,5,10,10b-hexahydro-1H-2,3a,10-triaza-cyclopenta[a]fluorene-4-car-
boxylic acid ethyl ester, (3)): 50% TFA (2mL, aq) was added to a solu-
tion of PheBB-Trp-OEt 2 (66 mg, 117 mmol) in acetonitrile (10 mL).
After reaction for 2h, the mixture was neutralized with solid NaHCO 3,
filtered by using acetonitrile for washing, and the solvent was removed in
vacuo. The crude product was purified by RP-HPLC and the product 3
(42mg, 92%) was obtained as a colorless glass film. 1H NMR (250 MHz,
CDCl3): d=7.58–7.45 (m, 3H), 7.45–7.32(m, 3H), 7.30–7.16 (m, 1H),
7.15–6.98 (m, 2H), 6.90 (d, J=7.2Hz, 1H), 5.82 (brs, 1H), 5.25 (brs,
1H), 5.22–5.04 (m, 2H), 4.23–3.88 (m, 3H), 3.44–3.21 (m, 2H), 3.18–2.95
(m, 2H), 1.21 ppm (t, J=7.1 Hz, 3H); 13C NMR (62.5 MHz, CDCl3): d=
173.4, 161.0, 136.5, 129.9, 129.7, 129.0, 127.8, 122.4, 119.7, 118.3, 110.8,
106.3, 61.5, 60.0, 55.6, 50.1, 42.2, 23.6, 14.1 ppm; HRMS: m/z: calcd for
C23H24N3O3: 390.1812[ M+H]+; found: 390.1830.
[b]
70[b,c]
>95[b]
>95[b]
>95[b]
>95[b]
>95[b]
>95[b]
95[b]
21
21
21
21
20
5
120
120
168
5
[a] The yield was determined as the yield of isolated pure compound.
[b] The purity of the crude product determined by HPLC, ESIMS, and
NMR spectroscopy. [c] The remaining ꢀ30% was the oxidized form
(22g) and several attempts to isolate the pure 21g failed. Compounds
19g and/or 22g were found as major impurities in all cases.
Solid-phase synthesis of Pictet–Spengler products and related products:
Dry PEGA800 resin (50–150 mg, 0.30–0.38 mmolgÀ1, 15–57 mmol) derivat-
ized with the HMBA linker and dipeptide (e.g. 6) was swelled in DMF.
The building block-OPfp ester (3.0 equiv) (1a–f, 11, 14, or 15) was dis-
solved in DMF (10 mLmgÀ1 resin) and the solution was added to the
resin. After 12h, the resin was washed with DMF (6”), dichloromethane
(6”), and lyophilized. The dry PEGA800 resin with HMBA linker, pep-
tide, and building block attached (4a–f, 7, 16a–k, or 19a–h) was swelled
in 10% TFA (aq, 1 h). The resin was washed with 10% TFA (aq, 2”).
Further reaction with TFA was applied when necessary to complete con-
version or side-chain deprotection (see below). The resin was then
washed with H2O until the eluate had a pH 5–7. It was then washed with
DMF (6”), dichloromethane (6”), and lyophilized. The compound was
cleaved from the resin by swelling the dry PEGA800 resin with HMBA
was used for the determination of purity (Æ2%) with the assumption
that identical chromophores have approximately the same extinction co-
efficient as determined from selected purified samples. Purities were con-
1
firmed by H NMR spectroscopy (see Supporting Information).
PheBB-Trp-OEt
(2-(1-{3-[1-ethoxycarbonyl-2-(1H-indol-3-yl)-ethyl]-
ureido}-2-phenyl-ethyl)-[1,3]oxazinane-3-carboxylic acid tert-butyl ester)
(2): PheBB-OPfp 1a (105 mg, 203 mmol) was added to a solution of
TrpOEt·HCl (54.6 mg, 203 mmol) and NEM (26 mL, 203 mmol) in DMF
(2mL). After stirring for 12h at RT, the solvent was removed in vacuo
at 408C and the crude product was purified by flash chromatography by
using ethyl acetate/petroleum ether 1:1. The product 2 (98 mg, 85%) was
8062
www.chemeurj.org
ꢁ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2006, 12, 8056 – 8066

Novel N-carbamyliminium Pictet–Spengler Reactions
FULL PAPER
linker and compound attached in NaOH (aq, 0.1m) (10 mLmgÀ1 resin).
After 2h, HCl (aq, 0.1 m) was used for neutralization. The resin was ex-
tracted with H2O (”2) and acetonitrile/H2O 70:30 (”2). The solvent from
the extract was removed in a speedvac giving the crude product (5a–f, 6,
8, 18a–k, 20h, or 21a–g) as a solid salt with a white to orange color.
Yield was in the range of 63–94%.
(62.5 MHz, [D6]DMSO): d=173.0, 168.3, 160.9, 136.3, 131.5, 127.5, 126.6,
124.6, 123.9, 121.4, 121.0, 118.3, 117.9, 111.4, 109.7, 106.4, 58.6, 55.7, 55.4,
51.5, 37.8, 25.3, 19.7, 15.9, 12.0 ppm; HRMS: m/z: calcd for
C29H31N5NaO4: 536.2268 [M+Na]+; found: 536.2247.
Pictet–Spengler product from AibBB-Trp-Ile-HMBA-PEGA800 (5 f):
T
Purity >95%; 1H NMR (250 MHz, [D6]DMSO): d=11.01 (s, 1H), 7.43–
7.31 (m, 3H), 7.09–6.93 (m, 3H), 4.75 (s, 1H), 4.69 (d, J=6.4 Hz, 1H),
3.74–3.69 (m, 1H), 3.45 (d, 1H), 2.79–2.66 (m, 1H), 1.54 (S, 3H), 1.43–
1.18 (m, 1H), 1.00–0.79 (m, 4H), 0.69 (t, J=7.3 Hz, 3H), 0.52ppm (d,
J=6.8 Hz, 3H); 13C NMR (62.5 MHz, [D6]DMSO): d=173.7, 169.0,
159.6, 137.0, 129.2, 127.8, 126.8, 124.1, 124.1, 121.5, 118.8, 118.3, 111.5,
107.5, 60.6, 58.6, 57.4, 50.9, 38.0, 28.6, 25.5, 24.3, 22.5, 16.0, 12.4 ppm;
HRMS: m/z: calcd for C23H30N4NaO4: 435.2003 [M+Na]+; found:
435.2003.
GlyBB-Trp-Ile-OH (4b): Compound 4b was synthesized and cleaved as
described above, with no TFA treatment applied. Purity >95%;
1H NMR (250 MHz, [D6]DMSO): d=10.78 (s, 1H), 7.51 (d, 7.7 Hz, 1H),
7.36–7.24 (m, 1H), 7.07 (s, 1H), 7.03 (t, J=8.0 Hz, 1H), 6.93 (t, J=
7.9 Hz, 1H), 6.27–6.14 (m, 2H), 5.20 (t, J=6.8 Hz, 1H), 4.34 (m, 1H),
3.93–3.70 (m, 2H), 3.67–3.42 (m, 2H), 3.38–2.99 (m, 3H), 2.98 (dd, J=7.0
and 14.8 Hz, 1H), 1.79–0.98 (m, 13H), 0.80 (t, J=7.4 Hz, 3H), 0.68 ppm
(dd, J=2.3 and 6.8, 3H); 13C NMR (62.5 MHz, [D6]DMSO): d=172.8,
170.4, 161.7, 157.4, 153.4, 136.0, 127.6, 123.5, 120.7, 118.5, 118.1, 111.1,
110.2, 80.5, 79.3, 59.6, 58.5, 54.5, 37.7, 37.4, 27.9, 25.3, 24.8, 15.7,
12.2 ppm; HRMS: m/z: calcd for C28H42N5O7: 560.3079 [M+H]+; found:
560.3077.
Pictet–Spengler product from PheBB-trp-ile-HMBA-PEGA800 (6): Purity
>95%; 1H NMR (250 MHz, [D6]DMSO): d=12.61 (brs, 1H), 10.97 (s,
1H), 7.90 (d, J=8.6 Hz, 1H), 7.42–7.25 (m, 2H), 7.24–6.90 (m, 7H), 6.64
(s, 1H), 5.42(d, J=8.0 Hz, 1H), 4.79 (d, J=6.8 Hz, 1H), 4.28–4.13 (m,
1H), 4.04 (dd, J=6.3 and 8.5 Hz, 1H), 2.84–2.62 (m, 2H), 2.19 (dd, J=
9.7 and 13.6 Hz, 1H), 1.86–1.64 (m, 1H), 1.39–1.20 (m, 1H), 1.16–0.94
(m, 1H), 0.84–0.65 ppm (m, 6H); 13C NMR (62.5 MHz, [D6]DMSO): d=
172.9, 171.2, 159.3, 137.7, 136.2, 129.2, 129.1 128.0, 126.5, 126.0, 121.1,
118.5, 117.8, 111.1, 107.1, 56.2, 55.0, 53.7, 49.6, 37.4, 36.1, 24.7, 23.4, 15.5,
11.1 ppm; HRMS: m/z: calcd for C27H31N4O4: 457.2340 [M+H]+; found:
457.2342.
Pictet–Spengler product from PheBB-Trp-Ile-HMBA-PEGA800 (5a):
G
Purity >95%; 1H NMR (250 MHz, [D6]DMSO): d=10.79 (s, 1H), 8.02
(d, J=8.3 Hz, 1H), 7.42–7.20 (m, 7H), 7.12–6.91 (m, 3H), 4.88–4.79 (m,
1H), 4.08 (dd, J=6.4, 8.2 Hz, 1H), 3.98–3.88 (m, 1H), 3.21–2.99 (m, 3H),
2.90–2.75 (m, 1H), 1.90–1.73 (m, 1H), 1.49–1.30 (m, 1H), 1.26–1.04 (m,
1H), 0.88–0.69 ppm (m, 6H). 13C NMR (62.5 MHz, [D6]DMSO): d=
173.0, 170.8, 160.7, 137.7, 136.2, 131.8, 129.5 128.4, 126.6, 126.4, 121.3,
118.7, 117.8, 111.2, 105.9, 56.5, 56.2, 55.2, 50.5, 41.1, 35.9, 25.0, 21.0, 15.6,
11.1 ppm; HRMS: m/z: calcd for C27H31N4O4: 457.2340 [M+H]+; found:
457.2373.
3-Methyl-2-[2-(2-oxo-2,3-dihydro-imidazol-1-yl)-3-phenyl-propionylami-
no]pentanoic acid (8): The imidazolone product (8) was obtained from
PheBB-Phe-Ile-HMBA-PEGA800 with
a
purity >95%; 1H NMR
Pictet–Spengler product from GlyBB-Trp-Ile-HMBA-PEGA800 (5b):
G
(250 MHz, [D6]DMSO): d=10.16 (s, 1H), 7.31–7.07 (m, 10H), 6.33 (s,
1H), 4.68 (dd, J=4.5, 11.1 Hz, 1H), 3.89 (dd, J=4.0, 8.0 Hz, 1H), 3.57
(d, J=3.3 Hz, 2H), 3.31–3.21 (m, 1H), 3.02 (dd, J=11.3, 14.3 Hz, 1H),
1.85–1.61 (m, 1H), 1.55–1.30 (m, 1H), 1.17–0.92(m, 1H), 0.90–0.59 ppm
(m, 6H); 13C NMR (62.5 MHz, [D6]DMSO): d=173.7, 168.3, 153.6, 138.7,
137.9, 129.0, 128.3 128.1, 128.0, 126.2, 106.0, 58.5, 56.1, 37.8, 35.9, 31.1,
24.8, 15.9, 12.0 ppm; HRMS: m/z: calcd for C25H30N3O4: 436.2231
[M+H]+; found: 436.2234.
Purity >95%; 1H NMR (250 MHz, [D6]DMSO): d=12.64 (brs, 1H),
10.92(s, 1H), 8.09 (d, J=8.0 Hz, 1H), 7.43–7.23 (m, 2H), 7.12–6.86 (m,
2H), 6.86 (s, 1H), 5.24–5.09 (m, 1H), 4.84 (d, J=7.2Hz, 1H), 4.12(dd,
J=6.5 and 8.4 Hz, 1H), 3.77 (t, J=8.9 Hz, 1H), 3.41 (dd, J=3.4 and
8.8 Hz, 1H), 3.17 (d, J=13.9 Hz, 1H), 2.92–2.74 (m, 1H), 1.91–1.71 (m,
1H), 1.48–1.27 (m, 1H), 1.27–1.01 (m, 1H), 0.87–0.71 ppm (m, 6H).
13C NMR (62.5 MHz, [D6]DMSO): d=172.9, 170.9, 161.5, 136.0, 132.5,
126.5, 122.6, 121.2, 118.6, 117.7, 111.2, 105.6, 56.5, 50.6, 50.0, 40.5, 35.9,
24.9, 21.3, 15.6, 11.1 ppm; HRMS: m/z: calcd for C20H25N4O4: 385.1876
[M+H]+; found: 385.1891.
3-Boc-2-(3-benzyloxycarbonylamino-propyl)-[1,3]oxazinane (10): A solu-
tion of Dess–Martin periodinane (12.9 g, 30.4 mmol) in dichloromethane
(150 mL) was added dropwise to a solution of N-benzyloxycarbonyl-3-
aminopropan-1-ol (9) (2.97 g, 15.2 mmol) in dichloromethane (50 mL).
After 1.5 h, a mixture of 2.1m aqueous Na2S2O3 (85 mL) and saturated
aqueous NaHCO3 (85 mL) were added. When a clear solution was ob-
tained, the organic phase was separated and the aqueous phase was ex-
tracted with diethyl ether (3”50 mL). The combined organic layers were
washed with saturated aqueous NaHCO3 (1”50 mL) and brine (1”
50 mL). After drying over MgSO4, the solvent was evaporated in vacuo
and the crude aldehyde (3.0 g, quant.) was obtained. Rf =0.47 (ethyl ace-
tate/petroleum ether 1:1); 1H NMR (250 MHz, CDCl3) was found to be
as reported.[42] 3-Amino-propanol (0.75 mL, 9.9 mmol) and anhydrous
sodium sulfate (14.1 g, 99 mmol) were added to a solution of the alde-
hyde (2.05 g, 9.9 mmol) described above in toluene (40 mL). The mixture
was stirred for 3 h, and then Boc2O (2.27 g, 10.4 mmol) was added. After
a further 3 h, the mixture was filtered and the solvent removed in vacuo.
The crude product was purified by flash-column chromatography by
using ethyl acetate/petroleum ether 1:2. The product 10 (2.66 g,
7.3 mmol, 74%) was obtained as a colorless syrup which solidified upon
Pictet–Spengler product from ValBB-Trp-Ile-HMBA-PEGA800 (5c):
G
Purity >95%; 1H NMR (250 MHz, [D6]DMSO): d=11.04 (s, 1H), 7.51
(d, J=7.1 Hz, 1H), 7.43–7.24 (m, 3H), 7.15–6.87 (m, 2H), 4.70 (s, 1H),
4.64 (d, J=6.9 Hz, 1H), 3.82(dd, J=3.6, 7.1 Hz, 1H), 3.55–3.30 (m, 2H),
2.82–2.62 (m, 1H), 2.06–1.82 (m, 1H), 1.72–1.56 (m, 1H), 1.54–1.34 (m,
1H), 1.16–0.91 (m, 7H), 0.89–0.68 (m, 4H), 0.65 ppm (d, J=6.8 Hz, 3H);
13C NMR (62.5 MHz, [D6]DMSO) d=173.6, 168.7, 161.6, 136.8, 132.3,
126.9, 121.7, 119.0, 118.2, 111.7, 106.6, 70.1, 60.7, 58.9, 53.8, 52.0, 38.4,
32.9, 25.7, 19.7, 19.0, 17.1, 16.3, 12.5 ppm; HRMS: m/z: calcd for
C23H31N4O4: 427.2340 [M+H]+; found: 427.2321.
Pictet–Spengler product of TyrBB-Trp-Ile-HMBA-PEGA800 (5d): 95%
E
TFA (aq, ”2, 15 min) was used for removal of the tert-butyl-protecting
1
group. Purity >95%; H NMR (250 MHz, [D6]DMSO): d=10.91 (s, 1H),
9.47 (brs, 1H), 7.73 (d, J=7.1 Hz, 1H), 7.41–7.28 (m, 2H), 7.22–7.12 (m,
3H), 7.06 (t, J=7.0 Hz, 1H), 6.96 (t, J=7.0 Hz, 1H), 6.74 (d, J=8.4 Hz,
2H), 4.73–4.67 (m, 2H), 3.94–3.81 (m, 2H), 3.03–2.72 (m, 3H), 1.78–1.62
(m, 1H), 1.50–1.66 (m, 1H), 1.14–0.99 (m, 1H), 0.88–0.73 (m, 4H),
0.68 ppm (d, J=2.8 Hz, 3H); 13C NMR (62.5 MHz, [D6]DMSO): d=
173.4, 168.8, 161.0, 156.1, 136.2, 131.5, 130.5, 127.3, 126.5, 121.3, 118.6,
117.8, 115.3, 111.3, 106.2, 58.4, 56.3, 54.9, 51.3, 37.5, 25.8, 15.9, 11.9 ppm;
HRMS: m/z: calcd for C27H31N4O5: 491.2289 [M+H]+; found: 491.2286.
1
storage in the refrigerator. H NMR (250 MHz, CDCl3): d=7.34–7.13 (m,
5H), 5.43 (t, J=7.0 Hz, 1H), 5.28–5.13 (m, 1H), 5.02 (s, 2H), 3.97–3.85
(m, 1H), 3.84–3.70 (m, 1H), 3.66–3.53 (m, 1H), 3.44–2.87 (m, 3H), 2.13–
1.51 (m, 3H), 1.51–1.23 ppm (m, 10H); 13C NMR (62.9 MHz,
[D6]DMSO): d=156.3, 153.7, 136.6, 128.4, 128.0, 80.8, 80.3, 66.5, 59.9,
37.4, 36.8, 29.2, 28.2, 25.0 ppm; HRMS: m/z: calcd for C20H25N2O5:
365.2071 [M+H]+; found: 365.2061.
Pictet–Spengler product from TrpBB-Trp-Ile-HMBA-PEGA800 (5e):
R
95% TFA (aq, ”2, 15 min.) was used for removal of the side chain Boc-
protecting group. Purity >95%; 1H NMR (250 MHz, [D6]DMSO) d=
10.96 (s, 1H), 10.91 (s, 1H), 7.72(d, J=7.5 Hz, 1H), 7.63 (d, J=7.2Hz,
1H), 7.42–7.30 (m, 4H), 7.14–6.94 (m, 6H), 4.85 (s, 1H), 4.68 (d, J=
6.9 Hz, 1H), 4.04–3.94 (m, 1H), 3.83–3.77 (m, 1H), 3.35–3.20 (m, 1H),
3.08 (dd, J=8.0 and 14.3 Hz, 1H), 2.81–2.66 (m, 1H), 1.70–1.63 (m, 1H),
1.47–1.33 (m, 1H), 1.06–0.93 (m, 1H), 0.86–0.61 ppm (m, 7H); 13C NMR
3-Boc-2-(1-pentafluorophenyloxycarbonylamino-propyl)-[1,3]oxazinane
(ApaBB-OPfp) (11): Compound 10 (2.37 g, 6.5 mmol) and 10% Pd/C
(200 mg) were mixed in ethanol (50 mL) and H2 (1 atm) was applied
under vigorous stirring for 4.5 h. After this time, the mixture was filtered
through Celite and the solvent was removed in vacuo. The crude product
Chem. Eur. J. 2006, 12, 8056 – 8066
ꢁ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
8063

M. Meldal et al.
was dissolved in dichloromethane (30 mL) and bispentafluorophenyl car-
bonate (2.57 g, 6.5 mmol) was added to the mixture. After reaction for
12h, the mixture was concentrated in vacuo. The crude product was puri-
fied by flash-column chromatography by using ethyl acetate/petroleum
ether 1:5. The product 11 (2.35 g, 5.3 mmol, 82%) was obtained as a col-
orless syrup which solidified upon storage in the refrigerator. 1H NMR
(250 MHz, CDCl3): d=6.07–5.86 (m, 1H), 5.56 (t, J=6.9 Hz, 1H), 4.08–
3.82 (m, 2H), 3.81–3.37 (m, 2H), 3.33–3.00 (m, 2H), 2.26–1.77 (m, 3H),
1.75–1.52(m, 1H), 1.48 ppm (s, 9H); 13C NMR (62.9 MHz, CDCl3): d=
81.1, 80.8, 66.5, 60.3, 38.3, 37.0, 29.0, 28.3, 25.1 ppm; HRMS: m/z: calcd
for C20H25N2O5: 441.1443 [M+H]+; found: 441.1438.
16 mg, 79%; purity >95%; 1H NMR (250 MHz, [D6]DMSO): d=11.27
(s, 1H), 8.10 (t, J=5.7, 1H), 7.55 (d, J=1.8 Hz, 1H), 7.25 (d, J=8.5 Hz,
1H), 7.15 (dd, J=1.9, 8.6 Hz, 1H), 6.70 (d, J=3.6 Hz, 1H), 5.50 (d, J=
5.6 Hz, 1H), 5.01–4.88 (m, 1H), 3.62(d, J=5.8 Hz, 2H), 3.45–3.09 (m,
4H), 2.72 (dd, J=6.6, 15.6 Hz, 1H), 2.58 (d, J=12.4, 1H), 1.65 ppm (dq,
J=4.6, 12.0 Hz, 1H); 13C NMR (62.5 MHz, [D6]DMSO): d=171.1, 155.1,
135.2, 134.9, 128.3, 123.2, 120.1, 117.6, 115.9, 113.0, 111.0, 105.2, 50.3,
49.3, 41.0, 37.7, 28.7, 22.1 ppm; HRMS: m/z: calcd for C17H17BrN4O4:
421.0506 [M+H]+; found: 421.0493.
Pictet–Spengler product from ApaBB-Ala(benzothiophen-3-yl)-Gly-
1
HMBA-PEGA800
A
3-Boc-2-(4,5-dimethoxy-2-nitro-phenyl)-[1,3]oxazinane (13): 3-Amino-
propanol (2.00 mL, 26.1 mmol) and anhydrous sodium sulfate (37.0 g,
261 mmol) were added to a solution of technical quality (80% pure) 4,5-
dimethoxy-2-nitrobenzaldehyde (12) (5.50 g, 20.84 mmol) in toluene
(150 mL). The mixture was stirred for 3 h and then Boc2O (5.69 g,
26.1 mmol) was added. After 3 h, the mixture was filtered and the solvent
removed in vacuo. The crude product was purified by flash-column chro-
matography by using ethyl acetate/petroleum ether 1:5. The product 13
(5.28 g, 14.3 mmol, 69%) was obtained as a light yellow liquid, which sol-
idified upon storage in the refrigerator. 1H NMR (250 MHz, CD3OD):
d=7.46 (s, 1H), 6.87 (s, 1H), 6.81 (s, 1H), 3.98 (dt, J=9.1, 17.9 Hz, 1H),
3.90 (s, 6H), 3.81–3.69 (m, 1H), 3.68–3.54 (m, 1H), 3.44–3.30 (m, 1H),
1.91–1.70 (m, 1H), 1.69–1.54 (m, 1H), 1.42ppm (s, 9H); 13C NMR
(62.5 MHz, CD3OD): d=156.0, 153.3, 150.3, 126.5, 111.5, 109.7, 82.2,
63.9, 57.0, 56.8, 41.2, 28.5, 25.9 ppm; HRMS: m/z: calcd for C17H25N2O7:
369.1656 [M+H]+; found: 369.1659.
d=8.23–8.11 (m, 1H), 7.92 (d, J=7.2Hz, 1H), 7.68 (d, J=7.0 Hz, 1H),
7.45–7.29 (m, 2H), 6.76 (d, J=3.4 Hz, 1H), 5.55 (d, J=5.9 Hz, 1H), 5.08–
4.96 (m, 1H), 3.65 (d, J=4.4 Hz, 2H), 3.51–2.88 (m), 2.87–2.74 (m, 1H),
2.44–2.32 (m, 1H), 1.90–1.69 ppm (m, 1H); HRMS: m/z: calcd for
C17H18N3O4S: 360.1018 [M+H]+; found: 360.1011.
Pictet–Spengler product from ApaBB-Ala(thiophen-3-yl)-Gly-HMBA-
1
PEGA800
A
(t, J=5.9 Hz, 1H), 7.35 (d, J=5.1 Hz, 1H), 6.81 (d, J=5.0 Hz, 1H), 6.67
(s, 1H), 5.40 (d, J=5.7 Hz, 1H), 4.97–4.85 (m, 1H), 3.68 (d, J=5.9 Hz,
2H), 3.44–3.02 (m, 3H), 2.76–2.63 (m, 1H), 2.37–2.24 (m, 1H), 1.82–
1.62ppm (m, 1H); HRMS: m/z: calcd for C13H16N3O4S: 310.0862
[M+H]+; found: 310.0852.
Pictet–Spengler product from ApaBB-Ala(thiophen-2-yl)-Gly-HMBA-
1
PEGA800
A
(t, J=5.8 Hz, 1H), 7.29 (d, J=5.1 Hz, 1H), 6.92(d, J=5.2Hz, 1H), 6.66
(s, 1H), 5.48 (d, J=5.4 Hz, 1H), 4.83–4.65 (m, 1H), 3.68 (d, J=5.8 Hz,
2H), 3.50–3.05 (m, 3H), 2.90–2.75 (m, 1H), 2.50–2.38 (m, 1H), 1.66–
1.38 ppm (m, 1H); HRMS: m/z: calcd for C13H16N3O4S: 310.0862
[M+H]+; found: 310.0853.
3-Boc-2-(4,5-dimethoxy-2-pentafluorophenyloxycarbonyl-aminophenyl)-
[1,3]oxazinane (14): Compound 13 (3.70 g, 10.0 mmol), DMAP (122 mg),
and 10% Pd/C (370 mg) in MeOH (100 mL) were stirred under a H2 at-
mosphere (1 atm) for 2.5 h. After this time, the mixture was filtered
through Celite and solvent removed in vacuo. The crude product was dis-
solved in dichloromethane (50 mL) and a solution of bispentafluorophen-
yl carbonate (3.94 g, 10.0 mmol) in dichloromethane (10 mL) was added.
After 5 h, the solvent was removed in vacuo and the crude product was
purified by column chromatography by using ethyl acetate/petroleum
ether 1:8. The product 14 (4.33 g, 7.9 mmol, 79%) was obtained as a light
yellow syrup, which solidified upon storage in the refrigerator. 1H NMR
(250 MHz, CD3OD): d=7.62(brs, 1H), 6.66 (s, 1H), 6.58 (s, 1H), 4.20–
4.03 (m, 1H), 3.93–3.71 (m, 8H), 3.42–3.26 (m, 1H), 2.05–1.81 (m, 1H),
1.69–1.50 (m, 1H), 1.49 ppm (s, 9H); 13C NMR (62.5 MHz, CD3OD): d=
112.9, 109.1, 82.9, 62.1, 57.0, 56.5, 39.9, 28.6, 26.0 ppm; HRMS: m/z: calcd
for C24H26F5N2O7: 549.1655 [M+H]+; found: 549.1651.
Pictet–Spengler product from ApaBB-Ala(furan-2-yl)-Gly-HMBA-
1
PEGA800
A
(t, J=4.8 Hz, 1H), 7.47 (d, J=1.8 Hz, 1H), 6.65 (s, 1H), 6.39 (d, J=
1.9 Hz, 1H), 5.51 (d, J=5.9 Hz, 1H), 4.67–4.55 (m, 1H), 3.72(d, J=
6.1 Hz, 2H), 3.56–3.02 (m, 3H), 2.80–2.68 (m, 1H), 2.40–2.16 (m, 1H),
1.61–1.37 ppm (m, 1H); HRMS: m/z: calcd for C13H16N3O5: 294.1090
[M+H]+; found: 294.1073.
Pictet–Spengler product from ApaBB-Phe-Gly-HMBA-PEGA800 (18h):
N
Purity >95%; 1H NMR (250 MHz, [D6]DMSO): d=8.18–8.06 (m, 1H),
7.27–7.04 (m, 4H), 6.64 (s, 1H), 5.29–5.25 (m, 1H), 4.81 (d, J=10.5 Hz,
1H), 3.50–3.30 (m, 3H), 3.16 (d, J=15.6 Hz, 1H), 2.90 (dd, J=5.7,
15.2 Hz, 1H), 2.42–2.33 (m, 1H), 1.72–1.58 ppm (m, 1H); HRMS: m/z:
calcd for C15H18N3O4: 304.1297 [M+H]+; found: 304.1298.
Pictet–Spengler product from ApaBB-Trp-Gly-HMBA-PEGA800 (18a):
R
Resin (154 mg, theoretical loading: 0.27 mmolgÀ1) was cleaved and yield-
ed 13.4 mg of product 17a (94%); Purity >95%; 1H NMR (250 MHz,
[D6]DMSO): d=12.72 (brs, 1H), 11.11 (s, 1H), 8.14 (t, J=5.8 1H), 7.38
(d, J=7.4 Hz, 1H), 7.28 (d, J=7.9 Hz, 1H), 7.09–6.90 (m, 2H), 6.67 (d,
J=3.7 Hz, 1H), 5.51 (d, J=5.6 Hz, 1H), 5.03–4.89 (m, 1H), 3.65 (d, J=
3.9 Hz, 2H), 3.57–3.01 (m), 2.81–2.68 (m, 1H), 2.64–2.53 (m, 1H), 1.74–
1.54 ppm (m, 1H); 13C NMR (62.5 MHz, [D6]DMSO): d=171.5, 171.2,
155.3, 136.3, 133.5, 126.5, 120.9, 118.4, 117.7, 111.0, 105.1, 50.5, 49.4, 40.8,
37.8, 28.9, 22.3 ppm; HRMS: m/z: calcd for C17H19N4O4: 343.1401
[M+H]+; found: 343.1452.
Pictet–Spengler product from ApaBB-Tyr-Gly-HMBA-PEGA800 (18i):
R
1
Purity >95%; H NMR (250 MHz, [D6]DMSO): d=9.18 (s, 1H), 8.07 (t,
J=6.0 Hz, 1H), 6.90 (d, J=8.9 Hz, 1H), 6.65–6.50 (m, 3H), 5.21 (dd, J=
2.7, 6.0 Hz, 1H), 4.76–4.64 (m, 1H), 3.63 (dd, J=2.0, 5.9 Hz, 1H), 3.21–
2.95 (m, 3H), 2.82–2.71 (m, 1H), 2.32–2.20 (m, 1H), 1.72–1.55 (m, 1H);
HRMS: m/z: calcd for C15H18N354 [M+H]+: 320.1246; found: 320.1227.
Pictet–Spengler product from ApaBB-3,4-Di-MeO-Phe-Gly-HMBA-
PEGA800
(18j): Purity >95%; 1H NMR (250 MHz, [D6]DMSO): d=8.04
U
(t, J=5.7 Hz, 1H), 6.77 (s, 1H), 6.65–6.62(m, 2H), 5.26 (m, 1H), 4.76–
4.68 (m, 1H), 3.71 (s, 3H), 3.60 (dd, J=2.7, 5.7 Hz, 1H), 3.42–3.07 (m,
4H), 2.78 (dd, J=6.2, 15.6 Hz, 1H), 2.46–2.37 (m, 1H), 1.62–1.46 ppm
(m, 1H); HRMS: m/z: calcd for C17H22N3O6 [M+H]+: 364.1509; found:
364.1499.
Pictet–Spengler product from ApaBB-Trp
A
ACHTREUNG
yield the product 17b (15 mg, 63%); Purity >95%; 1H NMR (250 MHz,
[D6]DMSO): d=11.08 (s, 1H), 8.11 (t, J=6.7, 1H), 7.27 (dd, J=4.5,
8.8 Hz, 1H), 7.13 (dd, J=2.5, 9.9 Hz, 1H), 6.87 (dt, J=2.4, 9.1 Hz, 1H),
6.69 (d, J=3.8 Hz, 1H), 5.50 (d, J=6.6 Hz, 1H), 5.00–4.87 (m, 1H), 3.63
(d, J=5.8 Hz, 2H), 3.45–3.09 (m, 4H), 2.72 (dd, J=9.9, 15.5 Hz, 1H),
2.46–2.39 (m), 1.65 ppm (dq, J=4.4, 12.0 Hz, 1H); 13C NMR (62.5 MHz,
[D6]DMSO): d=171.2, 158.5, 155.2, 154.9, 135.6, 132.9, 126.8, 126.6,
122.6, 122.6, 111.9, 111.7, 108.9, 108.5, 105.6, 105.5, 102.8, 102.5, 50.3,
49.3, 41.0, 37.7, 28.7, 22.2 ppm; HRMS: m/z: calcd for C17H17FN4O4:
361.1307 [M+H]+; found: 361.1304.
Pictet–Spengler product from ApaBB-1-pyrenyl-Ala-Gly-HMBA-
PEGA800
(18k): Purity >95%; 1H NMR (250 MHz, [D6]DMSO): d=
R
8.39–8.17 (m, 6H), 8.15–7.99 (m, 3H), 6.77 (d, J=3.2Hz, 1H), 5.62 (d,
J=4.2Hz, 1H), 5.34 (dd, J=2.5, 10.5 Hz, 1H), 4.14 (d, J=15.0 Hz, 1H),
3.63–3.20 (m, 5H), 2.67 (d, J=11.3 Hz, 1H), 1.91–1.70 ppm (m, 1H);
HRMS: m/z: calcd for C25H22N3O4: 428.1605 [M+H]+; found: 428.1611.
The cyclic N,O-acetal product from AbaBB-Phe-Gly-HMBA-PEGA800
A
1H), 9.96 (s, 1H), 7.46–7.13 (m, 8H), 7.01 (t, J=7.5 Hz, 1H), 6.88 (d, J=
8.0 Hz, 1H), 6.03 (s, 1H), 4.81 (t, J=6.9 Hz, 1H), 4.11 (d, J=18.0 Hz,
1H), 3.76 (d, J=18.0 Hz, 1H), 2.96 ppm (d, J=7.0 Hz, 1H); 13C NMR
Pictet–Spengler product from ApaBB-Trp
A
ACHTREUNG
8064
www.chemeurj.org
ꢁ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2006, 12, 8056 – 8066

Novel N-carbamyliminium Pictet–Spengler Reactions
FULL PAPER
(62.5 MHz, [D6]DMSO): d=172.6, 169.6, 152.3, 137.1, 136.4, 130.6, 129.3,
128.2, 127.5, 126.5, 121.7, 116.4, 114.6, 112.4, 70.3, 60.3, 41.6, 33.2 ppm;
HRMS: m/z: calcd for C19H18N3O4: 352.1292 [M+H]+; found: 352.1282.
Oxidation of Pictet–Spengler products on solid phase (22a–g): The dry
PEGA800 resin with HMBA linker and Pictet–Spengler product attached
(50–150 mg, 0.30–0.38 mmolgÀ1, 15–57 mmol) 21a–g was swelled in TFA
(95%, aq) in an open syringe fitted with a Teflon frit and the mixture
was shaken vigorously (reaction time see below). The resin was then
washed with H2O until the eluate had a pH 5–7. It was washed with
DMF (6”), dichloromethane (6”), and lyophilized. The compound was
cleaved from the resin by swelling the dry PEGA800 resin with HMBA
linker and the compound attached in NaOH (aq, 0.1m, 10 mLmgÀ1 resin).
After 2h, HCl (aq, 0.1 m) was used for neutralization. The resin was ex-
tracted with H2O (”2) and acetonitrile/H2O 70:30 (”2). The solvent from
the extract was removed in a speedvac to give the crude product (22a–g)
as a solid salt with a yellow to intensive red color.
Pictet–Spengler product from AbaBB-Trp-Gly-HMBA-PEGA800 (21a):
A
Purity >95%; 1H NMR (250 MHz, [D6]DMSO): d=10.86 (s, 1H), 9.78
(s, 1H), 7.54 (t, J=4.4 Hz, 1H), 7.47–7.31 (m, 3H), 7.25 (t, J=7.6 Hz,
1H), 7.11–6.89 (m, 4H), 6.12(s, 1H), 5.29 (d, J=6.1 Hz, 1H), 3.45–3.29
(m, 2H), 3.23 (dd, J=4.2, 16.7 Hz, 1H), 3.01–2.84 ppm (m, 1H);
13C NMR (62.5 MHz, [D6]DMSO): d=171.7, 169.1, 155.1, 137.3, 136.3,
130.6, 128.4, 126.6, 125.9, 121.6, 121.1, 119.8, 118.7, 117.8, 115.9, 113.7,
111.5, 106.4, 52.5, 51.7, 44.0, 21.2 ppm; HRMS: m/z: calcd for
C21H18N4O4: 391.1401 [M+H]+; found: 391.1426.
Pictet–Spengler product from AbaBB-Trp(5-F)-Gly-HMBA-PEGA800
H
Oxidized Pictet–Spengler product from AbaBB-Trp-Gly-HMBA-
A
PEGA800
(22a): Reaction time: 5 h; purity >95%; 1H NMR (250 MHz,
R
cleaved yield: 21 mg, 90%; 1H NMR (250 MHz, [D6]DMSO): d=10.89
(s, 1H), 9.77 (s, 1H), 7.54 (t, J=4.5 Hz, 1H), 7.40 (d, J=6.6 Hz, 1H),
7.33 (dd, J=4.6, 8.8 Hz, 1H), 7.29–7.13 (m, 2H), 7.01 (t, J=7.5 Hz, 1H),
[D6]DMSO): d=13.60 (brs, 1H), 12.74–12.39 (m, 2H), 8.84 (t, J=5.7 Hz,
1H), 8.67 (d, J=8.4 Hz, 1H), 8.11 (t, J=7.7 Hz, 1H), 7.85 (d, J=8.2Hz,
1H), 7.74–7.46 (m, 4H), 7.27 (t, J=7.0 Hz, 1H), 6.03 (d, J=6.6 Hz, 1H),
3.88 (d, J=18.0 Hz, 1H), 3.73–3.56 ppm (m, 4H); HRMS: m/z: calcd for
C21H17N4O4: 389.1244 [M]+; found: 389.1237.
6.95–6.82(m, 2H), 6.15 (s, 1H), 5.29 (d,
J=6.2Hz, 1H), 3.47–3.14 (m,
3H), 2.98–2.80 ppm (m, 1H); 13C NMR (62.5 MHz, [D6]DMSO): d=
171.5, 168.8, 158.7, 155.0, 154.9, 137.1, 132.9, 132.8, 128.5, 126.9, 126.7,
126.1, 121.6, 119.2, 113.6, 112.5, 112.3 109.2, 108.8, 106.7, 102.8, 102.4,
52.4, 51.6, 43.9, 20.8 ppm; HRMS: m/z: calcd for C21H17FN4O4: 409.1307
[M+H]+; found: 409.1297.
Oxidized Pictet–Spengler product from AbaBB-Trp
A
PEGA800
AHCTREUNG
[D6]DMSO): d=12.80 (s, 1H), 12.53 (brs, 1H), 8.86 (t, J=5.8 Hz, 1H),
8.68 (d, J=8.3 Hz, 1H), 8.13 (t, J=7.4 Hz, 1H), 7.73–7.53 (m, 4H), 7.46
(dt, J=5.1, 18.3 Hz, 1H), 6.04 (d, J=6.5 Hz, 1H), 4.01 (d, J=17.6 Hz,
1H), 3.68 (d, J=5.9 Hz, 2H), 3.62–3.48 ppm (m, 1H); HRMS: m/z: calcd
for C21H15N4O4: 407.1150 [M]+; found: 407.1145.
Pictet–Spengler product from AbaBB-Trp
A
ACHTREUNG
22 mg, 82%; purity >95%; 1H NMR (250 MHz, [D6]DMSO): d=11.06
(s, 1H), 9.78 (s, 1H), 7.62(d, J=1.8 Hz, 1H), 7.55 (t, J=4.6 Hz, 1H),
7.40 (d, J=7.4 Hz, 1H), 7.32(d, J=8.6 Hz, 1H), 7.25 (t, J=7.7 Hz, 1H),
7.17 (dd, J=1.9, 8.6 Hz, 1H), 7.00 (t, J=7.5 Hz, 1H), 6.92(d, J=7.8 Hz,
1H), 6.16 (s, 1H), 5.29 (d, J=6.2Hz, 1H), 3.47–3.12 (m, 3H), 2.97–
2.81 ppm (m, 1H); 13C NMR (62.5 MHz, [D6]DMSO): d=171.4, 168.7,
154.8, 137.1, 134.9, 132.5, 128.5, 128.4, 126.8, 123.5, 121.6, 120.1, 119.2,
113.7, 113.5, 111.3, 106.3, 52.4, 51.6, 44.0, 20.8 ppm; HRMS: m/z: calcd
for C21H17BrN4O4 [M+H]+: 469.0506; found: 469.0494.
Oxidized Pictet–Spengler product from AbaBB-Trp
A
PEGA800
AHCTREUNG
[D6]DMSO): d=12.86 (s, 1H), 8.84 (t, J=5.7 Hz, 1H), 8.67 (d, J=
8.3 Hz, 1H), 8.24–8.06 (m 2H), 7.75–7.52 (m, 4H), 6.04 (d, J=6.7 Hz,
1H), 4.04 (d, J=17.7 Hz, 1H), 3.75–3.52ppm (m, 3H); HRMS: m/z:
calcd for C21H15BrN4O4: 467.0349 [M]+; found: 467.0341.
Oxidized Pictet–Spengler product of AbaBB-Ala(thiophen-3-yl)-Gly-
HMBA-PEGA800
(22d): Reaction time: 5 d; purity >95%; 1H NMR
E
Pictet–Spengler product from AbaBB-Ala(3-thiophen-3-yl)-Gly-HMBA-
(250 MHz, [D6]DMSO): d=8.92–8.73 (m, 3H), 8.23–8.06 (m, 2H), 7.69
(dd, J=7.5, 8.5 Hz, 1H), 7.60 (d, J=7.6 Hz, 1H), 7.52(d, J=5.0 Hz, 1H),
6.02(d, J=6.5 Hz, 1H), 3.97–3.80 (m, 1H), 3.49 ppm (dd, J=1.8, 5.7 Hz,
1H), 3.65–3.55 ppm (m, 1H); HRMS: m/z: calcd for C17H14N3O4S:
356.0700 [M]+; found: 356.0702.
1
PEGA800
A
(s, 1H), 8.59 (t, J=6.0 Hz, 1H), 7.32–7.18 (m, 3H), 7.02 (dt, J=1.1,
7.5 Hz, 1H), 6.95–6.79 (m, 2H), 6.06 (s, 1H), 5.33 (d, J=6.9 Hz, 1H),
3.88 (dd, J=6.6, 17.3 Hz, 1H), 3.63 (dd, J=5.5, 17.3 Hz, 1H), 3.27 (d, J=
15.0 Hz, 1H), 2.97–2.81 ppm (m, 1H); 13C NMR (62.5 MHz, [D6]DMSO):
d=171.4, 170.0, 153.3, 136.2, 135.5, 133.3, 128.9, 128.1, 126.2, 123.7, 122.3,
119.4, 113.9, 52.7, 52.1, 23.2 ppm; HRMS: m/z: calcd for C17H16N3O4S:
358.0856 [M+H]+; found: 358.0832.
Oxidized Pictet–Spengler product from AbaBB-Ala(thiophen-2-yl)-Gly-
HMBA-PEGA800
(22e): Reaction time: 5 d; purity >95%; 1H NMR
G
(250 MHz, [D6]DMSO): d=8.80 (t, J=5.8 Hz, 1H), 8.50 (d, J=8.3 Hz,
1H), 8.18 (t, J=7.4 Hz, 1H), 8.01 (d, J=5.3 Hz, 1H), 7.85 (d, J=5.4 Hz,
1H), 7.59 (m, 2H), 6.00 (d, J=8.1 Hz, 1H), 4.03 (d, J=16.9 Hz, 1H),
3.76–3.63 ppm (m, 4H); HRMS: m/z: calcd for C17H14N3O4S: 356.0700
[M+H]+; found: 356.0725.
Pictet–Spengler product from AbaBB-Ala(thionphen-2-yl)-Gly-HMBA-
PEGA800
(21e): Purity >95%; 1H NMR (250 MHz, [D6]DMSO): d=
H
12.54 (brs, 1H), 9.68 (s, 1H), 8.54 (t, J=5.9 Hz, 1H), 7.31–7.18 (m, 3H),
6.99 (dt, J=1.1, 7.5 Hz, 1H), 6.86 (d, J=7.9 Hz, 1H), 6.65 (d, J=5.2Hz,
1H), 5.85 (s, 1H), 5.32(d, J=6.2Hz, 1H), 3.84 (dd, J=6.4, 17.3 Hz, 1H),
3.64 (dd, J=5.6, 17.3 Hz, 1H), 3.46 (d, J=22.2 Hz, 1H), 3.13–2.97 ppm
(m, 1H); 13C NMR (62.5 MHz, [D6]DMSO): d=171.3, 169.9, 153.2, 136.8,
134.2, 133.2, 128.4, 126.6, 124.9, 122.8, 121.4, 119.8, 113.8, 53.2, 52.2,
23.2 ppm; HRMS: m/z: calcd for C17H16N3O4S: 358.0856 [M+H]+; found:
358.0806.
Oxidized Pictet–Spengler product from AbaBB-Phe(3,4-di-MeO)-Gly-
HMBA-PEGA800
(22 f): Reaction time: 7 d.; purity >95%; 1H NMR
C
(250 MHz, [D6]DMSO): d=13.55 (brs, 1H), 8.75 (t, J=5.8 Hz, 1H), 8.45
(d, J=8.3 Hz, 1H), 8.15 (t, J=7.4 Hz, 1H), 7.68–7.52(m, 2H), 7.46 (s,
1H), 7.27 (s, 1H), 5.86–5.73 (m, 1H), 3.99 (s, 3H), 3.89 (s, 3H), 3.70–
3.55 ppm (m, 4H); HRMS: m/z calcd for C21H20N3O6: 410.1347 [M]+;
found: 410.1357.
Pictet–Spengler product from AbaBB-Phe(3,4-di-MeO)-Gly-HMBA-
Oxidized Pictet–Spengler product of DMAbaBB-Trp-Gly-OH (22g): Re-
action time: 5 h; purity >95%; 1H NMR (250 MHz, [D6]DMSO): d=
12.75 (s, 1H), 8.84 (t, J=5.7 Hz, 1H), 7.86 (s, 1H), 7.81 (d, J=8.2Hz,
1H), 7.65 (d, J=8.5 Hz, 1H), 7.51 (ddd, J=1.0, 7.0, 8.3 Hz, 1H), 7.51
(dd, J=7.2, 7.9 Hz, 1H), 7.07 (s, 1H), 6.54 (s, 1H), 5.95 (d, J=6.3 Hz,
1H), 4.07 (s, 3H), 4.04 (s, 3H), 3.69–3.55 ppm (m, 4H); HRMS: m/z:
calcd for C23H21N4O6: 449.1456 [M]+; found: 449.1447.
1
PEGA800
A
(s, 1H), 8.30 (m, 1H), 7.27 (t, 7.7 Hz, 1H), 7.21 (d, J=7.5 Hz, 1H), 7.01
(t, J=7.4 Hz, 1H), 6.94 (s, 1H), 6.92(d, J=7.7 Hz, 1H), 6.19 (s, 1H),
5.62(s, 1H), 4.79 (t, J=6.6 and 17.3 Hz, 1H), 3.76–3.67 (m, 5H), 3.50 (s,
3H), 3.23–3.09 (m, 1H), 3.00 ppm (dd, J=7.5, 15.6 Hz, 1H); HRMS:
m/z: calcd for C21H22N3O6: 412.1503 [M+H]+; found: 412.1481.
Pictet–Spengler product from DMAbaBB-Trp-Gly-HMBA-PEGA800
(21g): Purity 70%; 1H NMR (250 MHz, [D6]DMSO): d=12.62 (s, 1H),
10.53 (s, 1H), 9.41 (s, 1H), 8.45 (t, J=6.1 Hz, 1H), 7.42(d, J=7.4 Hz,
1H), 7.33 (d, J=7.8 Hz, 1H), 7.07–6.91 (m, 3H), 6.04 (s, 1H), 5.35 (d,
6.0 Hz, 1H), 3.84–3.70 (m, 8H), 3.31 (d, J=16.0 Hz, 1H), 3.73–3.56 ppm
(ddd, J=1.8, 7.0, 15.9 Hz, 1H); HRMS: m/z: calcd for C23H23N4O6:
451.1612[ M+H]+; found: 451.1599.
Acknowledgements
The financial support for this research from the Danish National Re-
search Foundation is gratefully acknowledged (F.D. and J.B.).
Chem. Eur. J. 2006, 12, 8056 – 8066
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Published online: July 31, 2006
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