Role of the peri-effect in synthesis and reactivity of highly substituted naphthaldehydes: A novel backbone amide linker for solid-phase synthesis

Article abstract of DOI:10.1039/b412971g

Handles (linkers) with an aldehyde functionality that permits the anchoring of substrates by reductive animation have, since their first report in the mid-1990s, become widely-used tools in solid-phase synthesis. In the synthesis of peptides, they allow a

Full text of DOI:10.1039/b412971g

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A R T I C L E
Role of the peri-effect in synthesis and reactivity of highly substituted
naphthaldehydes: a novel backbone amide linker for solid-phase
synthesis†
Michael Pittelkow,a Ulrik Boas,a,b Mikkel Jessing,a Knud J. Jensenb and Jørn B. Christensen*a
a Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100,
Denmark. E-mail: jbc@kiku.dk; Fax: +45 353220212; Tel: +45 35320194
b Department of Chemistry, Royal Veterinary and Agricultural University, Thorvaldsensvej 40,
DK-1870, Frederiksberg C, Denmark
Received 23rd August 2004, Accepted 11th November 2004
First published as an Advance Article on the web 23rd December 2004
Handles (linkers) with an aldehyde functionality that permits the anchoring of substrates by reductive amination
have, since their first report in the mid-1990s, become widely-used tools in solid-phase synthesis. In the synthesis of
peptides, they allow anchoring of the growing peptide chain through a backbone amide, thus giving easy access to
C-terminal modified or cyclic peptides. Recently, we described two new handles (NAL-1 and NAL-2) with
dialkoxynaphthaldehyde core structures. Here, we describe the design, synthesis and properties of a novel
trialkoxynaphthalene-based backbone amide linker (NAL-3). The NAL-3 handle is based on a
trialkoxynaphthaldehyde (NALdehyde-3) that was synthesized in nine high-yielding steps from
3-methoxyphenylacetic acid in 51% overall yield. The naphthalene ring system was constructed using a regioselective
methanesulfonic acid-catalyzed ring-closing reaction. The tetra-substituted naphthalene derivative
1,3,6-trimethoxynaphthalene-2-carbaldehyde (7) was selectively demethylated in the 1 position using BBr3. The
selectivity of this reaction is discussed, based on the crystal structures of reactant and product, 1-hydroxy-3,6-
dimethoxy-naphthalene-2-carbaldehyde (8), and in the context of the peri-effect. The new handle was anchored to an
aminomethylated poly(styrene) solid support, followed by assembly of a model dipeptide, then a study of the cleavage
properties under acidic conditions was carried out. Surprisingly, the trialkoxynaphthaldehyde-based handle proved
less acid-labile than the dialkoxynaphthaldehyde handles, and this fact is discussed with respect to handle
design.
then, BAL type handles with monoalkoxy-8, dialkoxy-9, and
Introduction
alkoxyhydroxybenzyl,10 as well as indole11 and thiophene12
The two defining steps in solid-phase synthesis are the anchoring
structures have been reported (Fig. 1).
of the first residue through a handle (linker) to the solid
support (resin) and the release of the fully assembled product.1–3
The handle carries a dual functionality: at one end it allows
permanent anchoring to the support, the other end has the
functionalityof a protecting group, which allows attachment and
eventually release of the final product.4 Acid-labile handles, i.e.
those able to release the final product with dilute trifluoroacetic
acid (TFA) or even milder conditions, remain the most widely
used. The acid-lability can to a large extent be correlated with
the stability of the carbenium ion, which forms after release of
the final product, for leaving groups with comparable electronic
properties.‡
Fig. 1 General backbone amide linkage (BAL) strategy for solid phase
synthesis.
Handles with an aldehyde functionality that allows the
The choice of handle for a particular solid-phase synthesis
anchoring of substrates by reductive amination, have, since
determines the chemistry that can be used for assembly and
their first report in the mid-1990s become widely used tools in
eventual release of the final product. The development of new
solid-phase synthesis.5–7 In the synthesis of peptides, they allow
handles therefore continues to be important.
anchoring of the growing peptide chain through a backbone
amide, thus giving easy access to C-terminal modified or cyclic
peptides. This backbone amide linker (BAL) concept was first
Design of naphthalene backbone amide linkers
The goal of our present research is to develop BAL-type linkers
with an increased acid-lability. One way of achieving this is to use
systems that can further stabilize the carbenium ion formed in
the cleavage process. Stabilization of such a carbenium ion can be
achieved either by increasing the number of electron donating
substituents on the aromatic nucleus,4 by using electron-rich
heteroaromatic compounds11,12 or by increasing the size of
the aromatic system.13§ Recently we reported the first two
implemented in a trialkoxybenzyl system, which allowed release
of final products by treatment with concentrated TFA.5 Since
† Electronic supplementary information (ESI) available: Crystal struc-
tures of 7 and 8, HPLC for Fmoc-Phe-Leu-OH, HMBC spectra of 7, 8,
10 and 11. See http://www.rsc.org/suppdata/ob/b4/b412971g/
‡ The PAL (peptide–amide linker) and BAL (backbone–amide linker)
handles generate the same trialkoxybenzyl carbenium ion upon cleavage,
however, the PAL handle releases a primary amide, while the corre-
sponding BAL handle releases a secondary amide. Interestingly, the PAL
handle requires TFA–H2O (19 : 1) for release while the BAL handle can
release the secondary amide with dilute TFA (as little as 1–2% TFA in
CH2Cl2). We ascribe this difference to steric factors, possibly steric relief.
§ While trityl-based (e.g., the 2-chlorotrityl chloride) handles are very
useful for C-terminal anchoring of the growing peptide chain, the steric
bulk they impose makes them less suitable for BAL-type anchoring.
5 0 8
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 5 0 8 – 5 1 4
T h i s j o u r n a l i s
T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 5
©

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naphthalene based handles, NALdehyde-1 and NALdehyde-2,
based on this strategy shown in Fig. 2 (NAL is an acronym
for naphthalene backbone amide linker).14 Here we describe a
new trialkoxynaphthaldehyde derivative (NALdehyde-3)¶ and
its application in solid-phase synthesis.
the ortho position to the aldehyde, thus giving a more common
ortho stabilizing effect on the stability of the carbenium ion
as used in the benzene derived BAL handles.7 NALdehyde-3
is a trialkoxynaphthaldehyde where the methoxy substituents
are placed such that it becomes an extended equivalent to
the trialkoxybenzaldehyde based BAL handles. Thus we antic-
ipated that NALdehyde-3 would produce a more acid-sensitive
handle than NALdehyde-1 and NALdehyde-2. Furthermore,
NALdehyde-3 has the aldehyde functionality in the 2-position
on the naphthalene core as compared to the 1-position of the
aldehyde in NALdehyde-1 and NALdehyde-2. This illustrates
the additional possibilities that are present when extending the
aromatic core from benzene to naphthalene.
Results and discussion
The synthesis of NALdehyde-3 (10) started from 3-
methoxyphenyl acetic acid, as outlined in Scheme 1. Treatment
of 3-methoxyphenylacetic acid with SOCl2 in CH2Cl2 gave the
acid chloride 2 that was conveniently reacted directly with the
magnesium salt of diethyl malonate to produce the adduct 3.18
Treatment of 3 with methanesulfonic acid at rt yielded the
ring-closed naphthalene compound 4 in a highly regioselective
manner. This selectivity followed the general trend for this type
of cyclisation, and was anticipated due to steric reasons.19 The
regiochemical outcome of the reaction was confirmed by X-
ray structure elucidation of derivatives (vide infra) and only
minor amounts (less than 1%) of the unwanted regioisomer
was observed. Methylation of 4 by treatment with excess
(CH3O)2SO2 in dry acetone yielded 5 in excellent yield. The
ester group in 5 was reduced to the alcohol 6 with LiAlH4 in
ether and subsequently oxidized to the aldehyde 7 with PDC in
84% yield. X-ray structure elucidation confirmed the structure
of 7 (Fig. 3).ꢀ Selective mono-demethylation in the 1-position
of 7 was achieved using BBr3 in CH2Cl2 at 0 ◦C yielding 8
in 86% yield as a yellow crystalline material. Performing the
reaction using the same reaction conditions at −78 ◦C did
not change either the yield or the selectivity. A small amount
of demethylation (approximately 1%) was observed in the 3-
position yielding naphthol 11 (Scheme 2). The regiochemistry of
the twodifferent naphthols was confirmedbysingle crystal X-ray
crystallography (Fig. 4) and 2D NMR spectroscopy (see crystal
data). Attachment of the valeric acid spacer, thus completing the
synthesis of the handle, was achieved by reacting the naphthol
compound 8 with ethyl 5-bromovalerate in DMF with K2CO3
as the base, followed by hydrolysis of the ethyl ester with dilute
NaOH in THF to yield 10 as the target handle, NALdehyde-3.
The high regioselectivity of the demethylation step from 7 to
8 was surprising. Methoxy groups placed in the ortho-position
to an aldehyde functionality, are known to increase reactivity
towards demethylation due to chelation control.20,21 Thus, we
expected the demethylation to take place in the 1- and 3-
positions of the naphthalene without pronounced selectivity.
The surprising selectivity led us to examine the crystal structure
of the reactant 7 and the main product 8. The crystal structure
of the 1,3,6-trimethoxynaphthalene-2-carbaldehyde (7) shows
interesting features (Fig. 3).
Fig. 2 Structure of two recently reported dialkoxynaphthaldehydes
(NALdehyde-1 and NALdehyde-2) and the new trialkoxynaphthalde-
hyde (NALdehyde-3).
Relatively few highly substituted naphthalenes are known.
The classical approach to aromatic compounds is electrophilic
aromatic substitution, but in the case of naphthalene (and
polycyclic aromatic compounds in general), this gives rise to
problems with the regiochemistry due to a much larger number
of possible isomers. Another problem is that many potential
starting materials of the polycyclic aromatic hydrocarbon type
with a suitable substitution pattern are no longer commercially
available.15
Substituted naphthalenes with the type of substitution pattern
needed in this study are furthermore difficult to synthesize by
classical methods due to the non-symmetrical nature of the
target. Many hydroxy substituted naphthalene compounds have
been synthesized by an alkali melting procedure via the corre-
sponding sulfonic acid derivative. This procedure is somewhat
harsh, and it does not introduce the desired functionalities
on the aromatic core. When dealing with highly substituted
naphthalene substrates one also has to take into account
the so called peri-effect, defined as the non-bonding repulsive
interaction between substituents in the 1- and the 8-positions on
the naphthalene core.16 The above considerations led us to look
into strategies based on building the naphthalene derivative from
a suitable benzene derivative. Several methods for synthesis of
naphthalenes from benzene derivatives exist and this topic has
been recently reviewed.17 In the development of the present pro-
cedure, scalability and ease was important, and this was achieved
by combining the reported procedure on naphthoresorcinol with
improvements disclosed in the recent literature on acid-catalyzed
ring-closing reactions with naphthalene systems.18,19
The methyl group of the 1-methoxy group is locked in a
position that is 76.9◦ out of plane with the aromatic core, and
the aldehyde oxygen is locked in a position pointing towards the
1-methoxy group. Also, the C–C bond connecting the aldehyde
The design of the three different NAL handles incorporates
distinct features. Common for NALdehyde-1 and NALdehyde-
2 is that they are dialkoxynaphthaldehydes. In NALdehyde-
1 one of the alkoxy-groups is placed in the peri-position,
enabling investigation of the effect of the closer proximity of the
alkoxy group [the 1–8 distance (peri-distance) in naphthalenes
is shorter than the 1–2 distance (ortho-distance)] on stability of
the carbenium ion formed during the cleavage of the products
from the solid phase.16 NALdehyde-2 has a methoxy group in
ꢀX-Ray data for 7. C14H14O4, M = 246.25, monoclinic system, space
˚
group P21/c, a = 3.9760(3), b = 11.8250(10), c = 24.3230(16) A, b =
◦
3
95.029(7) , Z = 4, V = 1139.17(16) A , Dc = 1.436 g cm−3, l(Mo Ka) =
˚
0.105 cm−1, crystal dimensions of 0.16 × 0.14 × 0.54 mm, Rinit = 0.1484,
final R = 0.0548 using 3941 independent reflections.
CCDC reference numbers 248480 and 248481. See http://www.
rsc.org/suppdata/ob/b4/b412971g/ for crystallographic data in .cif
format.
¶ In naming the new naphthaldehyde ‘NALdehyde-3’ we follow the
precedent set by the name ‘PALdehyde’; see ref. 5.
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 5 0 8 – 5 1 4
5 0 9

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Scheme 1 Reagents and conditions: (a) SOCl2, CH2Cl2; (b) Mg, EtOH, CH2(CO2Et)2, ether; (c) CH3SO3H, 74% (3 steps); (d) (CH3O)2SO2, K2CO3,
acetone, 98%; (e) LiAlH4, ether, quant.; (f) PDC, CH2Cl2, 84%; (g) BBr3, CH2Cl2, 0 ◦C, 86%; (h) ethyl 5-bromovalerate, K2CO3, DMF, 60 ◦C, quant.;
(i) 2M NaOH (aq), THF, 97%.
Scheme 2 Reagents and conditions: BBr3, CH2Cl2, 0 ◦C.
molecule, and they are an illustrative example of the effect of
peri-substitution.16 The protons in the 4- and 8-positions on the
naphthalene core ‘flank’ the methoxy groups in the 3- and 1-
positions, respectively, and these methoxy groups again ‘flank’
the aldehyde group in the 2-position. As a consequence, there
is not enough space to place both the methoxy groups and the
aldehyde group in the plane of the aromatic core. This forces
the methyl moiety of the methoxy group in the 1-position out of
the aromatic plane, because of steric repulsion (the peri-effect).
This, in turn, forces the C–C bond to the aldehyde slightly out
of the aromatic plane. In the demethylated product 8, some
of the steric strain has been relieved, as seen from the crystal
structure where all substituents are in plane with the aromatic
Fig. 3 Crystal structure of compound 7, illustrating that the methyl
moiety of the 1-methoxy group is twisted out of plane with the
naphthalene ring. Also, the aldehyde moiety is twisted slightly out of
the plane.
to the aromatic part of the molecule is out of plane with the
aromatic core. The plane spanning C2, C3 and the aldehyde
carbon is 7.4◦ out of the aromatic plane. These deviations
from planarity are due to the sterically congested nature of the
5 1 0
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 5 0 8 – 5 1 4

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Fig. 4 Crystal structure of compound 8 illustrating the total planarity
of the compound.
core (Fig. 4).‡‡ Thus, we propose steric relief as the driving
force deciding the regioselectivity in the demethylation reaction,
although increased basicity of O1 due to reduced overlap of the
oxygen lone pairs with the aromatic ring could also play a role.
In order to evaluate NALdehyde-3 as
a handle for
solid-phase peptide synthesis, NALdehyde-3 (10) was an-
chored to a high loading aminomethylated poly(styrene) resin
with N-[(1H-benzotriazol-1-yl)(dimethylamino)methylene]-N-
methylmethanaminium hexafluorophosphate N-oxide (HBTU)
in the presence of N,N-diisopropylethylamine (DIEA) in
DMF. Unreacted free amine sites on the resin were capped with
acetic acid anhydride.
Then a reductive amination of the free aldehyde on NAL-
3 with H-Leu-OtBu·HCl and NaBH3CN in DMF was con-
ducted twice, followed by acylation of the secondary amine
functionality on the handle with the symmetric anhydride of
Fmoc-Phe-OH (also conducted twice) resulting in a loading of
0.21 mmol g−1 (initial loading was 0.36 mmol g−1). The solid-
phase synthesis of the model dipeptide Fmoc-Phe-Leu-OH on
the NAL-3 is outlined in Scheme 3.
To investigate the acid-lability of the NAL-3 handle, the
synthesized dipeptide was cleaved from the solid support using
different acidic conditions, as shown in Table 1. Using ‘high acid’
conditions (TFA–CH2Cl2 19 : 1) and long reaction times, the
dipeptide was released in up to 56% yield (Scheme 3). Acidolytic
release from the NAL-3 resin with TFA–CH2Cl2 (19 : 1) gave
the dipeptide in 44% (16 h), 47% (24 h) and 56% (48 h) with
high HPLC purity. The overall yield of released dipeptide was
around 13% under high acid conditions for 1 h. Applying lower
concentrations of TFA in CH2Cl2 or adding H2O resulted in
lower cleavage yields releasing only traces of the dipeptide after
1 h. At these ‘low acid’ conditions a relatively large amount of
the fully protected peptide was released, however also in modest
yield. Thus, to our surprise, the NAL-3 handle did not exhibit
as high acid-lability as the dialkoxynaphthalene based handles
previously reported.
Scheme 3 Solid-phase synthesis of Fmoc-Phe-Leu-OH on NAL-3.
Reagents and conditions: (a) HBTU–DIPEA–HOBt–DMF, 16 h, rt,
then 30% Ac2O, DMF–DIPEA (catalytic amount), 3 h, rt; (b) H-Leu-
OtBu·HCl–NaBH3CN–DMF, 16 h, rt; (c) Fmoc-Phe-OH–DIPCDI,
CH2Cl2–DMF 9 : 1, 16 h, rt; (d) acidolytic release from NAL-3 resin,
see Table 1.
with a Fmoc-Phe-OH standard curve. Fmoc-Phe-Leu-OH was
synthesized on a preparative scale using the NAL-3 handle
and cleaved using 95% TFA in CH2Cl2 in 50% crude yield (see
supplementary information for HPLC†).
We had anticipated that NAL-3 would be highly acid-labile,
and more so than the naphthalene based handles with fewer
alkoxy substituents (NAL-1 and NAL-2). Also, the differences
between NAL-3 vs. NAL-1 and NAL-2 were expected to
parallel the differences between the BAL handles based on
dialkoxybenzaldehyde and trialkoxybenzaldehyde.22 The crystal
structure of the intermediate aldehyde (7) clearly showed that the
methoxy group in the peri-position and the aldehyde group are
twisted out of the aromatic plane. Co-planarity is of paramount
The above mentioned cleavage yields were determined from
the HPLC areas of released Fmoc-Phe-Leu-OH in comparison
‡‡ X-Ray data for 8. C13H12O4, M = 232.23, monoclinic system, space
˚
group P21/c, a = 18.985(2), b = 3.9180(6), c = 14.404(3) A, b =
◦
3
103.443(11) , Z = 4, V = 1049.3(3) A , Dc = 1.470 g cm−3, l(Mo
˚
Ka) = 0.109 cm−1, crystal dimensions of 0.06 × 0.12 × 0.61 mm, Rinit
=
0.1449, final R = 0.0519 using 3078 independent reflections.
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 5 0 8 – 5 1 4
5 1 1

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Table 1 Cleavage yields using different acidic conditions
Bu¨chi B-140 apparatus and are uncorrected. Elemental analysis
was performed by Mrs Karin Linthoe. Fast-atom bombardment
(FAB) mass spectra were recorded on a Jeol JMS-HX 110A
Tandem Mass Spectrometer in the positive ion mode using m-
NBA as the matrix. HRMS were recorded on a Micromass Q-
TOF apparatus using electrospray ionisation (ESI) technique.
All column chromatography was performed on Merck Kieselgel
60 (0.015–0.040 mm) using the DCVC technique.24
Cleavage conditionsa
Time/h
Cleavage yield/%b
95% TFA–DCM
95% TFA–H2O
1
1
13
3
50% TFA–DCM
50% TFA–2% TIS–DCM
5% TFA–DCM
5% TFA–2% TIS–DCM
1% TFA–DCM
1% TFA–2% TIS–DCM
95% TFA–DCM
1
1
1
1
1
1
16
24
48
9
6
<1
<1
<1
<1
44
47
56
2-[2-(3-Methoxyphenyl)-acetyl]-malonic acid diethyl ester (3)
95% TFA–DCM
95% TFA–DCM
(3-Methoxyphenyl)-acetic acid 1 (30.0 g, 180.5 mmol) was
dissolved in CH2Cl2 (200 mL) and SOCl2 (25.8 g, 15.8 mL,
217 mmol) was added in one portion. The reaction mixture was
heated to reflux for 2 h until no more HCl gas evolved. The
reaction mixture was evaporated to dryness in vacuo to yield the
crude acid chloride 2 (33.4 g ca. 100%). This was sufficiently
a All cleavage experiments were conducted at rt. b Cleavage yields were
calculated by comparison with a standard curve.
1
importance for achieving maximum resonance stabilization of
the naphthylic carbenium formed during the cleavage process
from the solid phase, and hence the reduced acid lability we
observe. We suggest that steric hindrance (due to the peri-effect)
forces the carbenium ion into the naphthyl methylene position
that is formed during the cleavage process of the substrate from
the solid support, out of plane with the aromatic core.
pure for further use. H NMR (CDCl3, 300 MHz) d 7.23–7.32
(m, 1H), 6.81–6.91 (m, 3H), 4.12 (s, 2H), 3.82 (s, 3H); 13C NMR
(CDCl3, 75 MHz) d 160.2, 132.8, 130.2, 122.0, 121.9, 115.5,
113.8, 55.5, 52.3; m/z (GC-MS) 182 (M+).
In a three-necked round-bottomed flask (500 mL) equipped
with a dropping funnel and a reflux condenser, Mg turnings
(4.13 g, 175 mmol), dry CCl4 (0.3 mL) and freshly distilled
diethylmalonate (13.61 g, 12.90 mL, 85.0 mmol) were placed
under a N2 atmosphere. Anhydrous EtOH (30 mL) was added.
When the reaction started to subside, additional diethylmalonate
(13.61 g, 12.90 mL, 85.0 mmol) was added and the reaction
proceeded to completion. The reaction mixture was cooled to rt,
dry diethyl ether (60 mL) was added and the mixture was heated
to reflux for 60 min. Then 3-methoxyphenylacetyl chloride 2
(33 g, 178.8 mol) in dry ether (90 mL) was added over a 30 min
period. After the addition, the reaction mixture was heated to
reflux for 20 min. The reaction mixture was cooled to rt and
water (30 mL) was added dropwise over a period of 10 min.
The organic phase was washed with water (2 × 30 mL), dried
(Na2SO4) and concentrated in vacuo to yield an off-white semi-
solid material. TLC (EtOAc–heptane 1 : 1, Rf ca. 0.6) showed
a high purity. Crude yield 53.0 g, 100%. The crude product
The naphthyl methylene carbon atom becomes sp2 hybridized
when the carbenium is formed. The p-orbital in the naphthyl
methylene carbenium ion must have the maximum overlap with
the p-orbitals from the aromatic core in order to gain the
maximum carbenium stabilization. If planarity of the carbenium
ion is lost, the carbenium ion stability and the acid-lability of
the handle, are also reduced. A related type of behavior has
recently been observed in highly substituted derivatives of 1,8-
bis-(dimethylamino)naphthalene (‘proton sponge’).23
Conclusions
We have developed a convenient synthesis of a number of
novel highly substituted naphthalene compounds and a facile
synthetic route to the first backbone amide linker based on a
trialkoxynaphthaldehyde. The synthesis of novel NALdehyde-
3 (10) features a selective ring-closing reaction to form the
naphthalene system 4 and a highly selective demethylation
reaction to form the naphthol compound (8). Using standard
procedures for Fmoc-based solid phase synthesis a dipeptide
was prepared on the new NAL-3 handle. The final products
were cleaved with high crude purity using TFA in CH2Cl2
(19 : 1).
1
was sufficiently pure for further synthesis. H NMR (CDCl3,
300 MHz) d 13.25 (enol form of a-H, br s, 5% of 3H), 6.80–6.90
(m, 2H), 6.45–6.60 (m, 2H), 3.98 (a-H, br s, 95% of 3H), 3.45–
3.60 (m, 7H), 1.05–1.20 (m, 6H). 13C NMR (CDCl3, 100 MHz) d
159.1, 138.6, 129.4, 128.8, 121.5, 114.7, 113.7, 111.4, 61.7, 60.4,
54.8, 44.4, 13.9; m/z (FABMS) 639 (2M + Na+).
1,3-Dihydroxy-6-methoxy-naphthalene-2-carboxylic acid ethyl
ester (4)
Experimental
High-loading aminomethylated PS resin, all amino acids,
and HBTU were obtained from NovaBioChem, HOBt from
Quantum Richelieu. Solid-phase reactions were performed in
poly(propylene) syringes equipped with a poly(ethylene) filter,
placed on a shaker. HPLC-MS analysis was performed on a
Shimadzu 2010, using a Phenomenex Jupiter C5 column (5 l,
Anhydrous CH3SO3H (100 mL) was added to the malonic ester
derivative 3 (32.5 g, 3.24 mmol) without cooling and the ester
dissolved by gentle heating with a heat gun. The reaction mixture
was stirred overnight at rt giving a reddish inhomogeneous
slurry. The reaction mixture was poured into ice water (2 L)
resulting in precipitation of a pale yellow solid. The mixture
was extracted with CH2Cl2 (2 × 1 L) and the combined organic
extracts were dried and evaporated to dryness in vacuo yielding
a pale yellow solid material. This was purified by dry column
vacuum chromatography (from heptane to EtOAc–heptane 1 :
1 with 5% increments). Yield 20.46 g, 74% as an off-white solid;
mp 112–113 C (EtOH); H NMR (CDCl3, 400 MHz) d 11.15
(br s, 1H; OH), 9.05 (br s, 1H; OH), 8.13 (d, 1H, J = 9.2 Hz;
H8), 6.91 (dd, 1H, J = 9.2, 2.5 Hz; H7), 6.83 (d, 1H, J = 2.5 Hz;
H6), 6.67 (s, 1H; H4), 4.60 (q, 2H, J = 7.1 Hz, CH2), 3.91 (s, 3H,
OCH3), 1.52 (t, 3H, J = 7.1 Hz, CH3); 13C NMR (CDCl3, 100
MHz) d 170.0, 161.4, 139.9, 125.9, 115.5, 115.4, 114.4, 104.2,
104.1, 101.6, 95.6, 62.6, 55.2, 14.3; m/z (FABMS) 263.1 (M +
H+); Anal. calcd. for C14H14O5: C, 64.12; H, 5.38. Found: C,
63.89; H, 5.21.
−1
˚
300 A). Gradient: Linear 1 mL min from 3% to 95% buffer B
over 18 min (buffer A: 0.025% TFA in H2O; buffer B: 0.025%
TFA in 90% aq. CH3CN). UV analysis was performed at a Perkin
Elmer Lambda 2 UV-vis spectrometer. Solvents were HPLC
grade and were used as received. 1H NMR and 13C NMR spectra
were recorded on a 300 MHz NMR (Varian or Bruker Avance)
instrument (300 MHz for 1H NMR and 75 MHz for 13C NMR)
or on a 400 MHz NMR (Bruker) instrument (400 MHz for
1H NMR and 100 MHz for 13C NMR). Proton chemical shifts
are reported in ppm downfield from tetramethylsilane (TMS)
and carbon chemical shifts in ppm downfield of TMS using
the resonance of the deuterated solvent as internal standard.
Assignments of 1H NMR signals were based on HSQC, HMBC,
NOESY, and COSY spectra. Melting points were measured on a
◦
1
5 1 2
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 5 0 8 – 5 1 4

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1,3,6-Trimethoxynaphthalene-2-carboxylic acid ethyl ester (5)
161.9, 157.5, 140.9, 126.3, 115.4, 114.9, 106.7, 105.8, 95.4, 55.4,
55.3; m/z (FABMS) 233.0 (M + H+); Anal. calcd. for C13H12O4:
C, 67.23; H, 5.21. Found: C, 67.08; H, 5.06.; 11: mp 98–99 ◦C;
1H NMR (CDCl3, 300 MHz) d 10.96 (s, 1H; OH), 10.35 (s, 1H;
CHO), 7.89 (d, 1H, J = 9.2 Hz; H8), 6.91 (dd, 1H; J = 9.2,
2.5 Hz; H7), 6.84 (d, 1H, J = 2.6; H5), 6.83 (s, 1H; H4), 4.01 (s,
3H; CH3O1), 3.81 (s, 3H; CH3O6); 13C NMR (CDCl3, 75 MHz)
d 194.0, 164.0, 161.5, 157.3, 142.1, 125.0, 117.4, 112.5, 106.5,
104.6, 95.3, 66.0, 55.3; m/z (FABMS) 233.0 (M + H+); Anal.
calcd. for C13H12O4: C, 67.23; H, 5.21. Found: C, 67.02; H, 5.28.
Diol 4 (5.52 g, 21.05 mmol) was dissolved in dry acetone (150
mL) and K2CO3 (9.67 g, 69.5 mmol) was added followed by
addition of dimethylsulfate (46.64 mmol, 2.2 eq). The mixture
was stirred at rt for 48 h. Water (200 mL) was added and the
mixture extracted with CH2Cl2 (2 × 100 mL) and the combined
organic extracts dried (Na2SO4) and evaporated to dryness in
vacuo. Recrystallisation from EtOH yielded compound 5 as
colorless needles. Yield 5.97 g, 98%; mp 101–102 ◦C; 1H NMR
(CDCl3, 300 MHz) d 7.86 (d, 1H), 6.94–6.97 (m, 2H), 6.78 (s,
1H), 4.37 (q, 3H), 3.93 (s, 3H), 3.83 (s, 3H), 3.84 (s, 3H), 1.34 (t,
2H); 13C NMR (CDCl3, 75 MHz) d 166.4, 159.1, 154.9, 154.6,
137.0, 124.2, 118.0, 116.3, 115.2, 105.5, 101.1, 63.0, 61.4, 55.8,
55.2, 14.2; m/z (FABMS) 290.1 (M+); Anal. calcd. for C16H18O5:
C, 66.19; H, 6.25. Found: C, 66.53; H, 6.20.
5-(2-Formyl-3,6-dimethoxynaphthalen-1-yloxy)-pentanoic acid
ethyl ester (9)
Naphthol 8 (104 mg, 0.448 mmol) was dissolved in anhydrous
DMF (7 mL) and K2CO3 (75 mg, 0.54 mmol) was added. Ethyl 5-
bromovalerate (112 mg, 0.54 mmol) was added and the reaction
mixture was stirred overnight at 60 ◦C and then allowed to
reach rt. Water (15 mL) was added and the mixture extracted
with CH2Cl2 (3 × 15 mL). The combined organic extracts were
evaporated to dryness (oil pump, 2 mm Hg) and purified by
dry column vacuum chromatography to yield a yellow oil. Yield
0.157 g, 97%. 1H NMR (CDCl3, 400 MHz) d 10.54 (s, 1H), 8.02
(d, 1H), 6.99–7.03 (m, 2H), 6.84 (s, 3H), 4.14 (q, 2H, J = 7.14
Hz), 4.10 (t, 2H, J = 6.04 Hz, 2H), 3.97 (s, 3H), 3.92 (s, 3H),
2.42 (t, 2H, J = 6.59 Hz), 1.84–2.02 (m, 4H), 1.26 (t, 3H, J =
7.14 Hz); 13C NMR (CDCl3, 100 MHz) d 189.0, 173.2, 160.8,
160.5, 158.3, 139.7, 125.6, 118.6, 116.9, 115.7, 105.2, 101.2, 76.9,
60.2, 55.8, 55.3, 33.9, 29.6, 21.5, 14.1; m/z (FABMS) 361.11
(M + H+); Anal. calcd. for C20H24O6: C, 66.65; H, 6.71. Found:
C, 66.28; H, 6.75.
(1,3,6-Trimethoxynaphthalen-2-yl)-methanol (6)
Ester 5 (5.97 g, 20.56 mmol) was dissolved in dry ether (300 mL)
and added to an ice-cold stirring solution of LiAlH4 (2.34 g,
61.7 mmol) in dry ether (100 mL) over a 5 min period under
an N2 atmosphere. This mixture was stirred at rt for 4 h. Water
was added until the mixture became turbid (ca. 8 mL) and the
mixture was filtered through a plug of MgSO4. The MgSO4 was
washed with additional ether and the combined ethereal extracts
were evaporated to dryness in vacuo. This yielded the alcohol as
an off-white solid. Yield 5.10 g, 100%; mp 78–80 ◦C; 1H NMR
(CDCl3, 300 MHz) d 7.93 (d, 1H, J = 9.0 Hz), 7.01–7.07 (m,
2H), 6.89 (s, 1H), 4.89 (s, 2H), 3.98 (s, 3H), 3.96 (s, 3H), 3.91 (s,
3H), 2.53 (br s, 1H); 13C NMR (CDCl3, 75 MHz,) d 158.8, 157.2,
155.1, 136.1, 124.0, 119.3, 118.5, 116.0, 105.5, 101.1, 63.4, 55.7,
55.5, 51.1; m/z (FABMS) 248.1 (M+); Anal. calcd.. for C14H16O4:
C, 67.73; H, 6.50. Found: C, 67.75; H, 6.55.
5-(2-Formyl-3,6-dimethoxynaphthalen-1-yloxy)-pentanoic acid
(10, NALdehyde-3)
1,3,6-Trimethoxy-naphthalene-2-carbaldehyde (7)
Alcohol 6 (3.00 g, 12.08 mmol) was dissolved in CH2Cl2
(120 mL) and PDC (9.12 g, 24.16 mmol) was added in one
portion. The reaction mixture was stirred under N2 overnight
at rt. The reaction mixture was filtered through a plug of
celite, and the organic phase was washed with water (100 mL),
evaporated to dryness in vacuo and purified by dry column
vacuum chromatography (heptane to heptane–EtOAc 1 : 1 with
5% increments). Yield: 2.50 g, 84% as a white solid; mp 111–
112 ◦C; 1H NMR (300 MHz, CDCl3) d 10.57 (s, 1H; CHO), 8.06
(d, 1H, J = 8.7Hz; H8), 7.05 (dd, 1H, J = 8.7, 1.7 Hz; H7),
7.02 (d, J = 1.7 Hz; H5), 6.86 (s, 1H; H4), 4.04 (s, 3H; CH3O1),
3.98 (s, 3H; CH3O), 3.93 (s, 3H; CH3O); 13C NMR (75 MHz,
CDCl3) d 189.1, 161.4, 158.4, 139.8, 125.7, 118.4, 117.0, 115.6,
105.2, 101.3, 64.3, 55.8, 55.3; m/z (FABMS) 247.1 (M + H+);
Anal. calcd. for C14H14O4: C, 68.28; H, 5.73. Found: C, 67.87;
H, 5.71.
Ester 9 (762 mg, 2.11 mmol) was dissolved in THF (20 mL) and
dilute NaOH (1 M, 10 mL) was added. The reaction mixture
was stirred overnight at rt. The reaction mixture was acidified
by addition of 2 M aq. HCl (20 mL) resulting in a green solution.
This was extracted with CH2Cl2 (2 × 50 mL) and the combined
organic extracts were evaporated to dryness in vacuo. The residue
was dissolved in EtOH (96%, 30 mL), decolourised with charcoal
and filtered through a plug of celite. Evaporation of the EtOH
resulted in a bright yellow oil which was precipitated as a fine
yellow powder by stirring with ether at 0 ◦C. Yield 0.681 g, 97%;
mp 95–97 ◦C; 1H NMR (CDCl3, 300 MHz) d 10.56 (s, 1H), 8.04
(d, 1H, J = 9.0 Hz; H8), 7.04 (dd, 1H, J = 9.0, 2.5 Hz; H7), 7.01
(d, 1H, J = 2.3 Hz; H5), 6.86 (s, 1H; H4), 4.12 (t, 2H, J = 5.9 Hz),
3.98 (s, 3H; CH3O), 3.94 (s, 3H, CH3O), 2.52 (t, 2H, J = 6.5 Hz;
H2ꢁ), 1.90–2.04 (m, 4H; H3ꢁ, H4ꢁ); 13C NMR (100 MHz, CDCl3)
d 189.2, 178.9, 160.8, 160.4, 158.3, 139.7, 125.7, 118.6, 116.9,
105.2, 101.2, 76.8, 55.8, 55.3, 33.6, 29.5, 21.2; m/z (FABMS)
331.1 (M − H−); Anal. calcd. for C18H20O6: C, 65.05; H, 6.07.
Found: C, 64.83; H, 6.32.
1-Hydroxy-3,6-dimethoxynaphthalene-2-carbaldehyde (8)
Trimethoxy aldehyde 7 (1.00 g, 4.06 mmol) was dissolved in
◦
CH2Cl2 (140 mL) and cooled to 0 C under N2. Then BBr3 in
CH2Cl2 (0.406 mL, 1 M, 4.06 mmol) was added dropwise over
a period of 30 min resulting in a deep red color. The reaction
mixture was allowed to reach rt over a period of 30 min. Water
(150 mL) was added slowly to the vigorously stirred solution
and the phases were separated. The aqueous phase was extracted
with additional CH2Cl2 (2 × 100 mL) and the combined organic
extracts were evaporated to dryness in vacuo and purified by dry
column vacuum chromatography (heptane to EtOAc with 5%
increments) to yield two products; title product 8 (815 mg, 86%)
and the regioisomer 11 (15 mg, 1%). 8: mp 111–112 ◦C; 1H NMR
(CDCl3, 300 MHz) d 13.59 (s, 1H; OH), 10.18 (s, 1H; CHO),
8.08 (d, 1H, J = 8.7 Hz; H8), 6.87 (dd, 1H, J = 9.2, 2.3 Hz; H7),
6.82 (broad s, 1H; H5), 6.33 (s, 1H; H4), 3.86 (s, 3H; CH3O),
3.83 (s, 3H, CH3O); 13C NMR (CDCl3, 75 MHz) d 193.5, 164.1,
Solid-phase anchoring of NALdehyde-3
NALdehyde-3 (10) (0.28 g, 0.82 mmol) and HBTU (0.30 g,
0.80 mmol) were dissolved in DMF (5 mL), DIEA (0.28 mL,
1.64 mmol) was added and the mixture was gently shaken for
5 min at rt. The clear solution was added to aminomethyl-
poly(styrene) resin (0.57 g, 0.21 mmol, loading 0.36 mmol g−1)
and the suspension was shaken for 16 h at rt. The resin was
washed with DMF (10 times). Residual free amino groups on
the resin were capped using 30% acetic anhydride in DMF
(5 mL) together with a catalytic amount of DIEA followed by
shaking for 2 h at rt. Finally, the resin was washed with DMF
(10 times) and CH2Cl2 (10 times), shrunk with methanol and air
dried.
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 5 0 8 – 5 1 4
5 1 3

View Article Online
Solid-phase synthesis of Fmoc-Phe-Leu-OH
5 (a) K. J. Jensen, M. F. Songster, J. Va´gner, J. Alsina, F. Albericio and
G. Barany, J. Am. Chem. Soc., 1998, 120, 5441; (b) Initial accounts:
K. J. Jensen, M. F. Songster, J. Va´gner, J. Alsina, F. Albericio and G.
Barany, in Peptides: Chemistry, Structure and Biology, Proceedings of
the 14th American Peptide Symposium, Columbus, Ohio, June 18–23,
1995, ed., P. T. P. Kaumaya and R. S. Hodges, Mayflower Scientific,
Kingswinford, UK, 1996, p. 30; (c) K. J. Jensen, M. F. Songster, J.
Alsina, J. Va´gner, F. Albericio and G. Barany, in Innovation and
Perspectives in Solid Phase Synthesis & Combinatorial Libraries:
Peptides, Proteins and Nucleic Acids – Small Molecule Organic
Chemical Diversity, Collected Papers, 4th International Symposium,
Edinburgh, Sept. 12–16, 1995, ed., R. Epton, Mayflower Scientific,
Birmingham, UK, 1996, p. 187.
6 (a) C. G. Boojamra, K. M. Burow and J. A. Ellman, J. Org. Chem.,
1995, 60, 5742; (b) C. G. Boojamra, K. M. Burow, L. A. Thompson
and J. A. Ellman, J. Org. Chem., 1997, 62, 1240.
7 For reviews of BAL handles, see: (a) J. Alsina, K. J. Jensen, F.
Albericio and G. Barany, Chem. Eur. J., 1999, 5, 2787; (b) J. Alsina,
K. J. Jensen, M. F. Songster, J. Va´gner, F. Albericio and G. Barany, in
Solid-phase Organic Synthesis, Vol. 1, ed. A. W. Czarnik, John Wiley
& Sons, New York, 2001, pp. 121.
8 (a) G. T. Bourne, W. D. F. Meutermans, P. F. Alewood, R. P. McGeary,
M. Scanlon, A. A. Watson and M. L. Smythe, J. Org. Chem., 1999,
64, 3095.
9 (a) E. E. Swayze, Tetrahedron Lett., 1997, 38, 8465; (b) A. M. Fivush
and T. M. Wilson, Tetrahedron Lett., 1997, 38, 7151; (c) D. Sarantakis
and J. J. Bickler, Tetrahedron Lett., 1997, 38, 7325; (d) M. F. Gordeev,
G. W. Luehr, H. C. Hui, E. M. Gordon and D. V. Patel, Tetrahedron,
1998, 54, 15879.
10 T. Okayama, A. Burritt and V. J. Hruby, Org. Lett., 2000, 2, 1787.
11 (a) K. G. Estep, C. E. Neipp, L. M. Stramiello-Stephens, M. D.
Adam, M. P. Allen, S. Robinson and E. J. Roskamp, J. Org. Chem.,
1998, 63, 5300; (b) J. J. Scicinski, M. S. Congreve and S. V. Ley,
J. Comb. Chem., 2004, 6, 375.
12 For preliminary accounts of our thiophene based T-BAL handle, see:
(a) U. Boas, Ph.D. Thesis, University of Copenhagen, Copenhagen,
Denmark, 2002; (b) U. Boas, J. Brask, J. B. Christensen and K. J.
Jensen, in Peptides 2002, Proceedings of the 27th European Peptide
Symposium, Sorrento, Italy, Aug. 31–Sept. 6, 2002, ed., E. Benedetti
and C. Pedone, Edizioni Ziino, Castellammare di Stabia, Italy, 2002,
p. 50.
13 (a) A. Faldt, M. J. Pedersen, T. P. Krogh, L. Hviid and J. B.
Christensen, Synth. Met., 1998, 94, 307; (b) B. W. Laursen, F. C.
Krebs, M. F. Nielsen, K. Bechgaard, J. B. Christensen and N. Harrit,
J. Am. Chem. Soc., 1998, 120, 12255.
14 (a) U. Boas, J. B. Christensen and K. J. Jensen, in Peptides 2000,
Proceedings of the 26th European Peptide Symposium, Montpellier,
France, Sept. 10–15, 2000, Meeting Date 2000, ed., J. Martinez and
J.-A. Fehrentz, Editions EDK, Paris, 2001, p. 197; (b) U. Boas, J. B.
Christensen and K. J. Jensen, J. Comb. Chem., 2004, 6, 497.
15 A. Bader, Adventures of a Chemist Collector, Weidenfeld & Nicolson,
London, 1995.
16 (a) For a general review on the peri-effect in naphthalenes, see:
V. Balasubramaniyan, Chem. Rev., 1966, 66, 567; (b) For a recent
example, see: A. Karacar, M. Freytag, H. Tho¨nnesen, P. G. Jones,
R. Bartsch and R. Schmutzler, J. Organomet. Chem., 2002, 68,
643.
17 C. B. de Koning, R. L. Rousseau and W. A. L. van Otterlo,
Tetrahedron, 2003, 59, 7.
18 Our procedure is similar to: K. Meyer, H. S. Bloch, Org. Synth. Coll.
Vol. III , 637.
19 (a) K. H. Weisgaber and U. Weiss, J. Chem. Soc., Perkin Trans.
1, 1972, 83; (b) P. Mu¨ller, J. Seres, K. Steiner, S. E. Helali and
E. Hardegger, Helv. Chim. Acta, 1974, 57, 790; (c) A. Piettre, E.
Chevenier, C. Massardier, Y. Gimbert and A. E. Greene, Org. Lett.,
2002, 4, 3139.
20 (a) T. Mizutani, M. Takeshi, T. Kurahashi and H. Ogoshi, J. Org.
Chem., 1996, 61, 539; (b) R. L. Hannan, R. B. Barber and H.
Rapoport, J. Org. Chem., 1979, 44, 2153.
21 o-BAL: U. Boas, J. Brask, J. B. Christensen and K. J. Jensen, J. Comb.
Chem., 2002, 4, 223.
NALdehyde-3 derivatised resin (0.30g, 0.11 mmol, theoretical
load 0.36 mmol g−1) was swollen in DMF (2 mL), followed by
addition of HLeu-OtBu·HCl (0.24 g, 1.08 mmol) and NaBH3CN
(67 mg, 1.08 mmol). The mixture was shaken for 16 h at rt.
The resin was washed once with DMF and the procedure was
repeated. Hereafter, the resin was washed with DMF (10 times)
and CH2Cl2 (10 times). The resin was swollen in a CH2Cl2–DMF
9 : 1 mixture (5 mL), Fmoc-Phe-OH (0.42 g, 1.08 mmol) and
DIPCDI (85 lL, 0.54 mmol) were added and the mixture was
shaken for 16 h at rt creating a viscous suspension. The resin was
washed with DMF (10 times) and the procedure was repeated.
Hereafter, the peptidyl-resin was washed with DMF (10 times)
and CH2Cl2 (10 times), shrunk with methanol and air dried. To
release the dipeptide, the peptidyl-resin (0.100 g) was put in a
filter syringe and treated with TFA-CH2Cl2 (19 : 1) for 1 h, the
solvents were removed in vacuo and the remaining material was
triturated with diethyl ether. Yield (crude): 6 mg (50%). HPLC-
MS: tR: 24.3 min; m/z (ESI) 501 (M + H+), 523 (M + Na+).
Determination of resin substitution using the Fmoc-group
Three portions of resin (approximately 5 mg, 2 lmol theoretical
amount of peptide) were suspended in a 20% piperidine–DMF
solution (25 mL). The mixtures were shaken for 30 min, and the
absorbance (290 nm) was measured using piperidine–DMF (1 :
4) as blank scan reference. The loading of the peptidyl-resin was
measured to 0.21 mmol g−1.
Cleavage studies
Portions of peptidyl-resin (approximately 5 mg) were placed in
filter syringes and subjected to acidolytic cleavage for the time
stated in Table 1 by 0.5 mL of a cleavage mixture as stated in
Table 1. After removal of the solvents in vacuo the remaining
material was dissolved in acetonitrile–H2O (1 : 1, 0.5 mL) and
analysed by HPLC-MS at 220 nm. The cleavage yields were
determined from the HPLC areas of released Fmoc-Phe-Leu-
OH in comparison with a Fmoc-Phe-OH standard curve having
an initial concentration of 0.25 mg mL−1 Fmoc-Phe-OH in
acetonitrile–H2O (1 : 1). The results are shown in Table 1.
Acknowledgements
J.B.C. thanks the Lundbeck foundation and the Leo Pharma
foundation for financial support. M.J. acknowledges a student
grant from Novo Nordisk. M.P. thanks the University of
Copenhagen for a PhD fellowship. K.J.J. acknowledges financial
support from the Danish Natural Science Research Council.
Magnus Magnussen and Peter Hammershøj are acknowledged
for their assistance with solving the crystal structures. Flemming
Hansen is acknowledged for collecting the crystal structure data
and the Center for Crystallographic Studies for the use of their
equipment.
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5 1 4
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 5 0 8 – 5 1 4