The attachment and cleavage of phenols from solid supports and their single bead mass spectral analysis

Article abstract of DOI:10.1016/S0040-4039(00)01431-3

The attachment of simple phenols to chloromethyl polystyrene, and their rapid cleavage at room temperature is described. Further, phenols from single resin beads can be unequivocally identified by Fourier transform ion cyclotron resonance mass spectrometr

Full text of DOI:10.1016/S0040-4039(00)01431-3

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 8183–8187
The attachment and cleavage of phenols from solid supports
and their single bead mass spectral analysis
Matthew H. Todd and Chris Abell*
University Chemical Laboratory, Lensfield Road, Cambridge CB2 1EW, UK
Received 26 July 2000; accepted 23 August 2000
Abstract
The attachment of simple phenols to chloromethyl polystyrene, and their rapid cleavage at room
temperature is described. Further, phenols from single resin beads can be unequivocally identified by
Fourier transform ion cyclotron resonance mass spectrometry of the dansylates prepared in situ. © 2000
Elsevier Science Ltd. All rights reserved.
Keywords: phenol cleavage; single bead analysis; FTICR mass spectrometry.
Linkers used in solid phase organic chemistry should be stable to the chemistry used in the
library construction, but should usually be easily cleaved on demand.1 However, there are
occasions when it is useful to have highly inert linkages to resin which can only be cleaved under
more forcing conditions, e.g. to attach tagging molecules. The attachment of groups to the
benzylic position of the chloromethyl polystyrene resin is analogous to the solution-phase benzyl
protection of those groups. A range of chemistry has been developed for the deprotection of
benzyl groups in solution,2 but relatively little of this chemistry has been applied to the solid
phase. We report herein the use of a thioanisole-based system for the facile removal of phenols
from a solid support, and their identification using Fourier transform ion cyclotron resonance
mass spectrometry (FTICRMS). The use of GC-MS in the identification of phenols cleaved
from solid support with strong acid has been described in the patent literature.3
1. Phenol attachment to resin
The attachment of phenols to chloromethyl resin (resin used was 1.6 meq. 250–300 mm
chloromethyl polystyrene) using the conditions reported by Merrifield,4 proceeds very well using
* Corresponding author. Tel: 44 1223 336405; fax: 44 1223 336362; e-mail: ca26@cam.ac.uk
0040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)01431-3

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three equivalents of phenol in dimethylacetamide (DMA) at 50°C with sodium methoxide as
base, as judged by chlorine analysis of the resin (there is negligible competition from methoxide).
Typical phenols used are the alkyl phenols shown in Scheme 1. The use of a high boiling point
solvent like DMA allows the reaction vessel to be sealed to ensure dryness, whilst not developing
a hazardous vapour pressure at 50°C.
Scheme 1. General method for attachment of phenols to chloromethyl resin
A competition reaction was performed between 4-hydroxybenzyl alcohol (the Wang linker)
and 4-amylphenol, in order to test how well the hydrophilic Wang linker reacted when
compared with the more hydrophobic alkyl phenol. The reaction mixture consisted of a total of
three equivalents of varying ratios of the two phenols. Incorporation of the Wang linker onto
the resin was determined by subsequent derivatisation with Fmoc-leucine, followed by cleavage
of the Fmoc group with piperidine in DMF, and analysis of the supernatant at 290 nm. As
expected, the hydrophobic phenol reacted faster than the hydrophilic hydroxybenzyl alcohol.
The level of Fmoc attached to the resin, and hence the level of incorporation of the Wang linker,
increases more slowly than its concentration in the reagent mixture, implying that the reaction
between the resin and alkyl phenol is faster (Fig. 1). This observation has important implications
for functionalising beads with different ratios of different phenols.
Figure 1. Graph showing percentage of Fmoc loading, corresponding to incorporation of Wang linker, as a function
of the composition of the 4-hydroxybenzyl alcohol: 4-amylphenol mixture in the competition resin loading reaction

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2. Cleavage of phenols from resin
The effectiveness of the TFA cleavage conditions usually used for the cleavage of the Wang
linker was initially investigated using a solution phase model compound (1, Scheme 2)
constructed by simple benzylation of 4-amylphenol. When this compound was stirred in TFA,
starting material was consumed within an hour at room temperature, and as judged by TLC the
phenol was apparently released. Upon closer inspection, the product of the reaction was in fact
the rearranged product 2 (which gave the same RF on TLC). The apparent ease of this reaction
suggested that, whether this rearrangement were intra- or intermolecular, the release of phenol
from the solid support would be hindered.
Scheme 2. TFA-induced rearrangement of resin model benzyl ether
Attempts to release phenols from resin by hydrogenolysis with palladium acetate and hydrogen
in DMF at room temperature and elevated temperatures failed. This was disappointing, since a
number of palladium-catalysed processes have been performed on resin,5 and palladium acetate
has been reported to allow the release of peptides from solid support.6 It was assumed that the
problem was the diffusion of hydrogen into the matrix of the support. When 1 was subjected to
catalytic transfer hydrogenation7,8 with cyclohexene and palladium acetate at 60°C, pure phenol
was released. When similar conditions were applied to the resin-bound phenols, no reaction was
observed. Neither was there any success with other hydrogen sources such as ammonium formate
and cyclohexadiene.
Attention was therefore turned to the use of thioanisole in combination with a Lewis acid to
cleave the O-benzyl bond nucleophilically.9,10 When the model compound 1 was treated with excess
(approx. 10 equiv.) thioanisole and TMS–triflate in 10% DCM:TFA at room temperature,
immediate cleavage was observed with no trace of starting material by TLC after five minutes.
When the same conditions were applied to the support-bound amylphenol (3, Scheme 3), release
of phenol from the support began immediately, giving, aside from the reagents, clean phenol.
Scheme 3. Release and derivatisation of amylphenol from resin using thioanisole cleavage and dansylation

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3. Phenol identification
High resolution mass spectrometry was used to identify the cleaved phenols unambiguously.
Phenols, and in particular alkyl phenols, do not fly well under electrospray ionisation condi-
tions. Consequently they were derivatised to their dansylates, which fly very well owing to the
ease of protonation of the nitrogen atom. A rapid biphasic procedure was developed for the
derivatisation (to form 4, Scheme 3), which is simpler than has previously been reported.11 This
method is sensitive enough to be employed on single beads. In detail the protocol involves
initially adding a solution of 10% TFA in DCM (250 mL) containing TMS–triflate (4.5 mM) and
thioanisole (7 mM) to the bead. The suspension is allowed to stand, with occasional agitation,
for 30 min, then the supernatant is removed under a stream of nitrogen. Diethyl ether (0.5 mL)
containing dansyl chloride (15 mM) and triethylamine (5.7 mM) is added to the residue, followed
by aqueous sodium hydroxide (1 M, 0.5 mL) containing tetra-n-butyl ammonium iodide (0.25
mM). This biphasic solution is stirred vigorously for 3 h, then the organic phase is separated,
and concentrated under a stream of nitrogen, and resuspended in acetonitrile containing 0.1%
TFA (250 mL). This solution is then filtered and injected into a Bruker BioApex™ 47e FTICR
mass spectrometer, using 150 V capillary voltage, 8 scans, 60 mL h−1 injection rate, and 10 V
cone at a gas temperature of 300°C.
When a single bead of 3 was treated with this cleavage-derivatisation protocol, and analysed
using FTICRMS, excellent signal-to-noise was seen in the spectrum (Fig. 2). The use of this
instrument allowed the unequivocal identification of this dansylate with a mass error of 5 ppm.
We have applied the method to a range of phenols on solid phase, and the method gives
excellent reproducibility.
Figure 2. Mass spectrum obtained for the dansylate of amylphenol cleaved from a single resin bead

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Acknowledgements
We thank Zeneca Pharmaceuticals for a studentship (to M.H.T.). C.A. also acknowledges
funds from Iberdrola, Spain (Visiting Professorship).
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