Solid-phase synthesis of a new class of oligosaccharide analogues based on azasugars

Article abstract of DOI:10.1016/S0040-4039(02)00250-2

A strategy for the solid-phase synthesis of a new class of oligosaccharide analogues based on coupling of azasugar building blocks via carbamate bonds is described.

Full text of DOI:10.1016/S0040-4039(02)00250-2

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 2215–2221
Solid-phase synthesis of a new class of oligosaccharide analogues
based on azasugars
Bart Ruttens and Johan Van der Eycken*
Laboratory for Organic and Bio-organic Synthesis, Department of Organic Chemistry, Ghent University, Krijgslaan 281(S.4),
B-9000 Gent, Belgium
Received 6 January 2002; revised 1 February 2002; accepted 4 February 2002
Abstract—A strategy for the solid-phase synthesis of a new class of oligosaccharide analogues based on coupling of azasugar
building blocks via carbamate bonds is described. © 2002 Elsevier Science Ltd. All rights reserved.
Oligosaccharides play an important role in many bio-
chemical recognition processes.1 As a consequence, syn-
thetic analogues of these natural biopolymers could be
used to study, to influence or even to control these
biochemical processes. Although the solid-phase syn-
thesis of peptides and oligonucleotides is already well
established, three main problems, caused by the typical
properties of the sugar monomers, still complicate the
synthesis of oligosaccharides: (i) stereochemical control
of the coupling reactions is difficult due to the anomeric
centre in the building blocks; (ii) the polyfunctionality
of the monomers requires selective protection before
coupling and (iii) glycosidation occurs with unpre-
dictable and often very low yields.2,3 A possible solu-
tion for these synthetic problems is based on structural
modification of the carbohydrate building blocks in
such a way that they allow easy and efficient coupling
without however losing their typical properties. An
important example of this approach is the use of sugar
amino and imino acids for the synthesis of oligosaccha-
ride analogues.4
similar properties for the resulting oligomers and natu-
ral oligosaccharides due to the resemblance of the
monomeric building blocks. Finally, oligosaccharide
analogues based on carbamate-linked azasugars should
be resistant to proteases and glycosidases.
As an example, we describe the synthesis of three
protected derivatives of 1,5-dideoxy-1,5-imino-
(Scheme 1) starting from commercially available b-
L-iditol
D
-
glucose pentaacetate 1.6 Treatment with 4-penten-1-ol
under acidic conditions led directly to pentenyl glu-
coside 2.7 Selective protection of the primary and sec-
ondary hydroxyl functions as tert-butyldiphenylsilyl
and benzyl ethers, respectively, gave the fully protected
glucoside 3. Removal of the anomeric pentenyl group
according to the method of Fraser–Reid furnished 4.8
Reduction of hemiacetal 4 with NaBH4 afforded diol 5
which was converted to dimesylate 6. Gentle heating of
6 in allylamine induced double nucleophilic SN2 dis-
placement of both mesylates providing N-allyl pro-
tected azasugar 7. Similarly, reaction with neat
ethylenediamine led to N-(2-aminoethyl) azasugar
12.9,10 The SN2 mechanism of this ring closure causes
Our approach is based on the use of azasugars as
building blocks.5 The stereochemical diversity, confor-
mational rigidity and numerous possibilities for
derivatisation make these monomers ideal tools for the
construction of oligomeric combinatorial libraries. In
addition, in principle these sugar analogues can be
coupled in high yield via carbamate bonds, and the lack
of an anomeric centre eliminates the problem of stereo-
control during coupling. Meanwhile one can expect
inversion of the
and leads exclusively to the azasugar analogues with the
-configuration. Removal of the allylic protecting
D-configuration in the starting material
L
group in compound 7 with 1-chloroethyl chloroformate
(ACE-Cl) and subsequent solvolysis with methanol
gave azasugar 8.11,12 Electrophilic amination of 8 with
N - fluorenylmethoxycarbonyl - 3 - (2,4 - dichlorophenyl)-
oxaziridine furnished N-Fmoc protected N-amino aza-
sugar 9.13,14 Although N-amino azasugar 10 could not
be isolated due to its instability, the formation of this
compound was proven by conversion into its stable
dimethyl hydrazone 11.15
* Corresponding author. Tel.: +32-9-264.44.80; fax: +32-9-264.49.98;
e-mail: johan.vandereycken@rug.ac.be
0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)00250-2

2216
B. Ruttens, J. Van der Eycken / Tetrahedron Letters 43 (2002) 2215–2221
AcO
HO
TBDPSO
BnO
O
O
O
i
ii,iii
AcO
AcO
HO
HO
OAc
O
O
BnO
AcO
HO
BnO
3
1
2
3
iv
TBDPSO
TBDPSO
BnO
TBDPSO
OMs
OMs
OH
OH
O
vi
v
BnO
BnO
BnO
BnO
OH
BnO
BnO
BnO
BnO
6
5
4
xii
vii
N
N
N
BnO
BnO
BnO
BnO
BnO
BnO
NH2
N
BnO
BnO
BnO
OTBDPS
OTBDPS
OTBDPS
7
12
11
viii
xi
NH
N
N
BnO
BnO
BnO
NHFmoc
BnO
BnO
NH2
BnO
x
ix
BnO
BnO
BnO
OTBDPS
OTBDPS
OTBDPS
8
9
10
Scheme 1. Synthesis of the building blocks. Reagents and conditions: (i) 4-penten-1-ol, PTSA, CHCl3, 90°C, 70 h (85%); (ii) DBU,
TBDPSCl, THF, 22 h (94%); (iii) NaH, BnBr, 18-crown-6, THF, 70°C, 19 h (85%); (iv) NBS, 1% aq. CH3CN, 2 h (70%); (v)
NaBH4, EtOH, 20 h (92%); (vi) MsCl, Et3N, DMAP, CH2Cl2, −15°C, 3 h (80%); (vii) allylamine, 70°C, 39 h (73%); (viii) (1)
ACE-Cl, ClCH2CH2Cl, 90°C, 24 h, (2) MeOH, 80°C, 21 h (82%); (ix) N-Fmoc-3-(2,4-dichlorophenyl)oxaziridine, CHCl3, 0°C,
3 h (60%); (x) piperidine, DMF, 16 h; (xi) acetone, 40°C (70% for two steps); (xii) ethylenediamine, 70°C, 46 h (72%).
Our strategy for the solid-phase synthesis of oligosac-
charide analogues is based on a three-step coupling
procedure (Scheme 2). In the first step the primary
hydroxyl function of a resin-bound azasugar is released
after selective cleavage of the tert-butyldiphenylsilyl
ether with TBAF. The second step consists of activa-
tion of this free hydroxyl group as a reactive carbonate
or carbamate. In the final step the resulting activated
intermediate is treated with an azasugar. Nucleophilic
attack of this building block at the activated carbonyl
group leads to the formation of the carbamate bond
between the monomers. Repetition of this convenient
coupling cycle offers the possibility to construct
oligomeric carbohydrate analogues.
the instability of the activated linker,17 coupling of 13
with azasugar 12 (after activation as the isocyanate)
afforded preloaded linker 14 in good yield (Scheme 3).
Indeed, conversion of the primary amine of 12 into a
highly reactive isocyanate with di-tert-butyltricarbonate
offers the possibility for an alternative preloading pro-
cedure where the use of the unstable activated linker is
avoided.18 Saponification of methyl ester 14 led to
carboxylic acid 15 which was linked to the resin via an
amide bond using standard peptide coupling condi-
tions. The 2,4,6-trinitrobenzenesulfonic acid test (TNBS
test) indicated quantitative reaction after 24 h.19 All
further solid-phase reactions were monitored via TLC
and ES-MS after release of a small amount of product
from the resin by treatment with 1% TFA in
dichloromethane.
In order to demonstrate the viability of this solid-phase
coupling strategy we envisaged the synthesis of a repeti-
tive tetrasaccharide analogue on TentagelTM R NH2
resin. This resin features a better swelling and a lower
loading than the standard TentagelTM resins and was
especially developed for difficult syntheses and the syn-
thesis of oligomers. The acid-labile HMPB linker 13
offers perfect compatibility with the basic reaction con-
ditions during the coupling cycle and the protecting
groups in the building blocks.16 This enables monitor-
ing of each reaction step via TLC and ES-MS after
cleavage of a small amount of product from the resin.
Whereas preloading of the HMPB linker 13 (after
activation as a reactive carbonate) with azasugar 8
proved to be impossible due to steric hindrance of the
secondary amino function in this building block and
Scheme 2. Three-step coupling cycle.

B. Ruttens, J. Van der Eycken / Tetrahedron Letters 43 (2002) 2215–2221
2217
Scheme 3. Solid-phase synthesis. Reagents and conditions: (i) (1) azasugar 12, di-tert-butyltricarbonate, CHCl3, rt, 15 min, (2) add
13, 70°C, 24 h (84%); (ii) NaOH, MeOH, 16 h (88%); (iii) TentagelTM R NH2, DIC, HOBt, DMAP, CH2Cl2, 24 h; (iv) TBAF,
THF, 24 h; (v) (1) bis(4-nitrophenyl) carbonate, DMAP, DMF, 24 h, (2) azasugar 12, DIPEA, DMF, 24 h.
Table 1. Coupling reagents and conditions for the synthesis of 18 on solid phase (Scheme 3)
Entry
Reaction conditions
Yield (%)a
Yield (%)b
1
2
3
4
5
(i) N,N%-Disuccinimidyl carbonate (10 equiv.), DMAP (1 equiv.), DMF, 2 h; (ii) 12 (5 equiv.),
DIPEA (10 equiv.), DMF, 24 h
(i) 1,1%-Carbonyldiimidazole (10 equiv.), DMAP (1 equiv.), DMF, 2 h; (ii) 12 (5 equiv.), DIPEA
(10 equiv.), DMF, 24 h
(i) 4-Nitrophenyl chloroformate (10 equiv.), pyridine, CH2Cl2, 0°C, 2 h; (ii) 12 (5 equiv.), DIPEA
(10 equiv.), DMF, 24 h
(i) Bis(4-nitrophenyl) carbonate (10 equiv.), DMAP (1 equiv.), DMF, 24 h; (ii) 12 (5 equiv.),
DIPEA (10 equiv.), DMF, 24 h
72
33
89
–
41
–
100
45
–
(i) 12 (5 equiv.), Di-tert-butyltricarbonate (5 equiv.), 30 min; (ii) add resin 17, CHCl3, 70°C, 24 h
65
a After a single coupling.
b After repetition of the coupling sequence.

2218
B. Ruttens, J. Van der Eycken / Tetrahedron Letters 43 (2002) 2215–2221
Treatment of resin 16 with TBAF resulted in deprotec-
tion of the silyl ether and afforded primary alcohol 17.
In Table 1, the efficiency of four different reagents for
the activation of the primary hydroxyl group of 17
(entries 1–4) and one for activation of the amino group
of 12 (entry 5) was compared by measuring the yield of
the dimer after treatment of the activated intermediate
with building block 12 (entries 1–4) or with resin 17
(entry 5). Somewhat surprisingly, the best results were
obtained with bis(4-nitrophenyl) carbonate (entry 4).20
All other reagents led to lower yields (entries 1 and 5)
or to formation of side products (entries 2 and 3).
Hence, starting from resin 17, deprotection of the tert-
butyldiphenylsilyl ether with TBAF, followed by acti-
vation of the primary hydroxyl function with
bis(4-nitrophenyl) carbonate and treatment of the acti-
vated 4-nitrophenyl carbonate with the next building
block gave dimer 18.21 Repetition of this coupling cycle
led to trimer 20 and tetramer 22. ES-MS and TLC
analysis indicated nearly quantitative yields for each
reaction step and purities of >95%, impurities mainly
resulting from incomplete coupling. Compounds 19, 21
and 23 were cleaved from the resin under mild acidic
conditions and, after purification, debenzylated in solu-
tion using transfer hydrogenation on Pd/C with cyclo-
hexene as hydrogen donor (Scheme 4 and Fig. 1).22
Purification of the fully deprotected dimer 25, trimer
26 and tetramer 27 was performed on silicagel using
eluent mixtures of iso-propanol and 25% aqueous
ammonia.
are coupled via the 5a-nitrogen atom (carbohydrate
numbering), which in this case corresponds to the
pyranose ring oxygen in the real sugar, and not to the
anomeric carbon atom. Examples of biologically active
oligosaccharides incorporating a similar 5a-azasugar
unit coupled via the nitrogen atom are very scarce.23
In conclusion, we have developed a strategy for the
solid-phase synthesis of a new class of azasugar-based
oligosaccharide analogues. Our strategy relies on a
highly efficient three-step (deprotection–activation–cou-
pling) formation of a carbamate bond between aza-
sugar building blocks, and leads to a new class of
highly functionalised oligosaccharide mimics with
potential bioactivity. The viability of our approach was
illustrated for a derivative of 1,5-dideoxy-1,5-imino-L-
iditol, for which a synthesis was developed. Our
approach should be easily adaptable to the solid-phase
coupling of other types of aza-sugars, e.g. isofagomine
and its stereoisomers.
Acknowledgements
We thank the Fund for Scientific Research-Flanders
(Belgium) (F.W.O.-Vlaanderen) for a Research Pro-
gramme (No. G.0249.97), BOF (GOA96-009) and the
‘Ministerie voor Wetenschapsbeleid’ for financial sup-
port. B.R. is grateful to the ‘Vlaams Instituut voor de
Bevordering van het Wetenschappelijk-Technologisch
Onderzoek in de Industrie’ (IWT) for a scholarship.
It should be noticed that this type of azasugar
oligomers is quite remarkable, as the iminosugar units
i
N
HO
HO
12
19
ii (74%)
HO
NH2
HO
OH
24
25
iii (89%)
ii (64%)
N
N
N
N
HO
HO
HO
HO
HO
NH
NH2
OH
O
O
iii (88%)
ii (65%)
N
N
N
HO
HO
HO
HO
HO
HO
21
23
HO
NH HO
NH HO
NH2
OH
O
O
26
O
O
O
iii (82%)
ii (73%)
N
N
HO
HO
HO
HO
HO
HO
HO
HO
HO
NH
HO
HO
HO
NH
NH
NH2
OH
O
O
O
27
O
O
Scheme 4. Cleavage and deprotection. Reagents and conditions. (i) TBAF, THF, 20 h (95%); (ii) Pd/C, cyclohexene, MeOH, reflux,
24 h; (iii) 1% TFA in CH2Cl2, kt, 5×10 min.

B. Ruttens, J. Van der Eycken / Tetrahedron Letters 43 (2002) 2215–2221
2219
Figure 1. ESMS spectra for compounds 24, 25, 26 and 27 (API-ES, Positive mode, Flow Injection Analysis).

2220
B. Ruttens, J. Van der Eycken / Tetrahedron Letters 43 (2002) 2215–2221
J=9.1 Hz, J=10.3 Hz), 4.45 (1H, d, J=11.4 Hz), 4.52
References
(1H, d, J=11.4 Hz), 4.58 (1H, d, J=11.8 Hz), 4.62 (1H,
d, J=11.8 Hz), 4.65 (2H, s), 7.18 (2H, m), 7.34 (19H, m),
7.66 (4H, m) ppm. lC (125 MHz, CDCl3) 19.2, 26.9, 44.0,
56.3, 60.1, 72.1, 72.5, 74.2, 77.0, 77.3, 78.5, 127.4, 127.5,
127.5, 127.6, 127.7, 127.7, 127.8, 128.2, 128.3, 129.7,
133.4, 133.5, 135.5, 138.4, 138.5, 138.6 ppm.
1. Varki, A. Glycobiology 1993, 3, 97–130.
2. Hindsgaul, O. Cell Biology 1991, 2, 319–326.
3. For a recent overview on oligosaccharide solid-phase
synthesis, see: (a) Seeberger P. H. Solid Support Oligosac-
charide Synthesis and Combinatorial Carbohydrate
Libraries; Wiley Interscience: Weinheim, 2001; (b)
Osborn, H. M. I.; Khan, T. H. Tetrahedron 1999, 55,
1807–1850.
13. Vidal, J.; Damestoy, S.; Guy, L.; Hannachi, J.-C.; Aubry,
A.; Collet, A. Chem. Eur. J. 1997, 3, 1691–1709.
14. A similar diazasugar, but with nitrogen in place of the
anomeric carbon and glycosidic oxygen, was reported:
Søhoel, H.; Liang, X.; Bols, M. Synlett 2000, 347–348.
15. Selected data for 11: lH (500 MHz, CDCl3) 1.07 (9H, s),
1.86 (3H, s), 1.88 (3H, s), 2.94 (1H, dd, J=4.6 Hz,
J=13.2 Hz), 3.18 (2H, m), 3.90 (2H, m), 4.01 (2H, m),
4.12 (1H, dd, J=3.4 Hz, J=10.7 Hz), 4.63 (1H, d,
J=11.7 Hz), 4.65 (1H, d, J=11.8 Hz), 4.72 (1H, d,
J=11.8 Hz), 4.75 (1H, d, J=11.7 Hz), 4.78 (1H, d,
J=11.0 Hz), 4.85 (1H, d, J=11.0 Hz), 7.35 (21H, m),
7.77 (4H, br d, J=7.9 Hz) ppm. lC (125 MHz, CDCl3)
18.3, 19.2, 25.2, 26.9, 52.5, 60.8, 64.9, 72.9, 73.4, 75.1,
77.5, 78.2, 82.7, 127.5, 127.5, 127.7, 127.9, 128.0, 128.0,
128.3, 129.5, 129.6, 133.5, 133.6, 135.7, 135.8, 138.9,
139.1, 139.2, 164.4 ppm.
4. See e.g. the following papers and references cited therein:
(a) Liang, X.; Petersen, B. E.; Duus, J. Ø.; Bols, M. J.
Chem. Soc., Perkin Trans.
1 2001, 2764–2773; (b)
Chakraborty, T. K.; Jayaprakash, S.; Srinivasu, P.;
Govardhana Chary, M.; Diwan, P. V.; Nagaraj, R.; Ravi
Sankar, A.; Kunwar, A. C. Tetrahedron Lett. 2000 41,
8167–8171; (c) Bach, P.; Lohse, A.; Bols, M. Tetrahedron
Lett. 1999 40, 367–370; (d) Szabo, L.; Smith, B. L.;
McReynolds, K. D.; Parrill, A. L.; Morris, E. R.; Ger-
vais, J. J. Org. Chem. 1998, 63, 1074–1078; (e) Long, D.
D.; Smith, M. D.; Marquess, D. G.; Claridge, T. D. W.;
Fleet, G. W. J. Tetrahedron Lett. 1998, 39, 9293–9296; (f)
Byrgesen, E.; Nielsen, J.; Willert, M.; Bols, M. Tetra-
hedron Lett. 1997, 38, 5697–5700.
16. Flo¨rsheimer, A.; Riniker, B. Peptides 1990, 131–133.
17. The slow nucleophilic attack of the sterically hindered
building block 8 allows mesomerically induced decompo-
sition of the activated linker. Conjugate addition of 8 to
the highly reactive decomposition product i leads to
formation of a preloaded compound ii with a benzylic
CꢀN bond between the azasugar and the HMPB linker in
14% yield.
5. Bols, M. Acc. Chem. Res. 1998, 31, 1–8.
6. For other synthetic approaches to 1,5-dideoxy-1,5-imino-
L
-iditol, see: (a) Lee, B. W.; Jeong, I.-Y.; Yang, M. S.;
Choi, S. U.; Park, K. H. Synthesis 2000, 1305–1309; (b)
Schaller, C.; Vogel, P.; Jager, V. Carbohydr. Res. 1998,
314 25–35; (b) Hugel, H. M.; Hughes, A. B.; Khalil, K.
Aust. J. Chem. 1998, 51, 1149–1155; (c) Poitout, L.; Le
Merrer, Y.; Depezay, J.-C. Tetrahedron Lett. 1996, 37,
1609–1612.
TBDPSO
OBn
7. Wing, R. E.; Bemiller, J. N. Carbohydr. Res. 1969, 10,
441–448.
8. (a) Konradsson, P.; Udodong, U.; Fraser-Reid, B. J.
Chem. Soc., Chem. Commun. 1988, 823–825; (b) Mootoo,
D. R.; Date, V.; Fraser-Reid, B. J. Am. Chem. Soc. 1988,
110, 2662–2663.
OBn
OBn
N
MeO
MeO
O
O
+O
9. Hansen, A.; Tagmose, T. M.; Bols, M. Tetrahedron 1997,
53, 697–706.
OMe
O
OR
i
ii
10. Selected data for 12: lH (500 MHz, CDCl3) 1.06 (9H, s),
1.50 (2H, br s), 2.74 (4H, m), 2.91 (2H, m), 3.27 (1H, m),
3.56 (1H, m), 3.62 (1H, t, J=9.4 Hz), 3.68 (1H, dd,
J=5.7 Hz, J=9.4 Hz), 3.93 (1H, dd, J=2.4 Hz, J=11.2
Hz), 4.08 (1H, dd, J=7.2 Hz, J=11.2 Hz), 4.56 (1H, d,
J=11.6 Hz), 4.58 (1H, d, J=11.6 Hz), 4.65 (1H, d,
J=11.6 Hz), 4.73 (1H, d, J=11.6 Hz), 4.76 (1H, d,
J=10.9 Hz), 4.80 (1H, d, J=10.9 Hz), 7.22 (2H, m), 7.32
(17H, m), 7.42 (2H, m), 7.72 (4H, m) ppm. lC (125 MHz,
CDCl3) 18.9, 26.7, 39.4, 48.9, 57.5, 58.3, 61.9, 72.5, 72.8,
75.3, 79.0, 80.2, 82.9, 127.2, 127.3, 127.4, 127.5, 127.5,
127.8, 128.1, 128.2, 129.5, 129.5, 133.0, 133.1, 135.5,
138.3, 138.5, 138.9 ppm.
11. (a) Olofson, R. A.; Martz, J. T.; Senet, J.-P.; Piteau, M.;
Malfroot, T. J. Org. Chem. 1984, 49, 2081–2082; (b)
Cooley, J. H.; Evain, E. J. Synthesis 1989, 1, 1–7.
12. Selected data for 8: lH (500 MHz, CDCl3) 1.07 (9H, s),
2.84 (1H, dd, J=7.1 Hz, J=12.5 Hz), 2.99 (1H, dd,
J=4.1 Hz, J=12.5 Hz), 3.29 (1H, ddd, J=4.5 Hz, J=6.0
Hz, J=9.1 Hz), 3.47 (1H, dd, J=6.7 Hz, J=10.7 Hz),
3.62 (1H, t, J=6.7 Hz), 3.72 (1H, dd, J=4.5 Hz, J=6.7
Hz), 3.76 (1H, dd, J=6.0 Hz, J=10.3 Hz), 3.89 (1H, dd,
18. (a) Peerlings, H. W. I.; Meijer, E. W. Tetrahedron Lett.
1999, 40, 1021–1024; (b) Pope, B. M.; Yamamoto, Y.;
Tarbell, D. S. Org. Synth. 1978, 57, 45–50.
19. Hancock, W. S.; Battersby, J. E. Anal. Biochem. 1976, 71,
260–264.
20. Stirchak, E. P.; Summerton, J. E.; Weller, D. D. Nucl.
Acids. Res. 1989, 17, 6129–6141.
21. Typical experimental coupling procedure:
To resin 18 (0.3 mmol) were added dry THF (10 ml) and
a 1 M solution of TBAF in THF (3.0 ml, 3.0 mmol).
After shaking the resin for 20 h under inert atmosphere
the resin was washed with DMF (10 ml), EtOH (10 ml)
and CH2Cl2 (10 ml). This washing procedure was
repeated five times. After thorough drying of the resin,
DMAP (37 mg; 0.3 mmol) and a solution of bis(4-nitro-
phenyl) carbonate (915 mg; 3.0 mmol) in dry DMF (15
ml) were added. After shaking for 24 h under inert
atmosphere the resin was washed quickly five times with
dry DMF (10 ml), followed by addition of a solution of
building block 12 (1.07 g; 1.5 mmol) and DIPEA (0.51
ml; 3.0 mmol) in dry DMF (15 ml). The reaction was

B. Ruttens, J. Van der Eycken / Tetrahedron Letters 43 (2002) 2215–2221
2221
shaken for another 24 h under inert conditions affording
resin 20. Finally the resin was washed consecutively with
CH2Cl2 (5×10 ml) and MeOH (5×10 ml).
tion of the filtrate in vacuo was purified via column
chromatography (CH2Cl2/MeOH: 95/5). The resulting
pure product was dissolved in MeOH, and cyclohexene
(20% v/v) and 10% Pd/C (100% w/w) were added. After
refluxing for 24 h the catalyst was removed via filtration
over Celite, and the residue obtained after concentration
of the filtrate in vacuo was purified via column chro-
matography on silica gel (isopropanol/NH3 (25% in
H2O):55/45) affording 25 (57% overall yield).
22. Typical experimental cleavage and deprotection procedure:
A portion of resin 19 (600 mg) was treated with a freshly
prepared 1% solution of TFA in dry CH2Cl2 (10 ml).
After shaking the resin for 10 min the resin was filtered
and the filtrate was collected. This procedure was
repeated five times and the combined filtrates were neu-
tralised with a resin-bound proton scavenger (piperidi-
nomethyl polystyrene HL resin, 2% DVB, 200–400 mesh,
loading 3.57 mmol/g, Novabiochem). The residue
obtained after filtration of the scavenger and concentra-
23. (a) Wischnat, R.; Martin, R.; Wong, C.-H. J. Org.
Chem. 1998, 63, 8361–8365; (b) Shiozaki, M.; Yoshiike,
R.; Ando, O.; Ubukata, O.; Haruyama, H. Tetrahedron
1998, 54 (50), 15167–15182.