Convergent synthesis of an inner core GPI of sperm CD52

Article abstract of DOI:10.1016/S0960-894X(02)00301-3

An inner core of the GPI anchor of sperm CD52 antigens was synthesized by a highly convergent process using specially modified inositol, glucosamine and phospholipid as key building blocks. This paper also presents a new and efficient procedure to prepare

Full text of DOI:10.1016/S0960-894X(02)00301-3

Bioorganic & Medicinal Chemistry Letters 12 (2002) 2015–2018
Convergent Synthesis of an Inner Core GPI of Sperm CD52
Jie Xue and Zhongwu Guo*
Department of Chemistry, Case Western Reserve University, 10900 Euclid Avenue,
Cleveland, OH 44106, USA
Received 28 January 2002; accepted 1 April 2002
Abstract—An inner core of the GPIanchor of sperm CD52 antigens was synthesized by a highly convergent process using specially
modified inositol, glucosamine and phospholipid as key building blocks. This paper also presents a new and efficient procedure to
prepare 1,2,6-differentiated derivatives of inositol for GPIsyntheses. # 2002 Elsevier Science Ltd. All rights reserved.
CD52 antigens, which are expressed on virtually all
human lymphocytes1 and sperm cells,2 belong to a very
unique class of glycoproteins that are anchored to
plasma membranes by glycosylphosphatidylinositols
(GPIs). CD52 antigens play a fundamental role in
the recognition process of the immune system and the
specific interactions between eggs and sperms.1,2
for a key intermediate 5 in the synthesis of 3, which
made use of both enantiomers of a partially protected
derivative, 6, of inositol.
The preparation and resolution of 6 were achieved by a
reported method1b that could afford both enantiomers
in excellent yields and optical purity. The allylation of
(+)-6 by means of Bu2SnO and allyl bromide gave a
6-allyl product 7 in good yields (Scheme 1a) and only a
small amount (5%) of the 1-allyl isomer. This regio-
selectivity is proposed to result from steric effect, as the
1-position of the tin complex of 6 (Fig. 2) would be
more sterically hindered than the 6-position. An experi-
mental support for this rationale is that when a bulkier
reagent, such as p-methoxybenzyl chloride (MBnCl),
was utilized for the reaction, only 6-alkylation was
observed. Then, the free hydroxyl group in 7 was pro-
tected by an MBn group to give 8. When 8 was treated
by HCl in MeOH/CH2Cl2 for a short period (10 min),
the more strained trans ketal was selectively cleaved
to offer 9 (60%). This reaction should be monitored
closely, as the remaining ketal in 9 could also be removed
Despite the fact that GPI-anchoring is very common in
the eukaryotic world,3 it is fairly difficult to obtain
homogeneous GPIs and GPI-anchored glycoproteins
from nature. Thus, chemical synthesis of GPIs has been
a major research focus in the past decade.4À9 However,
there is no report yet about the synthesis of CD52 GPIs,
though a glycopeptide fragment of CD52 antigens has
been recently synthesized by solid-phase method.10 This
paper describes the first chemical synthesis of a short
GPIanchor ( 1) of sperm CD52.
As indicated by our retrosynthetic analysis (Fig. 1), the
convergent assembly of 1 would require three key
building blocks 2, 3, and 4, of which the specially
modified d-myo-inositol 3 was critical.
During the past decades, numerous methods have been
designed for the synthesis of optically pure derivatives
of inositol.11 However, most methods utilized only one
resolved enantiomer of a racemic intermediate, which
has severely affected the overall synthetic efficiency. To
solve this problem, we designed a new method (Scheme 1)
*Corresponding author. Tel.: +1-216-368-3736; fax: +1-216-368-
3006; e-mail: zxg5@po.cwru.edu
Figure 1. Retrosynthetic analysis.
0960-894X/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(02)00301-3

2016
J. Xue, Z. Guo / Bioorg. Med. Chem. Lett. 12 (2002) 2015–2018
Scheme 1. Reagents and conditions: (a) (i) Bu2SnO, toluene, reflux, 2.5 h; (ii) AllBr, DMF, rt, 16 h; (b) MBnCl, NaH, DMF, rt, 3 h; (c) AcCl,
MeOH–CH2Cl2, rt, 10 min; (d) BnBr, NaH, DMF, rt, 3 h; (e) AcCl, MeOH–CH2Cl2, rt, 2 h; (f) (i) Bu2SnO, toluene, reflux, 2.5 h; (ii) BnBr, DMF, rt,
15 h; (g) (i) Bu2SnO, toluene, reflux, 2.5 h; (ii) MBnCl, DMF, rt, 16 h.
Scheme 2. Reagents and conditions: (a) C15H31COOH, DCC, DMAP,
CH2Cl2, rt, overnight; (b) PdCl2, NaOAc, AcOH, THF, rt, 18 h.
Figure 2. The tin complex involved in the selective allylation of (+)-6.
on prolonged exposure to acid. Thereafter, 9 was
benzylated to produce 10. The ketal in 10 was finally
deblocked by HCl/MeOH (2 h), and the resulting diol
11 was regioselectively benzylated in the presence of
Bu2SnO to afford the key derivative 5. Only the
Scheme 3. Reagents and conditions: (a) Ac2O–AcOH (1:1), H2SO4
(cat), 0 ꢀC, 15 min; (b) BnNH2, Et2O, rt, 2 h; (c) DAST, CH2Cl2, 0 ꢀC,
30 min; (d) NaOMe, MeOH, rt, 2 h; (e) BnBr, NaH, DMF, rt, 3 h.
3-benzylation was observed in this reaction. Thus, 5 was
effectively prepared on a multigram scale from (+)-6 in
six separate steps and a 32% overall yield.
PdCl2/HOAc without affecting benzyl groups at other
positions. Therefore, once 5 was obtained, it was quite
straightforward to transform 5 to 3. 2-Acylation of 5
using DCC as a condensation reagent and then selective
deallylation of the product 17 afforded 3 in an excellent
yield (Scheme 2).
To further improve the overall yield of 5 from inositol,
we studied the feasibility to make use of the remaining
enantiomer of 6. In fact, as inositol itself is symmetric
and the temporary chiral property of 6 was created by
derivatization, if we introduce new protecting groups to
(À)-6 (in replacement of the exiting ones) by a sequence
reverse to that of Scheme 1a, it is possible to derive the
same synthetic target from both enantiomers.
The glycosyl donor 2, having its amino group protected
as an azido group, was then prepared from d-glucal by
a procedure shown in Scheme 3. d-Glucal was readily
transformed to 18 by a reported method.12 Selective
breaking of the 5-membered ring in 18 by a mixture of
acetic acid and acetic anhydride (1:1) containing a trace
amount of concentrated sulfuric acid gave the diacetate
19 (a/b=3.5:1), which was selectively deprotected by
benzylamine to yield a hemiacetal 20. Treatment of 20
with DAST afforded the glycosyl fluoride 21 (a/b=1:6).
Finally, 2 was obtained by replacing the 6-acetyl group
in 21 for a benzyl group in two steps using conventional
protocols.
Therefore, we designed another synthetic procedure for
5 starting from (À)-6 (Scheme 1b). The reactions used
herein were similar to those used in the former procedure.
First, (À)-6 was benzylated to produce 12. Then, its
trans ketal was selectively cleaved under an acidic con-
dition described above to give a diol 13 (68%). Regio-
selective allylation of 13 was achieved again under the
influence of Bu2SnO to afford 14 (71%). Then, benzy-
lation of 14, removal of the ketal in the resulting 15 and
selective methoxybenzylation of 16 in the presence of
Bu2SnO finally offered 5 in an excellent yield (84%).
Thus, 5 was prepared from (À)-6 in 6 separate steps and
a 42% overall yield.
The phospholipid block 4 was prepared according to
Scheme 4. First, d-mannitol was converted to 23 by a
reported procedure.13 Then, phosphorylation of 23 by
24 afforded 4. It has a defined R-configuration. Com-
pound 4 was proved during chromatography to be
unstable on a silica gel column. Thus, crude 4 was
directly used for the next step.
While 5 contains a free 2-hydroxyl to which various
functional or protecting groups can be attached, its 1-
and 6-positions are respectively protected by MBn and
All that can be selectively removed by CAN/H2O and
The glycosylaction of 3 by 2 was achieved in anhydrous
Et2O using dichlorohafnocene and silver perchlorate as

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References and Notes
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Scheme 4. Reagents and conditions: (a) (NCCH2CH2O)P(NPri2)2 (24),
diisopropylaminium tetrazolide, CH2Cl2–MeCN, rt, 2 h.
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Scheme 5. Reagents and conditions: (a) Cp2HfCl2, AgOTf, MS 4A,
Et2O, À15 ꢀC to rt, overnight; (b) CAN, H2O–MeCN (1:9), rt, 2 h; (c)
4, tetrazole, MS 3A, CH2Cl2–MeCN, rt, 6 h; O2, light.
a promoter (Scheme 5), and a mixture of the expected
product 25 and its b-anomer (4:3) was obtained in 70%
yield. The anomers were easily separable on a silica gel
column. Meanwhile, it was interesting to notice that the
same reaction carried out in CH2Cl2 gave a mixture in
favor of the b-isomer (a/b=4:5). After removal of the
MBn in 25 by oxidation with CAN, 26 was obtained in
a good yield (80%). Finally, phosphorylation of 26 by 4
in the presence of tetrazole proceeded very smoothly to
give a product (67% yield after column chroma-
tography) that was projected to be a phosphite. How-
ever, the 31P NMR (d 8.71, 7.35) and MS spectra clearly
indicated that the product was a diastereoisomeric
mixture of the expected final product 114 that might
result from the oxidation of the phosphite by oxygen on
the column, which was previously observed by Frier15 as
well.
In conclusion, this paper described a highly convergent
synthesis of a GPIanchor ( 1) of sperm CD52. Since the
4-position of its glucosamine residue, which is protected
by an allyl group, can be selectively exposed, it will be
possible to further modify the glycan for the prepara-
tion of intact CD52 GPIanchors. In the meantime, this
paper also presented a new and effective method to
prepare an enantiomerically pure derivative (5) of
d-myo-inositol with its 1,2,6-positions well differ-
entiated. Because the inositol residue in many GPI
anchors has its 2-position modified by various lipid
chains, 5 can be widely useful to the synthesis of other
GPIanchors. The synthesis of 5 is highlighted by
making use of both enantiomers of an intermediate.
Enantiomers of inositol derivatives were previously
employed by Chen11a and Fraser-Reid5a in their works,
but these studies, like ours, used both enantiomers of a
certain intermediate in different ways and for different
purposes.
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Acknowledgements
This project was supported by the Research Cor-
poration. We thank Mr. Jim Faulk for the MS
measurements.

2018
J. Xue, Z. Guo / Bioorg. Med. Chem. Lett. 12 (2002) 2015–2018
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30H), 5.76 (m, 1H), 5.60 (bs, 1H), 5.40 (d, J=3.6 Hz, 1H), 5.10
(d, J=17.4 Hz, 1H), 5.04 (d, J=10.2 Hz, 1H), 4.89 (d,
J=11.4 Hz, 1H), 4.72 (d, J=11.4 Hz, 1H), 4.69 (m, 2H), 4.60–
4.64 (m, 3H), 4.50–4.54 (m, 3H), 4.44 (d, J=11.2 Hz, 1H), 4.39
(d, J=12.6 Hz, 1H), 3.30–4.22 (m, 19H), 3.17 (br, 1H), 3.13 (t,
J=9.0 Hz, 1H), 2.94 (d, J=9.6 Hz, 1H), 2.63 (t, J=6.0 Hz,
2H), 2.26 (m, 2H), 1.56 (m, 4H), 1.01–1.18 (m, 54H), 0.91 (m,
6H); 31P NMR (CDCl3) d 8.71, 7.35; FABMS: calcd for
C97H137N4O16P, 1645.0. Found, 1684.1 (M+K+), 1668.0
(M+Na+), 1640.1 (MÀN2+Na+), and 1549.1 (MÀN2–
Bn+Na+).
13. Hirth, G.; Barner, R. Helv. Chim. Acta 1982, 65, 1059.
14. 5: [a]D=À8ꢀ (c 1.0, CHCl3); 1H NMR (300 MHz, CDCl3)
d 7.31–7.36 (m, 17H), 6.92 (d, J=8.5 Hz, 2H), 5.96–6.08 (m,
1H), 5.32 (dd, J=17.3, 1.6 Hz, 1H), 5.19 (d, J=10.4 Hz, 1H),
4.85–4.94 (m, 4H), 4.73 (s, 2H), 4.68 (d, J=11.4 Hz, 1H), 4.64
(d, J=11.4 Hz, 1H), 4.42 (m, 2H), 4.19 (br, 1H), 3.98 (t,
J=9.5 Hz, 1H), 3.85 (m, 1H), 3.84 (s, 3H), 3.42 (t, J=9.3 Hz,
1H), 3.39 (dd, J=9.6, 2.5 Hz, 1H), 3.33 (dd, J=9.6, 2.5 Hz,
1H), 2.48 (s, 1H). (À)-6: [a]À16.5ꢀ (c 1.0, CHCl3); Lit.:11a À16ꢀ
(c 1.0, CHCl3). 1: 1H NMR (600 MHz, CDCl3) d 7.12–7.37 (m,
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