A convergent total synthesis of the mucin related F1α antigen by one-pot sequential stereoselective glycosylation

Article abstract of DOI:10.1246/cl.2001.840

A convergent total synthesis of F1α antigen, a member of the tumor-associated O-linked mucin glycosyl amino acids, was accomplished by one-pot sequential glycosylation. In the first step, the corresponding disaccharide was formed from galactosyl phenylcarbonate 2 or fluoride 3 and thioglycoside 4 by the promotion of TrB(C₆F₅)₄ or TfOH, and following glycosylation of glycosyl amino acid 5 by further addition of N-iodosuccinimide(NIS) afforded protected F1α in 80 or 89% overall yield, respectively. By the subsequent deprotection, F1α antigen was obtained in good yield.

Full text of DOI:10.1246/cl.2001.840

840
Chemistry Letters 2001
A Convergent Total Synthesis of the Mucin Related F1α Antigen
by One-Pot Sequential Stereoselective Glycosylation
Teruaki Mukaiyama,* Kazuhiro Ikegai, Hideki Jona, Takashi Hashihayata, and Kazuya Takeuchi
Department of Applied Chemistry, Faculty of Science, Science University of Tokyo, Kagurazaka, Shinjuku-ku, Tokyo 162-8601
(Received June 11, 2001; CL-010542)
A convergent total synthesis of F1α antigen, a member of
the tumor-associated O-linked mucin glycosyl amino acids, was
accomplished by one-pot sequential glycosylation. In the first
step, the corresponding disaccharide was formed from galacto-
syl phenylcarbonate 2 or fluoride 3 and thioglycoside 4 by the
promotion of TrB(C6F5)4 or TfOH, and following glycosylation
of glycosyl amino acid 5 by further addition of N-iodosuccin-
imide(NIS) afforded protected F1α in 80 or 89% overall yield,
respectively. By the subsequent deprotection, F1α antigen was
obtained in good yield.
During the last decade, mucins have been regarded as
important substances in antitumor immunological studies due to
their high expression on epithelial cell surfaces and high con-
of 7 with 6 was examined by using various catalysts and
solvents, and it was revealed that the glycosylation proceeded
not in good yield when a combination of a catalytic amount of
protic acid (such as TfOH or HClO4) and NIS11 was used
because partial deprotection of benzylidene acetal took place
during the glycosylation, and α-selectivity was not observed
when the reaction was carried out in Et2O. When trityl
tetrakis(pentafluorophenyl)borate [TrB(C6F5)4] was used as a
Lewis acid together with NIS4 in toluene, the glycosyl amino
acid 10 was obtained in high yield, however, the undesirable β-
glycoside was predominantly formed (Scheme 1). On the
other hand, the TrOTf (generated in situ from TrCl and
AgOTf)-catalyzed glycosylation in toluene afforded 10 in high
yield with good α-selectivity. It is interesting to note that the
stereoselectivity was reversed only by changing the counter
anion of the catalyst.12 After separation of these isomers,
regioselective benzylidene ring opening of 10α was carried out
by using a combination of BH3·Et3N and BF3·OEt213 to give
alcohol 5 in moderate yield.
tent of clustered α-O-linked carbohydrates. Mucins on epithe-
lial tumors often contain aberrant α-O-linked carbohydrates,
and recently found F1α antigens, Galβ-(1→4)GlcNAcβ-
(1→6)GalNAcα-(1→3)Ser and -(1→3)Thr, represent examples
of aberrant carbohydrate epitopes found on mucins associated
with gastric adenocarcinomas.1 Therefore, chemical total syn-
thesis of F1α antigens is an interesting topic because of their
structures and biological properties, and their total synthesis has
already been accomplished by R. R. Koganty et al. (in 1997)2
and S. J. Danishefsky et al. (in 1998).3 In our previous
papers,4,5 two types of one-pot sequential glycosylation for the
synthesis of trisaccharides were reported, in which glycosyl flu-
oride or phenylcarbonate and thioglycoside were used in the
first step, and methylglycoside was used in the second step.6 It
was thought then that the above method would be effectively
applied to rapid total synthesis of F1α. In this communication,
we would like to report a convergent total synthesis of F1α
antigen (1, serine type) by one-pot sequential glycosylation.
The retro synthetic analysis of F1α (1) is shown in Figure
1. The results of our previously reported procedure indicated
that the fully protected F1α would be synthesized by one-pot
sequential glycosylation using galactosyl donor 2 or 3, thiogly-
coside 4 and glycosyl amino acid 5. Galactosyl donor 2 or 3
having 2-O-p-methylbenzoyl (p-MeBz) group was prepared
according to the same procedure as glucosyl donors.4,7,8
Thioglycoside 4 having 4,5-dichlorophthaloyl (DCPhth) amino
function9 was chosen by considering its easiness in removing
the amino protecting group and was prepared by standard pro-
tecting group manipulations. It was thought that 5 would be
prepared by stereoselective glycosylation of the protected serine
7 with thioglycoside 6 using trityl salt and NIS.4
Next, stereoselective glycosylation of thioglycoside 4 with
galactosyl donor 2 or 3 was examined in detail according to our
In the first place, stereoselective synthesis of 5 was exam-
ined (Scheme 1): that is, 1-O-hydroxy sugar 8, prepared by the
known procedure,10 was treated with SOCl2 in DMF, followed
by anomeric substitution with NaSPh to afford the corresponding
thioglycoside 6 in good yield. Then, α-selective glycosylation
Copyright © 2001 The Chemical Society of Japan

Chemistry Letters 2001
841
previously reported one-pot glycosylation procedure in order to
optimize the reaction conditions (Table 1).
Thus, a convergent total synthesis of F1α antigen was
accomplished by one-pot sequential glycosylation and whose
method proved to be applicable for the rapid assembly of vari-
ous types of complex oligosaccharides. Also, it should be
noted that the control of the stereoselectivity of glycosylation is
influenced by the kind of the counter anion of the catalyst.
The TrB(C6F5)4-catalyzed glycosylation of 4 with phenyl-
carbonate donor 2 proceeded more smoothly in trifluoro-
methylbenzene (BTF) compared with that in CH2Cl2 and the
desired disaccharide 11 was obtained in good yield (Table 1,
Entries 1–3).4 After optimization, the best condition was deter-
mined as shown in Entry 4 (84%, in BTF, MS 5A, at –15 °C,
1.8 equivalent of donor). The glycosylation of 4 with glycosyl
fluoride 3 was further examined by using 20 mol% of TfOH
and 11 was obtained in excellent yield (94%).5,14
Then, two types of one-pot sequential glycosylation were
attempted as shown in Scheme 2. In the first step, glycosyl
phenylcarbonate 2 or fluoride 3 was treated with thioglycoside
4 in the presence of a catalyst such as TrB(C6F5)4 or TfOH, and
4 was almost completely consumed within 5 h or 1 h, respec-
tively, which was confirmed by TLC monitoring. Next, the
second glycosylation of glycosyl amino acid 5 with thus formed
disaccharide was tried by successive addition of NIS in one-pot
operation, and fully protected F1α 12 was stereoselectively
obtained in high yield (80 or 89%, respectively).
The present research is partially supported by Grant-in-Aids
for Scientific Research from Ministry of Education, Culture,
Sports, Science and Technology. The authors wish to thank Mr.
Hirokazu Ohsawa, Dr. Shigeru Nakajima, and Ms. Aya Mitsuya,
Banyu Pharmaceutical Company, for their kind help concerning
mass-spectrometry, NMR, and elementary analysis.
References and Notes
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Transformation of 12 into F1α is demonstrated as in
Scheme 3. In the first place, azido group of 12 was reduced
with thioacetic acid to give 13 in 85% yield. Successive de-
protection of DCPhth group of 13 smoothly proceeded to afford
the desired diacetamido glycosyl amino acid 14 in high yield
after acetylation (2 steps, 82%) only when hydrazine acetate15
was used in ethanol at 70 °C. On the other hand, the deprotection
under standard conditions (hydrazine hydrate, ethylenediamine,
and NaBH4-reduction followed by AcOH) gave complicated
mixtures. Then, removal of benzyl and benzyloxycarbonyl
groups of compound 14 by hydrogenolysis, and careful saponi-
fication of p-MeBz group afforded the final product F1α (1) in
84% yield.16
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15 H. H. Lee, D. A. Schwartz, J. F. Harris, J. P. Carver, and J. J.
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16 Selected 1H NMR (500 MHz, D2O, δ = TMS); δ 4.26 (1H, d, J = 7.9
Hz, H-1"), 4.36 (1H, d, J = 7.6 Hz, H-1'), 4.67 (1H, d, J = 3.7, H-1);
Selected 13C NMR (125 MHz, D2O, δ = TMS); δ 97.7 (C-1), 101.1
(C-1'), 102.5 (C-1"); HRMS (m/z): [M + H]+ calcd for C25H44N3O18,
674.2626; found 674.2620). [α]20 = +54.0° (c 0.1, H2O); FT-IR
D
(KBr): 1643, 1072, 1049 cm–1; Anal. Calcd for C25H49N3O21 3H2O:
C, 41.26; H, 6.79; N, 5.77%. Found: C, 41.46; H, 7.17; N, 5.97%.