An efficient synthesis of base-substituted analogues of S-adenosyl-dl-homocysteine

Article abstract of DOI:10.1016/j.tetlet.2009.04.076

An efficient method for the preparation of base-substituted S-adenosyl-dl-homocysteine analogues as well as of 2-chloro-N⁶-alkylated S-adenosyl-dl-homocysteine analogues is described. The method uses a convergent strategy that employs a common

Full text of DOI:10.1016/j.tetlet.2009.04.076

Tetrahedron Letters 50 (2009) 3939–3941
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
An efficient synthesis of base-substituted analogues of
S-adenosyl-DL-homocysteine
*
David B. Llewellyn, Amal Wahhab
Department of Medicinal Chemistry, MethylGene Inc., 7220 Frederick-Banting, Montreal, Quebec, Canada H4S 2A1
a r t i c l e i n f o
a b s t r a c t
Article history:
An efficient method for the preparation of base-substituted S-adenosyl-DL-homocysteine analogues as
well as of 2-chloro-N6-alkylated S-adenosyl-DL-homocysteine analogues is described. The method uses
a convergent strategy that employs a common intermediate late in the overall synthesis and allows small
libraries of SAH analogues to be prepared in a relatively short period of time.
Received 14 April 2009
Revised 17 April 2009
Accepted 20 April 2009
Available online 24 April 2009
Ó 2009 Elsevier Ltd. All rights reserved.
S-Adenosyl-
L
-homocysteine (L-SAH) and its analogues are a po-
sioned synthesizing an intermediate similar to b-D-ribofuranose
tent class of inhibitors for a number of biologically important en-
1,2,3,5-tetraacetate (TAR). TAR is a common starting material for
the syntheses of ribose-based nucleosides and can be glycosidated
under mild conditions with a high degree of stereocontrol.6 The
intermediate developed in this study, 1, possesses the same 10, 20
and 30 acetate groups as TAR as well as a protected homocysteine
unit (Fig. 1). As a result, SAH analogues can be prepared from 1
in only two synthetic steps; glycosidation, using standard proce-
dures developed for the glycosidation of TAR, followed by a single
deprotection under basic conditions.
zymes including S-adenosyl-L
-homocysteine hydrolase1 and S-
adenosylmethionine-dependant methyltransferases.2 However, in
spite of their biological importance there are relatively few distinct
synthetic pathways to SAH analogues which are both convenient
and reliable.3,4
Currently, SAH analogues are synthesized using one of two gen-
eral methods. In the most common approach, the 50 position of a
nucleoside is activated as the corresponding 50-chloro derivative
and then displaced with homocysteine, or a homocysteine ana-
logue1c,3 (Scheme 1, path A). Alternatively, the 50 position of a
nucleoside can be activated in situ and coupled to a protected
homocysteine unit which is then deprotected (Scheme 1, path
B).4 Both of these approaches are convenient and provide the de-
sired SAH analogue in two steps so long as the parent nucleoside
is commercially available. If, however, the nucleoside is not com-
mercially available, these two approaches become inefficient. This
is because the entire reaction sequence, starting from the synthesis
of the nucleoside, must be performed from beginning to end for
each different SAH analogue that is desired.5 In fact, prior to the
study presented here there were no reported synthetic pathways
to SAH analogues which allowed the addition of the base substitu-
ent at the end of the reaction sequence rather than at the begin-
ning. This is surprising since such an approach would enable
base-modified SAH analogues to be synthesized from a common
intermediate, significantly reducing the amount of time required
to prepare large numbers of these compounds.
O
R
O
R
HO
SOCl2
Path A
Cl
HO
S
OH
HO
OH
O
F3C NH
MeO2C
-
CO2
Path B
-S
NH2
2
n-Bu3P
HO2C
H2N
O
O
MeO2C
HN
F3C
Ba(OH)2
R
R
S
S
HO
OH
HO
OH
O
Scheme 1. Current methods used for synthesis of SAH analogues.
In order to develop a convenient synthesis for SAH analogues
which would allow the base substituent to be added at the end
of the reaction sequence rather than at the beginning, we envi-
CO2Me
O
O
OAc
OAc
OAc
AcO
S
HN
Fmoc
AcO
OAc
AcO
β−D-ribofuranose 1,2,3,5-tetraacetate
1
* Corresponding author.
E-mail address: wahhaba@methylgene.com (A. Wahhab).
Figure 1. Comparison of TAR and intermediate 1.
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.04.076

3940
D. B. Llewellyn, A. Wahhab / Tetrahedron Letters 50 (2009) 3939–3941
when 4 is treated with TFA/H2O.7 Treatment of 5 with a 50/50 mix-
ture of TFA/H2O, followed by precipitation with water and acetyla-
tion with acetyl chloride provided 1 in 77% yield as a mixture of
anomers (1.7:1of b/a).
O
O
MeO2C
O
O
O
TsO
NaOMe
MeOH
S
H
N
+
HN
Boc
S
Boc
O
O
O
O
2
3
4
As representative examples of how 1 can be used to synthesize
a range of SAH analogues, 1 was coupled to a variety of adenine
substituents using TMSOTf and SnCl4 as catalysts in CH3CN
(Scheme 3). As shown in Table 1, the addition of benzimidazole,
8-azaadenine and phenol to 1 (entries 1, 2 and 4, respectively) oc-
curred with similar regio- and stereochemistry as the addition of
these bases to TAR under the same reaction conditions.8 However,
in our hands the addition of 7-nitroimidazo[4,5-b]pyridine to 1, in
the presence of SnCl4, did not result in any of the desired product.
This is despite a report that 7-nitroimidazo[4,5-b]pyridine can cou-
ple with TAR using similar conditions.9 Nevertheless, the reaction
between 7-nitroimidazo[4,5-b]pyridine and 1 proceeded smoothly
using TMSOTf as a catalyst and gave 6c in 53% yield (Table 1, entry
3). Reduction of the nitro group in 6c with Fe/HCl, followed by
deprotection with KOH, provided S-(1-deazaadenosyl)-DL-homo-
cysteine (7c) in 34% overall yield from 1. Compared to the synthe-
1) TMSOTf
2)Fmoc-Cl,
NaHCO3
O
O
MeO2C
HN
MeO2C
HN
OAc
OAc
O
S
S
1) TFA/H2O
AcO
O
O
2) CH3COCl
pyridine
Fmoc
Fmoc
1
5
Scheme 2. Synthesis of intermediate 1.
The synthesis of 1 was accomplished in three steps from com-
mercially available methyl 2,3-isopropylidene-5- -p-tolylsulfo-
nyl-b- -ribofuranoside (2) and racemic N-Boc-DL-homocysteine
O
D
thiolactone (3) in 55% overall yield (Scheme 2). In the first step,
the tosylate group of 2 was displaced with the sodium salt of N-
Boc-DL-homocysteine methyl ester, generated in situ by the addi-
tion of sodium methoxide to 3, affording 4 in 83% yield. The BOC
protecting group of 4 was then selectively cleaved using TMSOTF
and the resulting free nitrogen was reprotected with the acid-sta-
ble Fmoc protecting group (5, 94% yield). The introduction of the
Fmoc group allows all of the protecting groups in the final products
to be removed under basic conditions. In addition, the replacement
of the acid-labile Boc group in 4 with the acid-stable Fmoc group
prevents decomposition of the intermediate triol that is generated
sis of S-(1-deazaadenosyl)-L-homocysteine from TAR, which
required five steps with a reported overall yield of 20%,9,10 the syn-
thesis of S-(1-deazaadenosyl)-DL-homocysteine (7c) from 1 occurs
in less synthetic steps and also gives higher yields.
To further demonstrate the utility of 1 as an intermediate for
the syntheses of SAH analogues, a number of N6-alkylated ana-
logues were also prepared (Scheme 4). The coupling of 2,6-dichlo-
ropurine to 1, using TMSOTf as a catalyst, provided 8 in good
overall yield (81%).11c Treatment of 8 with a variety of primary or
secondary amines in 1,2-dichloroethane resulted in the in situ dis-
placement of the chloride at the 6-position of the purine base.11a–c
Evaporation of the solvent and deprotection of the aminated inter-
mediate provided the N6-alkylated SAH analogues 9a–e in 54–72%
yield (Table 2).
CO2Me
CO2H
glycosidation
KOH
O
O
R1
THF/H2O
R2
The ability to synthesize N6-alkylated SAH analogues from 8 in
only two steps, one of which is performed in situ, is a significant
improvement over published procedures that have been used to
prepare similar analogues11d and substantially reduces the amount
of time required to prepare libraries of these compounds. For
1
S
S
H2N
HN
Fmoc
SnCl4
or
HO
OH
7a-d
AcO
OAc
6a-d
TMSOTf
Scheme 3. Synthesis of 6a–d and 7a–d.
Table 1
Yields and conditions for the synthesis of 6a–d and 7a–d
Entry
R1
R2
Catalyst
TMSOTf
Yield 6a–d (%)
Yield 7a–d (%)
N
N
N
N
1
76
73
6a
7a
NH
NH2
N
2
N
N
N
2
N
SnCl4
62a
79
N
N
N
N
N
7b
6b
NO2
NH2
N
N
N
N
3
4
TMSOTf
SnCl4
53
65b
97
N
N
6c
7c
O
O
38c
7d
6d
Only isolated yields are reported.
a
Yield of N7-isomer only.
The nitro group in 6c was reduced before deprotection.
Yield for the b isomer only.
b
c

D. B. Llewellyn, A. Wahhab / Tetrahedron Letters 50 (2009) 3939–3941
3941
Cl
N
Considering the divergent nature of the synthesis and the ease
with which libraries of SAH analogues can be generated, the con-
cepts presented here should find wide spread use in both academic
research and drug development.
CO2Me
N
O
1) BSA
N
N
Cl
N
1
+
S
HN
2) TMSOTf
N
N
H
N
Cl
Fmoc
AcO
OAc
8
, 81%
Cl
Acknowledgements
1) NHR3R
2) KOH
4
CO2H
N
O
N
NR3R4
The authors are grateful to Dr. Daniel Delorme and Dr. Ronny
Preifer for their useful discussions.
S
H2N
N
N
HO OH
Cl
9a-e
Supplementary data
Scheme 4. Synthesis of 9a–e.
Supplementary data (experimental procedures and spectro-
scopic characterizations of compounds 1–5, 6a–d, 7a–d and 9a–
e) associated with this paper can be found, in the online version,
at doi:10.1016/j.tetlet.2009.04.076.
Table 2
Yields for the synthesis of 9a–ea
Entry
1
Product
HNR3R4
Yield (%)
58
References and notes
N
H
9a
1. (a) Montgomery, J. A.; Clayton, S. J.; Thomas, J. H.; Shannon, W. M.; Arnette, G.;
Bodner, A. J.; Kion, I.; Cantoni, G. L.; Chiang, P. K. J. Med. Chem. 1982, 25, 626–
629; (b) Ueland, P. M. Pharmacol. Rev. 1982, 34, 223–253; (c) Miles, R. W.;
Nielsen, L. P. C.; Ewing, G. J.; Yin, D.; Borchardt, R. T.; Robins, M. J. J. Org. Chem.
2002, 67, 8258–8260; (d) Steere, J. A.; Sampson, P. B.; Honek, J. F. Bioorg. Med.
Chem. Lett. 2002, 12, 457–460.
2. Several representative examples include: (a) Borchardt, R. T.; Wu, Y. S. J. Med.
Chem. 1974, 17, 863–868; (b) Borchardt, R. T.; Huber, J. A.; Wu, Y. S. J. Med.
Chem. 1974, 17, 868–873; (c) Coward, J. K.; Bussolotti, D. L.; Chang, C. J. Med.
Chem. 1974, 17, 1286–1289; (d) Borchardt, R. T. Biochem. Pharmacol. 1975, 24,
1542–1544; (e) Borchardt, R. T.; Wu, Y. S. J. Med. Chem. 1975, 18, 300–304; (f)
Borchardt, R. T.; Huber, J. A.; Wu, Y. S. J. Med. Chem. 1976, 19, 1094–1099; (g)
Borchardt, R. T.; Wu, Y. S.; Wu, B. S. J. Med. Chem. 1978, 21, 1099–1307; (h)
Houston, D. M.; Matuszewaka, B.; Borchardt, R. T. J. Med. Chem. 1985, 28, 478–
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2276–2285.
H2N
N
2
3
4
9b
9c
54
63
N
NH
H2N
HN
9d
9e
69
71
O
5
H2N
a
Only isolated yields are reported.
3. (a) Kikugawa, K.; Ichino, M. Tetrahedron Lett. 1971, 2, 87–90; (b) Borchardt, R.
T.; Huber, J. A.; Wu, Y. S. J. Org. Chem. 1976, 41, 565–567; (c) Ramalingam, K.;
Woodard, R. W. J. Org. Chem. 1984, 49, 1291–1293; (d) Robins, M. J.; Hansske,
F.; Wnuk, S. F.; Kanai, T. Can. J. Chem. 1991, 69, 1468–1474.
4. (a) Serafinowski, P. Synthesis 1985, 926–928; (b) Serafinowski, P.; Dorland, E.;
Harrap, R. J. Med. Chem. 1992, 35, 4576–4583.
N
N
O
N
N
O
Cl
N
N
1) Me2NH
2) NH3/MeOH
HO
BzO
5. For several examples of nucleotide syntheses see: (a) Antonini, I.; Cristalli, G.;
Franchetti, P.; Grifantini, M.; Martelli, S.; Petrelli, F. J. Pharm. Sci. 1984, 73, 366–
369; (b) Sági, G.; Szu˜cs, K.; Vereb, G.; Ötvös, L. J. Med. Chem. 1992, 35, 4549–
4556; (c) Devlin, T. A.; Jebaratnam, D. J. Synth. Commun. 1995, 255, 711–718;
(d) Bookser, B. C.; Raffaele, N. B. J. Org. Chem. 2007, 72, 173–179.
6. Vorbrüggen, H.; Ruh-Pohlenz, C. In Handbook of Nucleoside Synthesis; John
Wiley and Sons: New York, 2001.
7. LRMS analysis of 4, after treatment with TFA/H2O, indicated that the expected
triol was formed (ES-MS calcd (M+H)+ m/z 282, found 282). However, attempts
to isolate the product were unsuccessful. Neutralization of the reaction mixture
with sodium bicarbonate and re-analysis by LRMS indicated that the product
underwent a dehydration (ES-MS calcd (MꢀH2O+H)+ m/z 264, found 264). One
possible explanation for this loss of water is imine formation between the un-
protected homocysteine nitrogen and the aldehyde of the open-chain ribose
unit.
N
N
N
HO
OH
BzO
OBz
11
Cl
10
Cl
1) SOCl2, Pyr.
2) NH4OH
N
N
HO2C
H2N
O
N
N
O
N
N
NaOH
S
Cl
N
N
N
HO
OH
N
DL-homocysteine
HO
OH
Cl
Cl
9a
12
Scheme 5. Alternative synthesis of 9a from its parent nucleoside.
8. (a) Seela, F.; Münster, I.; Löchner, U.; Rosemeyer, H. Helv. Chim. Acta 1998, 81,
1139–1155; (b) Boryski, J.; Grynkiewicz, G. Synthesis 2001, 14, 2170–2174; (c)
Parsch, J.; Engels, J. W. J. Am. Chem. Soc. 2002, 124, 5664–5672.
9. Cristalli, G.; Franchetti, P.; Grifantini, M.; Vittori, S.; Bordoni, T.; Geroni, C. J.
Med. Chem. 1987, 30, 1686–1688.
10. Itoh, T.; Sugawara, T.; Mizuno, Y. Nucleosides Nucleotides 1982, 12, 179–190.
11. For examples of the use of N6-alkylated adenosine and SAH analogues see: (a)
Thompson, R. D.; Secunda, S.; Daly, J. W.; Olsson, R. A. J. Med. Chem. 1991, 34,
3388–3390; (b) Keeling, S. E.; Albinson, D. F.; Ayres, B. E.; Butchers, P. R.;
Chambers, C. L.; Cherry, P. C.; Ellis, F.; Ewan, G. B.; Gregson, M.; Knight, J.; Mills,
K.; Ravencroft, P.; Reynolds, L. H.; Sanjar, S.; Sheenhan, M. J. Bioorg. Med. Chem.
Lett. 2000, 10, 403–406; (c) van Tilburg, E. W.; van der Klein, P. A. M.; von
Frijtag Drabbe Künzel, J.; de Groote, M.; Stannek, C.; Lorenzen, A.; Ijzerman, A.
P. J. Med. Chem. 2001, 44, 2966–2975; (d) Lin, Q.; Jiang, F.; Schultz, P. G.; Gray,
N. J. Am. Chem. Soc. 2001, 123, 11608–11613.
example, the preparation of 9a from 10,12 via intermediates 11 and
12 (Scheme 5) required four steps and occurred with an overall
yield of 11%, which is a significantly longer and lower yielding
route than that used for the synthesis of 9a from intermediate 1
(Table 2, entry 1).
In conclusion, a new method to synthesize base-substituted
analogues of SAH is described. By using 1 as a starting point, SAH
analogues can be prepared in as few as two synthetic steps rather
than the usual four steps that would be required to prepare the
same analogues from TAR. In addition, the synthesis of SAH
analogues from 1 offers milder reaction conditions and generally
gives higher overall yields than previously developed procedures.
12. Hocek, M.; Holy, A.; Dvorakova, H. Collect. Czech. Chem. Commun. 2002, 67, 325–
335.