Synthesis of enantiomerically pure (purin-6-yl)phenylalanines and their nucleosides, a novel type of purine-amino acid conjugates

Article abstract of DOI:10.1021/jo051110x

Enantiomerically or diastereomerically pure 4-(purin-6-yl)phenylalanines, a novel type of stable amino acid-purine conjugates, were synthesized by palladium-catalyzed cross-coupling reactions of protected 4-boronophenylalanines or 4-(trimethylstanyl)pheny

Full text of DOI:10.1021/jo051110x

Synthesis of Enantiomerically Pure (Purin-6-yl)phenylalanines
and Their Nucleosides, a Novel Type of Purine-Amino Acid
Conjugates
Petr Cˇ apek, Radek Pohl, and Michal Hocek*
Centre for New Antivirals and Antineoplastics, Institute of Organic Chemistry and Biochemistry,
Academy of Sciences of the Czech Republic, CZ-16610, Prague 6, Czech Republic
hocek@uochb.cas.cz
Received June 2, 2005
Enantiomerically or diastereomerically pure 4-(purin-6-yl)phenylalanines, a novel type of stable
amino acid-purine conjugates, were synthesized by palladium-catalyzed cross-coupling reactions
of protected 4-boronophenylalanines or 4-(trimethylstanyl)phenylalanines with diverse 6-halopurines
(9-benzyl-6-halopurines and 9-(tetrahydropyran-2-yl)-6-halopurines as well as acyl- and silyl-
protected 6-halopurine ribonucleosides and 2-deoxyribonucleosides). Free purine bases and nucleo-
sides bearing (S)- or (R)-phenylalanine in position 6 were obtained after complete deprotection of
the products of cross-coupling reactions. Reactivity trends for both of these cross-coupling and
deprotection protocols have been compared in terms of practicability, efficiency, and stereoselectivity.
Purine bases and nucleosides bearing C-substituents
in position 6 are known to possess cytostatic1 or antimi-
crobial2 activity, and some of them were used as artificial
nucleobases in the extension of the genetic alphabet.3
Cross-coupling reactions are a powerful tool4 for the
synthesis of these compounds but, in most cases, were
used only for an introduction of simple unfunctionalized
carbon substituents. On the other hand, purines bearing
C-substituents containing functional groups (OH, NH2,
COOH, etc.) would provide new exploitable moieties for
interactions with complementary nucleobases, enzymes,
or receptors while still preserving higher stability toward
enzymatic degradation due to the presence of a C-C bond
in position 6. However, because of limited synthetic
accessibility, such compounds are scarcely reported in the
literature. Therefore, an extension of the cross-coupling
methodology to functionalized C-substituents represents
a useful and challenging goal. Very recently we have
reported the synthesis of 6-(hydroxymethyl)purines5 that
exert cytostatic activity and inhibit adenosine deaminase.
Another important subclass of these modified purines is
carbon-carbon-linked conjugates of purines and amino
acids. Such compounds, as hybrids between nucleobases/
nucleosides and amino acids, could interact with a
number of receptors or enzymes or could serve as building
blocks for construction of stable conjugates of nucleic
acids and peptides/proteins.6
* To whom correspondence should be addressed. Phone: +420
220183324. Fax: +420 220183559.
(1) (a) Hocek, M.; Holy´, A.; Votruba, I.; Dvorˇa´kova´, H. J. Med. Chem.
2000, 43, 1817-1825. (b) Hocek, M.; Holy´, A.; Votruba, I.; Dvorˇa´kova´,
H. Collect. Czech. Chem. Commun. 2001, 66, 483-499.
(2) (a) Bakkestuen, A. K.; Gundersen, L.-L.; Langli, G.; Liu, F.;
Nolsøe, J. M. J. Bioorg. Med. Chem. Lett. 2000, 10, 1207-1210. (b)
Andresen, G.; Gundersen, L.-L.; Nissen-Meyer, J.; Rise, F.; Spilsberg,
B. Bioorg. Med. Chem. Lett. 2002, 12, 567-569. (c) Gundersen, L.-L.;
Nissen-Meyer, J.; Spilsberg, D. J. Med. Chem. 2002, 45, 1383-1386.
(3) Hirao, I.; Ohtsuki, T.; Fujiwara, T.; Mitsui, T.; Yokogawa, T.;
Okuni, T.; Nakayama, H.; Takio, K.; Yabuki, T.; Kigawa, T.; Kodama,
K.; Yokogawa, T.; Nishikawa, K.; Yokoyama, S. Nat. Biotechnol. 2002,
20, 177-182.
(5) Sˇilha´r, P.; Pohl, R.; Votruba, I.; Hocek, M. Org. Lett. 2004, 6,
3225-3228.
(6) Recent reviews: (a) Da Ros, T.; Spalluto, G.; Prato, M.; Saison-
Behmoaras, T.; Boutorine, A.; Cacciari, B. Curr. Med. Chem. 2005, 12,
71-88. (b) Pierce, T. L.; White, A. R.; Treager, G. W.; Sexton, P. M.
Mini-Rev. Med. Chem. 2005, 5, 41-55.
(4) For reviews, see: (a) Hocek, M. Eur. J. Org. Chem. 2003, 245-
254. (b) Agrofoglio, L. A.; Gillaizeau, I.; Saito, Y. Chem. Rev. 2003,
103, 1875-1916.
10.1021/jo051110x CCC: $30.25 © 2005 American Chemical Society
Published on Web 09/03/2005
J. Org. Chem. 2005, 70, 8001-8008
8001

Cˇ apek et al.
SCHEME 1. General Synthesis of
(Purin-6-yl)phenylalanines
The first type of such conjugates were protected (purin-
6-yl)glycines7 prepared by Pd-catalyzed R-arylation of
ethyl N-(diphenylmethylidene)glycinate by 6-iodopurines.
Unfortunately, as a result of limited stability of the
system, we were unable to cleave protective groups. We
have succeeded with another type of conjugates, i.e.,
(purin-6-yl)alanines.8 They were prepared by Pd(0)-
catalyzed cross-coupling reactions of protected iodozin-
calanines with 6-iodopurines followed by a deprotection
sequence. This approach allowed us a simple single-step
introduction of a chiral alanine moiety to the target
molecule. Enantiomericaly pure (purine-6-yl)alanines
were obtained when optically pure amino acid building
blocks were used. This paper reports on the synthesis of
4-(purin-6-yl)phenylalanines, the third example of this
type of compounds, combining the structural features of
cytostatic 6-arylpurines1 and amino acids and having a
longer and bulkier spacer between the amino acid and
purine moieties.
Results and Discussion
Our approach toward the synthesis of 4-(purin-6-yl)-
phenylalanines is based on palladium-catalyzed cross-
coupling reactions of 6-halopurines (1, 2) with organobo-
ron reagent9,10 (3) or organostannane11 (4) derived from
phenylalanine (Scheme 1). In general, organoboron1,12-14
or organotin15 compounds are established reagents for the
introduction of aryl substituents into position 6 of the
purine scaffold by cross-coupling reactions. Stannanes are
practical because of their good accessibility and solubility
in organic solvents and tolerance to most functional
groups, while their major drawbacks are toxicity and
complicated separation of the products. On the other
hand, boronic acids, as well as byproducts of the cross-
coupling reactions, are nontoxic, but in some cases there
might be problems with stability of functional and/or
protecting groups in the basic polar reaction media.
Therefore we have decided to explore both of these
approaches and compare them in terms of practicability,
efficiency, and stereoselectivity.
The starting 4-boronophenylalanine9 3 is known in (S)-
enantiomeric form, and (R)-3 was prepared analogously.
New organostannane 4 was prepared in (S)- and (R)-
forms by application of methodology reported for related
Boc/Me-protected (S)-4-(trimethylstanyl)phenylalanine.11
There are a few literature examples of use of 4-borono-
phenylalanines16,10d-g as well as the Boc/Me-protected
analogue11a of organostannane 4 in cross-coupling reac-
tions with simple aryl or hetaryl halides, but in most
cases the products were not deprotected and their enan-
tiopurity was not studied thoroughly. No example of
cross-coupling reactions of these Phe-derived organome-
tallics with highly functionalized biomolecular systems
of limited stability, i.e., nucleosides, was reported so far.
Apparently, suitable protecting groups both for amino
acid and purine or nucleoside moieties must be selected
and the reactions should be performed under mild
conditions. The protecting groups must be easily intro-
duced, survive under cross-coupling conditions, and be
cleavable under mild conditions. Therefore, the groups
of choice were benzyl carbamates for the amino group,
benzyl esters for carboxylic function, and acyl (acetyl or
4-toluoyl) or silyl (tert-butyldimethylsilyl, TBDMS) ethers
for sugar hydroxyl groups.
(7) Hocek, M. Heterocycles 2004, 63, 1673-1677.
(8) Cˇ apek, P.; Pohl, R.; Hocek, M. J. Org. Chem. 2004, 69, 7985-
7988.
(9) Nakanuta, H.; Fujiwara, M.; Yamamoto, Y. Bull. Chem. Soc. Jpn.
2000, 73, 231-235.
(10) For other related 4-(borono)phenylalanine derivatives, see: (a)
Nakamura, H.; Fujiwara, M.; Yamamoto, Y. J. Org. Chem. 1998, 63,
7529-7530. (b) Malan, C.; Morin, C. Synlett 1996, 167-168. (c) Malan,
C.; Morin, C. J. Org. Chem. 1998, 63, 8019-8020. (d) Firooznia, F.;
Gude, C.; Chan, K.; Marcopulos, N.; Satoh, Y. Tetrahedron Lett. 1999,
40, 213-216. (e) Firooznia, F.; Gude, C.; Chan, K.; Fink, C. A.; Qiao,
Y.; Satoh, Y.; Marcopoulos, N.; Savage, P.; Beil, M. E.; Bruseo, C. W.;
Trapani, A. J.; Jeng, A. Y. Bioorg. Med. Chem. Lett. 2001, 11, 375-
378. (f) Firooznia, F.; Gude, C.; Chan, K.; Satoh, Y. Tetrahedron Lett.
1998, 39, 3985-3988. (g) Satoh, Y.; Gude, C.; Chan, K.; Firooznia, F.
Tetrahedron Lett. 1997, 38, 7645-7648.
(11) Related organostannane protected with Boc/Me was reported:
(a) Morera, E.; Ortar, G. Synlett 1997, 12, 1403-1405. (b) Morera, E.;
Ortar, G. Bioorg. Med. Chem. Lett. 2000, 10, 1815-1818.
(12) (a) Havelkova´, M.; Hocek, M.; Cˇ esnek, M.; Dvorˇa´k, D. Synlett
1999, 1145-1147. (b) Hocek, M.; Holy´, A.; Votruba, I.; Dvorˇa´kova´, H.
Collect. Czech. Chem. Commun. 2000, 65, 1683-1697. (c) Havelkova´,
M.; Dvorˇa´k, D.; Hocek, M. Synthesis 2001, 1704-1710.
(13) (a) Lakshman, M. K.; Hilmer, J. H.; Martin, J. Q.; Keeler, J.
C.; Dinh, Y. Q. V.; Ngassa, F. N.; Russon, L. M. J. Am. Chem. Soc.
2001, 123, 7779-7787. (b) Lakshman, M. K.; Thomson, P. F.; Nuqui,
M. A.; Hilmer, J. H.; Sevova, N.; Boggess, B. Org. Lett. 2002, 4, 1479-
1482. (c) Gunda, P.; Russon, L. M.; Lakshman, M. K. Angew. Chem.,
Int. Ed. 2004, 43, 6372-6377.
Suzuki-Miyaura Cross-Coupling Reactions. The
Suzuki-Miyaura cross-coupling reactions of 4-(borono)-
phenylalanines 3 with 6-halopurines 1 and 2 were
investigated at first17 (Scheme 2, Table 1). Reaction of
organoboron 3 with model 6-iodopurine 1a was performed
in the presence of Pd(PPh3)4 and K2CO3 in toluene
(16) Gong, Y.; He, W. Org. Lett. 2002, 4, 3803.
(17) Although direct use of pinacol esters of boronic acids in Suzuki-
Miyaura cross-coupling reactions is well-known,10d-g reaction of pinacol
ester of 3 with 6-iodopurine 1a gave cross-coupling product in poor
yields in preliminary experiments (for details see Supporting Informa-
tion at http://pubs.acs.org). Therefore, our attention turned to more
reactive free boronic acids.
(14) (a) Liu, J.; Robins, M. J. Org. Lett. 2004, 6, 3421-3423. (b) Liu,
J.; Robins, M. J. Org. Lett. 2005, 7, 1149-1151.
(15) Gundersen, L. L.; Bakkestuen, A. K.; Aasen, A. J.; Overas, H.;
Rise, F. Tetrahedron 1994, 50, 9743-9756.
8002 J. Org. Chem., Vol. 70, No. 20, 2005

Synthesis of (Purin-6-yl)phenylalanines
TABLE 1. Cross-Coupling Reactions of 4-(Borono)phenylalanines with 6-Halopurines
entry
purine reagent
product
catalyst
additives
solvent
T [°C]
reaction time [h]
yield [%]a
1
2
3
4
5
1a
1a
2a
2b
2c
5a
5a
5a
5b
5c
Pd(PPh3)4
K2CO3
K3PO4
K3PO4
K3PO4
K3PO4
toluene
dioxane
dioxane
dioxane
dioxane
100
100
100
100
100
10
6
6.5
3
5
25
Pd(OAc)2 + 1.5 Lb
Pd(OAc)2 + 1.5 Lb
Pd(OAc)2 + 1.5 Lb
Pd(OAc)2 + 1.5 Lb
66
67
74 (S)d
71 (S)e
70 (R)e
70 (S)e
73 (R)e
25
6
1d
5d
Pd(OAc)2 + 1.5 Lb
K3PO4
CuTCc
dioxane
100
4
7
1a
5a
Pd(PPh3)4
THF
rt
8
a If no stereochemical descriptor is given, the reactions were performed with racemic organoboron reagent. b L ) 2-(dicyclohexylphos-
phino)biphenyl. c Copper(I) 2-thiophenecarboxylate. d 81% de. e Approximately 80% de.
SCHEME 2. Suzuki-Miyaura Cross-Coupling
Reactions
in the original mixture. In our case, protecting groups of
sample (S)-5b were cleaved by TFA and hydrogenolysis
to give free (S)-4-(9-H-purin-6-yl)phenylalanine. Deriva-
tization of this product by Marfey’s reagent and following
HPLC analysis showed that the diastereomerical purity
of sample (S)-5b was only 81% de. Going one more step
backward, we examined optical purity of starting boronic
acid (S)-3 by same methodology. The observed 93% ee
indicated that partial racemization took place already
during preparation of organoboron reagent (S)-3 accord-
ing to literature procedure.20 Optical purity of (S)-5b is
then the result of an accumulation of the undesired
enantiomer in the two steps.
Stille Cross-Coupling Reactions. The Stille cross-
coupling reactions of 4-(trimethylstanyl)phenylalanine 4
with 6-halopurines 1 and 2 (Scheme 3, Table 2) were
investigated in order to develop an alternative, racem-
ization-free route toward the protected (purin-6-yl)-
phenylalanines 5. Initial experiments with 6-iodopurine
1a and traditional Pd(PPh3)4 as catalyst in DMF or
toluene did not give any product even after 24 h of
heating (Table 2, entries 1 and 2). CuI as cocatalyst in
DMF (but not in toluene) resulted in formation of the
desired product 5a in low yield (entries 3 and 4). Inspired
by work of Morera on the reactivity of 4-(trimethylsta-
nyl)phenylalanines11 and Farina’s studies on positive
effects of AsPh3 ligand and polar solvents on Pd(0)-
catalyzed Stille reactions,21,22 we tried a catalytic system
consisting of Pd2dba3 (0.03 equiv) and AsPh3 (0.12 equiv)
in DMF. However, only traces of the desired product 5a
were isolated (9%, entry 5). Then the influence of CuI as
cocatalyst in the previous reaction was examined. In
contrast to Morera’s results,11 where it was reported that
there was virtually no influence of CuI on reactions of
organotin reagent 4, in our case the use of CuI (0.2 equiv)
and Pd2dba3 (0.03 equiv)/AsPh3 (0.12 equiv) in DMF
resulted in complete consumption of the starting iodopu-
rine 1a after 24 h, and the cross-coupling product 5a was
isolated in 42% yield (entry 6). To verify the influence of
the leaving group, the use of 6-chloropurine 2a was
examined under the same conditions. In this case (unlike
with boronic acids) a significant difference in reactivity
of 6-chloro- and 6-iodopurines was found. The conversion
(traditional conditions for the synthesis of 6-arylpu-
rines1,12) to give the product 5a in unsatisfactory yield of
25% (Table 1, entry 1). A more efficient catalytic system
based on Pd(OAc)2 and 2-(dicyclohexylphosphino)biphe-
nyl ligand with K3PO4 base in dioxane reported by
Lakshman13 as an alternative for the synthesis of 6-phe-
nylpurines was examined in our further experiment with
6-iodopurine 1a to afford a good isolated yield of product
5a (66%, entry 2). Similarly to previous findings in the
Suzuki-Miyaura reactions of 6-halopurines,1,12 the use
of more readily available 6-chloropurine 2a gave the same
good yield as in the case of the 6-iodo derivative 1a (entry
3). These conditions were found to be applicable to the
cross-coupling reactions of other 9-substituted 6-halopu-
rines (2b, 2c, 1d) affording the desired products 5b-d
in good yields (entries 4-6). Finally, we explored use of
a nonbasic palladium-catalyzed copper-mediated varia-
tion of the Suzuki-Miyaura coupling reaction.18 Reaction
of 6-iodopurine 1a with boronic acid 3 was carried out in
the presence of Pd(PPh3) catalyst and 2 equiv of CuTC
in DMF to furnish the product 5a in a yield of only 25%
(entry 7). These findings showed that the Lakshman’s
methodology13 was superior in terms of product yields
(entries 2-6).
The optical purity of product (S)-5b prepared under
Lakshman’s conditions (entry 4) was studied by an
indirect method based on use of Marfey’s derivatizating
reagent (1-fluoro-2,4-dinitrophenyl-5-L-alanine amide), a
standard method for determination of optical purity used
in peptide chemistry.19 Reaction of optically pure Marfey’s
reagent with mixture of (R)- and (S)-enantiomers of
amino acids leads to mixture of epimers. Baseline sepa-
ration of these epimers by HPLC on C18 column is then
used for determination of ratio of (R)- and (S)- amino acid
(20) Organoboron reagent 3 was prepared by palladium catalyzed
reaction of bis(pinacol)diboron and protected 4-iodophenylalanine (see
Jung, M. E.; Lazarova, T. I. J. Org. Chem. 1999, 64, 2979-2977)
followed by hydrolysis of pinacol ester intermediate.9
(21) Farina, V.; Krishnan, B. J. Am. Chem. Soc. 1991, 113, 9585-
9595.
(18) Savarin, C.; Liebeskind, L. S. Org. Lett. 2001, 3, 2149-2152.
(19) Szo´ka´n, G.; Mezo¨, G.; Hudec, F. J. Chromatogr. 1988, 444, 115-
122.
(22) Farina, V.; Krishnan, B.; Marshall, D. R.; Roth, G. P. J. Org.
Chem. 1993, 58, 5434-5444.
J. Org. Chem, Vol. 70, No. 20, 2005 8003

Cˇ apek et al.
TABLE 2. Cross-Coupling Reactions of 4-(Trimethylstannyl)phenylalanines with 6-Halopurines
entry
purine
product
catalyst
additives (equiv)
T [°C]
reaction time [h]
yield [%]a
1
1a
1a
1a
1a
1a
1a
2a
1a
1b
1a
1b
5a
5a
5a
5a
5a
5a
5a
5a
5b
5a
5b
Pd(PPh3)4
none
80
100
80
24
24
24
24
24
24
24
4
0
2b
3
Pd(PPh3)4
none
0
Pd(PPh3)4
Pd(PPh3)4
CuI (0.2)
CuI (0.2)
none
CuI (0.2)
CuI (0.2)
CuI (0.2) + CsF (2)
CuI (0.2) + CsF (2)
CuI (1.2)
CuI (1.2)
9
4b
5
100
80
0
Pd2dba3 + 4AsPh3
Pd2dba3 + 4AsPh3
Pd2dba3 + 4AsPh3
Pd2dba3 + 4AsPh3
Pd2dba3 + 4AsPh3
Pd2dba3 + 4AsPh3
Pd2dba3 + 4AsPh3
9
6
80
42
7
80
3
8
80
68
9
80
4.5
1
4
69 (S)c
70
10
11
80
70
72 (S)d
69 (R)d
72 (S)d
74 (S)d
63 (S)d
67 (R)d
71 (S)d
66 (R)d
12
13
14
1c
1d
1e
5c
5d
5e
Pd2dba3 + 4AsPh3
Pd2dba3 + 4AsPh3
Pd2dba3 + 4AsPh3
CuI (1.2)
CuI (1.2)
CuI (1.2)
70
70
65
2.5
2.5
1.75
15
1f
5f
Pd2dba3 + 4AsPh3
CuI (1.2)
65
1.75
a If no stereochemical descriptor is given, the reactions were performed with racemic organotin reagent. b Experiments were performed
in toluene. c 63% de. d >99.8% de.
SCHEME 3. Stille Cross-Coupling Reactions
then involved in the second Cu f Pd transmetalation
with palladium catalyst. Therefore, an experiment em-
ploying an excess of CuI (1.2 equiv, to ensure all orga-
nostannane could be transmetalated) and Pd2dba3/AsPh3
catalyst in DMF was performed. The results (entry 10)
showed a significant enhancement of reaction rate in
comparison to the experiment with CsF (entry 8). These
results employing CuI in DMF with Pd2dba3/AsPh3 as
catalyst (entries 6-10) are in agreement with Farina’s
Sn f Cu f Pd transmetalation hypothesis.25 Analo-
gously, another sample of (S)-5b was prepared by this
method (entry 11) and deprotected, and its enantiopurity
was determined by derivatization with Marfey’s reagent.
In this case, the enantiopurity of the final product was
>99.8 ee, and therefore this was finally the method of
choice for the synthesis of the enantiopure conjugates.
Thus a complete series of diastereomerically pure
derivatives 5b-f varying in substituent in position 9 and
in configuration at the amino acid moiety (two sets of
(R)- and (S)-epimers) was prepared by this method using
1.2 equiv of CuI and catalytic system based on Pd2dba3/
AsPh3 in DMF in yields of ca. 70% (entries 11-15).
Protecting Groups Cleavage. Having optimized
reaction conditions for synthesis of optically pure pro-
tected (purin-6-yl)phenylalanines 5, we turned our at-
tention to cleavage of the protecting groups. As men-
tioned before, the deprotection sequence should ideally
be carried out under mild conditions to prevent racem-
ization of the amino acid moiety and decomposition of
acid-labile nucleoside part of molecules 5c-f. The diverse
deprotection sequences are discussed below.
of 2a was very low and only traces of product formed after
24 h (entry 7).
Recently, a significant positive influence of CsF in
combination with CuI on Stille coupling reactions23 has
been reported. Therefore, we have explored the effect of
2 equiv of CsF24 and 0.2 equiv of CuI on our reaction in
DMF in the presence of Pd2dba3/AsPh3 and observed a
substantial increase in reaction rate and preparative
yield (68%, entry 8). Analogously, the reaction of 6-io-
dopurine 1b gave product 5b in similar yield (69%, entry
9). A sample of (S)-5b prepared by this method was
deprotected and the free (S)-4-(9H-purine-6-yl)phenyl-
alanine was tested by derivatization with Marfey’s
reagent as described above. The result showed that
excess of rather basic CsF caused partial epimerization
of the coupling product (63% de), and therefore this
method is not suitable for the synthesis of enantiopure
amino acids.
A mechanistic hypothesis on the influence of CsF
assumes a reversible transmetalation between the orga-
nostannane and CuI and the role of CsF is to shift the
equilibrium toward the formation of more reactive orga-
nocopper species.23 The same Sn f Cu transmetalation
in the presence of CuI and polar solvents was described
by Farina.25 The intermediate organocopper species is
4-(9H-Purin-6-yl)phenylalanines (S)-7 and (R)-7 were
prepared from (S)-5b and (R)-5b derivatives (Scheme 4).
At first, (S)-5b and (R)-5b were treated by 10% TFA in
DCM to give crystalline 9H-purines (S)-6 and (R)-6 in
almost quantitative yield. Hydrogenolytic cleavage of
benzyl ester and benzyl carbamate in DMF/H2O provided
final products (S)-7 and (R)-7 in very good overall yields
(Scheme 4).
Free purine nucleosides 10 and 11 could be prepared
both from acylated 5c and 5d and from silylated 5e and
5f nucleosides. In contrast to silyl ethers, cleaving of acyl
groups requires basic hydrolysis, which could be ac-
(23) Mee, S. P. H.; Lee, V.; Baldwin, J. E. Angew. Chem., Int. Ed.
2004, 43, 1132-1136.
(24) Other sources of fluoride anion (KF, TBAF) were ineffective in
this reaction.
(25) Farina, V.; Kapadia, S.; Krishnan, B.; Wang, C. J.; Liebeskind,
L. S. J. Org. Chem. 1994, 59, 5905-5911.
8004 J. Org. Chem., Vol. 70, No. 20, 2005

Synthesis of (Purin-6-yl)phenylalanines
SCHEME 4. Deprotection of 5b
SCHEME 6. Deprotection of Silylated Nucleosides
SCHEME 5. Deprotection of Acylated Nucleosides
(inhibition of cell growth of mouse leukemia L1210 cells
(ATCC CCL 219); human promyelocytic leukemia HL60
cells (ATCC CCL 240); human cervix carcinoma HeLaS3
cells (ATCC CCL 2.2) and human T lymphoblastoid
CCRF-CEM cell line (ATCC CCL 119)) and antiviral
activity (HCV subgenomic replicon assay27). None of the
tested compounds showed any considerable effect in these
assays.
Conclusions
A new practical method for synthesis of enantiomeri-
cally pure (purin-6-yl)phenylalanines by palladium-
catalyzed, copper-mediated Stille cross-coupling reactions
of protected 4-(trimethylstanyl)phenylalanines and 6-io-
dopurines was developed. This method was used in the
synthesis of a series of (purin-6-yl)phenylalanines (bases
and nucleosides) varying in substitution in position 9 of
the purine scaffold and in absolute configuration on the
R-carbon of phenylalanine. For the first time, the limited
applicability of the alternative routes using a Suzuki-
Miyaura coupling reaction of protected 4-boronophenyl-
alanine and CsF-mediated Stille coupling of 4-(trimeth-
ylstanyl)phenylalanines was shown as a result of partial
epimerization of the amino acid moiety under the cross-
coupling conditions. Furthermore, a mild, racemization-
free method for cleavage of protecting groups was
developed and applied to give both free (S)- and (R)-
phenylalanines bearing a purine base or a purine nucleo-
side. These compounds may serve as building blocks for
the synthesis of novel nondegradable nucleic acid/peptide
conjugates.
companied by partial racemization/epimerization (vide
infra) of the amino acid moiety. LiOH was deemed the
weakest possible base causing no racemization, yet still
able to simultaneously hydrolyze all ester groups in
molecule.26 Treatment of compounds (S)-5c and (S)-5d
with aqueous LiOH (0.8 equiv for one ester group) in
THF/MeOH mixture at ambient temperature was per-
formed until all esters were cleaved (Scheme 5). Then,
carbamate groups of the intermediates (S)-8 and (S)-9
were cleaved by hydrogenolysis to give desired free
nucleosides (S)-10 and (S)-11. Diastereomeric purity of
nucleosides was 96% and 98% de, respectively (deter-
mined by derivatization with Marfey’s reagent), proving
that a minute epimerization took place during the basic
hydrolysis of esters. The diastereomeric purity was not
improved even after employing lower concentrations of
LiOH or lowering of reaction temperature to 0 °C.
Second alternative route toward nucleosides 10 and 11
started from the silyl-protected derivatives 5e and 5f.
Treatment of 5e and 5f in THF by Et3N‚3HF (1.5 equiv
for one silyl ether) provided the partly deprotected
derivatives 12 and 13 (Scheme 6). These products were
sufficiently lipophilic to allow purification by flash chro-
matography on silica gel column. Finally, simultaneous
hydrogenolysis of benzyl esters and benzyl carbamates
gave the free nucleosides (S)-10, (R)-10, (S)-11, and (R)-
11 in good yields. No undesired diastereomers were
observed in this case (>99.8 de), showing that silyl ethers
are the superior protecting groups for sugar moiety.
Biological Activity Screening. Title (purin-6-yl)-
phenylalanines 7, 10, and 11 were subjected to biological
activity screening, which comprised cytostatic effect1a
Experimental Section
Method A. Stille Cross-Coupling Reaction of 4-(Tri-
methylstanyl)phenylalanine 4 with 6-Iodopurines 1a-
f. DMF (15 mL) was added through a septum to an argon-
purged flask containing a 6-iodopurine (1.65 mmol), 4-(tri-
methylstanyl)phenylalanine 4 (1.03 g, 1.87 mmol), Pd2dba3 (92
mg, 0.1 mmol), AsPh3 (61 mg, 0.2 mmol), and CuI (380 mg,
2.0 mmol). The mixture was stirred at 65-80 °C for 1-4.5 h
until consumption of starting 6-halopurine was reached ac-
cording to TLC (for reaction temperature and time, see Table
(27) Stuyver, L. J.; Whitaker, T.; McBrayer, T. R.; Hernandez-
Santiago, B. I.; Lostia, S.; Tharnish, P. M.; Ramesh, M.; Chu, C. K.;
Jordan, R.; Shi, J. X.; Rachakonda, S.; Watanabe, K. A.; Otto, M. J.;
Schinazi, R. F. Antimicrob. Agents Chemother. 2003, 47, 244-254.
(26) Nicolaou, K. C.; Bella, M.; Chen, D. Y.-K.; Huang, X.; Ling, T.;
Snyder, S. A. Angew. Chem., Int. Ed. 2002, 41, 3495-3499.
J. Org. Chem, Vol. 70, No. 20, 2005 8005

Cˇ apek et al.
2). The reaction mixture was diluted by ethyl acetate (200 mL),
filtrated though a Celite pad, and washed with water (200 mL).
The organic phase was evaporated and the residue was
chromatographed on a silica gel column (ethyl acetate/hexane,
1:5 f 2:1) to give products 5a-f.
OBn); 136.2 (C-i-Ph-Cbz); 139.0 (C-p-phenylene); 142.4 (CH-
8); 152.0 (C-4); 152.7 (CH-2); 155.0 (C-6); 155.6 (CO-carbam-
ate); 169.4, 169.6 and 170.3 (CO-Ac); 171.1 (CO-ester). IR
(CHCl3): 3432, 3032, 3012, 1749, 1584, 1509, 1450, 1375, 1329,
1228, 1206, 1059, 645. [R]20D ) -18.3 (c ) 3.77, CHCl3). Anal.
Calcd for C40H39N5O11 (765.8): C 62.74, H 5.13, N 9.15.
Found: C 62.35, H 5.13, N 8.86. Method B. Prepared from
6-chloropurine 2c (413 mg, 1.0 mmol) and (S)-4-(borono)-
phenylalanine (S)-3 (520 mg, 1.2 mmol): yield 542 mg (71%)
of 5c.
Method B. Suzuki-Miyaura Cross-Coupling Reaction
of 4-Boronophenylalanine 3 with 6-Iodopurines 1a, 1d,
and 2a-c. Dioxane (6 mL) was added through a septum to
an argon-purged flask containing a 6-halopurine (1.0 mmol),
4-boronophenylalanine 3 (520 mg, 1.2 mmol), and K3PO4 (425
mg, 2 mmol). Additionally the solution of Pd(OAc)2 (23 mg,
0.1 mmol) and 2-(dicyclohexylphosphino)biphenyl (52 mg, 0.15
mmol) in dioxane (3 mL) was prepared under argon, and two
drops of water (20 µL) were added. The mixture was stirred
at 100 °C for 3-6 h until consumption of starting 6-halopurine
was reached according to TLC (for reaction time see Table 1).
The reaction mixture was filtrated though a Celite pad,
evaporated in vacuo, and chromatographed on a silica gel
column (ethyl acetate/hexane, 1:5 f 2:1) to give products 5a-
d.
Benzyl (S)-3-{4-[9-(Tetrahydropyran-2-yl)purin-6-yl]-
phenyl}-2-[(benzyloxycarbonyl)amino]propanoate ((S)-
5b). Method A. Prepared from 9-THP-6-iodopurine (1b) (544
mg, 1.65 mmol) and (S)-4-(trimethylstannyl)phenylalanine
(S)-4 (1.03 g, 1.87 mmol): yield 701 mg (72%) of (S)-5b. Product
was isolated as amorphous colorless solid. MS (FAB): 592 (62,
M + 1); 508 (100, M - THP + 2). HRMS (FAB): for C34H34N5O5
calculated 592.2560, found 592.2582. 1H NMR (400 MHz,
CDCl3): 1.64-1.89 (m, 3H, CH2-THP); 2.04-2.23 (m, 3H, CH2-
Benzyl (S)-3-{4-[9-(3,5-Di-O-p-toluoyl-2-deoxy-â-D-eryth-
ro-pentofuranosyl)purin-6-yl]phenyl}-2-[(benzyloxycar-
bonyl)amino]propanoate ((S)-5d). Method A. Prepared
from 6-iodopurine 1d (987 mg, 1.65 mmol) and (S)-4-(tri-
methylstannyl)phenylalanine (S)-4 (1.03 g, 1.87 mmol): yield
1.05 g (74%) of (S)-5d. Product was isolated as amorphous
colorless solid. MS (FAB): 860 (70, M + 1); 508 (68, M - ToldRf
+ 2); 424 (100). HRMS (FAB): for C50H46N5O9 calculated
1
860.3296, found 860.3302. H NMR (400 MHz, CDCl3): 2.36
and 2.45 (2 × s, 2 × 3H, CH3); 2.91 (ddd, 1H, Jgem ) 14.2, J2′b1′
) 5.8, J2′b3′ ) 2.2, H-2′b); 3.15-3.26 (m, CH2 and H-2′a); 4.66-
4.72 (m, 2H, H-4′ and H-5′b); 4.77 (m, 1H, CH); 4.80 (dd, 1H,
Jgem ) 13.1, J5′a,4′ ) 5.0, H-5′a); 5.10 (s, 2H, CH2-Cbz); 5.13
and 5.18 (2 × d, 2 × 1H, Jgem ) 12.0, CH2-Bn); 5.29 (d, 1H,
JNH,CH ) 8.1, NH); 5.86 (dt, 1H, J3′2′a ) 6.2, J3′4′ ) 2.2, J3′2′b
)
2.2, H-3′); 6.65 (dd, 1H, J1′2′a ) 8.2, J1′2′b ) 5.7, H-1′); 7.19 (m,
2H, H-m-Tol); 7.22 (m, 2H, H-m-phenylene); 7.27-7.38 (m,
12H, H-m-Tol and 2 × Ph); 7.89 and 7.99 (2 × m, 2 × 2H,
H-o-Tol); 8.29 (s, 1H, H-8); 8.62 (m, 2H, H-o-phenylene); 8.96
(s, 1H, H-2). 13C NMR (100.6 MHz, CDCl3): 21.6 and 21.8
(CH3); 37.9 (CH2-2′); 38.1 (CH2); 54.7 (CH); 64.0 (CH2-5′); 67.0
(CH2-Cbz); 67.4 (CH2-Bn); 75.1 (CH-3′); 83.2 (CH-4′); 85.0 (CH-
1′); 126.4 and 126.6 (C-i-Tol); 128.1, 128.2, 128.50, 128.57 and
128.63 (CH-Ph), 129.26 and 129.30 (CH-m-Tol); 129.6 (CH-o-
Tol); 129.7 (CH-m-phenylene); 129.8 (CH-o-Tol); 130.0 (CH-o-
phenylene); 131.6 (C-5); 134.4 (C-i-phenylene); 134.9 (C-i-Ph-
OBn); 136.2 (C-i-Ph-Cbz); 138.9 (C-p-phenylene); 142.3 (CH-
8); 144.2 and 144.6 (C-i-Tol); 151.9 (C-4); 152.4 (CH-2); 154.7
(C-6); 155.6 (CO-carbamate); 166.0 and 166.2 (CO-Tol); 171.1
(CO-ester). IR (CHCl3): 3432, 3032, 3012, 1721, 1612, 1548,
THP); 3.18 and 3.23 (2 × dd, 2 × 1H, Jgem ) 13.7, JCH ,CH
)
2
5.8, CH2); 3.82 (td, 1H, J ) 11.7, 2.5, bCH2O-THP); 4.21 (ddt,
1H, J ) 11.7, 4.4, 1.7, aCH2O-THP); 4.77 (dt, 1H, JCH,NH
)
8.3, JCH,CH ) 5.8, CH); 5.10 (s, 2H, CH2-Cbz); 5.13 and 5.17 (2
× d, 2 × 12H, Jgem ) 12.2, CH2-Bn); 5.31 (d, 1H, JNH,CH ) 8.3,
NH); 5.86 (dd, 1H, J ) 10.3, 2.8, CHO-THP); 7.22 (m, 2H, H-m-
phenylene); 7.26-7.37 (m, 10H, 2 × Ph); 8.34 (s, 1H, H-8); 8.67
(m, 2H, H-o-phenylene); 9.01 (s, 1H, H-2). 13C NMR (100.6
MHz, CDCl3): 22.8, 24.9 and 31.8 (CH2-THP); 38.1 (CH2); 54.7
(CH); 67.0 (CH2-Cbz); 67.4 (CH2-Bn); 68.9 (CH2O-THP); 82.0
(CHO-THP); 128.1, 128.1, 128.5, 128.5, 128.6 and 128.6 (CH-
Ph), 129.7 (CH-m-phenylene); 130.0 (CH-o-phenylene); 131.0
(C-5); 134.5 (C-i-phenylene); 135.0 (C-i-Ph-OBn); 136.2 (C-i-
Ph-Cbz); 138.8 (C-p-phenylene); 142.0 (CH-8); 151.7 (C-4);
152.4 (CH-2); 154.5 (C-6); 155.6 (CO-carbamate); 171.1 (CO-
ester). IR (CHCl3): 3432, 3092, 3069, 3034, 1722, 1584, 1561,
1509, 1455, 1344, 1329, 1086, 1059, 1046, 911, 698, 647.
Method B. Prepared from 9-THP-6-chloropurine (2b) (239 mg,
1.0 mmol) and (S)-4-(borono)phenylalanine (S)-3 (520 mg, 1.2
mmol): yield 440 mg (74%) of (S)-5b.
1509, 1450, 1387, 1269, 1179, 1103, 1021, 926, 698, 646. [R]20
D
) -62.4 (c ) 4.34, CHCl3). Anal. Calcd for C50H45N5O9
(859.9): C 69.84, H 5.27, N 8.14. Found: C 69.73, H 5.21, N
7.99. Method B. Prepared from 6-iodopurine 1d (598 mg, 1.0
mmol) and (S)-4-(borono)phenylalanine (S)-3 (520 mg, 1.2
mmol): yield 604 mg (70%) of 5d.
Benzyl (S)-3-{4-[9-(2,3,5-Tri(tert-butyldimethylsilyloxy)-
â-D-ribofuranosyl)purin-6-yl]phenyl}-2-[(benzyloxycar-
bonyl)amino]propanoate ((S)-5e). Method A. Prepared
from 6-iodopurine 1e (1.19 g, 1.65 mmol) and (S)-4-(trimeth-
ylstannyl)phenylalanine (S)-4 (1.03 g, 1.87 mmol): yield 1.02
g (63%) of (S)-5e. Product was isolated as amorphous colorless
solid. MS (FAB): 982 (95, M + 1); 508 (100, M - TBDMSO-Rf
+ 2). HRMS (FAB): for C52H76N5O8Si3 calculated 982.5002,
found 982.5005. 1H NMR (400 MHz, CDCl3): -0.25, -0.03,
0.12, 0.13, 0.16 and 0.17 (6 × s, 6 × 3H, CH3-Si); 0.79, 0.95
and 0.98 (3 × s, 3 × 9H, CH3-t-Bu); 3.19 (dd, 1H, Jgem ) 13.8,
JbCH ,CH ) 5.6, bCH2); 3.23 (dd, 1H, Jgem ) 13.8, JaCH ,CH ) 5.7,
Benzyl (S)-3-{4-[9-(2,3,5-Tri-O-acetyl-â-D-ribofurano-
syl)purin-6-yl]phenyl}-2-[(benzyloxycarbonyl)amino]pro-
panoate ((S)-5c). Method A. Prepared from 6-iodopurine 1c
(832 mg, 1.65 mmol) and (S)-4-(trimethylstannyl)phenylala-
nine (S)-4 (1.03 g, 1.87 mmol): yield 913 mg (72%) of (S)-5c.
Product was isolated as amorphous colorless solid. MS (FAB):
766 (100, M + 1); 508 (42, M - AcRf + 2). HRMS (FAB): for
C40H40N5O11 calculated 766.2724, found 766.2722. 1H NMR
(400 MHz, CDCl3): 2.10, 2.14 and 2.17 (3 × s, 3 × 3H, CH3);
2
2
aCH2); 3.83 (dd, 1H, Jgem ) 11.4, J5′b,4′ ) 2.9, H-5′b); 4.05 (dd,
1H, Jgem ) 11.4, J5′a,4′ ) 4.1, H-5′a); 4.17 (dt, 1H, J4′,5′ ) 4.1,
2.9, J4′,3′ ) 3.3, H-4′); 4.35 (dd, 1H, J3′,2′ ) 4.4, J3′,4′ ) 3.3, H-3′);
4.75 (dd, 1H, J2′,1′ ) 5.4, J2′,3′ ) 4.4, H-2′); 4.77 (dt, 1H, JCH,NH
3.18 and 3.24 (2 × dd, 2 × 1H, Jgem ) 13.9, JCH ,CH ) 5.9, CH2);
2
4.41 (dd, 1H, Jgem ) 12.8, J5′b,4′ ) 5.2, H-5′b); 4.45-4.52 (m,
2H, H-4′ and H-5′a); 4.77 (dt, 1H, JCH,NH ) 8.1, JCH,CH ) 5.9,
2
CH); 5.10 (s, 2H, CH2-Cbz); 5.13 and 5.18 (2 × d, 2 × 1H, Jgem
) 12.2, CH2-Bn); 5.30 (d, 1H, JNH,CH ) 8.1, NH); 5.72(dd, 1H,
J3′,2′ ) 5.6, J3′,4′ ) 4.5, H-3′); 6.02 (t, 1H, J2′,3′ ) 5.6, J2′,1′ ) 5.3,
H-2′); 6.30 (d, 1H, J1′2′ ) 5.3, H-1′); 7.23 (m, 2H, H-m-
phenylene); 7.26-7.38 (m, 10H, 2 × Ph); 8.28 (s, 1H, H-8); 8.65
(m, 2H, H-o-phenylene); 9.02 (s, 1H, H-2). 13C NMR (100.6
MHz, CDCl3): 20.4, 20.5 and 20.8 (CH3); 38.1 (CH2); 54.7 (CH);
63.0 (CH2-5′); 67.0 (CH2-Cbz); 67.4 (CH2-Bn); 70.6 (CH-3′); 73.1
(CH-2′); 80.4 (CH-4′); 86.4 (CH-1′); 128.1, 128.2, 128.5, 128.6
and 128.6 (CH-Ph), 129.7 (CH-m-phenylene); 130.0 (CH-o-
phenylene); 131.5 (C-5); 134.3 (C-i-phenylene); 134.9 (C-i-Ph-
) 8.2, JCH,CH ) 5.7, 5.6, CH); 5.11 (s, 2H, CH2-Cbz); 5.13 and
2
5.19 (2 × d, 2 × 1H, Jgem ) 12.1, CH2-Bn); 5.28 (d, 1H, JNH,CH
) 8.2, NH); 6.17 (d, 1H, J1′2′ ) 5.4, H-1′); 7.21 (m, 2H, H-m-
phenylene); 7.25-7.36 (m, 10H, 2 × Ph); 8.46 (s, 1H, H-8); 8.68
(m, 2H, H-o-phenylene); 8.99 (s, 1H, H-2). 13C NMR (100.6
MHz, CDCl3): -5.4, -5.3, -5.1, -4.7, -4.6 and -4.4 (CH3-
Si); 17.8, 18.1 and 18.5 (C-t-Bu); 25.6, 25.8 and 26.1 (CH3-t-
Bu); 38.1 (CH2); 54.7 (CH); 62.6 (CH2-5′); 67.0 (CH2-Cbz); 67.4
(CH2-Bn); 72.1 (CH-3′); 75.8 (CH-2′); 85.7 (CH-4′); 88.2 (CH-
1′); 128.1, 128.2, 128.50, 128.55, 128.60 and 128.62 (CH-Ph),
8006 J. Org. Chem., Vol. 70, No. 20, 2005

Synthesis of (Purin-6-yl)phenylalanines
129.68 (CH-m-phenylene); 130.0 (CH-o-phenylene); 131.5 (C-
5); 134.6 (C-i-phenylene); 135.0 (C-i-Ph-OBn); 136.1 (C-i-Ph-
Cbz); 138.7 (C-p-phenylene); 143.2 (CH-8); 152.26 (CH-2);
152.29 (C-4); 154.4 (C-6); 155.6 (CO-carbamate); 171.1 (CO-
ester). IR (CHCl3): 3432, 3035, 2956, 2931, 1721, 1581, 1508,
(10 wt %, 49 mg) for 10 h. The catalyst was filtered off on a
Celite pad and the filtrate was evaporated in vacuo. Crude
product was purified by preparative HPLC on C18 column with
water/methanol as mobile phase. Then, 2% aqueous HCl was
added to the aqueous solution of product to adjust pH to 4
and product was lyophilized from this solution to give 128 mg
(72%) of (S)-7 as white solid. MS (FAB): 284 (100, M + 1).
HRMS (FAB): for C14H14N5O2 calculated 284.1148, found
284.1155. 1H NMR (500 MHz, D2O, refdioxane) 3.75 ppm): 3.27
1390, 1344, 1328, 1257, 1113, 1066, 839, 698, 647. [R]20
-32.9 (c ) 2.10, CHCl3).
)
D
Benzyl (S)-3-{4-[9-(3,5-Di(tert-butyldimethylsilyloxy)-
2-deoxy-â-D-erythro-pentofuranosyl)purin-6-yl]phenyl}-
2-[(benzyloxycarbonyl)amino]propanoate ((S)-5f). Method
A. Prepared from 6-iodopurine 1f (975 mg, 1.65 mmol) and
(S)-4-(trimethylstannyl)phenylalanine (S)-4 (1.03 g, 1.87
mmol): yield 1.0 g (71%) of (S)-5f. Product was isolated as
amorphous colorless solid. MS (FAB): 852 (20, M + 1); 508
(100, M - TBDMSO-dRf +2). HRMS (FAB): for C46H62N5O7-
(dd, 1H, Jgem ) 14.5, JbCH ,CH ) 7.4, bCH2); 3.36 (dd, 1H, Jgem
2
) 14.5, JaCH ,CH ) 5.9, aCH2); 4.23 (dd, 1H, JCH,CH ) 7.4, 5.9,
2
2
CH); 7.44 (m, 2H, m-H-phenylene); 7.94 (m, 2H, o-H-phe-
nylene); 8.59 (s, 1H, H-8); 8.84 (s, 1H, H-2). 13C NMR (125.8
MHz, D2O, refdioxane) 67.19 ppm): 36.6 (CH2); 55.5 (CH); 127.7
(C-5); 130.4 and 130.7 (CH-phenylene); 131.4 (C-i-phenylene);
139.8 (C-p-phenylene); 148.2 (CH-8); 150.5 (CH-2); 152.2 (C-
6); 155.0 (C-4); 173.1 (CO). IR (KBr): 2912, 2866, 1739, 1616,
1595, 1565, 1516, 1387, 1320, 795, 636, 524. Anal. Calcd for
C14H15Cl2N5O2 (356.2): C 47.21, H 4.24, N 19.66. Found: C
1
Si2 calculated 852.4188, found 852.4226. H NMR (400 MHz,
CDCl3): 0.10 and 0.12 (2 × s, 12H, CH3-Si); 0.92 and 0.93 (2
× s, 2 × 9H, CH3-t-Bu); 2.50 (ddd, 1H, Jgem ) 13.1, J2′b,1′
)
6.1, J2′b,3′ ) 3.7, H-2′b); 2.72 (ddd, 1H, Jgem ) 13.1, J2′a,1′ ) 6.9,
47.32, H 4.61, N 16.98. [R]20 ) -6.9 (c ) 2.02, H2O).
J2′a,3′ ) 5.8, H-2′a); 3.18 (dd, 1H, Jgem ) 14.0, JbCH ,CH ) 6.2,
2
D
bCH2); 3.23 (dd, 1H, Jgem ) 14.0, JaCH ,CH ) 5.6, aCH2); 3.80
2
Benzyl (S)-3-{4-[9-(â-D-Ribofuranosyl)purin-6-yl]phe-
nyl}-2-[(benzyloxycarbonyl)amino]propanoate ((S)-12).
Et3N‚3HF (320 µL, 1.95 mmol) was added to a solution of (S)-
5e (380 mg, 0.39 mmol) in THF (2.5 mL), and the reaction
mixture was stirred at ambient temperature for 14 h. Solvents
were evaporated in vacuo and crude product was purified by
flash chromatography on silica gel column with EtOAc/MeOH
(95/5) eluent to give 243 mg (97%) of (S)-12 as colorless
amorphous solid. MS (FAB): 640 (20, M + 1); 508 (100, M -
Rf +2). HRMS (FAB): for C34H34N5O8 calculated 640.2407,
found 640.2389. 1H NMR (400 MHz, DMSO-d6): 3.03 (dd, 1H,
(dd, 1H, Jgem ) 11.2, J5′b,4′ ) 3.2, H-5′b); 3.90 (dd, 1H, Jgem
11.2, J5′a,4′ ) 4.2, H-5′a); 4.06 (dt, 1H, J4′,5′ ) 4.2, 3.2, J4′,3′
)
)
3.2, H-4′); 4.66 (dt, 1H, J3′,2′ ) 5.8, 3.7, J3′,4′ ) 3.2, H-3′); 4.77
(dt, 1H, JCH,NH ) 8.3, JCH,CH ) 6.2, 5.6, CH); 5.10 (s, 2H, CH2-
2
Cbz); 5.12 and 5.18 (2 × d, 2 × 1H, Jgem ) 12.2, CH2-Bn); 5.28
(d, 1H, JNH,CH ) 8.3, NH); 6.58 (d, 1H, J1′2′ ) 6.9, 6.1, H-1′);
7.21 (m, 2H, H-m-phenylene); 7.25-7.37 (m, 10H, 2 × Ph);
8.43 (s, 1H, H-8); 8.67 (m, 2H, H-o-phenylene); 8.99 (s, 1H,
H-2). 13C NMR (100.6 MHz, CDCl3): -5.5, -5.4, -4.8 and -4.7
(CH3-Si); 18.0 and 18.4 (C-t-Bu); 25.8 and 26.0 (CH3-t-Bu); 38.1
(CH2); 41.2 (CH2-2′); 54.7 (CH); 62.8 (CH2-5′); 67.0 (CH2-Cbz);
67.4 (CH2-Bn); 72.0 (CH-3′); 84.5 (CH-1′); 88.0 (CH-4′); 128.09,
128.15, 128.50, 128.55, 128.59 and 128.63 (CH-Ph), 129.7 (CH-
m-phenylene); 130.0 (CH-o-phenylene); 131.5 (C-5); 134.6 (C-
i-phenylene); 135.0 (C-i-Ph-OBn); 136.2 (C-i-Ph-Cbz); 138.7 (C-
p-phenylene); 142.7 (CH-8); 151.9 (C-4); 152.2 (CH-2); 154.4
(C-6); 155.6 (CO-carbamate); 171.1 (CO-ester). IR (CHCl3):
3432, 2957, 2931, 1722, 1583, 1509, 1390, 1344, 1328, 1258,
1186, 1114, 1065, 1028, 968, 838, 698, 647. [R]20D ) -2.9 (c )
2.39, CHCl3).
Jgem ) 13.8, JbCH ,CH ) 10.1, bCH2); 3.19 (dd, 1H, Jgem ) 13.8,
2
JaCH ,CH ) 5.2, aCH2); 3.61 (ddd, 1H, Jgem ) 12.0, J5′b,OH ) 5.9,
J5′b,42′ ) 4.0, H-5′b); 3.72 (dt, 1H, Jgem ) 12.0, J5′a,OH ) 5.3, J5′a,4′
) 4.0, H-5′a); 4.01 (q, 1H, J4′,5′ ) 4.0, J4′,3′ ) 4.0, H-4′); 4.22 (q,
1H, J3′,OH ) 5.0, J3′,2′ ) 4.7, J3′,4′ ) 4.0, H-3′); 4.44 (ddd, 1H,
JCH,CH ) 10.1, 5.2, JCH,NH ) 8.1, CH); 4.67 (q, 1H, J2′,OH ) 5.7,
J2′,1′ )25.6, J2′,3′ ) 4.7, H-2′); 4.97 and 5.01 (2 × d, 2 × 1H, Jgem
) 12.6, CH2-CBz); 5.13 (s, 2H, CH2-Bn); 5.15 (t, 1H, JOH,5′
)
5.9, 5.3, OH-5′); 5.28 (d, 1H, JOH,3′ ) 5.0, OH-3′); 5.59 (d, 1H,
JOH,2′ ) 5.7, OH-2′); 6.10 (d, 1H, J1′2′ ) 5.6, H-1′); 7.22-7.36
(m, 10H, 2 × Ph); 7.48 (m, 2H, H-m-phenylene); 7.97 (d, 1H,
JNH,CH ) 8.1, NH); 8.75 (m, 2H, H-o-phenylene); 8.93 (s, 1H,
H-8); 9.01 (s, 1H, H-2). 13C NMR (100.6 MHz, DMSO-d6): 36.6
(CH2); 55.6 (CH); 61.4 (CH2-5′); 65.6 (CH2-Bn); 66.3 (CH2-Cbz);
70.5 (CH-3′); 73.9 (CH-2′); 85.9 (CH-4′); 87.9 (CH-1′); 127.8,
127.95, 127.99, 128.2, 128.5 and 128.6 (CH-Ph), 129.5 (CH-
m-phenylene); 129.7 (CH-o-phenylene); 130.9 (C-5); 133.8 (C-
i-phenylene); 136.0 (C-i-Ph-Bn); 137.0 (C-i-Ph-Cbz); 141.0 (C-
p-phenylene); 145.0 (CH-8); 152.1 (CH-2); 152.4 (C-4); 153.1
(C-6); 156.2 (CO-carbamate); 171.8 (CO-ester). IR (KBr): 3325,
3034, 1723, 1699, 1586, 1559, 1535, 1515, 1454, 1331, 1286,
Benzyl (S)-3-{4-[9-H-Purin-6-yl]phenyl}-2-[(benzyloxy-
carbonyl)amino]propanoate ((S)-6). Compound (S)-5b (300
mg, 0.51 mmol) was dissolved in DCM (6 mL) and a solution
of TFA (1.5 mL) in DCM (4.8 mL) was slowly added. The
reaction mixture was stirred at ambient temperature for 1.25
h. The solution was diluted by EtOAc and washed by an
aqueous solution of NaHCO3. The organic part was evaporated
and crude product was recrystallizated from EtOAc to give
(S)-6 (253 mg, 98%) as white crystals, mp 168-171 °C. MS
(FAB): 508 (100, M + 1); 210 (48). HRMS (FAB): for
C29H26N5O4 calculated 508.1985, found 508.1987. 1H NMR (400
MHz, DMSO-d6): 3.02 (dd, 1H, Jgem ) 13.7, Jvic ) 9.9, bCH2);
3.18 (dd, 1H, Jgem ) 13.7, Jvic ) 5.4, aCH2); 4.43 (ddd, 1H,
JCHb,CH ) 9.9, JCH,NH ) 8.2, J Cha,CH2 ) 5.4, CH); 4.97 and 5.01
(2 × d,22 × 1H, Jgem ) 12.6, CH2-Cbz); 5.13 (s, 2H, CH2-Bn);
7.22-7.35 (m, 10H, 2 × Ph); 7.47 (bm, 2H, m-H-phenylene);
7.96 (d, 1H, JNH,CH ) 8.2, NH-Cbz); 8.64 (s, 1H, H-8); 8.75
(bm, 2H, o-H-phenylene); 8.94 (s, 1H, H-8); 13.62 (bs, 1H, NH-
9). 13C NMR (100.6 MHz, DMSO-d6): 36.6 (CH2); 55.6 (CH);
65.6 (CH2-Cbz); 66.3 (CH2-Bn); 127.7, 127.9, 128.0, 128.2,
128.46 and 128.53 (CH-Ph); 129.4 (CH-o-phenylene); 129.6
(CH-m-phenylene), 129.9 (C-5); 134.2 (C-i-phenylene); 135.9
(C-i-Ph-OBn); 137.0 (C-i-Ph-Cbz); 140.6 (C-p-phenylene); 144.8
(CH-8); 152.0 (CH-2); 152.3 (C-6); 153.6 (C-4); 156.2 (CO-
carbamate); 171.8 (CO-ester). IR (KBr): 3331, 3065, 2832,
1724, 1686, 1584, 1562, 1532, 1455, 1323, 1286, 1184, 1054,
1259, 1216, 1058, 800, 743, 697. [R]20 ) -43.7 (c ) 3.90,
D
DMSO).
Benzyl (S)-3-{4-[9-(2-Deoxy-â-D-erythro-pentofurano-
syl)purin-6-yl]phenyl}-2-[(benzyloxycarbonyl)amino]pro-
panoate ((S)-13). Et3N‚3HF (240 µL, 1.47 mmol) was added
to the solution of (S)-5f (360 mg, 0.42 mmol) in THF (2.5 mL),
and the reaction mixture was stirred at ambient temperature
for 14 h. Solvents were evaporated in vacuo and crude product
was purified by flash chromatography on silica gel column with
EtOAc/MeOH (95/5) eluent to give 238 mg (91%) of (S)-13 as
colorless amorphous solid. MS (FAB): 624 (15, M + 1); 508
(100, M - dRf +2). HRMS (FAB): for C34H34N5O7 calculated
624.2458, found 624.2448. 1H NMR (400 MHz, DMSO-d6): 2.39
(ddd, 1H, Jgem ) 13.4, J2′b,1′ ) 6.4, J2′b,3′ ) 3.6, H-2′b); 2.82 (ddd,
1H, Jgem ) 13.4, J2′a,1′ ) 7.2, J2′a,3′ ) 6.0, H-2′a); 3.02 (dd, 1H,
922, 696, 639. [R]20 ) -21.7 (c ) 3.13, DMSO).
D
Jgem ) 13.9, JbCH ,CH ) 10.1, bCH2); 3.18 (dd, 1H, Jgem ) 13.9,
(S)-3-{4-[9-H-Purin-6-yl]phenyl}-2-aminopropanoic acid
dihydrochloride ((S)-7). Hydrogen was bubbled through
solution of (S)-6 (245 mg, 0.50 mmol) in DMF (10 mL), water
(5 mL), and HCl (0.2 mL, 10%) in the presence of Pd/C catalyst
2
JaCH ,CH ) 5.4, aCH2); 3.56 (dt, 1H, Jgem ) 11.6, J5′b,OH ) 5.5,
J5′b,42′ ) 4.7, H-5′b); 3.65 (dt, 1H, Jgem ) 11.6, J5′a,OH ) 5.5, J5′a,4′
) 5.0, H-5′a); 3.92 (td, 1H, J4′,5′ ) 5.0, 4.7, J4′,3′ ) 3.1, H-4′);
J. Org. Chem, Vol. 70, No. 20, 2005 8007

Cˇ apek et al.
4.43 (ddd, 1H, JCH,CH ) 10.1, 5.4, JCH,NH ) 8.1, CH); 4.47 (m,
(S)-3-{4-[9-(2-Deoxy-â-D-erythro-pentofuranosyl)purin-
6-yl]phenyl}-2- aminopropanoic Acid Dihydrate ((S)-11).
Hydrogen was bubbled through a solution of (S)-13 (207 mg,
0.33 mmol) in DMF (7 mL) and water (7 mL) in the presence
of Pd/C catalyst (10 wt %, 25 mg) for 2 h. The catalyst was
filtered off on a Celite pad and the filtrate was evaporated in
vacuo. Crude product was purified by preparative HPLC on
C18 column with water/methanol as mobile phase. Product
was lyophilized from water to give 102 mg (71%) of (S)-11 as
white solid. MS (FAB): 400 (61, M + 1); 284 (100, M - dRf +
2). HRMS (FAB): for C19H22N5O5 calculated 400.1621, found
400.1613. 1H NMR (400 MHz, D2O, refdioxane ) 3.75 ppm): 2.52
(ddd, 1H, Jgem ) 14.0, J2′b1′ ) 6.3, J2′b3′ ) 3.8, H-2′b); 2.74 (dt,
1H, Jgem ) 14.0, J2′a1′ ) 6.5, J2′a3′ ) 6.5, H-2′a); 3.19 (dd, 1H,
2
1H, H-3′); 4.97 and 5.02 (2 × d, 2 × 1H, Jgem ) 12.5, CH2-
CBz); 5.03 (bt, 1H, JOH,5′ ) 5.5, OH-5′); 5.13 (s, 2H, CH2-Bn);
5.39 (bd, 1H, JOH,3′ ) 4.1, OH-3′); 6.53 (d, 1H, J1′2′ ) 7.2, 6.4,
H-1′); 7.22-7.36 (m, 10H, 2 × Ph); 7.47 (m, 2H, H-m-
phenylene); 7.96 (d, 1H, JNH,CH ) 8.1, NH); 8.74 (m, 2H, H-o-
phenylene); 8.89 (s, 1H, H-8); 9.99 (s, 1H, H-2). 13C NMR (100.6
MHz, DMSO-d6): 36.6 (CH2); 39.5 (CH2-2′); 55.6 (CH); 61.7
(CH2-5′); 65.6 (CH2-Bn); 66.3 (CH2-Cbz); 70.8 (CH-3′); 83.9 (CH-
1′); 88.2 (CH-4′); 127.8, 127.94, 127.98, 128.2, 128.48 and
128.55 (CH-Ph), 129.5 (CH-m-phenylene); 129.7 (CH-o-phe-
nylene); 130.9 (C-5); 133.9 (C-i-phenylene); 136.0 (C-i-Ph-Bn);
137.0 (C-i-Ph-Cbz); 140.9 (C-p-phenylene); 144.9 (CH-8); 152.0
(CH-2); 152.1 (C-4); 152.9 (C-6); 156.2 (CO-carbamate); 171.8
(CO-ester). IR (KBr): 3416, 3064, 3033, 1720, 1584, 1515, 1454,
1329, 1259, 1214, 1185, 1056, 803, 742, 645. [R]20D ) -24.5 (c
) 2.76, DMSO).
Jgem ) 14.5, JbCH ,CH ) 7.8, bCH2); 3.32 (dd, 1H, Jgem ) 14.5,
2
JaCH ,CH ) 5.3, aCH2); 3.78 (dd, 1H, Jgem ) 12.6, J5′b4′ ) 4.5,
2
H-5′b); 3.84 (dd, 1H, Jgem ) 12.6 J5′a4′ ) 3.4, H-5′a); 4.04 (dd,
1H, JCH,CH b ) 7.8, JCH,CH a ) 5.3, CH); 4.17 (q, 1H, J4′5′b ) 4.5,
J4′3′ ) 3.5,2J4′5′a ) 3.4, H-4′); 4.63 (dt, 1H, J3′2′a ) 6.5, J3′2′b
)
2
(S)-3-{4-[9-(â-D-Ribofuranosyl)purin-6-yl]phenyl}-2-ami-
nopropanoic Acid Dihydrate((S)-10). Hydrogen was bubbled
through a solution of (S)-12 (210 mg, 0.33 mmol) in DMF (7
mL) and water (7 mL) in the presence of Pd/C catalyst (10 wt
%, 25 mg) for 2 h. The catalyst was filtered off on a Celite pad
and the filtrate was evaporated in vacuo. Crude product was
purified by preparative HPLC on C18 column with water/
methanol as mobile phase. Product was crystallized from water
to give 103 mg (69%) of (S)-10 as white crystals, mp ) 208-
212 °C. MS (FAB): 416 (100, M +1); 284 (97, M - Rf + 2).
HRMS (FAB): for C19H22N5O6 calculated 416.1570, found
416.1566. 1H NMR (400 MHz, D2O, refdioxane ) 3.75 ppm): 3.18
3.8, J3′4′ ) 3.5, H-3′); 6.40 (t, 1H, J1′2′a ) 6.5, J1′2′b ) 6.3, H-1′);
7.40 (m, 2H, m-H-phenylene); 7.98 (m, 2H, o-H-phenylene);
8.51 (s, 1H, H-8); 8.62 (s, 1H, H-2). 13C NMR (100.6 MHz, D2O,
refdioxane ) 67.19 ppm): 36.9 (CH2); 39.6 (CH2-2′); 56.3 (CH);
62.1 (CH2-5′); 71.6 (CH-3′); 85.2 (CH-1′); 88.0 (CH-4′); 130.3
and 130.4 (CH-phenylene); 131.0 (C-5); 133.4 (C-i-phenylene);
139.5 (C-p-phenylene); 145.3 (CH-8); 151.4 (C-4); 152.0 (CH-
2); 155.1 (C-6); 174.2 (CO). IR (KBr): 3402, 3260, 3106, 2924,
1636, 1585, 1516, 1449, 1395, 1329, 1216, 1096, 1056, 802, 645.
[R]20 ) -28.1 (c ) 5.20, DMSO). Anal. Calcd for C19H25N5O7
D
(435.4): C 52.41, H 5.79, N 16.08. Found: C 52.20, H 5.62, N
15.72.
(dd, 1H, Jgem ) 14.6, JbCH ,CH ) 7.8, bCH2); 3.32 (dd, 1H, Jgem
2
) 14.6, JaCH ,CH ) 5.3, aCH2); 3.85 (dd, 1H, Jgem ) 12.9, J5′b,4′
2
Acknowledgment. This work is a part of the
research project Z4 055 0506. It was supported by the
Grant Agency of the Czech Republic (a grant No. 203/
03/0035), by the “Centre for New Antivirals and Anti-
neoplastics” (1M6138896301) and by Sumitomo Chemi-
cal, Inc. (Osaka, Japan).
Supporting Information Available: General methods,
synthesis of starting compounds, characterization data of
compounds 5a, (R)-5b-f, 9a, (R)-6, (R)-7, (S)-8, (S)-9, (R)-10,
(R)-11, (R)-12, (R)-13, and (S)-4 and details about study on
the reactivity of pinacol ester of 4-(borono)phenylalanines with
1a. This material is available free of charge via the Internet
at http://pubs.acs.org.
) 3.8, H-5′b); 3.94 (dd, 1H, Jgem ) 12.9, J5′a,4′ ) 2.9, H-5′a);
4.03 (dd, 1H, JCH,CH ) 7.8, 5.3, CH); 4.28 (bq, 1H, J4′,3′ ) 4.0,
2
J4′,5′ ) 3.8, 2.9, H-4′); 4.43 (dd, 1H, J3′,2′ ) 5.2, J3′,4′ ) 4.0, H-3′);
4.71 (t, 1H, J2′,1′ ) 5.5, J2′,3′ ) 5.2, H-2′); 6.09 (d, 1H, J1′2′
)
5.5, H-1′); 7.40 (m, 2H, H-m-phenylene); 7.99 (m, 2H, H-o-
phenylene); 8.55 (s, 1H, H-8); 8.66 (s, 1H, H-2). 13C NMR (100.6
MHz, D2O, refdioxane ) 67.19 ppm): 37.0 (CH2); 56.4 (CH); 61.9
(CH2-5′); 71.0 (CH-3′); 74.5 (CH-2′); 86.2 (CH-4′); 89.1 (CH-
1′); 130.3 and 130.5 (CH-phenylene); 131.1 (C-5); 133.4 (C-i-
phenylene); 139.6 (C-p-phenylene); 145.6 (CH-8); 151.6 (C-4);
152.2 (CH-2); 155.4 (C-6); 174.4 (CO). IR (KBr): 3240, 3112,
2926, 1613, 1587, 1561, 1516, 1451, 1399, 1330, 1215, 1101,
1057, 802, 643. [R]20D ) -53.0 (c ) 3.50, DMSO). Anal. Calcd
for C19H25N5O8 (451.4): C 50.55, H 5.58, N 15.51. Found: C
50.58, H 5.68, N 15.43.
JO051110X
8008 J. Org. Chem., Vol. 70, No. 20, 2005