Synthesis of enantiomers of N-(2-aminopurin-6-yl)amino acids

Article abstract of DOI:10.1007/s11172-015-1125-x

Nonracemic N-(2-aminopurin-6-yl)-substituted amino acids were synthesized by nucleophilic substitution of chlorine atom in 2-acetamido-6-chloropurine upon treatment with amino acid tert-butyl esters and subsequent removal of protecting groups. Their enant

Full text of DOI:10.1007/s11172-015-1125-x

Russian Chemical Bulletin, International Edition, Vol. 64, No. 9, pp. 2106—2113, September, 2015
2106
Synthesis of enantiomers of Nꢀ(2ꢀaminopurinꢀ6ꢀyl)amino acids*
V. P. Krasnov, A. Yu. Vigorov, D. A. Gruzdev, G. L. Levit, A. M. Demin, I. A. Nizova,
A. A. Tumashov, L. Sh. Sadretdinova, E. B. Gorbunov, and V. N. Charushin
I. Ya. Postovsky Institute of Organic Synthesis, Ural Branch of the Russian Academy of Sciences,
22/20 ul. S. Kovalevskoi, 620137 Ekaterinburg, Russian Federation.
Fax: +7 (343) 374 1189. Eꢀmail: ca@ios.uran.ru
Nonracemic Nꢀ(2ꢀaminopurinꢀ6ꢀyl)ꢀsubstituted amino acids were synthesized by nucleoꢀ
philic substitution of chlorine atom in 2ꢀacetamidoꢀ6ꢀchloropurine upon treatment with amino
acid tertꢀbutyl esters and subsequent removal of protecting groups. Their enantiomeric compoꢀ
sition was determined by HPLC on chiral stationary phases.
Key words: purines, amino acids, enantiomers, racemization, high performance liquid
chromatography.
Purine derivatives are known to exhibit various types of
possibility for the preparation of Nꢀ(2ꢀaminopurinꢀ6ꢀyl)ꢀ
substituted amino acids with a free NH group at position 9
by nucleophilic substitution of the chlorine atom in 2ꢀacetꢀ
amidoꢀ6ꢀchloropurine with subsequent removal of proꢀ
tecting groups by alkaline hydrolysis.37 It was shown that
in the case of valine, this twoꢀstep process was accompaꢀ
nied by partial racemization.
biological activity and are active pharmaceutical compoꢀ
nents of different medicines.1—4 Chemical modification
of purine and 2ꢀaminopurine furnished compounds posꢀ
sessing tuberculostatic,5—9 antitumor,10—12 and other types
of biological activity.13
Modification of biologically active compounds via the
introduction of amino acid fragments in their structure is
one of the important pathways of optimization of pharꢀ
macokinetic and pharmacodynamic characteristics of
potential drugs.14—23 In particular, the introduction of
amino acid fragments into the molecular structure is
a valuable tool for the design of physiologically active comꢀ
pounds based on purine derivatives.24—28 Recently, it was
shown that dipeptides bearing a theophylline moiety exꢀ
hibit high activity against the Mycobacterium tuberculosis
H37Rv strain and are a new class of antituberculosis
agents.29 Purine conjugates bearing amino acid fragments
are of interest not only as compounds with potential bioꢀ
logical activity, but also as objects for further chemical
modification, in particular, they can be a basis for the
preparation of glycosides, compounds with peptide naꢀ
ture, etc. In this case, enantiomeric purity of conjugates is
of exceptional importance.
The purpose of the present work is to extend this apꢀ
proach on the preparation of other Nꢀ(2ꢀaminopurinꢀ6ꢀ
yl)ꢀsubstituted amino acids, development of synthetic
procedure and assessment of enantiomeric purity of the
indicated compounds. The main attention was paid to the
synthesis of natural amino acid derivatives with (S)ꢀconꢀ
figuration. Derivatives with (R)ꢀ and (RS)ꢀconfiguration
were synthesized and used as references for the analysis of
enantiomeric composition of the compounds.
The reaction of chloropurine 1 with amino esters 2a—h
(3 equiv.) upon heating in N,Nꢀdimethylacetamide (DMA)
in the presence of triethylamine (TEA) led to Nꢀ(2ꢀacetꢀ
amidopurinꢀ6ꢀyl)ꢀsubstituted amino acid tertꢀbutyl esters
3a—h (Scheme 1).
After isolation from the reaction mixture, compounds
3a—h were purified by either recrystallization (3a,b from
aqueous ethanol, 3e from ethyl acetate) or flashꢀchromaꢀ
tography on silica gel (3c,d,f—h). The yields of compounds
3a—e varied from 56 to 83% (Table 1). In the cases of
aspartic acid (2f) and lysine (2g,h) derivatives, the yields
were moderate (32–48%).
The enantiomeric composition of tertꢀbutyl esters 3b—f,h
was determined by HPLC on a chiral stationary phase
(CSP): a S,SꢀWhelkꢀO 1 column in the reversedꢀphase
mode (the mobile phase MeOH—H2O—AcOH). The sepꢀ
aration of enantiomers of these compounds on other chiral
columns (Chiralcel ODꢀH, Chiralpak AD, Kromasil Ceꢀ
lucoat, ChiraDex, Nautilus E) was not satisfactory. The
Most of known approaches to the introduction of amino
acid moieties at position 6 of purine are based on the
reactions of amino acids or their derivatives with N(9)ꢀ
substituted purines.30—36 This fact complicates the variaꢀ
tion of the structure of substituents at position 9 of purine
fragment.
Earlier, we took (S)ꢀ and (R)ꢀvaline and (S)ꢀphenylꢀ
alanine derivatives as examples to demonstrate a principal
* Dedicated to Academician of the Russian Academy of Sciences
N. S. Zefirov on the occasion of his 80th birthday.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 2106—2113, September, 2015.
1066ꢀ5285/15/6409ꢀ2106 © 2015 Springer Science+Business Media, Inc.

Nꢀ(2ꢀAminopurinꢀ6ꢀyl)ꢀsubstituted amino acids
Russ.Chem.Bull., Int.Ed., Vol. 64, No. 9, September, 2015 2107
Scheme 1
Table 1. Enantiomeric purity (ee) and yields of tertꢀ
butyl esters of Nꢀ(2ꢀacetamidopurinꢀ6ꢀyl)ꢀsubstitutꢀ
ed amino acids 3a—h
Comꢀ
pound
Yield
(%)
Enantiomeric
excess (ee) (%)a
3a
80
65
70
56
60
65
60
83
60
32
39
35
32
47
48
—
>99
>99
>99
>99
96
0
>99
0
50
0
—
—
(S)ꢀ3b
(R)ꢀ3b
(S)ꢀ3c
(R)ꢀ3c
(S)ꢀ3d
(RS)ꢀ3d
(S)ꢀ3e
(RS)ꢀ3e
(S)ꢀ3f
(RS)ꢀ3f
(S)ꢀ3g
(RS)ꢀ3g
(S)ꢀ3h
(RS)ꢀ3h
>99
0
a Chiral HPLC data (see Experimental).
found that a simultaneous hydrolysis of N(2)ꢀacetamide
and ester groups took place in the excess of 1 M NaOH
at 60 C for 2 h (method A). The reaction products
(S)ꢀ4a—e,g,h were isolated in 59—86% yields (Table 2).
Scheme 2
2, 3
a
R1
H
R1—R2
R2
H
b
c
H
H
Me
Pri
d
H
CH2Ph
e
f
g
(CH2)3
H
H
CH2CO2But
(CH2)4NHCbz
Reagents and conditions: TEA, DMA, 100 C, 12 h.
results of determination of enantiomeric purity of comꢀ
pounds 3b—f,h (see Table 1) showed that the nucleophilic
substitution of the chlorine atom in 2ꢀacetamidoꢀ6ꢀchloroꢀ
purine (1) upon treatment with most amino acid tertꢀbutyl
esters (2b—e,h) proceeded without significant racemizaꢀ
tion (ee of esters 3b,c,e,h was above 99%; ee of (S)ꢀ3d
96%). The reaction of diꢀtertꢀbutyl (S)ꢀaspartate ((S)ꢀ2f)
with chloropurine 1 was accompanied by considerable raꢀ
cemization (ee of (S)ꢀ3f 50%). It was impossible to select
conditions for the analysis of enantiomeric purity of comꢀ
pound (S)ꢀ3g.
4
a
b
c
d
e
g
R1
H
H
H
H
R1—R2
R2
H
Me
Pri
CH2Ph
(CH2)3
H
(CH2)4NHCbz
The removal of acetyl and ester protecting groups in
compounds (S)ꢀ3a—e,g,h was carried out by the alkaline
hydrolysis in aqueous solution of NaOH (Scheme 2). It was
Reagents and conditions: A: 1 M NaOH (6 equiv.), 60 C, 2 h;
B: 1 M NaOH (6 equiv.), 20 C, 5 days.

2108
Russ.Chem.Bull., Int.Ed., Vol. 64, No. 9, September, 2015
Krasnov et al.
Table 2. Enantiomeric purity (ee) and yields of Nꢀ(2ꢀacetꢀ
amidopurinꢀ6ꢀyl)ꢀsubstituted amino acids 4a—e,g,h
It was found that the alkaline hydrolysis of protecting
groups in compounds (S)ꢀ3b, (R)ꢀ3b, and (S)ꢀ3e (method A)
gave the corresponding Nꢀ(2ꢀaminopurinꢀ6ꢀyl)ꢀsubstitutꢀ
ed amino acids (S)ꢀ4b, (R)ꢀ4b, and (S)ꢀ4e without loss of
enantiomeric purity (see Table 2). Hydrolysis of valine
(S)ꢀ3c, phenylalanine (S)ꢀ3d, and lysine (S)ꢀ3h derivaꢀ
tives by method A was accompanied by partial racemizaꢀ
tion, the enantiomeric excess of the reaction products was
80—90% (see Table 2). Thus, a partial racemization occurꢀ
ing in the course of a twoꢀstep procedure developed by us
earlier for the preparation of Nꢀ(2ꢀaminopurinꢀ6ꢀyl)ꢀ
valine37 takes place in the step of deblocking of functional
groups by alkaline hydrolysis.
It was also shown that the removal of protecting groups
in compounds (S)ꢀ3c,d,h by alkaline hydrolysis under
milder conditions (see Scheme 2, method B: 1 M NaOH,
20 C, 5 days) gave the corresponding acids (S)ꢀ4c,d,h in
56—65% yields without loss of enantiomeric purity. These
conditions were used to obtain enantiomerically pure deꢀ
rivatives 4c,d,h (according to the HPLC data for acid 4h
and methyl esters 5c,d). The preparation of enantiomeriꢀ
cally pure Nꢀ(2ꢀaminopurinꢀ6ꢀyl)phenylalanine ((S)ꢀ4d)
(ee > 99%) starting from scalemic tertꢀbutyl ester (S)ꢀ3d
(ee 96%) is apparently related to the enantiomeric enrichꢀ
ment of the reaction product (S)ꢀ4d in the process of isoꢀ
lation. Conditions for the analysis of enantiomeric purity
of compound (S)ꢀ4g were not found. However, running its
synthesis under conditions providing the minimal racemꢀ
ization allowed us to suggest that derivative (S)ꢀ4g is enanꢀ
tiomerically pure.
Comꢀ
pound
Method
Yield
(%)
Enantiomeric
excess (ee) (%)a
b
4a
A
A
A
A
B
A
B
A
B
A
B
83
82
69
81
65
70
61
86
51
59
56
—
(S)ꢀ4b
(R)ꢀ4b
(S)ꢀ4c
(S)ꢀ4c
(S)ꢀ4d
(S)ꢀ4d
(S)ꢀ4e
(S)ꢀ4g
(S)ꢀ4h
(S)ꢀ4h
>99b
>99b
86b
>99b
80b
>99b
>99b
b
—
90b
>99b
a Chiral HPLC data (see Experimental).
b HPLC data for the corresponding methyl esters.
To assess enantiomeric purity of compounds 4b—e,h
by HPLC on CSP, a similar method (A) was used to synꢀ
thesize reference compounds, the corresponding (R)ꢀenanꢀ
tiomers or racemates. Earlier, we have found conditions
for the HPLC analysis of enantiomeric composition of
Nꢀ(2ꢀaminopurinꢀ6ꢀyl)valine (4c) on a ChiraDex column.37
We succeeded in the separation of enantiomers of comꢀ
pound 4h on a S,SꢀWhelkꢀO 1 column in the reversedꢀ
phase mode. Our attempts to select conditions for the
analysis of enantiomeric composition of amino acids 4b,d,e
using HPLC on different chiral columns were unsuccessꢀ
ful. Therefore, to determine enantiomeric purity of the
indicated compounds they were converted to the correꢀ
sponding methyl esters 5b—e (Scheme 3) with subsequent
HPLC analysis on CSP in the normalꢀphase mode (moꢀ
bile phase: hexane—PriOH—MeOH).
The removal of the Cbz protecting group in compound
(S)ꢀ4h by hydrogenation on palladium catalyst in acetic
acid led to free amino acid (S)ꢀ6h in moderate yield
(Scheme 4). Attempted isolation of amino acid (S)ꢀ6g in
the individual state from the hydrogenolysis products of
compound (S)ꢀ4g was unsuccessful. These results indicate
that the presence of the 2ꢀaminopurine fragment in the
molecule complicates hydrogenolysis of the Cbz groups in
this compound.
Scheme 3
Scheme 4
Compound
(S)ꢀ5b
(R)ꢀ5b
(S)ꢀ5c
(R)ꢀ5c
(S)ꢀ5d
(RS)ꢀ5d
(S)ꢀ5e
R1
H
H
H
H
H
H
R1—R2
R2
Me
Me
Pri
Pri
Bn
Bn
Yield (%)
58
53
95
99
92
88
88
70
Reagents and conditions: H2 (18 atm.), Pd/C, AcOH, 70 C, 4 h.
The structure of 2ꢀaminopurine conjugates 3a—h, 4a—h,
(CH2)3
(CH2)3
(RS)ꢀ5e
1
5b—e, and (S)ꢀ6h was confirmed by H and 13C NMR
Reagents and conditions: SOCl2, MeOH, 20 C, 16 h.
spectroscopy, elemental analysis, and high resolution mass

Nꢀ(2ꢀAminopurinꢀ6ꢀyl)ꢀsubstituted amino acids
Russ.Chem.Bull., Int.Ed., Vol. 64, No. 9, September, 2015 2109
spectrometry. The 1H NMR spectra of compounds 3a—h
recorded in DMSOꢀd6 at 25C exhibited a double set of
signals, apparently, because of the N(7)H—N(9)Hꢀproꢀ
totropic tautomerism of purine fragment.38—40 When
1H NMR spectra were recorded at 100 C, some signals
collapsed and some signals remained broadened.
In conclusion, we synthesized a number of Nꢀ(2ꢀ
aminopurinꢀ6ꢀyl)ꢀsubstituted amino acids and determined
their enantiomeric purity. The synthesized compounds are
of interest as potential biologically active compounds and
building blocks for the preparation of structurally various
nucleosides, as well as (2ꢀaminopurinꢀ6ꢀyl)ꢀsubstituted
peptides.
tion times ( ) are given in particular procedures. Column flashꢀ
R
chromatography was carried out on Silica gel 60 0.063—0.040 mm
(Alfa Aesar). Sorbfil plates (Imid Ltd., Russia) were used for
TLC; visualized under UV light (254 nm); sprayed with 0.2%
solution of ninhydrin in acetone (for compound (S)ꢀ6h).
tertꢀButyl N()ꢀbenzyloxycarbonylꢀ(S)ꢀlysinate ((S)ꢀ2h) was
synthesized according to the modified procedure.44,46 N()ꢀ
Benzyloxycarbonylꢀ(S)ꢀlysine (1.00 g, 3.57 mmol) was dispersed
in tertꢀbutyl acetate (50 mL) over 10 min in an ultrasonic bath
(100 W), then 58% aqueous HClO4 (0.45 mL, 3.93 mmol) was
added, and the mixture was stirred for 16 h. A precipitate formed
was filtered, the filtrate was concentrated, compound (S)ꢀ2h was
isolated by flashꢀchromatography (CHCl3—MeOH from 30 : 1
to 30 : 3.6). The yield was 0.913 g (76%), a colorless oil. 1H NMR
(400 MHz, DMSOꢀd6, 25 C), : 1.42 (m, 2 H, H2C(4)); 1.39
(s, 9 H, But); 1.43—1.70 (m, 4 H, H2C(3), H2C(5)); 2.75 (t, 2 H,
H2C(6), J = 7.2 Hz); 3.85 (ddd, 1 H, C(2)H, J = 9.3 Hz, J = 7.9 Hz,
J = 5.5 Hz); 5.04 (m, 2 H, CH2, Cbz); 7.25—7.40 (m, 5 H, Ph);
7.55 (br.s, 2 H, NH2); 7.78 (d, 1 H, NH, J = 7.8 Hz). 13C NMR
(100 MHz, DMSOꢀd6, 25 C), : 21.48 (C(4)); 26.21, 27.19,
28.18 (3 Me, But); 30.17 and 31.34 (C(3) and C(5)); ~40 (C(6),
overlapped with the signal of DMSO); 55.12 (C(2)); 65.51 (CH2,
Cbz); 80.64 (C, But); 127.73, 127.78, 128.00, 128.52, 129.07
(2 Co, 2 Cm, Cp); 136.91 (Ci); 156.23 (CO, Cbz); 171.61 (CO2But).
HRMS (ESI): found: m/z 337.2120 [M + H]+; C18H29N2O4;
calculated: M = 337.2122.
tertꢀButyl N()ꢀbenzyloxycarbonylꢀ(RS)ꢀlysinate ((RS)ꢀ2h)
was synthesized similarly to compound (S)ꢀ2h. The yield was
0.865 g (72%), a colorless oil. 1H NMR (400 MHz, DMSOꢀd6,
25 C, ) is similar to the spectrum of compound (S)ꢀ2h.
Synthesis of Nꢀ(2ꢀacetamidopurinꢀ6ꢀyl)ꢀsubstituted amino
acid tertꢀbutyl esters 3a—h (general procedure). Amino acid tertꢀ
butyl ester hydrochloride or hydrogen acetate (3.0 mmol,
3 equiv.) and TEA (0.56 mL, 4.0 mmol) were added to a suspension
of 2ꢀacetamidoꢀ6ꢀchloropurine (1) (0.212 g, 1.0 mmol) in DMA
(2.5 mL). When amino acid tertꢀbutyl esters were used as free
bases (3 equiv.), the amount of TEA was decreased to 1 equiv.
The mixture was allowed to stand at 100 C for 12 h and quenched
with water (25 mL), a precipitate formed was separated and
dried in vacuo. Compounds 3a, 3b, and 3e were isolated by reꢀ
crystallization, 3c, 3d, and 3f—h by flashꢀchromatography on
silica gel.
Experimental
2ꢀAcetamidoꢀ6ꢀchloropurine41 (1), N()ꢀbenzyloxycarbonꢀ
ylꢀ(RS)ꢀlysine,42 tertꢀbutyl (R)ꢀvalinate acetate43 ((R)ꢀ2c•
•AcOH), tertꢀbutyl (RS)ꢀphenylalaninate acetate ((RS)ꢀ2d•
•AcOH), diꢀtertꢀbutyl (S)ꢀaspartate acetate ((S)ꢀ2f•AcOH), and
(RS)ꢀaspartic acid acetate ((RS)ꢀ2f•AcOH)44 were obtained acꢀ
cording to the known procedures. tertꢀButyl (RS)ꢀprolinate
((RS)ꢀ2e) was synthesized according to the known method45 and
used as a free base. Compounds (S)ꢀ3c, (R)ꢀ3c, (S)ꢀ4c, and (S)ꢀ4d
were described earlier.37 Other reagents are commercially availꢀ
able. 1H and 13C NMR spectra were recorded on Bruker DRXꢀ400
(400 and 100 MHz, respectively) or Bruker Avance 500 spectroꢀ
meters (500 and 125 MHz, respectively) in DMSOꢀd6 relative to
SiMe4 (as an internal reference) at 25 C or 100 C. Melting
points of compounds were determined on a Stuart SMP3 appaꢀ
ratus (Barloworld Scientific, UK). IR spectra were recorded on
a FTꢀIR Spectrometer Spectrum One instrument (Perkin—Elmer
Instruments, USA) with a diffuse reflectance sampling accessory
(DRA) or by frustrated total internal reflection (FTIR). Specific
rotation was determined on a Perkin—Elmer Model 341 polaꢀ
rimeter (given in deg mL g–1 dm–1, concentration of solution in
g per 100 mL). Elemental analysis was performed on a Perkin—
Elmer 2400 II automated CHNS/Oꢀanalyzer. Individuality of
compounds was confirmed by HPLC on an Agilent 1100 chroꢀ
matograph (Agilent Technologies, USA) with a 4.6×250ꢀmm
column packed with the Phenomenex Luna C(18) sorbent, 5 m
(Phenomenex, Inc., USA); detection was carried out at 220,
230, and 254 nm; mobile phase MeCN—0.2% aqueous TFA
(gradient from 1 : 9 to 9 : 1 over 30 min); the rate of elution
0.8 mL min–1 (if not stated otherwise). Electrospray ionization (ESI)
high resolution mass spectra (HRMS) were recorded on a Bruker
tertꢀButyl Nꢀ(2ꢀacetamidopurinꢀ6ꢀyl)glycinate (3a). The yield
was 0.245 g (80%), colorless crystals, m.p. 307—310 C (decomp.,
EtOH). IR (DRA), /cm–1: 3328, 3139, 3092, 2940, 1728, 1708,
1627, 1593, 1518. 1H NMR (400 MHz, DMSOꢀd6, 100 C),
: 1.41 (s, 9 H, But); 2.26 (s, 3 H, Me, Ac); 4.19 (d, 2 H, CH2,
J = 6.1 Hz); 7.24 (s, 1 H, C(6´)NH); 7.87 (s, 1 H, C(8´)H); 9.08
maXis Impact HD instrument, carrier gas nitrogen at 4 L min–1
nebulizer pressure 0.4 Bar, needle potential 4.5 kV. Enantiomerꢀ
ic composition of compounds 3b—f,h and 4h was determined
using chiral HPLC on an Agilent 1100 chromatograph with
a 4.6×250ꢀmm column, packed with the S,SꢀWhelkꢀO 1 sorbent,
5 m (Regis Technologies Inc., USA); detection was carried out
at 220, 230, and 254 nm; mobile phases, elution rates, and retenꢀ
,
(s, 1 H, NH, Ac); 12.43 (br.s, 1 H, N(9´)H). 13C NMR (125
MHz, DMSOꢀd6, 25 C), : 24.65 (Me, Ac); 27.68 (Me, But);
42.31 (CH2); 80.44 (C, But); 115.75 (C(5´)); 138.32 (C(8´));
150.62, 152.50, 154.12 (C(4´), C(2´), C(6´)); 169.38 and 169.58
(CO, Ac and CO2But). HPLC (Phenomenex Luna C(18)),

= 12.2 min. Found (%): C, 51.12; H, 6.17; N, 27.74.
R
C13H18N6O3. Calculated (%): C, 50.97; H, 5.92; N, 27.44.
tertꢀButyl Nꢀ(2ꢀacetamidopurinꢀ6ꢀyl)ꢀ(S)ꢀalaninate ((S)ꢀ3b).
The yield was 0.208 g (65%), colorless crystals, m.p. 156—157 C
(EtOH—water), []D20 — 34.0 (c 0.5, MeOH), ee > 99%, HPLC
(S,SꢀWhelkꢀO 1, MeOH — 0.25% aqueous AcOH 48 : 52,
0.8 mL min–1), R = 26.6 min. HPLC (Phenomenex Luna C(18)),
tion times ( ) are given in particular procedures. Enantiomeric
R
composition of methyl esters 5b—e was determined using chiral
HPLC on a Knauer Smartlineꢀ1100 instrument (Knauer Wisꢀ
senschaftliche Geräte, Germany) with a 4.6×250ꢀmm column
packed with the S,SꢀWhelkꢀO 1 (5b—d) or Chiralcel ODꢀH sorꢀ
bent, 5 m (Daicel Corp., Japan) (5e); detection was carried out
at 220, 230, and 254 nm; mobile phases, elution rates, and retenꢀ

= 13.3 min. IR (DRA), /cm–1: 3208, 3142, 3026, 2977,
R
2935, 1743, 1658, 1614, 1525. 1H NMR (400 MHz, DMSOꢀd6,

2110
Russ.Chem.Bull., Int.Ed., Vol. 64, No. 9, September, 2015
Krasnov et al.
100 C), : 1.40 (s, 9 H, But); 1.46 (d, 3 H, Me, Ala, J = 7.2 Hz);
2.26 (s, 3 H, Me, Ac); 4.81 (br.s, 1 H, CH, Ala); 7.02 (br.s, 1 H,
N(6´)H); 7.89 (br.s, 1 H, C(8´)H); 9.06 (br.s, 1 H, NH, Ac);
12.45 (br.s, 1 H, N(9´)H). 13C NMR (125 MHz, DMSOꢀd6, 25 C),
: 17.19 (Me, Ala); 24.63 (Me, Ac); 27.56 (Me, But); 49.23 (CH,
Ala); 80.31 (C, But); 115.56 (C(5´)); 138.46 (C(8´)); 150.80,
152.43 and 153.53 (C(4´), C(6´) and C(2´)); 169.66 (CO, Ac),
172.28 (CO2But). Found (%): C, 52.39; H, 6.28; N, 25.95.
C14H20N6O3. Calculated (%): C, 52.49; H, 6.29; N, 26.23.
tertꢀButyl Nꢀ(2ꢀacetamidopurinꢀ6ꢀyl)ꢀ(R)ꢀalaninate ((R)ꢀ3b)
was synthesized similarly to (S)ꢀ3b. The yield was 0.224 g (70%),
colorless crystals, m.p. 154—155 C. []D25 +32.7 (c 0.5, MeOH),
ee > 99%, HPLC (S,SꢀWhelkꢀO 1, MeOH—0.25% aqueous
AcOH 48 : 52, 0.8 mL min–1), R = 25.3 min. Found (%): C, 52.31;
H, 6.31; N, 26.02. C14H20N6O3. Calculated (%): C, 52.49; H, 6.29;
N, 26.23. NMR spectra are similar to the spectra of compound
(S)ꢀ3b.
(Me, Ac, A and В); 27.51 (Me, But, В); 27.55 (Me, But, A);
28.63 (C(3), В); 30.71 (C(3), A); 47.35 (C(5), A); 48.93 (C(5),
B); 60.15 (C(2), B); 60.85 (C(2), A); 79.96 (C, But, A); 80.30
(C, But, B); 116.06 (C(5´), B); 116.41 (C(5´), A); 137.69 (C(8´), A);
138.20 (C(8´), B); 151.37 (C(4´) or C(2´), or C(6´), A); 151.93
(C(4´) or C(2´), or C(6´), B); 152.10 (C(2´) or C(4´), or C(6´), B);
152.18 (C(2´) or C(4´), or C(6´), A); 152.41 (C(6´) or C(2´), or
C(4´), A); 152.69 (C(6´) or C(2´), or C(4´), B); 169.46 (CO, Ac, B);
170.16 (CO, Ac, A); 171.35 (CO2But, A); 171.64 (CO2But, B).
Found (%): C, 55.42; H, 6.71; N, 24.28. C16H22N6O3. Calculatꢀ
ed (%): C, 55.48; H, 6.40; N, 24.26.
tertꢀButyl Nꢀ(2ꢀacetamidopurinꢀ6ꢀyl)ꢀ(RS)ꢀprolinate ((RS)ꢀ3e).
The yield was 0.208 g (60%), colorless crystals, m.p. 244—245 C
(decomp.) (EtOAc), HPLC (S,SꢀWhelkꢀO 1, MeOH—H2O
60 : 40, 0.9 mL min–1), R = 18.6 ((R)ꢀ3e) and 20.4 min ((S)ꢀ3e).
1H NMR spectrum is similar to that of compound (S)ꢀ3e. HRMS
(ESI): found: m/z 347.1826 [M + H]+; C16H23N6O3; calculated:
M = 347.1832.
tertꢀButyl Nꢀ(2ꢀacetamidopurinꢀ6ꢀyl)ꢀ(S)ꢀvalinate ((S)ꢀ3c)
was described earlier,37 ee > 99%, HPLC (S,SꢀWhelkꢀO 1,
MeOH—H2O 60 : 40, 0.9 mL min–1), R = 20.5 min.
tertꢀButyl Nꢀ(2ꢀacetamidopurinꢀ6ꢀyl)ꢀ(R)ꢀvalinate ((R)ꢀ3c)
was described earlier,37 ee > 99%, HPLC (S,SꢀWhelkꢀO 1,
MeOH—H2O 60 : 40, 0.9 mL min–1), R = 19.2 min.
Diꢀtertꢀbutyl Nꢀ(2ꢀacetamidopurinꢀ6ꢀyl)ꢀ(S)ꢀaspartate ((S)ꢀ3f)
was isolated by flashꢀchromatography (eluent CHCl3—MeOH
from 10 : 0.05 to 10 : 0.40). The yield was 0.135 g (32%), colorꢀ
20
less crystals, m.p. 106—108 C, []D +8.9 (c 1.0, CHCl3),
ee 50%, HPLC (S,SꢀWhelkꢀO 1, MeOH—0.25% aqueous AcOH
60 : 40, 1.0 mL min–1), R = 21.0 ((R)ꢀ3f) and 22.5 min ((S)ꢀ3f).
tertꢀButyl Nꢀ(2ꢀacetamidopurinꢀ6ꢀyl)ꢀ(S)ꢀphenylalaninate
((S)ꢀ3d) was isolated by flashꢀchromatography (eluent CHCl3—
MeOH from 10 : 0.05 to 10 : 0.30). The yield was 0.258 g (65%),
colorless crystals, m.p. 130—133 C, []D20 +1.7 (c 2.0, CHCl3),
ee 96%, HPLC (S,SꢀWhelkꢀO 1, MeOH—0.25% aqueous AcOH
HPLC (Phenomenex Luna C(18)),  = 17.3 min. IR (FTIR),
R
/cm–1: 3413, 3315, 3122, 2978, 2930, 1730, 1714, 1669, 1645,
1622, 1528, 1455. 1H NMR (400 MHz, DMSOꢀd6, 100 C),
: 1.38 (s, 9 H, But); 1.39 (s, 9 H, But); 2.26 (s, 3 H, Me, Ac); 2.82
(dd, 1 H, HВC(3), J = 16.2 Hz, J = 6.3 Hz); 2.90 (m, 1 H,
HAC(3)); 5.17 (s, 1 H, C(2)H); 7.00 (s, 1 H, N(6´)H); 7.90 (s, 1 H,
C(8´)H); 9.14 (s, 1 H, NH, Ac); 12.42 (br.s, 1 H, N(9´)H).
13C NMR (125 MHz, DMSOꢀd6, 25 C), : 24.60 (Me, Ac); 27.49
(Me, But); 27.62 (Me, But); 37.09 (CH2, Asp); 50.07 (CH, Asp);
80.36 (C, But); 80.94 (C, But);115.46 (C(5´)); 138.85 (C(8´));
151.08, 152.36, 153.35 (C(2´), C(4´), C(6´)); 169.34, 169.56 and
170.13 (CO, Ac and 2 CO2But). HRMS (ESI): found: m/z
421.2198 [M + H]+; C19H29N6O5; calculated: M = 421.2194.
Diꢀtertꢀbutyl Nꢀ(2ꢀacetamidopurinꢀ6ꢀyl)ꢀ(RS)ꢀaspartate
((RS)ꢀ3f). The yield was 0.164 g (39%). HPLC (S,SꢀWhelkꢀO 1,
MeOH—0.25% aqueous AcOH 60 : 40, 1.0 mL min–1), R = 21.0
63 : 37, 0.8 mL min–1),  = 20.5 min ((R)ꢀ3d), 21.7 ((S)ꢀ3d).
R
HPLC (Phenomenex Luna C(18)),  = 17.7 min. IR (FTIR),
R
/cm–1: 3206, 3132, 2977, 2933, 1732, 1660, 1608, 1524, 1497,
1456, 1426. 1H NMR (500 MHz, DMSOꢀd6, 100 C), : 1.34
(s, 9 H, But); 2.25 (s, 3 H, Me, Ac); 3.19 (dd, 1 H, HBC(3),
J = 14.5 Hz, J = 6.6 Hz); 3.22 (dd, 1 H, HAC(3), J = 14.5 Hz,
J = 6.9 Hz); 5.09 (br.s, 1 H, C(2)H); 6.93 (br.s, 1 H, N(6´)H);
7.17 (tt, 1 H, Hp, J = 7.0 Hz, J = 1.8 Hz); 7.22—7.29 (m, 4 H,
Ho and Hm); 7.91 (s, 1 H, C(8´)H); 9.16 (s, 1 H, NH, Ac); 12.38
(br.s, 1 H, N(9´)H). 13C NMR (125 MHz, DMSOꢀd6, 25 C),
: 24.61 (Me, Ac); 27.47 (Me, But); 36.56 (CH2); 54.95 (CH);
80.62 (C, But); 115.77 (C(5´)); 126.31 (Cp); 128.08 (Co); 129.11
(Cm); 137.78 and 138.44 (C(8´) and Cipso); 150.69, 152.38, 153.84
(C(4´), C(2´), C(6´)); 169.41 and 171.11 (CO, Ac and CO2But).
Found (%): C, 57.60; H, 6.22; N, 18.20. C20H24N6O3. Calculatꢀ
ed (%): C, 57.89; H, 6.18; N, 18.41.
1
((R)ꢀ3f) and 22.5 min ((S)ꢀ3f). H NMR spectrum is similar to
that of compound (S)ꢀ3f.
tertꢀButyl N()ꢀ(2ꢀacetamidopurinꢀ6ꢀyl)ꢀN()ꢀbenzyloxyꢀ
carbonylꢀ(S)ꢀlysinate ((S)ꢀ3g) was isolated by flashꢀchromatoꢀ
graphy (eluent CHCl3—MeOH from 10 : 0.05 to 10 : 0.40). The
tertꢀButyl Nꢀ(2ꢀacetamidopurinꢀ6ꢀyl)ꢀ(RS)ꢀphenylalaninate
((RS)ꢀ3d) was obtained similarly to (S)ꢀ3d. The yield was 0.238 g
(60%), m.p. 118—121 C, HPLC (S,SꢀWhelkꢀO 1, MeOH—
20
yield was 0.179 g (35%), colorless crystals, m.p. 92—94 C, []D
–20.9 (c 1.0, CHCl3). HPLC (Phenomenex Luna C(18)),
0.25% aqueous AcOH 63 : 37, 0.8 mL min–1),  = 20.5 min

= 17.0 min. IR (DRA), /cm–1: 3351, 3214, 3065, 2934,
R
R
((R)ꢀ3d), 21.7 ((S)ꢀ3d). NMR spectra are similar to the spectra
of compound (S)ꢀ3d.
2865, 1699, 1648, 1617, 1548. 1H NMR (500 MHz, DMSOꢀd6,
100 °C), : 1.37—1.53 (m, 4 H, 2 CH2); 1.39 (s, 9 H, But);
1.80—1.92 (m, 2 H, CH2); 2.26 (s, 3 H, Me, Ac); 3.01 (m, 2 H,
CH2); 4.77 (br.s, 1 H, CH); 5.00 (s, 2 H, CH2, Cbz); 6.94 (br.s, 1 H,
N(6´)H); 7.24—7.35 (m, 6 H, Ph and NH, Cbz); 7.91 (s, 1 H,
C(8´)H); 9.09 (br.s, 1 H, NH, Ac); 12.2 (br.s, 1 H, N(9´)H).
13C NMR (125 MHz, DMSOꢀd6, 25 C), : 22.29 (C(4)); 24.66
(Me, Ac); 27.56 (Me, But); 28.98 and 30.48 (C(5) and C(3));
~40 (C(6), overlapped with the signal of DMSO); 53.64 (C(2),
Lys); 65.03 (CH2, Cbz); 80.40 (C, But); 115.74 (C(5´)); 127.24,
127.61, 128.24 (Cp, Co, Cm); 137.22 and 138.27 (Ci and C(8´));
150.68, 152.42, 154.03 (C(2´), C(4´) and C(6´)); 156.02 (CO,
Cbz); 169.62 (CO, Ac); 171.83 (CO2But). HRMS (ESI): found:
m/z 512.2628 [M + H]+; C25H34N7O5; calculated: M = 512.2616.
tertꢀButyl Nꢀ(2ꢀacetamidopurinꢀ6ꢀyl)ꢀ(S)ꢀprolinate ((S)ꢀ3e).
The yield was 0.288 g (83%), colorless crystals, m.p. 242—244 C
20
(decomp.) (EtOAc), []D –118 (c 0.5, MeOH), ee > 99%,
HPLC (S,SꢀWhelkꢀO 1, MeOH—H2O 60 : 40, 0.9 mL min–1),

R = 20.4 min. HPLC (Phenomenex Luna C(18)), R = 14.3 min.
IR (DRA), /cm–1: 3146, 2975, 1735, 1658, 1595, 1523. 1H NMR
(400 MHz, DMSOꢀd6, 100 C), : 1.35 (s, 9 H, But); 1.87—2.08
(m, 3 H, HВC(3), H2C(4)); 2.30 (s, 3 H, Me, Ac); 2.23—2.36
(m, 1 H, HAC(3)); 3.95 (br.s, 2 H, H2C(5)); 4.96 (br.s, 1 H,
C(2)H); 7.83 (s, 1 H, C(8´)H); 8.97 (br.s, 1 H, NH, Ac); 12.42
(br.s, 1 H, N(9´)H). 13C NMR (125 MHz, DMSOꢀd6, 25 C),
 tautomers A and B (6 : 4): 21.89 (C(4), A); 24.23 (C(4), B); 24.68

Nꢀ(2ꢀAminopurinꢀ6ꢀyl)ꢀsubstituted amino acids
Russ.Chem.Bull., Int.Ed., Vol. 64, No. 9, September, 2015 2111
tertꢀButyl N()ꢀ(2ꢀacetamidopurinꢀ6ꢀyl)ꢀN()ꢀbenzyloxyꢀ
carbonylꢀ(RS)ꢀlysinate ((RS)ꢀ3g). The yield was 0.164 g (32%).
1H NMR spectrum is similar to that of compound (S)ꢀ3g.
tertꢀButyl N()ꢀ(2ꢀacetamidopurinꢀ6ꢀyl)ꢀN()ꢀbenzyloxyꢀ
carbonylꢀ(S)ꢀlysinate ((S)ꢀ3h) was isolated by flashꢀchromatoꢀ
graphy (eluent CHCl3—MeOH from 10 : 0.1 to 10 : 0.6). The
yield was 0.240 g (47%), light yellow crystals, m.p. 193—194 C,
1H NMR (400 MHz, DMSOꢀd6, 25 C), : 1.42 (d, 3 H, Me,
J = 7.3 Hz); 4.70 (m, 1 H, CH); 5.73 (s, 2 H, NH2); 7.09 (br.s,
1 H, N(6´)H); 7.69 (s, 1 H, C(8´)H); 12.16 (br.s, 2 H, N(9´)H
and CO2H). 13C NMR (125 MHz, DMSOꢀd6, 25 C), : 17.72
(Me); 47.89 (CH); 112.42 (C(5´)); 135.80 (C(8´)); 152.55 and
153.69 (C(2´) and C(4´)); 159.72 (C(6´)); 174.76 (CO2H).
Found (%): C, 42.83; H, 4.72; N, 37.28. C8H10N6O2. Calculatꢀ
ed (%): C, 43.24; H, 4.54; N, 37.82.
Nꢀ(2ꢀAminopurinꢀ6ꢀyl)ꢀ(R)ꢀalanine ((R)ꢀ4b). The yield was
0.153 g (69%), colorless crystals, m.p. 280—283 C (decomp.),
[]D20 –55.3 (c 0.54, 1 M NaOH). 1H NMR spectrum is similar
to that of compound (S)ꢀ4b.
25
[]D –13.6 (c 1.0, DMF), ee > 99%, HPLC (S,SꢀWhelkꢀO 1,
MeOH—0.25% aqueous AcOH 78 : 22, 0.85 mL min–1), 
=
R
= 15.9 min. HPLC (Phenomenex Luna C(18)),  = 18.3 min.
R
IR (FTIR), /cm–1: 3318, 3265, 3207, 3144, 3026, 2977, 2938,
2856, 1726, 1711, 1692, 1666, 1626, 1606, 1537. 1H NMR
(400 MHz, DMSOꢀd6, 25 C), : 1.28 (s, 2 H, H2C(4)); 1.35
(s, 9 H, But); 1.72—1.49 (m, 4 H, H2C(3) and H2C(5)); 2.22
(s, 3 H, Me, Ac); 3.43 (m, 2 H, H2C(6), overlapped with the
signal of water); 3.86 (ddd, 1 H, C(2)H, J = 13.6 Hz, J = 8.2 Hz,
J = 5.9 Hz); 5.00 (d, 1 H, HВ, Cbz, J = 12.6 Hz); 5.05 (d, 1 H,
HA, Cbz, J = 12.6 Hz); 7.39—7.25 (m, 5 H, Ph); 7.60 (d, 1 H,
NH, Cbz, J = 7.6 Hz); 7.68 (br.s, 1 H, N(6´)H); 7.93 (s, 1 H,
C(8´)H); 9.70 (s, 1 H, NH, Ac); 12.72 (s, 1 H, N(9´)H). 13C
NMR (125 MHz, DMSOꢀd6, 25 C), : 22.81 (C(4)); 24.59
(Me, Ac); 27.52 (Me, But); 28.52 and 30.51 (C(5) and C(3));
~39.5 (C(6), overlapped with the signal of DMSO); 54.53 (C(2));
65.31 (CH2, Cbz); 80.29 (C, But); 115.70 (C(5´)); 127.15, 127.66,
128.24 (Cp, Co and Cm); 136.96 and 137.69 (Ci and C(8´)); 150.14,
152.69, 154.41 (C(2´), C(4´), C(6´)); 156.02 (CO, Cbz); 169.22
(CO, Ac); 171.58 (CO2But). HRMS (ESI): found: m/z 512.2612
[M + H]+; C25H34N7O5; calculated: M = 512.2616.
Nꢀ(2ꢀAminopurinꢀ6ꢀyl)ꢀ(S)ꢀvaline ((S)ꢀ4c). Method A. The
yield was 0.203 g (81%). 1H NMR spectrum is similar to that
described earlier.37
Method B. The yield was 0.163 g (65%). IR (FTIR), /cm–1
:
3319, 3086, 2961, 2874, 1732, 1654, 1636, 1588, 1564, 1469.
1H NMR spectrum is similar to that described earlier.37
Nꢀ(2ꢀAminopurinꢀ6ꢀyl)ꢀ(S)ꢀphenylalanine ((S)ꢀ4d). Method A.
The yield was 0.209 g (70%). HPLC (Phenomenex Luna C(18)),

= 11.4 min. 1H NMR spectrum is similar to that described
R
earlier.37
Method B. The yield was 0.182 g (61%). IR (DRA), /cm–1
:
3271, 3124, 3026, 2932, 2852, 1633, 1609, 1464, 1454, 1438.
1H NMR spectrum is similar to that described earlier.37
Nꢀ(2ꢀAminopurinꢀ6ꢀyl)ꢀ(RS)ꢀphenylalanine ((RS)ꢀ4d). The
yield was 0.194 g (65%). 1H NMR spectrum is similar to that of
compound (S)ꢀ4d. HRMS (ESI): found: m/z 299.1244 [M + H]+;
C14H15N6O2; calculated: M = 299.1251.
tertꢀButyl N()ꢀ(2ꢀacetamidopurinꢀ6ꢀyl)ꢀN()ꢀbenzyloxyꢀ
carbonylꢀ(RS)ꢀlysinate ((RS)ꢀ3h). The yield was 0.246 g (48%).
HPLC (S,SꢀWhelkꢀO 1, MeOH—0.25% aqueous AcOH 78 : 22,
Nꢀ(2ꢀAminopurinꢀ6ꢀyl)ꢀ(S)ꢀproline dihydrate ((S)ꢀ4e). The
yield was 0.244 g (86%), colorless crystals, m.p. 220—230 C
20
0.85 mL min–1),  = 14.6 ((R)ꢀ3h) and 15.9 min ((S)ꢀ3h).
(decomp.), []D –85.0 (c 1.0; DMF). HPLC (Phenomenex
R
1H NMR spectrum is similar to that of compound (S)ꢀ3h.
Synthesis of Nꢀ(2ꢀaminopurinꢀ6ꢀyl)amino acids 4a—e,g,h
(general procedure). Method A. 1 M aqueous NaOH (6 mL,
6 equiv.) was added to tertꢀbutyl ester 3 (1.0 mmol), and the
mixture was stirred at 60 C for 2 h. A precipitate was filtered,
the filtrate was acidified with 0.1 M aqueous HCl to pH ~5,
a precipitate formed after cooling was separated, washed with
water, and dried in vacuo at 80 C.
Luna C(18)),  = 4.9 min. IR (FTIR), /cm–1: 3331, 3124,
R
2959, 2849, 2794, 1667, 1622, 1564, 1477, 1458. 1H NMR
(500 MHz, DMSOꢀd6, 100 C), : 1.93 (m, 2 H, H2C(4)); 2.07
(m, 1 H, HBC(3)); 2.22 (m, 1 H, HAC(3)); 3.44 (br.s, 6 H,
CO2H, N(9´)H, 2 H2O); 3.89 (m, 1 H, HBC(5)); 3.96 (m, 1 H,
HAC(5)); 4.97 (s, 1 H, C(2)H); 5.26 (s, 2 H, NH2); 7.57 (s, 1 H,
C(8´)H). 13C NMR (125 MHz, DMSOꢀd6, 25 C),  tautomers
A and В: 21.93 (C(4), A); 24.32 (C(4), В); 28.61 (C(3), В); 30.61
(C(3), A); 46.89 (C(5), A); 48.80 (C(5), В); 59.11 (C(2), В);
60.16 (C(2), A); 113.55 (C(5´), В); 113.74 (C(5´), A); 135.00
(C(8´), A); 135.50 (C(8´), В); 152.57 (C(4´) or C(2´), В); 152.68
(C(4´) or C(2´), A); 153.04 (C(2´) or C(4´), A); 153.32 (C(2´) or
C(4´), В); 159.28 (C(6´), В); 159.77 (C(6´), A); 174.10 (CO2H,
A and В). Found (%): C, 42.08; H, 5.74; N, 29.70. C10H12N6O2•
•2H2O (284.28). Calculated (%): C, 42.25; H, 5.68; N, 29.56.
Nꢀ(2ꢀAminopurinꢀ6ꢀyl)ꢀ(RS)ꢀproline ((RS)ꢀ4e). The yield
was 0.227 g (80%), colorless crystals, m.p. 217—235 C (deꢀ
comp.). 1H NMR spectrum is similar to that of compound (S)ꢀ4e.
HRMS (ESI): found: m/z 249.1093 [M + H]+; C10H13N6O2;
calculated: M = 249.1100.
Method B. 1 M aqueous NaOH (6 mL, 6 equiv.) was added
to tertꢀbutyl ester 3 (1.0 mmol), and the mixture was stirred at
20 C for 5 days. A precipitate was filtered, the filtrate was acidꢀ
ified with 0.1 M aqueous HCl to pH ~5, a precipitate formed
after cooling was separated, washed with water, and dried in vacuo
at 80 C.
Nꢀ(2ꢀAminopurinꢀ6ꢀyl)glycine (4a). The yield was 0.173 g
(83%), colorless crystals, m.p. 250—255 C (decomp.). HPLC
(Phenomenex Luna C(18), elution rate 1.0 mL min–1), R = 5.0 min.
IR (DRA), /cm–1: 3434, 3335, 3280, 3197, 3122, 2840, 1635,
1617, 1591, 1463. 1H NMR (400 MHz, DMSOꢀd6, 25 C), :
4.06 (br.s, 2 H, CH2); 5.72 (s, 2 H, NH2); 7.21 (br.s, 1 H,
C(6´)NH); 7.67 (s, 1 H, C(8´)H); 12.10 (br.s, 2 H, N(9´)H and
CO2H). 13C NMR (125 MHz, DMSOꢀd6, 25 C), : 41.33 (CH2);
112.97 (C(5´)); 135.61 (C(8´)); ~154 (br.s, C(2´) and C(4´));
159.79 (C(6´)); 171.99 (CO2H). Found (%): C, 40.38; H, 3.92;
N, 40.30. C7H8N6O2. Calculated (%): C, 40.39; H, 3.87; N, 40.37.
Nꢀ(2ꢀAminopurinꢀ6ꢀyl)ꢀ(S)ꢀalanine ((S)ꢀ4b). The yield was
0.182 g (82%), colorless crystals, m.p. 285—290 C (decomp.),
N()ꢀ(2ꢀAminopurinꢀ6ꢀyl)ꢀN()ꢀbenzyloxycarbonylꢀ(S)ꢀlysꢀ
ine ((S)ꢀ4g). The yield was 0.211 g (51%), colorless crystals, m.p.
20
1
225—226 C (decomp.), []D +20.0 (c 0.4; DMF). H NMR
(500 MHz, DMSOꢀd6, 25 C), : 1.26—1.52 (m, 4 H, 2 CH2);
1.74—1.90 (m, 2 H, CH2); 2.98 (m, 2 H, H2C(6)); 4.06 (br.s, 1 H,
C(2)H); 4.99 (s, 2 H, CH2, Cbz); 5.72 (br.s, 2 H, NH2); 7.00
(br.s, 1 H, N(6´)H); 7.20—7.40 (m, 6 H, Ph and NH, Cbz); 7.69
(s, 1 H, C(8´)H); ~12.2 (br.s, 2 H, N(9´)H and CO2H). 13C NMR
(125 MHz, DMSOꢀd6, 25 C), : 22.86 (C(4)); 29.06 and 30.93
(C(5) and C(3)); ~40 (C(6), overlapped with the signal of
25
[]D +49.7 (c 0.5, 1 M NaOH). HPLC (Phenomenex Luna
C(18)),  = 6.4 min. IR (FTIR), /cm–1: 3372, 3313, 3094,
R
2993, 2946, 2851, 2797, 2645, 1633, 1589, 1548, 1492, 1453.

2112
Russ.Chem.Bull., Int.Ed., Vol. 64, No. 9, September, 2015
Krasnov et al.
20
DMSO); 52.28 (C(2)); 65.03 (CH2, Cbz); 112.67 (C(5´)); 127.25,
127.62, 128.25 (Cp, Co, Cm); 135.61 (C(8´)); 137.23 (Ci); 152.14
and 154.07 (C(4´) and C(2´)); 156.01 (CO, Cbz); 159.80 (C(6´));
174.35 (CO2H). HRMS (ESI): found: m/z 414.1879 [M + H]+;
C19H24N7O4; calculated: M = 414.1884.
(95%), a colorless amorphous compound, []D –28.7 (c 0.8,
DMF), ee 86%. HPLC (S,SꢀWhelkꢀO 1, hexane—PriOH—
MeOH 20 : 1 : 1, 1.0 mL min–1), R = 26.2 ((R)ꢀ5c) and 27.4 min
((S)ꢀ5c). 1H NMR (500 MHz, DMSOꢀd6, 25 C),  tautomers A
and B (9 : 1): 0.94 (d, 3 H, Me, J = 6.8 Hz, A and B); 0.96 (d, 3 H,
Me, J = 6.8 Hz, A and B); 2.32 (m, 1 H, CH, A and B); 3.64 (s, 3 H,
MeO, A); 3.67 (s, 3 H, MeO, В); 4.50 (br.s, 1 H, CH, B); 4.62
(br.s, 1 H, CH, A); 5.52 (br.s, 2 H, NH2, B); 5.72 (br.s, 2 H,
NH2, A); 6.50 (br.s, 1 H, N(6´)H), B); 6.83 (br.s, 1 H, N(6´)H, A);
7.69 (s, 1 H, C(8´)H, A); 7.94 (s, 1 H, C(8´)H, B); 11.75 (s, 1 H,
N(9´)H, B); 12.14 (s, 1 H, C(9´)H, A). HRMS (ESI): found: m/z
265.1412 [M + H]+; C11H17N6O2; calculated: M = 265.1408.
Acid (S)ꢀ4c obtained by method B gave (S)ꢀ5c, The yield
was 0.104 g (79%), colorless amorphous compound, ee > 99%,
HPLC (S,SꢀWhelkꢀO 1, hexane—PriOH—MeOH 20 : 1 : 1,
1.0 mL min–1), R = 27.4 min.
N()ꢀ(2ꢀAminopurinꢀ6ꢀyl)ꢀN()ꢀbenzyloxycarbonylꢀ(S)ꢀlysꢀ
ine ((S)ꢀ4h). Method A. The yield was 0.244 g (59%), colorless
20
crystals, m.p. 163—167 C, []D –9.5 (c 0.4; DMF), ee 90%.
HPLC (S,SꢀWhelkꢀO 1, MeOH—0.25% aqueous AcOH 48 : 52,
0.85 mL min–1), R = 22.0 ((R)ꢀ4h) and 24.1 min ((S)ꢀ4h). HPLC
(Phenomenex Luna C(18)), R = 13.0 min. IR (FTIR), /cm–1
:
3305, 3132, 2937, 2860, 1701, 1632, 1582, 1497, 1453. 1H NMR
(400 MHz, DMSOꢀd6, 25 C), : 1.37 (m, 2 H, H2C(4)); 1.78—1.48
(m, 4 H, H2C(3) and H2C(5)); ~3.5 (m, 2 H, H2C(6), overꢀ
lapped with the signal of H2O); 3.94 (m, 1 H, C(2)H); 5.03 (m, 2 H,
CH2, Cbz); 5.89 (br.s, 2 H, NH2); 7.09 (br.s, 1 H, N(6´)H);
7.39—7.27 (m, 5 H, Ph); 7.58 (d, 1 H, NH, Cbz, J = 7.7 Hz);
7.71 (s, 1 H, C(8´)H); 12.27 (s, 2 H, N(9´)H and CO2H).
13C NMR (125 MHz, DMSOꢀd6, 25 C), : 23.02 (C(4)); 28.75
and 30.56 (C(5) and C(3)); ~40 (C(6), overlapped with the sigꢀ
nal of DMSO); 53.92 (C(2)); 65.34 (CH2, Cbz); 111.78 (C(5´));
126.99, 127.67, 128.29 (Cp, Co, Cm); 135.93 (C(8´)); 136.99 (Ci);
151.36 and 154.29 (C(4´) and C(2´)); 156.14 (CO, Cbz); 159.32
(C(6´)); 174.10 (CO2H). HRMS (ESI): found: m/z 414.1892
[M + H]+; C19H24N7O4; calculated: M = 414.1884.
Methyl Nꢀ(2ꢀaminopurinꢀ6ꢀyl)ꢀ(RS)ꢀvalinate ((RS)ꢀ5c). The
yield was 0.131 g (99%), a colorless amorphous compound,
HPLC (S,SꢀWhelkꢀO 1, hexane—PriOH—MeOH 20 : 1 : 1,
1.0 mL min–1), R = 26.2 ((R)ꢀ5c) and 27.4 min ((S)ꢀ5c). 1H NMR
spectrum is similar to that of compound (S)ꢀ5c. HRMS (ESI): found:
m/z 265.1409 [M + H]+; C11H17N6O2; calculated: M = 265.1408.
Methyl Nꢀ(2ꢀaminopurinꢀ6ꢀyl)ꢀ(S)ꢀphenylalaninate ((S)ꢀ5d).
The yield was 0.144 g (92%), a colorless amorphous compound,
20
[]D –5.7 (c 0.8, DMF), ee > 99%, HPLC (S,SꢀWhelkꢀO 1,
Method B. The yield was 0.232 g (56%), []D20 –9.8 (c 0.4;
DMF), ee > 99%. HPLC (S,SꢀWhelkꢀO 1, MeOH—0.25% aqueꢀ
ous AcOH 48 : 52, 0.85 mL min–1), R = 24.1 min.
hexane—PriOH—MeOH 6 : 1 : 1, 1.0 mL min–1), R = 14.1 min.
1H NMR (500 MHz, DMSOꢀd6, 100 C), : 3.14—3.24 (m, 2 H,
CH2); 3.62 (s, 3 H, MeO); 5.21 (br.s, 1 H, CH); 5.37 (br.s, 2 H,
NH2); 6.60 (br.s, 1 H, NH); 7.17 (m, 1 H, Hp); 7.20—7.28 (m, 4 H,
Ho, Hm); 7.62 (s, 1 H, C(8´)H); 11.84 (br.s, 1 H, N(9´)H). 13C NMR
(125 MHz, DMSOꢀd6, 25 C), : 36.69 (C(3)); 51.77 (OMe);
53.88 (C(2)); 112.66 (C(5´)); 126.36 (Cp); 128.15 (Co); 129.00
(Cm); 135.84 (C(8´)); 137.78 (Ci); 152.44 and 153.77 (C(2´) and
C(4´)); 159.74 (C(6´)); 172.77 (CO2Me). HRMS (ESI): found:
m/z 313.1408 [M + H]+; C15H17N6O2; calculated: M = 313.1408.
Methyl Nꢀ(2ꢀaminopurinꢀ6ꢀyl)ꢀ(RS)ꢀphenylalaninate ((RS)ꢀ
5d). The yield was 0.137 g (88%), a colorless amorphous comꢀ
pound, HPLC (S,SꢀWhelkꢀO 1, hexane—PriOH—MeOH
6 : 1 : 1, 1.0 mL min–1), R = 12.6 ((R)ꢀ5d) and 14.1 min ((S)ꢀ5d).
1H NMR spectrum is similar to that of compound (S)ꢀ5d.
Methyl Nꢀ(2ꢀaminopurinꢀ6ꢀyl)ꢀ(S)ꢀprolinate ((S)ꢀ5e). The
N()ꢀ(2ꢀAminopurinꢀ6ꢀyl)ꢀN()ꢀbenzyloxycarbonylꢀ(RS)ꢀ
lysine ((RS)ꢀ4h). The yield was 0.243 g (59%), colorless crystals,
m.p. 160—165 C. HPLC (MeOH—0.25% aqueous AcOH
48 : 52, 0.85 mL min–1), R = 22.0 ((R)ꢀ4h) and 24.1 min ((S)ꢀ4h).
1H NMR spectrum is similar to that of compound (S)ꢀ4h.
Synthesis of Nꢀ(2ꢀaminopurinꢀ6ꢀyl)ꢀsubstituted amino acid
methyl esters 5b—e (general procedure). Thionyl chloride (0.09 mL,
2.5 equiv.) was added dropwise to a suspension of amino acid 4
(0.50 mmol) in methanol (2 mL) at 0 C. The solution was stirred
at 20 C for 16 h and concentrated. The residue was diluted with
a mixture of EtOAc—5% aqueous Na2CO3 (2 : 1), the organic
layer was separated, washed with saturated aqueous NaCl until
neutrality, dried with Na2SO4, and concentrated. The residue
was dried in vacuo.
20
yield was 0.115 g (88%), a colorless amorphous compound, []D
Methyl Nꢀ(2ꢀaminopurinꢀ6ꢀyl)ꢀ(S)ꢀalaninate ((S)ꢀ5b). The
yield was 0.068 g (58%), a colorless amorphous compound, []D
–96.7 (c 0.8, CHCl3), ee > 99%, HPLC (Chiralcel ODꢀH, hexꢀ
ane—PriOH—MeOH 2 : 0.7 : 0.3, 0.8 mL min–1), R = 11.0 min.
1H NMR (500 MHz, DMSOꢀd6, 100 C), : 1.87—2.03 (m, 3 H,
H2C(4), HBC(3)); 2.27 (m, 1 H, HAC(3)); 3.61 (s, 3 H, MeO);
3.85—4.03 (m, 2 H, H2C(5)); 4.98 (br.s, 1 H, C(2)H); 5.22 (br.s,
2 H, NH2); 7.57 (s, 1 H, C(8´)H); 11.78 (br.s, 1 H, N(9´)H).
HRMS (ESI): found: m/z 263.1253 [M + H]+; C11H15N6O2;
calculated: M = 263.1251.
Methyl Nꢀ(2ꢀaminopurinꢀ6ꢀyl)ꢀ(RS)ꢀprolinate ((RS)ꢀ5e).
The yield was 0.092 g (70%), a colorless amorphous compound,
HPLC (Chiralcel ODꢀH, hexane—PriOH—MeOH 2 : 0.7 : 0.3,
0.8 mL min–1), R = 9.0 ((R)ꢀ5e) and 11.0 min ((S)ꢀ5e). 1H NMR
spectrum is similar to that of compound (S)ꢀ5e. HRMS (ESI): found:
m/z 263.1252 [M + H]+; C11H15N6O2; calculated: M = 263.1251.
N()ꢀ(2ꢀAminopurinꢀ6ꢀyl)ꢀ(S)ꢀlysine ((S)ꢀ6h). A catalyst 10%
Pd/C (35 mg) was added to a solution of (S)ꢀ4h (0.17 g,
0.42 mmol) in AcOH (10 mL), and the mixture was stirred under
atmosphere of H2 (18 atm.) at 70 C for 4 h. A precipitate was
filtered off, the filtrate was concentrated, the residue was puriꢀ
20
–37.3 (c 0.5, DMF), ee > 99%, HPLC (S,SꢀWhelkꢀO 1, hexꢀ
ane—PriOH—MeOH 10 : 1 : 1, 1.0 mL min–1),  = 19.3 min.
R
1H NMR (500 MHz, DMSOꢀd6, 100 C), : 1.45 (d, 3 H, Me,
J = 7.2 Hz); 3.64 (s, 3 H, MeO); 4.94 (m, 1 H, CH); 5.31 (br.s,
2 H, NH2); 6.69 (br.s, 1 H, N(6´)H); 7.61 (s, 1 H, C(8´)H);
11.79 (br.s, 1 H, N(9´)H). 13C NMR (125 MHz, DMSOꢀd6,
25 C), : 17.47 (Me); 47.94 (CH); 51.72 (OMe); 112.57 (C(5´));
135.79 (C(8´)); 152.48 and 153.62 (C(2´) and C(4´)); 159.73
(C(6´)); 173.74 (CO2Me). HRMS (ESI): found: m/z 237.1098
[M + H]+; C9H13N6O2; calculated: M = 237.1095.
Methyl Nꢀ(2ꢀaminopurinꢀ6ꢀyl)ꢀ(R)ꢀalaninate ((R)ꢀ5b). The
25
yield was 0.063 g (53%), a colorless amorphous compound, []D
+33.3 (c 0.5, DMF), ee > 99%. HPLC (S,SꢀWhelkꢀO 1, hexꢀ
ane—PriOH—MeOH 10 : 1 : 1, 1.0 mL min–1),  = 18.0 min.
R
1H NMR spectrum is similar to that of compound (S)ꢀ5b.
Methyl Nꢀ(2ꢀaminopurinꢀ6ꢀyl)ꢀ(S)ꢀvalinate ((S)ꢀ5c). Acid
(S)ꢀ4c obtained by method A gave (S)ꢀ5c, The yield was 0.126 g

Nꢀ(2ꢀAminopurinꢀ6ꢀyl)ꢀsubstituted amino acids
Russ.Chem.Bull., Int.Ed., Vol. 64, No. 9, September, 2015 2113
fied by flashꢀchromatography (eluent CHCl3—MeOH—AcOH,
7 : 3 : 0.5 (30 mL), then MeOH (80 mL)). The yield was 0.055 g
Charushin, O. N. Chupakhin, V. N. Kalinin, J. Organomet.
Chem., 2006, 690, 2761.
19. M. Gynther, K. Laine, J. Ropponen, J. Leppänen, A. Manꢀ
nila, T. Nevalainen, J. Savolainen, T. Järvinen, J. Rautio,
J. Med. Chem., 2008, 51, 932.
20
(47%), colorless crystals, m.p. 244—248 C (decomp.), []D
1
+18.9 (c 0.4, AcOH). H NMR (400 MHz, DMSOꢀd6, 25 C),
: 1.38 (m, 2 H, H2C(4), Lys); 1.47—1.65 (m, 3 H, Lys); 1.83
(m, 1 H, Lys); 3.17 (s, 2 H, H2C(6), Lys); 3.20—3.55 (br.m, 3 H,
C(2)H and NH2, Lys, overlapped with the signal of H2O); 5.69
(s, 2 H, N(2´)H2); 7.19 (br.s, 1 H, N(6´)H); 7.64 (s, 1 H, C(8´)H);
8.3 (br.s, 2 H, N(9´)H and CO2H). 13C NMR (125 MHz,
DMSOꢀd6, 25 C), : 22.51 (C(4), Lys); 29.00 and 30.86 (C(5)
and C(3) Lys); 48.53 (C(6), Lys); 54.26 (C(2), Lys); 111.94
(C(5´)); 135.77 (C(8´)); 153.03, 154.23, 160.11 (C(2´), C(4´),
C(6´)); 171.00 (CO2H). HRMS (ESI): found: m/z 280.1518
[M + H]+; C11H18N7O2; calculated: M = 280.1516.
20. B. S. Chhikara, K. Parang, Expert Opin. Drug Delivery, 2010,
7, 1399.
21. D. A. Gruzdev, G. L. Levit, I. V. Bazhov, A. M. Demin,
L. Sh. Sadretdinova, V. A. Ol´shevskaya, V. N. Kalinin, V. P.
Krasnov, O. N. Chupakhin, Russ. Chem. Bull. (Int. Ed.),
2010, 59, 110 [Izv. Acad. Nauk, Ser. Khim., 2010, 110].
22. S. Dutta, S. Ray, K. Nagarajan, Saudi Pharm. J., 2013, 21, 337.
23. L. Peura, K. Malmioja, K. Huttunen, J. Leppänen, M. Hä-
mäläinen, M. M. Forsberg, J. Rautio, K. Laine, Pharm. Res.,
2013, 30, 2523.
The authors are grateful to M. I. Kodess and M. A.
Ezhikova for recording NMR spectra, O. V. Koryakova
for recording IR spectra, I. N. Ganebnykh for recording
high resolution mass spectra, and M. S. Toporova for
HPLC studies on chiral stationary phases.
24. B. Zacharie, L. Gagnon, G. Attardo, T. P. Connolly, Y. Stꢀ
Denis, C. L. Penney, J. Med. Chem., 1997, 40, 2883.
25. B. S. Anand, S. Katragadda, Y. E. Nashed, A. K. Mitra,
Curr. Eye Res., 2004, 29, 153.
26. V. P. Kamath, J. Zhang, P. E. Morris Jr., Y. S. Babu, Bioorg.
Med. Chem. Lett., 2006, 16, 2662.
This work was financially supported by the Russian
Science Foundation (Grant 14ꢀ13ꢀ01077).
27. I. S. Krylov, B. A. Kashemirov, J. M. Hilfinger, C. E.
McKenna, Mol. Pharmaceutics, 2013, 10, 445.
28. S. Pande, P. Utale, S. Gholse, P. Tekade, J. Gunjate, Int. J.
ChemTech Res., 2013, 5, 3023.
References
29. Y. Voynikov, V. Valcheva, G. Momekov, P. Peikov, G. Stavꢀ
rakov, Bioorg. Med. Chem. Lett., 2014, 24, 3043.
30. Eur. Pat. 349242; Chem. Abstrs, 1990, 113, 59783.
31. A. F. Sayed Ahmed, Indian J. Chem., 1995, 34B, 135.
32. PCT Appl. WO2000039143; Chem. Abstrs, 2000, 133, 89744.
33. PCT Appl. WO2001058894; Chem. Abstrs, 2001, 135, 167014.
1. S. R. Ross, D. McTavish, D. Faulds, Drugs, 1993, 45, 737.
2. M. Legraverend, D. S. Grierson, Bioorg. Med. Chem., 2006,
14, 3987.
3. A. M. Roecker, A. Stockert, D. F. Kisor, Clin. Med. Insights:
Oncol., 2010, 4, 133.
4. P. Robak, T. Robak, Cancer Treat. Rev., 2013, 39, 851.
5. L.ꢀL. Gundersen, J. NissenꢀMeyer, B. Spilsberg, J. Med.
Chem., 2002, 45, 1383.
6. M. C. Long, W. B. Parker, Biochem. Pharmacol., 2006, 71, 1671.
7. A. Vik, E. Hedner, C. Charnock, L. W. Tangen, Ø. Samuelsꢀ
en, R. Larsson, L. Bohlin, L.ꢀL. Gundersen, Bioorg. Med.
Chem., 2007, 15, 4016.
´
34. A. Holý, I. Votruba, E. Tlouštova, M. Masojhdková, Collect.
Czech. Chem. Commun., 2001, 66, 1545.
35. J. A. Modi, K. C. Patel, Med. Chem. Res., 2012, 21, 1660.
36. H.ꢀY. Niu, S.ꢀX. Bai, S. Wu, G.ꢀR. Qu, H.ꢀM. Guo, Asian J.
Org. Chem., 2012, 1, 238.
37. A. Yu. Vigorov, V. P. Krasnov, D. A. Gruzdev, A. A.
Men´shikova, A. M. Demin, G. L. Levit, V. N. Charushin,
Mendeleev Commun., 2014, 24, 35.
8. M. Brændvang, L.ꢀL. Gundersen, Bioorg. Med. Chem., 2007,
15, 7144.
v
38. P. Se ká ová, R. Marek, K. Malináková, E. Kolehmainen,
D. Hocková, M. Hocek, V. Sklená , Tetrahedron Lett., 2004,
45, 6259.
39. T. Bartl, Z. Zacharová, P. Se ká ová, E. Kolehmainen,
R. Marek, Eur. J. Org. Chem., 2009, 1377.
40. T. A. Mohamed, I. A. Shabaan, W. M. Zoghaib, J. Husband,
R. S. Farag, A. E.ꢀN. M. A. Alajhaz, J. Mol. Struct., 2009,
938, 263.
41. W. A. Bowles, F. H. Schneider, L. R. Lewis, R. K. Robins,
J. Med. Chem., 1963, 6, 471.
42. P. H. Bentley, A. V. Stachulski, J. Chem. Soc., Perkin Trans.
1, 1983, 1187.
43. V. P. Krasnov, G. L. Levit, I. M. Bukrina, A. M. Demin, O. N.
Chupakhin, J. U. Yoo, Tetrahedron: Asymmetry, 2002, 13, 1911.
44. E. Taschner, A. Chimiak, B. Bator, T. Soko owska, J. Liebigs
Ann. Chem., 1961, 646, 134.
9. A. K. Pathak, V. Pathak, L. E. Seitz, W. J. Suling, R. C.
Reynolds, Bioorg. Med. Chem., 2013, 21, 1685.
10. L. Meijer, E. Raymond, Acc. Chem. Res., 2003, 36, 417.
11. M. Samsel, K. Dzierzbicka, P. Trzonkowski, Bioorg. Med.
Chem. Lett., 2014, 24, 3587.
12. Z. Demir, E. B. Guven, S. Ozbey, C. Kazak, R. C. Atalay,
M. Tuncbilek, Eur. J. Med. Chem., 2015, 89, 701.
13. M. T. Cancilla, M. M. He, N. Viswanathan, R. L. Simmons,
M. Taylor, A. D. Fung, K. Cao, D. A. Erlanson, Bioorg.
Med. Chem. Lett., 2008, 18, 3978.
14. G. L. Levit, L. B. Radina, V. P. Krasnov, Pharm. Chem. J.,
1995, 29, 518 [Khim. Farm. Zh. (Engl. Transl.), 1995, 29 (8), 10].
15. V. P. Krasnov, E. A. Zhdanova, L. I. Smirnova, Russ. Chem.
Rev., 1995, 64, 1049.
16. Membrane Transporters as Drug Targets, Eds G. L. Amidon,
W. Sadée, Kluwer Academic Publishers, New York, Boston,
Dordrecht, London, Moscow, 2002, 528 pp.
45. W. Rittel, Helv. Chim. Acta, 1962, 45, 2465.
46. M. J. Milewska, A. Chimiak, Tetrahedron Lett., 1987, 28, 1817.
17. A. Balakrishnan, B. JainꢀVakkalagadda, C. Yang, D. Pal,
A. K. Mitra, Int. J. Pharm. (Amsterdam, Neth.), 2002, 247, 115.
18. V. A. Ol´shevskaya, R. Ayuob, Z. G. Brechko, P. V. Petroꢀ
vskii, E. G. Kononova, G. L. Levit, V. P. Krasnov, V. N.
Received April 23, 2015